Carbon Isotope Chemostratigraphy, the Baby and the Bathwater
Abstract
Secular variations in the carbon isotopic values of carbonate sediments and rocks and their individual components have been applied successfully to problems of stratigraphic correlation and for interpretation of past changes in the global carbon cycle. However, this methodology is not without problems. A major tenet of stable isotope chemostratigraphy involves sampling and analyzing multiple, widely separated sequences, and, if possible, multiple carbon-bearing components (e.g., carbonate and organic carbon) in order to demonstrate a global signal. In some cases, this methodology has been short-circuited in the zeal to reveal a new event or excursion, particularly for time intervals for which adequate sequences are somewhat rare. Likewise, although most carbonate researchers are quite aware of the possible importance of diagenesis, particularly in organic-carbon rich sequences or in shoal-water carbonate sequences with longer-term subaerial exposure events, such overprints commonly go unrecognized or are considered of minor impact. Studies of stable isotope variations in carbonate sequences should always employ textural and geochemical methodologies for detecting and even quantifying diagenesis, if possible. Although some diagenetically overprinted or misinterpreted geochemical data have undoubtedly appeared in the literature, there are many excellent examples of global carbon isotope variations in records expressed in pelagic biogenic carbonate, marine organic carbon, platform carbonates, and terrestrial organic matter. Arguably, one of the best-documented examples is the Cenomanian-Turonian (ca. 93 Ma) positive carbon isotope excursion. The amplitude of the Cenomanian-Turonian carbon isotope excursion is similar among all types of records, but there are subtle pattern differences that arise from differences in sedimentation rate among and within sequences. Organic carbon and carbonate carbon isotope signals also may differ in phasing and amplitude for certain events, which may provide information regarding changes in atmospheric- oceanic carbon isotope variations versus changes in the partial pressure of carbon dioxide in the atmosphere. The early Aptian carbon isotope excursions serve as a key example of this. The platform-basin carbonate carbon isotope correspondence undoubtedly works best during times of "calcite seas," such as during the Cretaceous when the complication of greater dominance of more 13-C enriched aragonite in platform settings, such as during the late Neogene, is generally absent. The baby remains robust. Indeed, carbon isotopes still have great utility for reconstructing water-column isotope gradients, global changes in carbon cycling, and for pattern-based long-distance correlation. However, we do need to be judicious in our choice of samples and to temper our enthusiasm for seeking out and interpreting extreme signals.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP31D..02A
- Keywords:
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- 4912 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 1615;
- 4805);
- 4924 Geochemical tracers