Copper concentrations and isotopic composition in the North Pacific

Shun-Chung Yang1, Xiaopeng Bian1, Tim M Conway2 and Prof Seth John1, (1)University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States, (2)University of South Florida, St. Petersburg, FL, United States
Copper (Cu) is a key cofactor for photosynthesis in marine primary producers, but it can also cause acute toxicity to phytoplankton when the free ion concentration exceeds 10 pM. Thus, the interactions between biological recycling, ligand complexation, scavenging, and water mass mixing/advection play a critical role in shaping community structure and productivity in the ocean. Here we study total Cu concentration (dCu) and isotopic composition (d65Cu) in the seawater of the GEOTRACES GP15 Pacific Meridional Transect (along 152°W from 56°N to 20°S), as well as a Gradients cruise (along 158°W from 23.5° to 37.3°N), in order to decipher the relative importance of the processes on controlling Cu cycling in various biogeochemical settings including strong margin fluxes, HNLC and oligotrophic waters, the oldest deep water, hydrothermal plumes, and oxygen minimum zones. dCu overall ranges from 0.4 nM to 5.8 nM and increases continuously with depths along the entire cruise track, with the exception of stations in Alaska shelf area, indicating that biological recycling and scavenging are primary processes controlling vertical Cu distributions. In the Alaska shelf area, we’ve found relatively elevated dCu in the top 200 m zonation (1.5 to 3.9 nM), which is likely associated with local hydrothermal or sedimentary inputs. In the equatorial upwelling region, dCu in surface water ranges from 0.6 to 1.1 nM (roughly twice as high as in oligotrophic waters), reflecting physical forcing.

We have developed a novel, semi-automated, anion-exchange purification method for Cu isotopic analysis. Our preliminary d65Cu dataset shows consistent values in the top 600 m from 23.5° to 37.3°N which covers oligotrophic and HNLC waters, with +0.6±0.2 ‰ (2SD) in top 200 m, +0.7±0.2 ‰ at 400 m and +0.6±0.2 ‰ at 600 m, despite that dCu increases from 0.7±0.3 nM to 1.7±0.2 nM over this depth range. The consistent d65Cu in top 200 m indicates an insignificant net isotopic fractionation effect by biological uptake, recycling, and scavenging, as well as indicating similar d65Cu from various Cu sources. Seawater dissolved d65Cu below 200 m are comparable with those in the South Atlantic and NW and NE Pacific, which can be explained by a homogeneous interior oceanic Cu pool (Takano et al., 2014; Little et al., 2018).