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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]

Marine isotope stage (MIS) 16 (676-621 ka) and MIS 12 (478-424 ka) stand out as two significant glacial periods during the late Quaternary, demonstrating consistent continental ice volume or sea level, as discerned from benthic δ18O records. Intriguingly, the average sea surface temperature (SST) during MIS 16 surpasses that of MIS 12 by approximately 0.54–0.79 °C, while the atmospheric CO 2 concentration during MIS 16 is approximately 16 ppmv lower than that of MIS 12. The underlying mechanism driving these discrepancies remains enigmatic. In this study, we present new high-resolution X-ray fluorescence (XRF) and grain size records covering the interval from 700 to 400 ka from Ocean Drilling Program (ODP) Site 1090 in the South Atlantic Ocean. We also integrated benthic δ13C, ice-rafted debris (IRD) and carbonate ion concentration ([CO 3 2−]) records globally, to elucidate the distinct climate evolution during the MIS 16 versus MIS 12. Our findings suggest that MIS 12 is distinguished by a more substantial Antarctic ice-sheet, coupled with a weakened and/or equatorward displacement of southern westerly winds, and expansion of southern source water (SSW) alongside contractive northern source water (NSW). Conversely, the opposite climate state can be observed in MIS 16. The elevated atmospheric CO 2 in MIS 12 and diminished atmospheric CO 2 in MIS 16 may be attributed to reconfigurations in the ocean carbon reservoir regulated by the reorganization of deep-water masses and meridional ocean circulation, as a result of the divergent evolution of bipolar ice-sheets. Our study underscores the critical role of the interplay between the two polar regions in modulating deep-sea carbon storage on the orbital timescale and atmospheric CO 2. This interplay is essential for a comprehensive understanding of past and future carbon cycle adjustments in response to climate change. • We present a comparative analysis of MIS 16 and MIS 12 climates. • SST in MIS 16 surpasses that of MIS 12, yet CO 2 levels in MIS 16 are lower. • Grain size and XRF data from ODP Site 1090 (700-400 ka) are provided. • The reduced CO 2 in MIS 16, compared to MIS 12, is ascribed to bipolar carbon cycle control. [ABSTRACT FROM AUTHOR]

The Middle Miocene (~16–11.6 Ma) had atmospheric CO 2 levels comparable to the present day, and thus the mechanisms controlling the interactions of the climate system and the carbon cycle during the Middle Miocene may have significant implications for future climate change. In this study, we examined the phase relationship between benthic foraminiferal δ18O and δ13C records, within the 100-kyr band, during the Late Quaternary and the Middle Miocene. Our results suggest that benthic foraminiferal δ18O and δ13C were highly coherent and in-phase within the 100-kyr band during the Middle Miocene, in contrast to the anti-phased relationship during the Late Quaternary. However, the causes of this contrast remain elusive. We used a biogeochemical box model to explore the mechanisms of the in-phase relationship in the 100-kyr band during the Middle Miocene. The model results show that the in-phase relationship can be attributed to two mechanisms: (1) a shelf-basin carbonate shift regulated by sea-level fluctuations which originated from the growth and decay of the Antarctic ice sheet, within the 100-kyr band; and (2) changes in terrestrial carbon inputs and the biological pump driven by the discharge of riverine nutrients, which was regulated by precipitation variability in low latitudes, within the eccentricity-modulated precessional band. Thus, we propose that both high- and low-latitude processes played critical roles in the Middle Miocene carbon cycle, within the 100-kyr band. We further show that chemical weathering played a potential role in regulating the atmospheric p CO 2 on the orbital time scale during the Middle Miocene. • Phase relationships between benthic δ18O and δ13C records in the 100-kyr band are reinvestigated. • Terrestrial inputs explain the in-phase relationship of benthic δ18O and δ13C in the 100-kyr band over the Middle Miocene. • Both changes of Antarctic ice sheet and low-latitude precipitation played critical roles in the Middle Miocene carbon cycle. [ABSTRACT FROM AUTHOR]

[Display omitted] Global warming during the Miocene Climate Optimum (MCO, ∼17–14 million years ago) is associated with massive carbon emissions sourced from the flood basalt volcanism and ocean crustal production. However, the perturbation of tectonic carbon degassing on the interaction between climate change and carbon cycle remains unclear. Here, through time-evolutive phase analysis of new and published high-resolution benthic foraminiferal oxygen (δ 18O) and carbon (δ 13C) isotope records from the global ocean, we find that variations in the marine carbon cycle lead the climate-cryosphere system (δ 13C-lead- δ 18O) on 405,000-year eccentricity timescales during the MCO. This is in contrast to the previously reported climate-lead-carbon (δ 18O-lead- δ 13C) scenario during most of the Oligo-Miocene (∼34–6 million years ago). Further sensitivity analysis and model simulations suggest that the elevated atmospheric CO 2 concentrations and the resulting greenhouse effect strengthened the low-latitude hydrological cycle during the MCO, accelerating the response of marine carbon cycle to eccentricity forcing. Tropical climate processes played a more important role in regulating carbon-cycle variations when Earth's climate was in a warm regime, as opposed to the dominant influence of polar ice-sheet dynamics during the Plio-Pleistocene (after ∼6 million years ago). [ABSTRACT FROM AUTHOR]