![]() ![]() Subtle differences in export further influence biological food webs with regard to biodiversity and fisheries abundance (e.g., Beaugrand et al., 2010) as well as the fate of other biologically associated and particle-scavenged elements, such as heavy metals and organic contaminants ( Shine et al., 1995 Gustafsson et al., 1997). Even small changes in the magnitude of downward transport of C in the ocean, or export, in global Earth system models can have major impacts on ocean C sequestration and thus on predicted atmospheric CO 2 concentrations ( Kwon et al., 2009). Yet the mechanisms that influence the movement of fixed C from well-lit surface waters to the ocean’s interior remain elusive ( Siegel et al., 2016). 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). This work sets the foundation for understanding controls on the biological carbon pump during this EXPORTS campaign. 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). 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). 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. 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. Below 100 m, NSS and SS models overlapped. 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%. Thorium-234 profiles were remarkably consistent, with steady-state (SS) 234Th fluxes reaching 1,450 ± 300 dpm m −2 d −1 at 100 m. High-resolution vertical sampling enabled observations of the spatial and temporal evolution of particle flux in Lagrangian fashion. More than 950 234Th measurements were made during August–September 2018 at Ocean Station Papa in the northeast Pacific Ocean. ![]() Here, the naturally occurring radionuclide thorium-234 was used as a tracer of sinking particle flux. 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. ![]()
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