Biogenic Phosphate Pollution Inducing Eutrophication as a Catalyst for the Decline of Obolid-Dominated Brachiopod Communities in the Early Tremadocian of East Baltica

During the early Tremadocian (Cordylodus lindstromi and Cordylodus angulatus zones), the Baltoscandian epicratonic basin displayed environmental heterogeneity. The basin included a black shale depocentre, bordered in North Estonia by coastal plains and shoal complexes featuring extensive brachiopod shell accumulations. At this juncture, obolid-dominant brachiopod communities near the shore had become extinct. Allochthonous shell beds, manifested as biogenic phosphorites, were remnants eroded from Furongian shoal bioaccumulations.

Unconsolidated quartzose sand packages, displaying bidirectional cross laminae and intermittently punctuated by thin black shale intercalations and contemporaneous glendonite clusters (a pseudomorph of ikaite), were deposited in tidally influenced foreshore-to-shoreface settings. The nearshore reworking and condensation of sand phosphorites served as indicators of coastal eutrophication. This eutrophication likely led to a significant seasonal enrichment of the water column with nutrients, linked to fluctuations in dissolved oxygen. An overlooked potential contributor to this eutrophication and water pollution, coinciding with the widespread deposition of kerogenous clay and the extinction of shallow marine biota, was the rise in phosphate nutrients and an increased biomass of phytoplankton.

Toxic effects were likely triggered by the extensive Furongian obolid shelly substrates, submerged during the marine transgression. The presence of substantial amounts of dissolved phosphate in the water during this period was evidenced by the deposition of concretions and crusts of chemogenic phosphorites outlining the periphery of the black shale depocentre.