Ichnological trends along an open-water transect across a large marginal-marine epicontinental basin, the modern Baltic Sea

Late Holocene sediments in the Baltic Sea provide an opportunity to study lateral changes in the assemblages of identifiable biogenic sedimentary structures (ichnofossils) in a large, high-latitude semi-enclosed sea with instrumentally determined gradients in biodiversity and environmental factors such as salinity and oxygen availability. Integrated sedimentological and ichnological analysis is carried out on 6 long cores collected along an open-sea, declining salinity transect across the basin. Muddy sediments in euhaline (Kattegat) and polyhaline (Mecklenburg Bight) sites are characterized by the archetypal Cruziana Ichnofacies, portrayed by subsurface deposit-feeding structures (Scolicia and Planolites), surface deposit-feeding structures (Skolithos), and structures that reflect both these feeding strategies (Palaeophycus, Arenicolites/Polykladichnus and unnamed biodeformational structures produced by bivalves). The ichnofossils are tiered to 3 levels. The Cruziana Ichnofacies is impoverished in the higher mesohaline Arkona Basin and even more so with declining salinity farther inland. The deepest, oxygen-restricted study sites (Gotland Deep and the western Gulf of Finland) below a permanent halocline are characterized by very small and shallow deposit-feeding structures (Planolites and rare flat Arenicolites/Polykladichnus), and poorly developed tiering. The nearly freshwater eastern Gulf of Finland is characterized by the Cenozoic archetypal Mermia Ichnofacies, dominated by narrow and shallow subsurface and surface deposit-feeding structures (Planolites and flat Arenicolites/Polykladichnus). Large Planolites (3–7 mm in diameter) at this site are untypical of Mermia Ichnofacies assemblages. These results confirm the earlier observations that marine forms dominate brackish-water ichnoassemblages, with the ichnofossil size and diversity decreasing with declining salinity. The results also confirm the predicted decreases in the ichnofossil size and vertical extent at low-oxygen levels. Poorly developed tiering is a particularly useful indicator of oxygen stress in the salinity-restricted system.

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► We study ichnofossils in X-radiographs of late-Holocene muds across open Baltic Sea.
► Ichnofossil size, vertical extent and diversity decrease with declining salinity.
► Ichnofossil size, vertical extent and tiering further decrease in low-oxygen areas.
► Marine ichnofossils dominate, but lacustrine forms increase in low salinities.