GONDWANAN SUPERGLACIATION (CARBONIFEROUS) - GLACIAL II (GSG-II): INTEGRATION OF STRONTIUM, OXYGEN AND CARBON ISOTOPE SEAWATER RECORDS

The Carboniferous – Permian glaciation is characterized by three glacial events. Isotope seawater records of high-resolution (TOP) proxy material (brachiopods) of the late Mississippian and early Pennsylvanian clearly define the Gondwanan Superglaciation - Glacial II (GSG-II). The strontium isotope seawater record suggests initiation of significant continental tectonic activity by an increase in the radiogenic component at about 327.7 Ma or the latest Visean (GTS 2004). During the initial 1.8 Ma the 87Sr/86Sr changed at a rate of about 0.000114/Ma, subsequently this rate of change diminished to an overall of 0.000042/Ma for the next 7.9 Ma. This was followed by an essentially invariant trend for the next 3.4 Ma, but punctuated by perturbations that may be related to astronomical forcing. Glaciation GSG-II onset, according to the oxygen isotope seawater record at Arrow Canyon, followed a late Visean interglacial, was during the earliest Serpukhovian (GTS 2004), and lasted for about 11.4 Ma (until mid Bashkirian). The GSG-II glacial event can be subdivided into two phases, which are punctuated by four distinct stadials. The Interglacial immediately preceding the GSG-II has oxygen isotope characteristics of –3.35 ‰ (mean) consistent with isotopic values observed in modern brachiopods and considering local environmental variations and the temporal difference between the two datasets. The first glacial phase (GSG-II/p1) covers the first 8.1 Ma with ?18O characteristics of –2.32 ‰ (mean), whereas the second glacial phase (GSG-II/p2) covering 3.2 Ma has characteristics of –1.42 ‰ (mean). These two phases (GSG-II/p1 and GSG-II/p2) are further characterized by four episodes of climatic cooling probably related to secondary advances of glaciers (stadials) at 323.4 Ma (-0.74 ‰, ?18O), at 317.8 Ma (+0.55 ‰, ?18O), at 316.3 Ma (+0.20 ‰, ?18O) and at 314.6 Ma (-0.12 ‰, ?18O). In contrast to the distinct and pronounced oxygen isotope trends, the carbon isotope seawater record is not pronounced. Overall the carbon isotope values during GSG-II are slightly more positive than those of the immediately preceding interglacial period. Based on the shift in oxygen isotope composition from the interglacial to the GSG-II of about 1.93 ‰ (mean) sealevel changes during the Mid-Carboniferous glacial GSG-II may have approached 150 m.