Continental chemical weathering during the Early Cretaceous Oceanic Anoxic Event (OAE1b): a case study from the Fuxin fluvio-lacustrine basin, Liaoning Province, NE China

Xiao-Tao Xu,Jian-Yi Qin,Jie Zhao,Long-Yi Shao,Jason Hilton,Bo Lan,Hai-Hai Hou,Shuai Wang

doi:10.1186/s42501-020-00056-y

Abstract

This study focuses on Early Cretaceous mudstones from the Shahai and Fuxin formations in the Fuxin continental basin. We analyse chemical weathering, land surface temperatures and palaeoclimates based on chemical weathering indices, and emphasize the implications of continental chemical weathering on nutrient fluxes into lakes and oceans. According to Cr and Ni abundance, Al2O3-TiO2, La/Sc-Th/Co and V-Ni-Th×10 plots, as well as rare earth element (REE) analysis, mudstone samples from the Shahai and Fuxin formations were derived from the same type of provenance comprising mainly felsic igneous rocks. Chemical weathering trends reflected by the Chemical Index of Alteration (CIA), Weathering Index of Parker (WIP) and the Mafic Index of Alteration for Oxidative weathering environments (MIA(O)) are consistent with each other and allow the geological succession to be divided into four stages. Land surface temperatures of the Shahai and Fuxin formations are estimated based on the linear relationship of CIA to temperature, and also can be divided into four stages consistent with those determined from chemical weathering trends. During Stage A (early part of the late Aptian) chemical weathering and land surface temperatures were relatively low and showed characteristic high fluctuations, while Stage B (latest Aptian) represented a transitional period where weathering rates and temperatures increased, and high amplitude fluctuations continued. Conditions changed markedly in Stage C (early Albian) with very high and stable weathering, and warm, humid climates, while in Stage D (middle and late Albian) conditions returned to low chemical weathering and land surface temperatures. These stages of chemical weathering and land surface temperature fluctuations represent responses to global climate fluctuations during the Early Cretaceous, with the early Albian high weathering intensities and warm, humid climates combining to create high nutrient levels that would have flushed through rivers into lakes and ultimately oceans. This correlates stratigraphically with the development of Early Cretaceous black shales during Ocean Anoxic Event 1b, showing the importance of continental weathering regimes as a causal mechanism for lake and ocean anoxia.

Highlights

The Cretaceous is widely recognised as a greenhouse period in Earth history (Wang 2006; Keller 2008; Wang et al 2014b), with temperatures in the Early Cretaceous warmer and the latitudinal temperature gradient much lower than the present day (Huber et al 2002; Pucéat et al 2003)
Some studies suggest that episodic cooling might have occurred during the Early Cretaceous (e.g., Frakes 1999; Price 1999; Pucéat et al 2003; Amiot et al 2011), and short-term variations of atmospheric CO2 suggest that equable climates may have been interrupted by periods of climatic change including Oceanic Anoxic Events (OAEs) (Jenkyns 2003; Wang et al 2014b)
The variations of major elements normalized by Post Archean Australian Shale (PAAS) from the DY-1 core indicate that most of the SiO2, TiO2 and Al2O3 contents are lower than the PAAS, but the Na2O content is higher than the PAAS

Summary

Introduction

The Cretaceous is widely recognised as a greenhouse period in Earth history (Wang 2006; Keller 2008; Wang et al 2014b), with temperatures in the Early Cretaceous warmer and the latitudinal temperature gradient much lower than the present day (Huber et al 2002; Pucéat et al 2003). Other rapidly changing geological phenomena were likely to contribute to climatic variation at this time, including eruptions of Large Igneous Provinces (LIPs) (Coffin and Eldholm 1994), rapid increases in ocean crust production (Larson 1991) and Cretaceous Oceanic Red Beds (CORBs) (Wang et al 2005; Hu et al 2006). These differing palaeoclimate signals suggest that the greenhouse climates of the Cretaceous may have been transient rather than persistent (Retallack 2009). Systematic investigations of continental sediments during the Cretaceous deserve further attention in order to evaluate the influence of palaeoclimatically driven chemical weathering on the flux of nutrients entering into the oceans through time, and to analyse the relationship between the changing nutrient inputs and the oceanic processes such as ocean anoxic events

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