Quantifying carbon export and remineralization during a spring bloom using 238U-234Th disequilibrium in the EXPORTS Program

The goal of NASA's EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) project is to develop a predictive understanding of the fate of global ocean primary productivity and export of carbon from the surface to the deep. To accomplish this, thorium-234 (234Th, t1/2 = 24.1 d) was used to measure sinking particle export during the EXPORTS 2021 North Atlantic cruise (May 2021) at the Porcupine Abyssal Plain, following an anticyclonic eddy for four weeks during the spring bloom.

The disequilibrium between the particle-reactive 234Th and its soluble parent, uranium-238 (238U, t1/2 = 4.5 × 109 y), integrated over depth, results in 234Th export flux in dpm/m2/d. This value can be multiplied by a carbon to 234Th ratio in particulate samples to obtain carbon export flux. During the course of the EXPORTS 2021 cruise, 985 234Th seawater samples were collected over 52 CTD casts with high depth resolution from the surface to 500 m, along with particle size-fractioned particulate carbon to 234Th ratio samples from in situ McLane pumps. This data gives insight into carbon export out of the euphotic zone, primarily in the core and the edge of the eddy. Additionally, 33 underway seawater samples were taken from the mixed layer during a transect along the eddy edge. The results are a high vertical and horizontal spatial resolution study of carbon export throughout an eddy as it evolves over the course of four weeks during a spring bloom.

Preliminary results show the development of the eddy in a non-steady state model with increasing carbon export over the course of the cruise, which is consistent with high carbon export values of previous studies during the North Atlantic bloom. Results also show a deepening of the carbon remineralization layer during the demise of the bloom nearing the end of the sampling period.