EARLY PALEOZOIC NEARSHORE BLACK SHALES FROM BALTICA – EUTROPHICATION FRONT TRIGGERED BY NUTRIENT MIXING?

Contradicting biological, sedimentary and geochemical records characterize Tremadocian metalliferous black shales and associated shelly phosphorite deposits from the marginal part of Baltic Paleobasin (Estonia and NW Russia). Abundant bioavailable P and free O in the innermost shelf are evidenced by the thriving of phosphatic brachiopod fauna. Cyclic variations of black shales and subaqueous brachiopod coquinas and cross-bedded sandstones with ripped-up fragments and interbeds of organic-rich muds point to a very shallow water environment. Furthermore, various microfabrics of the black shales suggest that mud accumulation took place in a dynamic environment. The enrichment of pyrite in coarse-grained black shales and strongly pyritized sandstone intervals are widespread in these settings.

Detailed geochemical study of several North Estonian localities reveals U-Mo enrichment ratios and V-hyperenrichment in the black shales, similar to O-deprived oceanic upwelling settings under highly productive stratified water column. Nevertheless, the absence of regular Zn-Cd enrichment and negative Ce-anomalies (Ce/Ce*=0.8-1.2), both in black shales and phosphorites, signal sediment accumulation from generally poorly oxygenated seawater and possible photic zone anoxia. The Baltica paleocontinent was located in the southern Hemisphere mid-latitudes in Tremadocian, characterized by high global temperatures and sea levels. The shallow epicontinental Baltic Paleobasin, which holds extensive organic-rich deposits in its middle Cambrian to Lower Ordovician complexes, suffered from highly diminished terrigenous input in Tremadocian when transgressive seas flooded most of the low-relief continent. Supply and mixing of marine and continental nutrient fluxes could have been critical factors triggering elevated primary productivity nearshore and leading to the accumulation of organic-rich muds in shallow water facies. Excessive P flux to the inner shelf might have been a cumulative effect of upwelling of P-rich water at the Iapetus margin and anoxic recycling of P in deeper part of the paleobasin, whereas continental runoff from the nearby terrestrial areas (Baltic Shield) delivered biolimiting micronutrients such as Fe.