Paleoceanography and Paleoclimatology

2020

1. PaleoOcean

   Ocean_circulation

   Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes

   Jianghui Du, Brian A. Haley, and Alan C. Mix

   Quaternary Science Reviews, 2020

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   The Global Overturning Circulation is linked to climate change on glacial-interglacial and multi-millennial timescales. The understanding of past climate-circulation links remains hindered by apparent conflicts among proxy measures of circulation. Here we reconstruct circulation changes since the Last Glacial Maximum (LGM) based on a global synthesis of authigenic neodymium isotope records (epsilon(Nd)). We propose the bottom-up framework of interpreting seawater and authigenic epsilon(Nd) considering not only conservative watermass mixing, but also the preformed properties and the non-conservative behavior of epsilon(Nd), both subject to sedimentary influences. We extract the major spatial-temporal modes of authigenic epsilon(Nd) using Principal Component Analysis, and make a first-order circulation reconstruction with budget-constrained box model simulations. We show that during the LGM, the source region of North Atlantic overturning shifted southward, which led to more radiogenic preformed epsilon(Nd) of glacial Northern Source Water (NSW). Considering this preformed effect, we infer that glacial deep Atlantic had a similar proportion of NSW as today, although the overall strength of glacial circulation appears reduced from both North Atlantic and Southern Ocean sources, which increased the relative importance of nonconservative behavior of epsilon(Nd) and may have facilitated accumulation of respired carbon in the deep ocean. During the deglaciation, we find that Southern Ocean overturning increased, which offset suppressed North Atlantic overturning and resulted in a net stronger global abyssal circulation. Faster global scale deglacial circulation reduced the relative importance of non-conservative effects, resulting in AtlanticPacific convergence of abyssal epsilon(Nd) signatures. Variations of Southern Ocean overturning likely drove a significant fraction of deglacial changes in atmospheric CO2 and oceanic heat budget. (C) 2020 The Author(s). Published by Elsevier Ltd.

2018

1. PaleoOcean

   Ocean_circulation

   Flushing of the deep Pacific Ocean and the deglacial rise of atmospheric CO2 concentrations

   Jianghui Du, Brian A. Haley, Alan C. Mix, Maureen H. Walczak, and Summer K. Praetorius

   Nature Geoscience, 2018

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   During the last deglaciation (19,000-9,000 years ago), atmospheric CO2 increased by about 80 ppm. Understanding the mechanisms responsible for this change is a central theme of palaeoclimatology, relevant for predicting future CO2 transfers in a warming world. Deglacial CO2 rise hypothetically tapped an accumulated deep Pacific carbon reservoir, but the processes remain elusive as they are underconstrained by existing tracers. Here we report high-resolution authigenic neodymium isotope data in North Pacific sediment cores and infer abyssal Pacific overturning weaker than today during the Last Glacial Maximum but intermittently stronger during steps of deglacial CO2 rise. Radiocarbon evidence suggestive of relatively ‘old’ deglacial deep Pacific water is reinterpreted here as an increase in preformed C-14 age of subsurface waters sourced near Antarctica, consistent with movement of aged carbon out of the deep ocean and release of CO2 to the atmosphere during the abyssal flushing events. The timing of neodymium isotope changes suggests that deglacial acceleration of Pacific abyssal circulation tracked Southern Hemisphere warming, sea-ice retreat and increase of mean ocean temperature. The inferred magnitude of circulation changes is consistent with deep Pacific flushing as a significant, and perhaps dominant, control of the deglacial rise of atmospheric CO2.

2016

1. PaleoOcean

   Ocean_circulation

   Global deepwater circulation between 2.4 and 1.7Ma and its connection to the onset of Northern Hemisphere Glaciation

   Jianghui Du, Baoqi Huang, and Liping Zhou

   Paleoceanography, 2016

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   We have generated an early Pleistocene benthic isotopic record for the Ocean Drilling Program Site 807 (2804m) from the western equatorial Pacific. Between 2.4 and 1.7Ma, the benthic C-13 of this site and a few other deep Pacific sites was consistently higher than the Southern Ocean Site MV0502-4JC (4286m), pointing to a reversal relative to the current gradient and hence implying a different circulation regime. We reconstructed the deepwater mass distribution of this interval by using a collection of benthic isotope records from 15 Pacific and 10 Atlantic sites and a C-13-O-18 mixing model. A two-end-member mixing regime between the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), with properties very different from today, was identified. The Southern Ocean showed strong signs of stratification and AABW with low benthic C-13, but high O-18 values reached out to other basins only below similar to 4000m. In contrast, NADW ventilated most of the ocean interior, contributing similar to 70% to the Pacific Deep Water volumetrically. Our model results also reveal a strong remineralization effect at the bottom sites of the Pacific and the Atlantic, suggesting significant accumulation of respired carbon in the bottom water between 2.4 and 1.7Ma. We propose that such a circulation pattern was initiated by the reversal of salinity gradient between AABW and NADW from 3.0 to 2.4Ma, possibly linked to Antarctic sea ice expansion and reduced southward heat transport during the onset of Northern Hemisphere Glaciation.

2022

1. PaleoOcean

   Deoxygenation

   Volcanic trigger of ocean deoxygenation during Cordilleran ice sheet retreat

   Jianghui Du, Alan C. Mix, Brian A. Haley, Christina L. Belanger, and  Sharon

   Nature, 2022

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   North Pacific deoxygenation events during the last deglaciation were sustained over millennia by high export productivity, but the triggering mechanisms and their links to deglacial warming remain uncertain(1-3). Here we find that initial deoxygenation in the North Pacific immediately after the Cordilleran ice sheet (CIS) retreat(4) was associated with increased volcanic ash in seafloor sediments. Timing of volcanic inputs relative to CIS retreat suggests that regional explosive volcanism was initiated by ice unloading(5,6). We posit that iron fertilization by volcanic ash(7-9) during CIS retreat fuelled ocean productivity in this otherwise iron-limited region, and tipped the marine system towards sustained deoxygenation. We also identify older deoxygenation events linked to CIS retreat over the past approximately 50,000 years (ref. (4)). Our findings suggest that the apparent coupling between the atmosphere, ocean, cryosphere and solid-Earth systems occurs on relatively short timescales and can act as an important driver for ocean biogeochemical change. Deoxygenation in the North Pacific immediately after the Cordilleran ice sheet retreat was shown to be linked with volcanism, suggesting that coupling between atmosphere, ocean, cryosphere and solid-Earth systems can drive biogeochemical change.

2021

1. PaleoOcean

   Deoxygenation

   Reconstructing Paleo-oxygenation for the Last 54,000 Years in the Gulf of Alaska Using Cross-validated Benthic Foraminiferal and Geochemical Records

   Sharon, Christina Belanger, Jianghui Du, and Alan Mix

   Paleoceanography and Paleoclimatology, 2021

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   Holocene and Pleistocene marine sediment records in the North Pacific record multiple dysoxic events proximal to continental margins and oxygen minimum zones (OMZs). High-resolution paleoenvironmental studies in the Gulf of Alaska (GoA) were previously restricted to the last similar to 17,000 years, limiting our knowledge of oxygenation in the high latitude North Pacific. Here we develop a similar to 54,000-year-long record of co-registered benthic foraminiferal assemblages and redox sensitive metal concentrations (Mo/Al and U/Al) at Site U1419 in the upper OMZ of GoA to reconstruct the history of OMZ extent and intensity at multi-centennial resolution. Using multivariate analyses of total benthic foraminiferal assemblages, we develop quantitative dissolved oxygen estimates that are robust to differences in the benthic foraminiferal size fraction analyzed, replicate modern oxygenation patterns in the GoA, and are cross-validated by redox sensitive metal concentrations. We identify dysoxic events in the early Holocene and in the Bolling-Allerod (B/A), consistent with previous studies, as well as two dysoxic events during MIS 3 that are comparable in severity to the B/A event and lower in oxygen than the modern GoA OMZ. We further record short-duration (<300 years) dysoxic events during glacial times similar to those recorded at more southern latitudes. Rates of oxygenation change can be abrupt with transitions exceeding 1 ml/L O-2 in 100 years. Quantitative estimates of paleo-oxygenation, such as those possible with benthic foraminiferal assemblages, are important for forecasting future oxygenation changes in OMZs and their potential impacts on the marine ecosystems.

2020

1. PaleoOcean

   Deoxygenation

   North Pacific deep-sea ecosystem responses reflect post-glacial switch to pulsed export productivity, deoxygenation, and destratification

   Christina L. Belanger,  Sharon, Jianghui Du, Calie R. Payne, and Alan C. Mix

   Deep-Sea Rearch Part I-Oceanographic Research Papers, 2020

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   Deep-sea ecosystems are highly sensitive to changes in organic matter export and oxygenation driven by climate change. Here we document past ecological changes in benthic foraminiferal assemblages indicative of deglacial changes in deep-sea oxygenation and the character of organic matter fluxes from sedimentary records retrieved at intermediate (692 m) and abyssal (3667m) depths in the Gulf of Alaska. Constrained multivariate ordination combining faunal and geochemical data over the past similar to 22,000 years distinguishes the impacts of pulsed productivity, which exports carbon to the abyss, from extreme dysoxia across the deglacial warming transition. At both depths, opportunistic species are more prevalent in interglacial conditions, reflecting higher pulsed organic matter export to the seafloor developed in response to warming and reduced sea-ice cover. Benthic foraminiferal species tolerant of low-oxygen conditions increased in abundance during the deglacial transition at both intermediate and abyssal depths. Authigenic trace metals reveal sulfidic sedimentary conditions indicative intermittent anoxia, but only at intermediate-depths. Benthic foraminiferal richness and evenness are also highest during this deglacial low-oxygen interval, likely due to high food availability. Last Glacial Maximum faunas were distinctly different at the two sites, consistent with a more stratified deep Pacific, but the faunas become more similar during Holocene time, suggesting destratification of the abyss during deglaciation. These ecosystem responses support that carbon fluxes increased during warm intervals in subpolar regions and underscores the importance of considering the effects of transient biological blooms on paleoceanographic interpretations and in model projections of future deep carbon export.

2020

1. PaleoOcean

   Ice_sheet

   The role of Northeast Pacific meltwater events in deglacial climate change

   Summer K. Praetorius, Alan Condron, Alan C. Mix, Maureen H. Walczak, Jennifer L. McKay, and Jianghui Du

   Science Advances, 2020

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   Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bolling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.

2. PaleoOcean

   Ice_sheet

   Phasing of millennial-scale climate variability in the Pacific and Atlantic Oceans

   Maureen H. Walczak, Alan C. Mix, Ellen A. Cowan, Stewart Fallon, L. Keith Fifield, Jay R. Alder, Jianghui Du, Brian Haley, Tim Hobern, June Padman, Summer K. Praetorius, Andreas Schmittner, Joseph S. Stoner, and Sarah D. Zellers

   Science, 2020

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   New radiocarbon and sedimentological results from the Gulf of Alaska document recurrent millennial-scale episodes of reorganized Pacific Ocean ventilation synchronous with rapid Cordilleran Ice Sheet discharge, indicating close coupling of ice-ocean dynamics spanning the past 42,000 years. Ventilation of the intermediate-depth North Pacific tracks strength of the Asian monsoon, supporting a role for moisture and heat transport from low latitudes in North Pacific paleoclimate. Changes in carbon-14 age of intermediate waters are in phase with peaks in Cordilleran ice-rafted debris delivery, and both consistently precede ice discharge events from the Laurentide Ice Sheet, known as Heinrich events. This timing precludes an Atlantic trigger for Cordilleran Ice Sheet retreat and instead implicates the Pacific as an early part of a cascade of dynamic climate events with global impact.