Abstract
The stable isotope (δ13C, δ18O) composition of a collection of Lower Jurassic brachiopods and oysters from the Andean Basin of northern Chile was analyzed. The results allow the first reconstruction of absolute water temperatures for several ammonite zones in the Lower Jurassic of South America. The temperature record starts with comparatively high values in the Late Sinemurian (average: 27.0 °C; Raricostatum Zone). Just before the Sinemurian–Pliensbachian transition, temperatures dropped to an average of 24.3 °C. The lowest temperature value in the dataset was recorded for a brachiopod shell of the latest Pliensbachian Spinatum Zone (19.6 °C). No data are available for the Early Toarcian, but results for the late Toarcian show again comparatively warm conditions (average: 24.4 °C; Thouarsense–Levesquei zones). Even though more material and analyses are necessary to corroborate the recorded temperatures, the present dataset seems to indicate the global nature of the Late Pliensbachian Cooling Event. In contrast, the global warming during the Toarcian Oceanic Anoxic Event has not been recorded due to a lack of Early Toarcian material. The δ13C record of brachiopods and oysters documents a gradual increase in values representing background conditions. Oyster shells were used for high-resolution stable isotope analyses and show seasonal temperature fluctuations over a period of around 3 years in the life time of the bivalves. If explained only by temperatures, the δ18O values point to a minimum estimate for the seasonality in the late Toarcian of slightly more than 3 °C.
Highlights
The climate conditions of the Jurassic world have traditionally been described as warmer than today, relatively stable through time, and with weak latitudinal temperature gradients leading to a lack of polar glaciations (e.g., Chandler et al 1992; Valdes and Sellwood 1992; Rees et al 2000; Hallam 2001; Bailey et al 2003; Sellwood and Valdes 2006)
A collection of Sinemurian to Toarcian oyster and brachiopod shells from the Andean Basin of Chile has been analyzed for their stable isotope (δ13C, δ18O) content in a first step to remedy this lack of data
3 Material and methods Fifty-nine shells of oysters and brachiopods from the Lower Jurassic strata of northern Chile were used in the current study
Summary
The climate conditions of the Jurassic world have traditionally been described as warmer than today, relatively stable through time, and with weak latitudinal temperature gradients leading to a lack of polar glaciations (e.g., Chandler et al 1992; Valdes and Sellwood 1992; Rees et al 2000; Hallam 2001; Bailey et al 2003; Sellwood and Valdes 2006). The Early Jurassic was an epoch which experienced several events with a global impact Most of these have been identified via geochemical analyses of sedimentary rocks or their fossil content (e.g., Jenkyns 1988; Hesselbo et al 2000a; Jenkyns et al 2002; Korte et al 2009; Bodin et al 2016) and could be connected to pulses in the volcanic activity of the Karoo-Ferrar large igneous province influencing the atmospheric composition and the global climate. A collection of Sinemurian to Toarcian oyster and brachiopod shells from the Andean Basin of Chile has been analyzed for their stable isotope (δ13C, δ18O) content in a first step to remedy this lack of data
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