The pre-Littorina diatom stratigraphy and sediment sulphidisation record from the west-central Baltic Sea: implications of water column salinity variations

The pre-Littorina section of a sediment core collected from the eastern slope of the Landsort Deep has been analysed with respect to diatoms and sedimentologic parameters. The occurrences of different Fe-sulphide phases are related to the depositional and diagenetic conditions that are reflected by Corg and S contents and the illite to quartz ratio. The later parameter is applied as a proxy for the sediment grain size. Diatom documentation allows the sub-division of the pre-Littorina stratigraphy into intervals corresponding to the Yoldia Sea and Ancylus Lake. The Yoldia Sea interval is additionally divided into three phases, with the middle phase corresponding to the period when the entire water column was brackish. During the first phase the bottom waters were brackish and surface waters fresh and during the last phase rapid freshening took place. The occurrence of distinct pockets and lenses with FeS in two portions of the pre-Littorina sediments is explained by the elevated Corg content in these intervals. The relatively high Corg favoured sulphide generation through sulphate reduction and fixation as FeS. However, the sulphidic precipitation itself is interpreted to be sulphide limited, consistent with FeS preservation. The upper portion of the pre-Littorina sequence is affected by an additional sulphide influx by downward diffusion from Littorina Sea sediments above, facilitating the transformation of FeS to pyrite in these sediments. The variable Corg content and the grain size are primarily controlled by the proportions of sediment contribution from two different sediment sources: locally supplied, relatively organic-rich and coarse-grained littoral and fluvial sediments, and fine-grained, organic-poor sediments supplied from the melting ice-sheet.