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Cross-Cutting Project 3

The Role of the Deep Sub-Seafloor Biosphere for the C-Cycle and Climate

Objectives

Quantification of the dissolved organic carbon (DIC) flux from deeply buried sediments back to the ocean, and establishment of geochemical and climate models to reveal potentially unrecognized feedback mechanisms.

Project CCP3 has started investigations in April 2015. The postdoc positition is filled by Marshall Bowles.
There is now compelling evidence that microbially mediated diagenesis of organic matter extends to great sub-seafloor depths. Although metabolic rates in deeply buried formations are substantially lower than in surface environments, the enormous size of the deep biosphere suggests that its potential impact on the cycles of C and other elements should not be ignored.
Currently, the rate of production of DIC by the deep biosphere and the flux of DIC back into the ocean are largely unconstrained. Likewise, it is unclear how the remineralization of buried organic matter scales with its concentration and fluxes into the sub-seafloor, and how fluxes of carbon and other elements from the sub-seafloor back to the ocean relate to the activity and size of the deep biosphere. We test if the the long-term return of DIC via the deep sub-seafloor microbial pump significantly influences the carbon budget of the ocean and its isotopic composition on time scales of 10.000-100.000 years. The largely unrecognized feedback mechanism could be responsible for temporal offsets between variations of isotopic compositions of sedimentary carbonate species and ice volume during different intervals of the Cenozoic. To tackle the associated questions, we will utilize archived geochemical data indicative of organic carbon turnover for constraining fluxes of carbon into the modern ocean. A new module describing sub-seafloor biosphere processes will be incorporated into an isotope-enabled model of the geological carbon cycle, to quantify the impact of the sub-seafloor biosphere, and to investigate its potential lead-lag relationship with climate dynamics.

Key Hypothesis

The slow but pervasive organic matter turnover in sub-seafloor sediments significantly influences long-term variations in the oceanic reservoirs of carbon.