Abstract
Abstract Results of clumped isotope, oxygen isotope and elemental (Mg/Ca, Sr/Ca) analyses of exceptionally well-preserved belemnite rostra and ammonite shells from the uppermost Callovian–Upper Kimmeridgian (Lamberti–Mutabilis zones) of the Russian Platform are presented. Despite a significant decrease in belemnite δ18O values across the Upper Oxfordian–Lower Kimmeridgian, the clumped isotope data show a constant seawater temperature (ca. 16 °C) in the studied interval. The decrease in belemnite δ18O values and lower δ18O values measured from ammonite shells are interpreted as a result of the salinity decline of the Middle Russian Sea of ca. 12‰, and salinity stratification of the water column, respectively. The postulated secular palaeoenvironmental changes are linked to the inflow of subtropical, saline waters from the Tethys Ocean during a sea-level highstand at the Middle–Late Jurassic transition, and progressive isolation and freshening of the Middle Russian Sea during the Late Oxfordian–Kimmeridgian. The obtained clumped isotope data demonstrate relative stability of the Late Jurassic climate and a paramount effect of local palaeoceanographic conditions on carbonate δ18O record of shallow epeiric seas belonging to the Subboreal Province. Variations in Mg/Ca and Sr/Ca ratios of cylindroteuthid belemnite rostra, which are regarded by some authors as temperature proxies, are, in turn, interpreted to be primarily dependent on global changes in seawater chemistry. The paleoenvironmental variations deduced from clumped and oxygen isotope records of the Russian Platform correspond well with changes in local cephalopod and microfossil faunas, which show increasing provincialism during the Late Oxfordian and the Early Kimmeridgian. Based on the review of literature data it is suggested that the observed salinity decrease and restriction of Subboreal basins during the Late Jurassic played a major role in the formation of periodic bottom water anoxia and sedimentation of organic rich facies.
Highlights
A short-term global climate cooling at the Middle–Late Jurassic transition (Late Callovian–Middle Oxfordian) or the incursion of cold Boreal waters are inferred from oxygen isotope records of marine calcareous shells and tooth phosphates of Western and Central Europe, and the Russian Platform (Dromart et al, 2003; Lécuyer et al, 2003; Nunn et al, 2009; Price and Rogov, 2009; Wierzbowski et al, 2009; Wierzbowski and Rogov, 2011; Wierzbowski et al, 2013; Wierzbowski, 2015)
Pervasive calcite cementation of fossils could not have occurred in fine-grained siliciclastic sediments of low permeability and carbonate content, which predominate in the Jurassic of the Russian Platform
A Late Oxfordian–Early Kimmeridgian bottom water warming of 6.5–9.0 °C was suggested for the Middle Russian Sea as based on the changes in δ18O values and Sr/Ca ratios of cylindroteuthid belemnite rostra (Wierzbowski et al, 2013)
Summary
A short-term global climate cooling at the Middle–Late Jurassic transition (Late Callovian–Middle Oxfordian) or the incursion of cold Boreal waters are inferred from oxygen isotope records of marine calcareous shells and tooth phosphates of Western and Central Europe, and the Russian Platform (Dromart et al, 2003; Lécuyer et al, 2003; Nunn et al, 2009; Price and Rogov, 2009; Wierzbowski et al, 2009; Wierzbowski and Rogov, 2011; Wierzbowski et al, 2013; Wierzbowski, 2015). The Late Oxfordian–Early Kimmeridgian warming is regarded as a supra-regional, common phenomenon (cf Dromart et al, 2003; Lécuyer et al, 2003; Brigaud et al, 2008; Nunn et al, 2009; Žak et al, 2011; Alberti et al, 2012a, 2012b; Wierzbowski et al, 2013; Arabas, 2016) its magnitude in marginal marine basins seems to be overestimated due the effect of increasing freshwater runoff under a global sealevel fall (cf Wierzbowski et al, 2013; Wierzbowski, 2015). A particular case is the oxygen isotope record of belemnite rostra from the Russian Platform showing a dramatic decrease in δ18O values of ca. 3‰ throughout the Oxfordian–Kimmeridgian (cf. Riboulleau et al, 1998; Dromart et al, 2003; Price and Rogov, 2009), which may be only partially attributed to a temperature rise (Wierzbowski et al, 2013)
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