234Th in surface waters: Distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE

Abstract

As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100 m, derived from the depletion of 234Th with respect to its parent 238U, ranged from 621±105 to 1773±90 dpm m −2 d −1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2 mmol C m −2 d −1 (2.1–9.0 mmol C m −2 d −1) using POC/ 234Th ratio of bulk (respectively >50 μm) suspended particles at the export depth (100 m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO 2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers. In a zone of very low algal abundance (54–58°S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations. An ice-edge bloom that had developed in December/January in the zone 60–65°S as studied during the previous leg had caused a high export flux at 64.5°S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–69°S evident from uptake of CO 2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements.