Black shales contamination and depositional paleoenvironment during the Early Aptian OAE 1a in the Eastern Russian Platform

Abstract

Late Jurassic-Late Cretaceous is considered a time of increasing temperatures and "greenhouse conditions" (Scotese 2016). This period is marked by several episodes of widespread anoxia and enhanced productivity in the oceans (Jenkyns 2010, Scotese 2014), resulting in black shale deposits known in the Cretaceous as Oceanic Anoxic Events (OAE) (Schlanger & Jenkyns 1976). The Early Aptian OAE 1a (Selli Event, ~120 Ma) lasting ~1-1.4 Ma (Li et al. 2008, Malinverno et al. 2010) coincided with both the beginning of significant warming and a short-term drop in δ 13 C values in marine carbonates (e.g., Menegatti et al. 1998, Zakharov et al. 2013. The event could be caused by an activation of large igneous provinces (Larson & Erba 1999, Tejada et al. 2009, Keller et al. 2011, Percival et al. 2021 or release of methane gas hydrate trapped in oceanic sediments (e.g., Jahren et al. 2001, Van Breugel et al. 2007) leading to sudden addition of isotopically light volcanic CO 2 to the atmosphere and oceans. Similar events are known from the Jurassic, e.g. the Toarcian anoxic event (French et al. 2014) and the "Late Jurassic anoxic event" (Trabucho-Alexandre et al. 2012, Nozaki et al. 2013.At the moment, deposits rich in organic matter (OM) are of increasing interest since they are a potential source of hydrocarbons and an indicator for climate and environmental changes, but also may comprise itself a source of greenhouse gases, such as methane (Maksyutova et al. 2018).There is a general acceptance confirmed by the results from the Tethys, Atlantic, and Pacific oceans, North Sea, Russian Platform, Lower Saxony, and many other basins that major warming characterized OAE 1a (Selli Event), which began before and reached a maximum during initial OAE 1a (e.g., Erba et al. 2010, Zakharov et al. 2013, Bottini et al. 2015. The rest of the OAE 1a interval was marked by subsequent "cold snap", and a further cooling took place when the uppermost part of the organic carbonrich level was deposited (e.g., Bottini et al. 2015).

The extinct shark, Ptychodus (Elasmobranchii, Ptychodontidae) in the Upper Cretaceous of central-western Russia—The road to easternmost peri-Tethyan seas

“…The northeastern peri-Tethys represented a crucial connection between the various peripheral areas of the Neo-Tethys Ocean with the Russian Platform during the Late Cretaceous that had a major role in shaping aquatic environments, and possibly the distribution of marine faunas, across these epicontinental sea areas (e.g., Baraboshkin et al, 2003 ; Solonin et al, 2021b ; Zorina, 2022 ). Sharks from the Upper Cretaceous of central Russia have not yet been extensively studied and those studies available mainly are limited to Cenomanian deposits ( Kiprijanoff, 1852 , 1881 ; Glikman, 1980 ; Eremin, 1998 ; Olferev and Alekseev, 2005 ; Starodubtseva et al, 2008 ).…”