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1. Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

   https://doi.org/10.5194/bg-17-6051-2020  

   Rixen, Tim ; Cowie, Greg ; Gaye, Birgit ; Goes, Joaquim ; do Rosário Gomes, Helga ; Hood, Raleigh R. ; Lachkar, Zouhair ; Schmidt, Henrike ; Segschneider, Joachim ; Singh, Arvind ( January 2020 , Biogeosciences)

   null (Ed.)

   Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growthof higher organisms. They also alter the marine nitrogen cycle, which isstrongly bound to the carbon cycle and climate. While higher organisms ingeneral start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygenconcentration below a threshold of about 20 µM (microbial hypoxia),whereas anoxic processes dominate the nitrogen cycle at oxygenconcentrations of < ∼ 0.05 µM (functionalanoxia). The Arabian Sea and the Bay of Bengal are home to approximately21 % of the total volume of ocean waters revealing microbial hypoxia.While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionallyanoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Eventhough there are a few isolated reports on the occurrence of anoxia prior to1960, anoxic events have so far not been reported from the open northernIndian Ocean (i.e., other than on shelves) during the last 60 years.Maintenance of functional anoxia in the Arabian Sea OMZ with oxygenconcentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses thesurface ocean circulation twice a year and turns vast areas of the ArabianSea from an oligotrophic oceanic desert into one of the most productiveregions of the oceans within a few weeks. Thus, the comparably lowvariability of oxygen concentration in the OMZ implies stable balancesbetween the physical oxygen supply and the biological oxygen consumption,which includes negative feedback mechanisms such as reducing oxygenconsumption at decreasing oxygen concentrations (e.g., reduced respiration).Lower biological oxygen consumption is also assumed to be responsible for aless intense OMZ in the Bay of Bengal. According to numerical model results,a decreasing physical oxygen supply via the inflow of water masses from thesouth intensified the Arabian Sea OMZ during the last 6000 years, whereas areduced oxygen supply via the inflow of Persian Gulf Water from the northintensifies the OMZ today in response to global warming. The first issupported by data derived from the sedimentary records, and the latterconcurs with observations of decreasing oxygen concentrations and aspreading of functional anoxia during the last decades in the Arabian Sea.In the Arabian Sea decreasing oxygen concentrations seem to have initiated aregime shift within the pelagic ecosystem structure, and this trend is alsoseen in benthic ecosystems. Consequences for biogeochemical cycles are asyet unknown, which, in addition to the poor representation of mesoscalefeatures in global Earth system models, reduces the reliability of estimatesof the future OMZ development in the northern Indian Ocean. 

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2. Modeling Neodymium Isotopes in the Ocean Component of the Community Earth System Model (CESM1)

   https://doi.org/10.1029/2018MS001538  

   Gu, Sifan ; Liu, Zhengyu ; Jahn, Alexandra ; Rempfer, Johannes ; Zhang, Jiaxu ; Joos, Fortunat ( March 2019 , Journal of Advances in Modeling Earth Systems)

   Neodymium (Nd) isotopic composition (ε_Nd) is an important tracer for water mass mixing and the reconstruction of past ocean circulation. To allow for a direct model‐data comparison, we have implemented Nd isotopes in the ocean component of the Community Earth System Model (CESM1.3). The model is able to capture the major features of the observed modern distribution of bothε_Ndand Nd concentrations. Our model provides a useful tool for the interpretation ofε_Ndreconstructions. For example, we show that in an idealized North Atlantic freshwater hosing experiment,ε_Ndchanges in the Atlantic are documenting primarily the changes in water mass mixing and are hardly affected by the concomitant and large changes in the marine biological productivity and organic matter fluxes. However, the hosing experiment also shows that the end‐member changes due to the change of ocean circulation can influence the interpretation ofε_Ndin the Atlantic, depending on the location. The implementation of Nd, together with other existing tracers, such as δ¹⁸O,²³¹Pa/²³⁰Th, δ¹³C, and radiocarbon in the same model, can improve our understanding of past ocean circulation significantly.

    

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3. Evidence for a Northern Hemispheric trigger of the 100,000-y glacial cyclicity

   https://doi.org/10.1073/pnas.2020260118  

   Yehudai, Maayan ; Kim, Joohee ; Pena, Leopoldo D. ; Jaume-Seguí, Maria ; Knudson, Karla P. ; Bolge, Louise ; Malinverno, Alberto ; Bickert, Torsten ; Goldstein, Steven L. ( November 2021 , Proceedings of the National Academy of Sciences)

   The causes of the Mid-Pleistocene Transition, the shift from ∼41-ky to 100-ky interglacial–glacial cycles and more intense ice ages, remain intensely debated, as this fundamental change occurred between ∼1,250 and 650 ka without substantial changes in astronomical climate forcings. Recent studies disagree about the relative importance of events and processes in the Northern and Southern Hemispheres, as well as whether the shift occurred gradually over several interglacial–glacial cycles or abruptly at ∼900 ka. We address these issues using a north-to-south reconstruction of the Atlantic arm of the global meridional overturning ocean circulation, a primary means for distributing heat around the globe, using neodymium (Nd) isotopes. Results reveal a period of intense erosion affecting the cratonic shields surrounding the North Atlantic between Marine Isotope Stages (MIS) 27 and 25 (∼980 and 950 ka), reflected by unusually low Nd isotope ratios in deep North Atlantic seawater. This episode preceded a major ocean circulation weakening between MIS 25 and 21 (950 and 860 ka) that coincided with the first ∼100-ky-long interglacial–glacial onset of Northern Hemisphere glaciation at around 2.4 to 2.8 Ma. The data point to a Northern Hemisphere–sourced initiation for the transition, possibly induced through regolith loss and increased exposure of the crystalline bedrock, which would lead to increased friction, enabling larger ice sheets that are characteristic of the 100-ky interglacial–glacial cycles.

    

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4. GEOTRACES: Accelerating Research on the Marine Biogeochemical Cycles of Trace Elements and Their Isotopes

   https://doi.org/10.1146/annurev-marine-010318-095123  

   Anderson, Robert F. ( January 2020 , Annual Review of Marine Science)

   null (Ed.)

   The biogeochemical cycles of trace elements and their isotopes (TEIs) constitute an active area of oceanographic research due to their role as essential nutrients for marine organisms and their use as tracers of oceanographic processes. Selected TEIs also provide diagnostic information about the physical, geological, and chemical processes that supply or remove solutes in the ocean. Many of these same TEIs provide information about ocean conditions in the past, as their imprint on marine sediments can be interpreted to reflect changes in ocean circulation, biological productivity, the ocean carbon cycle, and more. Other TEIs have been introduced as the result of human activities and are considered contaminants. The development and implementation of contamination-free methods for collecting and analyzing samples for TEIs revolutionized marine chemistry, revealing trace element distributions with oceanographically consistent features and new insights about the processes regulating them. Despite these advances, the volume and geographic coverage of high-quality TEI data by the end of the twentieth century were insufficient to constrain their global biogeochemical cycles. To accelerate progress in this field of research, marine geochemists developed a coordinated international effort to systematically study the marine biogeochemical cycles of TEIs—the GEOTRACES program. Following a decade of planning and implementation, GEOTRACES launched its main field effort in 2010. This review, roughly midway through the field program, summarizes the steps involved in designing the program, its management structure, and selected findings. 

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5. Expedition 374 Preliminary Report: Ross Sea West Antarctic Ice Sheet History

   https://doi.org/10.14379/iodp.pr.374.2018  

   McKay, R.M. ; De Santis, L. ; Kulhanek, D.K. ( May 2018 , Preliminary report)

   null (Ed.)

   The marine-based West Antarctic Ice Sheet (WAIS) is currently retreating due to shifting wind-driven oceanic currents that transport warm waters toward the ice margin, resulting in ice shelf thinning and accelerated mass loss of the WAIS. Previous results from geologic drilling on Antarctica’s continental margins show significant variability in marine-based ice sheet extent during the late Neogene and Quaternary. Numerical models indicate a fundamental role for oceanic heat in controlling this variability over at least the past 20 My. Although evidence for past ice sheet variability has been collected in marginal settings, sedimentologic sequences from the outer continental shelf are required to evaluate the extent of past ice sheet variability and the associated oceanic forcings and feedbacks. International Ocean Discovery Program Expedition 374 drilled a latitudinal and depth transect of five drill sites from the outer continental shelf to rise in the eastern Ross Sea to resolve the relationship between climatic and oceanic change and WAIS evolution through the Neogene and Quaternary. This location was selected because numerical ice sheet models indicate that this sector of Antarctica is highly sensitive to changes in ocean heat flux. The expedition was designed for optimal data-model integration and will enable an improved understanding of the sensitivity of Antarctic Ice Sheet (AIS) mass balance during warmer-than-present climates (e.g., the Pleistocene “super interglacials,” the mid-Pliocene, and the late early to middle Miocene). The principal goals of Expedition 374 were to • Evaluate the contribution of West Antarctica to far-field ice volume and sea level estimates; • Reconstruct ice-proximal atmospheric and oceanic temperatures to identify past polar amplification and assess its forcings and feedbacks; • Assess the role of oceanic forcing (e.g., sea level and temperature) on AIS stability/instability; • Identify the sensitivity of the AIS to Earth’s orbital configuration under a variety of climate boundary conditions; and • Reconstruct eastern Ross Sea paleobathymetry to examine relationships between seafloor geometry, ice sheet stability/instability, and global climate. To achieve these objectives, we will • Use data and models to reconcile intervals of maximum Neogene and Quaternary Antarctic ice advance with far-field records of eustatic sea level change; • Reconstruct past changes in oceanic and atmospheric temperatures using a multiproxy approach; • Reconstruct Neogene and Quaternary sea ice margin fluctuations in datable marine continental slope and rise records and correlate these records to existing inner continental shelf records; • Examine relationships among WAIS stability/instability, Earth’s orbital configuration, oceanic temperature and circulation, and atmospheric pCO2; and • Constrain the timing of Ross Sea continental shelf overdeepening and assess its impact on Neogene and Quaternary ice dynamics. Expedition 374 was carried out from January to March 2018, departing from Lyttelton, New Zealand. We recovered 1292.70 m of high-quality cores from five sites spanning the early Miocene to late Quaternary. Three sites were cored on the continental shelf (Sites U1521, U1522, and U1523). At Site U1521, we cored a 650 m thick sequence of interbedded diamictite, mudstone, and diatomite, penetrating the Ross Sea seismic Unconformity RSU4. The depositional reconstructions of past glacial and open-marine conditions at this site will provide unprecedented insight into environmental change on the Antarctic continental shelf during the early and middle Miocene. At Site U1522, we cored a discontinuous upper Miocene to Pleistocene sequence of glacial and glaciomarine strata from the outer shelf, with the primary objective to penetrate and date seismic Unconformity RSU3, which is interpreted to represent the first major continental shelf–wide expansion and coalescing of marine-based ice streams from both East and West Antarctica. At Site U1523, we cored a sediment drift located beneath the westerly flowing Antarctic Slope Current (ASC). Cores from this site will provide a record of the changing vigor of the ASC through time. Such a reconstruction will enable testing of the hypothesis that changes in the vigor of the ASC represent a key control on regulating heat flux onto the continental shelf, resulting in the ASC playing a fundamental role in ice sheet mass balance. We also cored two sites on the continental slope and rise. At Site U1524, we cored a Plio–Pleistocene sedimentary sequence on the continental rise on the levee of the Hillary Canyon, which is one of the largest conduits of Antarctic Bottom Water delivery from the Antarctic continental shelf into the abyssal ocean. Drilling at Site U1524 was intended to penetrate into middle Miocene and older strata but was initially interrupted by drifting sea ice that forced us to abandon coring in Hole U1524A at 399.5 m drilling depth below seafloor (DSF). We moved to a nearby alternate site on the continental slope (U1525) to core a single hole with a record complementary to the upper part of the section recovered at Site U1524. We returned to Site U1524 3 days later, after the sea ice cleared. We then cored Hole U1524C with the rotary core barrel with the intention of reaching the target depth of 1000 m DSF. However, we were forced to terminate Hole U1524C at 441.9 m DSF due to a mechanical failure with the vessel that resulted in termination of all drilling operations and a return to Lyttelton 16 days earlier than scheduled. The loss of 39% of our operational days significantly impacted our ability to achieve all Expedition 374 objectives as originally planned. In particular, we were not able to obtain the deeper time record of the middle Miocene on the continental rise or abyssal sequences that would have provided a continuous and contemporaneous archive to the high-quality (but discontinuous) record from Site U1521 on the continental shelf. The mechanical failure also meant we could not recover sediment cores from proposed Site RSCR-19A, which was targeted to obtain a high-fidelity, continuous record of upper Neogene and Quaternary pelagic/hemipelagic sedimentation. Despite our failure to recover a shelf-to-rise transect for the Miocene, a continental shelf-to-rise transect for the Pliocene to Pleistocene interval is possible through comparison of the high-quality records from Site U1522 with those from Site U1525 and legacy cores from the Antarctic Geological Drilling Project (ANDRILL). 

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