Abstract
The EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program of National Aeronautics and Space Administration focuses on linking remotely sensed properties from satellites to the mechanisms that control the transfer of carbon from surface waters to depth. Here, the naturally occurring radionuclide thorium-234 was used as a tracer of sinking particle flux. More than 950 234Th measurements were made during August–September 2018 at Ocean Station Papa in the northeast Pacific Ocean. High-resolution vertical sampling enabled observations of the spatial and temporal evolution of particle flux in Lagrangian fashion. Thorium-234 profiles were remarkably consistent, with steady-state (SS) 234Th fluxes reaching 1,450 ± 300 dpm m−2 d−1 at 100 m. Nonetheless, 234Th increased by 6%–10% in the upper 60 m during the cruise, leading to consideration of a non-steady-state (NSS) model and/or horizontal transport, with NSS having the largest impact by decreasing SS 234Th fluxes by 30%. Below 100 m, NSS and SS models overlapped. Particulate organic carbon (POC)/234Th ratios decreased with depth in small (1–5 μm) and mid-sized (5–51 μm) particles, while large particle (>51 μm) ratios remained relatively constant, likely influenced by swimmer contamination. Using an average SS and NSS 234Th flux and the POC/234Th ratio of mid-sized particles, we determined a best estimate of POC flux. Maximum POC flux was 5.5 ± 1.7 mmol C m−2 d−1 at 50 m, decreasing by 70% at the base of the primary production zone (117 m). These results support earlier studies that this site is characterized by a modest biological carbon pump, with an export efficiency of 13% ± 5% (POC flux/net primary production at 120 m) and 39% flux attenuation in the subsequent 100 m (POC flux 220 m/POC flux 120m). This work sets the foundation for understanding controls on the biological carbon pump during this EXPORTS campaign.
Highlights
Marine ecosystems are of fundamental importance with regard to their capacity to influence the storage, transformation, and fate of carbon (C) and associated elements in the Earth’s biosphere (Falkowski et al, 2000; Sarmiento and Gruber, 2002; Chavez et al, 2011; Doney et al, 2012)
Particulate organic carbon (POC) flux estimates during EXPORTS at Ocean Station Papa The POC flux derived from our extensive 234Th survey, using C/Th ratios in mid-sized particles and the average of SS and NSS 234Th models, was 5.5 mmol C mÀ2 dÀ1 at 50 m decreasing to 1.7 mmol C mÀ2 dÀ1 by 120 m (Figure 6 and Table 3), which was near the depth of the primary production zone” (PPZ) (117 + 5 m) and Ez0.1 (118 + 9 m)
Our finding of a POC flux maximum during EXPORTS of 5.5 mmol C mÀ2 dÀ1 at 50 m, decreasing to a flux of 1–2 mmol C mÀ2 dÀ1 at the base of the Ez and below, places the Station P site as one of the more modest C flux settings
Summary
Marine ecosystems are of fundamental importance with regard to their capacity to influence the storage, transformation, and fate of carbon (C) and associated elements in the Earth’s biosphere (Falkowski et al, 2000; Sarmiento and Gruber, 2002; Chavez et al, 2011; Doney et al, 2012). The primary goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program of the National Aeronautics and Space Administration is to develop a predictive understanding of the export and fate of fixed C from global ocean primary production (Siegel et al, 2016). Inherent in these models is the need to reduce uncertainties in C export estimates and to develop constructs that facilitate future C export estimates across a variety of spatial and temporal scales. Critical components of the EXPORTS Program are quantitative measurements of sinking particle fluxes and their attenuation versus depth at scales similar to the physical and biological processes that influence fixed C export
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