T161 Understanding High-Amplitude Stable Isotope Variations in Proterozoic Strata

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 207

PILOT STUDY USING CLUMPED ISOTOPE PALEOTHERMOMETRY TO ASSESS DIAGENETIC HISTORY OF CARBONATES, NEOPROTEROZOIC CLEMENTE FORMATION, CABORCA, MEXICO

Paper number: T161-11
Program: Technical Sessions
Time: 10:30 AM – 10:45 AM

Authors

Abstract

The veracity of the Neoproterozoic δ13C record from stratigraphic sections around the world has recently been questioned. In particular, negative carbon isotopic anomalies are under close scrutiny; do they result from primary changes in the global carbon cycle, later effects during burial diagenesis, or perhaps meteoric diagenesis in association with a developing terrestrial biosphere? Burial and meteoric diagenesis are difficult to isotopically untangle from one another, as both drive δ18O (and potentially δ13C) of carbonates to isotopically depleted values. Was the original δ18O of the fluid isotopically light (as for meteoric fluids), near seawater, or isotopically enriched, indicating an evaporative brine? The relationship between the δ18O of the mineral phase, the fluid phase, and the temperature is well-known, but two of the variables must be known to calculate the third; without prior knowledge of the δ18O of the fluid (no longer measureable in ancient carbonate systems) or the temperature, the isotopic puzzle remains.

Here, we apply a new paleothermometer based on the temperature dependence on the distribution of 13C-18O “clumps” in the carbonate lattice (independent of the original fluid composition). As a test case, several limestones and dolostones from Caborca, Mexico, taken from the “Shuram Anomaly” interval, a prominent Neoproterozoic δ13C excursion, were analyzed for their clumped isotope composition. Results from clumped isotope paleothermometry reveal that the δ18O of the limestones and dolostones was equilibrated at high temperature (>100 degrees C). Using the relationship between the δ18O of the mineral (measured) and the temperature (now known), the fluid of equilibration was calculated to be near 0 per mil (SMOW) for the limestones, and ~13 per mil (e.g., a brine) for the dolostones. Thus, the carbonates did not equilibrate with meteoric fluids.

Our findings suggest caution must be used when interpreting δ18O/δ13C data in ancient carbonates without further knowledge of the equilibration temperature. We emphasize that the sources of the δ13C values remain equivocal, given the rock-buffering potential in carbonates. However, given the recurrence of the Shuram Anomaly in multiple, distant sedimentary successions it seems most reasonable that the δ13C records near primary conditions.