Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment

The Silurian of Gotland is characterized by repeated changes in depositiona l facies development. The deposition of uniform sequences of micritic limes tones and marls was interrupted four times by the growth of reef complexes and the formation of expanded carbonate platforms. Coinciding with these, o ften abrupt, facies changes extinction events occurred which predominantly affected nektonic and planktonic organisms. Ratios of carbon- and oxygen-is otopes covary with the facies development. Periods in which the deposition of limestone-marl alternations prevailed are characterized by relatively lo w C- and O-isotope values. During periods of enhanced reef growth isotope v alues are high. For these changes, BICKERT et al. (1997) assume climatic ch anges between humid "H-periods", with estuarine circulation systems and eut rophic surface waters with decreased salinity in marginal seas, and arid "A -periods", with an antiestuarine circulation and oligotrophic, stronger sal ine surface waters. In order to separate local and regional influences on the isotopic developm ent from the global trend, the interactions between facies formation and is otope record have to be clarified. For this purpose, the patterns of isotop e values in the upper part of the Silurian sequence on Gotland (upper Wenlo ck - upper Ludlow) has been determined and stratigraphically correlated alo ng four transects through different facies areas. Facies formation during t his time interval was investigated by differentiation and mapping of twelve facies complexes in the southern part of Gotland. These include shelf area s, reef complexes with patch reefs and biostromes, backreef facies, and mar ginal-marine deposits. The good correspondence between the carbon-isotope r ecords of the four transects suggests that local environmental conditions i n the different facies areas did not influence the delta(13)C values. There fore, a supra-regional or even global mechanism for the C-isotope variation s is likely. In contrast to carbon isotopes, the oxygen-isotope values of the four trans ects generally show parallel trends, but higher variabilities and in parts distinctly deviating developments with a trend to more negative values. The se are interpreted as an effect of local warming in small shallow-water are as which developed during arid periods in reef complexes and backreef areas . The boundaries between A-periods and H-periods, as defined by delta(13)C va lues, which an interpreted as isochrones, can be mapped. From the upper Hom erian to the Pridolian six parastratigraphic isotope zones are defined whic h only partly match the stratigraphic division of HEDE (1942, 1960). The is otope stratigraphy results in an improved correlation between the shallow a nd marginal-marine areas in the eastern part of Gotland and the uniform she lf areas at the west coast of the island. Furthermore, a detailed relationship between the development of carbon and oxygen isotope ratios, the carbonate facies formation, and the succession o f palaeontological events could be observed. At the transition from H-perio ds to A-periods, major extinction events occurred prior to the first increa se of delta(13)C and delta(18)O values. Extinction events affected conodont s, graptolites, acritarchs, chitinozoans, and vertebrates and resulted in i mpoverished nektonic and planktonic communities. The reef-building benthos was less affected. Parallel to a first slight increase of isotope values, f acies began to change, and reefs developed in suitable locations. The subse quent rapid increase of C- and O-isotope values occurred contemporarily wit h strong facies changes and a short-term drop of sea-level. Oligotrophic co nditions in the later stages of A-periods led to strong reef growth and to an expansion of carbonate platforms. The transitions from A-periods to H-periods were more gradual. The delta(13 )C values decreased slowly, but reef growth continued. Later the reefs retr eated and were covered by the prograding depositional facies of shelf areas . The diversity of planktonic and nektonic communities increased again. The close relationship between facies formation, palaeontological events an d isotope records in the Silurian suggests common steering mechanisms but g ives no indication of the causes for the repeated extincion events related to H-period/A-period transitions. Especially the observation, that strong e xtinctions occurred prior to changes of isotope values and facies, points t o causes that left no signals in the geological record. Hypothetical causes like collapse of trophical nets, anoxias, or cooling events are not eviden t in the sediment record or do not fit into the regular succession of perio d transitions.