Microbes, mud and methane: cause and consequence of recurrent Early Jurassic anoxia following the end‐Triassic mass extinction

Abstract

The end-Triassic mass extinction (c. 201.6 Ma) was one of the five largest mass-extinction events in the history of animal life. It was also associated with a dramatic, long-lasting change in sedimentation style along the margins of the Tethys Ocean, from generally organic-matter-poor sediments during the Triassic to generally organic-matterrich black shales during the Jurassic. New core material from Germany provides biomarker evidence of persistent photiczone euxinia during the Hettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the Hettangian and the better-known Toarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the Sinemurian and Pliensbachian. Both the Hettangian and Toarcian events are marked by important changes in phytoplankton assemblages from chromophyte-to chlorophytedominated assemblages within the European Epicontinental Seaway. Phytoplankton changes occurred in association with the establishment of photic-zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the d 13 C of atmospheric CO 2 and variation in the sources of organic carbon. Regardless of the causes of d 13 C variability, long-term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.

Ichthyosaurs from the French Rhaetian indicate a severe turnover across the Triassic–Jurassic boundary

“…Severe environmental changes have also been reported in the same interval, notably a meteorite impact (Olsen et al 2002;Smit et al 2014) and flood volcanism (Schoene et al 2010;Whiteside et al 2010). However, precise stratigraphic correlation in between all these biotic events and between the biotic and abiotic events is still wanting (see reviews of Hesselbo et al 2002;Deenen et al 2010;Van de Schootbrugge et al 2013). On the other hand, close analyses reveal that many groups, especially among invertebrates and microvertebrates, have a rather diffuse, non-catastrophic Late Triassic turnover characterized by elevated extinction rates during the entire NorianRhaetian interval (Cuny 1995;Tanner et al 2004;Bambach 2006;Mander et al 2008).…”

Episodic photic zone euxinia in the northeastern Panthalassic Ocean during the end-Triassic extinction

“…2) indices, indicate reduced bottom-water oxygen across the duration of ecological stress estimated as ~600 k.y. based on correlation to the Newark Basin presented in Whiteside et al (2010) and into the Early Jurassic relative to Late Triassic background values, with a temporary return to oxic conditions immediately after the ETE, as observed elsewhere (e.g., van de Schootbrugge et al, 2013;Richoz et al, 2012). Notably, biomarker concentrations indicative of PZE became established following the development of reducing conditions in bottom waters (e, d in Fig.…”

A new Early Jurassic gastropod from Ellesmere Island, Canadian Arctic - an ancient example of holoplanktonic gastropods

“…Carbon cycle anomalies and recurrent anoxia persisted throughout much of the Hettangian (Bartolini et al 2012), and further anoxic and dysoxic episodes are reported from the Sinemurian and Pliensbachian (van de Schootbrugge et al 2013). Finally, the Pliensbachian-Toarcian Anoxic Event is well known as the most likely cause of the end-Pliensbachian extinction event.…”

“…During and after the end-Triassic biotic crisis, anoxic or dysoxic bottom waters were wide-spread, likely related to changing oceanographic patterns due to the breakup of Pangaea and the opening of the Atlantic Ocean (Hallam 1981, Hallam & Wignall 1997, van de Schootbrugge et al 2013. Carbon cycle anomalies and recurrent anoxia persisted throughout much of the Hettangian (Bartolini et al 2012), and further anoxic and dysoxic episodes are reported from the Sinemurian and Pliensbachian (van de Schootbrugge et al 2013).…”

“…Bailey et al (2003) suggested that global warming boosted continental weathering, freshwater runoff and the hydrologic cycle in general, resulting in a higher content of organic matter in the sea during anoxia. Van de Schootbrugge et al (2013) also stated, "long-term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans" and hypothesized that this is "triggered by flood basalt volcanism".…”