Further evidence for the development of photic-zone euxinic conditions during Mesozoic oceanic anoxic events

Abstract

Periods in Earth history characterized by extensive organic carbon deposition, so-called oceanic anoxic events (OAEs), are the subject of considerable scrutiny. Insight into the extent of anoxic conditions in the water column has been provided by the reported occurrence of derivatives of isorenieratene, a carotenoid found only in green sulphur bacteria and thus indicative of photic-zone euxinia (i.e. a water column containing free H 2 S), in North Atlantic deep-sea sediments deposited during the Cenomanian-Turonian boundary OAE2 (Bonarelli Event). In this study, we have used the distributions of chlorophyll and bacteriochlorophyll degradation products, maleimides (1-H-pyrrole-2,5-diones) and high molecular weight porphyrins, to examine further the Cenomanian-Turonian boundary OAE2 as well as other OAEs of the early Toarcian (Posidonienschiefer Event) and early Aptian (Selli Event). In particular, methyl isobutyl (Me,i-Bu) maleimide, on structural grounds, appears to be diagnostic of green sulphur bacteria. This compound occurs in five of seven examined marls that record the early Toarcian OAE (Marche-Umbria, Italy), further expanding the geographical range of Toarcian sediments where evidence for photic-zone euxinic conditions has been found. Me,i-Bu maleimide occurs in three of six black shales spanning the Livello Selli (Marche-Umbria, Italy), the type locality for the Aptian OAE1a, providing the first evidence for photic-zone euxinic conditions during this event. With respect to the Cenomanian-Turonian OAE, maleimide evidence for photic-zone euxinic conditions was found in all the North Atlantic sites investigated, including those characterized by relatively organic-lean sediments, several Tethyan sites and one site off western Australia. These data indicate that euxinic conditions were common in the water column during Mesozoic oceanic anoxic events.

Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)

“…Higher atmospheric pCO 2 and higher temperatures are indicated from oxygen isotopes on bulk carbonate and TEX 86 data from organic matter (Ando et al, 2008;Dumitrescu et al, 2006;Schouten et al, 2003). Biomarkers and redox-sensitive trace metals suggest the development of poorly oxygenated to anoxic watermasses and local photic-zone euxinia (Pancost et al, 2004;Westermann et al, 2013). Oxygendeficiency favoured the burial of huge amounts of organic matter in the sedimentary record as globally distributed marine black shales, which ultimately caused the drawdown of atmospheric CO 2 .…”

A new method for the rapid analysis of 1H-Pyrrole-2,5-diones (maleimides) in environmental samples by two-dimensional gas chromatography time-of-flight mass spectrometry

“…Maleimides have also been detected in environmental samples, where they have been used as indicators of algal productivity, water column properties (i.e. redox conditions, stratification, chemocline depth) and depositional conditions [19,20,[26][27][28][29][30][31][32][33][34][35]. However, the precursors of many maleimides have still not been fully identified, the processes leading to their formation are incompletely understood and their environmental significance needs further investigation [29].…”

Evidence for photic‐zone euxinia in the Early Albian Santana Formation (Araripe Basin, NE Brazil)

“…The episodic occurrence of argillaceous, organic-rich sediments devoid of or impoverished in benthic fauna, and their global distribution, resulted in the concept of oceanic anoxic events or OAEs (Schlanger and Jenkyns, 1976). There is no doubt that OAEs were accompanied by peculiar palaeoceanographic conditions, which included the establishment of anoxic bottom waters and at times oceanwide stratification with sulphidic conditions reaching up into the photic zone (Sinninghe Damste´and Ko¨ster, 1998;Kuypers et al, 2002b;Pancost et al, 2004;Wagner et al, 2004). Although the geochemical record of OAEs has been studied intensively in oceanic pelagic and hemipelagic settings, the expression of these episodic events in interior marine environments has received less attention.…”