Study of the processes by which surface carbon is transported into the deep Earth is key to understanding modern volatiles recycling and longer-term Earth degassing and atmosphere evolution. Although subduction is considered the primary means by which carbon is returned to the mantle, little is known regarding the efficiency of this process and the chemical forms of this deeply subducted carbon (presumably as carbonate and as reduced carbon, the latter representing organic matter). Through geologic time, changes in the return of deeply subducted carbon to the atmosphere, via volcanic eruptions, could have led to significant change in the carbon dioxide content of the atmosphere and thus change in global surface temperatures. Anthropogenic (human) carbon dioxide additions to the atmosphere, largely due to burning of fossil fuels, are superimposed on these longer-timeframe, "natural" variations in atmospheric carbon dioxide concentration related in part to subduction zone inputs and volcanic outputs.
This research will examine the subduction pathway for carbon via field studies of high- and ultrahigh-pressure metamorphic rocks, providing valuable field-based "ground truthing" of recent theoretical and experimental studies of carbon cycling. This study will represent a first step in more fully characterizing the forearc-to-subarc metamorphic carbon flux in key lithologies (sedimentary, mafic, and ultramafic rocks) thought to convey carbon to depths beneath volcanic fronts and greater. Work will mostly be concentrated on localities in the French and Italian Alps allowing examination of ophiolitic rocks and related seafloor sediments metamorphosed over a wide range of high-pressure (HP)/ultrahigh-pressure (UHP) conditions. For each lithology and locality, field observations, petrologic study, and stable isotope analyses will focus on identifying any decarbonation that was experienced, relating these observations from study of the rocks with theoretical assessments of devolatilization history.