Isotopic signals from late Jurassic-early Cretaceous (Volgian-Valanginian) sub-Arctic belemnites, Yatria River, Western Siberia

Journal of the Geological Society, Nov 2004 by Price, G D, Mutterlose, J

Abstract:

This contribution presents the first detailed oxygen and carbon isotope record from the latest Jurassic-early Cretaceous interval of the Yatria River, subpolar Urals, Siberia. Isotopic compositions have been determined on well-preserved belemnite samples from the genera Lagonibelus, Cylindroteuthis and Acroteuthis. These new data indicate a shift to lower temperatures from the late Volgian into the late Valanginian, with some warmer phases recognized within the Ryazanian and earliest Valanginian. The lowest temperatures of the late Valanginian, consistent with subfreezing polar temperatures, are coincident with an inferred eustatic sea-level fall. A late Valanginian positive shift in carbon isotopes correlates with the carbon isotope excursion recorded from Tethyan successions. The most positive carbon isotope values correspond to the most positive oxygen isotope values (and hence lowest palaeotemperatures). In the absence of widespread Valanginian organic-rich black shale deposition, the carbon isotope excursion may point to increased storage of organic carbon in coastal areas and/or enhanced preservation within stratified waters in high-latitude basins. At these higher latitudes, where rates of weathering were presumably much lower because of the prevalent cold climate, the isotopic data may point to pulses of productivity being brought about by increased riverine nutrient transfer and also by nutrients being released by the melting of ice. The correlation between positive carbon isotopes and cool climates may indicate the effectiveness of these high-latitude carbon sinks and their ability to draw down atmospheric CO2, resulting in an 'inverse greenhouse' effect.

Keywords: Jurassic, Valanginian, Siberia, belemnites, stable isotopes.

Subfreezing polar temperatures during the Jurassic and Cretaceous are considered incompatible with widely accepted palaeoclimate data. This period is commonly viewed as a time of warm global climates with a low global temperature gradient, giving rise to weakly defined climatic zonation and warm polar regions (e.g. Frakes 1979; Hallam 1993). Particularly compelling evidence for polar warmth is provided by physiognomic analysis of mid- and late Cretaceous floras from Alaska and northeastern Asia (e.g. Herman & Spicer 1997) and has also been provided by oxygen isotope derived temperature determinations (e.g. Pirrie & Marshall 1990; Huber 1998).

A feature of the early Cretaceous is a series of positive carbon isotope excursions (e.g. the late Valanginian) identified within Tethyan and Atlantic areas (e.g. Lini et al. 1992; Wortmann & Weissert 2001) and also from sediments from the Pacific Ocean (e.g. Bartolini 2003). Positive carbon isotope excursions within the early Cretaceous have been attributed to greenhouse climate conditions (e.g. Lini et al. 1992; Föllmi et al. 1994; Weissert et al. 1998). A number of palaeoclimatological studies (e.g. Kemper 1987; Frakes & Francis 1988; Weissert & Lini 1991; Price 1999; Alley & Frakes 2003) indicate at least seasonally low ocean temperatures and the possibility of limited polar ice during the early Cretaceous. Such data are, however, not widely distributed in the Cretaceous (Bennett & Doyle 1996; Price 1999) and hence other palaeoproxies are required to lend support to glacial phases in a Jurassic-Cretaceous greenhouse Earth.

A number of workers, including Ditchfield (1997) and Pucéat et al. (2003), have postulated, on the basis of oxygen isotopic analyses, that seasonally low ocean temperatures and limited polar ice caps were present within the early Cretaceous. Decreases in carbon isotope values from the marine record across the Jurassic-Cretaceous boundary (e.g. Weissert 1989; Weissert & Mohr 1996; Ruffell et al. 2002) have also been interpreted as evidence of a transition from a warm and humid greenhouse climate to increasing aridity and possibly cooler conditions within the earliest Cretaceous. These published marine isotope records have often been constructed from outcrop exposures within Europe using a composite of different localities with varying diagenetic histories and sometimes poor sample resolution. This contribution presents a late Jurassic (Volgian)- early Cretaceous (early Hauterivian) carbonate (belemnite) isotope record from Western Siberia. A number of studies (e.g. Pirrie & Marshall 1990; Ditchfield 1997; Podlaha et al. 1998; Price et al. 2000; van de Schootbrugge et al. 2000) have demonstrated that, with appropriate constraints placed upon the isotopic composition of seawater, credible palaeotemperature trends can be derived from the isotopic analysis of belemnites. With respect to the early Cretaceous, isotopic data are derived largely from mid- to low latitudes and may therefore not necessarily reflect ambient climatic conditions at higher latitudes. Hence our understanding of Cretaceous climate has been hampered by a lack of data from northern high latitudes. It is the purpose of this contribution to provide robust, biostratigraphically constrained, oxygen and carbon isotopic data from high latitudes that contribute to the debate on whether the early Cretaceous was at times characterized by sub-freezing polar temperatures. A concurrent analysis of the belemnite fauna will also be undertaken to provide important information regarding the palaeoecological and palaeoceanographic setting.