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21A new Re-Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Brazil, 22 coupled with geological observations, provide evidence for late Mesoproterozoic glacial episodes, 23 conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time.

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 δ 18 O and δ 13 C 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 δ 18 O 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 δ 18 O and δ 13 C 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 δ 18 O 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 δ 18 O 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 δ 13 C 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.

A sample set of 164 calcitic brachiopod shells, covering the entire Silurian Period (~ 30 m.y.) with a resolution of about 0.7 m.y., was collected from stratotype sections at Anticosti Island (Canada), Wales (United Kingdom), Gotland (Sweden), Podolia (Ukraine), Latvia, and Lithuania. They show 87 Sr/ 86 Sr values ranging from 0.707930 to 0.708792 that progressively increase with time. This may indicate an increasing riverine flux of radiogenic Sr into the ocean from weathering of continental sialic rocks due to progressive warming of the climate. Exceptionally high increases in 87 Sr/ 86 Sr values were observed in early Llandovery (Rhuddanian), late Llandovery (Telychian), and late Ludlow (Gorstian-Ludfordian boundary) samples. Partial linear regressions, based on a stepwise climbing pattern, with local drops around the Llandovery-Wenlock boundary and in latest Ludlow time, were used to estimate relative ages with a resolution of about ±2 biozones (~1.5-2 m.y.). The Sr-isotope curve shows distinct inflection points in earliest Wenlock and mid-PÍídolí time. These may be used to correlate the Llandovery-Wenlock boundary in the United Kingdom, Gotland, and Lithuania, and the Kaugatuma-Ohesaare boundary in the Baltic states and Podolia.