AN ALTERNATIVE VIEW OF C ISOTOPE VARIATIONS IN NEOPROTEROZOIC CARBONATES

Nearly all Neoproterozoic stable isotope data for carbonates plot near or to the left of a line defined by δ¹³C = 2.4 δ¹⁸O + 6. This line is coincident with the classic limestone stabilization trajectory produced when originally precipitated marine aragonite, metastable calcite, and high Mg calcite convert to limestone and dolostone during early diagenesis (e.g. Gross and Tracey, 1965). During mineralogical stabilization, δ¹⁸O and δ¹³C are lowered in proportion to the amounts of coastal meteoric waters and photosynthetic C introduced into the pore fluids. Following stabilization, the system is mostly rock-buffered with respect to δ¹³C, but δ¹⁸O can decrease in response to re-equilibration during later alteration. Altered samples therefore plot to the left of the stabilization line. Most Neoproterozoic carbonate stable isotope data can thus be explained in terms of typical early carbonate stabilization followed by later diagenesis. Samples with values lying near the stabilization line became rocks in mixed marine/meteoric waters while those lying to the left underwent later alteration.

δ¹³C of Neoproterozoic carbonates is now typically considered mainly in terms of global “excursions” used to correlate strata and infer aspects of the global marine biomass. The unstated assumption is that individual analyses are indicative of the global marine bicarbonate reservoir. If so, the data should not all lie on or to the left of the stabilization line; at least some relatively unaltered low ¹³C samples should have high ¹⁸O. Since they do not, it is likely that most δ¹³C variations are due to varying amounts of photosynthetic C introduced during mixed meteoric/marine stabilization rather than to global negative excursions. “Excursions” to lower δ¹³C in units below exposure surfaces are particularly obvious examples. With the possible exception of some Cap Carbonates, δ¹³C ”chemostratigraphy” of Neoproterozoic carbonates is generally invalid, cannot be used to correlate units, and misses important, useful, and interesting information about the origin and alteration history of carbonates as well as the nature of the photosynthetic land cover at this important time in Earth history.