High-resolution spatial and temporal measurements of particulate organic carbon flux using thorium-234 in the northeast Pacific Ocean during the EXport Processes in the Ocean from RemoTe Sensing field campaign

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.

Export of particulate organic carbon (POC) in the eddy region of the tropical northwest Pacific

“…We speculate that the production in the deep SCM contributed to the POC flux in the deeper layer. The depth of the SCM in our study sites (~120 m) was greater than that at Station Papa (~75 m; Buesseler et al, 2020). Since the deep layer is biogeochemically less dynamic, our results suggest that such higher POC export may occur in the deep layer of the oligotrophic ocean.…”

“…An interesting finding from the observation in 2020 is the high POC flux at depths deeper than 100 m. We compared our deep POC flux data (> 100 m) with that from Station Papa, where POC fluxes based on the same method are available at several depths below the euphotic zone (Buesseler et al, 2020). The PP at Station Papa is about two times that in our study region (Buesseler and Boyd, 2009a).…”

“…Steady-state and no lateral advection are a required condition. However, deviation from this condition results in small errors in the open ocean (Buesseler et al, 2020). The fluxes of 234 Th at depths of 300 and 500 m in 2020 ranged from 250 to 1450 dpm m −2 d −1 .…”

“…In 2019, the POC fluxes at 100 m depth were calculated as 4.2 ± 0.3, 0.7 ± 0.1, and −3.0 ± 0.1 mmol m −2 d −1 at the reference site, the edge, and the core of the anticyclonic eddy, respectively (Figure 4F). Ideally, the non-steady state two-dimensional model together with the physical convection model based on high-resolution vertical and horizontal sampling will be needed to properly estimate the vertical and horizontal fluxes of POC (Buesseler et al, 2020). Zhou et al (2013) proposed that particle redistribution within the eddy by sub-mesoscale advection is important for POC export and should be considered in flux estimation.…”

Carbon cycling in the East Sea (Japan Sea): A review

“…The mean POC export in the UB based on the 234 Th/ 238 U disequilibrium method (Savoye et al, 2006;Waples et al, 2006;Buesseler et al, 2020) was 13 ± 6.3 mmolC m −2 d −1 (i.e., 4.9 molC m −2 yr −1 ), from the results of four cruises during spring 2006, summer 2007, and the fall and winter of 2008. (Kim et al, 2011).…”