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Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth. A detailed reconstruction of the calcium carbonate compensation depth—at which calcium carbonate is dissolved—in the equatorial Pacific Ocean over the past 53 million years shows that it tracks ocean cooling, increasing as the ocean cools. A history of carbon cycles and climate change The carbonate compensation depth — the oceanic depth at which carbonate is dissolved — reflects the amount of carbon dioxide present in the atmosphere, and thus gives clues about climate on geological timescales. This paper reports a detailed reconstruction of the carbonate compensation depth in the equatorial Pacific over the past 53 million years. The compensation depth is found to track ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (56–53 million years ago) to 4.6 kilometres today. Rapid fluctuations observed in the carbonate compensation depth around 46–34 million years ago could be explained, in part, by changes in weathering and in the type of organic carbon supplied to the sea floor.

Large-igneous-province volcanic activity during the mid-Cretaceous triggered a global-scale episode of reduced marine oxygen levels known as Oceanic Anoxic Event 2 approximately 94.5 million years ago. It has been hypothesized that this geologically rapid degassing of volcanic carbon dioxide altered seawater carbonate chemistry, affecting marine ecosystems, geochemical cycles and sedimentation. Here we report on two sites drilled by the International Ocean Discovery Program offshore of southwest Australia that exhibit clear evidence for suppressed pelagic carbonate sedimentation in the form of a stratigraphic interval barren of carbonate minerals, recording ocean acidification during the event. We then use the osmium isotopic composition of bulk sediments to directly link this protracted ~600 kyr shoaling of the marine calcite compensation depth to the onset of volcanic activity. This decrease in marine pH was prolonged by biogeochemical feedbacks in highly productive regions where elevated heterotrophic respiration added carbon dioxide to the water column. A compilation of mid-Cretaceous marine stratigraphic records reveals a contemporaneous decrease of sedimentary carbonate content at continental slope sites globally. Thus, we contend that changes in marine carbonate chemistry are a primary ecological stress and important consequence of rapid emission of carbon dioxide during many large-igneous-province eruptions in the geologic past.Volcanic activity led to ocean acidification at the onset of Oceanic Anoxic Event 2, which then persisted for 600,000 years due to biogeochemical feedbacks, according to marine osmium isotope and carbonate sedimentation records offshore from southwest Australia.

The globally averaged calcite compensation depth has deepened by several hundred metres in the past 15 Myr. This deepening has previously been interpreted to reflect increased alkalinity supply to the ocean driven by enhanced continental weathering due to the Himalayan orogeny during the late Neogene period. Here we examine mass accumulation rates of the main marine calcifying groups and show that global accumulation of pelagic carbonates has decreased from the late Miocene epoch to the late Pleistocene epoch even though CaCO3 preservation has improved, suggesting a decrease in weathering alkalinity input to the ocean, thus opposing expectations from the Himalayan uplift hypothesis. Instead, changes in relative contributions of coccoliths and planktonic foraminifera to the pelagic carbonates in relative shallow sites, where dissolution has not taken its toll, suggest that coccolith production in the euphotic zone decreased concomitantly with the reduction in weathering alkalinity inputs as registered by the decline in pelagic carbonate accumulation. Our work highlights a mechanism whereby, in addition to deep-sea dissolution, changes in marine calcification acted to modulate carbonate compensation in response to reduced weathering linked to the late Neogene cooling and decline in atmospheric partial pressure of carbon dioxide.

The Atlantic is the only ocean basin almost entirely surrounded by passive margins, and a major global long‐term sink of carbonate carbon that has evaded subduction. Quantifying the history of carbonate accumulation in the Atlantic has been limited by the absence of well‐defined regional carbonate compensation depth (CCD) models. We determine the CCD for the northern North Atlantic, central North Atlantic, and South Atlantic, and use these reconstructions to compute the carbonate carbon mass and carbonate carbon flux in a tectonic framework at 0.5 m.y. intervals since 66 Ma. We find that the total carbonate carbon mass of the Atlantic has grown 2.5‐fold from ∼1,500 Mt at 66 Ma to ∼3,800 Mt at present day. The overall Cenozoic increase in carbonate carbon flux toward the present day is punctuated by “carbonate crash” phases in the mid‐Eocene at ∼44–38 Ma and in the mid‐late Miocene at ∼19–8 Ma. During these times the flux decreases from ∼45 to ∼25 Mt C/yr, likely caused by carbonate dissolution and reductions in productivity. Reduced carbonate carbon flux in the mid‐Eocene also coincides with reduced calcification rates of small coccolithophores previously observed offshore Africa. After ∼8 Ma the carbonate carbon flux rises to a Cenozoic maximum of ∼75 Mt C/yr at ∼3 Ma, possibly driven by enhanced flux of nutrients into the ocean. Our CCD curves and the resulting carbonate accumulation history are useful for calibrating ocean chemistry models, and constraining global terrestrial weathering rates, climate perturbations, and carbon cycle models. Key Points Carbonate carbon flux and storage has been computed for the Atlantic Ocean spanning the entire Cenozoic at 0.5 m.y. intervals The total carbonate carbon mass of the Atlantic has grown 2.5‐fold from ∼1,500 Mt at 66 Ma to ∼3,800 Mt at present day Carbonate carbon flux fluctuations are linked to carbonate crash and bloom phases, and changes in deep‐water circulation

During the Eocene‐Oligocene Transition (ca. 34 Ma), the Earth underwent a dramatic decline in atmospheric CO2, global cooling, a deepening of the carbonate compensation depth (CCD), and the formation of a permanent ice sheet on Antarctica. The expansion of Antarctic glaciers eroded the underlying bedrock and increased the weathering flux to the ocean. However, the role silicate and carbonate weathering play in atmospheric CO2 removal and the CCD through Ca2+ and alkalinity production is poorly understood. Magnesium isotopes (δ26Mg) are fractionated during carbonate and clay mineral formation and can be used to quantify the relative flux from silicate and carbonate weathering. Here, we report the δ26Mg composition of the carbonate, reactive (ferromanganese coatings), and residual (silicate) fraction of marine sediments from the Kerguelen Plateau (Ocean Drilling Program Site 738), near a major drainage system of the East Antarctic Ice Sheet, to explore the response of subglacial and shelf weathering to ice sheet expansion. The δ26Mg of the carbonate fraction (−2.29‰ to −0.95‰), reactive fraction (−0.36‰ to 0.10‰), and residual fraction (−0.05‰ to 0.55‰) display similar values to surface‐dwelling calcareous nannofossils, deep‐water ferromanganese nodules, and Antarctic bedrock, respectively. Isotope fluctuations in all three phases suggest that the formation of the Antarctic ice sheet drove efficient chemical weathering of underlying silicate bedrock, which was rapidly transported to the Southern Ocean, resulting in further CO2 drawdown, while a local sea‐level low stand exposed carbonates on the Antarctic continental shelf to weathering, contributing to a deepening of the CCD.

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 CO2 and SO2 emissions separately and add them together afterward. In this study, we test this assumption by analyzing the joint effect of CO2 and SO2 on 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. Plain Language Summary Large Igneous Province (LIP) eruptions are among the largest volcanic events in Earth history and have been linked with environmental catastrophes such as mass extinctions and oceanic anoxic events. One of the main ways these volcanic events affect the environment is through the emission of climate‐active gases, primarily carbon dioxide (CO2) and sulfur dioxide (SO2). These gases are often thought of as behaving independently, as CO2 causes long‐term climate warming, while SO2, which turns into sulfate aerosols, causes short‐term climate cooling. However, in addition to directly causing climate change, both gases also cause more complex environmental changes, including changes to the ocean carbon cycle (e.g., ocean acidification and the amount and chemical species of dissolved carbon). Our study uses a long‐term marine carbon cycle box model to investigate these complex effects. We find that the assumption that the effects of each type of gas are independent is not accurate. Instead, we show that the carbon‐cycle effects of sulfur emissions, in particular, can persist on long timescales (>1,000 years) in addition to short‐term cooling. Our results provide a new perspective for understanding the environmental effects of large volcanic eruptions over Earth history. Key Points Volcanic CO2 and SO2 emissions have complex and interconnected effects on the ocean‐atmosphere system and biosphere Results show sulfate aerosol driven cooling has long‐lasting (>1,000 years) effects on the ocean carbon cycle Conventional assumption of strict timescale separation between the effects of volcanic CO2 and SO2 emissions is incorrect

Valdivia Bank (VB) is an oceanic plateau in the South Atlantic that formed from hotspot‐ridge volcanism during the Late Cretaceous at the Mid‐Atlantic Ridge (MAR). It is part of Walvis Ridge (WR), a quasi‐linear seamount chain extending from offshore Namibia to Tristan da Cunha and Gough Islands. To understand Valdivia Bank evolution, we interpret the seismic stratigraphy from multichannel seismic data paired with coring results from International Ocean Discovery Program (IODP) Expedition 391, which recovered mostly pelagic nannofossil ooze and chalks. The seismic section can be divided into three seismic units (SU), a lower transparent interval which is faulted and conforms to basement, a middle, moderate to high amplitude interval which is thick in local depocenters such as rifts, and an upper, subparallel transparent interval. Notable features include regional unconformities, dipping clinoforms, mass transport and contourite deposits, and volcanic structures. Additionally, three infilled rifts are observed across the plateau. Our analysis implies that following a period of sedimentation in the Campanian, the edifice was faulted through the Paleocene, coinciding with a South Atlantic tectonic reorganization. Local depocenters formed as a result of rifting. Subsequently, the plateau experienced thermal rejuvenation and regional uplift during the Eocene. Volcanic mounds were emplaced atop Cretaceous sediments and intrusives were emplaced within the sediments. During the Cenozoic, sedimentation was punctuated, likely in response to changes in the carbonate compensation depth and bottom current intensification. VB sedimentation was complex and largely influenced by the paleoceanographic context of the plateau, as well as thermal rejuvenation and tectonism. Plain Language Summary Valdivia Bank is an oceanic plateau in the South Atlantic that formed by hotspot volcanism at the MAR. It is part of Walvis Ridge, a chain of submarine mountains extending from offshore Namibia to Tristan and Gough Islands. To understand the formation and history of the plateau, we interpret seismic data which image the sub‐seabed sediments, and pair observations with drilling results from International Ocean Discovery Program (IODP) Expedition 391. Sediments on the plateau are subdivided into three seismic layers of mainly oozes and chalks, which are faulted in the bottom layer. Other observed features include gaps in the deposition of sediment, dipping formations, submarine landslides and ocean current deposits, and volcanoes. Three rifts are also observed which are filled by sediment. Our analysis suggests that after a period of sediment buildup in the Late Cretaceous, the plateau was faulted. Later volcanic activity uplifted the plateau and formed volcanoes in the Eocene. During the Cenozoic era, sedimentation was mostly intermittent and influenced by ocean conditions and currents. Valdivia Bank sedimentation was complex, influenced by changes in ocean conditions around the plateau, volcanic activity and tectonism. Key Points Seismic stratigraphy observations suggest rejuvenated magmatism led to widespread plateau uplift and erosion as an Eocene island The basement and lower sediments are offset by extensional faults, indicating faulting from the Late Cretaceous to Paleocene Seismic profiles show carbonate accumulation, sediment movement, and contourite deposition

The Carbonate Compensation Depth (CCD) refers to the depth within the ocean where the production and dissolution rates of carbonates reach equilibrium, widely likened to the oceanic calcareous ‘snowline’. The reconstruction of deep-time CCD has significant implications for understanding ocean circulation, seawater chemical conditions, sediment distribution, and the surface carbon cycle. This paper critically reviews the methods for CCD reconstruction, summarizes the driving mechanisms of the Cenozoic CCD evolution and its association with the carbon cycle, and offers insights into future directions for CCD research. CCD reconstruction has evolved over the past half century from early qualitative to quantitative methods. These methodological improvements have markedly improved the accuracy and resolution of CCD. Existing studies have indicated a general trend of the CCD deepening across major ocean basins since the Cenozoic, interspersed with a minor shallowing phase during the mid-Miocene. The variations in the CCD are primarily influenced by factors such as ocean productivity, weathering, and shelf-basin partitioning. During climate events such as the Paleocene-Eocene Thermal Maximum, the CCD exhibits pulselike fluctuations. Future research should focus on precision and quantification while integrating model simulations to further explore the correlations and response mechanisms between the CCD and the paleoclimate as well as the carbon cycle.

Coherent variation in CaCO3 burial is a feature of the Cenozoic eastern equatorial Pacific. Nevertheless, there has been a long-standing ambiguity in whether changes in CaCO3 dissolution or changes in equatorial primary production might cause the variability. Since productivity and dissolution leave distinctive regional signals, a regional synthesis of data using updated age models and high-resolution stratigraphic correlation is an important constraint to distinguish between dissolution and production as factors that cause low CaCO3. Furthermore, the new chronostratigraphy is an important foundation for future paleoceanographic studies. The ability to distinguish between primary production and dissolution is also important to establish a regional carbonate compensation depth (CCD). We report late Miocene to Holocene time series of XRF-derived (X-ray fluorescence) bulk sediment composition and mass accumulation rates (MARs) from eastern equatorial Pacific Integrated Ocean Drilling Program (IODP) sites U1335, U1337, and U1338 and Ocean Drilling Program (ODP) site 849, and we also report bulk-density-derived CaCO3 MARs at ODP sites 848, 850, and 851. We use physical properties, XRF bulk chemical scans, and images along with available chronostratigraphy to intercorrelate records in depth space. We then apply a new equatorial Pacific age model to create correlated age records for the last 8 Myr with resolutions of 1–2 kyr. Large magnitude changes in CaCO3 and bio-SiO2 (biogenic opal) MARs occurred within that time period but clay deposition has remained relatively constant, indicating that changes in Fe deposition from dust is only a secondary feedback to equatorial productivity. Because clay deposition is relatively constant, ratios of CaCO3 % or biogenic SiO2 % to clay emulate changes in biogenic MAR. We define five major Pliocene–Pleistocene low CaCO3 % (PPLC) intervals since 5.3 Ma. Two were caused primarily by high bio-SiO2 burial that diluted CaCO3 (PPLC-2, 1685–2135 ka, and PPLC-5, 4465–4737 ka), while three were caused by enhanced dissolution of CaCO3 (PPLC-1, 51–402 ka, PPLC-3, 2248–2684 ka, and PPLC-4, 2915–4093 ka). Regional patterns of CaCO3 % minima can distinguish between low CaCO3 caused by high diatom bio-SiO2 dilution versus lows caused by high CaCO3 dissolution. CaCO3 dissolution can be confirmed through scanning XRF measurements of Ba. High diatom production causes lowest CaCO3 % within the equatorial high productivity zone, while higher dissolution causes lowest CaCO3 percent at higher latitudes where CaCO3 production is lower. The two diatom production intervals, PPLC-2 and PPLC-5, have different geographic footprints from each other because of regional changes in eastern Pacific nutrient storage after the closure of the Central American Seaway. Because of the regional variability in carbonate production and sedimentation, the carbonate compensation depth (CCD) approach is only useful to examine large changes in CaCO3 dissolution.

At the beginning of the Cenozoic, the atmospheric CO 2 concentration increased rapidly from ~2000 ppmv at 60 Ma to ~4600 ppmv at 51 Ma, which is 5–10 times higher than the present value, and then continuous declined from ~51 to 34 Ma. The cause of this phenomenon is still not well understood. In this study, we demonstrate that the initiation of Cenozoic west Pacific plate subduction, triggered by the hard collision in the Tibetan Plateau, occurred at approximately 51 Ma, coinciding with the tipping point. The water depths of the Pacific subduction zones are mostly below the carbonate compensation depths, while those of the Neo-Tethys were much shallower before the collision and caused far more carbonate subducting. Additionally, more volcanic ashes erupted from the west Pacific subduction zones, which consume CO 2 . The average annual west Pacific volvano eruption is 1.11 km 3 , which is higher than previous estimations. The amount of annual CO 2 absorbed by chemical weathering of additional west Pacific volcanic ashes could be comparable to the silicate weathering by the global river. We propose that the initiation of the western Pacific subduction controlled the long-term reduction of atmospheric CO 2  concentration.