Alternative hypotheses for why the δ13C value of carbonate sediments has remained relatively constant and elevated compared to mantle sources over Earth history
Abstract
The δ13C value of carbonate sediments has been used extensively to reconstruct the history of the global carbon cycle over Earth history. A large compilation of this data indicates that, on average, the δ13C value of the carbonate reservoir has remained both relatively constant and elevated ( 0‰) compared to mantle sources (-5‰) over 3+ billion years. As biological carbon sinks are characterized by low δ13C values, the canonical explanation for the δ13C record in sedimentary carbonates is that it reflects a constant partitioning of carbon sinks over Earth history; 20% being as organic carbon (-25‰) and 80% as carbonate carbon (0‰). This result appears paradoxical as it suggests that all of the dramatic biological evolution that has occurred on Earth's surface over the last 3+ billion years has had no measurable effect on the overall partitioning of carbon fluxes between organic and inorganic sinks. Here I examine this model and argue that a steady-state δ13C value of 0‰ for carbonate sediments can be maintained for a wide range of organic carbon burial fractions as long as the δ13C value of the carbonate reservoir begins at 0‰, rates of carbonate recycling are sufficiently high, and there are feedbacks that maintain a tight balance between sedimentary organic carbon oxidation and new organic carbon burial. I then explore potential mechanisms for creating a carbonate reservoir with a δ13C value of 0‰ on the early Earth, focusing on the relationship between crustal growth and subduction (e.g. Bjerrum & Canfield, 2004).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP41E1904H
- Keywords:
-
- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 3675 Sedimentary petrology;
- MINERALOGY AND PETROLOGYDE: 4863 Sedimentation;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL