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

Abstract

In the subarctic Northwest Pacific and adjacent Siberia mean climate changes between the Last Glacial Maximum and the Holocene are poorly understood since climate 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 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 is found that SSTs in both settings are determined by rapid atmospheric teleconnections with abrupt climate changes in the North Atlantic (N Atlantic) since 15 ka BP. Before 15 ka BP, only the Bering Sea was connected to N-Atlantic climate change. The NW Pacific remained disconnected from the N-Atlantic until 15 ka BP due to an oceanic linkage with the NE Pacific through the Alaskan Stream. The second part investigates the LGM-to-Holocene evolution of mean air temperature (MAT) of the Kamchatka Peninsula where climate archives do not reach beyond 12 ka BP. 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. 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. The deglacial temperature development was characterized by abrupt millennial-scale temperature oscillations during the past 15 ka BP. 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 degreeday-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, confirming the hypothesis that summer temperature limited ice-sheet expansion in NE Russia during the LGM. The fourth part of the thesis contributes to an ongoing debate about the sources of old, (14Cdepleted) carbon dioxide (CO2) which increased atmospheric CO2-levels (CO2atm) and concurrently decreased the atmospheric radiocarbon signature (14Catm) 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 (14C) of leafwax-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 14Cdepleted 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/14Catm 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/14Catm changes during the YD suggesting that permafrost may have contributed to the atmospheric carbon-pool at that time.