Nonmonotonic Variation of Seawater 87Sr/86Sr Across the Ivorian/Chadian Boundary (Mississippian, Osagean): Evidence from Marine Cements within the Irish Waulsortian Limestone

Abstract

ABSTRACT Detailed analysis of compositionally unaltered marine fibrous cements (MFC) from a single core through the Mississippian Irish Waulsortian Limestone indicates that the variation of seawater 87Sr/86Sr is nonmonotonic across the Ivorian-Chadian boundary. This nonmonotonic variation has not been recognized by previous studies. Furthermore, marine cements yielded 87Sr/86Sr ratios lower than previously reported values for the Ivorian-Chadian (Osagean). Marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope (C, O) composition comparable to previous estimates of Mississippian marine calcite. The isotope chemistry (C, O, Sr) and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high-Mg calcite to low-Mg calcite + microdolomite, a conversion that probably took place in marine water during precipitation of Zone 1 calcite cement, the oldest non-MFC cement. High stratigraphic resolution was obtained by restricting the sample set to a single core, 429 m long, thereby eliminating chronostratigraphic correlation errors. The MFC cements are pre-Arundian (based on previous study), and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones. The core is estimated to represent about 9.8 million years of Waulsortian Limestone deposition. 87Sr/86Sr ratios of pristine MFC samples range from a high of 0.707908 in the early Ivorian to a low of 0.707650 in the late Ivorian and mid-Chadian, with an early Chadian maximum at 0.707800. The maximum rate of change in seawater 87Sr/86Sr is -0.00012/Ma, comparable in magnitude to Tertiary values. Our data document the presence of fine-scale seawater 87Sr/86Sr modulations for the Ivorian/Chadian, in contrast to the previously published monotonic seawater 87Sr/86Sr curve for this interval, and emphasize the importance of well characterized intraformational isotopic baselines.