THE ISOTOPIC RECORD OF OCEAN CHEMISTRY AND DIAGENESIS PRESERVED IN NONLUMINESCENT BRACHIOPODS FROM MISSISSIPPIAN CARBONATE ROCKS, ILLINOIS AND MISSOURI
Jl. Banner et J. Kaufman, THE ISOTOPIC RECORD OF OCEAN CHEMISTRY AND DIAGENESIS PRESERVED IN NONLUMINESCENT BRACHIOPODS FROM MISSISSIPPIAN CARBONATE ROCKS, ILLINOIS AND MISSOURI, Geological Society of America bulletin, 106(8), 1994, pp. 1074-1082
Fossil brachiopod shells are commonly used as a faithful record of the
chemical and isotopic composition of the ancient ocean water in which
they grew, owing to their stable mineralogy and their presence in mar
ine strata throughout Phanerozoic time. The utility of these fossils f
or preserving ocean-water compositions is evaluated through analyses o
f multiple specimens that grew in contemporaneous sea water at individ
ual stratigraphic horizons in the Mississippian-age Burlington-Keokuk
Formation of mid-continent North America. Significant variations in Sr
, C, and O isotopic compositions are observed in contemporaneous, non-
luminescent shells, indicating that apparently pristine brachiopod she
ll material has been altered by post-depositional processes of diagene
sis. The least-altered compositions at each horizon are used to estima
te secular variations of Sr-87/Sr-86 in Early Mississippian ocean wate
r. The estimated rates of change of ocean-water Sr-87/Sr-86 are compar
able in magnitude to the rapid Sr isotopic changes documented for Ceno
zoic ocean water, for which high-frequency glacial cycles are inferred
as a driving mechanism. The rigorous assessment of diagenetic effects
on the isotopic signatures of marine phases is requisite for the adva
ncement of high-resolution reconstructions of paleo-ocean chemistry.