Insights into carbon export and remineralization fluxes using 234Th distribution along the GEOTRACES Pacific Meridional transect

Jennifer An Kenyon1, Ken Buesseler2, Phoebe J Lam3, Steven M Pike1, Nick Carracino4 and Sophie Rojas4, (1)Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, United States, (2)Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, MA, United States, (3)University of California Santa Cruz, Department of Ocean Sciences, Santa Cruz, CA, United States, (4)University of California Santa Cruz, Ocean Sciences, Santa Cruz, CA, United States
Abstract:
Th-234 (T½ = 24.1 d) is a naturally occurring radioisotope whose disequilibrium with parent U-238 can be utilized to quantify fluxes of carbon and other elements out of the upper ocean with sinking particulate matter. This study focuses on the distribution of Th-234, and similarly Th-228 (T½ = 1.91 y), on the GEOTRACES GP15 Tahiti to Alaska meridional transect (Sept-Nov 2018) to provide high-resolution results for total and particulate 234-Th as well as organic carbon export and remineralization fluxes in the Pacific Ocean basin.

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.

Back to: Revealing Biogeochemical Processes on Basin Scales through Ocean Transects II