Carbon Sequestration Through Time and Its Role as an Overlooked Driver of Earth's Long-Term Climate History
Modeling of Earth’s long term (> 109 y) climate history is largely dependent on our ability to decipher clues preserved in ancient sedimentary rock and fossils. Advances in micron-scale analytical techniques, such as secondary ion mass spectrometry (SIMS) analysis of δ18O variability at the 10μm-scale, EPMA and synchrotron-based trace metal concentration and redox mapping, and crystallographic orientation mapping at the micron scale, allow us to clearly parse diagenetic effects and determine an original marine isotopic signature, or at least a signature locked in during the first stages of turning sedimentary particles into sedimentary rock while still bathed in marine water. A result of these efforts is a new high-resolution δ18O curve for the past 1.2 billion years that relies on data from shallowly buried sedimentary successions. When these δ18O data are combined with data from similarly selected clumped-isotope analyses, it is possible to untangle the confounding effects of the δ18O composition of ancient seawater and its temperature. In so doing, we provide a respite from an entanglement that has occupied paleoclimatologists for over 50 years. We deploy this new combined δ18O-clumped-isotope temperature curve to elucidate the following. First, δ18O variability in ancient minerals at the scale of 108 - 109 my is best explained by varying sea surface temperatures rather than through changes in the δ18O composition of seawater itself; the latter fails to explain a conspicuous lower bound in clumped-isotope calculated water δ18O composition of around -1.2‰ SMOW, the generally accepted value for recent oceans on an ice-free Earth, that is robust across 1.2 billion years’ worth of data. Second, Earth’s climate appears to have overall cooled in a step-wise fashion through time which requires a powerful mechanism. When our new best estimate of temperature, using both moving minima from clumped-isotope temperatures and temperatures calculated from mineral δ18O data, are compared to the age distribution of carbonate rock, it reveals that Earth’s periods of significant climatic cooling correspond closely to biological innovations in eukaryotic clades—first at the invention of calcareous biomineralization by the metazoa at the Proterozoic-Phanerozoic transition, and again at the evolutionary expansion of calcareous plankton in the Mesozoic to Cenozoic. A common assumption in long term climate models is to treat bulk marine carbonate rock formation as a steady-state, responsive, sink for CO2; a treatment that is predicated, in part, on the assumption that Precambrian marine carbonate sediments behaved (vis a vis distribution, volumes and fluxes) more or less like Cenozoic carbonates. Far from being a passive player in the climate system, biomineralization appears to have exerted significant influence on Earth’s long-term climate through changes in carbon sequestration through time.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019