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Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ¹³C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ¹³C and δ¹⁸O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ¹³C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ¹³C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ¹³C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ¹³C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.

 

Abstract. We extend the ecological component (ECOGEM) of the carbon-centric Grid-Enabled Integrated Earth system model (cGEnIE) to include a diatom functional group. ECOGEM represents plankton community dynamics via a spectrum of ecophysiological traits originally based on size and plankton food web (phyto- and zooplankton; EcoGEnIE 1.0), which we developed here to account for a diatom functional group (EcoGEnIE 1.1). We tuned EcoGEnIE 1.1, exploring a range of ecophysiological parameter values specific to phytoplankton, including diatom growth and survival (18 parameters over 550 runs) to achieve best fits to observations of diatom biogeography and size class distribution as well as to global ocean nutrient and dissolved oxygen distributions. This, in conjunction with a previously developed representation of opal dissolution and an updated representation of the ocean iron cycle in the water column, resulted in an improved distribution of dissolved oxygen in the water column relative to the previous EcoGEnIE 1.0, with global export production (7.4 Gt C yr−1) now closer to previous estimates. Simulated diatom biogeography is characterised by larger size classes dominating at high latitudes, notably in the Southern Ocean, and smaller size classes dominating at lower latitudes. Overall, diatom biological productivity accounts for ∼20 % of global carbon biomass in the model, with diatoms outcompeting other phytoplankton functional groups when dissolved silica is available due to their faster maximum photosynthetic rates and reduced palatability to grazers. Adding a diatom functional group provides the cGEnIE Earth system model with an extended capability to explore ecological dynamics and their influence on ocean biogeochemistry.

 

Climate and continental configuration combined to make Early Paleozoic animals susceptible to extinction. 

The largest mass extinction event is associated with changes in degassing style of the Siberian Traps volcanism. 

Paleontological reconstructions of plankton community structure during warm periods of the Cenozoic (last 66 million years) reveal that deep-dwelling ‘twilight zone’ (200–1000 m) plankton were less abundant and diverse, and lived much closer to the surface, than in colder, more recent climates. We suggest that this is a consequence of temperature’s role in controlling the rate that sinking organic matter is broken down and metabolized by bacteria, a process that occurs faster at warmer temperatures. In a warmer ocean, a smaller fraction of organic matter reaches the ocean interior, affecting food supply and dissolved oxygen availability at depth. Using an Earth system model that has been evaluated against paleo observations, we illustrate how anthropogenic warming may impact future carbon cycling and twilight zone ecology. Our findings suggest that significant changes are already underway, and without strong emissions mitigation, widespread ecological disruption in the twilight zone is likely by 2100, with effects spanning millennia thereafter.

 

Abstract The fossil record of marine invertebrates has long fuelled the debate as to whether or not there are limits to global diversity in the sea 1–5 . Ecological theory states that, as diversity grows and ecological niches are filled, the strengthening of biological interactions imposes limits on diversity 6,7 . However, the extent to which biological interactions have constrained the growth of diversity over evolutionary time remains an open question 1–5,8–11 . Here we present a regional diversification model that reproduces the main Phanerozoic eon trends in the global diversity of marine invertebrates after imposing mass extinctions. We find that the dynamics of global diversity are best described by a diversification model that operates widely within the exponential growth regime of a logistic function. A spatially resolved analysis of the ratio of diversity to carrying capacity reveals that less than 2% of the global flooded continental area throughout the Phanerozoic exhibits diversity levels approaching ecological saturation. We attribute the overall increase in global diversity during the Late Mesozoic and Cenozoic eras to the development of diversity hotspots under prolonged conditions of Earth system stability and maximum continental fragmentation. We call this the ‘diversity hotspots hypothesis’, which we propose as a non-mutually exclusive alternative to the hypothesis that the Mesozoic marine revolution led this macroevolutionary trend 12,13 . 

Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric pCO2, respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmosphericpCO2. Here we extend and calibrate a temperature-dependent representation ofmarine carbon cycling in the cGENIE.muffin Earth system model, intended forboth past and future climate applications. In this, we combine atemperature-dependent remineralisation scheme for sinking organic matterwith a biological export production scheme that also includes a dependenceon ambient seawater temperature. Via a parameter ensemble, we jointlycalibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopicsignature (δ13C) of dissolved inorganic carbon in the oceanwith modern observations. We additionally explore the role of temperature inthe creation and recycling of dissolved organic matter (DOM) and hence itsimpact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version showsa fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact ofaccounting for temperature-dependent remineralisation of POM is in drivinghigher rates of remineralisation in warmer waters, in turn driving a morerapid return of nutrients to the surface and thereby stimulating organicmatter production. As a result, more POM is exported below 80 m but onaverage reaches shallower depths in middle- and low-latitude warmer waterscompared to the standard model. Conversely, at higher latitudes, colderwater temperature reduces the rate of nutrient resupply to the surface andPOM reaches greater depth on average as a result of slower subsurface ratesof remineralisation. Further adding temperature-dependent DOM processeschanges this overall picture only a little, with a slight weakening ofexport production at higher latitudes. As an illustrative application of the new model configuration andcalibration, we take the example of historical warming and briefly assessthe implications for global carbon cycling of accounting for a more completeset of temperature-dependent processes in the ocean. We find that betweenthe pre-industrial era (ca. 1700) and the present (year 2010), in response to asimulated air temperature increase of 0.9 ∘C and an associatedprojected mean ocean warming of 0.12 ∘C (0.6 ∘C insurface waters and 0.02 ∘C in deep waters), a reduction inparticulate organic carbon (POC) export at 80 m of just 0.3 % occurs (or 0.7 % including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80 m) to the deep ocean (at 1040 m) is reduced by 5 %. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80 m falls by 2.9 % between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport.