[Houghton Lecture] Ann Pearson (Harvard): Phytoplankton-mediated carbon isotope fractionation: Implications for pCO2 paleobarometry

Abstract:
Better estimates of past carbon dioxide levels in the atmosphere (pCO2) are critical to the advancement of climate science. While instrumental and ice-core records enable understanding of the current radiative forcing, geochemical archives offer insights into ancient pCO2. One of the most frequently used proxy methods is the discrimination of stable carbon isotopes by marine phytoplankton  (εp ≈δ13CCO2(aq) – δ13Cphyto), i.e., algal paleobarometry.  Estimating CO2 from εp requires quantifying the relationship between the supply of carbon from seawater and the intracellular demand for that carbon. The most common approach is to assume that εp is set by the ratio of diffusive demand:supply, that the fractionation primarily is governed by the kinetic isotope effect (KIE) of the carbon-fixing enzyme RuBisCO, and that the long-chain unsaturated ketones (“alkenones”) typical of the modern taxon, Emiliania huxleyi, and its close relatives are a robust recorder of these processes.  However, recent work on the KIE of E. huxleyi RuBisCO in tandem with environmental and laboratory studies have significantly altered the standard view, suggesting that photosynthetic irradiance independently influences 𝜀p, and/or that non-diffusive supply of CO2 is common. These factors may explain why alkenone εp does not accurately preserve Late Pleistocene glacial cycles. Our current efforts aim to address these discrepancies, both through models that incorporate additional intracellular compartmentalization of inorganic carbon, and through direct experiments with pure cultures. The results point toward future strategies to refine pCO2 reconstructions and improve climate models.

Houghton Lecture Series

Supported by the Houghton Fund, Houghton Lecturers are distinguished visitors from outside MIT invited by the EAPS Program in Atmospheres, Oceans and Climate (PAOC) to spend a period of time, ranging from a week to several months, as scientists-in-residence.

Contact: kbauer@mit.edu