CIFAR Postdoctoral Fellow
The alteration of our current climate is one of the greatest problems of this generation. Given the importance of carbon dioxide and the carbon cycle in determining global climate, much is unknown about the dynamics of this system. The removal and storage of carbon, dominantly through the burial of carbon-bearing material in the ocean, is a critical component of this system that is less understood. This subsurface burial environment is a dynamic chemical and biological reactor, with bacteria and archaea consuming organic carbon resulting in the alternation of the organic carbon to dissolved inorganic carbon. The fate and impact of this dissolved inorganic carbon is largely unknown, with potential alternatives including the precipitation of carbonate minerals (resulting in long-term storage of the carbon) and the diffusion of this carbon back to the ocean and eventually the atmosphere (resulting in a greater contribution to climate change).
My postdoctoral research focuses on understanding the fate of this inorganic carbon, specifically comparing the conditions for modern formation with those found in the Cretaceous Ocean. In contrast to the modern, the Cretaceous oceans had high calcium concentrations (double modern), low sulfate concentrations (10% of modern) and high pCO2 (~10x modern) and therefore lower marine pH. Lower sulfate concentrations suggest less subsurface organic matter oxidation and thus less subsurface DIC production. And yet high calcium concentrations suggest more calcium for precipitation of DIC in the subsurface. Does this lead to lower, or higher, overall formation of authigenic carbonate? Understanding the differences and the importance of this process during both time periods should have important contributions to our understanding of the carbon cycle. Ongoing work to address these questions involve the construction of a flow through reactor to simulate these reactions in the laboratory, along with analysis of materials from hardground material from both modern and Cretaceous environments.
Lead (Pb) isotopes in ferromanganese minerals and residual silicate fractions is one such tracer that has the potential to characterize changes in these different sources. The first of these, Pb found in the ferromanganese coatings of sediment grains, has been shown to form in equilibrium with bottom water chemistry. As such, changes in source waters and/or hydrothermal fluxes can be recorded by changes in the Pb isotopic ratio at a particular site through time. Additionally, dust deposition in the equatorial open ocean is controlled by the location of the Intertropical Convergence Zone (ITCZ). Changes in the position of the ITCZ over time may therefore be tracked by monitoring changes in dust deposition and provenance. Since mineral dust particles from different source regions typically have distinct Pb isotopic signatures, the isotopic ratios of the dust-derived component (e.g. residual silicate fraction) in open ocean sediments could be used to reconstruct dust provenance. During my dissertation work, I have constructed records of these proxies for samples across the Pacific Ocean, with a particular focus on the Eastern Equatorial Pacific.
In Erhardt et al. (2013) we used the accumulation rates of marine barite, an inorganic precipitate whose accumulation correlates to the export of decayed organic matter, as a proxy for export production. In this paper, we investigated the barite accumulation rate (BAR) during the Eocene-Oligocene transition (EOT), a period marked by a switch from a greenhouse to an icehouse climate. We saw that during the EOT export production declined, implying that the biological pump did not contribute to carbon sequestration and the cooling over this transition. However, before the EOT, we observed a sharp increase in BAR. This enhanced export production and the associated carbon sequestration may have allowed for the additional sequestration of carbon in the Late Eocene, potentially contributing to the pCO2 drawdown at the onset of Antarctic glaciation.