Deep Ocean Particle Flux in the Northern South China Sea: Variability on Intra-Seasonal to Seasonal Timescales

Abstract

Based on a time-series sediment trap observation at a depth of 1,003-m in the northern South China Sea from 2014 to 2015, we used sinking particle flux combined with remote sensing-derived environmental data to infer the mechanisms of the biological carbon pump. Total particle flux, particulate organic carbon, CaCO 3 , and lithogenic (inorganic) fluxes peaked in winter and autumn but showed minima in summer, ranging from 59.7 to 413.2 mg m −2 d −1 , 2.1 to 18.2 mg m −2 d −1 , 32.9 to 197.9 mg m −2 d −1 , and 12.0 to 73.6 mg m −2 d −1 , respectively. Similarly, opal flux varied from 6.9 to 109.6 mg m −2 d −1 , with a prominent peak in winter and minimum in summer. However, a secondary maximum of opal in autumn was barely noticeable compared with other components. A deeper mixed-layer depth related to the northeast monsoon and surface cooling, overlapping with the effect of cyclonic eddies, is responsible for strong winter fluxes. When an anticyclonic eddy suppresses the subsurface nutrient supply, primary production and subsequent export flux in summer were reduced substantially. With respect to maximum lithogenic and CaCO 3 fluxes in autumn, a large aerosol optical depth suggests scavenging and mineral ballast effects as potential causes. This study highlights the importance of intra-seasonal physical processes (e.g., mesoscale eddies and aerosol deposition) to the deep particle flux in the South China Sea, as well as monsoonal transition.

Laterally Transported Particles From Margins Serve as a Major Carbon and Energy Source for Dark Ocean Ecosystems

“…Particles were collected to estimate sinking POC flux from a combination of lateral and vertical processes using bottom‐anchored time‐series sediment traps (McLane Mark 7G‐21 and Technicap PPS 3/3) deployed at 374–3250 m (one to four depth layers) at three stations in the NSCS and two in the SSCS (Table S2). Additional trap data (367 values for POC and 350 values for PN) from previous studies were also used (Gaye et al, 2009; Kao et al, 2012; Lahajnar et al, 2007; Ran et al, 2015; Tan et al, 2020; Wei et al, 2017; Zhang et al, 2019; Table S2). These trap fluxes were also fitted to a Martin curve using Equation .…”

“…Additional trap data (367 values for POC and 350 values for PN) from previous studies were also used (Gaye et al, 2009;Kao et al, 2012;Lahajnar et al, 2007;Ran et al, 2015;Tan et al, 2020;Wei et al, 2017;Zhang et al, 2019; Table S2). These trap fluxes were also fitted to a Martin curve using Equation 2.…”

Sediment Resuspension as a Major Contributor to Sinking Particles in the Northwestern South China Sea: Evidence From Observations and Modeling

“…As discussed before, the nearest river runoff from Hainan Island was very limited, and dust deposition (dry + wet) in the study area (16.5 • -18.5 • N, 109.5 • -111.5 which contributed only ∼0.45% of the lithogenic matter collected in the sediment trap (360.2 ± 331.8 mg•m −2 •d −1 ). On the other hand, the content of lithogenic matter in sinking particles in the northwestern SCS traps was much higher than that in the northern SCS basin (22.7 ± 10.3% at ∼1,000 m, Ran et al, 2015a;Tan et al, 2020) and the central SCS basin (26.5 ± 9.2% at ∼1,000 m, Li et al, 2017) but closer to the lithogenic matter content in the surface sediment near the SCS-NW trap location in the northwestern SCS (75.2 and 79.2%, data in Supplementary Dataset 4). Therefore, the resuspended surface sediment near the trap location was likely one of the most important sources of the sinking particles collected in the sediment trap.…”

“…Moreover, monsooninduced upwelling, mesoscale eddies, internal solitary waves (ISWs), typhoons, and atmospheric dust deposition frequently influence the SCS, resulting in highly complicated and variable marine environments and biogeochemical processes in the SCS (e.g., Wang et al, 2012Wang et al, , 2016Wang et al, , 2018Zhou et al, 2013;Li et al, 2017;Yu et al, 2019Yu et al, , 2020Zhang et al, 2019). To understand the complex biogeochemical processes and biological carbon pump in the SCS, bottom-tethered sediment traps have been widely used since the 1980s, particularly in the northern and central parts of the basin (Wiesner et al, 1996;Lahajnar et al, 2007;Gaye et al, 2009;Ran et al, 2015a,b;Li et al, 2017;Zhang et al, 2019;Tan et al, 2020).…”

Active Carbon Flux of Mesozooplankton in South China Sea and Western Philippine Sea

“…The proportion of zooplankton active flux in the total carbon flux is generally higher in the oligotrophic marine systems than in the meso-and eutrophic regions (Al-Mutairi and Landry, 2001;Steinberg and Landry, 2017;Yebra et al, 2018;Hernández-León et al, 2019b). The passive carbon fluxes (particle organic carbon, POC) in the SCS and WPS have been well documented (Chen et al, 1998;Hung and Gong, 2007;Ma et al, 2008;Cai et al, 2015;Zhang et al, 2019;Tan et al, 2020). In summer, the passive carbon flux in the oligotrophic basin region (51.6 mg C m −2 d −1 ) was lower than that in the costal (368.4 mg C m −2 d −1 ) and shelf (128.4 mg C m −2 d −1 ) regions in the SCS (Cai et al, 2015), and it was higher than that in the WPS (20.1 mg C m −2 d −1 ; Hung and Gong, 2007).…”