A late Jurassic carbon-isotope record from the Qiangtang Basin (Tibet), eastern Tethys, and its palaeoceanographic implications

Abstract

An Upper Jurassic succession of deep-marine limestones, marls and claystones preserved in the Ambo section of the Qiangtang Basin (Tibet) contains information concerning the response of the eastern Tethys Ocean to multiple carbon cycle perturbations and to palaeoceanographic changes. Three positive δ 13 C excursions (P-CIE 1–3), dated as latest Callovian, middle Oxfordian and early Late Kimmeridgian, respectively, are observed in our section and related to high organic carbon burial episodes; they are correlatable with profiles from the Atlantic, western Tethys and western Paleo-Pacific regions. A significant decrease in δ 13 C values in our section in late Oxfordian to early Kimmeridgian times is attributed to the onset of upwelling of 13 C-depleted watermasses caused by the opening of new seaways during the progressive collapse of Pangaea. A fourth positive δ 13 C excursions (P-CIE 4), dated as Late Kimmeridgian-Early Tithonian, is unlikely to be due to accelerated burial of organic carbon, but may be associated with the establishment of a stable low-latitude thermohaline circulation that replaced the upwelling zones. Due to the oceanic changes linked to the breakup of Pangaea, incursions of cold Arctic waters and associated changes in water mass density triggered a collapse in thermohaline circulation in the Tethys Ocean during the middle Early Tithonian (ca. ~151–150 Ma). As a consequence, an upwelling cold bottom current was formed, triggering global cooling and a significant reduction in the δ 13 C values. This study is the first to show continuous carbon isotope profiles for the eastern Tethys Ocean, and provides an inter-regional correlation and new paleoceanographic information. • The first high-resolution carbon isotope record of the Upper Jurassic from the Tibet, eastern Tethys. • Remarkable P-CIEs during the earliest Tithonian are associated with the major reorganization of Tethys oceanography. • A collapse in thermohaline circulation likely occurred in the middle Early Tithonian.