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

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.