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There is increasing evidence that abrupt vegetation shifts and large-scale erosive phases occurred in Central Africa during the third millennium before present. Debate exists as to whether these events were caused by climate change and/or intensifying human activities related to the Bantu expansion. In this study, we report on a multi-proxy investigation of a sediment core (KZR-23) recovered from the Congo submarine canyon. Our aim was to reconstruct climate, erosion and vegetation patterns in the Congo Basin for the last 10,000yrs, with a particular emphasis on the late Holocene period. Samples of modern riverine suspended particulates were also analyzedto characterize sediment source geochemical signatures from across the Congo watershed. We find that a sudden increase of bulk sediment aluminium-to-potassium (Al/K) ratios and initial radiocarbon ages of bulk organic matter occurred after 2,200yrs ago, coincident with a pollen-inferred vegetation change suggesting forest retreat and development of savannas. Although hydrogen isotope compositions of plant waxes (δDwax) do not reveal a substantial hydroclimate shift during this period, neodymium isotopes and rare earth elements in detrital fractions indicate provenance changes for the sediment exported from the Congo Basin at that time, hence suggesting a reorganization of spatial rainfall patterns across Central Africa during this event. Taken together, these findings provide evidence for changing landscapes in Central Africa from about 2,200yrs ago, associated with synchronous events of vegetation changes and enhanced erosion of pre-aged and highly weathered soils. These events coincided remarkably well with the arrival of Iron Age communities into the rainforest, as inferred from comparison to regional archaeological syntheses. While the human impact on the environment remains difficult to quantify at the scale of the vast Congo Basin, we tentatively propose that strengthening of El Niño-Southern Oscillation (ENSO

Atmospheric circulation over the North Atlantic has undergone significant fluctuations during the Holocene. To better constrain these changes and their impacts on the Fennoscandian subarctic, we investigated molecular and inorganic proxies as well as plant wax D/H isotopes (D-C28) in a Holocene sedimentary record from Lake Tornetrask (Sweden). These data indicate a thermal maximum c. 8100 to 6300cal. a BP with reduced soil organic matter input, followed by a long-term cooling trend with increasing soil erosion. D data suggest a stable atmospheric circulation with predominance of westerly flow and North Atlantic moisture sourcing during the Early and Middle Holocene. Asubstantial depletion in D followed by increased flood frequency starting at c. 5300cal. a BP and intensifying c. 1500cal. a BP suggests a reorganization of the atmospheric circulation from zonal towards meridional flow with predominantly Arctic Ocean and Baltic Sea moisture sourcing.

The western tropical Atlantic plays an important role in the interhemispheric redistribution of heat during millennial-scale changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The proper evaluation of this role depends on a clear understanding of sea surface temperature (SST) variations during AMOC slowdown periods like Heinrich Stadials (HS) in the western tropical Atlantic. However, published SST records from the western tropical Atlantic between ca. 4°S and 7°N show inconsistencies that are apparently related to the employed temperature proxy (i.e., Mg/Ca versus alkenone unsaturation index ). In general, while Mg/Ca values indicate warming during Heinrich Stadials, values show cooling. To assess this issue, we sampled core GeoB16224-1 retrieved off French Guiana (i.e., 6°39.38′N) and reconstructed water temperatures at high resolution using Mg/Ca on the foraminifera species Globigerinoides ruber, , TEX86 and modern analogue technique (MAT) transfer functions using planktonic foraminifera assemblages calibrated for 50 m water depth. Our results show that Mg/Ca and TEX86 values recorded an increase in SST related to AMOC slowdown. Conversely, and MAT values registered a decrease in temperatures during HS3 and HS1. Our and Mg/Ca results thus confirm the previously reported inconsistency for the period between 48–13 cal ka BP. We suggest that several non-thermal physiological effects probably imparted a negative temperature bias on the temperatures during Heinrich Stadials. However, MAT-based temperatures show similar variability with -based temperatures. Hence, we also suggest that during severe slowdown periods of the AMOC, a steeper meridional temperature gradient together with a southward shift of the Intertropical Convergent Zone produced not only an increase in SST but also a stronger upper water column stratification and a shoaling of the thermocline, decreasing subsurface temperatures. Our new high resolution temperature record

Organic geochemical and micropaleontological analyses of surface sediments collected in the southern Drake Passage and the Bransfield Strait, Western Antarctic Peninsula, enable a proxy-based reconstruction of recent sea ice conditions in this climate-sensitive area. We study the distribution of the sea ice biomarker IPSO25, and biomarkers of open marine environments such as more unsaturated highly branched isoprenoid alkenes and phytosterols. Comparison of the sedimentary distribution of these biomarker lipids with sea ice data obtained from satellite observations and diatom-based sea ice estimates provide for an evaluation of the suitability of these biomarkers to reflect recent sea surface conditions. The distribution of IPSO25 supports earlier suggestions that the source diatom seems to be common in near-coastal environments characterized by annually recurring sea ice cover, while the distribution of the other biomarkers is highly variable. Offsets between sea ice estimates deduced from the abundance of biomarkers and satellite-based sea ice data are attributed to the different time intervals recorded within the sediments and the instrumental records from the study area, which experienced rapid environmental changes during the past 100 years. To distinguish areas characterized by permanently ice-free conditions, seasonal sea ice cover and extended sea ice cover, we apply the concept of the PIP25 index from the Arctic Ocean to our data and introduce the term PIPSO25 as a potential sea ice proxy. While the trends in PIPSO25 are generally consistent with satellite sea ice data and winter sea ice concentrations in the study area estimated by diatom transfer functions, more studies on the environmental significance of IPSO25 as a Southern Ocean sea ice proxy are needed before this biomarker can be applied for semi-quantitative sea ice reconstructions.

Sea ice proxies are used to reconstruct the climate and environmental history in both polar regions. In the Southern Ocean, the biomarker IPSO25 ‒a highly branched isoprenoid (HBI) diene‒ is produced by sea ice diatoms (Belt et al., 2016). We evaluated the abundance and distribution of IPSO25 in recent ocean surface sediments through comparisons with satellite data and diatom assemblages for sea ice studies in the Western Antarctic Peninsula area. Further, analyses of bulk data and several biomarkers (HBIs, phytosterols, GDGTs) as well as XRF scans were conducted on a sediment core from the Bransfield Basin (add lat&long and core name?) and an age model was developed based on acid-insoluble organic 14C dating. The piston core provides insights on the development of spring sea ice, primary production and sea surface temperature (SST) over the past 20.000 years. The rapid decrease of IPSO25 and slight increase of open ocean indicators from HBI trienes and phytosterols as well as the SST reflect the warming after the Last Glacial Maximum. There is clear evidence for the Antarctic Cold Reversal and a mid-Holocene cooling event about 5.000 years BP. Seasonal sea ice cover remains high during the early Holocene and high variability occurs since the Holocene Climatic Optimum with an overall decrease of sea ice towards the present. Belt et al., 2016. Nature Communications, v. 7, p. 12655.

Recent paleoclimatic studies suggest that changes in the tropical rainbelt across the Atlantic Ocean during the past two millennia are linked to a latitudinal shift of the Intertropical Convergence Zone (ITCZ) driven by the Northern Hemisphere (NH) climate. However, little is known regarding other potential drivers that can affect tropical Atlantic rainfall, mainly due to the scarcity of adequate and high-resolution records. In this study, we fill this gap by reconstructing precipitation changes in Northeastern Brazil during the last 2,300 years from a high-resolution lake record of hydrogen isotope compositions of plant waxes. We find that regional precipitation along the coastal area of South America was not solely governed by north-south displacements of the ITCZ due to changes in NH climate, but also by the contraction and expansion of the tropical rainbelt due to variations in sea surface temperature and southeast trade winds in the tropical South Atlantic Basin.

The temporal succession of changes in Amazonian hydroclimate during Heinrich Stadial 1 (HS1) (ca. 18–14.7 cal ka BP) is currently poorly resolved. Here we present HS1 records based on isotope, inorganic and organic geochemistry from a marine sediment core influenced by the Amazon River discharge. Our records offer a detailed reconstruction of the changes in Amazonian hydroclimate during HS1, integrated over the basin. We reconstructed surface water hydrography using stable oxygen isotopes (δ18O) and Mg/Ca-derived paleotemperatures from the planktonic foraminifera Globigerinoides ruber, as well as salinity changes based on stable hydrogen isotope (δD) of palmitic acid. We also analyzed branched and isoprenoid tetraether concentrations, and compared them to existing bulk sediment ln(Fe/Ca) data and vegetation reconstruction based on stable carbon isotopes from n-alkanes, in order to understand the relationship between continental precipitation, vegetation and sediment production. Our results indicate a two-phased HS1 (HS1a and HS1b). During HS1a (18–16.9 cal ka BP), a first sudden increase of sea surface temperatures (SST) in the western equatorial Atlantic correlated with the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and the associated southern hemisphere warming phase of the bipolar seesaw. This phase was also characterized by an increased delivery of terrestrial material. During HS1b (16.9–14.8 cal ka BP), a decrease in terrestrial input was, however, associated with a marked decline of seawater δ18O and palmitic acid δD. Both isotopic proxies independently indicate a drop in sea surface salinity (SSS). A number of records under the influence of the North Brazil Current, in contrast, indicate increases in SST and SSS resulting from a weakened AMOC during HS1. Our records thus suggest that the expected increase in SSS due to the AMOC slowdown was overridden by a two-phased positive precipitation anomaly in Amazonian hydroclimate.

The combined use of plant wax n-alkane delta D values and branched glycerol dialkyl glycerol tetraether lipid (brGDGT) distributions provides a novel approach for paleoaltitude reconstruction. Studies from East Africa revealed, however, inconsistent results between the proxy estimates and altitudinal parameters. Here, we explore these proxies in soils of different land use (forest, cropland and pasture) along an altitudinal transect in the Jimma zone of the southwest Ethiopian highlands to better understand environmental and plant-specific factors controlling the isotopic composition and distribution of n-alkanes and brGDGTs. The hydrogen isotope composition of individual n-alkanes does not unambiguously reflect the altitude effect on precipitation delta D, but seems largely influenced by the specific land use. Only forest soil-derived n-C-27 and n-C-29 alkane delta D values exhibit a significant linear relationship with altitude (r -0.87, p < 0.05), likely reflecting the most stable ecosystem. The resulting lapse rate of -17 parts per thousand/1000 m is comparable with that of local precipitation in the southwest Ethiopian highlands. In addition, the linear correlation of the average chain length (ACL) and delta C-13 values of forest soil n-alkanes suggests a physiological adaptation of the specific plant type waxes to altitude-induced environmental changes in the study area. The distribution of brGDGTs also reveals a significant linear correlation with altitude (r -0.97, p < 0.01), reflecting the decrease in temperature with higher elevation, independent of land use. In addition, brGDGT-based mean annual air temperature (MAT) estimates of 19.5-14.0 degrees C and temperature lapse rate of -6 degrees C/1000 m are in good agreement with direct measurements in the Jimma zone. In contrast to previous studies from East Africa, our results show that both soil n-alkane dD values and brGDGT-based MAT distributions track present day altitude effects on local environmental g