Closing the geologic carbon cycle

Area of Science:

• Geochemistry
• Earth Science
• Paleoclimatology

Background:

• Discrepancies exist between inventory and isotope mass balance methods for estimating sedimentary organic carbon burial fluxes.
• Previous models did not fully account for all significant inputs to the ocean-atmosphere carbon system.
• Understanding these fluxes is crucial for comprehending global carbon cycling and climate regulation.

Purpose of the Study:

• To resolve the discrepancy in sedimentary organic carbon burial flux estimates.
• To refine the isotope mass balance model by incorporating a revised assessment of carbon inputs.
• To investigate the impact of past climatic events and geological processes on the carbon cycle.

Main Methods:

• Revised isotope mass balance calculations incorporating weathering of carbonate and old kerogen, geogenic methane oxidation, and volcanic/metamorphic degassing.
• Analysis of isotopically light carbon inputs (OCpetro and CH4) to determine the mean isotopic composition of total carbon input.
• Monte Carlo evaluation to assess the certainty of late Cenozoic reservoir growth for carbonate and organic carbon.

Main Results:

• The revised isotope mass balance model yields a modern sedimentary organic carbon burial flux of 15.9 ± 6.6 Tmol y⁻¹.
• Volcanic and metamorphic degassing contribute approximately 23% to the total carbon input.
• The mid-Miocene Climatic Optimum isotope anomaly resulted in an integrated excess deposition of ~4.3 × 10⁶ Tmol, exceeding estimates for major volcanic events.

Conclusions:

• Sedimentary organic carbon burial flux estimates are reconciled through a refined isotope mass balance approach.
• The late Cenozoic likely saw net growth in both carbonate and organic carbon reservoirs, suggesting increasing sedimentary carbon masses.
• Increasing sedimentary carbon implies surface environment oxidation and potential increases in atmospheric oxygen (pO2).