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Abstract An important role in the cycling of marine trace elements is scavenging, their adsorption and removal from the water column by sinking particles. Boundary scavenging occurs when areas of strong particle flux drive preferential removal of the trace metals at locations of enhanced scavenging. Due to its uniform production and quick burial via scavenging,230Th is used to assess sedimentary mass fluxes; however, these calculations are potentially biased near regions where net lateral transport of dissolved230Th violates the assumption that the flux of particulate230Th to the seabed equals its rate of production in the water column. Here, we present a water column transect of dissolved230Th along 152° W between Alaska and Tahiti (GEOTRACES GP15), where we examine230Th profiles across multiple biogeochemical provinces and, novelly, the lateral transport of230Th to distal East Pacific Rise hydrothermal plumes. We observed a strong relationship between the slope of dissolved230Th concentration‐depth profiles and suspended particle matter inventory in the upper‐mid water column, reinforcing the view that biogenic particle mass flux sets the background230Th distribution in open ocean settings. We find that, instead of the region of enhanced particle flux around the equator, hydrothermal plumes act as a regional boundary sink of230Th. At 152° W, we found that the flux‐to‐production ratio, and thereby error in230Th‐normalized sediment flux, is between 0.80 and 1.50 for hydrothermal water, but the error is likely larger approaching the East Pacific Rise.
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Constraining the Global Ocean Cu Cycle With a Data‐Assimilated Diagnostic Model
Abstract 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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- Award ID(s):
- 1658392
- PAR ID:
- 10452822
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 34
- Issue:
- 11
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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