234Th distribution along the Eastern Pacific GEOTRACES transect and implications for export and remineralization fluxes of carbon and TEIs

Authors: Black, E.E., Buesseler, K.O., Pike, S.M., Lam, P.J., Charette, MA.

²³⁴Th (t_½ ~ 24 days) is a naturally occurring radioisotope whose disequilibrium with parent ²³⁸U can be exploited to quantify fluxes of other elements traveling out of the upper ocean with sinking particulate matter (>51 µm). Tracers like ²³⁴Th play a vital role in the international GEOTRACES program, one goal of which is “to identify the processes and quantify the fluxes that control the distribution of trace elements and isotopes in the ocean”.

Unparalleled, high-resolution ²³⁴Th sampling during the U.S. GEOTRACES Eastern Pacific Zonal Transect (EPZT, Oct-Dec 2013) showed a relatively consistent vertical flux of ²³⁴Th out of surface ocean (0 -100 m) across most of the EPZT at 1700 dpm m^-2 day^-1 (± 400 std. dev.). When compared with the U.S. North Atlantic section (NAZT), the Pacific surface deficits in ²³⁴Th (relative to salinity-derived ²³⁸U) are much larger in magnitude as well as extent. Both transects crossed productive coastal zones with substantial surface deficits and greater export, but the NAZT export flux rapidly declines in the oligotrophic gyre while the offshore EPZT export remained high.

Total and particulate ²³⁴Th activities and preliminary particulate organic carbon results for the EPZT were used to calculate the carbon flux out of the surface ocean via sinking particles. Peruvian coastal shelf waters boasted carbon export over 10 mmolC m^-2 day^-1. A steady decline in carbon (~8 to 0.2 mmolC m^-2 day^-1) export was observed from 85 to 155 °W. The most pronounced subsurface excesses of ²³⁴Th (over ²³⁸U) and potential regions of remineralization for carbon and TEIs were found at stations 7, 8, and 9 just below the transition to suboxic water (<45 umol/kg). Further analyses using ²³⁴Th to track carbon and trace metal export will follow as particulate metal data is made available. In particular, we will attempt to quantify metal fluxes across the large productivity gradient in this region and above, in, and through the oxygen minimum zone.