A mid Mesozoic revolution in the regulation of ocean chemistry

Abstract The Phanerozoic has seen fundamental changes in the global biogeochemical cycling of calcium carbonate (CaCO 3 ), particularly the advent of biomineralization during the early Cambrian when the products of weathering could first be removed through metabolic expenditure, and the subsequent ecological success of planktic calcifiers during the Mesozoic which shifted the locus of deposition from the continental shelves to the deep open ocean. These biologically-driven CaCO 3 depositional 'mode' changes along with geochemical and tectonic variations in boundary conditions such as sea-level and calcium ion concentrations all affected the carbonate chemistry of the ocean. I employ a model of atmosphere-ocean-sediment carbon cycling to explore the impact of these factors on the saturation state and carbonate chemistry of the global ocean during the Phanerozoic. The model results highlight that overall; the time evolution and regulation of Phanerozoic ocean chemistry is dominated by a Mid Mesozoic Revolution in the marine carbonate cycle. Prior to this transition, it was possible for the ocean to attain states of extreme saturation during the Permian and Triassic as well as during the late Precambrian. This is primarily a consequence of low sea-level in restricting the potential area for the deposition of shallow water carbonates, thus requiring a more saturated ocean and higher rate of precipitation per unit area is then required in order to balance weathering input. This is consistent with the occurrence of mineralogically 'anomalous' carbonates during these periods but not commonly at other times. That the modern carbon cycle does not respond to similar tectonic forcings is due to the ecological success of calcifying planktic taxa during the Mesozoic, which in facilitating the creation of a responsive deep-sea carbonate sink enabled a much greater degree of regulation of saturation state than was 3 previously possible

Development of a novel empirical framework for interpreting geological carbon isotope excursions, with implications for the rate of carbon injection across the PETM

AbstractAs an episode of rapid global warming associated with the release of massive quantities of carbon to the atmosphere and oceans, the Paleocene–Eocene Thermal Maximum (PETM, ∼56 Ma) is considered a potential analog for modern anthropogenic carbon emissions. However, the prevailing order of magnitude uncertainty in the rate of carbon release during the PETM precludes any straightforward comparison between the paleo-record and the modern. Similar barriers exist to the interpretation of many other carbon isotope excursions in the geological record. Here we use the Earth system model cGENIE to quantify the consequences of differing carbon emissions rates on the isotopic record of different carbon reservoirs. We explore the consequences of a range of emissions scenarios – from durations of carbon input of years to millennia and constant versus pulsed emissions rates, and trace how the isotopic signal is imprinted on the different carbon reservoirs. From this, we identify a characteristic relationship between the difference in carbon isotope excursion sizes between atmospheric CO2 and dissolved inorganic carbon (DIC) and the duration of carbon emissions. To the extent that available isotopic data spanning the PETM constrain the size of the marine and atmospheric carbon isotopic excursions, applying this empirical relationship suggests the duration of the component of carbon emissions that dominates the isotopic signal could be less than 3000 yr. However, utilizing the ratio of excursion size in the atmosphere to ocean as a metric to constrain duration of carbon emissions highlights the necessity to strengthen estimates for these two measurements across the PETM. Our general interpretive framework could be equally applied in assessing rates of carbon emissions for other geological events

Can organic matter flux profiles be diagnosed using remineralisation rates derived from observed tracers and modelled ocean transport rates?

he average depth in the ocean at which the majority of sinking organic matter particles remineralise is a fundamental parameter in the ocean's role in regulating atmospheric CO2. Observed spatial patterns in sinking fluxes and relationships between the fluxes of different particles in the modern ocean have widely been used to invoke controlling mechanisms with important implications for CO2 regulation. However, such analyses are limited by the sparse spatial sampling of the available sediment trap data. Here we explore whether model ocean circulation rates, in the form of a transport matrix, can be used to derive remineralisation rates and infer sinking particle flux curves from the much more highly resolved observations of dissolved nutrient concentrations. Initially we show an example of the method using a transport matrix from the MITgcm model and demonstrate that there are a number of potential uncertainties associated with the method. We then use the Earth system model GENIE to generate a synthetic tracer data set to explore the method and its sensitivity to key sources of uncertainty arising from errors in the tracer observations and in the model circulation. We use a 54-member ensemble of different, but plausible, estimates of the modern circulation to explore errors associated with model transport rates. We find that reconstructed re-mineralisation rates are very sensitive to both errors in observations and model circulation rates, such that a simple inversion cannot provide a robust estimate of particulate flux profiles. Estimated remineralisation rates are particularly sensitive to differences between the "observed" and modelled circulation because remineralisation rates are 3–4 magnitudes smaller than transport rates. We highlight a potential method of constraining the uncertainty associated with using modelled circulation rates, although its success is limited by the observations currently available. Finally, we show that there are additional uncertainties when inferring particle flux curves from reliable estimates of remineralisation rates due to processes that are not restricted to the vertical water column transport, such as the cycling of dissolved organic matter

Evaluation of coral reef carbonate production models at a global scale

Calcification by coral reef communities is estimated to account for half of all carbonate produced in shallow water environments and more than 25% of the total carbonate buried in marine sediments globally. Production of calcium carbonate by coral reefs is therefore an important component of the global carbon cycle; it is also threatened by future global warming and other global change pressures. Numerical models of reefal carbonate production are needed for understanding how carbonate deposition responds to environmental conditions including atmospheric CO₂ concentrations in the past and into the future. However, before any projections can be made, the basic test is to establish model skill in recreating present-day calcification rates. Here we evaluate four published model descriptions of reef carbonate production in terms of their predictive power, at both local and global scales. We also compile available global data on reef calcification to produce an independent observation-based data set for the model evaluation of carbonate budget outputs. The four calcification models are based on functions sensitive to combinations of light availability, aragonite saturation (Ω_a) and temperature and were implemented within a specifically developed global framework, the Global Reef Accretion Model (GRAM). No model was able to reproduce independent rate estimates of whole-reef calcification, and the output from the temperature-only based approach was the only model to significantly correlate with coral-calcification rate observations. The absence of any predictive power for whole reef systems, even when consistent at the scale of individual corals, points to the overriding importance of coral cover estimates in the calculations. Our work highlights the need for an ecosystem modelling approach, accounting for population dynamics in terms of mortality and recruitment and hence calcifier abundance, in estimating global reef carbonate budgets. In addition, validation of reef carbonate budgets is severely hampered by limited and inconsistent methodology in reef-scale observations

Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet

The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources

Quantifying the influence of the terrestrial biosphere on glacial–interglacial climate dynamics

The terrestrial biosphere is thought to be a key component in the climatic variability seen in the palaeo-record. It has a direct impact on surface temperature through changes in surface albedo and evapotranspiration (so-called biogeophysical effects) and, in addition, has an important indirect effect through changes in vegetation and soil carbon storage (biogeochemical effects) and hence modulates the concentrations of greenhouse gases in the atmosphere. The biogeochemical and biogeophysical effects generally have opposite signs, meaning that the terrestrial biosphere could potentially have played only a very minor role in the dynamics of the glacial–interglacial cycles of the late Quaternary. Here we use a fully coupled dynamic atmosphere–ocean–vegetation general circulation model (GCM) to generate a set of 62 equilibrium simulations spanning the last 120 kyr. The analysis of these simulations elucidates the relative importance of the biogeophysical versus biogeochemical terrestrial biosphere interactions with climate. We find that the biogeophysical effects of vegetation account for up to an additional −0.91 °C global mean cooling, with regional cooling as large as −5 °C, but with considerable variability across the glacial–interglacial cycle. By comparison, while opposite in sign, our model estimates of the biogeochemical impacts are substantially smaller in magnitude. Offline simulations show a maximum of +0.33 °C warming due to an increase of 25 ppm above our (pre-industrial) baseline atmospheric CO2 mixing ratio. In contrast to shorter (century) timescale projections of future terrestrial biosphere response where direct and indirect responses may at times cancel out, we find that the biogeophysical effects consistently and strongly dominate the biogeochemical effect over the inter-glacial cycle. On average across the period, the terrestrial biosphere has a −0.26 °C effect on temperature, with −0.58 °C at the Last Glacial Maximum. Depending on assumptions made about the destination of terrestrial carbon under ice sheets and where sea level has changed, the average terrestrial biosphere contribution over the last 120 kyr could be as much as −50 °C and −0.83 °C at the Last Glacial Maximum