Investigating Bering Sea oceanographic response to the Milankovitch orbital cycle climatic shift during the middle Pleistocene

The transition of Earth’s glacial-interglacial cycles from 41 kyr to 100 kyr periodicity during the middle Pleistocene (the Mid-Pleistocene Transition (MPT); ~1.2–0.6 Ma) marks one of the largest climate events of the Cenozoic, but the causes of this cooling transition remain unclear, as the emergence of the 100 kyr Milankovitch orbital ‘eccentricity’ in climate records occurred without a long term change in external orbital forcing. Hypotheses for this transition have so far remained largely untested due to a lack of detailed, high resolution climate proxy information from critical regions on the planet. Major hypotheses infer changes to North American Ice Sheet dynamics, an early expansion of subpolar sea ice, and decreasing atmospheric CO2. Using sediment geochemistry and palaeontological proxies, this thesis assesses how the variability in sea ice, nutrient upwelling and primary productivity in the Northern Bering Sea impacted regional and global climate through the MPT, via their impact on North Pacific Intermediate Water expansion, regional carbon cycling and the subpolar biological pump. Through calculation of a semi-quantitative nutrient upwelling index (based on nitrogen isotopes and opal accumulation), key findings of this thesis indicate that sea ice played a dominant role on orbital scale variability in nutrient upwelling at the Bering slope, following global changes in atmospheric pCO2, continental ice sheet accumulation and sea level fluctuations. This is supported by fossil diatom assemblages which distinguish how sea ice dynamics evolved through the MPT, including high resolution variability in response to atmospheric teleconnections in the early Pleistocene. Principally, results support the notion that enhanced glacial formation of NPIW since the 900 kyr event (0.9 Ma) acted to cause region-wide suppression of deep water CO2 ventilation in the subarctic Pacific Ocean. Preliminary assessment of diatom silicon isotopes also suggests that sea ice delivery of iron, in combination with changes to nutrient cycling, may have additionally contributed to lowering glacial pCO2 which promoted increased duration of post-MPT glacials. Overall, this thesis calls for increased attention to subarctic Pacific palaeoceanography in Quaternary climate studies

Sea-ice response to climate change in the Bering Sea during the Mid-Pleistocene Transition

Sea-ice is believed to be an important control on climatic changes through the Mid-Pleistocene Transition (MPT; 0.6–1.2 Ma). However, the low resolution/short timescale of existing reconstructions prevents a full evaluation of these dynamics. Here, diatom assemblages from the Bering Sea are used to investigate sea-ice evolution on millennial timescales. We find that sea-ice was primarily controlled by ice-sheet/sea level fluctuations that modulated warm water flow into the Bering Sea. Facilitated by an amplified Walker circulation, sea-ice expansion began at ∼1.05 Ma with a step-increase during the 900 kyr event. Maximal pack ice was simultaneous with glacial maxima, suggesting sea-ice was responding to, rather than modulating ice-sheet dynamics, as proposed by the sea-ice switch hypothesis. Significant pack ice, coupled with Bering Strait closure at 0.9 Ma, indicates that brine rejection played an integral role in the glacial expansion/deglacial collapse of intermediate waters during the MPT, regulating subarctic ocean-atmospheric exchanges of CO2

Coupled climate and subarctic Pacific nutrient upwelling over the last 850,000 years

High latitude deep water upwelling has the potential to control global climate over glacial timescales through the biological pump and ocean-atmosphere CO2 exchange. However, there is currently a lack of continuous long nutrient upwelling records with which to assess this mechanism. Here we present geochemical proxy records for nutrient upwelling and glacial North Pacific Intermediate Water (GNPIW) formation in the Bering Sea over the past 850 kyr, which demonstrates that glacial periods were characterised by reduced nutrient upwelling, when global atmospheric CO2 and temperature were also lowered. We suggest that glacial expansion of sea ice in the Bering Sea, and the simultaneous expansion of low nutrient GNPIW, inhibited vertical mixing and nutrient supply across the subarctic Pacific Ocean. Our findings lend support to the suggestion that high latitude sea ice and the resultant intermediate water formation, modulated deep water upwelling and ocean-atmosphere CO2 exchange on glacial-interglacial timescales

Silicic Acid Cycling in the Bering Sea During the Mid‐Pleistocene Transition

The rate of deep-ocean carbon burial is considered important for modulating glacial-interglacial atmospheric CO2 concentrations and global climate during the Quaternary. It has been suggested that glacial iron fertilization and increased efficiency of the biological pump in the Southern Ocean since the Mid-Pleistocene Transition (MPT) was key in lowering atmospheric pCO2 and facilitating rapid land ice accumulation. There is growing evidence that a similar mechanism may have existed in the subarctic Pacific Ocean, although this has not yet been assessed. Here, the silicon isotope composition of diatoms (δ30Sidiatom) from the Bering Sea upwelling region is used to assess the role of nutrient cycling on the subarctic Pacific biological pump during the MPT. Results show that during and after the “900 kyr event,” the high productivity green belt zone was characterized by low silicic acid utilization but high supply, coincident with the dominance of diatom resting spores. We posit that as nutrient upwelling was suppressed following pack ice growth and expansion of glacial North Pacific Intermediate Water (GNPIW), primary productivity became nitrate-limited and enhanced opal remineralization caused a relative increase in silicic acid supply. However, preferential preservation and higher cellular carbon content of diatom resting spores, as well as increased supply of iron from expanded sea ice, likely sustained the net efficiency of the Bering Sea biological pump through the MPT. Remnant iron and silicic acid may also have propagated into the lower subarctic Pacific Ocean through GNPIW, aiding a regionally efficient biological pump at 900 kyr and during post-MPT glacials

Factors affecting consistency and accuracy in identifying modern macroperforate planktonic foraminifera

Planktonic foraminifera are widely used in biostratigraphic, palaeoceanographic and evolutionary studies, but the strength of many study conclusions could be weakened if taxonomic identifications are not reproducible by different workers. In this study, to assess the relative importance of a range of possible reasons for among-worker disagreement in identification, 100 specimens of 26 species of macroperforate planktonic foraminifera were selected from a core-top site in the subtropical Pacific Ocean. Twenty-three scientists at different career stages – including some with only a few days experience of planktonic foraminifera – were asked to identify each specimen to species level, and to indicate their confidence in each identification. The participants were provided with a species list and had access to additional reference materials. We use generalised linear mixed-effects models to test the relevance of three sets of factors in identification accuracy: participant-level characteristics (including experience), species-level characteristics (including a participant’s knowledge of the species) and specimen-level characteristics (size, confidence in identification). The 19 less experienced scientists achieve a median accuracy of 57 %, which rises to 75 % for specimens they are confident in. For the 4 most experienced participants, overall accuracy is 79 %, rising to 93 % when they are confident. To obtain maximum comparability and ease of analysis, everyone used a standard microscope with only 35× magnification, and each specimen was studied in isolation. Consequently, these data provide a lower limit for an estimate of consistency. Importantly, participants could largely predict whether their identifications were correct or incorrect: their own assessments of specimen-level confidence and of their previous knowledge of species concepts were the strongest predictors of accuracy

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

Investigating Bering Sea oceanographic response to the Milankovitch orbital cycle climatic shift during the middle Pleistocene

The transition of Earth’s glacial-interglacial cycles from 41 kyr to 100 kyr periodicity during the middle Pleistocene (the Mid-Pleistocene Transition (MPT); ~1.2–0.6 Ma) marks one of the largest climate events of the Cenozoic, but the causes of this cooling transition remain unclear, as the emergence of the 100 kyr Milankovitch orbital ‘eccentricity’ in climate records occurred without a long term change in external orbital forcing. Hypotheses for this transition have so far remained largely untested due to a lack of detailed, high resolution climate proxy information from critical regions on the planet. Major hypotheses infer changes to North American Ice Sheet dynamics, an early expansion of subpolar sea ice, and decreasing atmospheric CO2. Using sediment geochemistry and palaeontological proxies, this thesis assesses how the variability in sea ice, nutrient upwelling and primary productivity in the Northern Bering Sea impacted regional and global climate through the MPT, via their impact on North Pacific Intermediate Water expansion, regional carbon cycling and the subpolar biological pump. Through calculation of a semi-quantitative nutrient upwelling index (based on nitrogen isotopes and opal accumulation), key findings of this thesis indicate that sea ice played a dominant role on orbital scale variability in nutrient upwelling at the Bering slope, following global changes in atmospheric pCO2, continental ice sheet accumulation and sea level fluctuations. This is supported by fossil diatom assemblages which distinguish how sea ice dynamics evolved through the MPT, including high resolution variability in response to atmospheric teleconnections in the early Pleistocene. Principally, results support the notion that enhanced glacial formation of NPIW since the 900 kyr event (0.9 Ma) acted to cause region-wide suppression of deep water CO2 ventilation in the subarctic Pacific Ocean. Preliminary assessment of diatom silicon isotopes also suggests that sea ice delivery of iron, in combination with changes to nutrient cycling, may have additionally contributed to lowering glacial pCO2 which promoted increased duration of post-MPT glacials. Overall, this thesis calls for increased attention to subarctic Pacific palaeoceanography in Quaternary climate studies