Insights into carbon export and remineralization fluxes using 234Th distribution along the GEOTRACES Pacific Meridional transect
Fluxes of total and particulate Th-234 and Th-228 out of the surface ocean (calculated here as the base of the euphotic zone) varied across a wide range of biogeochemical environments. The highest flux was observed at the Alaskan Shelf (1600 ± 100 dpm m-2 d-1), and the lowest flux observed out of the high-nutrient, low-chlorophyll center in the North Pacific (900 ± 200 dpm m-2 d-1). Th-234 surface flux at equator (1500 ± 100) was similar to those observed during the JGOFS Fall 1992 cruise (1500 ± 150) and the equatorial GA03 Fall 2011 cruise (1200 ± 100). Similarly, the surface flux at Subtropical South Pacific Gyre (1400 ± 20) was comparable to that observed during the GEOTRACES EPZT Fall 2010 transect (1200 ± 400).
Th-234 and Th-228 results will be combined with particulate organic carbon data. This will give insight to the different controls dominating upper ocean export within the Pacific Ocean basin (such as zooplankton-mediated particle disaggregation, local light conditions, shallow oxygen minima, and more) and across the large productivity gradient along this transect. Further analyses using Th-234 to track trace metal export will follow as particulate metal data is made available. The quantification of export and remineralization within the world’s largest ocean basin provided by this study will help to inform gaps in knowledge regarding carbon cycling, as well as better inform climate models with high-resolution data.