218 / 2019-06-24 22:21:18

Reconstructing Early Jurassic sea water temperatures in the Andean Basin of northern Chile based on stable isotope analyses of oyster and brachiopod shells: a preliminary assessment

Early Jurassic,Chile,stable isotopes,palaeoclimate,water temperatures,seasonality

The stable isotope (δ13C, δ18O) composition of a collection of Early Jurassic brachiopods and oysters from the Andean Basin of northern Chile was analysed. The material allows the first reconstruction of absolute water temperatures for several ammonites zones in the Early Jurassic of South America. The record starts with comparatively warm temperatures in the Late Sinemurian (average: 27.0°C; Raricostatum Zone). Just below the Sinemurian–Pliensbachian boundary, temperatures dropped to an average of 24.3°C. The lowest temperature value in the dataset was recorded in the latest Pliensbachian Spinatum Zone (19.6°C). No data are available from the Early Toarcian, but the Late Toarcian shows 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 confirm the global nature of the Late Pliensbachian Cooling Event. In contrast, the global warming during the Toarcian Oceanic Anoxic Event could not be seen due to a lack of Early Toarcian material. The δ13C record of brachiopods and oysters represents background conditions and does not show any isotope excursions. Oyster shells were used for high-resolution stable isotope analyses and show seasonal temperature fluctuations over a period of around three 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.