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1. Toward a Better Understanding of the Global Ocean Copper Distribution and Speciation Through a Data‐Constrained Model

   https://doi.org/10.1029/2023GB007769  

   Liang, Hengdi ; Moffett, James W. ; John, Seth G. ( September 2023 , Global Biogeochemical Cycles)

   Copper (Cu) is an important micronutrient for marine organisms, which can also be toxic at elevated concentrations. Here, we present a new model of global ocean Cu biogeochemical cycling, constrained by GEOTRACES observations, with key processes including sources from rivers, dust, and sediments, biological uptake and remineralization of Cu, reversible scavenging of Cu onto sinking particles, conversion of Cu between labile and inert species, and ocean circulation. In order for the model to match observations, in particular the relatively small increase in Cu concentrations along the global “conveyor belt,” we find it is necessary to include significant external sources of Cu with a magnitude of roughly 1.3 Gmol yr⁻¹, having a relatively stronger impact on the Atlantic Ocean, though the relative contributions of river, dust, and sediment sources are poorly constrained. The observed nearly linear increase in Cu concentrations with depth requires a strong benthic source of Cu, which includes the sedimentary release of Cu that was reversibly scavenged from the water column. The processes controlling Cu cycling in the Arctic Ocean appear to be unique, requiring both relatively high Cu concentrations in Arctic rivers and reduced scavenging in the Arctic. Observed partitioning of Cu between labile and inert phases is reproduced in the model by the slow conversion of labile Cu to inert in the whole water column with a half‐life of ∼250 years, and the photodegradation of inert Cu to labile in the surface ocean with a minimum half‐life of ∼2 years at the equator.

    

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2. Constraining the Global Ocean Cu Cycle With a Data‐Assimilated Diagnostic Model

   https://doi.org/10.1029/2020GB006741  

   Roshan, Saeed ; DeVries, Tim ; Wu, Jingfeng ( October 2020 , Global Biogeochemical Cycles)

   Copper (Cu) is a biologically important trace metal for marine plankton, but it is also toxic at high concentrations. Understanding the global distribution of Cu and the processes controlling its cycling in the ocean is important for understanding how the distribution of this important element can respond to climate change. Here, we use available observations of dissolved copper, an artificial neural network, and an ocean circulation inverse model, to derive a global estimate of the three‐dimensional distribution and cycling of dissolved Cu in the ocean. We find that there is net removal by bio‐assimilation and/or scavenging of dissolved Cu in the surface ocean at a rate of ~1.7 Gmol yr⁻¹and that both the concentration and export of dissolved Cu are highest in the Southern Ocean. In the subsurface above the near‐sediment layer, dissolved Cu is removed at a net rate of ~2.4 Gmol yr⁻¹, consistent with scavenging onto sinking particles, contributing to an increase in the flux of particulate Cu with depth. This removal of Cu by scavenging in the interior ocean is balanced by a net near‐sediment source of dissolved Cu, which sustains a gradual increase in the concentration of dissolved Cu with depth. Globally, this net near‐sediment source is estimated at ~2.6 Gmol yr⁻¹in the deep ocean and ~0.8 Gmol yr⁻¹along continental shelves and slopes. Our results suggest an active oceanic dissolved Cu cycle with a mean internal ocean residence time of ~530 years, highlighting the potential for climate‐driven changes in the marine Cu cycle.

    

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3. Distribution and stability of Mn complexes in the ocean: Influence of hydrothermal plumes and weather events

   https://doi.org/10.1002/lno.12285  

   Thibault de Chanvalon, Aubin ; Luther, III, George W. ; Oldham, Véronique E. ; Tebo, Bradley M. ; Coffey, Nicole R. ; Shaw, Timothy F. ( December 2022 , Limnology and Oceanography)

   We measured the speciation of dissolved Mn from the surface to just above the hydrothermal vents at 9°50′N East Pacific Rise in the open ocean of the Pacific over a 3‐week period. Total dissolved Mn concentrations ranged from 2.2 to 135 nmol L⁻¹with a significant contribution of dissolved Mn(III) bound to humic acid in one third of our samples representing up to 64% of the total dissolved Mn. These humic complexes were mostly detected in the hydrothermal vent plume and at the redox boundaries of the oxygen minimum zone in the water column. In the hydrothermal plume, the Mn(III)‐humic acid stabilized the manganese in solution up to a ~ 10,000‐fold dilution of the venting water. In the upper water column, Mn(III)‐humic acid was only detected after a squall and rain event, which indicates that it is a transient species, persistent over days to weeks. This temporal variability highlights the importance of non‐steady‐state processes in the open ocean, which may help to explain previous observations of a dissolved Mn maximum within oceanic oxygen minimum zones.

    

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4. Particulate and dissolved metabolite distributions along a latitudinal transect of the western Atlantic Ocean

   https://doi.org/10.1002/lno.12275  

   Johnson, Winifred M. ; Kido Soule, Melissa C. ; Longnecker, Krista ; Bhatia, Maya P. ; Hallam, Steven J. ; Lomas, Michael W. ; Kujawinski, Elizabeth B. ( December 2022 , Limnology and Oceanography)

   Metabolites, or the small organic molecules that are synthesized by cells during metabolism, comprise a complex and dynamic pool of carbon in the ocean. They are an essential currency in interactions at the population and community levels of biological organization. Characterizing metabolite distributions inside microbial cells and dissolved in seawater is essential to understanding the controls on their production and fate, as well as their roles in shaping marine microbial food webs. Here, we apply a targeted metabolomics method to quantify particulate and dissolved distributions of a suite of biologically relevant metabolites including vitamins, amino acids, nucleic acids, osmolytes, and intermediates in biosynthetic pathways along a latitudinal transect in the western Atlantic Ocean. We find that, in the upper 200 m of the water column, most particulate or intracellular metabolites positively covary with the most abundant microbial taxa. In contrast, dissolved metabolites exhibited greater variability with differences in distribution between ocean regions. Although fewer particulate metabolites were detected below 200 m, the particulate metabolites identified in the deep ocean may be linked to adaptive physiological strategies of deep‐sea microbes. Based on the identified metabolite distributions, we propose relationships between certain metabolites and microbial populations, and find that dissolved metabolite distributions are not directly related to their particulate abundances.

    

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5. Sensitivity of Steady State, Deep Ocean Dissolved Organic Carbon to Surface Boundary Conditions

   https://doi.org/10.1029/2021GB007102  

   Matsumoto, Katsumi ; Tanioka, Tatsuro ; Gilchrist, Maya ( January 2022 , Global Biogeochemical Cycles)

   We use the transport matrices of a data‐constrained circulation model to efficiently compute the steady state distribution of the deep ocean dissolved organic carbon (DOC) at a 1° horizontal resolution by propagating the surface DOC boundary conditions into the ocean interior. An equivalent simulation in the traditional forward modeling approach would be prohibitively computationally expensive. Our model simulates the total DOC as the sum of two DOC pools, the refractory and the semi‐labile. The model is able to simulate the large‐scale features of the deep ocean DOC without local sources or sinks of DOC in the ocean interior. The deep ocean DOC in the model is sensitive to the preformed DOC concentrations in the formation sites of deep and bottom waters, where observations are lacking. Furthermore, our model experiments indicate that the deep Atlantic DOC gradient is sensitive to the mixing of deep waters with different concentrations of preformed refractory DOC, the transport of semi‐labile DOC from the surface North Atlantic, and the decay rate of semi‐labile DOC. These, combined with the observation that much of the deep ocean DOC gradient is in the Atlantic, suggests that the semi‐labile DOC may be an important component of the deep Atlantic DOC. Finally, we show that DOC export depends substantially on the depth level where it is evaluated.

    

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