Application of δ13c Values Recorded in Neoproterozoic Marine Dolomite As a Marker for Global Correlations: Significance of Major δ13c Variations for the Carbon Cycle Based on Studies of Modern Dolomite Precipitating Environments

Abstract:

Since the early Paleozoic, the average bulk δ¹³C value of marine carbonates has remained relatively positive varying between 0 and +4‰ with distinctive positive excursions that are associated with global changes in the carbon cycle. Unlike the Phanerozoic δ¹³C data for marine limestones, a major δ¹³C excursion has been recorded in a globally deposited Neoproterozoic marine dolomite formation, known as the cap dolostone. This excursion with δ¹³C values ranging systematically between -3 and -5‰ represents a global chronstratigraphic marker used to correlate the end of the major Marinoan glaciation at 636 Ma¹. Does this excursion signify a primary seawater value and how might it be interpreted as a primary carbon cycle signal, considering the widespread distribution of the cap dolostone?

Studies of modern dolomite precipitating environments, such as supratidal sabkhas of Abu Dhabi, U.A.E. and Qatar and coastal hypersaline lagoons of Rio de Janiero State, Brazil, indicate that microbial activity or the biological products, thereof, influence or mediate mineral formation. The precipitating solutions are sourced from normal seawater, which has experienced variable stages of concentration through evaporative processes. Comparison of δ¹³C values of sabkha dolomite with that formed in the hypersaline lagoons reveals that the former are always rather positive (approx. +2 to +7 ‰), whereas the latter are always negative (approx. -5‰ to -11‰). During very early diagenesis, the original δ¹³C value of the initial precipitate is not necessarily retained, indicating that synsedimentary processes can alter the carbon signal prior to burial and later diagenesis. However, the potential for very early lithification of microbial dolomite promotes the preservation of original δ¹³C values, which, thus, can be useful for evaluation of the ancient carbon cycle.

¹Halverson, G.P. et al., 2005. Toward a Neoproterozoic composite carbon-isotope record, GSA Bulletin, v. 117, p. 1181-1207.