Stable Isotopes in Marine Carbonates: Their Implications for the Paleoenvironment with Special Reference to the Proterozoic Vindbyan Carbonates (Central India)

The carbon and oxygen isotopic compositions of the minerals of carbonate rocks depend on the composition of the precipitating water and the temperature of precipitation. The carbon isotopes suffer little fractionation during precipitation of carbonate minerals and their carbon isotopic composition reflects that of the precipitating water. On the other hand, carbonate minerals become enriched in ¹⁸O with respect to the precipitating water and the degree of enrichment depends on the temperature of precipitation - lower the temperature, higher the enrichment.

Isotopic studies of marine carbonate rock successions have the potential to reveal the temporal trends in the isotopic composition and temperature of seawater. The oxygen isotopic composition of seawater is a function of a number of processes such as, evaporation, rainfall, runoff and mixing of surface and deep ocean water. Evaporation leads to enrichment in heavier isotope whereas the other processes cause enrichment in lighter isotope. The oxygen isotopic composition of seawater also varies between glacial and non-glacial periods. The seawater becomes enriched in heavier isotope during glacial period due to preferential removal of the lighter isotope in glaciers. Melting of glaciers, on the other hand, leads to dilution of the seawater isotopic composition. The oxygen isotopic composition of marine carbonate rocks has been found to be decreasing with increasing geologic ages. This may either indicate that the seawater composition has progressively become heavier or that the seawater temperature has decreased through time. There is a general opinion that the seawater composition did not vary much throughout geological time and the seawater temperature may have been relatively higher in past geological periods.

The carbon isotopic composition of seawater is a function of (1) terrestrial input of dissolved inorganic carbon and particulate organic carbon through rivers, (2) organic carbon productivity in the ocean, and (3) organic carbon deposition and burial in the ocean. The lighter river water dilutes the overall carbon isotopic composition of seawater. On the other hand, increased organic carbon productivity and burial enriches the seawater in heavier isotope. The present day zero permit carbon isotopic composition of seawater is maintained by a balance between the terrestrial input and oceanic output through organic carbon production and burial, and carbonate precipitation. On a long-term scale (10⁹a), the average δ¹³C values of carbonates of Precambrian and Phanerozoic ages have been found to be fairly constant around zero permil. However, δ¹³C values of marine carbonates indeed show short-term (0.2 to 10 Ma) excursions from the zero permil value, both in the positive and negative directions. For a constant terrestrial input of inorganic carbon, two processes can lead to positive excursion ill the isotopic record of shallow marine carbonates: (1) increased rate of preservation of organic matter in the ocean and (2) increased rate of photosynthetic organic carbon production in the ocean. These processes preferentially extract lighter carbon from the seawater and make it isotopically heavier. Negative excursion would occur if the above processes move in the opposite direction.

Isotopic studies of Precambrian-Cambrian boundary interval in different parts of the world have revealed that the transition is marked by a positive excursion close to the boundary and a swing back to more negative-less positive values in the early Cambrian strata. The similarity of the carbon isotopic record across the transition in different boundary sections of the world indicates that the isotopic excursion is of chronostratigraphic significance and may be used as an aid to correlation. The carbon isotope chronostratigraphic marker for the Precambrian-Cambrian boundary has been identified in the Krol section of the Himalayas. The present study reveals that the chronostratigraphic marker is also present in the Vindhyan Basin of central India.