Tectonic control of the carbon cycle and climate: Resolving the effects of global spreading-rate variations with high temporal resolution over the past 20 Myr

Brown University, Providence RI

Investigators

Abstract

Plate tectonics elevates mountain chains, rearranges connections between the ocean basins, redistributes gasses between the Earth's mantle and surface, and can affect the Earth's climate at a variety of timescales. Volcanism, most of which occurs as a consequence of plate tectonics, releases carbon dioxide (CO2) into the atmosphere. The first-order control on this degassing comes from volcanism associated with seafloor spreading and crustal construction at mid-ocean ridges and volcanism at subduction zones. Therefore, it is likely that changes in the rate of seafloor spreading can cause changes in the atmospheric CO2 concentration over time. This project seeks to investigate the role of seafloor spreading in contributing to a global cooling of 8-10 degrees C that began around 16 million years ago and continued for at least 12 million years. Variations in spreading rate along the global mid-ocean ridge system over the past 20 Myr will be reconstructed. These records will be used to estimate the rates of oceanic crustal production and CO2 degassing during crustal production. The estimates of CO2 degassing can be compared to changes in ocean temperature over this time. Results will be put into models of the carbon cycle to assess the impact on atmospheric CO2 levels. This project supports the training of a graduate student in geophysics and paleoclimatology. It also supports the training of undergraduate students, and the project will lead to a new course at Brown University. This project will produce the first synthesis of global seafloor spreading rates and ocean crustal production for the past 20 Myr using newly available plate reconstructions at high temporal resolution, global compilations of seafloor fabric and magnetic anomalies, and the recently derived astronomical timescale for magnetic polarity reversals over this time period. The astronomical timescale provides a highly precise independent dating system that avoids the need to assume constancy of seafloor spreading between (coarsely spaced) radiometric tie points and the need to choose a particular ridge system as a template. New paleotemperature estimates via the alkenone method will permit a continuous reconstruction of ocean temperatures 0-20 Myr. Results will be integrated into a global carbon cycle model with the aim of testing the hypothesis that a global slowing of spreading rates caused atmospheric CO2 levels to fall by several hundred ppm since ~14 Myr and led to global cooling.