Redox conditions and depositional environment of the Lower Jurassic Bächental bituminous marls (Tyrol, Austria)

Abstract

A suite of trace elements (TEs) characterized by an affinity to reducing environments, including molybdenum (Mo), uranium (U), vanadium (V), copper (Cu), and nickel (Ni), were used to investigate secular variations in environmental redox conditions during deposition of the Bächental bituminous marls (Bächentaler Bitumenmergel). These marls, which belong to the Sachrang Member of the Lower Jurassic Middle Allgäu Formation in the Northern Calcareous Alps, are subdivided on the basis of Al-normalized TE concentrations and concentration ratios into three units that correspond to major shifts in depositional redox conditions. The lower and upper units (Units 1 and 3, respectively) were deposited under suboxic to transiently anoxic conditions. The removal of TEs to the sediment was connected to siliciclastic input, diagenetic processes, and organic-matter accumulation in the sediment. In contrast, the middle unit (Unit 2) was characterized by intense anoxia as reflected in strong TE enrichments and peak total organic carbon (TOC) contents up to 13 %. Enrichment of TEs was linked mainly to adsorption onto organic matter in Unit 2, reflecting control of TE enrichment by the availability of an organic substrate. Comparison of inorganic and organic (e.g., pristane/phytane ratio, gammacerane index) geochemical proxies for environmental conditions permits detection of periods with elevated surface-water bioproductivity. A flourishing of algal and planktonic organisms contributed to the significant increase of TOC content at the base of Unit 2. This increase corresponds to the time-equivalent early Toarcian oceanic anoxic event characterized by the global occurrence of organic-rich sediments.Eine Auswahl an Spurenelementen (Mo, U, V, Cu, Ni), charakterisiert durch ihre Affinität zu reduzierenden Milieus, wurde verwendet um Variationen der vorherrschenden Redoxbedingungen während der Ablagerung der unterjurassischen Bächentaler Bitumenmergel (Sachrang-Member der Mittleren Allgäu-Formation in den Nördlichen Kalkalpen) zu untersuchen. Die Bitumenmergelabfolge wurde auf Basis dieser Spurenelementdaten in drei Einheiten unterteilt. Anorganisch-geochemische Parameter reflektieren wechselnde Umweltbedingungen während der Ablagerung der Bitumenmergel. Der untere und der obere Bereich (Units 1 und 3) der Bächentaler Bitumenmergel wurden unter suboxischen bis teils anoxischen Bedingungen abgelagert. Die Anreicherung der Spurenelemente im Sediment erfolgte durch siliziklastischen Eintrag, diagenetische Prozesse und Akkumulation von organischem Material. Maximalgehalte an Spurenelementen und von organischem Detritus (bis zu 13 %) bestätigen durchgehend anoxische Bedingungen während der Ablagerung des Mittelteils des Profils (Unit 2). Die Fixierung der Spurenelemente im Sediment erfolgte in diesem Bereich ausschließlich durch Adsorption an organischem Material. Folglich war die Verfügbarkeit von organischem Material der limitierende Faktor für die Spurenelementanreicherung. Der Vergleich der anorganisch-geochemischen und organisc...

The taphonomic fate of isorenieratene in Lower Jurassic shales—controlled by iron?

“…Only high sulfide concentrations may overcome this effect (Farrimond et al., ), and an S/Fe of 1.15 (stoichiometric pyrite) is considered as the threshold for iron limitation during pyrite formation (Dean & Arthur, ). However, S/Fe in our samples is always lower (Table ; see also Neumeister et al., , for Bächental), implying that excess iron may have outcompeted organic matter in the reaction with reduced sulfur.…”

“…Indeed, isorenieratane and further diaromatic carotenoids from Upper Devonian organic‐rich carbonates appear to be preserved rather within the kerogen than in the free bitumen (Duvernay Formation, Western Canada Basin and Williston Basin; Hartgers et al., ). The Bächental basin was also periodically anoxic (Köster et al., ; Neumeister et al., , ) and thus expectedly favorable for the production and preservation of aromatic carotenoids via sulfur bonds in kerogen. However, kerogen pyrolysis yields only very low amounts of isorenieratene derivatives (Figure ) and sulfurized compounds are virtually absent (except small amounts of XXIII to XXV in bitumens; see 3.2.2 and Figure S4).…”

“…The Bächental oil shales were deposited under temporarily anoxic conditions (Köster et al., ; Neumeister et al., , ) prevailing in a local basin on the Tethys shelf (Spieler & Brandner, ). The findings of 13 C enriched isorenieratane (about 15‰ compared to algal biomass, Köster et al., ; this study, Table S2) confirm brown‐colored green sulfur bacteria as the biological source and also imply photic zone anoxia.…”

“…The Bächental basin was also F I G U R E 6 Partial ion chromatograms (a = m/z 237; b = m/z 287) of the aromatic hydrocarbons (F2) in the bitumen from Bäch-1383. Triangles represent 2,3,5′,6-tetramethyl-2′-alkylbiphenyls; IV, VI, VII, VIII, IX, XI and XII = C 40 isorenieratene derivatives with up to three additional aromatic rings; XV = C 33 isorenieratene derivative with one additional aromatic ring; XVII, XVIII and XIX = C 32 isorenieratene derivatives with either one or two additional aromatic rings periodically anoxic (Köster et al, 1995;Neumeister et al, 2015Neumeister et al, , 2016 and thus expectedly favorable for the production and preservation of aromatic carotenoids via sulfur bonds in kerogen. However, kerogen pyrolysis yields only very low amounts of isorenieratene derivatives ( Figure 5) and sulfurized compounds are virtually absent (except small amounts of XXIII to XXV in bitumens; see 3.2.2 and Figure S4).…”

“…The Bächental oil shales were deposited under temporarily anoxic conditions (Köster et al, 1995;Neumeister et al, 2015Neumeister et al, , 2016 prevailing in a local basin on the Tethys shelf (Spieler & Brandner, 1989).…”