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Arctic permafrost soils have recently been identified as the largest mercury (Hg) reservoir on Earth. Today, rapid warming in the high latitudes may be altering the Arctic Hg cycle by accelerating permafrost thaw, leading to changes including deepening of the active layer, increasing organic matter decay, and increasing seasonal groundwater flow. However, few studies have investigated how the Hg cycle has responded to past changes in climate, and there is a lack of Arctic records that span the late glacial to early Holocene when climate conditions changed abruptly. We propose that the geochemical and physical changes in the sediment record of Burial Lake (68.43ºN, 159.17ºW; 460 m ASL), which document climatic and environmental changes in northwestern Alaska after the Last Glacial Maximum (LGM), can be used as an analog to investigate how today’s rapid warming affects Hg mobilization from permafrost soils to surficial waters. Warming in the Northern Hemisphere between ~15.0 and 8.0 ka resulted in rapid changes in northwest Alaska, including the submergence of the Bering Land Bridge that reconnected the Pacific and Arctic Oceans (~11.0 ka), in addition to changes in the hydroclimate. Our results indicate that the Hg concentration was relatively low and stable in the Burial Lake record during the transition from the LGM to the late glacial (20.0 and 16.0 ka) with a mean concentration of 64±7 μg/kg. Mercury concentrations begin to increase after 16.0 ka. Then, coinciding with a rapid temperature increase at the beginning of the Bølling Allerød (14.7 to 12.9 ka), Hg concentrations increased by ~20% and showed higher variability as temperatures fluctuated until the end of the Younger Dryas (12.9 to 11.7 ka). At 11.0 ka, the Hg concentration increased rapidly. It peaked at 140 µg/kg, with a mean Hg concentration of 119 μg/kg between 11.0 to 8.8 ka, coinciding with evidence of a rapid increase in regional precipitation and flooding of the Bering Land Bridge. From 8.8 to 0.1 ka, the mean Hg concentration decreased to 107 μg/kg and then increased rapidly over the last 100 years to a maximum concentration of 196 μg/kg occurring during the 1990s. Throughout the majority of the Burial Lake sediment record, the Hg concentration is most strongly correlated with total organic carbon content and geochemical proxies sensitive to changes in redox conditions. We interpret this finding as an indication that a large fraction of Hg is mobilized from the lake catchment along with dissolved organic matter (DOM), iron (Fe), and manganese (Mn) that are mobilized as a result of saturation and deepening of the active layer during periods of warmer, but most importantly, wetter climate. The Hg record from Burial Lake suggests that as the climate warmed after the LGM, organic-rich permafrost soils and Hg accumulated in the catchment. The sudden increase in Hg mobilization from permafrost soils was then initiated at the onset of the Holocene due to the rapid increase in precipitation that coincided with the flooding of the Bering Land Bridge.

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG [Abstract] Understanding the environmental response to the last glacial termination in regions located in transitional climate zones such as the Atlantic Iberian mountains is crucial to estimate potential changes in regions affected by current glacial melting. We present an 8.5 m-long, solid last deglaciation and Holocene chronostratigraphic record including detailed sediment analysis from Lake Peixão, a pro-glacial lake in the Serra da Estrela (Central Portugal). The age–depth model relies on a Bayesian approach that includes 16 AMS 14C dates and 210Pb–137CS measurements, robustly dating the lake formation at 14.7 ± 0.32 cal. ka BP. This chronological reconstruction shows an average sedimentation rate of ca. 0.07 cm yr−1 (15 yr cm−1), enabling proxy analyses at decadal timescales. The sediment sequence is composed of five lithological units: (U1) coarse and unsorted fluvioglacial lacustrine deposits; (U2) massive fluvioglacial lacustrine deposits (863–790 cm below surface [bsf]; 14.7 ± 0.32–13.8 ± 0.12 cal. ka BP); (U3) water current fluvioglacial lacustrine deposits (790–766 cm bsf; 13.8 ± 0.12–12.9 ± 0.29 cal. ka BP); (U4) laminated/banded lacustrine deposits characterized by terrigenous deposits from ice-covered lake periods and episodic events of ice and snow melting (766–752 cm bsf; 12.9 ± 0.29–11.7 ± 0.15 cal. ka BP); and (U5) massive muddy lacustrine deposits (752–0 cm bsf; 11.7 ± 0.15 cal. ka BP–present). The occurrence of U2 to U4 deposits defines the transition from glacial cold (U1) to net warm postglacial conditions (U5). These climate transitions are marked by changes in sediments and the presence of very low sedimentation rate periods, possibly related to the Intra-Allerød Cold Period and the coldest phase of the Younger Dryas. Our results support the previously proposed timing of the retreat of the Serra da Estrela glaciers ca. 13.8 ± 0.12 cal. ka BP. The robust chronology of Lake Peixão highlights the potential of Iberian pro-glacial lakes for dating deglaciation processes and will lead to unprecedented decadal-to-centennial timescale palaeoclimate reconstructions in this region since the last glacial–interglacial transition. This work was supported by the Portuguese Foundation for Science and Technology – FCT, through the HOLMODRIVE project (PTDC/CTA-GEO/29029/2017). A. Hernandez is funded by the Spanish Ministry of Science and Innovation through the Ramón y Cajal Scheme [RYC2020-029253-I]. C. Martin-Puertas is funded by UKRI-FLF MR/W009641/1. G. Gil-Romera is funded by the DFG project FOR2358 ‘The Mountain Exile Hypothesis’. Pedro M. Raposeiro was funded by the Portuguese Science Foundation through contract DL57/2016/ICETA/EEC2018/25. Alexandre M. Ramos is funded by the Helmholtz ‘Changing Earth’ programme. Funding for open access charge: Universidade da Coruña/CISUG. The Estrela UNESCO Global Geopark is thanked for their support with field activities and permissions. The Parque Natural da Serra da Estrela (ICNF) provided research permits. Deutsche Forschungsgemeinschaft; FOR2358 Portugal. Fundação para a Ciência e a Tecnologia; DL57/2016/ICETA/EEC2018/25 Portugal. Fundação para a Ciência e a Tecnologia; PTDC/CTA-GEO/29029/2017 UK Research and Innovation; UKRI-FLF MR/W009641/1

The Serra da Estrela range in Central Portugal is a key region to explore the Last Glacial termination in the Atlantic region of Iberia. We present the Late Glacial and Holocene record of Lake Peixão to reconstruct past environmental and climatic conditions at decadal timescales. We retrieved an 8.5m long core in the central and deepest zone of the lake. From the base to the top, the sediment core is composed of i) a thick layer (aprox. 0.5 m) of gravel-to-sand quartz clasts; ii) an alternation of one meter thick grey to pale-brown sandy deposits; and iii) homogeneous dark brown and pale-brown organic-rich muddy facies (ca. 7 meters). Using a Bayesian statistical model based on 16 radiocarbon dates using pollen concentrated samples and 210Pb measurements, we have developed an age-depth model covering the last ca. 14.6 ka, recording the onset of the lake at that time. Lake and mire records previously published in the area generally support this chronology for the formation of proglacial lakes. In addition, the proposed timing for the disappearance of the Serra da Estrela glaciers has been based on Cosmic-Ray Exposure dating (in situ cosmogenic 36Cl) of exposed moraine boulders and glacially polished bedrock surfaces. These results indicate that the end of the glacier’s presence in Serra da Estrela occurred at ca. 14.2 ka BP (during the Bølling-Allerød Interstadial). Therefore our results broadly suport the timing of the end of the deglaciation at the Serra da Estrela and highlight the value of lake sedimentary records. Moreover, the availability of last termination and Holocene highly-resolved sediments from Lake Peixão is likely to record very valuable evidence of climatic and environmental changes at decadal timescales extending from the last deglaciation, through the Holocene. This new multi-proxy data of the Lake Peixão sedimentary sequence will enable comparisons to other decadal-resolved records and support further investigations from the Mediterranean Iberian region.The financial support for this work was possible through the following FCT project: HOLMODRIVE—North Atlantic Atmospheric Patterns Influence on Western Iberia Climate: From the Late Glacial to the Present (PTDC/CTA-GEO/29029/2017).