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The carbon isotope (δ13C) in marine biogenic carbonate is a valuable tool for comprehending the marine carbon cycle. However, the absence of monthly resolved δ13C records before the instrumental era (since 1850) hinders us to fully understand the global carbon cycle. In this study, we present a δ13C record from a modern Tridacna gigas shell (δ13Cshell) collected from the southern South China Sea (SCS) with an average resolution of ∼10 days. The δ13C record illustrates seasonal and interannual cycles which are superimposed by long‐term decreasing trend. We investigate the environmental controls influencing the δ13Cshell on different time scales. We suggest that more positive δ13Cshell values observed during the juvenile stage (first 8–10 years), which correspond to a high growth rate, can be attributed to 13C enrichment of the internal dissolved inorganic carbon pool. This resulted from the exponential increase of symbiotic zooxanthellae, and preferential 12C sequestration by the accelerated buildup of productive tissues. On the interannual time scale, changes in δ13Cshell were likely affected by both internal (vital effect) and external processes (sea water δ13C), as well as being disturbed by the super El Niño‐La Niña event. Seasonal changes of δ13Cshell were mainly dominated by the intensity of photosynthesis and respiration of symbiotic zooxanthellae, which depend on the seasonal solar irradiance variations in association with precipitation and the South Asian summer monsoon. Our study sheds light on the controlling factors of δ13Cshell variations on seasonal timescale and the long‐term trend, providing insight into the carbon cycle over geological history. Plain Language Summary: Tridacna is a marine bivalve that forms symbiotic zooxanthellae in its mantle tissues and has a hard calcium shell. In this study, we analyzed the carbon isotope composition of the Tridacna shell collected from the southern South China Sea and compared it with various climatic and oceanographic records. Our results suggest that the variability of carbon isotope in the Tridacna shell is predominantly controlled by the activity of its symbiotic zooxanthellae in association with solar radiation and growth rate on various time scales. Furthermore, we found that changes in the carbon isotope of Tridacna shell can be influenced by fluctuations in the South Asian summer monsoon and strong El Niño‐La Niña events. Our study sheds light on the factors that control carbon isotope in the Tridacna shell on different time scales, thereby providing valuable insights into the carbon cycle and climate history. Key Points: A high‐resolution δ13C record of Tridacna shell from the southern South China Sea was presentedActivity and density of symbiotic zooxanthellae linked to the solar irradiance dominated the δ13C changes of Tridacna shellsChanges of δ13C of Tridacna shells are also affected by the South Asian summer monsoon and super El Niño‐La Niña event [ABSTRACT FROM AUTHOR]

• Seasonal variability of dissolved organic carbon (DOC) transport through major straits of the South China Sea (SCS) is largely controlled by the East Asian monsoon. • Cross-isobath transport is determined by both the pressure gradient along the isobath and the Ekman transport, with the contribution of the latter increasing shoreward. • DOC is transported into the SCS via the Luzon Strait and out via the Taiwan, Karimata and Mindoro straits. • A net export of labile and semilabile DOC from the SCS is quantified. The South China Sea (SCS) is the largest semienclosed marginal sea in the western Pacific. The advective supply of dissolved organic carbon (DOC) from the open ocean to the SCS influences its carbon cycle and biogeochemical processes. However, quantitative studies of DOC exchange across different straits and the shelf to the SCS are particularly limited. In this study, we investigated seasonal DOC transport across four major straits and cross-shelf breaks using a coupled physical–biogeochemical model. The modeling results show that the seasonal variations in DOC transport through the major straits are largely controlled by the monsoon. In winter, associated with the northeast monsoon, the magnitudes of DOC transport from the four straits are generally larger than those in summer with the southwest monsoon. DOC transport is eastward at the Luzon Strait and northward at the Taiwan and Karimata straits during the southwest monsoon and changes direction during the northeast monsoon. The annual mean of depth-integrated DOC transport is 86.7 Tg C yr−1 westward at the Luzon Strait, 19.1 Tg C yr−1 northward at the Taiwan Strait, 25.9 Tg C yr−1 southward at the Karimata Strait, and 46.0 Tg C yr−1 southward at the Mindoro Strait. DOC transport across the shelf break is a result of geostrophic or quasigeostrophic balance. In the north and west, the annual net cross-shelf break transports are onshore with magnitudes of 8.8 and 31.1 Tg C yr−1, respectively. In the southern SCS, the annual net transport is offshore with a magnitude of 18.3 Tg C yr−1. The net export of total labile and semilabile DOC from the SCS has a flux of 10.7 Tg C yr−1. This study provides seasonal budgets of DOC exchange between the SCS and its adjacent oceans, which contribute to a better evaluation of the carbon cycle in the SCS. [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]

Atmospheric CO 2 and global climate are closely coupled. Since 800 ka CO 2 concentrations have been up to 50% higher during interglacial compared to glacial periods. Because of its dependence on temperature, humidity, and erosion rates, chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition. Cooler and drier conditions and lower non-glacial erosion rates suppressed in situ chemical weathering rates during glacial periods. However, using systematic variations in major element geochemistry, Sr–Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea spanning the last 1.1 Ma, we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. We attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. Our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for ∼9% of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial–interglacial variation of ∼80 ppmv. As a result, if similar magnitudes can be identified in other tropical shelf-slope systems, the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric CO 2 during cold stages of the Quaternary. This in turn would have caused an intensification of glacial cycles. [ABSTRACT FROM AUTHOR]

While the benthic δ 18 O records from many South China Sea sites mimic the SPECMAP/LR04 standard, their paired planktic δ 18 O curves differ by comparable values at negative peaks corresponding to interstadials, especially between MIS 5.1, 5.3, 5.5, 6.5, and 7.3. Similar planktic δ 18 O records also occur in some other low-latitude oceans under monsoon influence. Because neither temperature nor salinity effects can fully account for the variations of such δ 18 O records after derivation from the SPECMAP/LR04 standard, variations in the rain water δ 18 O affected by regional hydroclimate changes are considered to have played an important role. In contrast to the SPECMAP/LR04 standard δ 18 O curve prevailed by 100-kyr cycles, these regional planktic δ 18 O curves are distinguished by 20-kyr precession signal, showing partial similarity to the δ 18 O records of atmospheric oxygen in polar ice-core bubbles and in stalagmite calcite. We speculate that the common features of these three independent δ 18 O records are indicative of the effect of evolving hydrological cycling driven by monsoon circulation in low latitudes. [ABSTRACT FROM AUTHOR]