IMPLICATIONS OF SAMPLING STRATEGY ON C AND O ISOTOPE CHEMOSTRATIGRAPHY: A TEST FROM THE ORDOVICIAN OF ESTONIA

Most Paleozoic chemostratigraphic studies use stable isotope values from bulk samples. The goal of this work is to quantify within and between sample isotope values to determine the accuracy of bulk sampling. 113 carbonate samples were collected for carbon and oxygen isotope analysis from the Middle to Upper Ordovician sequence in northern Estonia. Stage-level mean δ¹⁸O values steadily increased from -6.4 ‰VPDB in the Middle Ordovician Llanvirn (Volkhov Stage) to -4.0 ‰VPDB in the Upper Ordovician Caradoc (Haljala Stage). This may be in response to decreased depth of burial and/or changing paleotemperatures. Stage-level mean δ¹³C values varied more complexly from -1.5 ‰VPDB in the Middle Ordovician Darriwilian (Kunda Stage) to 1.1 ‰VPDB in the Upper Ordovician Caradoc (Haljala Stage). Stable isotopes also varied by their source. Brachiopods had mean δ¹³C and δ¹⁸O values of -1.2 and -5.3 ‰VPDB, respectively. Bryozoans had mean δ¹³C and δ¹⁸O values of -0.8 and -5.2 ‰VPDB, respectively. Cements had mean δ¹³C and δ¹⁸O values of -0.6 and -5.0 ‰VPDB, respectively. Matrix samples had mean δ¹³C and δ¹⁸O values of -0.8 and -5.6 ‰VPDB, respectively. These results suggest that bulk samples that do not take into consideration the relative amounts of the various sources of carbonate may yield misleading isotope values.