Palaeoenvironmental changes recorded in the oxygen and carbon isotope composition of Kimmeridgian (Upper Jurassic) carbonates from central Poland

Abstract

Oxygen and carbon isotope ratios of well-preserved calcitic bivalves from the Lower–lowermost Upper Kimmeridgian of Central Poland (SW margin of the Holy Cross Mountains) are studied to reconstruct palaeoenvironmental conditions and variations in ancient water chemistry. Low and scattered δ18O and δ13C values of bivalve shells from shallow carbonate deposits of the Hypselocyclum and the Hypselocyclum–Divisum zone boundary (-3.5 to -1.5, and 2.6 to 4.0‰, respectively) are a result of salinity changes, and local variations in the composition of dissolved inorganic carbon (DIC) under the restricted water circulation. A slight increase in bivalve δ13C values and more densely clustering of δ18O values is observed after the marine transgression at the Divisum–Mutabilis zone boundary.A global decrease of δ13C values of marine carbonates is partly recorded in carbon isotopes of Lower–lowermost Upper Kimmeridgian bulk carbonates from Central Poland (from the Radomsko Elevation and the Wieluń Upland). Local negative shifts and the data scatter are, however, observed in very shallow sediments deposited on carbonate platforms during the Platynota and Hypselocyclum chrons. This interval corresponds to the uppermost part of the lowstand systems track of a major regressive trend, which started already in the Late Oxfordian Oxygen and carbon isotope ratios of well-preserved calcitic bivalves from the Lower–lowermost Upper Kimmeridgian of Central Poland (SW margin of the Holy Cross Mountains) have been studied to reconstruct palaeoenvironmental conditions and variations in ancient water chemistry. Low and scattered δ18O and δ13C values of bivalve shells from shallow carbonate deposits of the Hypselocyclum and the Hypselocyclum–Divisum zone boundary (–3.5 to –1.5, and 2.6 to 4.0‰, respectively) are a result of salinity changes and local variations in the composition of dissolved inorganic carbon (DIC) in conditions of restricted water circulation. A slight increase in bivalve δ13C values and more densely clustering of δ18O values is observed after the marine transgression at the Divisum–Mutabilis zone boundary. A global decrease of δ13C values of marine carbonates is partly recorded in Lower–lowermost Upper Kimmeridgian bulk carbonates from central Poland (from the Radomsko Elevation and the Wieluń Upland). Local negative shifts and data scatter are, however, observed in rocks deposited in a very shallow environment of carbonate platforms during the Platynota and Hypselocyclum chrons. This interval corresponds to the uppermost part of the lowstand systems tract of a major regressive trend, which had started in the Oxfordian.