Physics
Scientific paper
Aug 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1679v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 8, pp.1679-1696
Physics
20
Scientific paper
Combined trace element and isotope studies of 319 brachiopods, covering the Ordovician to Permian time span, show that 13 C and 18 O in well preserved specimens varied during the Paleozoic. The overall 13 C secular trend is in accord with the previously published observations, but its details are obscured by vital isotopic fractionation effects at generic level. Nonetheless, the results suggest that the negative correlation between marine 13 C carbonate and 34 S sulphate deteriorates at time scales of 10 6 years, due to the long residence time, and thus slow response, of SO 4 2- in the ocean. For oxygen isotopes, all Devonian and older specimens have 18 O of -4%, while the well preserved Permian samples have near-present day 18 O of about -1% (PDB). This isotopic dichotomy is probably not due to post-depositional phenomena, salinity, or biogenic fractionation effects. This leaves open the perennial arguments for a change in 18 O / 16 O of sea water versus warmer ancient oceans. The present data are difficult to explain solely by the temperature alternative. The coincidence of the proposed shift in 18 O with the large Late Paleozoic changes in marine 87 Sr / 86 Sr , 13 C / 12 C , 34 S / 32 S , and "sea level stands" argues for a tectonic cause and for a change in 18 O / 16 O of sea water, although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites. Whatever the precise cause, or combination of causes, the implications for tectonism and/or paleoclimatology are of first order significance.
Fritz Peter
Jones Brian
Veizer Ján
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