Abstract
The 234Th–238U disequilibrium has been widely used to quantify the sinking flux of particulate organic carbon (POC) out of the upper ocean. However, the influence of the advection on the quantification is poorly understood due to the lack of in situ measured physical parameters. Here, a Lagrangian observation was deployed for 39 h to track the variability of 234Th along with the current on the slope of the northeastern South China Sea (SCS). Contrasting to the general ocean interior, 234Th showed deficits relative to 238U in the mesopelagic waters, indicating an enhanced removal of 234Th. Concurrently, elevated total particulate matter (TPM) and POC contents were observed in the mesopelagic waters, supporting the driving force of the cross-shelf dispersion of re-suspended shelf/slope sediments for the 234Th removal. The widely used 234Th-model (ignoring physical processes) produced a much lower sinking flux of POC than the sediment trap-derived POC flux during the same observation, indicating an unneglectable influence of advection and diffusion. By considering the main horizontal advection and vertical diffusion, the 234Th–238U method gave rise to comparable results to sediment trap. 234Th-derived POC fluxes showed an increased pattern from 300 to 1,000 m, consistent with the more abundant POC where. These results indicated that advection represents an unneglectable process during the quantification of the sinking flux of 234Th over the slope of the SCS.
Highlights
Particulate organic carbon (POC) settling largely dominates the ability of the oceans to sequestrate carbon dioxide
We evaluated the influence of physical processes by comparing the flux of 234Th at 200 m with that calculated through trap-derived particulate organic carbon (POC) at 200 m
A Lagrangian observation of 234Th was conducted over the slope of the northeastern South China Sea (SCS)
Summary
Particulate organic carbon (POC) settling largely dominates the ability of the oceans to sequestrate carbon dioxide. Lagrangian Observation of 234Th ocean (Roca-Martí et al, 2017; Umhau et al, 2019). This deficit has been used to quantify the sinking flux of particulate components (especially POC) out of the euphotic zone (Buesseler et al, 2009; Maiti et al, 2016). A few studies indicate that physical processes can largely affect the 234Th and POC fluxes in some oceanic settings, e.g., upwelling in the Equatorial Pacific (Bacon et al., 1996) and anticyclonic eddies (Zhou et al, 2013). The largest fractions of both 234Th and POC in medium-sized (10–15 μm) particles are observed in the northwestern Pacific Ocean (Hung et al, 2010). The influence of physical processes, temporal variability, and particle size on the 234Th–238U method needs extensive investigations to improve the application of 234Th–238U
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