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Estimates of sedimentary organic carbon burial fluxes based on inventory and isotope mass balance methods have been divergent. A new calculation of the isotope mass balance using a revised assessment of the inputs to the ocean-atmosphere system resolves the apparent discrepancy. Inputs include weathering of carbonate and old kerogen, geogenic methane oxidation, and volcanic and metamorphic degassing. Volcanic and metamorphic degassing comprise ≈23% of the total C input. Inputs from isotopically lightOCpetroandCH4-geodrive the mean δ13C of the input to =−8.0 ± 1.9‰, notably lower than the commonly assumed volcanic degassing value. The isotope mass balance model yields a modern burial flux =15.9 ± 6.6 Tmol y−1. The impact of the mid-Miocene Climatic Optimum isotope anomaly is an integrated excess deposition ≈ 4.3 × 106Tmol between 18 and 11 Ma, which is both longer and larger than estimates for the total degassing by the Columbia River Basalt eruptions, implying a complex carbon system response to large eruptive events. Monte Carlo evaluation finds that late Cenozoic net growth of the carbonate reservoir is very likely while net growth of theCorgreservoir is less certain but more likely than not. At present, subduction does not appear to keep up with net sedimentation and the overall masses of sedimentary carbonate and organic carbon are likely increasing. Growth in the sedimentaryCorgreservoir implies oxidation of the surface environment and likely increases in atmospheric pO2.
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This content will become publicly available on December 1, 2025
Re-Interpreting Sedimentary Carbon Isotope Mass Balances
Plain-Language Summary: The use of carbon isotopes to constrain the relative rates of carbonate and organic carbon cycling has a long history. Most workers have assumed that the inputs of C to the ocean atmosphere system have isotopic compositions close to that of mantle, but that leads to substantial lower estimate of organic carbon burial and potential oxygen generation than sediment inventory approaches have found. A re-evaluation of carbon input shows that oxidation of old organic carbon and methane result in a signibcantly lower value for the isotopic composition of inputs, implying larger rates of carbon burial that agree much more closely with the inventory approaches. The new results also show that the sedimentary reservoirs of carbonate and organic carbon are experiencing net growth over the last 35 million years and that should increase the oxidation state of the Earth surface environment. Consideration of carbon isotope cycling under low oxygen conditions that were characteristic of the Precambrian shows that the standard assumptions may be lead to substantial mass balance errors under such conditions
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- Award ID(s):
- 2141989
- PAR ID:
- 10563498
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Subject(s) / Keyword(s):
- carbon cycle, carbon burial
- Format(s):
- Medium: X
- Location:
- Washington DC
- Sponsoring Org:
- National Science Foundation
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