Varve microfabric record of seasonal sedimentation and bottom flow-modulated mud deposition in the coastal northern Baltic Sea

• Microfabric of brackish-water muds in the coastal Baltic Sea was investigated.
• Rhythmic biogenic and lithogenic lamina successions reflect seasonal sedimentation.
• Organic matter is introduced to the sea-bottom in rapid pulses during spring blooms.
• Near-bottom flows contribute to the formation of lithogenic lamina successions.
• The laminites provide a high-resolution tool for assessing environmental changes.

Eutrophication-induced hypoxia has resulted in preservation of laminated sedimentary fabric in coastal areas of the northern Baltic Sea. These laminites offer a potential technique for dating recently deposited organic-rich muds, but their formation mechanisms have been inadequately understood. In this study, microfabric of the modern laminated sediments in the coastal Gulf of Finland and Archipelago Sea was investigated through X-radiography and scanning electron microscopy of samples embedded in epoxy resin. The sedimentary fabric comprises rhythmic biogenic and lithogenic lamina successions reflecting seasonal changes in the composition of accumulating material. The diatom-rich biogenic lamina succession is formed during the vernal phytoplankton bloom through rapid sedimentation of aggregates composed of intact diatom frustules and minor proportions of clay-rich lithic material that are enclosed in a sticky matrix of amorphous phytodetritus, analogous to the formation of the marine snow. Accumulation of the lithogenic lamina succession results from decline in primary production towards late summer, enhanced recycling of organic carbon due to grazing by zooplankton and intensified resuspension of lithic-rich terrigenous material concomitantly with increased cyclonic activity towards late autumn and winter. The inclusion of silt grains and isolated diatoms and faecal pellets in the lithogenic laminae attests to lateral transport and deposition from near-bottom flows, contradicting the generally accepted view that laminated sediments accumulate by suspension settling under quiescent conditions only. Accumulation rates inferred from 137Cs dating are consistent with the biogenic–lithogenic lamina succession-couplet thickness, supporting the interpretation that the couplets reflect an annual cycle of deposition. The results indicate that these modern laminated sediments provide a high-resolution archive for assessing recent human-induced environmental changes as well as a robust tool for evaluating the lateral expansion of seafloor oxygen deficiency due to anthropogenic eutrophication in the area during the past decades.