ORDOVICIAN AND SILURIAN OCEAN AND CLIMATE CHANGES INTERPRETED FROM SR, ND, AND PARTICULARLY O ISOTOPE ANALYSES OF CONODONTS

Changes and events in Ordovician and Silurian paleoceans and paleoclimates have been interpreted from isotope studies of Sr, Nd, C, and O. Temporal trends in paleoceanography were interpreted from earlier Sr and Nd studies using small numbers of conodonts. Temperature is a key factor, with much effort undertaken at some labs to determine seawater paleotemperatures using oxygen isotope compositions of conodonts (given their limited susceptibility to diagenesis). Conventional δ¹⁸O phosphate analyses require tens to hundreds of conodonts, whereas recent advances have enabled high spatial resolution in-situ analysis within single specimens. This allows multiple analyses of individual conodont elements and can target specific mineralized tissues, avoid contaminants, and limit potential variables. Our initial work, based on in-situ d¹⁸O analyses of conodonts using ANU’s SHRIMP II ion microprobe, documented Ordovician paleotemperature trends from equatorial paleoplates. Within a larger conodont paleothermometry project, we have extended this through the Silurian. Specimens were selected from collections from long stratigraphic sections or cores with good age control and little tectonic or thermal alteration. They represent several paleoplates, mainly from equatorial to mid paleolatitudes and shelf to upper slope paleoenvironments, including Laurentia (Cornwallis and Anticosti islands, Canada), Avalonia (Welsh Borderlands, UK), and Baltica (Estonia and Latvia). O isotope compositions were determined on >650 conodonts from 158 samples (>2000 spot analyses), which documented both long-term trends and short-term excursions including: an overall Llandovery warming trend following the terminal Ordovician glaciation, cooling during the Wenlock, and returning to warmer conditions through the Ludlow and Pridoli. This primary trend is punctuated by numerous short-term climate cycles. Overall, Silurian equatorial seawater temperatures varied by ~12°C, but did not reach the excessively high temperatures we reported for the Early Ordovician (Science, 2008). Some of the isotope data support complementary studies of conodont community patterns, sea level changes, and paleoceanographic changes such as the closure of the Iapetus Ocean.