The end Ordovician is known to represent a period of considerable environmental change. Sedimentologists have presented clear evidence suggesting the growth of an ice cap on Gondwanaland which was situated at the south pole (e.g. Brenchley, 1988). In addition, during the Hirnantian (the latest stage of the Otrdovician) palaeontologists have recognised the second largest mass extinction during the Phanerozoic, when it is considered that 12 % of all families became extinct (Raup and Sepkoski, 1982; Brenchley, 1984). Subsequent research has produced carbon and oxygen stable isotopic curves across Ordovician Silurian Boundary intervals using both well preserved shells of articulate brachiopods and whole rock material. Sections where isotope curves have been produced using brachiopod shells include Sweden, situated on Baltica (positive shifts in d18O of >3 and d13C >3 ) (Marshall and Middleton, 1990) and new data from the Baltic States of Estonia and Latvia (positive shifts in d18O of 4 and d13C 5 ) also on Baltica, and Anticosti Island in eastern Canada (smaller positive shifts in d18O of 1 and d13C 3 ) located on Laurentia (Brenchley et al., 1995; Carden, 1995). Analyses of poorly preserved brachiopods from the Hirnantian of Argentina, situated at low latitudes on Gondwanaland (Marshall et al., in press) also record a carbon isotopic shift but the shift in oxygen values has been obscured by diagenetic alteration. This data correlates well with data produced by others. Analyses of whole rock material from the Ordovician-Silurian boundary produced by Wang et al. (1993a) for the Selwyn Basin in Canada, and for Anticosti Island (Long, 1993; Orth et al., 1986) both of which are located on Laurentia demonstrate positive shifts in d13C values of 2 and 3 respectively. In addition, analyses of organic carbon extracted from shales of Hirnantian age in China (Wang et al., 1996), located at low latitudes on Gondwanaland, demonstrate a 6 in d13C.
All sections document a positive shift in carbon isotope values and in the case of sections where well preserved brachiopods were analysed: Anticosti Island, the Baltic States and Sweden, a positive shift in oxygen isotope values as well. In each case, the excursions begin at the base of the Hirnantian Stage and then shift back to lower values in the mid Hirnantian Stage. The isotopic changes can be used to demonstrate the lower Hirnantian Stage, when all the late Ordovician environmental changes occurred. The correlation of excursions in sections from three different super continents at different, demonstrates the isotopic changes in the oceans were not localised but global. Furthermore, the smaller magnitude carbon and oxygen isotopic shifts recorded in brachiopods from Anticosti Island compared to other sections, suggest that either there was a stratigraphic hiatus in this section or localised environmental effects caused a smaller shift in values.
The positive excursion in oxygen isotope values was most likely produced by substantial lowering of sea water temperature, coupled together with an ice-volume fractionation effect associated with the onset of glacation. A drop in temperature at the south pole would have produced cool dense water which would have sunk disrupting the previously warm, stratified oceans causing more vigorous thermohaline oceanic circulation. The positive shift in carbon isotopes concomitant with the shift in oxygen values suggests an increase in the burial or storage of organic carbon and/or increased productivity levels caused by upwelling arising from the change in oceanic circulation.
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