Temperature controlled deposition of early Cretaceous (Barremian–early Aptian) black shales in an epicontinental sea

摘要: Abstract A continuous, 103m thick mudstone sequence of Barremian–early Aptian age, recently exposed in northern Germany, has been logged and analyzed with respect to its macrofaunal content geochemistry (δ13C, δ18O). Based on common belemnite rarer ammonite findings a detailed biozonation established covering the complete interval, including mid early “Fischschiefer”, deposition which occurred during Oceanic Anoxic Event 1a. Quite throughout are finely laminated black shales (Blatterton) high TOC contents up 7%. These CaCO3 60% interbedded dark carbonate-poor clays scale decimeters several meters. Oxygen depletion bottom waters is indicated for intervals since these barren any benthic fossils. salinity driven stratification water column more humid periods was caused by intense run-off from nearby hinterland. This increased surface productivity seen as possible cause lamination observations being mainly based micropalaeontological evidence (dinoflagellates, calcispheres, calcareous nannofossils). Stable isotope studies δ18O) performed guards show δ13C values 0‰ 2‰ Barremian. distinctive positive excursion ~ 4‰ directly above Fischschiefer correlates well global age following first record Boreal Realm therefore confirms nature signal. shift δ18O about late interpreted temperature decrease 8°C. cooling goes along (up 6.5m) Barremian Hauptblatterton thin (10cm 20cm) laminites During very warm conditions long lasting (> 500kyrs) stable stratification. The cooler temperatures reduced stability resulting rather short termed alternations clays. suggest controlled peak anoxic subsequently changes suboxic Aptian.