Search

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]

East Antarctic ice sheet expansion (EAIE) at ∼13.9 Ma in the middle Miocene represents a major climatic event during the long-term Cenozoic cooling, but ocean circulation and carbon cycle changes during this event remain unclear. Here, we present new fish teeth isotope ( ε Nd) and benthic foraminiferal B/Ca records from the South China Sea (SCS), newly integrated meridional Pacific benthic foraminiferal δ 18 O and δ 13 C records and simulated results from a biogeochemical box model to explore the responses of deep Pacific Ocean circulation and carbon cycle across EAIE. The ε Nd and meridional benthic δ 13 C records reveal a more isolated Pacific Deep Water (PDW) and a sluggish Pacific meridional overturning circulation during the post-EAIE with respect to the pre-EAIE owing to weakened southern-sourced deep water formation. The deep-water [CO 2 − 3 ] and calcium carbonate mass accumulation rate in the SCS display markedly similar increases followed by recoveries to the pre-EAIE level during EAIE, which were probably caused by a shelf–basin shift of CaCO 3 deposition and strengthened weathering due to a sea level fall within EAIE. The model results show that the ∼1‰ positive δ 13 C excursion during EAIE could be attributed to increased weathering of high- δ 13 C shelf carbonates and a terrestrial carbon reservoir expansion. The drawdown of atmospheric CO 2 over the middle Miocene were probably caused by combined effects of increased shelf carbonate weathering, expanded land biosphere carbon storage and a sluggish deep Pacific meridional overturning circulation. [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]

The ~400-kyr period of marine carbon isotope records, in concert with the long eccentricity cycle, is a meaningful signature of the global carbon cycle, and this period was prolonged to ~500 kyr in the last ~1.6 Ma. The cause of the elongation of the long-eccentricity carbon cycle at ~1.6 Ma has not been fully understood, partly due to the conundrum of its relationship to changes in the circulation of deep-water masses. In this study, we present the benthic foraminifera δ13C and δ18O and the relative abundances of selected benthic foraminifera, from the upper ~80 m of Site U1489 of IODP Expedition 363. First, we established an age model for our samples, covering the last 4 Ma, based on the correlation of benthic foraminifera δ18O to the LR04 global stack as well as ship-board magneto-stratigraphy. Then we compared the δ18O and δ13C between Cibicidoides and Uvigerina to reconfirm the inter-genus isotopic adjustments. The coarse fraction, benthic foraminifera δ13C and the abundances of selected benthic species/genera show detectable 400–500 kyr cycles, with minimal coarse fraction and maximal δ13C around eccentricity minima. The variations in the relative abundances of selected benthic foraminifera, especially Favocassidulina favus , and the comparisons among deep-Pacific δ13C records, reveal a shift to a more sluggish mode of Pacific deep-water circulation occurring coincidently at ~1.6 Ma. Therefore, it may be implied that the lengthening of the long-term δ13C cycle at ~1.6 Ma is linked to a deep Pacific circulation change that prolonged the cycling of carbon in the ocean. • A first depth-age model of IODP U1489 is established by benthic foraminifera δ18O. • The benthic foraminifera δ13C and species abundances, and coarse fraction content show significant long eccentricity cycling. • The deep-Pacific circulation is inferred to shift to a more sluggish mode at ~1.6 Ma, which may be linked to the elongation of the long-term δ13C cycle. [ABSTRACT FROM AUTHOR]