Dynamics of primary productivity in the northern South China Sea over the past 24,000 years

Abstract

In this study, paleoproductivity on millennial scales was precisely reconstructed from core MD12‐3428cq in the northern South China Sea (SCS) over the past 24 kyr, based on a transfer function derived from the strong exponential negative correlation between relative abundance of Florisphaera profunda (%FP) in core top sediments and basin‐wide satellite‐based primary productivity (PP) in the SCS. To detect the potential driving mechanisms of PP, correlation analyses were carried out among our PP records and other paleoenvironment parameters. PP peaked during 18–15 ka in parallel with the strong East Asian Winter Monsoon (EAWM). From 15 ka to the early Holocene, a decrease in PP coincided with sea level progradation and weakening of EAWM, which ultimately reduced fluvial nutrient levels and wind‐driven upper water column mixing. Since the middle Holocene, gradually more frequent El Niño‐Southern Oscillation (ENSO) events have taken place, further decreasing PP by injecting oligotrophic Kuroshio water masses into the northern SCS. Associated findings conclusively indicated that the main controlling factors of PP in the northern SCS have shifted from EAWM (glacial) to ENSO (interglacial) over the past 24 kyr.

Nitrogen Isotope Variations in the Northern South China Sea Since Marine Isotopic Stage 3: Reconstructed From Foraminifera‐Bound and Bulk Sedimentary Nitrogen

“…During the glacial period, there was higher input of fine‐grained terrestrial sediment from the South China due to low sea level (Liu et al, ; Zhang et al, ); this observation is consistent with the record of C:N ratios in our study of core MD12‐3433 (Figure c), which are relatively high during the glacial, especially in the last deglaciation and the LGM. The peak C:N ratios in the LGM are ~9, which are just above the marine end member and well below pure terrestrial end member value, indicating that while the majority of organic matter is of marine origin throughout our record, there may be significant contributions of N from land.…”

“…In addition, the sedimentation rate that is relatively high during the glacial period (Figure ) also supports that the high TOC and TN content can reasonably be interpreted as indicative of higher productivity. Other proxy records from the northern SCS support the idea that there was higher productivity in the last glacial period, including TOC and chlorine abundance records from ODP Site 1144 (Higginson et al, ), an alkenone content record from core SO50‐31KL (Huang et al, ), and a record of relative abundance of Florisphaera profunda (%FP) from cores MD12‐3428 (Zhang et al, ) and MD05‐2904 (Su et al, ). Higher productivity during glacials in the northern SCS is generally attributed to enhanced vertical mixing in response to a strengthened East Asian Winter Monsoon (EAWM) bringing subsurface nutrients into the euphotic zone (Wang & Li, ).…”

Two Production Stages of Coccolithophores in Winter as Revealed by Sediment Traps in the Northern South China Sea

“…The relative abundance of F. profunda in bore‐core sediments indicate the winter coccolithophore productivity throughout the water column (e.g., Sprengel et al, ; Triantaphyllou et al, ), thus reflecting the winter primary productivity in the northern SCS. This study supports our previous finding that in the northern SCS, the decline in annual primary productivity that is estimated by % F. profunda was a consequence of the high frequency of El Niño events in the late Holocene (Zhang et al, ). A higher correlation between F. profunda and surface chlorophyll occurs at the basinal TJ‐A ( R 2 = 0.7071) site, with lower levels at both the relatively coastal TJ‐E ( R 2 = 0.3754) and TJ‐G ( R 2 = 0.5645) sites (Figure a). This is due to the Noëlaerhabdaceae coccolithophores being one of the productive phytoplankton groups, and they contribute more to the surface organic carbon production at a further distance from coastal regions, which is confirmed by sea water biomarker (Li et al, ) and phytoplankton pigment (Zhai et al, ) investigations in the SCS.…”

“…G. caribbeanica and small Gephyrocapsa spp. may possibly have higher contributions to the primary productivity or organic carbon production (higher potential to assimilate nutrients), as driven by the evolution during this period. The correlations are built mostly based on the differences between monsoonal and nonmonsoonal seasons, and the correlation style is analogous to the case in surface sediments (Figure b; Zhang et al, )—a lower surface chlorophyll‐ a concentration (<0.2 mg/m 3 ) and productivity (<150 gC·m ‐2 ·year −1 ) can correspond to a large range of F. profunda percentages, that is, ranging from ~70% to 100% in sediment traps and ~40% to 80% in surface sediments (Figure ). For sediment traps, the high variability of % F. profunda during nonmonsoonal seasons can be due to a lateral transport as triggered by mesoscale eddies or typhoon events (Zhang et al, ; Y. Zhang, Liu, et al, ), even for the shallow (500‐m depth) traps.…”

“…For example, a G. caribbeanica dominance event occurred globally during marine isotope stages 8–12 (e.g., Bollmann et al, ). During this period, the relative abundance of F. profunda may have been as low as 20% in the western Pacific (Jin et al, ), and the primary productivity may have been as large as 161–237 gC·m −2 year −1 , based on the transfer functions in the equatorial Indian Ocean and SCS (Beaufort et al, ; Zhang et al, ) being approximately 50–120% higher than the oligotrophic modern western Pacific. Such a high phytoplankton productivity required a large quantity of nutrient inventory at the ocean's surface, which may have been fueled by a long‐term enhanced aeolian dust deposition (Guerreiro et al, ), weathering (Rickaby et al, ), terrestrial inputs (Flores et al, ), and a shallowing of the thermocline (mixed layer depth) and nutricline.…”