STABLE ISOTOPES IN MARINE CARBONATES - THEIR IMPLICATIONS FOR THE PALEOENVIRONMENT WITH SPECIAL REFERENCE TO THE PROTEROZOIC VINDHYAN CARBONATES (CENTRAL INDIA)

The carbon and oxygen isotopic compositions of the minerals of carbona te rocks depend on the composition of the precipitating water and the temperature of precipitation. The carbon isotopes suffer little fracti onation during precipitation of carbonate minerals and their carbon is otopic composition reflects that of the precipitating water On the oth er hand, carbonate minerals become enriched in O-18 with respect to th e precipitating water and the degree of enrichment depends on the temp erature of precipitation - lower the temperature, higher the enrichmen t. Isotopic studies of marine carbonate rock successions have the pote ntial to reveal the temporal trends in the isotopic composition and te mperature of seawater. The oxygen isotopic composition of seawater is a function of a number of processes such as, evaporation, rainfall, ru noff and mixing of surface and deep ocean water. Evaporation leads to enrichment in heavier isotope whereas the other processes cause enrich ment in lighter isotope. The oxygen isotopic composition of seawater a lso varies between glacial and non-glacial periods. The seawater becom es enriched in heavier isotope during glacial period due to preferenti al removal of the lighter isotope in glaciers. Melting of glaciers, on the other hand, leads to dilution of the seawater isotopic compositio n. 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 heavie r or that the seawater temperature has decreased through time. There i s a general opinion that the seawater composition did not vary much th roughout geological time and the seawater temperature may have been re latively higher in past geological periods. The carbon isotopic compos ition of seawater is a function of (1) terrestrial input of dissolved inorganic carbon and particulate organic carbon through rivers, (2) or ganic carbon productivity in the ocean, and (3) organic carbon deposit ion and burial in the ocean. The lighter river water dilutes the overa ll carbon isotopic composition of seawater. On the other hand, increas ed organic carbon productivity and burial enriches the seawater in hea vier isotope. The present day zero permil carbon isotopic composition of seawater is maintained by a balance between the terrestrial input a nd oceanic output through organic carbon production and burial, and ca rbonate precipitation. On a long-term scale (10(9)a), the average delt a(13)C values of carbonates of Precambrian and Phanerozoic ages have b een found to be fairly constant around zero permil. However, delta(13) C values of marine carbonates indeed show short-term (0.2 to 10 Ma) ex cursions 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 in the isotopic record of sh allow 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. Ne gative excursion would occur if the above processes move in the opposi te direction. Isotopic studies of Precambrian-Cambrian boundary interv al in different parts of the world have revealed that the transition i s marked by a positive excursion close to the boundary and a swing bac k 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 e xcursion 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 sect ion of the Himalayas. The present study reveals that the chronostratig raphic marker is also present in the Vindhyan Basin of central India.