Tracing continental input and organic matter export on the West Antarctic Shelf using the long-lived thorium isotopes

The long-lived thorium and uranium isotopes (²³²Th, ²³⁰Th, ²³⁸U and ²³⁴U) were analyzed in seawater samples collected during austral summers 2015 and 2016 from the Palmer Long Term Ecological Research (LTER) time-series grid offshore from the West Antarctic Peninsula (WAP). ²³²Th can be used as a tracer of continental material, and furthermore as an indicator of supply of the micronutrient Fe, which can limit primary productivity in the WAP seas. Data from 2015 and 2016 indicate a strong source of ²³²Th coming from Antarctic shelf sediments, likely also supplying Fe, being advected offshore at ~100-150 m depth. Additionally, we used ²³⁰Th to provide a quantitative estimate of the export flux of organic matter. This is done by multiplying the integrated production of ²³⁰Th by ²³⁴U decay by the ratio of organic carbon to ²³⁰Th in particulate material. For particulate organic carbon estimates, we used available prior time-series measurements. Collected at the site of long-term moored sediment traps, our Th results produce reasonable estimates of organic matter export in comparison to the sediment trap time-series, and can be compared to other published export estimates based on the shorter-lived ²³⁴Th. Finally, in the 2015 data set, we also measured dissolved uranium concentration (²³⁸U) and isotopic composition (²³⁴U/²³⁸U). These parameters are critical in the use of thorium isotopes (both ²³⁰Th and ²³⁴Th) to determine organic matter export. It was found that the isotopic composition was indistinguishable from the global average and U concentration had a linear relationship with salinity that was consistent with that in global average seawater.