Paleolimnological Signatures from Carbon and Oxygen Isotopic Ratios in Carbonates from Organic Carbon-Rich Lacustrine Sediments
M. R. Talbot, K. Kelts, 1990. "Paleolimnological Signatures from Carbon and Oxygen Isotopic Ratios in Carbonates from Organic Carbon-Rich Lacustrine Sediments", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
Download citation file:
Primary and early diagenetic carbonate minerals may be associated with organic carbon-rich lacustrine sediments. The stable oxygen and carbon isotopic compositions of these carbonates can yield useful paleoenvironmental information. Primary carbonates from hydrologically open lakes typically show little or only poorly correlated covariance between oxygen and carbon isotopic variations, whereas carbonates precipitated from surface waters of closed lakes display characteristic, highly correlated covariance. Oxygen isotopic ratios in early diagenetic phases reflect the isotopic composition of the waters in which the host sediments were deposited. The carbon isotopic composition of diagenetic carbonates is determined by the nature of the dominant microbiological processes involved in diagenesis of organic matter during the earliest stages of burial. In lakes poor in dissolved sulfate, bacterial methanogenesis is dominant and produces carbonates with markedly positive <513C values. Sulfate-rich lakes favor bacterial sulfate reduction that leads to precipitation of carbonates with negative <513C signatures. Using isotopic data from carbonates from Lake Bosumtwi, Ghana, as a case study, we show that characteristic relationships exist between isotopic compositions of primary and diagenetic carbonates of sulfate-poor and sulfate-rich lakes.
Figures & Tables
Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation