Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial

Abstract

[1] High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest 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 most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the midCretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from $2 g/m 2 Á yr prior to the OAE2 to $12 g/m 2 Á 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.Citation: Kolonic, S., et al. (2005), Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial, Paleoceanography, 20, PA1006,

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

“…The Gun Gardens section has become established as a reference for the Cenomanian/Turonian boundary succession in the chalk facies of NW Europe (Tsikos et al 2004;Amédro, Accarie & Robaszynski, 2005;Erbacher et al 2005;Kolonic et al 2005;Kuhnt et al 2005). Published data will be used here to illustrate the consistency of major carbon-isotope trends across the boundary interval.…”

“…The person (circled) is 1.8 m high. Robaszynski, 2005;Erbacher et al 2005;Kolonic et al 2005;Li et al 2006). Jenkyns, Gale & Corfield (1994) defined eight correlative events between east Kent and Italy for the Cenomanian-Lower Campanian.…”

“…The Eastbourne Gun Gardens section in West Sussex (Figs 1, 5) is the most expanded boundary section in chalk facies in NW Europe (Paul et al 1999), and is increasingly being used as an international reference for biostratigraphic and chemostratigraphic studies (Amédro, Accarie & Robaszynski, 2005;Erbacher et al 2005;Kolonic et al 2005;Kuhnt et al 2005). Carbon-isotope curves for the Cenomanian/Turonian Boundary Event at Culver, Dover, Trunch and Speeton are illustrated in Figure 3, and a detailed δ 13 C correlation between Eastbourne and Dover is shown in Figure 6, which compares these sections with the Turonian Global boundary Stratotype Section and Point at Pueblo, Colorado, USA.…”

Oceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco

“…Distinct intervals of black shale deposition with unusually high concentrations of total organic carbon (TOC) termed oceanic anoxic events (OAE) were deposited on a global scale during the Aptian-Albian, Cenomanian-Turonian and Coniacian-Santonian (e.g., Hart and Leary, 1989;Crowley, 1991;Johnson et al, 1996;Wilson et al, l998;Jones and Jenkyns, 2001;Norris et al, 2002;Leckie et al, 2002). At these times, the sequestration of the organic carbon due to high burial rates favored the formation of petroleum source rocks and reduced atmospheric PCO 2 , which affected climate (e.g., Kuypers et al, 1999;Kolonic et al, 2005). OAEs are commonly attributed to the formation of large igneous provinces (LIPs) and associated hydrothermal activity (Larson and Erba, 1999), enhanced upwelling and nutrient cycling driven by orbital forcing Kuhnt et al, 2004;Kolonic et al, 2005).…”

“…At these times, the sequestration of the organic carbon due to high burial rates favored the formation of petroleum source rocks and reduced atmospheric PCO 2 , which affected climate (e.g., Kuypers et al, 1999;Kolonic et al, 2005). OAEs are commonly attributed to the formation of large igneous provinces (LIPs) and associated hydrothermal activity (Larson and Erba, 1999), enhanced upwelling and nutrient cycling driven by orbital forcing Kuhnt et al, 2004;Kolonic et al, 2005).…”