The deglacial development of the Oxygen Minimum Zone in the Bering Sea: A study based on high-resolution laminated sediment records


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Ralf.Tiedemann [ at ] awi.de

Abstract

In the past decades several paleoceanographic studies found evidence for millennial scale climate variability during the late Pleistocene. While these climate fluctuations are well studied in the North Atlantic, the paleoceanographic history in the North Pacific is less well known. The last glacial termination is of special interest, as it represents the most recent transition from glacial to deglacoal conditions. In the North Pacific these changes led to a strengthening of the mid-depth oxygen minimum zone (OMZ) to anoxic bottom water conditions and the deposition of laminated sediments. The main objective of this thesis was to gain new insights into the deglacial mid-depth OMZ dynamics in the Bering Sea, using a suite of mid-depth, partly laminated sediment cores that were collected during R/V Sonne cruise SO202-INOPEX. It will be shown that during the Bølling–Allerød (BA) and early Holocene the Bering Sea OMZ strengthened to anoxic values of <0.1 ml/l, which led to the formation of laminations, and expanded vertically to water depths of >2100 m. Based on a correlation of 14C-dated, laminated sediment cores it is revealed that throughout the Bering Sea and the Gulf of Alaska the onset of deglacial anoxia at 14.6 ka and 11.7 ka was a synchronous event, while the disappearance of laminations was a diachronous process. A decadal-scale correlation of two 14C dated, layer counted sediment cores with seasonal resolution from the northeastern Bering Sea slope to the NGRIP δ18O record revealed, that (1) the formation of laminations was tightly coupled to warm phases of the B/A and the early Holocene, which implies an atmospheric teleconnection between the North Atlantic and Bering Sea, and (2) the presence of annually laminated sediments (varves). The established age model was partly independent from radiocarbon ages and allowed the calculation of surface reservoir ages, which are 770 yr, 910yr and 875 yr for the Holocene, Younger Dryas and B/A respectively. The anoxia were driven on millennial scales by basin-wide remineralization of organic matter in intermediate waters, in combination with decadal-scale export productivity increases during times of warm temperatures. Based on a stacked sea surface temperature (SST) record, SSTs of about 6 °C persisted during times of lamina formations, while SSTs showed values of about 5 °C during times were laminations were not widespread. Other factors that enhanced the export productivity during the B/A and Holocene were increased stratification, a change towards seasonal sea ice, and an input of nutrients and organic carbon from the Alaska hinterlands, which started at 16.6 ka in the Bering Sea. Spectral analyses of Xray-fluorescense-based Cl values of single laminae, which were used as a indicator for biosiliceous productivity, revealed a sharp spectral peak of 18.8 yr and a broader peak of 30 – 60 yr in the B/A and early Holocene. The first peak was related to the 18.6 yr nodal tidal cycle and the latter peak to the Pacific Decadal Oscillation. They influenced primary productivity through nutrient delivery to the surface via tidal mixing, SST variations and stratification. Both processes act in the modern subarctic Pacific, which suggests that productivity variations in the subarctic Pacific are consistently influenced by a combination of external and internal forcing mechanisms since the last deglacial.



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Thesis (PhD)
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42875
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Kühn, H. (2015): The deglacial development of the Oxygen Minimum Zone in the Bering Sea: A study based on high-resolution laminated sediment records , PhD thesis, Universität Bremen, Fachbereich 5, Geowissenschaften.


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