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