NONMONOTONIC VARIATION OF SEAWATER SR-87 SR-86 ACROSS THE IVORIAN CHADIAN BOUNDARY (MISSISSIPPIAN, OSAGEAN) - EVIDENCE FROM MARINE CEMENTS WITHIN THE IRISH WAULSORTIAN LIMESTONE/

Detailed analysis of compositionally unaltered marine fibrous cements (MFC) from a single core through the Mississippian Irish Waulsortian l imestone indicates that the variation of seawater Sr-87/Sr-86 nonmonot onic across the Ivorian-Chadian boundary. This nonmonotonic variation has not been recognized by previous studies. Furthermore, marine cemen ts yielded Sr-87/Sr-86 ratios lower than previously reported values fo r the Ivorian-Chadian (Osagean). Marine fibrous cements are interprete d to be compositionally unaltered on the basis of nonluminescent chara cter and stable isotope(C, O) composition comparable to previous estim ates of Mississippian marine calcite. The isotope chemistry (C, O, Sr) and cathodoluminescent character of the marine fibrous cements theref ore remained intact during their conversion from high-Mg calcite to lo w-Mg calcite + microdolomite, a conversion that probably took place in marine water during precipitation of Zone 1 calcite cement, the oldes t non-MFC cement. High stratigraphic resolution was obtained by restri cting the sample set to a single core, 429 m long, thereby eliminating chronostratigraphic correlation errors. The MFC cements are pre-Arund ian (based on previous study), and are interpreted to be in stratigrap hic order and to be approximately the same age as the host limestones. The core is estimated to represent about 9.8 million years of Waulsor tian Limestone deposition. Sr-87/Sr-86 ratios of pristine MFC samples range from a high of 0.707908 in the early Ivorian to a low of 0.70765 0 in the late Ivorian and mid-Chadian, with an early Chadian maximum a t 0.707800. The maximum rate of change in seawater Sr-87/Sr-86 is -0.0 0012/Ma, comparable in magnitude to Tertiary values. Our data document the presence of fine-scale seawater Sr-87/Sr-86 modulations for the I vorian/Chadian, in contrast to the previously published monotonic seaw ater Sr-87/Sr-86 curse for this interval, and emphasize the importance of well characterized intraformational isotopic baselines.