Carbon isotope 13C enrichment in Upper Silurian (Whitcliffian) marine calcareous rocks in Scania, Sweden

Abstract

Carbon and oxygen isotope data have been obtained from three drill cores, that together span the Öved-Ramsåsa Group (Ludlow–Pridoli) in Scania. Throughout most of the sequence the δ13C values of -1.7 to +2.7‰ correspond well to a modern marine composition. An enrichment in 13C is noted in the Lower Whitcliffian by δ13C values of between +3.6 to +11.2‰. The excursion, also noted in contemporary deposits elsewhere, coincides with a sea-level lowstand. The magnitude of the excursion implies that it cannot be explained solely by an increase in primary production and/or enhanced burial of organic carbon, but a combination of factors is suggested. Carbonates with δ13C values of +3 to +5‰ precipitated from a regionally or globally modified seawater δ13C-composition. The increasing δ13C values of +6 to +11‰ are suggested to be attributed to photosynthetic activity. The local and regional increase in photosynthesizing cyanobacteria and algae is demonstrated by the high abundance of oncoids in the Bjärsjölagård Limestone in Scania and the Eke beds of Gotland. The oxygen isotope composition ranging from -12.6 to -4.7‰ (PDB) is depleted in 18O compared to modern marine carbonates but is mainly in agreement with Silurian values. It is not excluded that the absolute δ18O values, reported here, reflect a diagenetic overprinting. It is, however, suggested that salinity fluctuations may have triggered the increase in cyanophytes. The less negative δ18O values and the unstable 18O/16O ratio, recorded during the δ13C excursion, may thus be explained by evaporation and restricted water circulation.