Search

Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4-18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37ĝF S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3-4 ĝC, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas..

Supplemental material, TPD_SM_Figures for Holocene atmospheric dust deposition in NW Spain by Antonio Martínez Cortizas, Olalla López-Costas, Lisa Orme, Tim Mighall, Malin E Kylander, Richard Bindler and Ángela Gallego Sala in The Holocene

Atmospheric mineral dust plays a dynamic role in the climate system acting both as a forcing and a feedback mechanism. The majority of paleodust studies to date have been conducted on marine sediments or polar ice cores, while terrestrial deposition has been less studied, leaving large regional gaps. Scandinavia is currently only represented by one paleodust study, conducted at Store Mosse, southern Sweden (Kylander et al. 2013; 2016). Within this project the overall aim is to compare the magnitude and timing of dust deposition at three sites located in a west to east transect in southern Sweden. Here, early results from two rain-fed bogs; Draftinge (57°06’27.6 N13°42’54.1 E) and Gällsered Mosse (57°10’27.4 N 12°35’52.3 E) are presented. The respective sequences have been analysed for botanical composition, bulk density, loss-on-ignition (LOI), organic chemistry (elemental C, N, and δ13C), and dated by AMS radiocarbon dating. Downcore elemental concentrations have been analysed by quantitative WD-XRF analysis every 3-5 cm coupled with mineralogical analysis by X-ray diffraction (XRD). Based on the concentrations of soil dust elements (Si, Al, and Ti), four episodes of increased dust deposition are recorded at Draftinge Mosse whereas six dust events (DEs) are noted at Gällsered Mosse. Three of these DEs are co-occurring at the two study sites, and two are recorded at all three sites. The mineralogy throughout the Draftinge sequence is dominated by quartz and feldspars, with the exception of sample from ca 5.6 ka BP where a distinct mineralogy of calcite, clays and quartz is noted. By comparing dust deposition at several study sites, based on multi-elemental datasets coupled with mineralogy, magnitude and timing of regional paleodust deposition can be constrained and an increased knowledge of past dust dynamics obtained.

Peatlands in northern latitudes sequester one third of the world’s soil organic carbon. Mineral dusts can affect the primary productivity of terrestrial systems through nutrient transport but this process has not yet been documented in these peat-rich regions. Here we analysed organic and inorganic fractions of an 8900-year-old sequence from Store Mosse (the “Great Bog”) in southern Sweden. Between 5420 and 4550 cal yr BP, we observe a seven-fold increase in net peat-accumulation rates corresponding to a maximum carbon-burial rate of 150 g C m−2 yr−1 – more than six times the global average. This high peat accumulation event occurs in parallel with a distinct change in the character of the dust deposited on the bog, which moves from being dominated by clay minerals to less weathered, phosphate and feldspar minerals. We hypothesize that this shift boosted nutrient input to the bog and stimulated ecosystem productivity. This study shows that diffuse sources and dust dynamics in northern temperate latitudes, often overlooked by the dust community in favour of arid and semi-arid regions, can be important drivers of peatland carbon accumulation and by extension, global climate, warranting further consideration in predictions of future climate variability. The Swedish Research Council is gratefully acknowledged for funding to MEK (2009-4426) SI

In peat research a decision is sometimes to be made of choosing between single (i.e. intensive) and multiple (i.e. extensive) peat cores studies. Sample and time constraints are also common when destructive and time consuming methods are applied. In this context, infrared spectroscopy is a promising alternative as it is non-destructive, easy to apply, requires none or minimum sample preparation and can yield valuable information on peat properties. Using FTIR-ATR, we analysed two peat cores: one from Store Mosse (SM) and one from the adjacent, smaller, Draftinge Mosse (DM). Total C and N contents and isotopic ratios (δ13C and δ15N) were also determined (n= 106 and 76 samples, respectively). Partial least squares (PLS) modelling was applied using the SM spectra as predictors and the other properties as response variables. Validation was assessed in two ways: i) two thirds of the SM samples were used as training set and one third as validation set, and ii) all DM samples were used as validation set. Highly significant models (R2 0.97–0.99), with 2–10 latent vectors (LV), were obtained for SM. Validation coefficients were very high (r2 >0.80) for C, N and δ15N and high (r2 0.77) for δ13C. For DM, models were highly accurate for C (r2 0.92, 2 LV) and N (r2 0.94, 3 LV) and accurate for the isotopic ratios (δ13C r2 0.69, 1 LV; δ15N r2 0.70, 2 LV). Despite the lower performance, the predicted δ13C and δ15N values reproduced the main features of the isotopic chronologies. Our results indicate that FTIR-ATR combined with PLS i) is able to accurately predict peat properties in the same bog (helpful for multiple cores studies), and ii) the models can be transferred to similar peatlands of the same region, although the accuracy may vary between peat properties.

Reconstructing surface wetness in peatlands is a challenging issue that, more often than desired, gives contrasting proxy evidence, making it difficult to clearly differentiate between dry and wet periods. The differences inherent to the different methods and their intrinsic weaknesses may be behind this situation. However, other circumstances such as seasonal contrast or microtopography development could account for some of the apparent disagreement. Draftinge Mosse is a raised peat bog located in south-central Sweden close to the extensively studied “Great Bog” (Store Mosse). Here, contrasting proxy evidence for moisture conditions was found for the period between ~5600 and ~4600 cal.BP. At this time, a high peat accumulation event (HPAE, with a maximum of 294 g m-2 y-1, also found at Store Mosse by Kylander et al., 2013) was accompanied by increased carbohydrates/lipids ratio indicating less decomposed peat and pointing to wetter conditions. In contrast, in the same peat section supposed dry NPP indicators also showed a large increase in abundance, suggesting that drier conditions prevailed. To complicate the interpretation further, suggested wet NPP indicators also increase from ~5000 cal. yr BP to present (with ups and downs). The sudden boost in peat accumulation, that started coinciding with warming in the North Atlantic right after the termination of Bond Event 4, suggests that changes in microtopography might have played a role in the creation of different microhabitats (drier hummocks and wetter hollows) and this would explain the variable response in some of the proxies. Although other factors affecting the ecological behavior of the detected NPP cannot be neglected, our hypothesis is that during the HPAE, despite generally wetter conditions in the region, the surface of the peatland might have been locally isolated of the water table because of the rapid peat growth, resulting in the creation of dry microhabitats.

Atmospheric mineral dust plays a dynamic role in the climate system affecting biogeochemical cycles, radiation budgets, and cloud formation. The amount of dust in the atmosphere is related to prevailing climate regimes, where glacial periods are known to be significantly dustier (x2.5) than interglacials. Despite recent increases in Holocene paleo-dust records, additional data from continental locations are required to fill regional gaps. During the last decades’ ombrotrophic (rain-fed) peat bogs have been increasingly used to reconstruct terrestrial dust deposition by geochemical methods such as XRF and ICP-MS. Derived elemental data are commonly used infer net deposition rates, source changes, grain size, and mineral composition. Currently, Scandinavia is only represented by one such paleo-dust record, and further studies are required from the region to distinguish local, regional and global dust events. Here we present ongoing work from an ombrothropic peat bog in southern Sweden where we combine elemental data with direct mineralogical observations through X-ray diffraction analysis (XRD), bulk density and C/N analysis, to build a dust record covering the last 8.3 kyr BP. The study aims are 1) to identifying periods of increased dust deposition; 2) detect potential source changes; 3) compare our results with past dust and paleoclimate studies from the region; and lastly, 4) verify and/or refine assumptions related to interpretation of dust deposition from bulk elemental data compared to direct observation of mineralogy (XRD).

It is generally accepted that mineral dusts play an important role in biological productivity, particularly in nutrient poor systems, through fertilization. We demonstrate the relationship between terrestrial ecosystem productivity and dust deposition in a northern temperate setting using a peat sequence covering the last 8900 cal yr BP from Store Mosse (N57°13’37’’, E13°55’17’’), an extensive (77 km2) ombrotrophic bog (i.e., atmospherically fed) located in southern Sweden. While this site is typical in terms of its organic matrix (bulk density, carbon and peat accumulation rates, C/N ratios, etc), we observe a seven-fold increase in peat accumulation rates, starting 5300 cal yr BP and reaching a maximum (291 g m-2 yr-1) just 800 years later. While the wetter climatic conditions at this time surely drove this high peat accumulation event (HPAE), the scale of this change implies another factor must contribute. The inorganic geochemistry from Store Mosse reveals three periods of increased dust deposition as estimated using Al mass accumulation, which do not co-occur with the HPAE. Rather, during the HPAE there is a distinct change in mineral character. There is a rapid increase in light and middle REE enrichment starting 5300 cal yr BP, expressed by La/Lu and Gd/Lu, respectively, which is interpreted to signal the presence of the primary phosphate minerals such as apatite, allanite and/or monazite. The Eu anomaly profile ([Eu/Eu*]UCC) follows a similar pattern and indicates that addition of plagioclase feldspar and less weathered minerals to the bog also increased. We suggest that this increased the P and K supply to this nutrient poor system, spiking productivity. This shows that dust dynamics in the temperate northern latitudes can play an important part in stimulating primary production and therefore carbon cycling and storage.

Although archaeological research suggests that mining/metallurgy already started in the Chalcolithic (3rd millennium BC), the earliest atmospheric metal pollution in SW Europe has thus far been dated to ~ 3500–3200 cal. yr. BP in paleo-environmental archives. A low intensity, non-extensive mining/metallurgy and the lack of appropriately located archives may be responsible for this mismatch. We have analysed the older section (> 2100 cal. yr. BP) of a peat record from La Molina (Asturias, Spain), a mire located in the proximity (35–100 km) of mines which were exploited in the Chalcolithic/Bronze Age, with the aim of assessing evidence of this early mining/metallurgy. Analyses included the determination of C as a proxy for organic matter content, lithogenic elements (Si, Al, Ti) as markers of mineral matter, and trace metals (Cr, Cu, Zn, Pb) and stable Pb isotopes as tracers of atmospheric metal pollution. From ~ 8000 to ~ 4980 cal. yr. BP the Pb composition is similar to that of the underlying sediments (Pb 15 ± 4 μg g− 1; 206Pb/207Pb 1.204 ± 0.002). A sustained period of low 206Pb/207Pb ratios occurred from ~ 4980 to ~ 2470 cal. yr. BP, which can be divided into four phases: Chalcolithic (~ 4980–3700 cal. yr. BP), 206Pb/207Pb ratios decline to 1.175 and Pb/Al ratios increase; Early Bronze Age (~ 3700–3500 cal. yr. BP), 206Pb/207Pb increase to 1.192 and metal/Al ratios remain stable; Late Bronze Age (~ 3500–2800 cal. yr. BP), 206Pb/207Pb decline to their lowest values (1.167) while Pb/Al and Zn/Al increase; and Early Iron Age (~ 2800–2470 cal. yr. BP), 206Pb/207Pb increase to 1.186, most metal/Al ratios decrease but Zn/Al shows a peak. At the beginning of the Late Iron Age, 206Pb/207Pb ratios and metal enrichments show a rapid return to pre-anthropogenic values. These results provide evidence of regional/local atmospheric metal pollution triggered by the earliest phases of mining/metallurgy in the area, and reconcile paleo-environmental and archaeological records. This work was funded by the projects CGL2010-20672 and HAR2008-06477-C03-03/HIST (Plan Nacional I + D + i, Spanish Ministerio de Economía y Competitividad), 10PXIB200182PR (General Directorate of I + D, Xunta de Galicia), and CDS-TCP (CSD2007-00058, Programa Consolider-Ingenio 2010). José Antonio López-Sáez, Fernando Gil Sendino, Carmen Fernández Ochoa and Roberto Zapico for their collaboration and assistance during the fieldwork, to José Rodríguez Racedo for helping with the geochemical analyses, and to Suzanne Leroy for perceptive comments on an earlier draft.