PALEOENVIRONMENTAL CHANGES IN THE SILURIAN INDICATED BY STABLE ISOTOPES IN BRACHIOPOD SHELLS FROM GOTLAND, SWEDEN

Ratios of stable carbon and oxygen isotopes in brachiopod shells (more than 370 specimens, esp. Atrypa reticularis) from the Silurian of Got land, Sweden, have been analysed. Preservation of biological skeletal ultrastructures, observed in SEM-micrographs, and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs. The Silurian of Gotland consists of 440 m carbonate deposits, spanning the late Llandovery to late Ludlow epochs (431-411 m.y.). Repeatedly, uni form sequences of micritic limestones and marls are interrupted by com plex-structured reefs and adjacent platform sediments. Previously, the alternation of facies is interpreted as the result of sea level fluct uations caused by a gradual regression with superimposed minor transgr essive pulses. The Silurian sequence of Gotland exhibits principally p arallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units. Lower values occur in periods dominated by d eposition of marry sequences. Higher values are observed in periods do minated by reefs and extended carbonate platforms. The isotope ratios are influenced by local as well as global factors. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than changes in paleotemperature. Consequent ly, the facies distribution of Gotland is interpreted as resulting fro m changes in terrigenous input caused by different rates of continenta l weathering and freshwater runoff rather than by sea revel fluctuatio ns. Periods of arid climate and, therefore, anti-estuarine downwelling of oxygenated surface water appear as short episodes of reef growing (less than or equal to 1.5 m.y.) in an epoche characterized by a tropi c humid climate, which causes an estuarine circulation and the upwelli ng of CO2-rich deep water. Carbon isotope ratios are obviously connect ed to these changes in circulation by the advection of C-13- rich surf ace water (arid episodes) or upwelling of C-13-depleted deep water (hu mid episodes) of a Silurian ocean which itself reveals generally euxin ic deep water conditions due to the burial of organic carbon in black shales. Copyright (C) 1997 Elsevier Science Ltd.