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

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 and its geochemistry (delta C-13, delta O-18). Based on common belemnite and rarer ammonite findings a detailed biozonation has been established covering the complete Barremian-early Aptian interval, including the mid early Aptian Fischschiefer, the deposition of which occurred during Oceanic Anoxic Event 1a. Quite common throughout the sequence are finely laminated black shales (Blatterton) with high TOC contents of up to 7%. These laminated black shales with CaCO3 contents of up to 60% are interbedded with dark and carbonate-poor clays on the scale of decimeters to several meters. Oxygen depletion of bottom waters is indicated for the laminated intervals since these are barren of any benthic fossils. A salinity driven stratification of the water column during more humid periods was caused by a more intense run-off from the nearby hinterland. This and increased surface water productivity are seen as a possible cause for the lamination with observations being mainly based on micropalaeontological evidence (dinoflagellates, calcispheres, calcareous nannofossils). Stable isotope studies (delta C-13, delta O-18) performed on belemnite guards show delta C-13 values of 0 parts per thousand to 2 parts per thousand for the Barremian. A distinctive positive excursion of similar to 4 parts per thousand directly above the early Aptian Fischschiefer correlates well with the global positive delta C-13 excursion of mid early Aptian age following the Oceanic Anoxic Event 1 a. This first record of the delta C-13 positive excursion from the Boreal Realm therefore confirms the global nature of the signal. A shift in delta O-18 values of about 2 parts per thousand in the late Barremian-early Aptian is interpreted as a temperature decrease by similar to 8 degrees C. This cooling goes along with a shift from the thick (up to 6.5m) early Barremian Hauptblatterton to thin (10cm to 20cm) laminites (Blatterton) in the late Barremian. During the deposition of the Hauptblatterton very warm conditions caused a long lasting (> 500kyrs) stable water stratification. The cooler temperatures of the late Barremian reduced the stability of the water stratification resulting in rather short termed alternations of thin laminites and dark clays. These observations suggest a temperature controlled peak of anoxic conditions in the early Barremian Hauptblatterton and subsequently short termed changes from anoxic to suboxic conditions in the late Barremian and early Aptian. (C) 2008 Elsevier B.V. All rights reserved.