Literature data derived from sediment trap studies (Details of data source and compilation can be found in Supplementary material) have been complied to show a general layout for total mass flux (TMF) and POC flux at mesopelagic SCS of current stage. On average, the TMF at ~1000 m depth ranges from 66.3 ± 16.2 to 558.9 ± 446.0 mg m⁻² d⁻¹, while the POC flux varies from 3.4 ± 1 to 14.6 ± 10.3 mg m⁻² d⁻¹. Both the TMF and POC flux decrease from the slope to the deep basin, with relatively higher values in the western boundary and the lowest values in the central basin, showing a consistent spatial pattern with the annual mean Chl-a derived from satellite sensors ( Figure 1A ). Compared with the global synthetic data of POC flux at 2000 m from JGOFS (Honjo et al., 2008), the POC flux in SCS is obviously higher than those in the Pacific Warm Pool (0.8 mg m⁻² d⁻¹), but lower than those in the divergent Arabian Sea (19.9 mg m⁻² d⁻¹). Temporally, one-year consecutive records (Zhang et al., 2019; Tan et al., 2020), multiyear time series (Lui et al., 2018) and climatological results (Li et al., 2017) have all confirmed that the variability in sinking POC in the mesopelagic layer is synchronized with the overlying Chl-a changes.

In addition to POC, calcium carbonate (CaCO₃) and opal are other two main biogenic components, which refer to biogenic minerals, and these are mainly sourced from the skeletons of calcareous and siliceous plankton, respectively. Significant relationships between biogenic minerals, POC and TMF have been observed in the mesopelagic SCS (Li et al., 2017; Zhang et al., 2019; Tan et al., 2020). In general, the CaCO₃ and opal fluxes exhibit a similar temporal pattern to that of POC flux at a certain depth for a given station (Honjo et al., 2008 and references therein); in turn, they act as vehicles and protection shielding for organic matter, increasing the settling rates and potentially decreasing degradation rates (Armstrong et al., 2002; Francois et al., 2002). Microscopic inspections of siliceous and calcareous plankton in sediment trap samples exhibited higher abundance during productive seasons with larger export fluxes and less abundance during periods of low productivity and fluxes (Chen et al., 2007; Ran et al., 2015; Priyadarshani et al., 2019; Ladigbolu et al., 2020). This biological information remaining in the mesopelagic particles is consistent with the previously reported significant temporal variations in phytoplankton biomass and community structures in the euphotic layer (Ning et al., 2004).

Notably, lithogenic matter occupies a large proportion of sinking particles (Jennerjahn et al., 1992; Zhang et al., 2019; Tan et al., 2020; Li H. et al., 2022), which raises the question of whether there exists strong advection of heterochthonous material in the water column of SCS basin. Micropaleontological investigations have provided some evidences. For example, extinct coccoliths were inspected as a perennial component in the mesopelagic sinking particles from the northern SCS basin, with average proportions of 3.3-5.1% in the annual total coccolith flux. These extinct coccoliths probably sourced from the Miocene and remaining in the sediment cover to the west and south of the Dongsha Islands, which somehow lateral transported to the northern basin (Priyadarshani et al., 2019). Again groups of benthic and freshwater diatoms were also found in the trap samples from the northwestern SCS, with the abundances of 1.5-6.9% (Ran et al., 2022). However, the lateral contribution is currently hard to quantify due to the lack of information as to the abundance of benthic diatoms and reworked coccoliths in the surface sediment in the northern and northwestern SCS.

Based on the analysis of POC source indicators, the C/N mole ratio and dual carbon isotope compositions (δ¹³C_org and radiocarbon content), the sinking POC characteristics in the mesopelagic layer were relatively homogeneous and comparable with the POC collected from the overlying surface water (Liu et al., 2007; Zhang et al., 2019; Zhang et al., 2022). Estimates of binary mixing end-member models demonstrated that POC incorporated in sinking particles was predominantly derived from primary productivity in the overlying surface ocean, no matter in the slope area (Zhang et al., 2019) or at the basin station (Zhang et al., 2022). The relatively high proportion of lithogenic matter, as well as the covarying trend between lithogenic matter and POC flux, is likely due to scavenging by sinking organic matter, and in turn, the lithogenic matter acts as ballast material to accelerate the settling process, especially during the highly productive seasons (Zhang et al., 2019). The contribution of lateral transported organic carbon is reported to be much more important in the offshore sediment or in the sinking particles collected from the near bottom layers (Kao et al., 2014; Blattmann et al., 2018; Wei et al., 2020), but the export flux of sinking biogenic matter in the mesopelagic layer of SCS basin is overall mainly controlled by the productive processes in the overlying layer with the nonsignificant impact of lateral advection.

Overall, the East Asian monsoon has been regarded as a first-order control on BCP strength in the SCS. It not only drives the basin-scale background circulation (Wyrtki, 1961; Hu et al., 2000) but also profoundly influences biogeochemical processes and particle export (Ning et al., 2004). Clear seasonal variabilities in total mass and main component (POC, CaCO₃, opal and amino acid) fluxes of trap-collected particles correspond to monsoon transition. Generally, the values of POC flux in the northern SCS basin are always highest in winter but remain at a relatively low level during summer and the intermonsoon seasons (Wiesner et al., 1996; Lui et al., 2018). The prevailing, strong northeast wind coupled with extensive cooling in winter enhances vertical mixing (i.e., r ² = 0.61, N=19, p<0.001, Tseng et al., 2009a), which entrains more subsurface nutrient into the upper layer (Tseng et al., 2005; 2009) and then fuels photosynthetic productivity (Chen, 2005; Liu et al., 2013; Wong et al., 2015; Du et al., 2017). Such controlling mechanism of mixed layer depth has also been quantitatively validated in further studies, and corresponding biogenic parameters including Chl-a, net primary production as well as all components of mesopelagic particles show significant correlations with it (Table 3 in Tan et al. (2020); Table 1 in Li et al. (2017). Statistical result indicates that the biogenic particles in winter accounts for nearly half of the annual measured fluxes (41.5-48.9%). In the central SCS, except for the winter peak, there is a secondary peak during the southwest monsoon period, showing a distinct bimodal seasonality in the monthly multiyear average POC and opal fluxes (Li et al., 2017; Figure 1B). The fluxes of total mass, POC and foraminifera at a station located further south also demonstrate a covaried bimodal pattern, with maximum values during monsoon seasons, and the values in summer seem to be slightly higher than those in winter (Wan et al., 2010). The spatial differences in the seasonal transition pattern of the particle export flux might be highly correlated with the strength discrepancy of the East Asian monsoon from north to south across the SCS.

Significantly, several extremely high-flux peaks coupled with cyclonic eddies have been observed in the long-term records of a central station of the SCS (Li et al., 2017; Figure 1C). Cyclonic eddies, typically upwelling nutrient-rich subsurface water and stimulating phytoplankton blooms (Tang et al., 1999; Ning et al., 2004), are mainly generated during the northeastern monsoon period (Wang et al., 2003). Their contributions to biogenic particle flux may overlap the regular winter flux peak, with higher mesopelagic POC and opal fluxes of 41% and 116% respectively, compared with fluxes during non–cyclonic eddy periods (Li et al., 2017). Therefore, the seasonal pattern of biogeochemical responses in the SCS cannot be solely attributed to the influence of the monsoon transition, winter cyclonic eddies would amplify the seasonal particle export cycle.

The temporal variability in biogenic particle flux does not always follow its climatological seasonal pattern in the SCS. In addition to the northeast monsoon maxima, some discrete high-flux events have been observed in both the northwestern slope and the northern basin (Ran et al., 2015; Zhang et al., 2019; Tan et al., 2020). These episodic flux pulses are likely to be driven by randomly occurring intraseasonal physical processes, such as subsurface upwelling, aerosol deposition or mesoscale eddies. Local upwelling along the western boundary of the SCS (e.g., the northern shelf and the Vietnam coast) is probably generated by southwest winds during the summer monsoon. These intraseasonal processes may serve to provide “new” nutrients from underlying water or atmospherically deposition into the euphotic zone, thereby stimulating phytoplankton growth and a subsequent particle rainout. In contrast, the occasional passage of anticyclonic eddies may likely suppress the subsurface nutrient supply, which would substantially reduce primary production and subsequent export flux (Tan et al., 2020). As statistically estimated, three discrete high-flux events in the northwestern SCS determined by upwelling, aerosol deposition, and the northeast monsoon occupied 40% of the total sampling period but contributed nearly 80% of the biogenic particles (Zhang et al., 2019). Notably, the events contributed nearly equally to the total POC flux with their own unique biogeochemical overprints, implying that in the SCS, the intraseasonal physical processes might be as important locally as the seasonal monsoon in modulating the strength of the BCP.

ENSO is a recurring ocean-atmosphere coupled phenomenon that occurs across the tropical Pacific, with extreme warmer (El Niño) and cooler phases (La Niña) that dominate the global weather and climate patterns at the interannual time scale. In 2009, Tseng et al. proposed that winter monthly Chl-a (and integrated primary production) in the northern SCS decreased by 42% (and 42%) and 13% (and 10%), respectively, during two El Niño events (1997/1998 and 2002/2003). The reduction was attributed to the diminished vertical mixing and strengthened stratification by statistical analysis (Tseng et al., 2009a). Later studies have further elucidated that ENSO could impact the SCS biogeochemical conditions not only via abnormal changes in atmospheric forcing (wind speed and heat flux), but also through basin-scale circulation (Kuroshio and SCS throughflow) (Xiao et al., 2017), ultimately resulting in an asymmetric response in both surface Chl-a and mesopelagic flux during the 1997-1999 ENSO cycle (Liu et al., 2013; Li H. et al., 2022). This response is different from the general responses in the equatorial Pacific Ocean: the BCP weakens during El Niño events and strengthens during La Niña events (Chavez et al., 1999; Gierach et al., 2012; Brainard et al., 2018).

By extension, during the 1997/1998 super El Niño event, weak wind and intense solar radiation strengthened the upper water stratification, the Luzon Strait transport was therefore enhanced and intensified the oligotrophic Kuroshio intrusion to dilute the nutrient inventory in the SCS basin (Du et al., 2013). Together, compared with the climatological means, these processes led to impoverished bioavailable nutrients for photosynthetic productivity (decreased by 17%), and in particular, they inhibited diatom growth (opal flux decreased by 31.7%), thus resulting in an inefficient BCP with low particle export flux (Li H. et al., 2022). When the climate conditions oscillated to the La Niña phase in 1998/1999, the atmospheric forcing and the ongoing water mixing rapidly recovered to the normal state, but no obvious biological activity occurred. The net primary production and mesopelagic POC flux were still lower (13% and 10.9%, respectively) than the climatological means (Li H. et al., 2022). Although the Kuroshio intrusion was weakened, the thermocline/nutricline deepened due to basin-scale downwelling circulation or episodic warm eddies (Wang et al., 2002), which debilitated the nutrient supply, subsequently restraining the rebound of primary production and deep biogenic export flux (Liu et al., 2013; Li H. et al., 2022). However, the studies explained only the extreme event within a whole ENSO cycle, from El Niño to La Niña, while dynamic ENSO events with individual phases may present processes that respond differently. For example, the interannual variation in Chl is modulated by the intensity of the monsoon and mesoscale eddies which exhibits a close relationship with ENSO (Wang et al., 2006; Tseng et al., 2009b; He et al., 2016; Xiu et al., 2018). As extreme El Niño events are predicted to occur more often due to anthropogenically forced global warming (Cai et al., 2014), systematic studies are needed to evaluate the alterations of marine ecosystems and carbon cycles in response to the combined multiscale physical forcings in the SCS.