Abstract
Copper (Cu) is both an essential micronutrient and toxic to photosynthesizing microorganisms at low concentrations. Its dissolved vertical distribution in the oceans is unusual, being neither a nutrient-type nor scavenged-type element. This distribution is attributed to biological uptake in the surface ocean with remineralisation at depth, combined with strong organic complexation by dissolved ligands, scavenging onto particles, and benthic sedimentary input. We present coupled dissolved and particulate phase Cu isotope data along the UK-GEOTRACES South Atlantic section, alongside higher resolution dissolved and particulate phase Cu concentration measurements. Our dissolved phase isotope data contribute to an emerging picture of homogeneous deep ocean δ65Cu, at about +0.65‰ (relative to NIST SRM 976). We identify two pools of Cu in the particulate phase: a refractory, lithogenic pool, at about 0‰, and a labile pool accessed via a weak acidic leach, at about +0.4‰. These two pools are comparable to those previously observed in sediments. We observe deviations towards lighter δ65Cu values in the dissolved phase associated with local enrichments in particulate Cu concentrations along the continental slopes, and in the surface ocean. Copper isotopes are thus a sensitive indicator of localised particle-associated benthic or estuarine Cu inputs. The measurement of Cu isotopes in seawater is analytically challenging, and we call for an intercalibration exercise to better evaluate the potential impacts of UV-irradiation, storage time, and different analytical procedures.
Highlights
Copper (Cu) is a bioessential element but the free Cu2+ ion is toxic at low concentrations, to prokaryotic photosynthesizing bacteria (e.g., Brand et al, 1986)
Other moderately heavy δ65Cu values are observed through the upper 2000 m of the water column at this station (Fig. 3). These values may indicate a different source of Cu along this margin, e.g., isotopically heavy Cu associated with the Aghulas current, which brings warm, salty water from the Indian Ocean into the southeastern Atlantic (e.g., Paul et al, 2015)
We present dissolved and particulate phase Cu concentration and Cu isotope data from the South Atlantic UK-GEOTRACES section GA10
Summary
Copper (Cu) is a bioessential element but the free Cu2+ ion is toxic at low concentrations (toxicity at > 10−11 M), to prokaryotic photosynthesizing bacteria (e.g., Brand et al, 1986). Aqueous Cu in seawater is predominantly complexed by strong organic ligands, which appear to have a detoxifying influence, as well as retaining Cu in the dissolved phase (e.g., Bruland et al, 2013). As for example in the Black Sea, Cu exists as the insoluble reduced Cu(I) species CuHS and Cu(HS)2− (Mountain and Seward, 1999). This array of behaviours make Cu isotopes a potential target as a (palae)oceanographic tracer (e.g., Chi Fru et al, 2016), contingent on an understanding of the key processes influencing their modern oceanic cycling
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