Oxygen and carbon isotopic composition of Silurian brachiopods: Implications for coeval seawater and glaciations

We collected 236 calcitic brachiopod shells, covering the entire Silurian Period (∼30 m.y.), at high temporal resolution from stratotype sections from Anticosti Island (Canada), Wales (United Kingdom), the Oslo region (Norway), Gotland (Sweden), and Podolia (Ukraine), Estonia, Latvia, and Lithuania. Data from petrography, scanning electron microscopy, cathodoluminescence, isotopes, and trace elements all confirmed that there was excellent preservation in most shells, thus arguing for retention of primary isotope signals; exceptions were samples from the Oslo region.

The δ¹⁸O and δ¹³C values for the well-preserved samples range from −2‰ to −6.5‰ and from −1‰ to 7.5‰ (Peedee belemnite), respectively. In terms of temporal trends, oxygen and carbon isotopes vary in parallel, with a slight decrease with declining age of ∼1‰ through Silurian time, with superimposed short-term oscillations that are negatively correlated with sea-level changes. Three successive positive δ¹⁸O shifts in early Aeronian, latest Aeronian, and early Wenlock time correlate with sea-level lowstands and with glacial diamictite deposits in the Amazon Basin and in Africa. The high δ¹⁸O and δ¹³C values are attributed to cold episodes with low sea levels and low values to warm episodes with high sea levels. During warm Silurian episodes, the δ¹⁸O value of seawater is suggested to have been about −3.5‰ standard mean ocean water (SMOW) and the global tropical temperatures of about 20–30 °C, similar to the present-day values in summer. During glacial episodes, seawater is proposed to have had a δ¹⁸O value of ∼−2.5‰ (SMOW) and temperature of about 14–23 °C, comparable to tropical temperatures proposed for the last glacial episode (14 to 28 °C). Three positive δ¹³C peaks, in early and late Wenlock and late Ludlow time, likely of regional to global significance, appear to coincide with sea-level lowstands, but we are as yet unable to propose a convincing causative geologic scenario that would explain their origin.