Search

Cores from Searsville Lake within Stanford University’s Jasper Ridge Biological Preserve, California, USA, are examined to identify a potential GSSP for the Anthropocene: core JRBP2018-VC01B (944.5 cm-long) and tightly correlated JRBP2018-VC01A (852.5 cm-long). Spanning from 1900 CE ± 3 years to 2018 CE, a secure chronology resolved to the sub-annual level allows detailed exploration of the Holocene-Anthropocene transition. We identify the primary GSSP marker as first appearance of 239,240Pu (372–374 cm) in JRBP2018-VC01B and designate the GSSP depth as the distinct boundary between wet and dry season at 366 cm (6 cm above the first sample containing 239,240Pu) and corresponding to October-December 1948 CE. This is consistent with a lag of 1–2 years between ejection of 239,240Pu into the atmosphere and deposition. Auxiliary markers include: first appearance of 137Cs in 1958; late 20th-century decreases in δ15N; late 20th-century elevation in SCPs, Hg, Pb, and other heavy metals; and changes in abundance and presence of ostracod, algae, rotifer, and protozoan microfossils. Fossil pollen document anthropogenic landscape changes related to logging and agriculture. As part of a major university, the Searsville site has long been used for research and education, serves users locally to internationally, and is protected yet accessible for future studies and communication about the Anthropocene., The Anthropocene Review, 10 (1), ISSN:2053-0196, ISSN:2053-020X

Despite extensive paleoenvironmental research on the postglacial history of the Kenai Peninsula, Alaska, uncertainties remain regarding the region's deglaciation, vegetation development, and past hydroclimate. To elucidate this complex environmental history, we present new proxy datasets from Hidden and Kelly lakes, located in the eastern Kenai lowlands at the foot of the Kenai Mountains, including sedimentological properties (magnetic susceptibility, organic matter, grain size, and biogenic silica), pollen and macrofossils, diatom assemblages, and diatom oxygen isotopes. We use a simple hydrologic and isotope mass balance model to constrain interpretations of the diatom oxygen isotope data. Results reveal that glacier ice retreated from Hidden Lake's headwaters by ca. 13.1 cal ka BP, and that groundwater was an important component of Kelly Lake's hydrologic budget in the Early Holocene. As the forest developed and the climate became wetter in the Middle to Late Holocene, Kelly Lake reached or exceeded its modern level. In the last ca. 75 years, rising temperature caused rapid changes in biogenic silica content and diatom oxygen isotope values. Our findings demonstrate the utility of mass balance modeling to constrain interpretations of paleolimnologic oxygen isotope data, and that groundwater can exert a strong influence on lake water isotopes, potentially confounding interpretations of regional climate.

Long and continuous lake sedimentary records offer enormous potential for interpreting the paleoenvironmental histories of the past and for understanding how terrestrial environments might adapt in the context of current global warming. However, sedimentary records that contain multiple glacialinterglacial cycles are scarce in continental basins. An ~80 m sediment core was recently obtained from Stoneman Lake (STL), Arizona, containing a unique record of the last ~1.3 Ma. Here we show a detailed pollen study of the topmost ~10 m of the record, covering the last climatic cycle since the Last Interglacial period (MIS5-MIS1; last ~130,000 years ¼ 130 kyr), with the goal of broadening our knowledge of the paleoenvironmental history of the arid North American Southwest in the past. The STL pollen record shows that the MIS5e interglacial was the warmest period of the last 130 kyr. This is deduced by the abundance of pollen types from plants that today exist at lower elevations that occurred around the STL at that time. These include Pinus edulis and other associated low elevation thermophilous plants such as Juniperus, Ambrosia, Amaranthaceae, Asteraceae and Artemisia. Climate cooled rapidly and dramatically at the MIS5-4 boundary, which triggered a displacement of forest species towards lower elevation, causing P. ponderosa to occupy the study area. MIS3 was characterized by relatively warmer climate conditions with 3 prominent climatic oscillations (MIS3a, b and c). The coldest conditions were reached during MIS2 (LGM), when a ~1000 m displacement towards lower elevations of the subalpine forest species relative to present is observed. This is deduced by the highest abundance of Picea (~20 e25%) and Abies in the STL record, indicating their occurrence in the study area.Warming during the last deglaciation is evidenced by a shift of vegetation towards higher altitudes and the development of a montane forest composed mainly of Pinus ponderosa and Quercus replacing the LGM subalpine species. This montane forest remained abundant throughout the Holocene. This study shows that orbital-scale climate changes (mainly precession and eccentricity changes) forced vegetation and lake-level oscillations, documenting that insolation had a main role in controlling environmental change in this area. Climate projections of enhanced warming predict that P. edulis and Juniperus forest species will occupy the study area in the near future., Spanish Government PRX18/00080 PRX21/00127 CGL2013-47038-R CGL2017- 85415-R, European Commission, Spanish Government CGL2013-47038-R CGL2017- 85415-R, Junta de Andalucia P-20-00059, FEDER Projects B-RNM-144-UGR18, UGR-FEDER B-RNM-144-UGR18, Arizona TRIFresearch program 1002654

Long, continuous records of terrestrial paleoclimate offer insights into natural climate variability and provide context for geomorphological studies, climate model reconstructions, and predictions of future climate change. STL14 is an 80 m lacustrine sediment core that archives paleoenvironmental changes at Stoneman Lake, Coconino County, Arizona, from the early Pleistocene (ca. 1.3 Ma) to present. Full-core sedimentology was analyzed using smear slides and core face observations. Lithofacies strongly correlate with wet bulk density and bulk magnetic susceptibility (MS), and these data resemble a sawtooth pattern characteristic of glacial-interglacial climate cycles. A linkage between deep to shallow lake depth transitions and glacial terminations is supported by an age model that incorporates accelerator mass spectrometry radiocarbon dates and tephrochronology of ashes from the Lava Creek B and multiple Long Valley, California, volcanic eruptions. We correlated middle and late Pleistocene glacial maxima to deep lake deposits defined by well-preserved bedding, increased biosilica, boreal pollen taxa (i.e., Picea), and lower density and MS. Interglacial periods are associated with shallow-water deposits characterized by banded-to-massive siliciclastic material, some authigenic calcite, the alga Phacotus, and higher density and MS. Prior to the marine isotope stage (MIS) 24–22 interval, smaller-amplitude changes in the lake environment suggest milder glacial conditions compared to those of the middle and late Pleistocene. Thus, abrupt intensification of glacial conditions may have occurred ca. 900 ka in the American Southwest, mirroring a global characteristic of the mid-Pleistocene transition. The STL14 record suggests that lake environments throughout the history of this small (3.5 km2), internally drained, basaltic catchment are sensitive to the regional hydrologic balance, which, at orbital time scales, is largely influenced by the northern cryosphere and associated changes in atmospheric circulation. The predominance of quartz in sediment throughout the record indicates significant eolian inputs. Few paleoclimate records from this region extend beyond the last glacial cycle, let alone the middle Pleistocene, making STL14 a valuable resource for studying environmental responses to a range of natural climate states and transitions throughout much of the Quaternary.

This study was supported by the action "Proyectos I + D + i del Programa Operativo FEDER 2018-Junta de Andalucia-UGR"& nbsp;[grant number B-RNM-144-UGR] , the Spanish Government (Ministerio de Economia y Competividad) and the European Regional Development Fund (ERDF-FEDER) [grant numbers CGL2013-47038-R and CGL2017- 85415-R] , and the Junta de Andalusia [grant numbers P18-RT-871 and Retos P20_00059, and research group RNM-190] . A.G.-A. was supported by a Ramon y Cajal Fellowship RYC of the Spanish Government (Ministerio de Economia y Competividad) [grant number 2015-18966] . A.L.-A PhD was funded by a fellowship of the Spanish Government (Ministerio de Economia y Competividad) [grant number BES-2018-084293] . F.G was financially supported by a Ramon y Cajal Fellowship (RYC2020-029811-I) of the Spanish Government (Ministerio de Economia y Competividad) . The Open Access Publication Charge was funded by the University of Granada/CBUA., Evaporitic lakes such as playa-lakes are characteristic of many arid regions and are unique environments with respect to fauna and flora, while being very vulnerable to climate and environmental fluctuations and threatened by the current global change scenario. Water balance oscillations in these systems can trigger the precipitation or dissolution of different evaporitic minerals, negatively impacting local biodiversity and economic activities. Here, we study the sedimentary record of a small saline pond from a playa-lake complex in southwestern Iberia in order to reconstruct the paleohydrological evolution of this area and assess potential anthropogenic disturbances. The different proxies studied in the ~11.9 ky old sedimentary record of the Laguna de la Ballestera suggest that the greatest lake extension and the highest water levels occurred during the Early Holocene, pointing to the wettest period of the record. Climate transitioned towards more arid conditions during the Middle Holocene, and even more dramatically during the Late Holocene. In this last stage the wetland surface and the water level largely diminished and gypsum precipitation gradually increased pointing towards a negative precipitation/ evapotranspiration balance and lowest water levels. Summer desiccation likely occurred under this scenario, especially after ~1.0–0.9 cal ky BP coeval with the Medieval Climate Anomaly, when gypsum content started to rise abruptly. However, this significant gypsum precipitation was only associated with a massive drop in the siliciclastic content and scarce carbonates (dolomite and calcite) during the last ~400 years. This evidence suggests a shift from a (semi) permanent to a temporal/seasonal hydrological regime. The environmental evolution of this wetland responded to the general climatic evolution of the western Mediterranean during the Holocene, being mostly controlled by changes in insolation. Our data also show that the environmental response of the studied wetland to natural climate variations was only significantly disturbed by human activities since the 20th century, especially in the second half of the century, deduced by abrupt fluctuations in the siliciclastic, gypsum and organic content in the sediments, as well as by the enhanced sedimentary accumulation rates, probably as a response to changes in the hydroperiod of the lake and in the catchment land use., Spanish Government B-RNM-144-UGR, Proyectos I + D + i del Programa Operativo FEDER 2018-Junta de Andalucia-UGR B-RNM-144-UGR, Spanish Government RNM-190, European Commission B-RNM-144-UGR BES-2018-084293, Junta de Andalusia CGL2013-47038-R CGL2017- 85415-R P18-RT-871, Ramon y Cajal Fellowship RYC of the Spanish Government (Ministerio de Economia y Competividad) Retos P20_00059 2015-18966, University of Granada/CBUA RYC 2020-029811-I

Holocene climate records from northern Europe improve our understanding of important North Atlantic ocean and atmospheric circulation systems to long-term insolation-driven changes, as well as more rapid forcing and feedback mechanisms. Here we assess Holocene climate and environmental changes in northern Norway based on the analysis of pollen, non-pollen palynomorphs, plant macrofossils, and plant wax biomarkers from a high latitude ombrotrophic bog. We define the extent and thickness of Hollabåttjønnen Bog (0.16 km2), which is located 10 km north of Tromsø. Several cores were analyzed, including a 5.16-m core that spans the last 9.5 cal ka BP. Vegetation changes from several sites were reconstructed and the distribution and hydrogen isotopic composition (δD) of n-alkanes (C21–C33) were analyzed. Our data show several distinct climate intervals that primarily indicate changes in bog surface moisture. In the early Holocene (c. 9.5–7.7 cal ka BP), wetter conditions are defined by the presence of wetland sedges and grasses, higher concentrations of mid-chain length n-alkanes, and a similarity in δD values among homologs. A dry mid-Holocene (c. 7.7–3.8 cal ka BP) is inferred from the presence of a heath shrubland, low peat accumulations rates, and significant differences between δD values of mid- and long-chain length n-alkanes. The late Holocene (c. 3.8 cal ka BP-present) is marked by the onset of wetter conditions, lateral bog expansion, and an increase in sedges and grasses. The Hollabåttjønnen Bog record is also significant because its margins were an important location for human settlement. We correlate early Holocene environmental conditions with changes in Stone Age structures recently excavated, and we identify the occurrence of coprophilous fungi, such as Sporormiella and Sordaria, likely associated with reindeer grazing activity beginning c. 1 cal ka BP. This site therefore provides important regional paleoclimate information as well as context for evaluating local prehistoric human-environment interactions.

In this study, we studied palynomorphs from the alpine record from Laguna Seca in Sierra Nevada, southern Spain to investigate the response of forests and lake environments in the western Mediterranean area to climate changes and to human impact during the latest Pleistocene and Holocene. Xerophyte herbs such as Artemisia, Ephedra, and Amaranthaceae are highest during the late Bölling-Allerod (BA) and Younger Dryas (YD) indicating aridity. Poaceae (grasses) are maximum in the BA and Early Holocene, probably indicating expansion in the barren areas left by glaciers after deglaciation. Forest abundance and composition, in particular of deciduous Quercus and Pinus species indicates maximum humidity during the Early Holocene and aridification in the Middle and Late Holocene. Microcharcoal analysis done on the palynological preparations agrees with the vegetation changes, showing maxima in the Early and Middle Holocene and a decrease in the Late Holocene when the Mediterranean vegetation, and thus fuel availability, diminished. The record of Laguna Seca is particularly interesting with respect to the algae and other aquatic microscopic remains. Pediastrum algae only occurred and showed very high abundances during the end of Heinrich Stadial 1 and beginning of the BA, probably indicating deepest lake conditions with deglaciation. Botryococcus, Debarya, Spirogyra and Zygnema algae also peaked at that climate transition, also supporting lake conditions. Botryococcus increased during the Early Holocene, while the rest of the algae almost vanished, which could indicate that the lake became shallower but very productive until 8200 cal yr BP. From then on, the lake level lowered and became a seasonal lake in the Middle-Late Holocene transition.

The Mediterranean region is a vulnerable area affected, among other environmental threats, by aridification and desertification processes that can cause soil salinization in areas rich in evaporite deposits. This is especially evident in the evaporite lake systems of southern Iberia, where a combination of both natural and anthropogenic causes gives rise to aridification and land degradation. In this study, we performed a multiproxy analysis (magnetic susceptibility, lithology, bulk organic geochemistry, inorganic geochemistry, and mineralogy) in the sedimentary record of the Laguna de la Ballestera playa-lake in southwestern Iberia in order to reconstruct the environmental evolution of this evaporitic area and potential human impacts during the Holocene. The most humid period registered in the Laguna de la Ballestera record was the Early Holocene with significant catchment runoff and high lake water levels. The lake size, lake level and catchment runoff decreased throughout the Middle and Late Holocene. This aridification trend is especially evident in the Late Holocene when gypsum precipitation enhanced. The highest gypsum contents were registered in the last millennium, especially after 1600 cal yr CE, probably related to important changes in the hydrology of the area, shifting from a (semi) permanent to a temporal and seasonal hydrological regime. These trends represent the local environmental responses to the western Mediterranean climate during the Holocene. The studied proxies did not evidence any potential human impact until the last century, when the catchment runoff and the playa-lake hydroperiod changed, probably related to changes in the land use.

This study is was supported by the I + D + i projects CGL2013- 47038-R and CGL2017-85415-R funded by Ministerio Ciencia e Innovación/Agencia Estatal de Investigación/ 10.13039/ 501100011033/ and Fondo Europeo de Desarrollo Regional “Una manera de hacer Europa”, I + D + i projects B-RNM-144-UGR18 and P20_00059 of the action “Proyectos I + D + i del Programa Operativo FEDER - Junta de Andalucía - UGR” and the research group RNM-190. Alejandro López Avilés acknowledges the predoctoral fellowship BES- 2018-084293 provide by the MCIN/ AEI/ 10.13039/501100011033/. Antonio García-Alix acknowledges the Ramón y Cajal fellowship RYC- 2015-18966 provided by the MCIN/ AEI/ 10.13039/501100011033/. Fernando García García collaboration in this study was supported by the I + D + I project CGL2017-89618-R funded by MCIN/ AEI/ 10.13039/ 501100011033/ and FEDER “Una manera de hacer Europa” and the research group RNM-178. José Carrión collaboration in this study was supported by the I + D + I project PID2019-1049449 GB-I00 funded by MCIN/ AEI/ 10.13039/501100011033/ and FEDER “Una manera de hacer Europa” and the fellowship 20788/PI/18 of Fundación Séneca., In an effort to understand how alpine environments from the western Mediterranean region responded to climate variations since the last glacial-interglacial transition, a detailed chronological control and sedimentological analysis, supported by magnetic susceptibility, total organic carbon and C/N data, were carried out on the sedimentary record of Laguna Seca (LS). This is a latitudinal and altitudinally (2259 masl) key alpine wetland site located in the easternmost area of the Sierra Nevada, southern Iberian Peninsula, where sediments accumulated during Heinrich Stadial 1, Bølling-Allerød (B-A) and the Younger Dryas (YD) - previously unrecorded in alpine Sierra Nevada. Climate controlled sedimentation in LS and three coarse-grained and one fine-grained facies association are differentiated, which help us decipher the paleoenvironmental evolution of LS: (1) subaerial cohesionless debris flows during a paraglacial stage; (2) till or nival diamicton during a small glacier/nivation hollow stage; (3) massive mudstone by suspension settling of clays into standing water during a lacustrine stage; and (4) frost-shattering breccia deposited inside the lacustrine stage, probably during the YD, and linked to a periglacial substage. The development of a previously existing small glacial cirque during the Last Glacial Maximum (LGM) in the LS basin at an elevation between 2500 and 2300 m could be supported by the important availability of slope sediments glacially-conditioned such as debris flows, reworked by paraglacial slope processes during the first deglaciation stages, confirming previous studies of landforms in the catchment area and the LGM-Equilibrium Line Altitude estimation above 2400 masl in Sierra Nevada. Mean sediment accumulation rates in the LS sedimentary units (4.21 and 0.28 mm/yr during the paraglacial - small glacier/nivation stage and the lacustrine stage, respectively) confirm that geomorphic activity accelerated just after glaciers retreated due to a slope adjustment and high availability of glacially conditioned sediments. An abrupt change in paleoenvironmental and paleoclimatic conditions occurred in LS at ∼ 15.7 cal kyr BP. This change was probably due to an increase in temperature and precipitation in the western Mediterranean region during the B-A. At LS, this resulted in significant ice-melt, forming a deep-water lake in LS with important organic matter contribution until the end of the Early Holocene (except in the YD when the lake level probably dropped), but elsewhere a general glacier recession in the Sierra Nevada and an expansion of the Mediterranean forest in the southern Iberian Peninsula. Finally, the general long-term aridification that occurred during the Middle Holocene until the present in the western Mediterranean region triggered an important environmental change transforming LS into an ephemeral wetland with an increase in aquatic productivity., I + D + i CGL2013-47038-R, CGL2017-85415-R, Ministerio Ciencia e Innovación, European Regional Development Fund B-RNM-144-UGR18, P20_00059, RNM-190, Junta de Andalucía, Agencia Estatal de Investigación 20788/PI/18, CGL2017-89618-R, PID2019-1049449 GB-I00, RYC-2015-18966, Fundación Séneca

Acknowledgments This study was supported by the project B-RNM-144-UGR18 and ARNM- 336-UGR20 of the action “Proyectos I + D + i del Programa Operativo FEDER 2018 - Junta de Andalucía-UGR”, the projects CGL2013-47038-R and CGL2017-85415-R, of the “Ministerio de Economía y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER”, and the research group RNM-190 (Junta de Andalucía), and the projects P18-RT-871 and Retos P20_00059 of Junta de Andalucia. A.G.-A. was also supported by a Ram´on y Cajal Fellowship RYC-2015-18966 of the Spanish Government (Ministerio de Economía y Competividad). A.L.-A PhD is funded by BES-2018-084293 (Ministerio de Economía y Competividad). We thank the Paleomagnetic Laboratory CCiTUB-Geo3Bcn CSIC for the support on paleomagnetic analysis. LV and EB thank the Geomodels Research Institute (UB). We are very grateful to two anonymous reviewers and to the editor Christian Zeeden., The Padul-15-05 sediment core provides an exceptional perspective of the paleoenvironmental and climate change in the Western Mediterranean region for the last ca. 200 kyr. However, even though a robust chronology mainly relying on radiometric dating is available for the last 50 ka, the chronology for the older sediments is not yet fully resolved. Ages for the bottom part of the core (>21 m) were previously inferred from amino-acid racemization dating and sediment accumulation rates. In this work, we provide a more accurate chronology for the older part (>100 kyr) of the Padul-15-05 sediment core record based on the recognition of past Earth's magnetic excursions. We identify an interval prone of reversed polarity samples close to MIS-5e/5 d transition that we correlate to the Blake geomagnetic excursion (116.5 kyr–112 kyr). In addition, we identify an interval of low inclinations and two reversed samples that we interpret as the Iceland Basin geomagnetic excursion (192.7 kyr–187.7 kyr: wide scenario of VGP, Project B-RNM-144-UGR18 and ARNM- 336-UGR20 of the action “Proyectos I + D + i del Programa Operativo FEDER 2018 - Junta de Andalucía-UGR”, Projects CGL2013-47038-R and CGL2017-85415-R, of the “Ministerio de Economía y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER”, Research group RNM-190 (Junta de Andalucía), Projects P18-RT-871 and Retos P20_00059 of Junta de Andalucia, Ramón y Cajal Fellowship RYC-2015-18966 of the Spanish Government (Ministerio de Economía y Competividad), Funded by BES-2018-084293 (Ministerio de Economía y Competividad)

This study was supported by the project P11-RNM 7332 of the "Junta de Andalucia", the projects CGL2013-47038-R and CGL2017-85415-R of the "Ministerio de Economia y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER" and the research group RNM0190 (Junta de Andalucia). We thank Julio Betancourt for thoughtful comments on a previous version of the manuscript and Frank Sirocko and Guillaume Leduc for providing 106KL and M772-059 data. This paper benefited from the thoughtful comments and suggestions of two anonymous reviewers., El Niño Southern Oscillation (ENSO) is one of the most important modes of variability in the climate system. However, ENSO instrumental records are too short to characterize its natural variability at long-term timescales. Paleoclimate records showing ENSO variability during the Holocene on centennial and millennial timescales are rare but critical for our understanding of long-term multidecadal-to millennial-scale variability. Here we used several climate sensitive piñon pine (Pinus edulis) pollen records from the Southern Rockies, USA, to produce a detailed continuous record of effective precipitation and ENSO-like variability for the last 11,000 yrs. La Niña conditions dominated the Early Holocene while El Niño conditions enhanced in an increasing trend over the last 6,000 yrs. This trend was modulated by millennial-scale and ENSO-like hydrological activity at prominent 900-1,000-yr cycles and the amplitude of these cycles increased until present. Enhanced La Niña and related multidecadal megadroughts occurred in the Southern Rockies centered at ca. 10, 8, 6.8, 5.8, 4.8, 4, 3, 2.2, 1 ka. Insolation and solar output changes are suggested here as the main triggers for ENSO climate and vegetation changes. Our analysis of recent strong La Niña events, representing modern climate analogs of past conditions, indicates anomalouslydry conditions persisting annually, leading to prolonged drought that impact piñon pine growth. Following the thermostat hypothesis and the Sun-ENSO link, such dry conditions are expected to prevail in the future, which combined with increasing temperatures, will most likely generate megadroughts in the SW USA., Junta de Andalucia P11-RNM 7332 RNM0190, Ministerio de Economia y Competitividad of Spain CGL2013-47038-R CGL2017-85415-R, European Commission CGL2013-47038-R CGL2017-85415-R

Learning how terrestrial environments responded to past temperature and precipitation variations can help anticipate how these environments will respond to natural climate variability upon which human induced climate change is superimposed. Here we present a detailed multi-proxy analysis of a sediment core from Emerald Lake, located at the montane-subalpine forest transition in west-central Colorado. The record tracks changes in the lake environment, vegetation and fire activity mostly related to climate change for the latest Pleistocene and Holocene, and complements the previously published lake-level reconstruction from the same site. Vegetation and lake level show similar patterns with insolation changes and other regional paleoclimate records for temperature, with coldest conditions during the Younger Dryas (YD), warming in the early Holocene - thermal maximum reached around 6800 cal yr BP – and cooling in the middle and late Holocene. This record also shows how subalpine environments reacted to Holocene moisture dynamics for both summer, related to the North American Monsoon (NAM), and winter precipitation that could be associated with El Nino Southern Oscillation (ENSO), indicating that Emerald Lake was sensitive to long-term and millennial-scale regional and global climate changes. Climatic perturbations, generally cold and/or arid events and low lake levels at ca. 8200, 4200, 1200-1000 cal yr BP influenced the vegetation around Emerald Lake in association with well-known worldwide climatic events.

Continuous sediment, pollen, and charcoal records were developed from an 8.46-m-long sediment core taken from Hermit Lake in the northern Sangre de Cristo mountain range of Colorado. Presently, vegetation around the lake is upper subalpine forest, consisting of Picea engelmannii (Englemann spruce) with some Abies lasiocarpa (subalpine fir), and the lake lies >200 m below present tree line. We used several pollen ratios to reconstruct the relative position of the tree line and the occurrence of clay layers to infer landscape instability through time. Deglaciation of the Hermit Lake drainage began during the Bølling-Allerød interval. Between ca. 13.5 and 12.4 ka, high Artemisia (sagebrush) pollen abundance, low Picea/Pinus (spruce/pine; S/P) ratios, and sporadic occurrence of Picea macrofossils indicate alpine tundra-spruce conditions. Though the pollen record shows no transition to the Younger Dryas, the subsequent absence of Picea needle fragments suggests a lowering of tree line. By ca. 10.2 ka, a subalpine forest of Picea and Pinus grew there. Based on pollen ratios, tree line was higher than today from ca. 9.0 to ca. 3.8 ka, after which the tree line began to lower to its present elevation. Maximum expansion of the Picea-Abies subalpine forest, determined from both pollen and macrofossils, was coincident with the highest influx of charcoal particles and maximum deposition of postfire erosion (clay layers) into the lake. The period ca. 7.8–6.2 ka was the driest period, as shown by aquatic indicators, but pollen ratios suggest that ca. 6.2–3.8 ka was the warmest period of the Holocene, accompanied by high rates of burning, and consequently elevated erosion of clays into the lake. During the late Holocene, declining S/P ratios are interpreted as declining alpine tree line, while decreases in both Picea to Artemisia (S/Art) and Pinus to Artemisia (P/Art) ratios suggest climate cooling. Pollen evidence suggests expansion of the lower-elevation Colorado piñon (Pinus edulis), which has been documented as part of a widespread phenomenon noted by other studies.

The Last Glacial Termination is marked by changing environmental conditions affected by abrupt and rapid climate oscillations, such as Heinrich Stadial 1 (HS1), which is characterized by extremely low sea surface temperatures (SST) and significant changes in northern hemisphere terrestrial landscape (e.g., vegetation) and human dispersion. Previous studies show that overall cold/dry conditions occurred during HS1, but the lack of high-resolution records precludes whether climate was stable or instead characterized by instability. A high-resolution paleoclimatic record from the Padul wetland (southern Iberian Peninsula), supported by a high-resolution chronology and contrasted with other records from southern Europe and the Mediterranean region, shows 1) that the age boundaries of HS1 in this area occurred at ∼18.0 kyr BP (median age = 17,970 cal yr BP; mean age = 18,030 ± 330 cal yr BP) and ∼15.2 kyr BP (median age = 15,210 cal yr BP; mean age = 15,200 ± 420 cal yr BP) and 2) that climate during HS1 was non-stationary and centennial-scale variability in moisture is superimposed on this overall cold climatic period. In this study, we improve the pollen sampling resolution with respect to previous studies on the same Padul-15-05 sedimentary core and suggest a novel subdivision of HS1 in 7 sub-phases, including: i) 3 sub-phases (a.1-a.3) during an arid early phase (HS1a; ∼18.4–17.2 kyr BP), ii) a relatively humid middle phase (HS1b; ∼17.2–16.9 kyr BP), and iii) 3 sub-phases (c.1-c.3) during an arid late phase (HS1c; ∼16.9–15.7 kyr BP). This climatic subdivision is regionally supported by SST oscillations from the Mediterranean Sea, suggesting a strong land-sea coupling. A cyclostratigraphic analysis of pollen data between 20 and 11 kyr BP indicates that the climate variability and the proposed subdivisions characterized by ∼2000 and ∼800-yr periodicities could be related to solar forcing controlling climate in this area., This research is supported by the projects CGL 2013-47038-R and CGL-2017-85415-R, B-RNM-144-UGR18, PhD funding BES-2014-069117 (J.C.) and Ramón y Cajal fellowship RYC-2015-18966 (A.G.-A.) provided by the Ministerio de Economía y Competitividad of the Spanish Government. Additional funding was provided by the project number 316702 from the Academy of Finland for a postdoctoral research contract (J.C.) and the research group RNM0190 and the project P11-RNM-7332 with a postdoctoral fellowship (M.J.R.-R.) from the Junta de Andalucía. M.J.R.-R. acknowledges the postdoctoral funding provided by the European Commission /H2020 (ERC-2017-ADG, project number 788616 ). Finally, we acknowledge two anonymous reviewers and the editor (Donatella Magri) for their very useful corrections and suggestions.

This study was supported by the project B-RNM-144-UGR18 of the action "Proyectos I+D+i del Programa Operativo FEDER 2018 -Junta de Andalucia-UGR", the projects CGL2013-47038-R and CGL201785415-R, of the "Ministerio de Economia y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER", and the research group RNM-190 (Junta de Andalucia). A.G.-A. was also supported by a Marie Curie Intra-European Fellowship of the 7th Framework Programme for Research, Technological Development and Demonstration of the European Commission (NAOSIPUK. Grant Number: PIEF-GA-2012-623027) and by a Ram ' on y Cajal Fellowship RYC-2015-18966 of the Spanish Government (Ministerio de Economia y Competividad). J.C. acknowledges the postdoctoral funding provided by the Academy of Finland (project number 316702). J.L.T. hosted the NAOSIPUK project (PIEFGA-2012-623027) at the University of Glasgow. A.L.-A PhD is funded by BES-2018-084293 (Ministerio de Economia y Competividad). M.J.R.R. acknowledges the postdoctoral funding by the European Research Council (ERC-2017-ADG-788616). This study was supported by an ERC Consolidator Grant (STEEPclim) to D.S. (Grant Agreement No. 647035). E.S. is supported by the DFG Cluster of Excellence 2077 >The Ocean Floor -Earth's Uncharted Interface < at MARUM. We thank Ralph Kreutz for analytical support., The transitional regions between the low and high latitudes of the Northern Hemisphere are highly vulnerable to future climate change yet most of the current climate models usually diverge in their projections. To better understand the dynamics in these regions, the reconstruction of past hydrological fluctuations and precipitation patterns is of paramount importance to accurately constrain present and future climate scenarios. In this study, we investigated paleohydrological dynamics in the western Mediterranean region, a transitional zone between low-mid latitudes and Atlantic - Mediterranean realms. We reconstruct precipitation and moisture source changes during the last -35 ka in order to propose the potential mechanisms driving these oscillations. To do so, we use hydrogen isotopes from sedimentary leaf waxes, more specifically the C31 n-alkane homologue, and a precipitation proxy based on previously published pollen data from a sedimentary core (Padul-15-05) in southern Iberia (Padul wetland -37-N). With this combination we disentangle the coupled effect of precipitation amount and source on the hydrogen isotopic signature of the studied C31 n-alkane record. Our results show three main periods characterized by different precipitation patterns. Low precipitation, mainly linked to a significant contribution from an isotopically-enriched Mediterranean precipitation source, occurred from -30 to -15.5 ka BP and during the last -5 ka, whereas enhanced precipitation with a predominant isotopically-depleted Atlantic precipitation source prevailed from -15.5 to -5 ka BP. This latter stage is here defined as the Western Mediterranean Humid Period (WMHP). In addition, some occasional millennial-scale opposite precipitation patterns can be observed during these climatically distinct periods. These changes in the source of precipitation were likely coupled to a shift in the main rainy season from winter, when Atlantic precipitation prevailed, to late winter-early spring, when the contribution of Mediterranean moisture is higher. Comparison between the studied mid-latitude terrestrial Padul-15-05 core and a low-latitude marine record offshore of northwestern Africa shows clear long-term synchronous responses of both western Mediterranean precipitation and western African monsoon systems to northern Hemisphere atmospheric dynamics, ultimately controlled by orbital forcing and ice-sheets fluctuations., action "Proyectos I+D+i del Programa Operativo FEDER 2018 -Junta de Andalucia-UGR" B-RNM-144-UGR18, Ministerio de Economia y Competitividad of Spain CGL2013-47038-R CGL201785415-R, Junta de Andalucia RNM-190, Marie Curie Intra-European Fellowship of the 7th Framework Programme for Research, Technological Development and Demonstration of the European Commission (NAOSIPUK) PIEF-GA-2012-623027, Spanish Government (Ministerio de Economia y Competividad) RYC-2015-18966, Academy of Finland, European Commission 316702, NAOSIPUK project at the University of Glasgow PIEFGA-2012-623027, Ministerio de Economia y Competividad BES-2018-084293, European Research Council (ERC), European Commission ERC-2017-ADG-788616, ERC Consolidator Grant (STEEPclim) 647035, DFG Cluster of Excellence 2077 >The Ocean Floor -Earth's Uncharted Interface < at MARUM, European Commission

This research has been supported by the Directorate for Geosciences of the National Science Foundation (grant nos. AGS-1602105 and AGS-1903548)., We thank the USGS John Wesley Powell Center for Analysis and Synthesis, which hosted a meeting that led to this synthesis effort. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government. We thank the original data generators who made their data available for reuse, and we acknowledge the data repositories for safeguarding these assets., Holocene climate reconstructions are useful for understanding the diverse features and spatial heterogeneity of past and future climate change. Here we present a database of western North American Holocene paleoclimate records. The database gathers paleoclimate time series from 184 terrestrial and marine sites, including 381 individual proxy records. The records span at least 4000 of the last 12 000 years (median duration of 10 725 years) and have been screened for resolution, chronologic control, and climate sensitivity. Records were included that reflect temperature, hydroclimate, or circulation features. The database is shared in the machine readable Linked Paleo Data (LiPD) format and includes geochronologic data for generating site-level time-uncertain ensembles. This publicly accessible and curated collection of proxy paleoclimate records will have wide research applications, including, for example, investigations of the primary features of oceanatmospheric circulation along the eastern margin of the North Pacific and the latitudinal response of climate to orbital changes. The database is available for download at https://doi.org/10.6084/m9.figshare.12863843.v1 (Routson and McKay, 2020)., National Science Foundation (NSF) AGS-1602105 AGS-1903548

Holocene climate reconstructions are useful for understanding the diverse features and spatial heterogeneity of past and future climate change. Here we present a database of western North American Holocene paleoclimate records. The database gathers paleoclimate time series from 209 terrestrial and marine sites, including 382 individual proxy records. The records span at least 4000 of the last 12 000 years (median duration = 10 603 years), and have been screened for resolution, chronologic control, and climate sensitivity. Records were included that reflect temperature, hydroclimate, or circulation features. The database is shared in the machine readable Linked Paleo Data (LiPD) format and includes geochronologic data for generating site-level time-uncertain ensembles. This publicly accessible and curated collection of proxy paleoclimate records will have wide research applications, including, for example, investigations of the primary features of ocean-atmospheric circulation along the eastern margin of the North Pacific and the latitudinal response of climate to orbital changes. The database is available for download at: https://doi.org/10.6084/m9.figshare.12863843.v1 (Routson and McKay, 2020).

Alpine regions of the Mediterranean realm are among the most climatically sensitive areas in the world. Thus, alpine wetlands from the southern Iberian Peninsula, in the westernmost part of the Mediterranean region, are highly sensitive sensors of environmental changes. Difficulties have surfaced in separating controls by temperature and/or precipitation in previous paleoenvironmental studies from alpine environments in this area. We present a Holocene biomarker record (n-alkanes and long-chain diols) from a high elevation lake, Laguna de Río Seco (LdRS), in the south of the Iberian Peninsula, which contributes to the identification of these forcing mechanisms. The hydrological history of the area, primarily water availability and evapotranspiration, is reconstructed by means of the n-alkane record, including the indices of average chain length, portion aquatic, and carbon preference index, as well as hydrogen isotopes (δD) of aquatic (δD) and terrestrial (δD) n-alkanes. Temperatures are also estimated using the algae derived long-chain diols. We interpret δD and δD fluctuations as showing changes in the source and amount of precipitation throughout the LdRS record. An Atlantic precipitation source appears to have predominated during the early-middle Holocene, but an occasional Mediterranean influence with an isotopic enrichment in precipitation is detected in the middle-late Holocene that is likely related to the setting of the current atmospheric pattern in southeastern Iberia under the joint control of the North Atlantic Oscillation (NAO) and the Western Mediterranean dynamics, such as the Western Mediterranean Oscillation (WeMO). Our new record from LdRS is consistent with a generalized trend of a humid early-middle Holocene with low temperature variability, evolving towards an arid middle-late Holocene with abrupt temperature changes. In addition to these long-term trends during the last ∼10,500 years, two phases of climate instability, evidenced by abrupt depletions in δD, have been identified at the end of these periods, one between ∼6500 and 5500 cal yr BP and another in the last ∼500 years. These episodes would represent strengthened winter cold conditions that favoured the persistence of snowpack and frozen soil in the catchment, causing reduced terrestrial plant growth and low lake evaporation. According to the long-chain diol record, temperatures during these phases were relatively low, but experienced abrupt increases at the end of each period., This study was supported by the project P11-RNM-7332 of the “Junta de Andalucía”, the projects CGL2013-47038-R and CGL-2017-85415-R of the “Ministerio de Economía y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER” and the research group RNM0190 (Junta de Andalucía). A.G.-A. was also supported by a Marie Curie Intra-European Fellowship of the 7th Framework Programme for Research, Technological Development and Demonstration of the European Commission (NAOSIPUK. Grant Number: PIEF-GA-2012-623027) and by a Ramón y Cajal Fellowship RYC-2015-18966 of the Spanish Government (Ministerio de Economía y Competividad). J.L.T. was also supported by a Small Research Grant by the Carnegie Trust for the Universities of Scotland and hosted the NAOSIPUK project (PIEF-GA-2012-623027). We also want to thank the Editor, P. Rioual and two anonymous reviewers for their useful comments that improved the final version of the manuscript.

Comprehending the effects of climate variability and disturbance on forested ecosystems is paramount to successfully managing forest environments under future climate scenarios (e.g., global warming, aridification increase). Changes in fossil pollen abundance in sedimentary archives record past vegetation dynamics at regional scales, mainly related to climate changes and, in the last few millennia, to human impact. Pollen records can thus provide long databases with information on how the environment reacted to climate change before the historical record. In this study, we synthesized fossil pollen data from seven sites from the Sierra Nevada in southern Spain to investigate the response of forests in the western Mediterranean area to millennial-scale climate changes and to human impact during the Holocene. In particular, here we focused on Cedrus pollen abundances, which most-likely originated from Northern Africa and were carried to Sierra Nevada by wind. Cedrus pollen has received little attention in the Iberian Peninsula palynological records, for it occurs in low concentrations and has an African source, and thus this article explores the potential to reconstruct its past history and climate. Although Cedrus abundances are generally lower than 1% in the studied pollen samples, a comparison with North African (Moroccan) Cedrus pollen records shows similar trends at long- and short-term time-scales. Therefore, this record could be used as a proxy for changes in this forest species in North Africa. As observed in the Sierra Nevada synthetic record, the increasing trend of Cedrus pollen during the Middle and Late Holocene closely correlates with decreasing summer insolation. This would have produced overall cooler annual temperatures in Northern Africa (Middle Atlas and Rif Mountains) as well as lower summer evaporation, benefiting the growth of this cool-adapted montane tree species while increasing available moisture during the summer, which is critical for this water-demanding species. Millennial-scale variability also characterizes the Sierra Nevada Cedrus synthetic pollen record. Cedrus abundance oscillations co-vary with well-known millennial-scale climatic variability that controlled cedar abundance and altitudinal distribution in montane areas of N Africa., Support for this study comes from projects CGL2013-47038-R and CGL2017-85415-R funded by Ministerio de Economía y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER; Séneca Project 20788/PI/18; Junta de Andalucía FEDER Project B-RNM-144-UGR18, UGR-FEDER B-RNM-144-UGR18 Proyectos I + D + i del Programa Operativo FEDER 2018 and the research group RNM-190 (Junta de Andalucía). M.J.R.R. acknowledges the postdoctoral funding provided by the European Commission/H2020 (ERC-2017-ADG, project number 788616). J.C. acknowledges the postdoctoral funding provided by the Academy of Finland (project number 316702). A.G.-A. acknowledges the Ramón y Cajal fellowship RYC-2015-18966 provided by the Ministerio de Economía y Competitividad of the Spanish Government. This paper greatly benefited from the reviews of two anonymous reviewers and from the editor Giovanni Zanchetta. Thanks to Rachid Cheddadi, William Fletcher and Nathalie Combourieu-Nebout for kindly providing pollen data from the Alboran Sea and N Morocco records.

A new sediment core from Padul, Padul-15-05, provides a long and continuous sedimentary and paleoenvironmental record of the last ∼200 kyr. Here we present a detailed palynological study from this sedimentary sequence with new vegetation and paleoenvironmental data and a climatic interpretation for the last two climatic cycles from the southern Iberian Peninsula in the western Mediterranean region. Pollen analysis from this core shows orbital-scale vegetation oscillations (mainly forced by precession and eccentricity) pointing into insolation as the principal factor controlling vegetation changes at this latitude. The last two glacial-interglacial cycles (from MIS 6 to the present) as well as stadial and interstadial phases are well represented and characterized by significant oscillations in Mediterranean forest in the area, but several differences with respect to other Mediterranean long pollen records can be recognized. Some of these disparities between regions seem to be more related to different precipitation rates linked to the specific geographical features (e.g. strong topographic relief) than to latitudinal differences between the sites. In addition, a lake level reconstruction based on palynological data was developed and compared with the previous reconstruction from this record, representing local environmental changes in the Padul wetland that seems to be related to precipitation/evapotranspiration balance, and therefore, to changes in regional climate conditions. During glacial/stadial conditions recorded by maxima in xerophytes, the p/e increased due to very low evapotranspiration, resulting in high lake levels, while during warmer interglacial/interstadial phases (except for the MIS 5e), mainly represented in this record by maxima in the Mediterranean forest, water level decreased as a result of high evapotranspiration exceeding precipitation input. The comparison of the new Padul-15-05 pollen record with the two previous palynological studies from Padul shows significant differences mainly due to disagreements in the age control of the sequences. This study shows that an accurate age-depth model becomes essential when interpreting long paleoenvironmental and paleoclimate records., This research is supported by the projects CGL2013-47038-R and CGL-2017-85415-R funded by Ministerio de Economía y Competitividad of the Spanish Government, project P11-RNM-7332 from the Junta de Andalucía and the research group RNM0190 (Junta de Andalucía). Jon Camuera acknowledges the PhD funding (BES-2014-069117) provided by the Ministerio de Economía y Competitividad of the Spanish Government under the project CGL2013-47038-R. María J. Ramos-Román also acknowledges the PhD and post-PhD fellowship (P11-RNM-7332) from the Conserjería de Economía, Innovación, Ciencia y Empleo of the Junta de Andalucía. Antonio García-Alix was supported by a Ramón y Cajal fellowship (RYC-2015-18966) from the Ministerio de Economía y Competitividad of the Spanish Government. Thanks to Javier Jaimez for helping with the coring (CIC-UGR). José Carrión thanks the Ministerio de Economía y Competitividad of the Spanish Government (grant number CGL-BOS-2012-34717 and CGL-BOS 2015-68604) and Fundación Séneca (grant number 19434/PI/14).

Remote small alpine lakes and wetlands from the Sierra Nevada mountain range (southern Spain) are exceptional ecological sensors of global change and preserve a complete post-glacial Holocene sedimentary record. Several organic and inorganic geochemical analyses carried out in the sedimentary record of the Borreguil de los Lavaderos de la Reina, a small bog located in the north face of the Sierra Nevada at 2421 m, have allowed us to reconstruct climate, vegetation and human interaction in the highest mountain range in southern Iberia in the last 3000 years. This study shows that during the Late Holocene there is a progressive climatic aridification, which produced a reduction in the aquatic environments in Sierra Nevada. The studied peat bog geomorphology and surrounding areas also conditioned the evolution of the local vegetation. An increasing trend in the detrital eolian input from northern Africa, as well as an important anthropic impact in the ecosystems (artificial drainage system among other activities) are observed in the studied record in the last century.

High-altitude peat bogs and lacustrine records are very sensitive to climate changes and atmospheric dust input. Recent studies have shown a close relationship between regional climate aridity and enhanced eolian input to lake sediments. However, changes in regional-scale dust fluxes due to climate variability at short scales and how alpine environments were impacted by climatic- and human-induced environmental changes are not completely understood.Here we present a multi-proxy (palynological, geochemical and magnetic susceptibility) lake sediment record of climate variability in the Sierra Nevada (southeastern Iberian Peninsula) over the Holocene. Magnetic susceptibility and geochemical proxies obtained from the high mountain lake record of Laguna Hondera evidence humid conditions during the early Holocene, while a trend towards more arid conditions is recognized since ĝ1/4 7000 cal yr BP, with enhanced Saharan eolian dust deposition until the present. This trend towards enhanced arid conditions was modulated by millennial-scale climate variability. Relative humid conditions occurred during the Iberian Roman Humid Period (2600-1450 cal yr BP) and predominantly arid conditions occurred during the Dark Ages and the Medieval Climate Anomaly (1450-650 cal yr BP). The Little Ice Age (650-150 cal yr BP) is characterized in the Laguna Hondera record by an increase in runoff and a minimum in eolian input. In addition, we further suggest that human impact in the area is noticed through the record of Olea cultivation, Pinus reforestation and Pb pollution during the Industrial Period (150 cal yr BP-present). Furthermore, we estimated that the correlation between Zr and Ca concentrations stands for Saharan dust input to the Sierra Nevada lake records. These assumptions support that present-day biochemical observations, pointing to eolian input as the main inorganic nutrient source for oligotrophic mountain lakes, are comparable to the past record of eolian supply to these high-altitude lakes., This study was supported by the project P11-RNM 7332 of the “Junta de Andalucía”, the projects CGL2013-47038-R and CGL2015-66830-R of the “Ministerio de Economía y Competitividad of Spain and Fondo Europeo de Desarrollo Regional FEDER”, and the research groups RNM0190 and RNM179 (Junta de Andalucía). We also thank Unidad de Excelencia (UCE-PP2016-05). Jose Manuel Mesa-Fernández acknowledges the PhD funding provided by Ministerio de Economía y Competitividad (CGL2015-66830-R). Antonio García-Alix was also supported by a Marie Curie Intra-European Fellowship of the Seventh Framework Programme for Research, Technological Development and Demonstration of the European Commission (NAOSIPUK; grant number: PIEF-GA-2012-623027) and by a Ramón y Cajal Fellowship RYC-2015-18966 of the Spanish Government (Ministerio de Economía y Competividad) and Marta Rodrigo-Gámiz from the Andalucía Talent Hub Program co-funded by the European Union's Seventh Framework Program (COFUND; grant agreement no. 291780) and the Junta de Andalucía. We thank Santiago Fernández, Maria Dolores Hernandez and Antonio Mudarra for their help recovering the core and Inés Morales for the initial core description and MS data. We thank Jaime Frigola (Universitat de Barcelona) for his help with XRF core scanning.

Padul is one of the few wetland sites in southern Europe and the Mediterranean region that exhibits an unusually large temporal span (>100 kyr) and continuous Quaternary sedimentary record. Previous core-based studies from Padul yielded paleoecological datasets (i.e., pollen and organic geochemistry), but with a poor age control that resulted in rather arbitrary climate inferences. Therefore, precise age control and a multidisciplinary approach is necessary to understand long-term regional environmental and climate change and the associated local response of the Padul wetland environment. Here we present a new long sediment record (Padul-15-05) from this wetland in the southern Iberian Peninsula with the aim of improving the age control of the sedimentary sequence and carrying out up-to-date high-resolution multiproxy analyses. In this study the age control is based on 61 AMS radiocarbon dates for the last ca. 50 kyr BP and on the extent of amino acid racemization (AAR) in mollusc shells extending back ∼118 kyr BP. No numerical ages are available for the bottom part of the core but the sediment accumulation rates (SAR) and the cyclostratigraphic analysis of the multiproxy data suggest that the core preserves a continuous record of the last ∼197 kyr (from late MIS 7 to present) with millennial-scale time resolution. Sedimentological (lithology, magnetic susceptibility, XRD, color), geochemical (XRF, TOC, C/N, % carbonate content) and paleontological (pollen, charophytes, gastropods) data show co-varying cyclical paleoenvironmental changes linked to orbital-scale climatic variability. Silicon, magnetic susceptibility (MS) and total organic carbon (TOC) data show periodicities between ∼26.2–19.6 kyr linked to insolation, which is strongly dominated by precession cycles at this latitude. High values of Si and MS data have been related to high siliciclastic/detrital input from Sierra Nevada range during minima in insolation due to enhanced soil weathering/erosion during regional aridity and lower forest cover recorded by the arboreal pollen, which could also be favored by a minor biogenic productivity. In addition, warm climate conditions during maxima in insolation mostly resulted in negative precipitation/evapotranspiration balance and low lake levels, while cold glacial and stadial periods were mainly characterized by positive precipitation/evapotranspiration balance, and therefore, high lake levels. The improved chronology of the Padul sedimentary sequence along with a multiproxy study permitted us to better relate environmental and vegetation changes to climatic events and to demonstrate how both local (i.e., lake level, sedimentation) and regional (i.e., vegetation) environments responded to orbital-scale climate changes.

A key task in fire-climate research in the western United States is to characterize potential future fire-climate linkages across different elevational gradients. Using thirty-seven sedimentary charcoal records, here we present a 1500-year synthesis of wildfire activity across different elevational gradients to characterize fire-climate linkages. From our results, we have identified three periods of elevated fire occurrence centered on the 20th century, 900 cal yr BP, and 1350 cal yr BP. During the 20th century, fire activity has occurred primarily in the northern Rocky Mountains, with mid-elevations experiencing the greatest increase in wildfire activity. While wildfires occurred primarily in the SRM region ∼900 cal yr BP, the greatest increase in high-elevations occurred in the NRM at this time. Finally, synchronous wildfires occurred in both northern and southern Rocky Mountain mid-elevations ∼1350 cal yr BP, suggesting a potential analog for future wildfire conditions in response to warmer temperatures and more protracted droughts. We conclude that wildfire activity increased in most elevations during periods of protracted summer drought, warmer-than-average temperatures, and based on modern climate analogs, reduced atmospheric humidity.

The composition and hydrogen isotope values of leaf wax components can be powerful tools in reconstructing past climate and environments. However, interpretation of past environmental conditions from such components in sediments is complicated by species-specific influences and there is a need to better understand how vegetation type affects leaf wax composition and isotope ratios in modern environments. In this study we analyzed leaf wax (n-alkane) distributions and hydrogen isotope values of plants from a high latitude ombrotrophic bog in northern Norway. The isotopic analysis of surface water was also conducted on samples from 15 lakes along a ca. 150 km transect to contextualize the bog water isotopic composition and constrain fractionation factors among n-alkane homologues. We identified 14 different plant types growing on the bog surface, including mosses, graminoids and other herbs, sub-shrubs and a tree. n-Alkanes from the leaves of the modern plants had average chain lengths from 25 to 30.5, with a variety of distributions, and with the dominant compound of longer chain lengths (C27, C29, or C31). δD values of n-C25 to n-C33 for the vegetation samples ranged from −197‰ to −116‰, with an average of −162‰. The data also revealed that the δD values for the homologues for half of the vegetation types had ranges that were ≥20‰. Using the average isotopic value of bog water samples, −60‰, we calculated apparent fractionation factors that ranged from −66‰ to −134‰ (avg. −108 ± 22‰), similar to other sites across Europe and to a global data compilation. Our results demonstrate the range of species-specific influences on leaf wax composition and isotopic values at this site and presumably other ombrotrophic bog environments, and provide a dataset to help evaluate the influence of vegetation type on regional sedimentary leaf wax records.

A series of large storms attributed to Atmospheric River conditions struck the California coast in the winter of 1861-2. Although historical accounts document inland flooding, little is known about how the 1861-2 storms impacted the now heavily-developed California coast. Here we show that the 1861-2 storms emplaced a deposit of beach sand up to 50 cm thick over 450 m inland within a southern California salt marsh. This deposit is unprecedented in the post-European sediments of the marsh and more extensive than that derived from any other historical event. It is comparable in scale to hurricane and tsunami washover fans in back-barrier environments along other coastlines. The presence of overwash deposits in Carpinteria suggests that the 1861-2 storm season was erosive enough to remove coastal barriers, allowing for inundation of parts of the coastline currently developed. Efforts to prepare for a recurrence of an 1861-2-like storm season should address potential coastal impacts; likewise, interpretations of past washover deposits should consider these unusually prolonged stormy periods in addition to hurricane and tsunami inundation.

Holocene centennial-scale paleoenvironmental variability has been described in a multiproxy analysis (i.e., lithology, geochemistry, macrofossil, and microfossil analyses) of a paleoecological record from the Padul Basin in Sierra Nevada, southern Iberian Peninsula. This sequence covers a relevant time interval hitherto unreported in the studies of the Padul sedimentary sequence. The ∼ 4700-year record has preserved proxies of climate variability, with vegetation, lake levels, and sedimentological change during the Holocene in one of the most unique and southernmost wetlands in Europe. The progressive middle and late Holocene trend toward arid conditions identified by numerous authors in the western Mediterranean region, mostly related to a decrease in summer insolation, is also documented in this record; here it is also superimposed by centennial-scale variability in humidity. In turn, this record shows centennial-scale climate oscillations in temperature that correlate with well-known climatic events during the late Holocene in the western Mediterranean region, synchronous with variability in solar and atmospheric dynamics. The multiproxy Padul record first shows a transition from a relatively humid middle Holocene in the western Mediterranean region to more aridity from ∼ 4700 to ∼ 2800 cal yr BP. A relatively warm and humid period occurred between ∼ 2600 and ∼ 1600 cal yr BP, coinciding with persistent negative North Atlantic Oscillation (NAO) conditions and the historic Iberian–Roman Humid Period. Enhanced arid conditions, co-occurring with overall positive NAO conditions and increasing solar activity, are observed between ∼ 1550 and ∼ 450 cal yr BP (∼ 400 to ∼ 1400 CE) and colder and warmer conditions occurred during the Dark Ages and Medieval Climate Anomaly (MCA), respectively. Slightly wetter conditions took place during the end of the MCA and the first part of the Little Ice Age, which could be related to a change towards negative NAO conditions and minima in solar activity. Time series analysis performed from local (Botryococcus and total organic carbon) and regional (Mediterranean forest) signals helped us determining the relationship between southern Iberian climate evolution, atmospheric and oceanic dynamics, and solar activity. Our multiproxy record shows little evidence of human impact in the area until ∼ 1550 cal yr BP, when evidence of agriculture and livestock grazing occurs. Therefore, climate is the main forcing mechanism controlling environmental change in the area until relatively recently.

The study of climate variability in especially sensitive areas is crucial for a better understanding of the response of Earth’s different components to abrupt changes and envisage future climate responses. In this regard, the southern Iberian and western Mediterranean regions have demonstrated hemispheric-scale teleconnections during the last glacial period. Long-records from continental sedimentary archives are scarce, and the Padul wetland represents one of the longest and most continuous continental record in this area, detecting climate variability at centennial to millennial-scale from the Pleistocene to the Holocene. The applicability of organic-based proxies in this organic rich continental archive is a promising tool because the variations in different biomarkers are closely related to biological sources and environmental factors such as temperature. Particularly interesting from a paleoclimatic point of view are glycerol dialkyl glycerol tetraethers (GDGTs), which are membrane lipids from Bacteria and Archaea, ubiquitous in a range of natural archives, including wetlands. Previous works have demonstrated their applicability as a significant past continental air temperature proxy, where the distribution of bacterial branched GDGTs (brGDGTs) is correlated with mean annual air temperature (MAAT) and soil pH. Here we present a first quantification of past temperatures using brGDGTs in the Padul sedimentary record. Preliminary results have evidenced substantial variations in derived-MAAT and distribution of the different brGDGTs during the last 36 kyr that are consistent with abrupt climate periods, such as Henrich Stadial 1 and the Holocene onset. Nevertheless, different absolute MAAT values using the peat-specific calibration and the mineral soil calibration have been obtained and they need to be evaluated.

Alpine ecosystems of the southern Iberian Peninsula are among the most vulnerable and the first to respond to modern climate change in southwestern Europe. While major environmental shifts have occurred over the last ∼1500 years in these alpine ecosystems, only changes in the recent centuries have led to abrupt environmental responses, but factors imposing the strongest stress have been unclear until now. To understand these environmental responses, this study, for the first time, has calibrated an algal lipid-derived temperature proxy (based on long-chain alkyl diols) to instrumental historical data extending alpine temperature reconstructions to 1500 years before present. These novel results highlight the enhanced effect of greenhouse gases on alpine temperatures during the last ∼200 years and the long-term modulating role of solar forcing. This study also shows that the warming rate during the 20th century (∼0.18 ∘C per decade) was double that of the last stages of the Little Ice Age (∼0.09 ∘C per decade), even exceeding temperature trends of the high-altitude Alps during the 20th century. As a consequence, temperature exceeded the preindustrial record in the 1950s, and it has been one of the major forcing processes of the recent enhanced change in these alpine ecosystems from southern Iberia since then. Nevertheless, other factors reducing the snow and ice albedo (e.g., atmospheric deposition) may have influenced local glacier loss, since almost steady climate conditions predominated from the middle 19th century to the first decades of the 20th century., This research has been supported by the Seventh Framework Programme (grant no. NAOSIPUK (623027)), the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación (grant no. CGL2017-85415-R), the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación (grant no. CGL2013-47038-R), the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación (grant no. CGL2011-23483), and the Consejería de Economía, Innovación, Ciencia y Empleo, Junta de Andalucía, Agencia de Innovación y Desarrollo de Andalucía (grant no. P11-RNM 7332). This research has also been supported by grant no. 87/2007 of the Organismo Autónomo Parques Nacionales (OAPN)-Ministerio de Medio Ambiente, the research group no. RNM-190 of the Plan Andaluz de Investigación, Desarrollo e Innovación (Junta de Andalucía), and the Ramón y Cajal Fellowship (fellowship no. RYC-2015-18966) of the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación.

EGU General Assembly 2020, Online, 4–8 May 2020, [EN] In this study, we synthesized pollen data from seven sites from the Sierra Nevada in southern Spain to investigate the response of forests in the western Mediterranean area to centennial- and millennial-scale climate changes and to human impact during the Holocene. In particular, here we focused in Cedrus pollen abundances, which most-likely originated from Northern Africa and were carried to Sierra Nevada by wind. Although Cedrus abundances are generally lower than 1% in the studied pollen samples, a comparison with North African pollen records shows similar trends at long- and short-term time-scales. Therefore, this record could be used as a proxy for changes in this forest species in North Africa. A Middle and Late Holocene Cedrus pollen increasing trend has been observed in the Sierra Nevada synthetic record, which seems to be the result of decreasing summer insolation. This would have produced overall cooler annual temperatures in Northern Africa (Atlas and Rif Mountains), benefiting the growth of this cool-adapted montane tree species, and lower summer evaporation, increasing available moisture during the summer, which is critical for this water-demanding species. Millennial- and centennial-scale variability also characterize the Sierra Nevada Cedrus synthetic record. Cedrus abundance oscillations could have been produced by well-known millennial-scale climatic variability that controlled cedar abundance in montane areas of N Africa.

Supplemental material, Supplemental(revised) for Vegetation changes and plant wax biomarkers from an ombrotrophic bog define hydroclimate trends and human-environment interactions during the Holocene in northern Norway by Nicholas L. Balascio, R. Scott Anderson, William J. D’Andrea, Stephen Wickler, Robert M. D’Andrea and Jostein Bakke in The Holocene

The Eurasian (née European) Modern Pollen Database (EMPD) was established in 2013 to provide a public database of high-quality modern pollen surface samples to help support studies of past climate, land cover, and land use using fossil pollen. The EMPD is part of, and complementary to, the European Pollen Database (EPD) which contains data on fossil pollen found in Late Quaternary sedimentary archives throughout the Eurasian region. The EPD is in turn part of the rapidly growing Neotoma database, which is now the primary home for global palaeoecological data. This paper describes version 2 of the EMPD in which the number of samples held in the database has been increased by 60% from 4826 to 8134. Much of the improvement in data coverage has come from northern Asia, and the database has consequently been renamed the Eurasian Modern Pollen Database to reflect this geographical enlargement. The EMPD can be viewed online using a dedicated map-based viewer at https://empd2.github.io and downloaded in a variety of file formats at https: //doi.pangaea.de/10.1594/PANGAEA.909130 (Chevalier et al., 2019)., Swiss National Science Foundation (SNSF) 200021_169598, University of Lausanne

Heinrich Stadial 1 (HS1) is one of the most extreme climate periods of the last glacial cycle, generating extremely low sea surface temperatures (SST) and significant changes in terrestrial landscape (e.g., vegetation). Previous studies show that overall cold/dry conditions occurred during HS1, but the lack of high-resolution records precludes whether climate was stable or instead characterized by instability. A high-resolution paleoclimatic record from Padul (southern Iberian Peninsula), supported by a robust chronology, shows that climate during HS1 was non-stationary and centennial-scale variability in moisture is superimposed on this overall cold climatic period. In this study we improve the resolution and suggest a novel subdivision of HS1 in 7 sub-phases, including: i) 3 sub-phases (a.1–a.3) during an arid early phase (HS1a; ∼18.4–17.2 kyr BP), ii) a humid middle phase (HS1b; ∼17.2–16.7 kyr BP), and iii) 3 sub-phases (c.1–c.3) during an arid late phase (HS1c; ∼16.7–15.6 kyr BP). This climatic subdivision is regionally supported by SST oscillations from the Mediterranean Sea, suggesting a strong land-ocean relationship. A cyclostratigraphic analysis of our pollen data indicates that HS1 climate variability, and thus this subdivision, is characterized by &sim:2000 and ∼800-yr periodicities, suggesting solar forcing controlling climate in this area.