Is the Cenomanian - Turonian Oceanic Anoxic Event 2 globally synchronous?
Abstract
Mesozoic oceanic anoxic events (OAEs), marked by secular variations in atmospheric [CO2], high atmospheric temperatures, and punctuated episodes of ocean acidification and deoxygenation, have received much attention due to the apparent synchronous deposition of organic-rich shales. A fingerprint of these events is δ13C isotopic excursions (CIE) recorded in both organic and inorganic carbon, and which are interpreted as perturbations in the long-term exogenic carbon cycle. The Cenomanian-Turonian, OAE2 is a globally recognized event that is characterized by a rapid shift to heavier δ13C values (by about 5‰). This first build-up is followed by a negative fluctuation (the trough), a second build-up of positive carbon values, immediately followed by a plateau, and finally a steady return to pre-excursion δ13C values. The transition from plateau to recovery phase coincides with the Cenomanian-Turonian boundary. To produce a globally synchronous CIE, rapid burial of organic matter and oceanic turnover must occur in large parts of the ocean geologically simultaneously. Thus, the CIE would represent a large-scale variation in the exogenic carbon cycle and not local variation within discrete basins. Of the localities where organic rich deposits or the positive CIE associated with OAE2 have been identified, only two, in Colorado and Texas, have been precisely dated using a combination of U/Pb, 40Ar/39Ar, and cyclostratigraphy. To test the idea that the initiation, internal variations of the CIE and duration of the CIE during OAE2 are globally synchronous, two sections exposed in the Upper Magdalena Valley, Colombia are being studied for their stratigraphy, carbon chemostratigraphy, and geochronology. In contrast to other OAE2 sections, these tropical sections are characterized by low deposition rates and the occurrence of ~70 bentonite layers interbedded with the organic rich rocks that clearly record the CIE associated to OAE2. These water-lain tuffs yield abundant zircons, but precise dating is complicated by numerous detrital and xeno- or antecryst populations. Prescreening of zircon grains by laser-ablation ICPMS is being used to complement TIMS dating for precise U-Pb dating.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP13C1447P
- Keywords:
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- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 4901 Abrupt/rapid climate change;
- PALEOCEANOGRAPHY;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY;
- 4948 Paleocene/Eocene thermal maximum;
- PALEOCEANOGRAPHY