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Abstract

We explore the relationship between atmospheric O2 and CO2 evolution and seawater chemistry, with particular focus on the CO2-carbonic acid system and ocean ventilation, over the Phanerozoic Eon using a coupled biogeochemical Earth system model (MAGic). This model describes the biogeochemical cycles involving the major components of seawater (Ca, Mg, Na, K, Cl, SO4, CO2HCO3CO3), as well as components (O2, Fe, P, organic C, reduced S) central to long-term ecosystem productivity. The MAGic calculations show that the first-order input fluxes from weathering of continental rocks of Ca, Mg, and dissolved inorganic carbon (DIC) to the ocean varied in a cyclical manner over the Phanerozoic. The cyclicity is mainly the result of the impact of changing atmospheric CO2 levels, and hence temperature and runoff, on these fluxes, reflecting the nature of hothouse (greenhouse, high CO2 and warm) versus icehouse (low CO2, cool, and continental glaciation) conditions during the Phanerozoic. Uptake of DIC by seafloor basalt-seawater reactions also varied in a corresponding fashion to the weathering fluxes. The fluxes of Ca, Mg, DIC and other seawater constituents removed in oceanic sinks were also calculated and hence with calculated inputs and outputs of seawater constituents, the changes in seawater chemistry through Phanerozoic time could be obtained. Seawater pH increased irregularly during the Phanerozoic from just above 7 in the Cambrian Period, approaching modern average values in the most recent several millions of years. Calcite saturation state also increased with decreasing age. Both pH and calcite saturation state trends exhibited a cyclic overprint of hothouse and icehouse environmental conditions. Dissolved sulfate changed in a cyclical manner reflecting mainly variations in weathering and accretion rates and redox conditions, whereas dissolved potassium exhibited little variation in concentration.

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... These reconstruction methods can be divided into forward and inversion models. Forward models include nutrient/weathering models (Bergman et al., 2004;Arvidson et al., 2013;Hansen and Wallmann, 2003) and isotope mass balance models Falkowski et al., 2005), while inversion models infer oxygen content from proxies such as charcoal (Glasspool and Scott, 2010), organic-carbon-to-phosphorus ratios (Algeo and Ingall, 2007) and plant resin δ 13 C (Tappert et al., 2013). Figure 1 shows the reconstructed oxygen content for a variety of these methods. ...
... A roughly 7 ‰ decline in pO 2 is consistent with the ability to change oxygen content by the order of a few percent in ∼ 10 Myr. The reconstructions of Bergman et al. (2004), Arvidson et al. (2013) and are the most plausible based on ice core data (Stolper et al., 2016). Considering these three models alone would still suggest a large uncertainty in oxygen content for most of the Phanerozoic, except for elevated levels in the late Carboniferous/early Permian and reduced levels in the late Devonian. ...
... Previous modelling studies have investigated which factor dominates with conflicting results. Goldblatt et al. (2009) presented radiative-convective model simulations for the Archean (∼ 3 Ga), which suggested that a nitrogen inventory around 3 times larger than present would help to keep the early Earth warm at a time when solar input was only around 75 % of what it is today, potentially solving the "Faint Young Sun" (2007), Arvidson et al. (2013), Bergman et al. (2004), Canfield (1989) and Glasspool and Scott (2010). The mean (black line) and range (grey shading) of the Arvidson et al. (2013), Bergman et al. (2004) and is indicated, as these reconstructions were most consistent with ice core evidence (Stolper et al., 2016). ...
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The amount of dioxygen (O2) in the atmosphere may have varied from as little as 5 % to as much as 35 % during the Phanerozoic eon (54 Ma–present). These changes in the amount of O2 are large enough to have led to changes in atmospheric mass, which may alter the radiative budget of the atmosphere, leading to this mechanism being invoked to explain discrepancies between climate model simulations and proxy reconstructions of past climates. Here, we present the first fully 3-D numerical model simulations to investigate the climate impacts of changes in O2 under different climate states using the coupled atmosphere–ocean Hadley Centre Global Environmental Model version 3 (HadGEM3-AO) and Hadley Centre Coupled Model version 3 (HadCM3-BL) models. We show that simulations with an increase in O2 content result in increased global-mean surface air temperature under conditions of a pre-industrial Holocene climate state, in agreement with idealised 1-D and 2-D modelling studies. We demonstrate the mechanism behind the warming is complex and involves a trade-off between a number of factors. Increasing atmospheric O2 leads to a reduction in incident shortwave radiation at the Earth's surface due to Rayleigh scattering, a cooling effect. However, there is a competing warming effect due to an increase in the pressure broadening of greenhouse gas absorption lines and dynamical feedbacks, which alter the meridional heat transport of the ocean, warming polar regions and cooling tropical regions. Case studies from past climates are investigated using HadCM3-BL and show that, in the warmest climate states in the Maastrichtian (72.1–66.0 Ma), increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Asselian (298.9–295.0 Ma), increasing oxygen content leads to a warmer global-mean surface temperature and reduced carbon storage on land, suggesting that high oxygen content may have been a contributing factor in preventing a “Snowball Earth” during this period of the early Permian. These climate model simulations reconcile the surface temperature response to oxygen content changes across the hierarchy of model complexity and highlight the broad range of Earth system feedbacks that need to be accounted for when considering the climate response to changes in atmospheric oxygen content.
... , Arvidson et al. (2013), Bergman et al. (2004), Berner (2009), Berner and Canfield (1989) and Glasspool and Scott (2010). The mean (black line) and range (grey shading) of the Arvidson et al. (2013), Bergman et al. (2004) and Berner (2009) is indicated as these reconstructions were most consistent with ice core evidence (Stolper et al., 2016). ...
... , Arvidson et al. (2013), Bergman et al. (2004), Berner (2009), Berner and Canfield (1989) and Glasspool and Scott (2010). The mean (black line) and range (grey shading) of the Arvidson et al. (2013), Bergman et al. (2004) and Berner (2009) is indicated as these reconstructions were most consistent with ice core evidence (Stolper et al., 2016). High and low limits on atmospheric oxygen are indicated by horizontal grey dashed lines. ...
... These reconstruction methods can be divided into forward and inversion models. Forward models include nutrient / weathering models (Bergman et al., 2004;Arvidson et al., 2013;Hansen and Wallmann, 2003) and isotope mass balance models (Berner 2009 andFalkowski et al. 2005) while inversion models infer oxygen content from proxies such as charcoal (Glasspool and Scott, 2010), organic carbon to phosphorus ratios (Algeo and Ingall, 2007) and plant resin δ 13 C (Tappert et al., 2013). Figure 1 shows the reconstructed oxygen contents for a 15 variety of these methods. ...
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The amount of dioxygen (O2) in the atmosphere may have varied from as little as 10% to as high as 35% during the Phanerozoic eon (541 Ma–Present). These changes in the amount of O2 are large enough to have lead to changes in atmospheric mass, which may alter the radiative budget of the atmosphere, leading to this mechanism being invoked to explain discrepancies between climate model simulations and proxy reconstructions of past climates. Here we present the first fully 3D numerical model simulations to investigate the climate impacts of changes in O2 during different climate states using the HadGEM3-AO and HadCM3-BL models. We show that simulations with an increase in O2 content result in increased global mean surface air temperature under conditions of a pre-industrial Holocene climate state, in agreement with idealised 1D and 2D modelling studies. We demonstrate the mechanism behind the warming is complex and involves trade-off between a number of factors. Increasing atmospheric O2 leads to a reduction in incident shortwave radiation at Earth's surface due to Rayleigh scattering, a cooling effect. However, there is a competing warming effect due to an increase in the pressure broadening of greenhouse gas absorption lines and dynamical feedbacks, which alter the meridional heat transport of the ocean, warming polar regions and cooling tropical regions. Case studies from past climates are investigated using HadCM3-BL which show that in the warmest climate states, increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Maastrichtian (72.1–66.0Ma), increasing oxygen content leads to a better agreement with proxy reconstructions of surface temperature at that time irrespective of the carbon dioxide content. For the Asselian (298.9–295.0Ma), increasing oxygen content leads to a warmer global mean surface temperature and reduced carbon storage on land, suggesting that high oxygen content may have been a contributing factor in preventing a Snowball Earth during this period of the early Permian. These climate model simulations reconcile the surface temperature response to oxygen content changes across the hierarchy of model complexity and highlight the broad range of Earth system feedbacks that need to be accounted for when considering the climate response to changes in atmospheric oxygen content.
... In lieu of direct proxy records, several carbon cycle models have been developed to reconstruct Phanerozoic seawater carbonate chemistry. Carbon cycle box models have solved for the Ω values necessary to drive carbonate burial that balances weathering inputs (Arvidson et al., 2006(Arvidson et al., , 2013Ridgwell, 2005). One such model incorporated different topologies hypothesized to characterize marine carbonate production before and after the Mid-Mesozoic Revolution-the diversification and ecological expansion of planktonic calcifiers that enabled a significant deep-water carbonate depositional flux (Ridgwell, 2005;Zeebe & Westbroek, 2003). ...
... For example, the hypothesis that a decrease in the abundance of microbial carbonates during Paleozoic time was driven by a trend of decreasing Ω Ca (Riding & Liang, 2005) conflicts with the hypothesis that the Ordovician diversification of carbonate biomineralizers was driven by a trend of increasing Ω Ca (Knoll, 2003;Pruss et al., 2010). Similarly, the hypothesis that Ω Ca has been stable and relatively low since the Mid-Mesozoic Revolution (Ridgwell, 2005) conflicts with the predictions of several models suggesting that Ω Ca was elevated above modern levels for most of the Cenozoic (Arvidson et al., 2006(Arvidson et al., , 2013Riding & Liang, 2005). ...
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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.
... B13). During the Phanerozoic, O 2 likely ranged from 10 % to 30 %, with lows during the early Paleozoic and early Triassic and highs during the Carboniferous to early Permian and Cretaceous Glasspool and Scott, 2010;Arvidson et al., 2013;Mills et al., 2016;Lenton et al., 2018). Assuming a present-day * of 40 ppm (at 21 % O 2 ), * would be 60 ppm at 30 % O 2 and 20 ppm at 10 % O 2 . ...
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Leaf gas-exchange models show considerable promise as paleo-CO2 proxies. They are largely mechanistic in nature, provide well-constrained estimates even when CO2 is high, and can be applied to most subaerial, stomata-bearing fossil leaves from C3 taxa, regardless of age or taxonomy. Here we place additional observational and theoretical constraints on one of these models, the “Franks” model. In order to gauge the model's general accuracy in a way that is appropriate for fossil studies, we estimated CO2 from 40 species of extant angiosperms, conifers, and ferns based only on measurements that can be made directly from fossils (leaf δ¹³C and stomatal density and size) and on a limited sample size (one to three leaves per species). The mean error rate is 28 %, which is similar to or better than the accuracy of other leading paleo-CO2 proxies. We find that leaf temperature and photorespiration do not strongly affect estimated CO2, although more work is warranted on the possible influence of O2 concentration on photorespiration. Leaves from the lowermost 1–2 m of closed-canopy forests should not be used because the local air δ¹³C value is lower than the global well-mixed value. Such leaves are not common in the fossil record but can be identified by morphological and isotopic means.
... The Late Ordovician is marked by both heightened explosive volcanism (Huff et al., 2010) and extensive deposition of C org -rich black shales (Pohl et al., 2017). Modelling studies and halite inclusion data suggest that seawater Ca 2+ concentrations were~3-4 times higher in the Ordovician than in the modern ocean (Arvidson et al., 2013;Ridgwell and Zeebe, 2005). When this is coupled with the fact that lower dissolved O 2 and higher alkalinity levels also favour C auth precipitation (Schrag et al., 2013), this suggests that C auth precipitation within Ordovician tephra layers would have been even more prevalent than those within the modern oceans. ...
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Preservation of organic carbon (C org ) in marine sediments plays a major role in defining ocean-atmosphere CO 2 levels, Earth climate, and the generation of hydrocarbons. Important controls over sedimentary C org preservation include; biological productivity, C org isolation from oxidants (mainly dissolved O 2 ) in the overlying water column and sediments, and C org – mineral association in sediments. Deposition of the products of explosive volcanism (tephra) in the oceans directly enhances C org burial through all these mechanisms, and indirectly through enhanced formation of authigenic carbonate (C auth ) derived from sedimentary C org . In the modern oceans, it is suggested that tephra deposition may account for 5–10% of the C org burial flux and 10–40% of the C auth burial flux. However, during certain periods in Earth's history, extensive explosive volcanism may have led to enhanced C auth precipitation on a sufficiently large scale to influence the global ocean-atmosphere carbon cycle. Changes in tephra-related C org preservation may also have played a role in increasing C org preservation rates in local marine basins, at the oxic-anoxic boundary and enhanced the generation of hydrocarbon deposits in these settings.
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Oxygen is essential for animal life, and while geochemical proxies have been instrumental in determining the broad evolutionary history of oxygen on Earth, much of our insight into Phanerozoic oxygen comes from biogeochemical modelling. The GEOCARBSULF model utilizes carbon and sulphur isotope records to produce the most detailed history of Phanerozoic atmospheric O2 currently available. However, its predictions for the Paleozoic disagree with geochemical proxies, and with non-isotope modelling. Here we show that GEOCARBSULF oversimplifies the geochemistry of sulphur isotope fractionation, returning unrealistic values for the O2 sourced from pyrite burial when oxygen is low. We rebuild the model from first principles, utilizing an improved numerical scheme, the latest carbon isotope data, and we replace the sulphur cycle equations in line with forwards modelling approaches. Our new model, GEOCARBSULFOR, produces a revised, highly-detailed prediction for Phanerozoic O2 that is consistent with available proxy data, and independently supports a Paleozoic Oxygenation Event, which likely contributed to the observed radiation of complex, diverse fauna at this time.
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The ‘COPSE’ (Carbon, Oxygen, Phosphorus, Sulphur and Evolution) biogeochemical model predicts the coupled histories and controls on atmospheric O₂, CO₂ and ocean composition over Phanerozoic time. The forwards modelling approach utilized in COPSE makes it a useful tool for testing mechanistic hypotheses against geochemical data and it has been extended and altered a number of times since being published in 2004. Here we undertake a wholesale revision of the model, incorporating: (1) elaboration and updating of the external forcing factors; (2) improved representation of existing processes, including plant effects on weathering and ocean anoxia; (3) inclusion of additional processes and tracers, including seafloor weathering, volcanic rock weathering and ⁸⁷Sr/⁸⁶Sr; (4) updating of the present-day baseline fluxes; and (5) a more efficient and robust numerical scheme. A key aim is to explore how sensitive predictions of atmospheric CO₂, O₂ and ocean composition are to model updates and ongoing uncertainties. The revised model reasonably captures the long-term trends in Phanerozoic geochemical proxies for atmospheric pCO₂, pO₂, ocean [SO₄], carbonate δ¹³C, sulphate δ³⁴S and carbonate ⁸⁷Sr/⁸⁶Sr. It predicts a two-phase drawdown of atmospheric CO₂ with the rise of land plants and associated cooling phases in the Late Ordovician and Devonian-early Carboniferous, followed by broad peaks of atmospheric CO₂ and temperature in the Triassic and mid-Cretaceous – although some of the structure in the CO₂ proxy record is missed. The model robustly predicts a mid-Paleozoic oxygenation event due to the earliest land plants, with OO₂ rising from ~ 5% to > 17% of the atmosphere and oxygenating the ocean. Thereafter, atmospheric O₂ is effectively regulated with remaining fluctuations being a Carboniferous–Permian O₂ peak ~ 26% linked to burial of terrestrial organic matter in coal swamps, a Triassic–Jurassic O₂ minimum ~ 21% linked to low uplift, a Cretaceous O₂ peak ~ 26% linked to high degassing and weathering fluxes, and a Cenozoic O₂ decline.
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Calcite and aragonite seas are commonly distinguished based on the prevailing primary mineralogy of ooids and carbonate cements over time. Secular oscillations of these seas are usually attributed to changes in ocean chemistry and paleoclimate. While the veracity of such oscillations has been verified by independent data and modeling approaches, the timing of the transition from one ocean state to the other remains poorly resolved. Here, the timing of the last aragonite–calcite sea transition is estimated by assessing the preservation of Early Jurassic ooids from the Trento Platform in northern Italy. Point counting of ooid-bearing limestones from four distinct stratigraphic levels provides a contrasting pattern: Hettangian and Sinemurian ooids are all poorly preserved and were probably predominantly originally aragonitic, whereas Pliensbachian and Toarcian ooids are excellently preserved, suggesting a primary calcitic mineralogy. Although calcitic ooids may have already been common in the Late Triassic, it is proposed that the last aragonite–calcite sea transition occurred in the Early Jurassic between the Sinemurian and Pliensbachian, at least in this subtropical region. Therefore, the selective extinction of aragonite-secreting organisms at the end-Triassic mass extinction cannot be attributed to secular changes in ocean chemistry.
Article
The primary mineralogy of marine carbonate precipitates has been a crucial factor in constraining the major element composition of ancient oceans. Secular changes in Phanerozoic marine chemistry, including Mg/Ca, have been well-documented using the original carbonate mineralogy of ooids, marine cements and biominerals. However, the history of Precambrian seawater chemistry is not as well constrained, partially due to the prevalence of dolomitisation in the Precambrian geological record. The Neoproterozoic (~ 1000 Ma to ~ 541 Ma) record of primary carbonate mineralogy is documented here using a combination of literature data and new analysis of marine carbonate precipitates from the Otavi Fold Belt, Namibia, the Death Valley succession, USA and the Adelaide Fold Belt, Australia. These data suggest that the last ~ 460 million years of the Proterozoic were dominated by aragonite and high-Mg calcite precipitation in shallow marine settings. In contrast, low-Mg calcite has only been recognised in a small number of formations. In addition to aragonite and calcite precipitation, marine dolomite precipitation was widespread in Neoproterozoic oceans, including mimetic (syn-sedimentary) dolomitisation and primary dolomite marine cementation. The combination of marine aragonite, high Mg-calcite and dolomite precipitation during the Neoproterozoic suggests extremely high seawater Mg/Ca conditions relative to Phanerozoic oceans. Marine dolomite precipitation may also be linked to widespread marine anoxia during this time.
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An antagonistic view of the relationship between microbialites and metazoans has long been inferred, in part because of the large scale anticorrelation of these two groups through geologic time. The nexus of this relationship occurs in the Early Paleozoic Era: stromatolites declined in abundance as complex animals and algae diversified, but thrombolites, a type of microbialite little known before the Proterozoic-Cambrian boundary, proliferated for the first time. Well-preserved parasequences in the basal portion of the Lower Ordovician Boat Harbour Formation, western Newfoundland, contain a succession of stromatolites and thrombolites that permit an investigation into the role metazoans played in shaping the nature and abundance of microbialites in Early Paleozoic carbonate seas. Sessile benthic animals colonized thrombolite surfaces, but are nearly absent from stromatolites. Bioturbation rarely co-occurred with microbialites, but is widespread in clastic carbonates that lack microbialites. Our results, thus, support the hypothesis of ecological antagonism between microbial communities and motile benthic animals, but also demonstrate biological facilitation between thrombolites and both sessile benthic animals and nekton.
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The relative influences of tectonics, continental weathering and seafloor weathering in controlling the geological carbon cycle are unknown. Here we develop a new carbon cycle model that explicitly captures the kinetics of seafloor weathering to investigate carbon fluxes and the evolution of atmospheric CO 2 and ocean pH since 100 Myr ago. We compare model outputs to proxy data, and rigorously constrain model parameters using Bayesian inverse methods. Assuming our forward model is an accurate representation of the carbon cycle, to fit proxies the temperature dependence of continental weathering must be weaker than commonly assumed. We find that 15-31 °C (1σ) surface warming is required to double the continental weathering flux, versus 3-10 °C in previous work. In addition, continental weatherability has increased 1.7-3.3 times since 100 Myr ago, demanding explanation by uplift and sea-level changes. The average Earth system climate sensitivity isK (1σ) per CO 2 doubling, which is notably higher than fast-feedback estimates. These conclusions are robust to assumptions about outgassing, modern fluxes and seafloor weathering kinetics.
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Changes in atmospheric oxygen concentration over Earth history are commonly related to the evolution of animals and plants. But there is no direct geochemical proxy for O2 levels, meaning that estimations rely heavily on modeling approaches. The results of such studies differ greatly, to the extent that today's atmospheric mixing ratio of 21% might be either the highest or lowest level during the past 200 m.y. Long-term oxygen sources, such as the burial in sediments of reduced carbon and sulfur species, are calculated in models by representation of nutrient cycling and estimation of productivity, or by isotope mass balance (IMB)—a technique in which burial rates are inferred in order to match known isotope records. Studies utilizing these different techniques produce conflicting estimates for paleoatmospheric O2, with nutrient-weathering models estimating concentrations close to, or above, that of the present day, and IMB models estimating low O2, especially during the Mesozoic. Here we re-assess the IMB technique using the COPSE biogeochemical model. IMB modelling is confirmed to be highly sensitive to assumed carbonate δ13C, and when this input is defined following recent compilations, predicted O2 is significantly higher and in reasonable agreement with that of non-IMB techniques. We conclude that there is no model-based support for low atmospheric oxygen concentrations during the past 200 m.y. High Mesozoic O2 is consistent with wildfire records and the development of plant fire adaptions, but links between O2 and mammal evolution appear more tenuous.
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see the figure). Under a scenario in which O 2 is at the low end of the range of estimates (see the figure), these differences between climate models and proxy data largely disappear. The results have important implications for assessing climate during other times in Earth history. The early Paleogene (66 to 50 million years ago) is one of the best-studied intervals of a hothouse planet, with proxy records indicating very warm and wet conditions in mid-latitude and polar regions (2, 7) and CO 2 levels below 2000 ppm (1) (see the figure). Despite decades of intensive research, climate models predict too cool and too dry climates , especially in polar regions (2), unless CO 2 concentrations greater than 2500 ppm are used. O 2 during this time was probably between ~15 and 25% (see the figure). If the low-end O 2 estimates are correct (~15%), the reduced density of the atmosphere relative to today would elevate surface temperatures and precipitation, particularly at the poles, likely reducing the model-proxy differences. During the late Carbon-iferous and Permian (315 to 255 million years ago), Earth experienced the largest and longest glacial period of the Phanerozoic (8), coincident with high O 2 (as much as 35%) and low CO 2 (~350 ppm) (see the figure). Climate models require CO 2 to drop below 560 ppm to initiate ice sheet growth (8) and are thus broadly compatible with CO 2 constraints from proxies and long-term carbon cycle models. Nevertheless , climate model results should be revisited in light of Poulsen et al.'s findings, because the high-pressure atmosphere at this time likely caused additional A
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Oscillations between the dominance of aragonite and calcite in abiotic marine CaCO3 precipitates throughout Earth history are closely coupled with the evolution of Earth's seawater composition and represent the environmental context in which organisms evolved their ability to biomineralize. The most important factor controlling these Phanerozoic oscillations in CaCO3 polymorph composition is the ratio of Mg:Ca in seawater, which is thought to separate aragonite and calcite precipitation along a distinct temperature-controlled threshold. A sharp threshold, however, is contradicted by overlapping aragonite and calcite precipitation fields at a range of experimental conditions. Here we present experimental data that enable us to quantify the proportions of CaCO3 polymorphs as a function of Mg:Ca ratio and temperature. This allows us to convert published Mg:Ca ratio proxy data and models of the Phanerozoic Mg:Ca ratio into proportions of abiotic CaCO3 polymorphs at a given temperature, and thus provides a temperature-corrected view of aragonite-calcite sea conditions. In this revised view, abiotic calcite precipitation was inhibited during aragonite sea intervals at temperatures above 20 degrees C, whereas calcite sea intervals were characterized by the co-precipitation of aragonite and calcite in environments above 20 degrees C. This continuous prominence of aragonite precipitation in Phanerozoic warm-water environments explains the Phanerozoic increase of aragonite over calcite skeletal composition in calcifying organisms.
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Experimental burial of polychaete (Nereis) and crustacean (Crangon) carcasses in kaolinite, calcite, quartz, and montmorillonite demonstrates a marked effect of sediment mineralogy on the stabilization of nonbiomineralized integuments, the first step in producing carbonaceous compression fossils and Burgess Shale-type (BST) preservation. The greatest positive effect was with Nereis buried in kaolinite, and the greatest negative effect was with Nereis buried in montmorillonite, a morphological trend paralleled by levels of preserved protein. Similar but more attenuated effects were observed with Crangon. The complex interplay of original histology and sediment mineralogy controls system pH, oxygen content, and major ion concentrations, all of which are likely to feed back on the preservation potential of particular substrates in particular environments. The particular susceptibility of Nereis to both diagenetically enhanced preservation and diagenetically enhanced decomposition most likely derives from the relative lability of its collagenous cuticle vs. the inherently more recalcitrant cuticle of Crangon. We propose a mechanism of secondary, sediment-induced taphonomic tanning to account for instances of enhanced preservation. In light of the marked effects of sediment mineralogy on fossilization, the Cambrian to Early Ordovician taphonomic window for BST preservation is potentially related to a coincident interval of glauconite-prone seas.
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The LOSCAR model is designed to efficiently compute the partitioning of carbon between ocean, atmosphere, and sediments on time scales ranging from centuries to millions of years. While a variety of computationally inexpensive carbon cycle models are already available, many are missing a critical sediment component, which is indispensable for long-term integrations. One of LOSCAR's strengths is the coupling of ocean-atmosphere routines to a computationally efficient sediment module. This allows, for instance, adequate computation of CaCO<sub>3</sub> dissolution, calcite compensation, and long-term carbon cycle fluxes, including weathering of carbonate and silicate rocks. The ocean component includes various biogeochemical tracers such as total carbon, alkalinity, phosphate, oxygen, and stable carbon isotopes. LOSCAR's configuration of ocean geometry is flexible and allows for easy switching between modern and paleo-versions. We have previously published applications of the model tackling future projections of ocean chemistry and weathering, p CO<sub>2</sub> sensitivity to carbon cycle perturbations throughout the Cenozoic, and carbon/calcium cycling during the Paleocene-Eocene Thermal Maximum. The focus of the present contribution is the detailed description of the model including numerical architecture, processes and parameterizations, tuning, and examples of input and output. Typical CPU integration times of LOSCAR are of order seconds for several thousand model years on current standard desktop machines. The LOSCAR source code in C can be obtained from the author by sending a request to loscar.model@gmail.com.
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We present a new model of biogeochemical cycling over Phanerozoic time. This work couples a feedback-based model of atmospheric O2 and ocean nutrients (Lenton and Watson, 2000a, 2000b) with a geochemical carbon cycle model (Berner, 1991, 1994), a simple sulfur cycle, and additional components. The resulting COPSE model (Carbon-Oxygen-Phosphorus-Sulfur-Evolution) represents the co-evolution of biotic and abiotic components of the Earth system, in that it couples interactive and evolving terrestrial and marine biota to geochemical and tectonic processes. The model is forced with geological and evolutionary forcings and time-dependent solar insolation. The baseline model succeeds in giving simultaneous predictions of atmospheric O2, CO2, global temperature, ocean composition, δ13C and δ34S that are in reasonable agreement with available data and suggested constraints. The behavior of the coupled model is qualitatively different to single cycle models. While atmospheric pCO2 (CO2 partial pressure) predictions are mostly determined by the model forcings and the response of silicate weathering rate to pCO2 and temperature, multiple negative feedback processes and coupling of the C, O, P and S cycles are necessary for regulating pO2 while allowing δ13C changes of sufficient amplitude to match the record. The results support a pO2 dependency of oxidative weathering of reduced carbon and sulfur, which raises early Paleozoic pO2 above the estimated requirement of Cambrian fauna and prevents unrealistically large δ34S variation. They do not support a strong anoxia dependency of the C:P burial ratio of marine organic matter (Van Cappellen and Ingall, 1994, 1996) because this dependency raises early Paleozoic δ13C and organic carbon burial rates too high. The dependency of terrestrial primary productivity on pO2 also contributes to oxygen regulation. An intermediate strength oxygen fire feedback on terrestrial biomass, which gives a pO2 upper limit of ∼1.6PAL (present atmospheric level) or 30 volume percent, provides the best combined pO2 and δ13C predictions. Sulfur cycle coupling contributes critically to lowering the Permo-Carboniferous pCO2 and temperature minimum. The results support an inverse dependency of pyrite sulfur burial on pO2 (for example, Berner and Canfield, 1989 , which contributes to the shuttling of oxygen back and forth between carbonate carbon and gypsum sulfur. A pO2 dependency of photosynthetic carbon isotope fractionation (Berner and others, 2000; Beerling and others, 2002) is important for producing sufficient magnitude of δ13C variation. However, our results do not support an oxygen dependency of sulfur isotope fractionation in pyrite formation (Berner and others, 2000) because it generates unrealistically small variations in δ34S. In the Early Paleozoic, COPSE predicts pO2=0.2-0.6PAL and pCO2>10PAL, with high oceanic [PO3-4] and low [SO=4]. Land plant evolution caused a 'phase change' in the Earth system by increasing weathering rates and shifting some organic burial to land. This change resulted in a major drop in pCO2 to 3 to 4PAL and a rise in pO2 to ∼1.5PAL in the Permo-Carboniferous, with temperatures below present, ocean variables nearer present concentrations, and PO4:NO3 regulated closer to Redfield ratio. A second O2 peak of similar or slightly greater magnitude appears in the mid-Cretaceous, before a descent towards PAL. Mesozoic CO2 is in the range 3 to 7PAL, descending toward PAL in the Cretaceous and Cenozoic.
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The global CO2-carbonic acid-carbonate system of seawater, although certainly a well-researched topic of interest in the past, has risen to the fore in recent years because of the environmental issue of ocean acidification (often simply termed OA). Despite much previous research, there remain pressing questions about how this most important chemical system of seawater operated at the various time scales of the deep time of the Phanerozoic Eon (the past 545 Ma of Earth's history), interglacial-glacial time, and the Anthropocene (the time of strong human influence on the behaviour of the system) into the future of the planet. One difficulty in any analysis is that the behaviour of the marine carbon system is not only controlled by internal processes in the ocean, but it is intimately linked to the domains of the atmosphere, continental landscape, and marine carbonate sediments. For the deep-time behaviour of the system, there exists a strong coupling between the states of various material reservoirs resulting in an homeostatic and self-regulating system. As a working hypothesis, the coupling produces two dominant chemostatic modes: (Mode I), a state of elevated atmospheric CO2, warm climate, and depressed seawater Mg/Ca and SO4/Ca mol ratios, pH (extended geologic periods of ocean acidification), and carbonate saturation states (Omega), and elevated Sr concentrations, with calcite and dolomite as dominant minerals found in marine carbonate sediments (Hothouses, the calcite-dolomite seas), and (Mode II), a state of depressed atmospheric CO2, cool climate, and elevated seawater Mg/Ca and SO4/Ca ratios, pH, and carbonate saturation states, and low Sr concentrations, with aragonite and high magnesian calcites as dominant minerals found in marine carbonate sediments (Icehouses, the aragonite seas). Investigation of the impacts of deglaciation and anthropogenic inputs on the CO2-H2O-CaCO3 system in global coastal ocean waters from the Last Glacial Maximum (LGM: the last great continental glaciation of the Pleistocene Epoch, 18,000 year BP) to the year 2100 shows that with rising sea level, atmospheric CO2, and temperature, the carbonate system of coastal ocean water changed and will continue to change significantly. We find that 6,000 Gt of C were emitted as CO2 to the atmosphere from the growing coastal ocean from the Last Glacial Maximum to late preindustrial time because of net heterotrophy (state of gross respiration exceeding gross photosynthesis) and net calcification processes. Shallow-water carbonate accumulation alone from the Last Glacial Maximum to late preindustrial time could account for similar to 24 ppmv of the similar to 100 ppmv rise in atmospheric CO2, lending some support to the "coral reef hypothesis''. In addition, the global coastal ocean is now, or soon will be, a sink of atmospheric CO2, rather than a source. The pH(T) (pH values on the total proton scale) of global coastal seawater has decreased from similar to 8.35 to similar to 8.18 and the CO32- ion concentration declined by similar to 19% from the Last Glacial Maximum to late preindustrial time. In comparison, the decrease in coastal water pH(T) from the year 1900 to 2000 was similar to 8.18 to similar to 8.08 and is projected to decrease further from about similar to 8.08 to similar to 7.85 between 2000 and 2100. During these 200 years, the CO32- ion concentration will fall by similar to 45%. This decadal rate of decline of the CO32- ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene! In terms of the modern problem of ocean acidification and its effects, the "other CO2 problem", we emphasise that most experimental work on a variety of calcifying organisms has shown that under increased atmospheric CO2 levels (which attempt to mimic those of the future), and hence decreased seawater CO32- ion concentration and carbonate saturation state, most calcifying organisms will not calcify as rapidly as they do under present-day CO2 levels. In addition, we conclude that dissolution of the highly reactive carbonate phases, particularly the biogenic and cementing magnesian calcite phases, on reefs will not be sufficient to alter significantly future changes in seawater pH and lead to a buffering of the CO2-carbonic acid system in waters bathing reefs and other carbonate ecosystems on timescales of decades to centuries. Because of decreased calcification rates and increased dissolution rates in a future higher CO2, warmer world with seas of lower pH and carbonate saturation state, the rate of accretion of carbonate structures is likely to slow and dissolution may even exceed calcification. The potential of increasing nutrient and organic carbon inputs from land, occurrences of mass bleaching events, and increasing intensity (and perhaps frequency of hurricanes and cyclones as a result of sea surface warming) will only complicate matters more. This composite of stresses will have severe consequences for the ecosystem services that reefs perform, including acting as a fishery, a barrier to storm surges, a source of carbonate sediment to maintain beaches, and an environment of aesthetic appeal to tourist and local populations. It seems obvious that increasing rates of dissolution and bioerosion owing to ocean acidification will result in a progressively increasing calcium carbonate (CaCO3) deficit in the CaCO3 budget for many coral reef environments. The major questions that require answers are: will this deficit occur and when and to what extent will the destructive processes exceed the constructive processes?
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The `dolomite problem' has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite. We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (∼3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase. Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.
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In this chapter, a new Earth system model termed MAGic (Mackenzie, Arvidson, Guidry interactive cycles) model is used to examine how the weathering fluxes of calcium (Ca), magnesium (Mg), carbon (C), sulfur (S), and phosphorus (P) have influenced biogeochemical cycles in the ocean over the past 500 million years. In addition, the fluxes of these five components in relation to their sink reservoirs and the effect of the changes in these fluxes and those from basalt-seawater reactions on the chemistry of seawater are calculated. The chapter then goes on to show that the age distributions of inorganic and biogenic carbonate phases in the Phanerozoic carbonate rock record is related to the kinetics of the precipitation rates of these phases that in turn are controlled by changing atmosphere-seawater composition. Such an Earth system model is a global biogeochemical model, and in MAGic, each elemental cycle is explicITAy coupled to corresponding cycles of other elements via a reaction network. This network incorporates the basic reactions controlling atmospheric carbon dioxide and oxygen concentrations, continental and seafloor weathering of silicate and carbonate rocks, net ecosystem productivity, basalt-seawater exchange reactions, precipitation and diagenesis of chemical sediments and authigenic silicates, oxidation-reduction reactions involving C, S, and Fe, and subduction-decarbonation reactions. Geological and biological processes have acted in concert to alter atmospheric and seawater chemistry over the Phanerozoic. The evolution and rise of various planktonic siliceous and calcareous organisms over this period are direct evidence to this interplay of these processes.
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The mineral dolomite and the uncertainties surrounding its origin have attracted the attention of earth scientists for over a century, The core of the dolomite "problem" is the apparent paradox posed by the paucity of dolomite in modern marine depositional environments versus its relative abundance in the sedimentary rock record, Solving this problem requires knowledge of the conditions under which the mineral forms and the rate of precipitation under those conditions, As a working hypothesis, it is suggested that the precipitation rate of dolomite may be quantified and modeled in a manner similar to other carbonate minerals through application of a rate law that represents the rate as a simple function of saturation index, r = k(Omega - 1)(n). This hypothesis is tested in a series of experiments by measuring the steady state rate of dolomite precipitation in a dolomite-seeded now reactor through analysis of reacted fluid chemistry. By varying temperature from approx 100 degrees to 200 degrees C and saturation index (Omega) from near saturation to similar to 100, sufficient data were collected to solve for the reaction order and Arrhenius rate constant (k = A exp {-(epsilon(A)/RT)}) Of this rate law, The dolomite produced in these experiments was variable in composition but typically a calcium-rich protodolomite, forming syntaxial overgrowths on the seed material. tit the highest supersaturations obtained, formation of distinct nucleation centers was observed. These experiments do confirm a strong temperature dependence for the precipitation reaction (activation energy epsilon(A) = 31.9 kcal mol(-1)) and moderate dependency on saturation index (n = 2.26, log A = 1.05). The experimental findings of this paper suggest that the abundance of dolomite in the sedimentary rock record reflects, at least in part, environmental changes in temperature and seawater chemistry over geologic time.
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A dynamical model (MAGic) is presented that describes the elemental cycling of sedimentary materials involving sodium, potassium, calcium, magnesium, chloride, carbon, oxygen, iron, sulfur and phosphorous through much of the Phanerozoic. The model incorporates the basic reactions controlling atmospheric carbon dioxide and oxygen concentrations, continental and seafloor weathering of silicate and carbonate rocks, net ecosystem productivity, basalt-seawater exchange reactions, precipitation and diagenesis of chemical sediments and authigenic silicates, oxidation-reduction reactions involving carbon, sulfur, and iron, and subduction-decarbonation reactions. Although MAGic contains feedback and forcing functions adapted from the GEOCARB models (Berner, 1991, 1994; Berner and Kothavala, 2001), these functions are incorporated in a reservoir-reaction scheme that is considerably more detailed. Coupled reservoirs include shallow and deep cratonic silicate and carbonate rocks and sediments, seawater, atmosphere, oceanic sediments and basalts, and the shallow mantle. Model results are reasonably consistent with recently published constraints provided by fluid inclusion, isotopic, floral, and mineralogical records. We have used these results to evaluate sensitivity to uncertainties in the history of the earth-ocean-atmosphere system over the past 500 Ma: the advent of pelagic carbonate sedimentation, the importance of burial versus early diagenetic dolomite formation, the importance of reverse weathering, and the relationship of these processes to seafloor spreading rates. Results include a general pattern of dolomite abundance during periods of elevated seafloor spreading and alkalinity production, elevated atmospheric CO2 concentrations for most of the Phanerozoic similar to those predicted by GEOCARB, and covariance of seawater sulfate to calcium ratios with magnesium to calcium ratios. These trends are broadly consistent with proxies for seawater composition and the mass-age data of the rock record itself.
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A steady-state quantitative model for the sedimentary rock cycle is presented. The cycling of 11 major elements through the oceanic, atmospheric, biospheric, and rock reservoirs is shown. Flux rates are based on the estimated average geologic rates of transfer; the total flux of material through the oceans is about that of today. The model is consistent with current estimates of the chemical composition of the average dissolved and suspended loads of streams, with the present-day composition of the oceans, that of the average sedimentary rock, and that of the average composition of precipitation. The mean residence time of the major elements in the cycle is about 400 million years; estimates are given for the cycling times of the individual elements.
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Stomatal characteristics of an extinct Cretaceous conifer, Pseudofrenelopsis parceramosa (Fontaine) Watson, are used to reconstruct atmospheric carbon dioxide (pCO2) over a time previously inferred to exhibit major fluctuations in this greenhouse gas. Samples are from nonmarine to marine strata of the Wealden and Lower Greensand Groups of England and the Potomac Group of the eastern United States, of Hauterivian to Albian age (136 100 Ma). Atmospheric pCO2 is estimated from the ratios between stomatal indices of fossil cuticles and those from four modern analogs (nearest living equivalent plants). Using this approach, and two calibration methods to explore ranges, results show relatively low and only slightly varying pCO2 over the Hauterivian Albian interval: a low of ˜560 960 ppm in the early Barremian and a high of ˜620 1200 ppm in the Albian. Data from the Barremian Wealden Group yield pCO2 values indistinguishable from a soil-carbonate based estimate from the same beds. The new pCO2 estimates are compatible with sedimentological and oxygen-isotope evidence for relatively cool mid-Cretaceous climates.
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The essential state of the Phanerozoic ocean-atmosphere system with respect to major lithophile and organic components can be bounded by sedimentary observational data and relatively few model assumptions. The model assumptions are in turn sufficient to constrain and compute the remaining fluxes that result in a comprehensive model describing atmospheric and oceanic evolutionary history over the past 500 m.y. that is in accord with the sedimentary observational data. Two central themes emerge. First, there is a strong coupling of the state of various reservoirs throughout the entire system imposed mainly by negative physical, chemical and biological feedbacks. Second, there is a significant overprint of 'physical' processes, such as weathering, by biologically-mediated processes and ecosystem evolution. Ultimately, the Phanerozoic is characterized by two modes of sea water major-ion chemistry, pH and carbonate saturation state, and atmospheric CO2. Importantly, the transition between these two modes may result from the previous state of the system whose impacts lag by tens of millions of years. Thus, the instantaneous state of the system at any given point in time may reflect in part the 'memory' of a previous period when fluxes and processes were not in balance. The modern-day problem of ocean acidification mainly reflects the fact that human activities of fossil fuel burning and land use changes are resulting in geologically rapid releases of CO2 to the atmosphere and its absorption by the surface ocean and does not reflect the longer term processes and feedbacks that led to the acidic oceans of the past.
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CaCl2 basinal brines, which are present in most Phanerozoic sedimentary basins, inherited their chemistries and salinities from evaporated paleoseawaters when the world oceans were Ca rich and SO4 poor (CaCl2 seas). CaCl2 seas coincided with periods of rapid seafloor spreading, high influxes of mid-ocean-ridge brines rich in CaCl2, and elevated sea levels, conditions that favored accumulation of marine CaCl2 brines in marginal and interior continental basins. Typical basinal brines in Silurian Devonian formations of the interior Illinois basin, United States, show the same compositional trends as those of progressively evaporated CaCl2-rich Silurian seawater. Chemical deviations can be accounted for quantitatively by brine-rock reactions during burial (dolomitization, dolomite and K-feldspar cement). This explanation for the origin of CaCl2 basinal brines contrasts with others that assume constancy of seawater chemistry and involve more complex brine-rock interactions.
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The suggestion of Sarmiento and Toggweiler (1984) that glacial-to-interglacial changes in P(CO2) are related to changes in the nutrient content of high latitude surface water leads to the present development of a four-box model of the ocean and atmosphere that includes low and high latitude surface boxes, an atmosphere, and a deep ocean. The model equations indicate that the CO2 content of high latitude surface water is directly connected to the large CO2 reservoir in deep water through the nutrient content of high latitude surface water. It is proposed that the low ice age P(CO2) value can be generated by a reduction in local exchange between high latitude surface water and deep water.
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The LOSCAR model is designed to efficiently compute the partitioningof carbon between ocean, atmosphere, and sediments on time scalesranging from centuries to millions of years. While a varietyof computationally inexpensive carbon cycle models are alreadyavailable, many are missing a critical sediment component,which is indispensable for long-term integrations. One of LOSCAR'sstrengths is the coupling of ocean-atmosphere routines to a computationallyefficient sediment module. This allows, for instance, adequate computationof CaCO 3 dissolution, calcite compensation, and long-term carbon cyclefluxes, including weathering of carbonate and silicate rocks.The ocean component includes various biogeochemical tracers such astotal carbon, alkalinity, phosphate, oxygen, and stable carbonisotopes. LOSCAR's configuration of ocean geometry is flexible andallows for easy switching between modern and paleo-versions.We have previously published applications of the modeltackling future projections of ocean chemistry and weathering, pCO 2 sensitivityto carbon cycle perturbations throughout the Cenozoic, andcarbon/calcium cycling during the Paleocene-Eocene Thermal Maximum.The focus of the present contribution is the detailed descriptionof the model including numerical architecture, processes andparameterizations, tuning, and examples of input and output.Typical CPU integration times of LOSCAR are of order secondsfor several thousand model years on current standarddesktop machines. The LOSCAR source code in C can be obtainedfrom the author by sending a request [email protected] /* */
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The intersection between geological sciences and human health, termed medical geology, is gaining significant interest as we understand more completely coupled biogeochemical systems. An example of a medical geology problem largely considered solved is that of lead (Pb) poisoning. With aggressive removal of the major sources of Pb to the environment, including Pb-based paint, leaded gasoline, and lead pipes and solder, the number of children in the United States affected by Pb poisoning has been reduced by 80%, down to a current level of 2.2%. In contrast to this national average, however, about 15% of urban children exhibit blood Pb levels above what has been deemed "safe" (10 pg per deciliter); most of these are children of low socio-economic-status minority groups. We have analyzed the spatial relationship between Pb toxicity and metropolitan roadways in Indianapolis and conclude that Pb contamination in soils adjacent to roadways, the cumulative residue from the combustion of leaded gasoline, is being remobilized. Developing strategies to remove roadway Pb at the source is a matter of public health and social justice, and constitutes perhaps the final chapter in this particular story of medical geology.
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Eustasy can be studied using a variety of methods, including areal plots of the changing temporal distribution of marine deposits, facies analysis of stratigraphic sequences, and seismic stratigraphy, allied with the best available means of biostratigraphic correlation. The results of these various methods are then compared for use in eliminating the complicating effects of local and regional tectonics in the interpretation of sea-level oscillations. The determination of the rate and amount of sea- level change is also discussed. Use is made of areal plots, in conjunction with hypsometric data and a variety of stratigraphic sequence evidence, to produce a eustatic curve for the pre-Quaternary Phanerozoic. Notwithstanding its necessarily tentative and provisional nature, this curve is considered to be a more accurate representation of Phanerozoic eustasy than that of Vail et al (1977).-from Author
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A compilation of lithosphere consumed globally in the past 180 m.y. reveals that the equivalent of the surface area of Earth has descended into the mantle, producing "lithospheric graveyards'. This subducted lithosphere lies concentrated in areas of cold mantle that correspond to gross mantle heterogeneities delineated by seismic tomography and the geoid and inversely related to the global distribution of hotspots. Differences in graveyard distribution result from displacements of trenches (assumed nearly fixed to the overriding plates) over the mantle. Net convergence at selected subduction zones also shows variability: in the past 150 m.y. Previous workers have shown that a decrease in global spreading rates (based on estimating the geometry and spreading history of ridges for the past 80 m.y.) is the primary cause of a volume decrease in mid-ocean ridges and is consisent with a corresponding lowering of eustatic sea level during this time interval. We present an extension of this hypothesis to the past 180 m.y. by using our rates of global lithospheric consumption to estimate total global spreading and calculate the inferred changes in mid-ocean ridge volumes. -from Authors
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... dioxide in the atmosphere and Chamberlin2 suggested a variety of geological processes that could affect atmospheric carbon dioxide concentra- tions ... established values for surface ocean pH and alkalinity, it is possible to calculate aqueous CO2 and atmospheric pCO2. ...
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The upper part of the igneous oceanic basement consists of basaltic lava. This basalt undergoes reaction with seawater over a range of temperatures, time and locale. This reaction is a major source and sink for various ions in seawater, and is a major process in buffering seawater composition and forming metalliferous ores in the marine environment. The nature of the chemical reaction and the fluxes of ions exchanged between the oceans and the igneous basement are mostly dependent on the temperature of reaction and the relative proportion of the reactants. These vary in respect to the location of the water circulation and distance from the heat source. Four examples of seawater-basalt interactions are considered and the net exchange fluxes are calculated; these examples cover the range of temperature and water:rock ratios typically found in the ocean floor. Low temperature, high water:rock ratio is typical of the exchange in the upper few meters of the oceanic basement. Only about 0.1% of new oceanic crust undergoes this reaction which extends over a time period of tens of millions of years. The annual fluxes produced are not very large. Low temperature, low water:rock ratio is typical of the reaction in the deeper parts of the oceanic basement. About 8% of newly formed oceanic crust can be expected to undergo this reaction over a period of a few million years. Reactions and fluxes on the flanks of spreading centers are at moderate temperatures and water:rock ratios. These reactions are relatively short lived, but the fluxes produced are quite high. High temperature reaction of seawater and basalt (in excess of 100°C) takes place at spreading centre axes. These reactions are fast but result in very high fluxes and formation of polymetallic sulfides or iron and manganese oxides. The products of this reaction and the direction of exchange for some ionic species are quite different compared to the lower temperature reactions. The net effect of the basalt-seawater exchange is the sum of all the reacton fluxes over the full temperature range. This calculated net flux indicates that the basalt is a source for ions such as Si, Ca, Ba, Li, Fe, Mn, Cu, Ni, Zn and hydrogen ions. It also is the sink for ions such as Mg, K, B, Rb, H2O, Cs and U. The annual fluxes calculated for some of these species is of the same order of magnitude as the annual river influxes into the ocean.
Chapter
Evaporites are probably the most significant of climatically sensitive sediments, because they form in only regions where the rate of evaporation greatly exceeds the rate of rainfall plus runoff. Evaporites, coals, carbonates, tillites, and thick clastic deposits are among some of the climate-sensitive sediments that have been used as qualitative and indirect evidence for paleoclimatic conditions.
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The mass of the Phanerozoic sediments is about 2.1 x 1018 metric tons, and between a quarter and one-third of it is distributed on the present continental margins and deep sea floor. The survival rate (surviving mass per unit time of deposition) seems to decrease exponentially with advancing age back to the Carboniferous, beyond which the tail of the distribution holds up and is somewhat irregular. The distribution for the past 300 Ma can be expressed by the equation log s = 10.01 - 0.24t, where s is the survival rate in metric tons per year and t is in units of 100 Ma. For a constant sediment mass with constant probability of destruction, this corresponds to a mean sedimentation rate since Devonian time of 101(} metric tons per year and a half-life for the post-Devonian mass of about 130 Ma. The overall distribution, however, is the sum of the distributions for three major realms, cratonic, marginal and pelagic, each with its own characteristic pattern. The last two account for most of the exponential-looking trend in the later Phanerozoic, but it is not clear (nor, in the case of the pelagic sediments, likely) that they are themselves exponential in character.The surviving masses (per unit time) of the Phanerozoic Systems tend to decrease with advancing age. This trend was revealed by a series of global volumetric estimates for the Devonian through Jurassic Systems (Ronov 1959) and extrapolations to the rest of the Phanerozoic based on a rough correlation between the volumes of the Systems surveyed and their maximum known thicknesses (Gregor 1967). Ronov and his colleagues have lately published (Ronov et al. 1980) complete volumetric estimates for those parts of the Phanerozoic Systems that underlie the global land surface. These estimates (after subtraction of the volcanic component and conversion from volume to mass) are summarized here in Table 3, Column 4. To them, in order to obtain the complete Phanerozoic mass-age distribution, must be added the sediments of the continental margins and deep sea. For convenience of study, the submarine realm can be divided into: 1. 'Passive' continental margins as defined by the US National Academy of Sciences (1979) and by Sclater et al. (1980). Examples are the margins bordering the Atlantic and Arctic Oceans and the Indian Ocean as far east as the Andaman Sea; 2. Marginal basins (Sclater et al. 1980) separated from ocean ridges by tectonic barriers (trenches, arcs, continental crust). They include such regions as the Caribbean and Mediterranean Seas and the marginal basins of the western Pacific; 3. The deep sea floor. The areal distribution of these submarine sedimentary environments, as determined by Sclater et al. (1980) is, in millions of km2: passive margins, 52.2; marginal basins, 26.9; deep sea floor, 281.7.
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The basic quantitative distinction between global oceanic ridge volume and the global rate of seafloor generation is made fully explicit. From this, the question of inversion over time from the former quantity into the latter is then posed using a generalized expression to approximate global subduction zone distribution. Two numerical methods are described. Then, assuming the hypothesis that long-term (108 yr) eustatic sealevel change is due primarily to changing ridge volume, an inversion of a widely cited Phanerozoic sealevel curve (Vail) is also presented. The approach taken here is expected to be of direct importance for quantitative models of the carbonate-silicate cycle which seek to develop scenarios for atmospheric carbon dioxide variation over geologic time scales. Indeed, the testing of sealevel inversion, as performed here, may ultimately come from its degree of correspondence with past climate variation.-Author
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Because it lies at the intersection of Earth's solid, liquid, and gaseous components, sea level links the dynamics of the fluid part of the planet with those of the solid part of the planet. Here, I review the past quarter century of sea-level research and show that the solid components of Earth exert a controlling influence on the amplitudes and patterns of sea-level change across time scales ranging from years to billions of years. On the shortest time scales (10(0)-10(2) yr), elastic deformation causes the ground surface to uplift instantaneously near deglaciating areas while the sea surface depresses due to diminished gravitational attraction. This produces spatial variations in rates of relative sea-level change (measured relative to the ground surface), with amplitudes of several millimeters per year. These sea-level "fingerprints" are characteristic of (and may help identify) the deglaciation source, and they can have significant societal importance because they will control rates of coastal inundation in the coming century. On time scales of 10(3)-10(5) yr, the solid Earth's time-dependent viscous response to deglaciation also produces spatially varying patterns of relative sea-level change, with centimeters-per-year amplitude, that depend on the time-history of deglaciation. These variations, on average, cause net seafloor subsidence and therefore global sea-level drop. On time scales of 10(6)-10(8) yr, convection of Earth's mantle also supports long-wavelength topographic relief that changes as continents migrate and mantle flow patterns evolve. This changing "dynamic topography" causes meters per millions of years of relative sea-level change, even along seemingly "stable" continental margins, which affects all stratigraphic records of Phanerozoic sea level. Nevertheless, several such records indicate sea-level drop of similar to 230 m since a mid-Cretaceous highstand, when continental transgressions were occurring worldwide. This global drop results from several factors that combine to expand the "container" volume of the ocean basins. Most importantly, ridge volume decrease since the mid-Cretaceous, caused by an similar to 50% slow-down in seafloor spreading rate documented by tectonic reconstructions, explains similar to 250 m of sea-level fall. These tectonic changes have been accompanied by a decline in the volume of volcanic edifices on Pacific seafloor, continental convergence above the former Tethys Ocean, and the onset of glaciation, which dropped sea level by similar to 40, similar to 20, and similar to 60 m, respectively. These drops were approximately offset by an increase in the volume of Atlantic sediments and net seafloor uplift by dynamic topography, which each elevated sea level by similar to 60 m. Across supercontinental cycles, expected variations in ridge volume, dynamic topography, and continental compression together roughly explain observed sea-level variations throughout Pangean assembly and dispersal. On the longest time scales (10(9) yr), sea level may change as ocean water is exchanged with reservoirs stored by hydrous minerals within the mantle interior. Mantle cooling during the past few billion years may have accelerated drainage down subduction zones and decreased degassing at mid-ocean ridges, causing enough sea-level drop to impact the Phanerozoic sea-level budget. For all time scales, future advances in the study of sea-level change will result from improved observations of lateral variations in sea-level change, and a better understanding of the solid Earth deformations that cause them.
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The carbonate-silicate geochemical cycle of Berner et al. (1983) is extended to include reactions involving organic carbon and sulfur. Affirms the link between CO2, paleoclimate, and tectonism. -from Authors
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Data on mass-age distributions and latitudinal limits of Mesozoic-Cenozoic shelf carbonate, on Cenozoic deepening of the CCD in all major ocean basins, and on the abundance of pelagic chalk in epicontinental carbonate sequences, indicate that the primary locus of global limestone deposition has gradually shifted from shallow cratonic to deep oceanic settings since Jurassic time. This change is generally coincident with progressive emergence of continents during global regression as well as with taxonomic diversification of calcareous plankton. -from Authors
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The distribution of area of the ocean floor with age, t, is approximately described by dA/dt=C0(1-(t/tm)), where C0 is the rate of crustal generation and tm the maximum age. A linear differential area versus age relation can be explained by a balance between generation and consumption where consumption is uniformly distributed with age. The present distribution of consumption with age was estimated from the isochron map used to derive the area-age relation and a recently published set of angular velocity vectors describing present plate motions. The trenches appear to be distributed randomly with respect to age provinces in the oceans. Changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area-age distribution. In turn, these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states. This coupling between driving forces and the area-age distribution provides a feedback mechanism limiting the extent of any changes. A measure of the magnitude of shifts in the area-age distribution is given by global changes in sea level. The area-age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution, parameterized in terms of age, and exact expressions for the heat loss from the ocean floor.-Author
Article
We present an atmospheric p CO2 (p is partial pressure) curve showing extreme fluctuations for the interval between ca. 77 and 63 Ma in southern Alberta, Canada, using a paleosol barometer. Paleosol carbonate nodules (micrite) were collected from 40 Bk horizons among 6 stratigraphic sections for stable carbon isotope analysis. Based on results from the study area, declining atmospheric p CO2 from 1200 ppmV (V is volume) in the Campanian to 780 ppmV in the Maastrichtian correlates with Late Cretaceous climate cooling and falling sea level as documented in global records. The remarkable rise in atmospheric p CO2 near 65.5 Ma (1440 ppmV) correlates with volcanic activity associated with the Deccan Traps, rising sea level, and warmer global climates. The decline in atmospheric p CO2 (760 ppmV) at the Cretaceous-Tertiary boundary and subsequent sharp rise into the Danian (1000 ppmV) occurred during static terrestrial temperatures and sea level. This work provides compelling evidence that atmospheric p CO2 curves modeled for the Phanerozoic do not offer the resolution needed to understand environmental conditions during catastrophic events in Earth's history.
Article
CO[sub 2] solubility, and the first and second stoichiometric dissociation constants of carbonic acid (K*[sub 1] and K*[sub 2]), have been determined in aqueous solutions containing Cl and SO[sub 4] salts of Na, K, Ca, and Mg over a wide range of ionic strength from 0 to 90[degrees]C at 0.1032 MPa (1 atm) pressure. Activity coefficients for CO[sub 2(aq)], HCO[sub 3][sup [minus]], and CO[sub 3][sup 2[minus]] ions were calculated from m[sub CO[sub 2]], K*[sub 1] and K*[sub 2]. These activity coefficients were then used to derive Pitzer parameters for the interaction between these carbonate species and other ions between 0 and 90[degrees]C. The resulting Pitzer model for the carbonic acid system was verified by its ability to accurately predict experimentally determined calcite solubility in brines from 0 to 90[degrees]C. However, calcite solubility measurements in complex brines initially supersaturated with respect to calcite indicate that Na[sup +], Mg[sup 2+], and SO[sub 4][sup 2[minus]] ions are probably incorporated into calcite lattice and significantly increase calcite solubility in brines. 50 refs., 8 figs., 7 tabs.
Article
Instead of having been more or less constant, as once assumed, it is now apparent that the major ion chemistry of the oceans has varied substantially over time. For instance, independent lines of evidence suggest that calcium concentration ([Ca2+]) has approximately halved and magnesium concentration ([Mg2+]) approximately doubled over the last 100 million years. On the other hand, the calcite compensation depth, and hence the CaCO3 saturation, has varied little over the last 100 My as documented in deep sea sediments. We combine these pieces of evidence to develop a proxy for seawater carbonate ion concentration ([CO32−]) over this period of time. From the calcite saturation state (which is proportional to the product of [Ca2+] times [CO32−], but also affected by [Mg2+]), we can calculate seawater [CO32−]. Our results show that [CO32−] has nearly quadrupled since the Cretaceous. Furthermore, by combining our [CO32−] proxy with other carbonate system proxies, we provide calculations of the entire seawater carbonate system and atmospheric CO2. Based on this, reconstructed atmospheric CO2 is relatively low in the Miocene but high in the Eocene. Finally, we make a strong case that seawater pH has increased over the last 100 My.
Article
Secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites occur in phase on a 100 200 m.y. time scale such that periods of “aragonite seas” are synchronized with MgSO4 evaporites and periods of “calcite seas” with KCl evaporites. It is proposed that these coupled changes are the result of secular variation in seawater chemistry controlled primarily by fluctuations in the mid-ocean ridge hydrothermal brine flux, which in turn have been driven by fluctuations in the rate of ocean crust production. Quantitative predictions based on this hypothesis yield secular variation in limestone and potash evaporite mineralogies that closely match the observed variation over the past 600 m.y., providing strong support for the thesis that seawater chemistry, rather than remaining constant, has oscillated significantly over geologic time.
Article
A reassessment of the abundance of dolomite in carbonate sediments has confirmed that carbonates deposited during the past 150 Ma contain, on average, less dolomite than Proterozoic and Paleozoic carbonates. The lower dolomite content of the more recent carbonate sediments results from the increase in the deposition of CaCO3 in deep-sea sediments, and to the difficulty of dolomitizing deep-sea CaCO3 by reaction with cold, unevaporated seawater. The decrease in the rate of dolomite formation during the past 150 Ma has led to an increase in the output of oceanic Mg+ by the reaction of seawater with clay minerals and with ocean-floor basalts. The increase in the output of marine Mg+ into these reservoirs has been brought about by an increase in the Mg+ concentration of seawater. During the past 40 Ma, the concentration of Mg+ in seawater has probably increased by ∼18 mmol/kg, and probably has been accompanied by an equimolar increase in the concentration of SO4−.
Article
The turnover of water and 18O in the outer terrestrial sphere is investigated considering mantle degassing, subduction zone processes, hydrothermal circulation at spreading centers, seafloor alteration, and continental weathering processes. Mass balances indicate that the ocean currently loses water to the mantle because the water emission by mantle degassing proceeds at a significantly slower rate than the subduction of water structurally bound in the down-going slab. The current input of 18O into the ocean through hydrothermal circulation and water emissions at arc volcanoes surpasses the fixation of 18O via low-temperature water/rock interactions at the seafloor and on continents, inducing an increase in marine δ18O values. Results of a box model simulating the Phanerozoic water and 18O cycles suggest that the mass of seawater decreased significantly causing a continuous drop in global sea-level by several hundred meters over the Phanerozoic. Model results and mass balances also allow for an enhanced estimate of current water fluxes in subduction zones consistent with the secular changes in sea-level and marine δ18O observed in the geological record. Moreover, the model generates a secular trend for seawater δ18O—produced by the surplus of 18O inputs and through internal feed-backs associated with isotopic exchange reactions at the seafloor—comparable to that observed in Phanerozoic carbonates. This coincidence suggests that the marine carbonates record a continuous change in isotopic composition of seawater with superimposed temperature-related fluctuations. A continuous record of near-surface temperatures was calculated using the model curve for seawater δ18O and the corresponding carbonate data. This new climate record indicates three icehouse-greenhouse cycles with a duration of 127 My between the Cambrian and the Triassic followed by an additional cycle with extended periodicity spanning the Jurassic to Cenozoic. Simulations of the Precambrian water and 18O cycles imply that the strong 18O depletion in seawater during the early Cambrian (δ18O around −8 ‰) was caused by enhanced weathering, diminished hydrothermal activity and extreme glaciations during the preceding late Neo- proterozoic.
Article
Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO2, SO4), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO2, Ca, K, Ba, B and CO2 were leached from basalt and enriched in solution. SiO2 concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.
Article
87 Sr/ 86 Sr measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest 87 Sr/ 86 Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low 87 Sr/ 86 Sr in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate.
Article
The relationships between the global carbon cycle and paleo-climates on short and long time scales have been based on studies of accumulation rate of the two main components of the sedimentary carbon reservoir, organic carbon and carbonate carbon. Variations in the rate and proportion of carbonate burial through Phanerozoic time have been attributed to the effects of tectonics on eustasy, atmospheric CO2 concentration, MOR (Mid-Ocean Ridge) hydrothermal flux, and weathering and riverine flux.This study addresses the history of variations in the state of the surface ocean and its degree of saturation with respect to calcite and aragonite, based on a geochemical model that considers the Phanerozoic atmospheric PCO2 and surface ocean temperature reconstructions as the main forcings on the system. The results show that, using near-present-day values of ocean salinity and alkalinity, the Early Paleozoic and Middle Mesozoic oceans are calculated to be undersaturated (or nearly undersaturated) with respect to CaCO3. For the near-present-day values of supersaturation (Ω=ICP/Ksp) of 3–5 with respect to calcite, paleo-alkalinity of ocean water would have been up to 2.5 times greater than at present, although the pH values of surface ocean water would have been somewhat lower than the present values. This alkalinity factor is consistent with a higher calcium concentration (up to ×2.5) due to increased circulation at ocean spreading-zones and also higher salinity (up to ×1.5) attributed by other authors to segments of the geologic past. Our model results indicate that although PCO2 was a contributing factor to shifts between calcite and aragonite saturation of seawater, additional changes in alkalinity were needed to maintain supersaturation at the level of 3–5, comparable to the present. Continental weathering of crystalline and older carbonate rocks, in addition to MOR (Mid-Ocean Ridge) circulation, was likely an important mechanism for maintaining supersaturation of surface ocean water, particularly during times of increased carbonate storage.
Article
The chemical evolution of seawater during the Phanerozoic is still a matter of debate. We have assembled and critically analyzed the available data for the composition of fluid inclusions in marine halite and for the mineralogy of marine evaporites. The composition of fluid inclusions in primary marine halite reveals two major long-term cycles in the chemistry of seawater during the past 600 myr. The concentration of Mg2+, Ca2+, and SO42− has varied quite dramatically. The Mg2+ concentration in seawater during most of the early Paleozoic and Jurassic to Cretaceous was as low as 30 to 40 mmol/kg H2O; it reached maximum values ≥50 mmol/kg H2O during the Late Neoproterozoic and Permian. The Ca2+ concentration in seawater during the Phanerozoic has reached maximum values two to three times greater than the concentration in seawater today (10.6 mmol/kg H2O), whereas SO42− concentrations may have been as low as 5 to 10 mmol/kg H2O (a third to a fifth of the modern value) during the Jurassic and Early Paleozoic. The Mg2+/Ca2+ ratio in seawater ranged from 1 to 1.5 during the early to middle Paleozoic and Jurassic-Cretaceous to a near-modern value of 5.2 during the Late Neoproterozoic and Permian. This change in seawater Mg2+/Ca2+ ratio is consistent with the notion of alternating “calcite-aragonite seas” recorded in oölites and marine carbonate cements.
Article
Phosphorus is probably the major limiting nutrient on marine primary productivity at geological timescales, although other elements (e.g., N, Fe) can be transiently biolimiting. It has been inferred that remineralization of organic P and upwelling of nutrient-rich deepwaters have the potential to stimulate primary productivity, and that measurements of sedimentary (C:P)org ratios in ancient sediments can yield insights on nutrient fluxes and productivity levels in paleoceans. However, recent work has shown that (1) sedimentary (C:P)org ratios exceed the Redfield ratio (106:1) in most environments as a result of preferential bacterial destruction of labile P-bearing compounds, and (2) the inorganic P fractions associated with Fe-bound and authigenic phosphate phases in anoxic sediments are largely of organic derivation, representing remineralized P that has been fixed through redox-related processes. These insights suggest that the most useful measure of nutrient regeneration is the nondetrital P fraction of sediments, a variable that is rarely determined but that can be adequately proxied by total P in anoxic facies, in which the detrital P fraction is typically small. In this study, Corg:P ratios were determined for 60 anoxic facies of Cambrian through Recent age. The Recent facies yield a mean Corg:P of 65±25, values similar to those for most anoxic facies of Mississippian and younger age. In contrast, many Cambrian-Devonian anoxic facies yield substantially higher Corg:P ratios, with the highest values (>400) in the Middle-Upper Cambrian and Middle-Upper Devonian. For the Phanerozoic as a whole, the logarithmic mean Corg:P ratio declines by a factor of four, from ~260:1 in the Cambrian to 65:1 in the Recent, a statistically robust result. Given the importance of redox controls on sedimentary P retention at a local scale, this pattern can be interpreted as a record of benthic redox conditions through time, i.e., paleocean ventilation. Although Recent anoxic facies are oxygen-depleted at present (i.e., an instantaneous condition), their relatively low Corg:P ratios suggest that, on a time-averaged basis at timescales associated with sedimentary P retention (i.e., 103-104 y), these environments are not nearly as anoxic as their Early-Middle Paleozoic counterparts. Anoxic facies of all ages experience episodic "freshening" events, in which small quantities of oxygen from surface waters are transferred below the chemocline by storm mixing, turbidite flows, or overspill into silled basins. Such freshening events have imparted an "oxic" Corg:P signature to Recent anoxic facies but not to Cambrian-Devonian anoxic facies. The key difference probably lies in atmospheric O2 levels, which may have been sufficiently low during the Cambrian-Devonian that "freshening" events did little to mitigate oxygen-poor conditions in contemporaneous deepwaters. The abrupt mid-Paleozoic decline in sedimentary Corg:P ratios can be attributed to burial of large quantities of organic matter as marine black shales during the Devonian and as freshwater coals during the Carboniferous, effecting a permanent increase in atmospheric pO2 and improved deep ocean ventilation.
Article
ROBERT A. BERNER. 2004. Oxford University Press, New York. 158 pp. $99.50. ISBN: 0-1951-7333-3. The global carbon cycle is a topic of great interest because it is a critical controller of earth's climate. Despite the great amount of research done on the global carbon cycle, many gaps remain in our