Carbon and silica fluxes during a declining North Atlantic spring bloom as part of 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 ocean. Thorium-234 (²³⁴Th, t_1/2 = 24.1 d) was used to measure sinking particle export from an anticyclonic eddy during the EXPORTS North Atlantic cruise (May 2021) at the Porcupine Abyssal Plain. The four-week sampling period was broken into three time periods (“epochs”) where 800 ²³⁴Th seawater samples were collected from over 50 CTD casts with high depth resolution over the upper 500 m. Size-fractioned particulate samples were collected to determine particulate organic carbon (POC) and biogenic silica (bSi) to ²³⁴Th ratios using in situ McLane pumps. A ²³⁴Th non-steady state model shows an eddy center epoch average progression of increasing ²³⁴Th export (∼2800 ± 300 (Epoch 1; standard deviation) to 4500 ± 700 (Epoch 3) dpm m⁻² d⁻¹) out of the top 110 m of the water column over the course of the cruise (29 d). This translates into an epoch average progression of ∼11 ± 1 to 14 ± 2 mmol C m⁻² d⁻¹ of sinking POC flux, and ∼ 3 ± 1 to 6 ± 1 mmol bSi m⁻² d⁻¹ of sinking bSi flux to deeper waters at 110 m. The overall efficiency of the biological carbon pump (amount of net primary production reaching 100 m below the euphotic zone) increases from ∼10% to ∼30% throughout the sampling period. The temporal trends discussed extensively in this paper show that POC and bSi export increase during diatom bloom evolution.