ABSTRACT: Structure in the Secular Variation of Seawater 87Sr/86Sr for the Ivorian/Chadian (Osagean, Lower Carboniferous)
Timothy L. Douthit, Gilbert N. Hanson, William
J. Meyers
The secular variations of 87Sr/86Sr in seawater for the Ivorian/Chadian (equivalent to the Osagean, Lower Carboniferous) were determined through detailed analysis of well-preserved marine cements from the Waulsortian facies of Ireland. The results indicate that marine cements have utility in characterizing marine paleochemistries.
Marine cements were judged pristine on the basis of nonluminescent character and stable isotopic composition comparable to previous estimates of Mississippian marine calcite. Analysis of the marine cements yielded 87Sr/86Sr ratios lower than previously reported values for the Ivorian/Chadian. Error resulting from chronostratigraphic correlation between different geographic areas was avoided by restricting the sample set to a single 1406-ft-long core (core P-1). The P-1 core is estimated to represent a minimum of 8.7 m.y. of continuous Waulsortian Limestone deposition.
The 87Sr/86Sr ratios of 11 nonluminescent cements document a non-monotonic variation in seawater 87Sr/86Sr along the length of the core. 87Sr/86Sr ranges from a high of 0.707908 in the early Ivorian to a low of about 0.707650 in the late Ivorian and middle Chadian, with an early Chadian maximum at 0.707800 (all data are adjusted to a value of 0.710140 for SRM 987). The indicated maximum rate of change in seawater 87Sr/86Sr is -0.00011/Ma, comparable in magnitude to Tertiary values.
The secular variation curve of seawater 87Sr/86Sr for the Ivorian/Chadian has previously been thought to decrease monotonically with decreasing age. These data suggest that the seawater 87Sr/86Sr variation over this interval may be sinusoidal in nature and emphasize the importance of well-characterized intraformational isotopic base lines.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990