publications
2023
- ChemOceanBe_isotopesDominance of benthic fluxes in the oceanic beryllium budget and implications for paleo-denudation recordsKai Deng, Jörg Rickli, Tim Jesper Suhrhoff, Jianghui Du, Florian Scholz, Silke Severmann, Shouye Yang, James McManus, and Derek VanceScience Advances, 2023
The ratio of atmosphere-derived 10Be to continent-derived 9Be in marine sediments has been used to probe the long-term relationship between continental denudation and climate. However, its application is complicated by uncertainty in 9Be transfer through the land-ocean interface. The riverine dissolved load alone is insufficient to close the marine 9Be budget, largely due to substantial removal of riverine 9Be to continental margin sediments. We focus on the ultimate fate of this latter Be. We present sediment pore-water Be profiles from diverse continental margin environments to quantify the diagenetic Be release to the ocean. Our results suggest that pore-water Be cycling is mainly controlled by particulate supply and Mn-Fe cycling, leading to higher benthic fluxes on shelves. Benthic fluxes may help close the 9Be budget and are at least comparable to, or higher ( 2-fold) than, the riverine dissolved input. These observations demand a revised model framework, which considers the potentially dominant benthic source, to robustly interpret marine Be isotopic records. Benthic fluxes from ocean sediments dominate the oceanic Be budget, complicating the use of 10Be/9Be as a paleo-denudation tracer.
- ChemOceanNi_isotopesThe nickel output to abyssal pelagic manganese oxides: A balanced elemental and isotope budget for the oceansSarah Fleischmann, Jianghui Du, Aditi Chatterjee, James McManus, Sridhar D. Iyer, Ankeeta Amonkar, and Derek VanceEarth and Planetary Science Letters, 2023
The development of nickel isotopes as a chemical tracer of past ocean environments requires a sound understanding of the modern oceanic budget. Our current understanding of this budget implies a large elemental and isotope imbalance between inputs to and outputs from the dissolved pool of the ocean. This imbalance is mainly caused by the dominant oxic sink of Ni to Mn oxide-rich sediments. Though the Ni isotope composition of Fe-Mn crusts has previously been used as proxy for the Ni isotope composition of these sediments, crusts and nodules represent a very small part of the total Mn oxide output. Instead, Mn oxide microparticle supply to pelagic and hemi-pelagic sediments dominates the removal of Mn to sediments, but there are very few isotope data for such samples. Here we present the first extensive Ni concentration and isotope dataset from fully oxic Mn-rich pelagic sediments, from 6 different sites across the open Pacific and 10 closely-spaced sites in the Indian Ocean. We also present data for one hemi-pelagic site representing a suboxic setting on the California Margin. Abyssal Pacific and Indian Ocean sediments have a Ni/Mn ratio of 0.02 (similar to Fe-Mn crusts) and their authigenic Ni is isotopically lighter (δ60Ni = +0.26 to +1.08‰) than seawater (+1.33‰) and crusts (+1.55±0.38‰). Data presented here for organic carbon-rich suboxic sediments of the Californian margin have lower Ni/Mn ratios (0.004 to 0.014 for the oxic top of the core, where Mn oxide is present in abundance) and even lighter authigenic Ni isotope compositions (δ60Ni = -0.08±0.11‰). We show that the Ni isotopes of nearly all Mn-rich sediments and deposits analysed to date, including the new data presented here, are correlated with Co/Mn ratios, suggesting that both are controlled by accumulation rate, progressive incorporation of Ni into the metal oxide structure and isotopic re-equilibration between the solid and aqueous phase. At sites where sediments are diagenetically processed, such as the California Margin, differential diagenetic remobilisation of Mn, Ni and Co cause deviations from this correlation. We present a new mass balance calculation that recognises the importance of scavenging of oceanic Ni to Mn oxide-rich proximal hydrothermal sediments, with low Ni/Mn and light isotope compositions. The mass balance produces a budget that can be simultaneously balanced for both amounts and isotope compositions of Ni. This result provides a strong basis for the application of Ni isotopes as records of the evolution of the metal sink from the oxic oceans through Earth history.
2022
- ChemOceanNd_isotopesWidespread lithogenic control of marine authigenic neodymium isotope records? Implications for paleoceanographic reconstructionsA. N. Abbott, S. C. Lohr, A. Payne, H. Kumar, and Jianghui DuGeochimica et Cosmochimica Acta, 2022
Our understanding of past ocean-climate dynamics is informed by multiple paleocirculation proxies including delta C-13, Pa-231/(230Th), and radiogenic neodymium isotopes (epsilon(Nd)). Of these, the epsilon(Nd) signature of marine authigenic phases is of particular importance as it is considered a robust circulation proxy applicable across timescales, permitting circulation reconstructions during periods of rapid, climatically-induced biological or chemical change (e.g. productivity, pH). However, growing evidence of non-conservative behavior and a widespread sedimentary source (benthic flux via pore water) of Nd to the global ocean suggests that authigenic epsilon(Nd) records do not strictly record a water mass signature, highlighting the need to reconsider interpretations of the authigenic record. To examine the impact of a sedimentary influence on the authigenic record, here we compile paired authigenic and detrital neodymium records from every major ocean basin and from 80 Ma to present. We then focus on just the North Atlantic Ocean basin to examine if this relationship holds up regionally and how authigenic epsilon(Nd) changes relate to sediment composition changes from two scientific ocean drill cores spanning the past 25 ka. We present a new conceptual framework to guide our discussion that examines the coupling or decoupling of authigenic and detrital epsilon(Nd) in terms of the relative importance of each of the three major potential controls as defined in the existing literature (bottom water, pore water, sediments) on the authigenic record. Our compilation reveals a strong linear relationship between detrital epsilon(Nd) and authigenic epsilon(Nd )(correlation coefficient = 0.86, n = 871), demonstrating a widespread influence of lithogenically sourced neodymium on authigenic epsilon(Nd). We find the same is true within the North Atlantic, with the authigenic records at both locations strongly influenced by the sediments and likely not recording bottom water neodymium values. Emerging evidence for a lithogenic or benthic flux influence on the budgets of a wide range of trace elements suggests that our interpretative framework will be broadly useful for understanding the behavior of trace elements and their isotopes at the sediment-water interface.(c) 2021 Elsevier Ltd. All rights reserved.
- ChemOceanNi_isotopesThe essential bioactive role of nickel in the oceans: Evidence from nickel isotopesNolwenn Lemaitre, Jianghui Du, Gregory F. Souza, Corey Archer, and Derek VanceEarth and Planetary Science Letters, 2022
The role of nickel (Ni) in ocean biogeochemical cycles is both under-studied and controversial. Strong correlations between Ni and organic carbon in modern and ancient marine sediments suggest a prominent biogeochemical role over a substantial portion of Earth history. Addition of Ni to culturing and seawater incubation experiments produces strong responses in terms of cell growth, particularly of nitrogen-fixing organisms. But the implied limiting role for phytoplankton growth is inconsistent with observations in the real ocean, specifically that photic zone Ni concentrations never descend to the very low values that characterise other bioactive, and often bio-limiting, metals like iron. These two observations can be reconciled if a large portion of the total dissolved Ni present in open-ocean surface waters is not bio-available on short timescales. Here we present new Ni concentration and stable isotope data from the GEOVIDE transect in the North Atlantic. We interpret these new data in the light of the growing database for Ni stable isotopes in the modern ocean, with implications for the biogeochemical importance of Ni. In the new North Atlantic dataset, the lowest Ni concentrations (1.8-2.6 nmol/L) and highest delta Ni-60 (up to +1.67 parts per thousand) are associated with low nitrate, south of the subarctic front (SAF). By contrast, stations at latitudes north of the SAF, with higher surface nitrate, show very subdued variation in Ni concentrations throughout the entire depth of the water column (3.6 +/- 0.3 nmol/L, mean and 2SD), and no variation in delta Ni-60 beyond the narrow global deep-ocean range (+1.33 +/- 0.13 parts per thousand). These North Atlantic Ni isotope data also show relationships with nitrogen isotope effects, observed in the same samples, that are suggestive of a link between Ni utilisation, isotope fractionation and nitrogen fixation. The global dataset, including the new data presented here, reveals a biogeochemical divide with Ni isotope fractionation only occurring in low latitude surface waters. A simple observationally constrained three-dimensional model of Ni cycling suggests that the creation of this isotopically heavy, Ni-poor, end-member, together with the physical circulation and remineralisation at depth, can explain the global Ni-delta Ni-60 systematics. Taken together, these findings hint at Ni-N co-limitation in the modern ocean. We advocate for more extensive and detailed culturing/incubation studies of this neglected metal in order to elucidate its potentially crucial biogeochemical role. (C) 2022 The Author(s). Published by Elsevier B.V.
- ChemOceanREEDominance of benthic flux of REEs on continental shelves: implications for oceanic budgetsK. Deng, S. Yang, Jianghui Du, E. Lian, and D. VanceGeochemical Perspectives Letters, 2022
Rare earth elements (REEs) are powerful tools to track oceanic biogeochemical proc-esses. However, our understanding of REE sources is incomplete, leading to contro-versial interpretations regarding their oceanic cycling. Continental margin sediments are often assumed to be a major source, but the sediment pore water data required to understand the processes controlling that potential source are scarce. Here, we mea-sure and compile pore water and estuarine REE data from the Changjiang (Yangtze) estuary-East China Sea shelf. We show that release of REEs, from shallow pore water to overlying seawater, is coupled to Mn reduction. In contrast, REEs are removed in deep pore water, perhaps via formation of an authigenic REE-bearing phase. This sedimentary source can potentially explain REE addition in the estuary at mid-high salinity. Our calculations suggest that the benthic flux is the largest Nd source (similar to 40 %) on the East China Sea shelf. Globally, however, despite a higher benthic Nd flux on the advection-dominated shelf, the much more extensive deep ocean still domi-nates the total area-integrated benthic flux. Our results call for a more extensive investigation of the magnitude of the benthic flux of REEs to the oceans.
- ChemOceanNd_isotopesReactive-transport modeling of neodymium and its radiogenic isotope in deep-sea sediments: The roles of authigenesis, marine silicate weathering and reverse weatheringJianghui Du, Brian A. Haley, Alan C. Mix, April N. Abbott, James McManus, and Derek VanceEarth and Planetary Science Letters, 2022
Dissolved Rare Earth Elements (REE) and radiogenic neodymium (epsilon(Nd)) isotope composition (ENd) of seawater are widely used geochemical tools in studying marine processes, but their modern ocean budgets are poorly understood. Recent discoveries of large benthic fluxes of REE with unique epsilon(Nd) signatures from marine sediments, particularly in the deep-sea, have led to a “bottom-up” hypothesis, which suggests that early diagenesis below the sediment-water interface (SWI) controls the ocean’s REE and epsilon(Nd) budgets. To investigate such sedimentary processes, we created a reactive-transport model for the biogeochemical cycling of Nd and epsilon(Nd) in marine sediments. Here, we attempt to quantify the roles of authigenesis, marine silicate weathering and reverse weathering in the diagenetic cycling of Nd and epsilon(Nd) at a deep-sea (3000 m) site on the Oregon margin. Our model predicts that, at this site, Nd carried by Fe/Mn oxides into sediments eventually transforms to authigenic Nd-phosphate, during which similar to 9% of the incoming solid Nd flux is released as a dissolved benthic flux back to the overlying bottom water. We also find that the classic reversible scavenging formulation applied to Nd co-cycling with Fe/Mn oxides is inconsistent with the data. Rather, a co-precipitation formulation, assuming Nd is structurally incorporated into Fe/Mn oxides, successfully simulates the data. The model also shows that authigenesis alone cannot explain the pore water and authigenic epsilon(Nd), which are both more radiogenic than bottom water at this site. However, the weathering of volcanic silicates sourced from the local subduction zone can successfully explain epsilon(Nd). We suggest that, because reverse weathering by authigenic clay formation maintains the under-saturation of primary silicates in pore water, marine silicate weathering can proceed. The processes we model likely affect the sedimentary cycling of many other trace elements and isotopes, with much broader implications for the understanding of ocean biogeochemistry. (C) 2022 The Author(s). Published by Elsevier B.V.
- PaleoOceanDeoxygenationVolcanic trigger of ocean deoxygenation during Cordilleran ice sheet retreatJianghui Du, Alan C. Mix, Brian A. Haley, Christina L. Belanger, and SharonNature, 2022
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
- PaleoOceanDeoxygenationReconstructing Paleo-oxygenation for the Last 54,000 Years in the Gulf of Alaska Using Cross-validated Benthic Foraminiferal and Geochemical RecordsSharon, Christina Belanger, Jianghui Du, and Alan MixPaleoceanography and Paleoclimatology, 2021
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
- PaleoOceanIce_sheetThe role of Northeast Pacific meltwater events in deglacial climate changeSummer K. Praetorius, Alan Condron, Alan C. Mix, Maureen H. Walczak, Jennifer L. McKay, and Jianghui DuScience Advances, 2020
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.
- PaleoOceanOcean_circulationEvolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopesJianghui Du, Brian A. Haley, and Alan C. MixQuaternary Science Reviews, 2020
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.
- PaleoOceanDeoxygenationNorth Pacific deep-sea ecosystem responses reflect post-glacial switch to pulsed export productivity, deoxygenation, and destratificationChristina L. Belanger, Sharon, Jianghui Du, Calie R. Payne, and Alan C. MixDeep-Sea Rearch Part I-Oceanographic Research Papers, 2020
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.
- PaleoOceanIce_sheetPhasing of millennial-scale climate variability in the Pacific and Atlantic OceansMaureen 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. ZellersScience, 2020
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.
2019
- PaleoOceanData_standardPaCTS 1.0: A Crowdsourced Reporting Standard for Paleoclimate DataD. Khider, J. Emile-Geay, N. P. McKay, Y. Gil, D. Garijo, V Ratnakar, M. Alonso-Garcia, S. Bertrand, O. Bothe, P. Brewer, A. Bunn, M. Chevalier, L. Comas-Bru, A. Csank, E. Dassie, K. DeLong, T. Felis, P. Francus, A. Frappier, W. Gray, S. Goring, L. Jonkers, M. Kahle, D. Kaufman, N. M. Kehrwald, B. Martrat, H. McGregor, J. Richey, A. Schmittner, N. Scroxton, E. Sutherland, K. Thirumalai, K. Allen, F. Arnaud, Y. Axford, T. Barrows, L. Bazin, S. E. Pilaar Birch, E. Bradley, J. Bregy, E. Capron, O. Cartapanis, H-W Chiang, K. M. Cobb, M. Debret, R. Dommain, Jianghui Du, K. Dyez, S. Emerick, M. P. Erb, G. Falster, W. Finsinger, D. Fortier, Nicolas Gauthier, S. George, E. Grimm, J. Hertzberg, F. Hibbert, A. Hillman, W. Hobbs, M. Huber, A. L. C. Hughes, S. Jaccard, J. Ruan, M. Kienast, B. Konecky, G. Le Roux, V Lyubchich, V. F. Novello, L. Olaka, J. W. Partin, C. Pearce, S. J. Phipps, C. Pignol, N. Piotrowska, M-S Poli, A. Prokopenko, F. Schwanck, C. Stepanek, G. E. A. Swann, R. Telford, E. Thomas, Z. Thomas, S. Truebe, L. Gunten, A. Waite, N. Weitzel, B. Wilhelm, J. Williams, M. Winstrup, N. Zhao, and Y. ZhouPaleoceanography and Paleoclimatology, 2019
The progress of science is tied to the standardization of measurements, instruments, and data. This is especially true in the Big Data age, where analyzing large data volumes critically hinges on the data being standardized. Accordingly, the lack of community-sanctioned data standards in paleoclimatology has largely precluded the benefits of Big Data advances in the field. Building upon recent efforts to standardize the format and terminology of paleoclimate data, this article describes the Paleoclimate Community reporTing Standard (PaCTS), a crowdsourced reporting standard for such data. PaCTS captures which information should be included when reporting paleoclimate data, with the goal of maximizing the reuse value of paleoclimate data sets, particularly for synthesis work and comparison to climate model simulations. Initiated by the LinkedEarth project, the process to elicit a reporting standard involved an international workshop in 2016, various forms of digital community engagement over the next few years, and grassroots working groups. Participants in this process identified important properties across paleoclimate archives, in addition to the reporting of uncertainties and chronologies; they also identified archive-specific properties and distinguished reporting standards for new versus legacy data sets. This work shows that at least 135 respondents overwhelmingly support a drastic increase in the amount of metadata accompanying paleoclimate data sets. Since such goals are at odds with present practices, we discuss a transparent path toward implementing or revising these recommendations in the near future, using both bottom-up and top-down approaches.
2018
- PaleoOceanOcean_circulationFlushing of the deep Pacific Ocean and the deglacial rise of atmospheric CO2 concentrationsJianghui Du, Brian A. Haley, Alan C. Mix, Maureen H. Walczak, and Summer K. PraetoriusNature Geoscience, 2018
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.
2017
- ChemOceanNd_isotopesThe Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the OceansBrian A. Haley, Jianghui Du, April N. Abbott, and James McManusFrontiers in Marine Science, 2017
Neodymium (Nd) isotopes are considered a valuable tracer of modern and past ocean circulation. However, the promise of Nd isotope as a water mass tracer is hindered because there is not an entirely self-consistent model of the marine geochemical cycle of rare earth elements (REEs, of which Nd is one). That is, the prevailing mechanisms to describe the distributions of elemental and isotopic Nd are not completely reconciled. Here, we use published [Nd] and Nd isotope data to examine the prevailing model assumptions, and further compare these data to emergent alternative models that emphasize benthic processes in controlling the cycle of marine REEs and Nd isotopes. Our conclusion is that changing from a “top-down” driven model for REE cycling to one of a “bottom-up” benthic source model can provide consistent interpretations of these data for both elemental and isotopic Nd distributions. We discuss the implications such a benthic flux model carries for interpretation of Nd isotope data as a tracer for understanding modern and past changes in ocean circulation.
2016
- ChemOceanNd_isotopesNeodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstructionJianghui Du, Brian A. Haley, and Alan C. MixGeochimica et Cosmochimica Acta, 2016
The isotopic composition of neodymium (epsilon(Nd)) extracted from sedimentary Fe-Mn oxyhydroxide offers potential for reconstructing paleo-circulation, but its application depends on extraction methodology and the mechanisms that relate authigenic epsilon(Nd) to bottom water. Here we test methods to extract authigenic epsilon(Nd) from Gulf of Alaska (GOA) sediments and assess sources of leachate Nd, including potential contamination from trace dispersed volcanic ash. We show that one dominant phase is extracted via leaching of core-top sediments. Major and trace element geochemistry demonstrate that this phase is authigenic Fe-Mn oxyhydroxide. Contamination of leachate (authigenic) Nd from detrital sources is insignificant (<1%); our empirical results are consistent with established kinetic mineral dissolution rates and theory. Contamination of extracted epsilon(Nd) from leaching of volcanic ash is below analytical uncertainty. However, the epsilon(Nd) of core-top leachates in the GOA is consistently more radiogenic than bottom water. We infer that authigenic phases record pore water epsilon(Nd), and the relationships of epsilon(Nd) among bottom waters, pore waters, authigenic phases and detrital sediments are primarily governed by the exposure time of bottom water to sea-floor sediments, rate of exchange across the sediment-water interface and the reactivity and composition of detrital sediments. We show that this conceptual model is applicable on the Pacific basin scale and provide a new framework to understand the role of authigenic phases in both modern and paleo-applications, including the use of authigenic epsilon(Nd) as a paleo-circulation tracer.(C) 2016 Elsevier Ltd. All rights reserved.
- PaleoOceanOcean_circulationGlobal deepwater circulation between 2.4 and 1.7Ma and its connection to the onset of Northern Hemisphere GlaciationJianghui Du, Baoqi Huang, and Liping ZhouPaleoceanography, 2016
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.
2010
- PaleoOceanMonsoonVariations in upper water structure during MIS 3 from the western South China SeaJianghui Du, and Baoqi HuangChinese Science Bulletin, 2010
Core 17954 is located in the modern summer upwelling area in western South China Sea, its sediments recorded the variations of upwelling generated by East Asia Summer Monsoon (EASM) during MIS 3. Based on the strict age model of AMS C-14 dating, the paleo-Sea Surface Temperature (SST) and Salinity (SSS) are reconstructed by pairing Mg/Ca-Paleothermometer and delta O-18 of planktonic foraminifera Globigerinoides ruber (white s.s.). Results show that in Core 17954, the delta O-18 record of G.ruber has significant millennium fluctuations as the delta O-18 records from NGRIP icecore and Hulu Cave stalagmites, this indicates that the climate changes of western SCS contains both signals from High Latitude of Northern Hemisphere as well as EASM. In order to get more information on upwelling changes, previous records of thermocline and foraminiferal primary productivity in Core 17954 are collected, restudied and compared. Five distinct shallowing periods of thermocline (referred to as S1-S5) are identified in this study. In S1-S4, SST is lower and productivity is higher, these indicate to an enhanced upwelling and strengthened EASM during these periods. And the lower SSS, caused by increasing precipitation or fresh water input, also prove this standpoint.