The expression of the Cenomanian–Turonian oceanic anoxic event in Tibet

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Abstract

The Gongzha section of Tibet, China is located at the northern margin of the Indian Plate (SE Tethys) and is characterized by hemipelagic grey marls and marly limestones, light grey limestones and silty limestones, but no organic-rich sediments. High-resolution biostratigraphy reveals an expanded Cenomanian–Turonian (CT) boundary interval and the δ13C record includes the main features of the classical positive carbon-isotope excursion that characterizes the CT oceanic anoxic event. The biotic response inferred from the foraminifera suggests that oxic to dysoxic conditions prevailed, except for a short interval marked by peak abundance of Heterohelix that indicates a significantly dysoxic environment during the δ13C “b” peak excursion. The overall decreasing trend in redox-sensitive trace elements (RSTE) during the maximum δ13C excursion confirms the absence of significant longer-lasting anoxia in the Gongzha section. Enrichments in RSTE are linked to phases of increased detrital input. Chemical weathering indices suggest that the upper Cenomanian sediments accumulated under an increasingly hot and humid climate that culminated near the CT boundary. In the early Turonian lower weathering indices suggest a warm, drier climatic regime with reduced continental runoff. Phosphorus mass-accumulation rates show a significant peak at the onset of the positive δ13C excursion, followed by a decrease up to the basal Turonian. This pattern is positively correlated with the long-term decrease in detrital index as also observed in numerous other CT boundary sections (e.g., Eastbourne, Pueblo, and Whadi El Ghaib, Sinaï). Long-term phosphorus accumulation in the Gongzha section is therefore associated with changes in detrital input. The overall decreased detrital input can be explained by the increasingly remote continental sources due to the major transgression at the end of Cenomanian, coupled with changes in continental weathering intensity linked to increasingly more arid climate conditions.

Highlights

► No strong anoxia prevailed during the CT interval at Gongzha, Tibet. ► CIA values suggest that climate change from hot and humid to drier conditions. ► P accumulation during most of OAE is primarily linked to changes in detrital input. ► Both eustatic/climatic and productivity/anoxic processes control P accumulation. ► Regardless the redox conditions, both models generate the same P trend.

Introduction

The latest Cenomanian Oceanic Anoxic Event 2 (OAE 2) represents one of the most important global environmental changes to have occurred during the Cretaceous (e.g., Jarvis et al., 2006, Jenkyns, 2010). In many sediments deposited in pelagic environments, OAE 2 records the interruption of normal sedimentation by laminated organic carbon-rich sediments, informally termed “black shales”, which are thought to reflect widespread oceanic anoxia (Schlanger and Jenkyns, 1976, Jenkyns, 1980, Arthur et al., 1990). The trends in carbon isotopes (δ13C) recorded during the event are characterized by a prominent positive excursion close to the Cenomanian–Turonian (CT) boundary that reflects a profound global perturbation in the carbon cycle (Scholle and Arthur, 1980). Most of the known localities characterized by black-shale deposition are concentrated in the western Tethys, Central Atlantic and the North American Western Interior Seaway (Arthur and Premoli-Silva, 1982, Schlanger et al., 1987, Jarvis et al., 1988, Kerr, 1998, Coccioni and Luciani, 2005, Jenkyns, 2010). The formation of large igneous provinces (Caribbean, Ontong Java and Madagascar Plateaus) occurring during this period is considered a strong candidate as a trigger of OAE 2 (Courtillot and Renne, 2003, Snow et al., 2005, Turgeon and Creaser, 2008). Large quantities of biolimiting metals would have been introduced into the ocean leading to enhanced primary productivity. The release of large quantities of volcanic CO2 into the atmosphere warmed the Earth, invigorated the hydrological cycle and increased continental weathering and nutrient supply, which also contributed to increased productivity. This magmatic activity was coeval with a major transgression, which culminated in the highest sea levels of the Phanerozoic (Haq et al., 1987). This may have accelerated the remobilization of nutrients and sustained productivity (Mort et al., 2007). The resulting increase in organic carbon production is thought to have initiated a period of widespread anoxia, through the enhanced oxidation of organic matter (Schlanger and Jenkyns, 1976).

These hypotheses on the onset of anoxic conditions during OAE 2 have received additional support from phosphorus (P) burial records. P is an element that is incorporated into organic matter (as phosphate, PO43 ) as well as into various mineral phases, and its formation is often dependent on the concentration of oxygen in the depositional environment. A peak in total P burial predates a significant increase in primary productivity inferred from δ13C records (Mort et al., 2007, Kraal et al., 2010). This is thought to relate to the transition from oxic to anoxic environments, in which the P burial efficiency was reduced. The size of the oceanic PO43  reservoir thus would thus have increased, sustaining new productivity and creating a possible positive feedback loop between productivity, anoxia and P remobilization. Trace-element concentrations in sedimentary successions are useful for reconstructing paleoredox fluctuations (e.g. U, V, Mo) and productivity (e.g. Ba, Ni, Cu, Zn) (Algeo and Maynard, 2004, Tribovillard et al., 2006, Hetzel et al., 2009). Redox-sensitive trace elements (RSTE) are less mobile under oxygen-depleted conditions and are hence generally enriched in sediments (Tribovillard et al., 2006). However, the spatial variability of total P and RSTE contents during OAE 2 is still poorly known, due to limited availability of data from sections outside the central Tethys.

The Gongzha section represents a remote part of the Tethys away from the main depocenters of organic-rich sediments. Previous research on the CT interval of the Gongzha area was mainly focused on the carbon-isotope and biostratigraphic records and the response of marine biota to the OAE 2 (Wan et al., 2003a, Wan et al., 2003b, Li et al., 2006, Wendler et al., 2009). A high-resolution multi-proxy study of the section is presented, integrating data from stable isotopes, bulk mineralogy, phosphorus, major and trace elements, and total organic carbon across the CT boundary. Unlike the classical pelagic sections in the central Tethys and Atlantic, the Gongzha sector of the Tethys remained largely oxic throughout the CT boundary, as indicated by the absence of organic-rich sediments and by RSTE distributions. RSTE pattern and P burial rates correlate well with the overall detrital flux rates. This suggests that P fluxes were directly dependent on detrital influx and were not modulated by the degree of oxygen-depletion in bottom waters.

Section snippets

Geological setting

The Gongzha section is located in Tibet (China) at the northern margin of the Indian Plate in the Tethys Himalayan Zone (Fig. 1). The area is subdivided into two domains: the North Tethys-Himalaya sub-belt characterized by a hemipelagic to pelagic environment, and the South Tethys-Himalaya sub-belt (Gamba–Tingri Basin) composed mainly of carbonate and terrigenous outer-shelf deposits (Willems et al., 1996, Wan et al., 2003b). The latter includes the Gongzha section which is itself divided in

Material and methods

A total of 221 samples were collected along a 73-m thick Cenomanian to lower Turonian interval at the Gongzha section with a sample spacing ranging from 20 to 30 cm. Bulk-rock sample powders were obtained using a mechanical agate crusher. Thin sections were obtained for each sample to aid the identification of the microfacies and microfossils.

Bulk-rock and clay mineralogical analyses were performed at the Geological Institute of the University of Neuchâtel (Switzerland) using a SCINTAG XRD 2000

Lithology and microfacies

The basal part of the Gongzha section exhibits a monotonous succession of limestone and marly limestone of the Lengqingre Formation, interrupted by several more silty intervals, which are commonly bioturbated (Fig. 2). The uppermost part of the section is more calcareous and starts with a thick limestone bed corresponding to the base of the Gamba Cunkou Fm. Most of the section is composed of dark grey marls. Neither laminated nor organic-rich sediments were observed. However, some thin sections

Faunal events and depositional environment

Although the Gongzha section is fairly homogenous in terms of lithology, several shifts in the depositional environment and faunal assemblages point to important changes in this pelagic setting. The agreement between enrichments in fine-grained quartz and DI values, suggests that this pelagic environment was significantly influenced by terrestrial run-off (Fig. 3, Fig. 4).

Two major biological changes highlight the environmental stress associated with OAE2 in the area. A “filament event”

Conclusions

  • 1

    The abundant fauna in the Gongzha section and the resulting biostratigraphic framework reveal a very expanded CT boundary interval. A detailed δ13C curve includes the main features of the classic positive δ13C excursion seen in other parts of the world during OAE2.

  • 2

    The biotic response inferred from the planktonic foraminifera suggests that no strong anoxia prevailed during the CT interval at Gongzha. The environment can therefore be classified as oxic to dysoxic, with the exception of a short

Acknowledgments

We would like to thank Michèle Caron for constructive discussions, training and determination of the planktonic foraminifera, Virginie Matera and Jean-Claude Lavanchy for major and trace-element analyses, Tiffany Monnier for laboratory assistance and André Villard for the preparation of thin sections. We would like to thank Guillaume Suan for helpful comments on the manuscript. We also thank Finn Surlyk and two anonymous reviewers for improving the manuscript, Ian Jarvis and Gerta Keller for

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