Geochemical Record of the Cenomanian‐Turonian Anoxic Event in Tunisia: Is it Correlative and Isochronous to the Biotic Signal?

Abstract

In Tunisia, five Bahloul spaced sections, Bargou, Jerisa, Guern Halfaya, Kherij and Gafsa were analyzed for biostratigraphy (foraminifera and radiolarians) and major and trace elements. This high‐resolution biostratigraphic and chemostratigraphic integrated analyses for the Late Cenomanian‐Early Turonian Bahloul Formation provide new insight into the palaeoceanographic evolution of the southern Tethyan margin. Relative low abundance of related terrigenous Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba, Cu, and Ni (organic matter related trace elements) suggests that the Bahloul, deposited during a relatively short period (0.5 Ma), was of relatively elevated primary productivity and minimal detrital input. While higher D* values concurrent with lower Ti/Al ratios are interpreted as caused by enhanced fluvial material contribution, due to more humid climate during the OAE‐2. Enhanced humidity triggered probably fluvial influxes, resulted in a sluggish water circulation and consequent anoxic/euxinic conditions favoring the preservation of organic matter at the bottom. Enrichments in redox‐sensitive trace metals U, V, and Mo in the Bahloul Formation deposits and redox indices, such as V/(V+Ni), U/Th, V/Cr, and Ni/Co, indicate that oxygen‐restricted conditions prevailed during the Late Cenomanian to earliest Turonian times and correlate well with relative abundances of some foraminiferal and radiolarian paleo‐environmental relevant indicators. High Baxs values and Uauth may indicate anoxic conditions at least at the water–sediment interface during the Bahloul Formation deposition and provide information about low to moderate sulfate‐reduction reactions.

Cretaceous oceanic anoxic events (OAEs) recorded in the northern margin of Africa as possible oil and gas shale potential in Tunisia: An overview

“…Confusion has arisen from the fact that Th occurs in close association with clay minerals, but detailed microprobe and SEM studies have shown that it only occurs in heavy minerals possibly of clay-silt grade (see Hurst 1999). In addition, the U/Th ratio, which is considered as a reliable redox index (Riquier et al 2006;Soua et al, 2011b;Hatch and Leventhal 1992;Jones and Manning 1994), may contradict value distribution between levels of black shale and adjacent sides.…”

“…According to some authors (e.g. Algeo and Maynard 2004;McManus et al 2005;Tribovillard et al 2006;Soua et al 2011b and references therein), U enrichment does not occur in the water column but rather takes place mainly in the sediment, while Zhang et al (2002) hypothesized that U may occur by the combination of iron and sulphate reduction. The oxidized soluble form of uranium is reduced and fixed in euxinic basin sediments as tetravalent U (Anderson et al 1989;Arthur and Sageman 1994;Lüning et al 2000a;Tribovillard et al 2006;Soua 2012Soua , 2014c.…”

“…Generally, under reducing conditions soluble U 6+ is converted to insoluble U 4+ (e.g. Wignall and Myers 1988;Lüning et al 2005;Tribovillard et al 2006;Soua 2010b;Soua et al, 2011b). Basically, uranium is removed from seawater and pores into organic-rich sediments (Anderson et al 1989; Barnes and Cochran 1990;Klinkhammer and Palmer 1991).…”

Trochamminaas opportunist foraminifera in the Lower Jurassic from north Siberia

“…In addition, the opportunist/ specialist behaviour of foraminifera and the diversity of the assemblages are related to nutrient input; a nutrient increase favours the proliferation of opportunist taxa (r-type strategy) and produces the diminution of foraminiferal diversity (Sjoerdsma & Van der Zwaan 1992). For this reason, foraminifera provide insights as to the controlling factors behind Jurassic and Cretaceous oceanic anoxic events and the response of microfaunal assemblages before and after an event (e.g., Bartolini et al 1992;Hylton & Hart 2000;Coccioni & Luciani 2004;Gebhardt et al 2004;Friedrich et al 2009;Mailliot et al 2009;Nikitenko 2009;Soua et al 2011;Reolid, Rodríguez-Tovar & Nagy 2012;. The r-strategist genera have an important role in such biotic crises, as subsequent survivors or colonizers (e.g., Rey et al 1994;Tyszka 1994;Reolid et al 2010;.…”

Planktonic foraminiferal turnover across the Cenomanian – Turonian boundary (OAE2) in the northeast of the Tethys realm, Kopet-Dagh Basin

“…Production of a lot of organic matter during the time of decreasing oceanic circulations leads to an anoxic condition and superimposition of three crises during the event: wide scale occurrence of black shales (Arthur & Premoli Silva 1982;Luciani & Cobianchi 1999;Coccioni & Luciani 2005), faunal turnovers (Leckie et al 1998;Keller et al 2001Keller et al , 2008Keller & Pardo 2004;Caron et al 2006;Soua et al 2009Soua et al , 2011) and a positive δ 13 C excursion (Paul et al 1999;). …”