Partitioning of carbon export in the euphotic zone of the oligotrophic South China Sea

Abstract

We conducted samplings of total and particulate 234Th, along with particulate organic carbon (POC), in the summer of 2017 to examine nutrient-dependent structures of export productivity within the euphotic zone (Ez) of the oligotrophic basin of the South China Sea (SCS). Nitrate concentrations throughout the study area were below detection limits in the nutrient-depleted layer (NDL) above the nutricline, while they sharply increased with depth in the nutrient-replete layer (NRL) across the nutricline until the base of the Ez. Based on our vertical profilings of 234Th-238U disequilibria, this study estimated for the first time POC export fluxes both out of the NDL and at the horizon of the Ez base. The total 234Th deficit relative to 238U was determined in the NDL at all study sites. By contrast, 234Th was mostly in equilibrium with 238U in the NRL, except at the northernmost station, SEATS (SouthEast Asian Time-series Study; 18 N, 116 E), where the 234Th deficit was also observed in the NRL. By combining 1D steady-state 234Th fluxes and POC/234Th ratios, we derived vertical patterns of POC export fluxes. The POC export fluxes at station SEATS were 1.6 ± 0.6 mmol C m-2 d-1 at the NDL base, accounting for approximately half of that at the base of the Ez. For the rest of the sampling sites, the POC export fluxes at the NDL base (averaged at 2.3 ± 1.1 mmol C m-2 d-1) were comparable with those at the base of the Ez (1.9 ± 0.5 mmol C m-2 d-1), suggesting rapid export of POC out of the NDL. This finding fundamentally changes our traditional view that the NDL, being depleted in nutrients, would not be a net exporter of POC. Furthermore, our results revealed a significant positive correlation between POC export fluxes at the NDL base and the potential of subsurface nutrient supplies, indicated by nutricline depth and nutrient concentrations obtained from both in situ measurements and numerical modeling. POC export fluxes (averaged at 3.4 ± 1.2 mmol C m-2 d-1) at the NDL base at stations with shallow nutriclines and high levels of subsurface nutrients approximately doubled those (averaged at 1.6 ± 0.5 mmol C m-2 d-1) at other stations. We subsequently used a two-endmember mixing model based on the mass and 15N isotopic balances to evaluate the relative contribution of different sources of new nitrogen that support the observed particle export at stations SEATS and SS1, located respectively in the northern and southern basin of the SCS with different hydrological features. We showed that more than 50 % of the particle flux out of the NDL was supported by nitrate sources likely supplied from depth and associated with episodic intrusions other than atmospheric deposition and nitrogen fixation. However, the exact mechanisms and pathways for subsurface nutrients to support the export production from the NDL merit additional careful and dedicated studies.