Physics
Scientific paper
Oct 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2911g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 10, pp.2911-2917
Physics
6
Scientific paper
Isotopic analyses of 227 limestone samples from the Cambro-Ordovician Arbuckle Group, Oklahoma, document slow secular changes in the chemistry of the limestones. From late Cambrian to early Ordovician, the 18 O values of the limestones increase from -10%. to -7%. (PDB); 13 C values decrease from 0%. to -2%. (PDB); and 87 Sr / 86 Sr ratios decrease from 0.7091 to 0.7088. The light 18 O values suggest that all Arbuckle limestones underwent diagenetic alteration, probably caused by meteoric water recharged during the development of the overlying, pre-middle Ordovician unconformity. The gradual 18 O increase from late Cambrian to early Ordovician reflects reduced 18 O depletion with decreasing burial temperature during alteration, although the presence of additional primary secular 18 O variation cannot be ruled out. The 13 C and 87 Sr / 86 Sr variations, in accord with 13 C and 87 Sr / 86 Sr variations in the literature, represent primary secular variations. The variations indicate that the 13 C value and 87 Sr / 86 Sr ratio of early Paleozoic surface seawater decreased from late Cambrian to early Ordovician. The 13 C variation during this time period seems to correlate with sea-level variation. Specifically, during sea-level fall, an increase in the rate of oxidation of organic matter caused 13 C depletion of inorganic bicarbonate in seawater. As a result, early Ordovician carbonates, probably deposited during the regression stage of the latest Precambrian to latest early Ordovician cycle, became 13 C depleted, relative to late Cambrian carbonates. The decrease of seawater 87 Sr / 86 Sr ratio from late Cambrian to early Ordovician may have resulted from decreased riverine Sr input caused by decreased rate of continental weathering.
Gao Guoqiu
Land Lynton S.
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