Collaborative research: The role of pCO2 in the astronomically-paced climatic cycles of the Miocene

Earth's geologic record is paced by cyclical variations in the geometry of our planet's orbit around the sun, generally called Milankovitch cycles (named for the Serbian mathematician who first predicted the orbital variations). Over the last ~1 million years, these orbital variations caused global climatic oscillations between cold Ice Ages, when large ice sheets covered much of North America, and warmer Interglacial periods with climates similar to today's. Measurements of air bubbles of ancient atmosphere trapped in Antarctic ice cores has shown that these Ice Age cycles coincide with significant swings in atmospheric carbon dioxide (CO2) levels. This suggests that the CO2 greenhouse effect plays a role climate change on orbital timescales. However, estimates of atmospheric CO2 from ice cores cover only the last 800,000 years, leaving open the question of whether atmospheric CO2 played a role in the Milankovitch forcing of climate during earlier times in Earth history. This research aims to fill this major knowledge gap by applying geochemical methods to reconstruct ancient CO2 at unprecedented resolution during a time interval with well-documented Milankovitch cycles ~14 million years ago, when global climate was warmer, and the continents were in different positions compared to today. The resulting high-resolution CO2 record will be coupled to a cutting-edge Earth System model to test hypotheses regarding how Milankovitch cycles control Earth's carbon cycle and climate. Together, new geochemical records and modeling undertaken during this project promise to refine understanding of the role of atmospheric CO2 during warm climates -- an issue of significant societal importance in light of currently increasing atmospheric CO2. Results will be communicated to the scientific community by publications in widely-read scientific journals, and to grade school students through by the creation and distribution of an illustrated book exploring the interaction between atmospheric CO2 and climate in Earth history.

More specifically, this project aims to generate the first high-resolution (1 sample per ~5 thousand years) records of atmospheric pCO2 using the foraminiferal boron isotope proxy system coupled with matching proxy records of global temperature and carbon cycle dynamics in the mid-Miocene. This time interval was selected as an example of a warmer-than modern Earth with slightly higher background pCO2, smaller continental ice sheets, and well-defined Milankovitch cycles in climate and carbon cycling. The primary, high-resolution boron isotope record will be generated at Site 926 (Ceara Rise, Atlantic Ocean) with supplementary, lower resolution data generated at Sites 608 (North Atlantic) and 806 (Western Pacific). Boron isotope measurements will include the development and rigorous analytical testing of a new microsublimation method for boron separation from carbonate samples, which offers the prospect of smaller sample sizes and faster throughput. At Site 926, boron isotopes will be complemented by measurements of carbon isotopes (placing constraints on carbon cycling), oxygen isotopes (as a measure of paleotemperature and ice volume) and trace metal concentration including Mg/Ca (a paleotemperature proxy) and B/Ca (an emerging carbonate chemistry proxy) on both planktic and benthic foraminifera. These combined datasets will be used to reconstruct ocean temperatures and carbon cycle dynamics, and by comparison to the boron isotope-based CO2 record, allow estimates of temperature sensitivity to changing CO2 levels on orbital timescales. The new records will be used to guide model simulations of Milankovitch climate forcing in an Earth system model of intermediate complexity (cGENIE), providing quantitative constraints on the mechanisms and feedbacks responsible for the Milankovitch control of climate and carbon cycling. Contrasting mid-Miocene and Pleistocene Milankovitch cycles will enable a direct assessment of the role of large continental ice-sheets in modulating the dynamics, and role, of orbitally-forced pCO2 variations.