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Variations in the Atlantic Meridional Overturning Circulation (AMOC) redistribute heat and nutrients, causing pronounced anomalies of temperature and nutrient concentrations in the subsurface ocean. However, exactly how millennial‐scale deglacial AMOC variability influenced the subsurface is debated, and the role of other deglacial forcings of subsurface temperature change is unclear. Here, we present a new deglacial temperature reconstruction, which, with published records, helps assess competing hypotheses for deglacial warming in the upper tropical North Atlantic. Our record provides new evidence of regional subsurface warming in the western tropical North Atlantic within the core of modern Antarctic Intermediate Water (AAIW) during Heinrich Stadial 1 (HS1), an early deglacial interval of iceberg discharge into the North Atlantic. Our results are consistent with model simulations that suggest subsurface heat accumulates in the northern high‐latitude convection regions and along the upper AMOC return path when the AMOC weakens, and with warming due to rising greenhouse gases. Warming of AAIW may have also contributed to warming in the tropics at modern AAIW depths during late HS1. Nutrient and ΔCO32− ${\Delta }\left[{\mathrm{C}\mathrm{O}}_{3}^{2-}\right]$ reconstructions from the same site suggest a link between AMOC intensity and the northward extent of AAIW in the northern tropics across the deglaciation and on millennial time scales. However, the timing of the initial deglacial increase in AAIW to the northern tropics is ambiguous. Deglacial trends and variability of ΔCO32− ${\Delta }\left[{\mathrm{C}\mathrm{O}}_{3}^{2-}\right]$ in the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is characterized by northward flow in the upper ocean and southward flow in the deep ocean. Understanding how the AMOC has changed in the past, and how such changes have affected surface climate and the distribution of ocean heat, carbon, and nutrients is important but challenging, as reconstructions of subsurface ocean properties are sometimes ambiguous. Here, we use the chemical composition of seafloor shells from a site in the western tropical Atlantic Ocean at ∼950 m water depth, within the northward‐flowing limb of the AMOC, to reconstruct temperature, nutrients, and carbon content during the end of the last Ice Age, an interval when AMOC strength is believed to have varied. Our results support a link between AMOC strength and tropical Atlantic nutrient content, and further suggest that both rising atmospheric CO2 and AMOC variations influenced temperatures and carbon in the subsurface tropical Atlantic Ocean. Key Points: Regional warming occurred in the tropical Atlantic at Antarctic Intermediate Water (AAIW) depths during Heinrich Stadial 1Deglacial nutrient and ΔCO32− ${\Delta }\left[{\mathrm{C}\mathrm{O}}_{3}^{2-}\right]$ trends and variability suggest a strong link between Atlantic Meridional Overturning Circulation intensity and AAIW northward extentDeglacial ΔCO32− ${\Delta }\left[{\mathrm{C}\mathrm{O}}_{3}^{2-}\right]$ trends and variability likely affected upper Atlantic temperature estimates based on Mg/Ca of foraminifera [ABSTRACT FROM AUTHOR]

Seawater carbonate chemistry links Earth's climate and carbon cycle through the production and preservation of carbonate sediments. Models and carbonate facies abundance records have generated hypotheses about trajectories of seawater carbonate chemistry, including responses to key events in the evolutionary history of carbonate biomineralizers. However, tests of these hypotheses have remained elusive. We applied a novel proxy for the carbonate mineral saturation state (Ω) of seawater based on the diameters of ooids—concentrically‐coated carbonate sand grains—to estimate Ω, dissolved inorganic carbon, alkalinity, and pH of seawater spanning Phanerozoic time. Reconstructed Ω values decreased sharply around ∼120 Ma, which we interpret as the fingerprint of the Mid‐Mesozoic Revolution of planktic calcifiers. Shifts in Ω across Ordovician time also suggest a possible causal relationship with the Great Ordovician Biodiversification Event. Our results demonstrate that ooid sizes are a useful tool for reconstructing Earth's ancient carbon cycle. Plain Language Summary: Earth's oceans play an important role in removing carbon from Earth's surface environments. One of the ways this happens is through the production and burial of calcium carbonate sediments, which include shells and calcium carbonate minerals formed in other ways. The chemistry of the oceans, including pH and the concentrations of carbonate ions, affect how easy it is for carbonate minerals to form, and whether they can survive long enough to be permanently buried on the seafloor. Tracking these aspects of the chemistry of ancient oceans can enable scientists to better understand the balance of carbon entering and exiting Earth's surface environments in the past. Until recently, we have not had the right tools to extract this information from sedimentary rocks. Here, we used a recently developed method that uses the sizes of ooids—sand grains that grow by accumulating concentric layers of calcium carbonate—to track ancient ocean carbonate chemistry. We compared the results of our new approach with previous efforts and found that our approach works well. Our work demonstrates that ooid size measurements can improve our understanding of ancient oceans. Key Points: We estimated seawater calcite saturation states spanning Phanerozoic time using a proxy based on ooid sizeWe combined our calcite saturation state values with pCO2 and seawater chemistry proxy data to calculate seawater carbonate chemistryDissolved inorganic carbon, alkalinity, and pH estimates based on the ooid size proxy agree with predictions from carbon cycle models and carbonate facies records [ABSTRACT FROM AUTHOR]

Large rivers export a large amount of dissolved inorganic carbon (DIC) and nutrients to continental shelves; and subsequent river-to-sea mixing, eutrophication, and seasonal hypoxia (dissolved oxygen < 2 mg⋅L–1) can further modify DIC and nutrient distributions and fluxes. However, quantitative studies of seasonal carbonate variations on shelves are still insufficient. We collected total alkalinity (TA), DIC, and NO3– data from nine cruises conducted between 2006 and 2010 on the northern Gulf of Mexico continental shelf, an area strongly influenced by the Mississippi and Atchafalaya Rivers. We applied a three-end-member model (based on salinity and potential alkalinity) to our data to remove the contribution of physical mixing to DIC and nitrate distribution patterns and to derive the net in situ removal of DIC and nitrate (ΔDIC and ΔNO3–, respectively). Systematic analyses demonstrated that the seasonal net DIC removal in the near-surface water was strong during summer and weak in winter. The peak in net DIC production in the near-bottom, subsurface waters of the inner and middle sections of the shelf occurred between July and September; it was coupled, but with a time lag, to the peak in the net DIC removal that occurred in the near-surface waters in June. A similar 2-month delay (i.e., January vs. November) could also be observed between their minima. A detailed examination of the relationship between ΔDIC and ΔNO3– demonstrates that net biological activity was the dominant factor of DIC removal and addition. Other effects, such as air–sea CO2 gas exchange, wetland exports, CaCO3 precipitation, and a regional variation of the Redfield ratio, were relatively minor. We suggest that the delayed coupling between eutrophic surface and hypoxic bottom waters reported here may also be seen in the carbon and nutrient cycles of other nutrient-rich, river-dominated ocean margins worldwide.

The precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform-building process during deposition of the 2560-2520 Ma Campbellrand-Malmani carbonate platform, South Africa. Aragonite fans and fibrous coatings are common in unrestricted, shallow subtidal to intertidal facies. They are also present in restricted facies, but are absent from deep subtidal facies. Decimetre-thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies. The proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0% to > 65% depending on the facies. Subtidal facies commonly contain 20-35%in situ precipitated carbonate, demonstrating that Neoarchaean sea water was supersaturated with respect to aragonite, carbonate crystal growth rates were rapid compared with sediment influx rates, and the dynamics of carbonate precipitation were different from those in younger carbonate platforms. The abundance of aragonite pseudomorphs suggests that sea-water pH was neutral to alkaline, whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans.

The Bohai Sea to the west of the North Yellow Sea in the Northwest Pacific is a semienclosed shallow‐water marginal sea on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism‐induced summertime dissolved oxygen (DO) decline and acidification, eight field surveys were conducted between August 2011 and September 2017, investigating seasonal and interannual variations in DO and carbonate system parameters in the central Bohai Sea. The historically lowest Bohai Sea DO was recorded at 66 μmol O2/kg in subsurface waters in early September 2015. By comparing our data set with literature data obtained in the 1980s, 1990s, and 2000s, the emergence history of summertime hypoxia in this marginal sea was shown. Associated with the seasonal DO shortage, the metabolism‐enhanced CO2 accumulation in subsurface waters in summer resulted in critical aragonite saturation states of close to or even less than 1.5, threatening calcifiers with potentially severe consequences for valuable shellfish fisheries. Based on model results combining the aerobic respiration (as indicated by apparent oxygen utilization) and the given wintertime/springtime atmospheric CO2 level of the time, this study suggests that combined stresses of the seasonal DO shortage and CO2 acidification capable of threatening Bohai Sea ecosystems could develop within decades. Plain Language Summary: More and more coastal oceans are subject to eutrophication‐induced seasonal deoxygenation and CO2 acidification. The Bohai Sea, a semienclosed and shallow‐water inland sea of China, was subject to stepwise decline in summertime bottom water dissolved oxygen after the launch of the China "Reform and Openness" policy in 1978. In early September 2015, the historically lowest bottom water dissolved oxygen concentration in the Bohai Sea was recorded at 2.14–2.21 mg/L, very close to a so‐called hypoxia. Associated with the seasonal dissolved oxygen shortage, the Bohai Sea subsurface waters accumulated respiration‐derived CO2 in summer, lowering bottom water pH and aragonite saturation state to critical levels for the Bohai Sea shellfish fisheries to be maintained. The combined stresses of the seasonal dissolved oxygen shortage and CO2 acidification may have changed the structure and function of the Bohai Sea benthic communities. Despite a regional research, this study demonstrated a simple procedure in the projection of future coastal carbonate mineral suppression under the combined stresses of global atmospheric CO2 intrusion and the regional metabolism enhancement. This approach may have applicability in many other shallow‐water coastal oceans with seasonal stratification and eutrophication‐related dissolved oxygen depletion. Key Points: The emergence history of metabolism‐induced summertime hypoxia in the Bohai Sea was documented for the first timeThe Bohai Sea accumulates respiration‐induced CO2 in summer, lowering aragonite saturation states of subsurface waters to a critical levelCarbonate mineral suppression in Bohai Sea may threaten calcifiers with potentially severe consequences for valuable shellfish fisheries [ABSTRACT FROM AUTHOR]

Abstract The concept of "Carbonate Factory" was introduced to describe areas of carbonate production and accumulation. In this paper, the "Ooid Factories" is used to analyze extensive Griesbachian (Triassic) oolite deposits in South China that, along with microbialites and lime-mud deposits from different carbonate depositional environments. Late Permian tropical shallow-water skeletal factories (dominant by calcareous algae, sponge, and other associated organisms) collapsed in pace with the most severe mass extinction event, and were rapidly replaced by mud-mound factories (microbialite and lime-mud deposits) and subsequent ooid factories in shallow-water circumstances of Griesbachian. Meantime, temporal nutrient-rich and high-temperature conditions probably delayed re-establishment of tropical shallow-water skeletal factories. As an elementary unit of ooid factories, Lower Triassic oolites have diagnostic characteristics of ooid fabrics (alternatively dark- and light-colored laminae), size ranges (1–5 mm), and grain compositions (pure ooids, and only a few gastropod and bivalve fragments). Continuous aggradational and progradational oolite sequences caused extensive oolite accumulations that significantly contribute to the development of platform architecture with the collaboration of mud-mound factories (mainly line-mud deposits) in South China, through the evolution of low-angle or homoclinal ramps in the early Griesbachian to distally steepened ramps or flat-topped shelves in the late Griesbachian. Moreover, ooid factories also developed in Western Tethyan, Cimmerian, and Arabian carbonate platforms within low latitudes of the Tethyan Ocean during the Griesbachian. It is suggested that extreme hothouse climate with (seasonal) dry conditions in low latitudes significantly facilitated the extensive carbonate production of ooid factories at that time due to the strong linear relationship between surface seawater temperature and carbonate saturation state. Highlights • Our study focuses on "ooid factory" and introduces its characteristics during the earliest Triassic. • Tropical shallow water (skeletal) factory was replaced by ooid and mud-mound factories after the latest Permian mass extinction event. • Ooid factories were widely distributed in low latitudes during the Griesbachian. • Ooid and associated mud-mound factories have a great influence on early-stage platform architecture. • The high temperature of surface seawater and arid climate may relate to the operation of ooid factories. [ABSTRACT FROM AUTHOR]

• Hatching duration and hatching rate in Haliotis discus hannai is susceptible to increasing pCO2 level. • CaCO3 deposition in larval abalone shell is susceptible to calcium carbonate saturation state (Ω) rather than pCO2 or pH. • Abalone shell formation relies heavily on inorganic carbon supply from seawater. • Different seawater carbonate parameters play different roles in affecting early embryonic development and shell formation of the Pacific abalone. pH or p CO 2 are usually taken to study the impact of ocean acidification on molluscs. Here we studied the different impact of seawater carbonate parameters on embryonic development and calcification of the Pacific abalone (Haliotis discus hannai). Early embryonic development was susceptible to elevated p CO 2 level. Larvae hatching duration was positively and hatching rate was negatively correlated with the p CO 2 level, respectively. Calcium carbonate (CaCO 3) deposition of larval shell was found to be susceptible to calcium carbonate saturation state (Ω) rather than p CO 2 or pH. Most larvae incubated in seawater with Ω arag = 1.5 succeeded in shell formation, even when seawater p CO 2 level was higher than 3700 μatm and pH T was close to 7.4. Nevertheless, larvae failed to generate CaCO 3 in seawater with Ω arag ≤ 0.52 and control level of p CO 2 , while seawater DIC level was lowered (≤ 852 μmol/kg). Surprisingly, some larvae completed CaCO 3 deposition in seawater with Ω arag = 0.6 and slightly elevated DIC (2266 μmol/kg), while seawater p CO 2 level was higher than 2700 μatm and pH T was lower than 7.3. This indicates that abalone may be capable of regulating carbonate chemistry to support shell formation, however, the capability was limited as surging p CO 2 level lowered growth rate and jeopardized the integrity of larval shells. Larvae generated thicker shell in seawater with Ω arag = 5.6, while adult abalone could not deposit CaCO 3 in seawater with Ω arag = 0.29 and DIC = 321 μmol/kg. This indicates that abalone may lack the ability to directly remove or add inorganic carbon at the calcifying sites. In conclusion, different seawater carbonate parameters play different roles in affecting early embryonic development and shell formation of the Pacific abalone, which may exhibit limited capacity to regulate carbonate chemistry. [ABSTRACT FROM AUTHOR]

The Permian–Triassic mass extinction (PTME) is not only a dramatic loss in biodiversity and major change in ecosystem structures, but also coincided with the formation of abundant unusual sedimentary structures. Of these, ooids were widespread in shallow marine carbonate settings during the Permian–Triassic (P–Tr) transition, and giant ooids occurred more frequently at this critical period than any time during the Phanerozoic. Global review of 43 oolite-bearing Permian–Triassic boundary (PTB) sections with reliable biostratigraphic controls indicates that ooids occurred mostly in coincidence with the latest Permian extinction (LPE) and its immediate aftermath. Ooids became widespread over extensive regions just after the LPE during the interval corresponding to the conodont Hindeodus changxingensis Zone. They persisted into the earliest Triassic until the conodont Isarcicella isarcica Zone. In addition, oolites are often found in association with microbialites in low-latitude shallow-marine settings. Proliferation of ooids over the P–Tr transition indicates an extensive range of warm waters with high level of carbonate saturation state that prevailed in the oceans during that time. The latest Permian ooids were usually small (0.3 to 0.7 mm in diameter), aragonitic, poorly preserved and recrystallized, while moderately to well-preserved morphology, bimineralic, and oversized forms usually occurred in the I. isarcica Zone of the earliest Triassic and afterwards. Widespread aragonitic ooids in the end of the Permian reinforce the scenario that an “aragonite sea” period may have resulted in the dramatic losses of skeletal organisms that precipitated low-Mg calcite and hampered their recovery in the aftermath. The anomalous primary mineralogy of Lower Triassic ooids implies that previously assumed stable seawater composition during the Early Triassic needs to be revaluated. [ABSTRACT FROM AUTHOR]

Ocean acidification can have negative repercussions from the organism to ecosystem levels. Octocorals deposit high-magnesium calcite in their skeletons, and according to different models, they could be more susceptible to the depletion of carbonate ions than either calcite or aragonite-depositing organisms. This study investigated the response of the gorgonian coral Eunicea fusca to a range of CO concentrations from 285 to 4,568 ppm (pH range 8.1-7.1) over a 4-week period. Gorgonian growth and calcification were measured at each level of CO as linear extension rate and percent change in buoyant weight and calcein incorporation in individual sclerites, respectively. There was a significant negative relationship for calcification and CO concentration that was well explained by a linear model regression analysis for both buoyant weight and calcein staining. In general, growth and calcification did not stop in any of the concentrations of pCO; however, some of the octocoral fragments experienced negative calcification at undersaturated levels of calcium carbonate (>4,500 ppm) suggesting possible dissolution effects. These results highlight the susceptibility of the gorgonian coral E. fusca to elevated levels of carbon dioxide but suggest that E. fusca could still survive well in mid-term ocean acidification conditions expected by the end of this century, which provides important information on the effects of ocean acidification on the dynamics of coral reef communities. Gorgonian corals can be expected to diversify and thrive in the Atlantic-Eastern Pacific; as scleractinian corals decline, it is likely to expect a shift in these reef communities from scleractinian coral dominated to octocoral/soft coral dominated under a 'business as usual' scenario of CO emissions. [ABSTRACT FROM AUTHOR]

Abstract: The potential influence of low latitude ocean primary productivity on glacial atmospheric carbon dioxide levels has proven challenging to deduce using mass accumulation rates (MARs) of biogenic particulates in deep sea sediment cores. Benthic foraminiferal B/Ca serves as a proxy for past seawater calcite saturation state, and thereby provides a fresh perspective on this outstanding paleoceanographic problem. Here we employ Cibicidoides wuellerstorfi B/Ca in the Panama Basin region of the eastern equatorial Pacific (EEP) to investigate the nature of deep tropical Pacific carbon storage over the past 50 ka BP. We present evidence for persistently lower deep Panama Basin calcite saturation state, reflecting an increase in total carbon dioxide storage, during the last ice age relative to the Holocene. These results reflect the modification of inflowing deep waters by overlying export productivity, and support the concept of an invigorated glacial EEP soft-tissue pump possibly driven by oceanic nutrient (iron and silica) redistribution. Benthic Cibicidoides spp. carbon-13 is consistent with this conclusion by exhibiting substantially lighter values during glacial time, reflecting the accumulation of metabolic carbon dioxide in the deep tropical Pacific. Counterintuitively, downcore application of the Globorotalia menardii calcite fragmentation index (MFI) reveals enhanced glacial sedimentary calcite preservation in the Panama Basin. Together these results point towards a systematic decoupling of bottom water chemistry from biogenic burial fluxes: the crux of the aforementioned traditional paleoproductivity problem. [Copyright &y& Elsevier]

Abstract: Shallow submarine gas vents in Levante Bay, Vulcano Island (Italy), emit around 3.6t CO2 per day providing a natural laboratory for the study of biogeochemical processes related to seabed CO2 leaks and ocean acidification. The main physico-chemical parameters (T, pH and Eh) were measured at more than 70 stations with 40 seawater samples were collected for chemical analyses. The main gas vent area had high concentrations of dissolved hydrothermal gases, low pH and negative redox values all of which returned to normal seawater values at distances of about 400m from the main vents. Much of the bay around the vents is corrosive to calcium carbonate; the north shore has a gradient in seawater carbonate chemistry that is well suited to studies of the effects of long-term increases in CO2 levels. This shoreline lacks toxic compounds (such as H2S) and has a gradient in carbonate saturation states. [Copyright &y& Elsevier]

Abstract: The health of coral reefs and calcareous plankton is strongly influenced by the carbonate saturation state of seawater. Calculations of carbonate saturation states currently require measurements of two CO2 system parameters, such as pH and total dissolved carbon, plus thermodynamic calculations that relate carbonate ion concentrations to directly measured parameters. In this work we report novel procedures for direct measurements of carbonate ion concentrations and saturation states in seawater. Measurements are obtained via ultraviolet spectroscopic observations of Pb(II) spectra as the relative concentrations of PbCO3 0 and an ensemble of lead chloride complexes vary in response to dissolved CO3 2−. Measurement precision is enhanced by parameterization in terms of absorbance ratios. The PbCO3 0 stability constant, and Pb(II) molar absorbance ratios in seawater, were determined at 25 °C over a range of salinity between 36 and 20. The procedures described in this work are well suited to measurements throughout the normal range of carbonate ion concentrations in the oceans. Rapid equilibration rates for Pb(II) carbonate complexation make the procedures described in this work well suited to rapid direct analysis in situ. [Copyright &y& Elsevier]

Ocean acidification is rapidly changing the carbonate system of the world oceans. Past mass extinction events have been linked to ocean acidification, and the current rate of change in seawater chemistry is unprecedented. Evidence suggests that these changes will have significant consequences for marine taxa, particularly those that build skeletons, shells, and tests of biogenic calcium carbonate. Potential changes in species distributions and abundances could propagate through multiple trophic levels of marine food webs, though research into the long-term ecosystem impacts of ocean acidification is in its infancy. This review attempts to provide a general synthesis of known and/or hypothesized biological and ecosystem responses to increasing ocean acidification. Marine taxa covered in this review include tropical reef-building corals, cold-water corals, crustose coralline algae, Halimeda, benthic mollusks, echinoderms, coccolithophores, foraminifera, pteropods, seagrasses, jellyfishes, and fishes. The risk of irreversible ecosystem changes due to ocean acidification should enlighten the ongoing CO2 emissions debate and make it clear that the human dependence on fossil fuels must end quickly. Political will and significant large-scale investment in clean-energy technologies are essential if we are to avoid the most damaging effects of human-induced climate change, including ocean acidification. [ABSTRACT FROM AUTHOR]