EFFECTS OF EARLY DIAGENESIS ON δ44/40CA RECORDS OF UPPER ORDOVICIAN DOLOSTONES

The end-Ordovician (Hirnantian) was a time of ice sheet growth and sea-level fall, and it corresponds with the second largest extinction event in Earth history. A positive excursion in carbon isotope ratios (δ¹³C) has played an important role in constraining environmental changes that accompanied the mass extinction and glaciation, but intrabasinal and global variability in δ¹³C records complicates interpretation. Like δ¹³C, calcium isotope ratios (δ^44/40Ca) are controlled by changes in ocean chemistry and subsequent diagenetic alteration. However, the residence time of calcium in the ocean is an order of magnitude greater than carbon, which means changes in calcium over the same period as carbon could not be due to gradual changes in oceanic values alone. Processes that have been considered to explain covariant δ¹³C and δ^44/40Ca in deep water Hirnantian limestones include changes in seawater circulation, submarine groundwater discharge, and early diagenesis. We seek to reconstruct the δ^44/40Ca record of shallow shelf Hirnantian dolostones in order to test these hypotheses.

We analyzed δ^44/40Ca in subtidal to peritidal dolostones from three stratigraphic sections that transect a shallow carbonate shelf in the eastern Great Basin. Two of the sections, from Pancake Range and Silver Island Range, contain a Hirnantian record, identified by stratigraphic correlation and the presence of the Hirnantian δ¹³C excursion; the Hirnantian is missing in Barn Hills due to non-deposition or erosion during glacioeustatic sea level fall. This shallowing is represented at Pancake Range by karstic features indicating meteoric exposure and at Silver Island Range by a thin interval with aeolian sand grains. We combine these field observations with δ^44/40Ca, δ¹³C, and trace element data to quantify the impact of early diagenetic processes in uppermost Ordovician strata across the carbonate platform. These data, supplemented by a hand-sample scale study of δ¹³C variability, should also aid in explaining minor variations in carbon isotope ratios observed across stratigraphically correlated units. These results demonstrate the utility of δ^44/40Ca records in ancient dolomitized carbonates and help to constrain the diagenetic history of these strata.