Black Shale Deposition on the Northwest African Shelf During the Cenomanian/Turonian Oceanic Anoxic Event: Climate Coupling and Global Organic Carbon Burial
Kolonic, Sadat1, T. Wagner1, A. Forster2, J. S. Sinninghe Damste2, B. Walsworth-Bell3, E. Erba3, S. Turgeon4, H. J. Brumsack4, El-H. Chellai5, H. Tsikos6, W. Kuhnt7, M. M. Kuypers8 (1) University of Bremen, Bremen, Germany (2) Royal Netherlands Institute for Sea Research, Den Burg, Netherlands (3) University of Milan, Milan, Italy (4) Carl von Ossietzky University of Oldenburg, Oldenburg, Germany (5) Cadi Ayyad University, Marrakech, Morocco (6) University of Oxford, Oxford, United Kingdom (7) Christian-Albrechts University of Kiel, Kiel, Germany (8) Max Planck Institute for Marine Microbiology, Bremen, Germany
High-resolution
geochemical records from a depth transect through the Cenomanian/Turonian
(C/T) Tarfaya Basin (NW African shelf) reveal
high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming
trace metals, and biomarkers indicative of photic
zone euxinia. These fluctuations are in general
coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records
and the absence of a prominent continental signature suggest a dynamic
depositional setting linked to orbital and higher frequency forcing.
Determining the dominant frequency depends on the definition of the OAE2 and
its duration. We propose that eccentricity is the main forcing factor at Tarfaya that controlled fluctuations in wind-driven
upwelling of nutrient-rich, oxygen-depleted intermediate waters
from the adjacent bottom
water euxinia on the mid-Cretaceous NW-African
Shelf. Accumulation records clearly identify the basin centre as the primary
site of sediment deposition with highest temporal variability and an up to
six-fold increase in OC burial from ~2 g/m2•a g/m2•yr prior to the OAE2 to ~12
g/m2•a g/m2•yr during the OAE2. Photic zone and
bottom
water euxinia alternated with periods of
greater oxygenation of the water column in response to climate forcing. Mass
balance calculations imply that ~2% of the overall global excess OC burial
associated with the OAE2 was deposited in the Tarfaya
Basin, an area that represented only ~0.05% of the total global C/T ocean
floor. In fact, the lateral extent of similar black shales
along the African continental margin indicates that this part of the ocean
contributed significantly to the global increase in organic carbon burial
during the OAE2.