Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco

Abstract

a b s t r a c tWith a multi-proxy approach, an attempt was made to constrain productivity and bottom-water redox conditions and their effects on the phosphorus accumulation rate at the Mohammed Plage section on the Tarfaya coast, Morocco, during the Cenomanian-Turonian Anoxic Event (OAE 2). A distinct d 13 C org isotope excursion of þ2.5& occurs close to the top of the section. The unusually abrupt shift of the isotope excursion and disappearance of several planktonic foraminiferal species (e.g. Rotalipora cushmani and Rotalipora greenhornensis) in this level suggests a hiatus of between 40-60 kyrs at the excursion onset. Nevertheless, it was possible to determine both the long-term environmental history as well as the processes that took place immediately prior to and during OAE 2. TOC% values increase gradually from the base of the section to the top (from w2.5% to w10%). This is interpreted as the consequence of a longterm eustatic sea-level rise and subsidence causing the encroachment of less oxic waters into the Tarfaya Basin. Similarly a reduction in the mineralogically constructed 'detrital index' can be explained by the decrease in the continental flux of terrigenous material due to a relative sea-level rise. A speciation of phosphorus in the upper part of the section, which spans the start and mid-stages of OAE 2, shows overall higher abundances of P reactive mass accumulation rates before the isotope excursion onset and lower values during the plateau. Due to the probable short hiatus, the onset of the decrease in phosphorus content relative to the isotope excursion is uncertain, although the excursion plateau already contains lower concentrations. The C org /P total and V/Al ratios suggest that this reduction was mostly likely caused by a decrease in the available bottom oxygen content (probably as a result of higher productivity) and a corresponding fall in the phosphorus retention ability of the sediment. Productivity appears to have remained high during the isotope plateau possibly due to a combination of ocean-surface fertilisation via increased aridity (increased K/Al and Ti/Al ratios) and/or higher dissolved inorganic phosphorus content in the water column as a result of the decrease in sediment P retention. The evidence for decreased P-burial has been observed in many other palaeoenvironments during OAE 2. Tarfaya's unique upwelling paleosituation provides strong evidence that the nutrient recycling was a global phenomenon and therefore a critical factor in starting and sustaining OAE 2.

Palaeoenvironmental and climatic changes during the Palaeocene–Eocene Thermal Maximum (PETM) at the Wadi Nukhul Section, Sinai, Egypt

“…In general, P is released as PO 4 3 − from decaying organic matter during oxic, suboxic and anoxic bacterial degradation below the sediment-water interface . However, under anoxic conditions, P is not retained in the sediment and generally diffuses upward from the sediment and returns to the water column (Mort et al 2008). Phosphorus enrichment may result from high OM supply in highly productive marine environments such as upwelling areas.…”

“…The degree to which remineralized organic P is retained as a reactive fraction in sediments depends on the redox conditions of the depositional system. In environments with at least intermittently oxic bottom waters, redox cycling of Fe in the sediments limits the diffusive flux of demineralized P to the overlying water column Mort et al 2008).…”

A New Insight into Shale-Gas Accumulation Conditions and Favorable Areas of the Xinkailing Formation in the Wuning Area, North-West Jiangxi, China

“…The total thickness of these three members was 72.56 m. Fortunately, a water well (drilling began in the lower section) nearby PM1 revealed 18 m of black shale with sustained thickness and developed pyrite, which was a positive signal of un-weathered Xinkailing-formation shale. This suggested that Xinkailing formation shale developed in a deep-water shelf with a strong anoxic environment, which is favorable for the preservation of organic matter [25,26]. The lithology of the Xinkailing formation was mainly comprised of black mud shale and carbonaceous shale.…”

Palaeomagnetic and AMS study of the Tarfaya coastal basin, Morocco: an early Turonian palaeopole for the African plate

“…Mohammed Plage), Mid-Cenomanian-Early Turonian distal to proximal basin strata have been subdivided into various different microfacies and their palaeoenvironmental evolution has been described (El Albani et al 1999a). Different geo-, chemo-and biostratigraphical proxies have been studied for this anoxic period across the C/T boundary, which correlate these Tarfaya Basin sections among themselves and with other successions from the proto-North Atlantic and Tethys Oceans (Kuhnt et al 1990(Kuhnt et al , 1997Holbourn et al 1999;Kolonic et al 2002;Kuypers et al 2002;Tsikos et al 2004;Jenkyns et al 2007;Keller et al 2008;Mort et al 2008;Tantawy 2008). The Tarfaya Basin cyclicity has been interpreted as being forced by the Milankovitch precession (20 ka) and obliquity (39 ka) signals (Kuhnt et al 1997(Kuhnt et al , 2005), but short eccentricity (100 ka) origins (Kolonic et al 2005;Kuhnt et al 2009) for some of the observed cycles has also been proposed (each cycle representing a couplet of carbonate-rich and organic carbon-rich beds).…”