Reply on RC1

Abstract

We conducted high vertical resolution 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 limit 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 high resolution vertical profilings of 234Th/238U disequilibria, this study for the first time estimated POC export fluxes both out of the NDL and at the horizon of the Ez base. Total 234Th deficit relative to 238U occurred in the NDL at all study sites, while 234Th was mostly in equilibrium with 238U in the NRL except at the northmost station SEATS (116° E, 18° N), where the 234Th deficit could also be observed in the NRL. By combining 1D steady-state 234Th fluxes and POC/234Th ratios, we derived vertical patterns of POC export fluxes. Values were 1.6±0.6 mmol C m-2 d-1 at the NDL base, representing approximately half of the flux estimated at the base of the Ez at station SEATS; for the rest of the sampling sites, POC export fluxes at the NDL base (averaged at 2.3±1.1 mmol C m-2 d-1) were identical within error to 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. Based on the positive relationship between POC export fluxes at the NDL base and supply potential of subsurface nutrients (i.e., nutricline depth and nutrient concentrations), we found that 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 subsurface nutrient concentrations were ~100 % higher than the fluxes (averaged at 1.6±0.5 mmol C m-2 d-1) at other stations. We used a two-endmember mixing model based on the mass and 15N-isotopic balances to further evaluate the potential sources of new nitrogen that could 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 other than atmospheric deposition and nitrogen fixation: likely supply from depth associated with episodic intrusions. However, the exact mechanisms and pathways for subsurface nutrients to support the export production from the NDL merit careful and dedicated studies.