Evolution of temperature and mobilization of terrigenous organic matter in the subarctic Northwest Pacific and adjacent Beringia since the Last Glacial Maximum


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vera.meyer [ at ] awi.de

Abstract

In times of global warming a profound understanding of the climate system is necessary to develop mitigation strategies. Studying episodes of climate change during the Earth’s history (e.g. Glacial-Interglacial cycles) allows insights into the climate system and its feedback processes. In the subarctic Northwest Pacific (NW Pacific) and adjacent Northeast Siberia (NE Siberia) mean climate changes between the Last Glacial Maximum and the Holocene are poorly understood since climate records (e.g. temperature records) spanning the full LGM-Holocene transition are sparse. This thesis shall contribute to a better understanding of climate and environmental change since the LGM and the controlling mechanisms in the region by investigating the development of temperature, glaciation and export of terrigenous organic matter into the North Pacific (N Pacific). Biomarkers in sediment cores from the Western Bering Sea and the NW Pacific/continental margin off Siberia are applied as palaeoclimate archives. In the first part of the thesis LGM-to-Holocene sea surface temperature (SST) records for the marginal Northwest Pacific and the Western Bering Sea are established using the TEXL86 (Tetraether IndeX)-SST proxy. It focusses on the LGM and the early deglaciation since existing deglacial SST records from the region do not reach beyond 15 ka BP. TEXL86-based SSTs in both settings closely follow millennial-scale climate fluctuations known from Greenland ice-cores until 15 ka BP, confirming other SST-records from the region which point to rapid atmospheric teleconnections with abrupt climate changes in the North Atlantic (N Atlantic). During Heinrich Stadial 1 (HS1), Western Bering Sea SSTs decline, similar to the N Atlantic realm, suggesting the Bering Sea was connected to the N Atlantic climate change. Progressively rising SSTs in the NW Pacific differ from the Western Bering Sea and the N Atlantic climate. Similarities between the climate in the Gulf of Alaska and the NW Pacific suggest that the Alaskan Stream accumulated in the NW Pacific during the LGM connecting the climates of the eastern and western N Pacific. Deviating trends in the climate from 12-10 ka onwards point to reduced influence of the Alaskan Stream in the NW Pacific and the end of the oceanic linkage. The second part of the thesis investigates the LGM-to-Holocene evolution of mean air temperature (MAT) of the Kamchatka Peninsula. Climate archives, existing in Kamchatka, do not reach beyond 12 ka BP, so the climate evolution since the LGM is fairly unknown. Using the CBT/MBT’-palaeothermometry (Cyclisation of Branched Tetreathers and the Methylation of Branched Tetraethers indices) a continuous record in summer MAT is provided for the past 20 ka. It is found that glacial summers were as warm as at present. This is in line with summer conditions in continental Siberia but contrasts with the SST-development of the surrounding seas. Likely, strong southerly winds, associated with a pronounced North Pacific High pressure system (NPH) over the subarctic NW Pacific, accounted for the warm conditions on Kamchatka. A comparison with an Earth System Model reveals discrepancies between proxy-based inferences for temperature and atmospheric circulation. The deglacial temperature development was characterized by abrupt millennial-scale temperature oscillations. The Bølling/Allerød warm-phase (B/A) and the Younger Dryas cold-spell (YD) are pronounced events, providing evidence for a strong impact of N Atlantic climate variability on southeastern Siberia, at least during the past 15 ka BP. During HS1, similarities with the NW Pacific SST imply that the Alaskan Stream determined temperature change on the Peninsula rather than teleconnections with the N Atlantic. Considering that NE-Siberian glaciation is supposed to have been more extensive than at present but restricted to mountain ranges during the LGM, the warm glacial-summers of Siberia suggest that summer temperature may have been an important limiting factor for ice sheet growth in the region. In the third part of the thesis, mass balance calculations for the LGM-glaciers on Kamchatka and the Kankaren Range (NE Siberia) are performed by degree-day-modelling in order to estimate the precipitation needed to sustain the glaciers under warm summer conditions. It is found that precipitation at least must have equaled or even exceeded the modern average. The precipitation estimates confirm the hypothesis that summer temperature limited ice-sheet expansion in NE Russia during the LGM, thereby countering the prevailing view that increased aridity (relative to present) hampered ice-sheet growth. The fourth part of the thesis contributes to an ongoing debate about the sources of old, (14C-depleted) carbon dioxide (CO2) which increased atmospheric CO2-levels (CO2atm) and concurrently decreased the atmospheric radiocarbon signature (Δ14Catm) during the deglaciation. Permafrost-decomposition in the Northern Hemisphere (NH) triggered by deglacial warming and sea-level rise is considered as one possible source of 14C-depleted CO2, particularly at the onset of the B/A-interstadial (14.6 ka BP). However, the timing of carbon mobilization in permafrost areas of the NH is underconstrained. In order to investigate the potential role of permafrost decomposition in the subarctic N Pacific realm in the atmospheric, changes mass accumulation rates and the radiocarbon signature (Δ14C) of leaf-wax lipids are analyzed in order to identify intervals of intensified export of 14C-depleted terrigenous OM into the Western Bering Sea and the NW Pacific. Enhanced burial of nearly 14C-free carbon commenced during the HS1 and was likely triggered by increased runoff in the Yukon River due to retreating American ice-sheets. Since the B/A mobilization of 14C-depleted seems to have been dominantly controlled by sea-level rise and thus by erosion of permafrost-covered shelves. Enhanced OM-export associated with permafrost-thaw on Kamchatka likely initiated during the second half of the B/A-interstadial and peaked during the YD-stadial. Lagging the rapid CO2atm/Δ14Catm changes at 14.6 ka BP, the permafrost degradation in the Kamchatka region was probably irrelevant for the atmosphere. Instead, enhanced OM-export in the region coincided with abrupt CO2atm/Δ14Catm changes during the YD suggesting that permafrost may have contributed to the atmospheric carbon-pool at that time.



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Thesis (PhD)
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Eprint ID
42181
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Meyer, V. (2016): Evolution of temperature and mobilization of terrigenous organic matter in the subarctic Northwest Pacific and adjacent Beringia since the Last Glacial Maximum PhD thesis,


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