Utilization of Soot and 210 Po-210 Pb Disequilibria to Constrain Particulate Organic Carbon Fluxes in the Northeastern South China Sea

Abstract

Black carbon (BC) is believed to be refractory and thus affects the timescale of organic carbon conversion into CO2 and the magnitude of the sink of CO2. However, the fate of BC in the oceans remains poorly understood. Here, 210Po and 210Pb were measured to examine the export of soot in the northeastern South China Sea (SCS). Concentrations of soot decreased from 0.141 ± 0.021 μmol-C L–1 (mean ± SD) in the mixed layer (0–30 m) to 0.087 μmol-C L–1 at the euphotic base (150 m) due to potential photodegradation within the euphotic zone. In the twilight zone, however, the soot showed an increasing pattern along with the total particulate matter and total particulate organic carbon (POC) contents, corresponding to additions from the shelf/slope sediment resuspension through lateral transport. Using the deficits of 210Po, the export flux of soot from the euphotic zone was calculated to be 0.172 ± 0.016 mmol-C m–2 d–1 and increased with depth. Assuming that the soot is entirely refractory below the euphotic zone, the sediment-derived soot fluxes were estimated based on the increase in soot fluxes relative to the base of the euphotic zone, with values varying from 0.149 ± 0.030 to 0.96 ± 0.10 μmol-C L–1. This indicates that sediment resuspension is an important source of soot to the ocean interior in the SCS. Coupling the sediment-derived soot and 210Po-derived POC fluxes gave rise to a Martin Curve-like flux attenuation of local euphotic zone-derived POC in the twilight zone with b value of 0.70 ± 0.01. These results suggest that soot could be useful for constraining in situ POC fluxes and their transport.

Lagrangian Observation of 234Th and Its Application in Constraining the Sinking of Particulate Organic Carbon on the Slope of the Northeastern South China Sea

“…Notably, below the euphotic base (i.e., 150 m), both TPM and POC showed a downward increasing pattern (Figure 2B), increasing from 0.11 to 0.19 mg L −1 and 0.25 to 0.59 µmol L −1 , respectively. This phenomenon was also reported at a station near T1 during the same cruise (Yang et al, 2021b), pointing to the cross-shelf transport of re-suspended sediments. The activity concentrations of 234 Th T varied from 1.00 to 2.03 (avg.…”

“…The activity ratios of 234 Th T to 238 U (i.e., 234 Th/ 238 U) ranged from 0.42 to 0.84 at T1 and 0.54 to 0.86 at T2, averaging 0.67 and 0.70, respectively. Unlike the equilibrium between 234 Th and 238 U widely observed in mesopelagic waters, 234 Th/ 238 U showed deficits of 234 Th relative to 238 U at both T1 and T2 (Figure 3D), which was similar to the deficient 210 Po relative to 210 Pb observed near T1 during the same cruise (Yang et al, 2021b).…”

“…POC concentrations showed a range of 0.25-1.42 µmol L −1 , averaging 0.67 µmol L −1 . POC exhibited the highest values between 50 and 75 m (Figure 2B), corresponding to the widely observed depth of chlorophyll-a maximum in the SCS (Cai et al, 2015;Yang et al, 2021b). Notably, below the euphotic base (i.e., 150 m), both TPM and POC showed a downward increasing pattern (Figure 2B), increasing from 0.11 to 0.19 mg L −1 and 0.25 to 0.59 µmol L −1 , respectively.…”

“…With the influence of currents, re-suspended particles disperse over a large scope as supported by the elevated particle fluxes in deeper sediment traps in the basin of the SCS (Schroeder et al, 2015;Hong et al, 2021). This dispersion of the shelf/slope sediments, mainly driven by advection, results in enhanced scavenging of particle-reactive nuclides (e.g., 210 Po) in the mesopelagic water of the SCS (Figure 3; Ma et al, 2017) and thus the sinking flux of POC (Yang et al, 2021b).…”

“…In this study, the northeastern SCS was selected to investigate the influence of horizontal advection on the estimation of 234 Th flux (Figure 1). Over the past decade, increasing studies report the widely observed cross-shelf dispersion of particulate matter (Shih et al, 2019;Shen et al, 2020;Yang et al, 2021b;Ge et al, 2022) driven by the dynamic physical processes (Li et al, 2018;Jia et al, 2019). Thus, the northeastern SCS could be an ideal site for attesting to the influence of physical processes on the application of the 234 Th/ 238 U method.…”