Bringing the boron proxy pieces together to solve the ocean carbonate system puzzle: Miocene Monterey excursion and PETM case studies
Abstract
Geologic carbon cycle perturbations are often characterized by isotope excursions (CIEs) that signify large injections of carbon into global reservoirs. Carbon isotope (δ13C) records alone are insufficient to both fingerprint a C source(s) and quantify the ocean's response to carbon invasion. We use boron to calcium ratios (B/Ca) and boron isotopes (δ11B) proxies in planktonic foraminifera to generate novel contemporaneous records that allow the entire surface ocean carbonate system to be constrained. We present two case studies: Monterey Carbon Isotope Excursion (MCIE; 17-13.5 Ma) and Paleocene-Eocene Thermal Maximum (PETM; 56 Ma) characterized by a positive 1‰ and negative 3-5 ‰ CIE, respectively. The MCIE is long-lived event with a duration of several million years while the PETM is a rapid perturbation occurring over a couple hundred kilo years. Our records exhibit a similar B/Ca reduction of 30-40% and a δ11B decline of 1.0-1.2‰ (PETM) and 1.5-2.0‰ (MCIE) coeval with the CIE onset, followed by consistently depressed B/Ca and δ11B values that persist for the approximate duration of the CIEs themselves. Boron proxy and δ13C records are tightly coupled consistent with the interpretation that oceanic absorption of the carbon released during each of these events resulted in a decline of surface ocean pH and DIC addition, likely induced by elevated atmospheric pCO2. At the MCIE, ocean carbonate chemistry changes coincide with a positive CIE which we propose to be caused by volcanic degassing induced warming that resulted in sea-level rise and enhanced organic C burial on drowned continental shelves. The negative CIE at the PETM signifies a depleted carbon source which resulted in widespread surface ocean acidification. PETM planktonic foraminiferal δ13C values appear to recover faster than B proxy records. This divergence pattern can be explained by shifting the relative contributions of organic carbon burial vs. weathering to carbon sequestration during the CIE recovery phase. We conclude similar underlying carbon cycle processes are at play during these events, yet their behavior is fundamentally different as the magnitude, C emissions rate (slow vs. fast) and background climate state (icehouse vs greenhouse) resulted in distinct δ13C expressions and variable climatic responses to each carbon perturbation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP21C1422B
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHYDE: 4930 Greenhouse gases;
- PALEOCEANOGRAPHY