Paleoceanography of the Bering Sea across the Mid- to late Pleistocene

The Bering Sea represents the gateway between the Pacific and the Arctic Ocean. It is characterized by a seasonal sea ice cycle, which together with upwelling along the continental margin provides nutrients for primary producers. The resulting high sedimentation rates along the margin allow to study Quaternary climate change at an orbital to sub-orbital resolution, ideal to examine the evolution of continental ice volume across the Mid-Pleistocene transition (MPT, 1.2-0.7 Ma), the shift in glacial/interglacial (G/IG) frequency from 41-ka to 100-ka. Additionally, the Bering Sea provides a unique opportunity to study sea ice-ocean/land ice interactions. In the North Pacific, sea ice not only plays a significant role in climatic feedbacks, but also influences the ventilation of mid-depth waters via brine rejection. This thesis focuses on MPT climate change in the eastern Bering Sea, with particular emphasis on bottom water temperature (BWT), seawater oxygen isotopes (δ18Ow), sedimentary redox conditions, and sea ice dynamics. The MPT findings are reinforced by examining the same parameters across the last G/IG cycle. Further, the chemical composition of contamination phases in benthic foraminifera in eastern Bering Sea slope sediments is analysed. Foraminiferal contaminants are primarily composed of authigenic carbonates enriched in Mg, U, Mn, Fe, and Sr with consequences for geochemical proxies, such as Mg/Ca for BWT. Nevertheless, this study also demonstrates the opportunities accompanying authigenic carbonates, such as authigenic U/Mn for sedimentary redox chemistry

Multi-elemental composition of authigenic carbonates in benthic foraminifera from the eastern Bering Sea continental margin (International Ocean Discovery Program Site U1343)

Bering Sea sediments represent exceptional archives, offering the potential to study past climates and biogeochemistry at a high resolution. However, abundant hydrocarbons of microbial origin, especially along the eastern Bering Sea continental margin, can hinder the applicability of palaeoceanographic proxies based on calcareous foraminifera, due to the formation of authigenic carbonates. Nonetheless, authigenic carbonates may also bear unique opportunities to reconstruct changes in the sedimentary redox environment. Here we use a suite of visual and geochemical evidence from single-specimens of the shallow infaunal benthic foraminiferal species Elphidium batialis Saidova (1961), recovered from International Ocean Discovery Program (IODP) Site U1343 in the eastern Bering Sea, to investigate the influence of authigenic carbonates on the foraminiferal trace metal composition. Our results demonstrate that foraminiferal calcite tests act as a nucleation template for secondary carbonate precipitation, altering their geochemistry where organoclastic sulphate reduction and anaerobic oxidation of methane cause the formation of low- and high-Mg calcite, respectively. The authigenic carbonates can occur as encrusting on the outside and/or inside of foraminiferal tests, in the form of recrystallization of the test wall, or as banding along natural laminations within the foraminiferal test walls. In addition to Mg, authigenic carbonates are enriched in U/Ca, Mn/Ca, Fe/Ca, and Sr/Ca, depending on the redox environment that they were formed in. Our results demonstrate that site-specific U/Ca thresholds are a promising tool to distinguish between diagenetically altered and pristine foraminiferal samples, important for palaeoceanographic reconstructions utilising the primary foraminiferal geochemistry. Consistent with previous studies, U/Mn ratios of foraminifera at IODP Site U1343 increase according to their degree of diagenetic alteration, suggesting a potential response of authigenic U/Mn to the microbial activity in turn linked to the sedimentary redox environment

Holocene sea-ice dynamics in Petermann Fjord in relation to ice tongue stability and Nares Strait ice arch formation

The Petermann 2015 expedition to Petermann Fjord and adjacent Hall Basin recovered a transect of cores, extending from Nares Strait to underneath the 48&thinsp;km long ice tongue of Petermann glacier, offering a unique opportunity to study ice&ndash;ocean&ndash;sea ice interactions at the interface of these realms. First results suggest that no ice tongue existed in Petermann Fjord for large parts of the Holocene, raising the question of the role of the ocean and the marine cryosphere in the collapse and re-establishment of the ice tongue. Here we use a multi-proxy approach (sea-ice-related biomarkers, total organic carbon and its carbon isotopic composition, and benthic and planktonic foraminiferal abundances) to explore Holocene sea ice dynamics at OD1507-03TC-41GC-03PC in outer Petermann Fjord. Our results are in line with a tight coupling of the marine and terrestrial cryosphere in this region and, in connection with other regional sea ice reconstructions, give insights into the Holocene evolution of ice arches and associated landfast ice in Nares Strait. The late stages of the regional Holocene Thermal Maximum (6900&ndash;5500&thinsp;cal&thinsp;yr&thinsp;BP) were marked by reduced seasonal sea ice concentrations in Nares Strait and the lack of ice arch formation. This was followed by a transitional period towards Neoglacial cooling from 5500&ndash;3500&thinsp;cal&thinsp;yr&thinsp;BP, where a southern ice arch might have formed, but an early seasonal breakup and late formation likely caused a prolonged open water season and enhanced pelagic productivity in Nares Strait. Between 3500 and 1400&thinsp;cal&thinsp;yr&thinsp;BP, regional records suggest the formation of a stable northern ice arch only, with a short period from 2500&ndash;2100&thinsp;cal&thinsp;yr&thinsp;BP where a southern ice arch might have formed intermittently in response to atmospheric cooling spikes. A stable southern ice arch, or even double arching, is also inferred for the period after 1400&thinsp;cal&thinsp;yr&thinsp;BP. Thus, both the inception of a small Petermann ice tongue at &sim;&thinsp;2200&thinsp;cal&thinsp;yr&thinsp;BP and its rapid expansion at &sim;&thinsp;600&thinsp;cal&thinsp;yr&thinsp;BP are preceded by a transition towards a southern ice arch regime with landfast ice formation in Nares Strait, suggesting a stabilizing effect of landfast sea ice on Petermann Glacier. &nbsp;</p

Closure of the Bering Strait caused Mid-Pleistocene Transition cooling

The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600–1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards. Reduced high latitude upwelling, a process that retains CO2 within the deep ocean over glacials, could have aided drawdown but has so far not been constrained in either hemisphere over the MPT. Here, we find that reduced nutrient upwelling in the Bering Sea, and North Pacific Intermediate Water expansion, coincided with the MPT and became more persistent at ~900 ka. We propose reduced upwelling was controlled by expanding sea ice and North Pacific Intermediate Water formation, which may have been enhanced by closure of the Bering Strait. The regional extent of North Pacific Intermediate Water across the subarctic northwest Pacific would have contributed to lower atmospheric CO2 and global cooling during the MPT

Late quaternary sea-ice and sedimentary redox conditions in the eastern Bering Sea – Implications for ventilation of the mid-depth North Pacific and an Atlantic-Pacific seesaw mechanism

On glacial-interglacial and millennial timescales, sea ice is an important player in the circulation and primary productivity of high latitude oceans, affecting regional and global biogeochemical cycling. In the modern North Pacific, brine rejection during sea-ice freezing in the Sea of Okhotsk drives the formation of North Pacific Intermediate Water (NPIW) that ventilates the North Pacific Ocean at 300 m to 1000 m water depth. Glacial intervals of the late Quaternary, however, experienced a deepening of glacial NPIW to at least 2000 m, with the strongest ventilation observed during cold stadial conditions of the last deglaciation. However, the origin of the shifts in NPIW ventilation is poorly understood. Numerical simulations suggest an atmospheric teleconnection between the North Atlantic and the North Pacific, in response to a slowdown or shutdown of the Atlantic meridional overturning circulation. This leads to a build-up of salinity in the North Pacific surface ocean, triggering deep ventilation. Alternatively, increased sea-ice formation in the North Pacific and its marginal seas may have caused strengthened overturning in response to enhanced brine rejection. Here we use a multi-proxy approach to explore sea-ice dynamics, sedimentary redox chemistry, and benthic ecology at Integrated Ocean Drilling Program Site U1343 in the eastern Bering Sea across the last 40 ka. Our results suggest that brine rejection from enhanced sea-ice formation during early Heinrich Stadial 1 locally weakened the halocline, aiding in the initiation of deep overturning. Additionally, deglacial sea-ice retreat likely contributed to increased primary productivity and expansion of mid-depth hypoxia at Site U1343 during interstadials, confirming a vital role of sea ice in the deglacial North Pacific carbon cycle