The chemistry of copper (Cu) in seawater is well known to be dominated by complexation with organic ligands. The prevailing paradigm is that Cu forms strong but labile complexes. Recently, a novel procedure revealed that only a small fraction of dissolved Cu exists as labile complexes. The majority is present as a fraction that is relatively inert on timescales of weeks or more and probably does not participate in coordination exchange reactions, including biologically mediated processes. Samples collected from the 2018 GEOTRACES GP15 cruise show that throughout the interior of the Pacific Ocean, this inert fraction comprises about 90% of the dissolved Cu. Labile Cu accumulates in surface waters, probably arising from photochemical decomposition of the inert fraction. There is also a modest accumulation of labile Cu near deep sea sediments and along the Alaskan shelf and slope. The results have important implications for Cu transport and biological availability. Inert Cu may influence Cu transport throughout the water column and contribute to the linear increase in Cu with depth, a distribution which is hard to explain for a biologically active trace metal. The origins of inert Cu are unknown. It may be produced slowly within the water column on the timescale of meridional overturning circulation. In the Columbia River, between 92% and 98% of the dissolved Cu is in the inert fraction, suggesting a possible terrestrial source of inert Cu to the ocean.
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.
more » « less- NSF-PAR ID:
- 10465643
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 37
- Issue:
- 9
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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