Early Jurassic carbon-isotope perturbations in a shallow-water succession from the Tethys Himalaya, southern hemisphere

Abstract

The Early Jurassic was characterized by extreme carbon-cycle perturbations that are associated with abrupt environmental and climatic change. However, the evidence mainly derives from sections in the western Tethys and northern Europe: localities situated in the northern hemisphere. This paper presents new records of foraminiferal biostratigraphic, sedimentological and carbonate carbon-isotope (δ 13 Ccarb) data from the Tibetan Kioto Platform formed in the southeastern Tethys (southern hemisphere) during the Sinemurian-lowermost Toarcian interval. Six foraminiferal zones have been recognized: late Sinemurian Textulariopsis sinemuriensis, Pliensbachian Planisepta compressa, Bosniella oenensis, Cyclorbitopsella tibetica and Streptocyclammina liasica, and earliest Toarcian Siphovalvulina sp. A. Based on biostratigraphy, δ 13 Ccarb data allow correlation with coeval records from the western Tethys and northern Europe by the identification of both negative and positive δ 13 C excursions. The negative excursions characterize the Sinemurian-Pliensbachian boundary event (SPBE) and the margaritatus-spinatum zone boundary event (MSBE); the positive δ 13 C excursion characterizes the margaritatus zone event (ME). Facies evolution in the Early Jurassic indicates that the establishment of carbonate sedimentation on the Kioto Platform occurred in the context of a global sea-level rise partly coincident with the SPBE and that, in common with other coeval platforms, carbonate production following the negative shift was predominantly made up of skeletal carbonates. Furthermore, the spread of the Lithiotis Fauna on the Kioto Platform followed the rebound of isotopic values after the SPBE. This phenomenon has been observed in the Western Tethys and suggests that the global biocalcification event represented by the flourishing of the Lithiotis Fauna may have occurred synchronously across the Tethys, possibly reflecting the creation of more favourable marine conditions after the SPBE. Biostratigraphic data indicate that certain index larger benthic foraminifera became extinct around the onset level of the MSBE, likely due to the deleterious impact of this event. However, as in 3 more northerly localities, the Lithiotis Fauna persisted during the late Pliensbachian in the shallow-water platforms of the Tethys until its disappearance in the early Toarcian.

Early Jurassic long-term oceanic sulfur-cycle perturbations in the Tibetan Himalaya

“…At Wölong, the following lithostratigraphic units are exposed (from the bottom to the top): the Zhamure Formation (upper Sinemurian-lower Pliensbachian), the Pupuga Formation (upper Pliensbachian-lowest Toarcian) and the Nieniexiongla Formation (Toarcian-Aalenian?). Overall, the succession illustrates an evolution that sees a progressive decrease in the terrigenous content of the sediments and an increase of carbonate-rich sediment through time (Jadoul et al, 1998;Han et al, 2016Han et al, , 2021:…”

“…1A and B). The Lower Jurassic succession is composed of shallow-water carbonates and siliciclastics in the southern zone, whereas deep-water marls, calcareous shales and silty shales are found in the northern zone (Han et al, 2021 and references therein).…”

“…1B; 28°29′2″N, 87°02′3″E) is located in the southern zone of the Tibetan Tethys Himalaya. The Lower Jurassic has been well constrained in the section by larger benthic foraminifera, the occurrence of Lithiotis bivalves, and chemostratigraphy (Jadoul et al, 1998;Han et al, 2016Han et al, , 2018Han et al, , 2021. At Wölong, the following lithostratigraphic units are exposed (from the bottom to the top): the Zhamure Formation (upper Sinemurian-lower Pliensbachian), the Pupuga Formation (upper Pliensbachian-lowest Toarcian) and the Nieniexiongla Formation (Toarcian-Aalenian?).…”

“…In addition to these two periods of significant carbon-isotope disturbance, also identified by the volcanogenic fingerprint of mercury in associated organic-rich sediments (Percival et al, 2015(Percival et al, , 2017, an increasing number of studies have led to the recognition of other global events within this interval: namely, a Sinemurian-Pliensbachian boundary event (SPBE), a margaritatus-spinatum zone boundary event (MSBE) accompanied by a negative carbon-isotope excursion (CIE), and a margaritatus zone event (ME) characterized by a positive CIE in Europe, northern Africa, eastern Oregon, USA and Tibet, China (e.g. Korte and Hesselbo, 2011;De Lena et al, 2019;Franceschi et al, 2019;Baghli et al, 2020;Storm et al, 2020;Han et al, 2021). Although the carbon-isotope record for this time interval is becoming better documented, uncertainties remain with regard to these carbon-isotope perturbations.…”

“…These CIEs have been shown to be associated with major environmental and climatic changes such as warming and cooling, fluctuating redox conditions, ocean acidification, carbonate-platform demise and regional sea-level rises and falls that may relate to the impact of varying CO2 concentrations in the oceans and atmosphere (Hesselbo and Jenkyns, 1998;Franceschi et al, 2014Franceschi et al, , 2019Jenkyns, 2020;Müller et al, 2020;Han et al, 2021). The global carbon cycle is intimately tied to the biogeochemical sulfur cycle via organic-matter re-mineralization during microbial sulfate reduction (MSR) and associated pyrite burial (Garrels and Perry, 1974).…”