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Biological carbon pump (BCP) inefficiency was first observed in the South China Sea (SCS) throughout the 1997–1999 El Niño Southern Oscillation (ENSO) event, but the BCP usually recovers its efficiency when the climate conditions change from El Niño to La Niña conditions in the Pacific Ocean. Enhanced stratification and Kuroshio intrusion led to weak mixing and oligotrophic conditions in the sunlit layer of the SCS during the 1997/1998 El Niño phase, but the particulate organic carbon (POC) flux was comparable to the climatological mean due to the ballast effect of increased lithogenic material and CaCO3 flux. The deepened thermocline rendered the recovered mixing less effective in replenishing subsurface nutrients and subsequently lowered POC flux during the 1998/1999 La Niña phase. Both scenarios were characterized by decreased siliceous plankton but a stimulated calcareous plankton contribution, which reduced BCP efficiency, resulting in a unique asymmetric response to the ENSO in the SCS. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) in the Pacific Ocean is one of the most important ocean‐atmosphere coupled climatic phenomena on Earth, and the ENSO in turn influences marine biogeochemical cycles and the biological carbon pump (BCP). Usually, as climate conditions transition from the El Niño to La Niña phases, the weakened BCP rebounds to a highly productive level in the central and eastern Pacific. However, the BCP exhibits an asymmetric response to the ENSO in the South China Sea (SCS), the largest tropical marginal sea of the Pacific Ocean. We show that strengthened stratification of the upper water column and enhanced oligotrophic Kuroshio intrusion led to impoverished in bioavailable nutrients near the surface, caused fewer siliceous plankton compared to calcareous plankton, and thus resulted in an inefficient BCP in the 1997/1998 El Niño phase. In contrast, the deepened thermocline prevented the replenishment of nutrients from the subsurface, subsequently inhibiting the rebound of primary production and deep biogenic flux during the 1998/1999 La Niña phase. This study may aid in understanding the response of the BCP to ENSO events in marginal seas and improve predictions of global ocean carbon storage. Key Points: The South China Sea biological carbon pump does not rebound from the 1997/1998 El Niño to La Niña phase as expectedThe enhanced stratification and Kuroshio intrusion are responsible for the low opal:CaCO3 ratio of sinking particles in the El Niño phaseThe deepened thermocline prevents the diapycnal nutrient supply and the recovery of the biological carbon pump in the La Niña phase [ABSTRACT FROM AUTHOR]

The East Asian Monsoon and mesoscale eddies are known to regulate primary production in South China Sea (SCS), the largest tropical marginal sea; however, their contributions to the deep biogenic flux are yet to be quantified. Based on 7-year time series sediment trap observations at the depth of 1200 m in the central SCS, we used the monthly average sinking biogenic fluxes to evaluate the impact of the monsoon and mesoscale cyclonic eddies on biogenic fluxes in combination with remote sensing physical parameters. The monthly average particulate organic carbon (POC) and opal fluxes, ranging from 3.0 to 5.2 and 14.8–34.9 mg m −2 d −1 , respectively, were higher during the northeastern monsoon period. This corresponded to the deeper mixed layer depth and higher net primary production in this area, due to nutrient replenishment from the subsurface induced by monsoon transition and surface cooling. In contrast, lower POC and opal fluxes occurred during well-stratified inter-monsoon periods. In addition, CaCO 3 flux (23.6–37.0 mg m −2 d −1 ) exhibited less seasonality and was assumed to originate from foraminifera. In terms of the long-term record, the combined effect of cyclonic eddies and mixing in the upper ocean could effectively regulate the temporal variation in the biogenic flux. In particular, the opal and POC fluxes in cyclonic eddies were 116% and 41% higher on average, respectively, than those during the non-cyclonic eddy period. Since the cyclonic eddies mainly occurred during the northeastern monsoon period, their contributions to biogenic flux via diatom blooms might overlap the regular winter flux peak, which could make the biological carbon pump more efficient at CO 2 sequestration during this period thus amplifying the impact of seasonal transition. [ABSTRACT FROM AUTHOR]

In order to assess dominant settling processes that change the composition of the detrital clay fraction during transport from neighboring estuaries to a deep sea basin, we studied relative clay mineral abundances and absolute clay mineral fluxes of clay-sized sinking particulate matter collected by eight sediment trap systems deployed from shallow to deep water depth in the South China Sea. This is the first basin-wide study on recent sedimentation processes in the western Pacific marginal seas. Annual averages of relative clay mineral abundances at the shallow traps are temporally more variable and regionally more diverse, resembling those of surrounding drainage basins. In contrast, higher fluxes of material reach the deeper traps. Their characteristics trend temporally and spatially towards uniformity and are enriched with smectite in the entire deep basin. Sinking particulate matter that reaches the shallow traps spends less time in pelagic transport and is affected by monsoonal current reversals. The enrichment in smectite in the deeper traps is a result of longer duration in transport at low velocities, which may increase the effect of differential settling during transport. The trend is caused by lateral advection driven by the cyclonic deep circulation, and this is considered as the main transport process in the northern and central deep basin. The high fluxes in the south-western deep basin could be the result of laterally advected re-suspended sediments from the neighboring shelves. The effects on the composition of the detrital clay fraction caused by oceanographic control, which indirectly include those by differential settling, mask the climatic signal from surrounding drainage basins in the deep basin sediments. This strongly affects the interpretation of the clay mineralogical record in sediments deposited under recent conditions in the South China Sea deep basin. [ABSTRACT FROM AUTHOR]

Abstract: Settling particles collected by sediment traps deployed between 1987 and 1999 in the northern, central and southwestern South China Sea (SCS) were analysed to study seasonal, interannual and spatial variations in the composition and flux of labile particulate matter. Results were combined with remote-sensing and surface-sediment data in order to describe the factors controlling the preservation of organic matter en route from the upper ocean to the seafloor. Organic carbon, amino acid and hexosamine fluxes generally follow the fluxes of total particulate matter, with maxima during the SW and NE monsoon periods. During non-El Niño conditions spectral amino acid distributions show that degradation of organic matter in the water column decreases as the flux rates increase. This is suggested to be the combined result of enhanced primary productivity, greater input of lithogenics serving as ballast to increase settling rates, and sorption of labile components to clay minerals. During El Niño conditions, in contrast, the degree of organic matter degradation is at very high and comparable levels at all trap sites. Flux component seasonality is strongly reduced except for the coastal upwelling areas, particularly off central Vietnam, which show significantly higher fluxes of organic carbon and lithogenic matter as compared to the open SCS. This suggests that the fluxes are affected by lateral advection of reworked organic matter from riverine sources or resuspended sediments from the nearby shelf/slope. Comparison of the measured organic carbon fluxes in 1200m depth with those accumulating in surface sediments results in a more than 80% loss of organic matter before final burial in the sediments. The degree of organic matter preservation in the surface sediments of the deep SCS is distinctly lower than in other monsoonal oceans. This may be due to varying lithogenic input and almost complete dissolution of protective biogenic mineral matrices at greater water depth. [Copyright &y& Elsevier]

Abstract: The sediments in the upwelling area off central Vietnam are totally bioturbated and display a low-diverse assemblage of bioturbational structures. During interglacial times (Marine Isotope Stage MIS 1, 5a, 5c, 5e), summer monsoon leads to pronounced upwelling and seasonally pulsed arrival of organic matter on the seafloor. These deposits are characterized by a 4-tier bioturbated zone with Zoophycos. Zoophycos producers used this seasonal food source and show a cache behavioral strategy. During glacial periods upwelling was weak and oxygenation of bottom water decreased as evidenced by bioturbational structures that show a decrease in size, penetration depth and diversity. Enlarged freshwater influx during glacial times fuelled primary production and led to estuarine circulation. The resultant oxygen minimum layer favored an increased deposition of organic matter that in turn affected sediment properties. During MIS 2 and 5b oxygenation was probably a little lower than today. During MIS 3 and 5d Chondrites-like burrows point to a stronger oxygen deficiency. During MIS 4 bottom water oxygenation was even lower and only biodeformational structures occur in soft sediment rich in benthic food. Based on these findings an ichnofabric model for food-rich, fairly soft deposits experiencing oxygen deficiency is suggested having biodeformational structures as low-oxygen end member. [Copyright &y& Elsevier]

Abstract: A detailed interbasinal study of calcareous nannofossils in surface sediments and their environmental preferences was conducted across the South China Sea, from the upwelling area off Vietnam to the non-upwelling area off Luzon, Philippines. Emiliania huxleyi, Florisphaera profunda and Gephyrocapsa oceanica are the dominant taxa in all the stations, comprising 72%–100% of the total nannofossil assemblage. In the shallow coastal waters and nearshore environments in the western portion, the dominant taxa are E. huxleyi and G. oceanica. Offshore, towards the central (abyssal) portion of the SCS, F. profunda dominates the nannofossil assemblage. Distribution patterns observed in this study, as well as the results of the principal components analysis, confirmed and established environmental preferences not only of the three major taxa, but of several minor taxa as well. Based on these results and cluster analysis of the samples, three assemblages were recognized in the study area: (a) upwelling, (b) oceanic and (c) deep basin assemblages. The upwelling assemblage is associated with the Vietnam and Sunda Shelves upwelling areas and is dominated by E. huxleyi and G. oceanica. The oceanic assemblage encompasses the outer shelf to slope environments and is characterized by the common occurrence of F. profunda and E. huxleyi (western study area) and F. profunda and G. oceanica (eastern study area). The deep basin assemblage includes the abyssal portion of the SCS (below the inferred lysocline and calcite compensation depth) and is dominated by F. profunda (>80%). It is proposed in the present study that F. profunda is resistant to dissolution. [Copyright &y& Elsevier]

The South China Sea offers an excellent case for studying source-to-sink transport processes of fluvial sediments among the global marginal seas. This study synthesizes existing clay mineralogical and geochemical data from ~ 1500 samples from the seafloor and surrounding rivers, deepwater mooring observation results, and high-resolution glacial–cyclic clay mineralogy records from six high-quality sediment cores. Source-to-sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales: modern process and Late Quaternary glacial cycles. The results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the South China Sea, e.g., dominant illite and chlorite in Taiwan, overwhelming majority of smectite in Luzon, dominant kaolinite in South China, and similar amounts of the four clay mineral species in Red and Mekong river systems. The formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the East Asian monsoon climate with warm temperature and high precipitation, and subordinately by tectonic activity and specific lithological character. The basin-wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects. The differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the South China Sea. Through combining clay mineralogical distributions on the seafloor with observed oceanic current systems, the modern transport pathways can be well established, e.g., in the northern South China Sea, illite and chlorite from Taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies. High-resolution sediment dynamic analysis for the Late Quaternary reveals different sediment transport patterns in the northern, western, and southern South China Sea when the land–sea configuration dramatically changed during glacial conditions. In the north, the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport. In the west, the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds. In the south, the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods. The transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern South China Sea. The relative contributions from three provenances (South China, Luzon, and Taiwan) highlight their distinct variability over the last 28 ka, implying that the southward shift of the inter-tropical convergence zone (ITCZ) at 16 ka BP caused an increased sediment contribution from South China, the intensified influence of the Kuroshio Current intrusion controls more efficient westward transport of Luzon sediments, and the stronger deepwater current transports Taiwan-sourced sediments further westward from the last glaciation to the Holocene. [ABSTRACT FROM AUTHOR]

• Eddy current controls the movement of particles within the mesoscale eddies. • The source funnel of sediment traps within the mesoscale eddy is tilted vertically. • A great number of particles can be transported from eddy's edge to the center. • Slow-sinking particles collected by traps could be originated from hundreds of kilometers away. The mooring tethered time-series sediment traps (TS-traps) collect sinking particles in the ocean, enabling the estimation of biological pump (BP) efficiency based on the assumption of vertical settling. However, the advection of sea water can disperse particles over long distance during the sinking process, introducing uncertainties into the estimates of BP efficiency. In the South China Sea, mesoscale eddies generate significant lateral transport above the mesopelagic zone, which has a large impact on the collecting area and source funnel of TS-traps. We studied the role of eddies in the lateral transport of sinking particles using a Lagrangian Particle Tracking Model (LPTM) which was forced by three-dimensional currents of eddy composites derived from the global Hybrid Coordinate Ocean Model (HYCOM). The eddy reanalysis using the HYCOM outputs was validated using occurrence, genesis and dissipation statistics of eddies. Backward particle tracking was performed at three stations located in the northern, central and southwestern SCS, respectively. The results indicate that the vertical sinking assumption is invalid when the particle sinking velocity is relatively low. Furthermore, the fraction of surface sources from the edge of the eddies depends on both the sinking velocity and seasonal variability of the background current. The eddy current controls the movement of particles within the eddy, and many of the particles with a sinking velocity <80 m d−1 can be transported from the edge to the eddy center. The source funnel within the mesoscale eddy is tilted vertically, leading to an asymmetrical distribution of the particle source around the trap location. Finally, for slow-sinking particles, trap-collected particles could originate from the subsurface located tens to hundreds of kilometers away from the trap location. Overall, our results highlight the role of mesoscale eddies in interpreting trap measurements. [ABSTRACT FROM AUTHOR]

The East Asian monsoon system influences the local oceanographic and climatic conditions of South China Sea (SCS), making the habitat depths of planktonic foraminifera (PF) specific for this region.The previous paleo-ocean reconstructions of SCS have been based on calcification habitat depths derived from other oceans, which may have affected the accuracy of interpretations. In this study, the calcification depths of Globigerinoides ruber , Globigerinoides sacculifer , and Neogloboquadrina dutertrei from sediment traps deployed at 1000 m depth off southwest Hainan Island from July 2012 to April 2013 were analyzed with regard to the stable isotopes of oxygen (δ18O) and carbon (δ13C) to calculate their regional apparent calcification depths (ACDs) with the overall aim to improve the reconstruction of the upper ocean thermal structure. ACDs and temperature estimated from foraminifera δ18O ranged from 0 to 42 m and 25°C for G. ruber , 30-65 m and 23.3°C for G. sacculifer , and 75-100 m and 20.3°C for N. dutertrei , respectively. The difference between the thermocline-species (N. dutertrei) and mixed-layer species (G. ruber and G. sacculifer) δ18O (∆δ18O Th–ML) and the temperature (∆T) shows water column stratification. The periods of lower and higher ∆T and ∆δ18O were in phase with the changes in regional overlying wind speed. Additionally, this study reveals that N. dutertrei lives and calcifies its shells within the base of the mixed-layer depth (MLD) and upper thermocline depth in SCS. Overall, the results from this study provide a new insight to understand the mixed-layer and thermocline depth in the northern SCS. • δ18O, ACD and temperature of the observed species tracked the water column. • We estimate ACD from δ18Ocalcite and δ18Oequil. at different water depths. • We infer Upper ocean mixing intensity from difference between ∆δ18O(Ml-Th) species. • We observe upwelling in summer and a little stratification in winter Nov. 19-Dec. 4. [ABSTRACT FROM AUTHOR]