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1. Early Cenozoic Decoupling of Climate and Carbonate Compensation Depth Trends

   https://doi.org/10.1029/2019PA003601  

   Greene, S. E.; Ridgwell, A.; Kirtland Turner, S.; Schmidt, D. N.; Pälike, H.; Thomas, E.; Greene, L. K.; Hoogakker, B. A. A. (June 2019, Paleoceanography and Paleoclimatology)

   Abstract Our understanding of the long‐term evolution of the Earth system is based on the assumption that terrestrial weathering rates should respond to, and hence help regulate, atmospheric CO₂and climate. Increased terrestrial weathering requires increased carbonate accumulation in marine sediments, which in turn is expected to result in a long‐term deepening of the carbonate compensation depth (CCD). Here, we critically assess this long‐term relationship between climate and carbon cycling. We generate a record of marine deep‐sea carbonate abundance from selected late Paleocene through early Eocene time slices to reconstruct the position of the CCD. Although our data set allows for a modest CCD deepening, we find no statistically significant change in the CCD despite >3 °C global warming, highlighting the need for additional deep‐sea constraints on carbonate accumulation. Using an Earth system model, we show that the impact of warming and increased weathering on the CCD can be obscured by the opposing influences of ocean circulation patterns and sedimentary respiration of organic matter. From our data synthesis and modeling, we suggest that observations of warming, declining δ¹³C and a relatively stable CCD can be broadly reproduced by mid‐Paleogene increases in volcanic CO₂outgassing and weathering. However, remaining data‐model discrepancies hint at missing processes in our model, most likely involving the preservation and burial of organic carbon. Our finding of a decoupling between the CCD and global marine carbonate burial rates means that considerable care is needed in attempting to use the CCD to directly gauge global carbonate burial rates and hence weathering rates. 

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2. Large Igneous Province Sulfur Emissions Have Long‐Term (>1000 Years) Effects on the Ocean Carbon Cycle

   https://doi.org/10.1029/2024GC011893  

   Cheong, Hee_Jun; Mittal, Tushar; Sprain, Courtney_Jean; Fendley, Isabel_M (March 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Large Igneous Province (LIP) eruptions are thought to have driven environmental and climate change over wide temporal scales ranging from a few to thousands of years. Since the radiative effects and atmospheric lifetime of carbon dioxide (CO₂, warming) and sulfur dioxide (SO₂, cooling) are very different, the conventional assumption has been to analyze the effects of CO₂and SO₂emissions separately and add them together afterward. In this study, we test this assumption by analyzing the joint effect of CO₂and SO₂on the marine carbonate cycle using a biogeochemical carbon cycle box model (Long‐term Ocean‐atmosphere‐Sediment CArbon cycle Reservoir Model). By performing model runs with very fine temporal resolution (∼0.1‐year timestep), we analyze the effects of LIP carbon and sulfur gas emissions on timescales ranging from an individual eruption (hundreds to thousands of years) to the entire long‐term carbon cycle (>100,000 years). We find that, contrary to previous work, sulfur emissions have significant long‐term (>1,000 years) effects on the marine carbon cycle (dissolved inorganic carbon, pH, alkalinity, and carbonate compensation depth). This is due to two processes: the strongly temperature‐dependent equilibrium coefficients for marine carbonate chemistry and the few thousand‐year timescale for ocean overturning circulation. Thus, the effects of volcanic sulfur are not simply additive to the impact of carbon emissions. We develop a causal mechanistic framework to visualize the feedbacks associated with combined carbon and sulfur emissions and the associated timescales. Our results provide a new perspective for understanding the complex feedback mechanisms controlling the environmental effects of large volcanic eruptions over Earth history. 

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3. Earth System Model Analysis of How Astronomical Forcing Is Imprinted Onto the Marine Geological Record: The Role of the Inorganic (Carbonate) Carbon Cycle and Feedbacks

   https://doi.org/10.1029/2020PA004090  

   Vervoort, Pam; Kirtland Turner, Sandra; Rochholz, Fiona; Ridgwell, Andy (September 2021, Paleoceanography and Paleoclimatology)

   Abstract Astronomical cycles are strongly expressed in marine geological records, providing important insights into Earth system dynamics and an invaluable means of constructing age models. However, how various astronomical periods are filtered by the Earth system and the mechanisms by which carbon reservoirs and climate components respond, particularly in absence of dynamic ice sheets, is unclear. Using an Earth system model that includes feedbacks between climate, ocean circulation, and inorganic (carbonate) carbon cycling relevant to geological timescales, we systematically explore the impact of astronomically modulated insolation forcing and its expression in model variables most comparable to key paleoceanographic proxies (temperature, the δ¹³C of inorganic carbon, and sedimentary carbonate content). Temperature predominately responds to short and long eccentricity and is little influenced by the modeled carbon cycle feedbacks. In contrast, the cycling of nutrients and carbon in the ocean generates significant precession power in atmospheric CO₂, benthic ocean δ¹³C, and sedimentary wt% CaCO₃, while inclusion of marine sedimentary and weathering processes shifts power to the long eccentricity period. Our simulations produce reducedpCO₂and dissolved inorganic carbon (DIC) δ¹³C at long eccentricity maxima and, contrary to early Cenozoic marine records, CaCO₃preservation in the model is enhanced during eccentricity‐modulated warmth. Additionally, the magnitude of δ¹³C variability simulated in our model underestimates marine proxy records. These model‐data discrepancies hint at the possibility that the Paleogene silicate weathering feedback was weaker than modeled here and that additional organic carbon cycle feedbacks are necessary to explain the full response of the Earth system to astronomical forcing. 

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4. Increased Biogenic Calcification and Burial Under Elevated p CO ₂ During the Miocene: A Model‐Data Comparison

   https://doi.org/10.1029/2022GB007541  

   Si, Weimin; Herbert, Timothy; Wu, Mengxi; Rosenthal, Yair (May 2023, Global Biogeochemical Cycles)

   Abstract Ocean acidification due to anthropogenic CO₂emission reduces ocean pH and carbonate saturation, with the projection that marine calcifiers and associated ecosystems will be negatively affected in the future. On longer time scale, however, recent studies of deep‐sea carbonate sediments suggest significantly increased carbonate production and burial in the open ocean during the warm Middle Miocene. Here, we present new model simulations in comparison to published Miocene carbonate accumulation rates to show that global biogenic carbonate production in the pelagic environment was approximately doubled relative to present‐day values when elevated atmosphericpCO₂led to substantial global warming ∼13–15 million years ago. Our analysis also finds that although high carbonate production was associated with high dissolution in the deep‐sea, net pelagic carbonate burial was approximately 30%–45% higher than modern. At the steady state of the long‐term carbon cycle, this requires an equivalent increase in riverine carbonate alkalinity influx during the Middle Miocene, attributable to enhanced chemical weathering under a warmer climate. Elevated biogenic carbonate production resulted in a Miocene ocean that had carbon (dissolved inorganic carbon) and alkalinity (total alkalinity) inventories similar to modern values but was poorly buffered and less saturated in both the surface and the deep ocean relative to modern. 

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5. Earth System Model Analysis of How Astronomical Forcing Is Imprinted Onto the Marine Geological Record: The Role of the Inorganic (Carbonate) Carbon Cycle and Feedbacks

   https://doi.org/10.1029/2023PA004826  

   Vervoort, P.; Kirtland Turner, S.; Rochholz, F.; Ridgwell, A. (March 2024, Paleoceanography and Paleoclimatology)

   Abstract Astronomical cycles are strongly expressed in marine geological records, providing important insights into Earth system dynamics and an invaluable means of constructing age models. However, how various astronomical periods are filtered by the Earth system and the mechanisms by which carbon reservoirs and climate components respond, particularly in absence of dynamic ice sheets, is unclear. Using an Earth system model that includes feedbacks between climate, ocean circulation, and inorganic (carbonate) carbon cycling relevant to geological timescales, we systematically explore the impact of astronomically‐modulated insolation forcing and its expression in model variables most comparable to key paleoceanographic proxies (temperature, the δ¹³C of inorganic carbon, and sedimentary carbonate content). Temperature predominately responds to obliquity and is little influenced by the modeled carbon cycle feedbacks. In contrast, the cycling of nutrients and carbon in the ocean generates significant precession power in atmospheric CO₂, benthic ocean δ¹³C, and sedimentary wt% CaCO₃, while inclusion of marine sedimentary and weathering processes shifts power to the long eccentricity period. Our simulations produce reducedpCO₂and dissolved inorganic carbon δ¹³C at long eccentricity maxima and, contrary to early Cenozoic marine records, CaCO₃preservation in the model is enhanced during eccentricity modulated warmth. Additionally, the magnitude of δ¹³C variability simulated in our model underestimates marine proxy records. These model‐data discrepancies hint at the possibility that the Paleogene silicate weathering feedback was weaker than modeled here and that additional organic carbon cycle feedbacks are necessary to explain the full response of the Earth system to astronomical forcing. 

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