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1. Evidence for Stable Holocene Basin‐Scale Overturning Circulation Despite Variable Currents Along the Deep Western Boundary of the North Atlantic Ocean

   https://doi.org/10.1029/2018GL080187  

   Hoffmann, Sharon S. ; McManus, Jerry F. ; Swank, Emerson ( December 2018 , Geophysical Research Letters)

   While substantial changes in thermohaline circulation related to deglacial climate variability are well established, the role of this circulation in Holocene climate variability remains uncertain. Here we use two dynamical proxies,²³¹Pa/²³⁰Th ratios and mean sortable silt size (), to reconstruct Holocene bottom water circulation at the intermediate‐depth Carolina Slope. We find no substantial change in deep current speed or²³¹Pa export at this site during the Holocene, suggesting consistent²³¹Pa export via the Deep Western Boundary Current.shows increasing millennial‐scale variability in the middle‐late Holocene, which may reflect Labrador Sea Water contribution to current speed. We conclude that deepwater export from the North Atlantic has remained remarkably stable during the Holocene, decoupled from changing rates of specific water masses, while production of these water masses varied at millennial to centennial time scales. The persistence of the large‐scale overturning may reflect the ocean's stabilizing influence on Holocene climate.

    

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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. Insights into the deglacial variability of phytoplankton community structure in the eastern equatorial Pacific Ocean using [231Pa/230Th]xs and opal-carbonate fluxes

   https://doi.org/10.1038/s41598-022-26593-1  

   Schimmenti, Danielle ; Marcantonio, Franco ; Hayes, Christopher T. ; Hertzberg, Jennifer ; Schmidt, Matthew ; Sarao, John ( December 2022 , Scientific Reports)

   Fully and accurately reconstructing changes in oceanic productivity and carbon export and their controls is critical to determining the efficiency of the biological pump and its role in the global carbon cycle through time, particularly in modern CO₂source regions like the eastern equatorial Pacific (EEP). Here we present new high-resolution records of sedimentary²³⁰Th-normalized opal and nannofossil carbonate fluxes and [²³¹Pa/²³⁰Th]xs ratios from site MV1014-02-17JC in the Panama Basin. We find that, across the last deglaciation, phytoplankton community structure is driven by changing patterns of nutrient (nitrate, iron, and silica) availability which, in turn, are caused by variability in the position of the Intertropical Convergence Zone (ITCZ) and associated changes in biogeochemical cycling and circulation in the Southern Ocean. Our multi-proxy work suggests greater scrutiny is required in the interpretation of common geochemical proxies of productivity and carbon export in the EEP.

    

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4. Isopycnal Transport and Scavenging of 230 Th and 231 Pa in the Pacific Southern Ocean

   https://doi.org/10.1029/2020GB006760  

   Pavia, Frank J. ; Anderson, Robert F. ; Pinedo‐Gonzalez, Paulina ; Fleisher, Martin Q. ; Brzezinski, Mark A. ; Robinson, Rebecca S. ( November 2020 , Global Biogeochemical Cycles)

   The Southern Ocean hosts complex connections between ocean physics, chemistry, and biology. Changes in these connections are hypothesized to be responsible for significant alterations of ocean biogeochemistry and carbon storage both on glacial‐interglacial timescales and in the future due to anthropogenic forcing. Isotopes of thorium (²³⁰Th and²³²Th) and protactinium (²³¹Pa) have been widely applied as tools to study paleoceanographic conditions in the Southern Ocean. However, understanding of the chemical behavior of these isotopes in the modern Southern Ocean has been limited by a paucity of high‐resolution observations. In this study, we present measurements of dissolved²³⁰Th,²³¹Pa, and²³²Th on a meridional transect along 170°W from 67°S to 54°S in the Pacific sector of the Southern Ocean, with high vertical and meridional sampling resolution. We find Th/Pa fractionation factors below 1, highlighting the preferential removal of Pa relative to Th in a region with low lithogenic inputs where the particle flux is dominated by biogenic opal. We also find steep gradients in all three of these isotopes along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Our results suggest that²³¹Pa and²³⁰Th in the Southern Ocean are highly sensitive tracers of physical transport that may find use in studies of Southern Ocean biogeochemical‐physical connections in the past, present, and future.

    

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5. Biogeochemical Cycling of Dissolved Zinc in the Western Arctic (Arctic GEOTRACES GN01)

   https://doi.org/10.1029/2018GB005975  

   Jensen, L. T. ; Wyatt, N. J. ; Twining, B. S. ; Rauschenberg, S. ; Landing, W. M. ; Sherrell, R. M. ; Fitzsimmons, J. N. ( March 2019 , Global Biogeochemical Cycles)

   The biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient‐type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogeneous (sp.) with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by (1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes and (2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol,r² = 0.58) that is higher than the global slope (0.059 nmol/μmol,r² = 0.94) because of the shelf‐derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique preformed Zn:Si stoichiometries.

    

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