Carbon, oxygen, and strontium isotope geochemistry of carbonate rocks of the upper Miocene Kudankulam Formation, southern India: Implications for paleoenvironment and diagenesis

The carbon, oxygen, and strontium isotope Compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaecenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in O-18 and C-13 compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on delta C-13 and delta O-18 values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (delta C-13 from -7.8 parts per thousand to -6.0 parts per thousand and -9.0 parts per thousand to -7.0 parts per thousand; delta O-18 from -9.2 parts per thousand to -6.5 parts per thousand and -9.4 parts per thousand to -2.6 parts per thousand, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative delta O-18 values of various carbonate components (about -9.4 parts per thousand to -4.1 parts per thousand for whole rocks; about -8.4 parts per thousand to -2.6 parts per thousand for ooids) suggest that during the late Miocene the paleoclitnate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (-7.9 parts per thousand to -3.6 parts per thousand for whole rocks; -4.9 parts per thousand to -1.5 parts per thousand for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C-4 type plants, supporting a scenario of expansion Of C4 plants during the late Miocene in the Indian subcontinent as far south as the Southern tip of India. The Sr-87/Sr-86 ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic 87 Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene. (C) 2008 Elsevier GmbH. All rights reserved.