摘要
The stable isotope(δ13 C,δ18 O)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℃;Raricostatum Zone).Just before the Sinemurian–Pliensbachian transition,temperatures dropped to an average of 24.3℃.The lowest temperature value in the dataset was recorded for a brachiopod shell of the latest Pliensbachian Spinatum Zone(19.6℃).No data are available for the Early Toarcian,but results for the late Toarcian show again comparatively warm conditions(average:24.4℃;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.Theδ13 C 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,theδ18 O values point to a minimum estimate for the seasonality in the late Toarcian of slightly more than 3℃.
The stable isotope(δ13 C, δ18 O) 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 ℃; Raricostatum Zone）. Just before the Sinemurian–Pliensbachian transition, temperatures dropped to an average of 24.3 ℃. The lowest temperature value in the dataset was recorded for a brachiopod shell of the latest Pliensbachian Spinatum Zone（19.6 ℃）. No data are available for the Early Toarcian, but results for the late Toarcian show again comparatively warm conditions（average: 24.4 ℃; 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 δ13 C 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 δ18 O values point to a minimum estimate for the seasonality in the late Toarcian of slightly more than 3 ℃.
基金
supported financially by the German Research Foundation(DFG,project AL 1740/3-1).
