The dawn of calcifying plankton: a revolution in climate regulation

Climate is regulated by the carbon cycle. A key component of this is the biological pumping of organic and inorganic carbon into the deep ocean. This ocean carbon pumping underwent a fundamental transformation ~200 million years ago with the evolution and proliferation of calcifying plankton, whose calcium carbonate skeletons sink to the seafloor and accumulate as carbonate sediments. The ‘carbonate pump’ was a major evolutionary innovation, with repercussions for the regulation of climate by 1) altering the partitioning of carbon between the atmosphere, surface, and deep ocean and 2) creating a new, highly reactive deep sea carbonate sediment reservoir, available to dissolve and buffer the earth-climate system against perturbation. Dissolution of deep sea carbonates will ultimately remove anthropogenic CO2 from the atmosphere and help neutralise ocean acidification. How different were carbon cycle perturbations in a world without this buffer? Ultimately, this may hold the key to understanding mass extinctions, both past and present.

This studentship will firstly explore how the evolution of the carbonate pump changed the spatial patterns of carbon distribution in the ocean and partitioning of carbon between the ocean, atmosphere, and marine sediments. Building on this knowledge, the student will address how the carbonate pump affects the earth system response to a change in pCO2/climate. The rock record is peppered with carbon cycle perturbations caused by the rapid emission of CO2 to the atmosphere. Can the advent of the carbonate pump explain the differences observed between Mesozoic abrupt CO2 release events (end-Permian, end-Triassic mass extinctions) and the later Cenozoic hyperthermals? Did the lack of a carbonate pump in the early Mesozoic leave the earth more susceptible to abrupt climate change and mass extinction?

This project will use an Earth system model with a sophisticated representation of biogeochemistry to explore the implications of the development of the carbonate pump for the earth-ocean-climate system. In particular, the studentship will focus on interpreting the first records of ocean temperature and ocean pH across the end-Triassic mass extinction, a palaeo-ocean acidification event broadly coincident with the development of the carbonate pump (a collaboration between St. Andrews and the University of Birmingham).

The project will utilize cGENIE, an intermediate complexity Earth System Model with state-of-the-art representation of biogeochemistry and biogeochemical cycling, including weathering, sediment diagenesis and burial, and full, spatially-resolved representation of the carbonate system.

Initial suites of model experiments will compare the earth-climate system at steady-state with and without a carbonate pump. Building on this, a further ensemble of model experiments will explore the response of the earth system to carbon cycle perturbations of different magnitudes and rates in the presence or absence of a carbonate pump. Building on this knowledge, model experiments will be run to interpret the first records of carbonate chemistry (surface ocean pH) and surface ocean temperature across the Triassic-Jurassic mass extinction, an interval for which there is great uncertainty about the strength of the incipient carbonate pump.