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Lateral particle transport in shelf/slope settings of marginal sea systems may potentially supply allochthonous particulate organic carbon (POC) to the ocean interior. The magnitude and prevalence of such processes has implications for our understanding of the functioning and efficiency of modern biological carbon pump processes, as well as for interpretation of deep ocean sediment records. Yet, ages and relative contribution of POC from different sources to deep ocean basins are much less understood. Here, based on radiocarbon (Fm, fraction modern), stable carbon (δ13Corg), and other geochemical analysis of particles intercepted by time‐series sediment traps deployed at three different depths (1,000, 2,150, and 3,200 m), we constrain the source of the sinking POC in the deep basin of northern South China Sea (SCS). The results show that the modern POC derived from surface ocean productivity, acting as the vertical vector, accounts for 87% ± 4% of sinking POC on average, while the lateral vector (aged POC hosted on resuspended sediment) comprises the remainder (13%). The contribution of laterally supplied POC increases with depth and is composed of aged POC derived from several sources. The majority stems from sediment resuspension of the northeastern SCS slope, that entrains aged marine POC (dominant), fossil OC, and soil OC (least significant). The diminished proportion of fossil OC in deep basin sinking POC relative to Taiwan‐proximal areas is likely due to its dispersion via resuspension–deposition loops and dilution by POC stemming from modern surface ocean productivity during the delivery. Plain Language Summary: Quantification of fresh and aged sinking particulate organic carbon (POC) and their vertical and lateral transfer in the marginal sea systems is crucial to budget the global carbon cycle. The South China Sea (SCS) is one of the largest marginal seas in the world, the ages, and relative contribution of POC from different sources to deep basins are poorly constrained. In this study, we provide a first assessment on the age and source of sinking POC in the deep northern SCS basin based on radiocarbon (Fm), stable carbon (δ13Corg), and other geochemical parameters. The results show that 87% ± 4% of sinking POC in the deep basin (1,000–3200 m) derives from modern surface ocean productivity via vertical export. The aged POC supplied via lateral transport has minimal impact on the fluxes and temporal dynamics of the sinking POC. However, the contribution of laterally supplied POC increases with depth and is composed of aged POC derived from several sources: aged marine POC, fossil OC, and soil OC. The importance of laterally supplied POC and complexity of aged POC sources increase with depth, underlining the importance of considering sedimentary OC resuspension and dispersion in bathypelagic and sedimentary records of these dynamic marginal sea systems. Key Points: Multigeochemical data are used to constrain the age and source of sinking particulate organic carbon (POC) in the northern South China SeaThe sinking POC is dominated by vertically settled fresh marine POC, while the aged POC from lateral transport makes a minor contributionThe aged POC is composed of aged marine POC, fossil OC, and soil OC, its contribution and complexity increase with depth [ABSTRACT FROM AUTHOR]

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]

Deep ocean microorganisms consume particulate organic matter that is produced in the surface ocean and exported to deeper depths. Such consumption not only enriches inorganic carbon in the deep ocean but also transforms organic carbon into recalcitrant forms, creating an alternative type of carbon sequestration. However, estimates of deep microbial carbon demand substantially exceed the available particulate organic carbon exported from the euphotic zone, resulting in an unbalanced dark ocean carbon budget. Here, we combined field‐based microbial activity parameters, integrated multiyear particle export flux data, sinking particle fluxes measured by sediment traps, and optical data from Biogeochemical‐Argo floats to quantify the main sources of organic carbon to the dark ocean. Laterally transported particles (including sinking and suspended particles) serve as a major energy source, which directly provide organic carbon and enhance new organic carbon production by dark carbon fixation, reconciling the mismatch in the regional carbon budget. Plain Language Summary: Particulate organic matter, produced by phytoplankton in the upper ocean, can sink through the water column and act as a source of organic matter to the deep ocean. These particles are decomposed to carbon dioxide by microorganisms, resulting in dissolved inorganic carbon and organic carbon resistant to decomposition in the deeper ocean. This process controls the biological sequestration of CO2 by the oceans. However, there is an imbalance between the low amount of organic carbon exported from the photic zone and the high microbial demand for carbon in the dark ocean. We attempted to explain how the deep ocean carbon and energy supply can meet the microbial metabolic demand. Four main organic carbon sources were measured and quantified in the South China Sea: particles that come from the photic zone, particles that move laterally through the ocean, dark carbon fixation, and dissolved organic carbon. We found that laterally transported particles from the surrounding margins provide a direct source of organic carbon and also allow for much new organic carbon production through dark carbon fixation. These particles, which provide a major energy source to dark ocean ecosystems, help resolve the mismatch in the regional carbon budget. Key Points: Data from Biogeochemical‐Argo floats showed direct evidence of episodic pulses of laterally transported particles into the deep seaLaterally transported particles are a direct organic carbon source and also enhance new organic carbon production by dark carbon fixationLaterally transported particles provide major carbon and energy for deep ecosystems, reconciling the mismatch in the regional carbon budget [ABSTRACT FROM AUTHOR]