Rare earth and yttrium elements (REY) patterns of mesostructures of Miaolingian (Cambrian) thrombolites at Jiulongshan, Shandong Province, China

Abstract

The relationship between the thrombolitic mesostructures and their depositional environments is still poorly understood due to inconsistent results by sedimentary investigation. Rare earth elements plus yttrium (REY) in ancient microbialites have been extensively applied to paleoenvironmental studies owing to their fractionation in different depositional environments. In order to investigate the environmental controls on thrombolitic mesostructures, we present the REY concentrations and patterns of four types of mesostructures of the Miaolingian (Cambrian) thrombolites in the Changhia Formation at the Jiulongshan section, Shandong Province, China. The REY compositions of those thrombolites show two distinctive groups: (1) light REY depleted patterns with negative Ce anomalies in spotted (SM) and layered mesostructures (LM) of thrombolites; and (2) flat patterns with weak Ce anomalies in dendritic (DM) and meshed mesostructures (MM) of thrombolites. Controlling factors analysis reveals that terrigenous detritus inputs have stronger influence on REY in SM and LM. In contrast, early diagenetic porewaters from underlying sediments have more serious impacts on REY concentrations and patterns in DM and MM. Our results clearly indicate that SM and LM were formed under oxic marine settings with minor terrigenous inputs, whereas DM and MM formed under suboxic marine settings suffered from early diagenetic porewater from underlying sediments. This new geochemical evidence suggests that thrombolitic mesostructures were strongly influenced by paleoenvironment, and REY of thrombolites with controlling factors analysis can be utilized as effective proxies for paleoenvironments.

Introduction

Thrombolite is one kind of microbialites with distinctive clotted characteristics at mesostructure (Chen et al., 2019, Riding, 2000). The study of microbialites is usually undertaken at four scales of observation: mega-, macro-, meso-, and microstructure (Shapiro, 2000). Many previous studies have documented water depth and energy conditions controlling the thrombolitic megastructure (Lee et al., 2010, Lee et al., 2012), macrostructure (Zhang et al., 1985, Grotzinger et al., 2005, Wang et al., 2012, Tang et al., 2013), meso- and microstructures (Liuet al., 2007, Woo et al., 2008, Woo and Chough, 2010, Wang et al., 2012, Ezaki et al., 2017). Yan et al. (2017) integrated multiscopic observations for thrombolites to interpret the depositional environments. Nonetheless, the relationship between the thrombolitic mesostructures and their depositional paleoenvironment is still poorly understood. Geochemical investigation from the thrombolites themselves would be therefore crucial.

The rare earth elements and yttrium (REY) in ancient microbialites have been broadly used to reconstruct paleomarine environments (Nothdurft et al., 2004, Olivier and Boyet, 2006, Loope et al., 2013, Della Porta et al., 2015, Nutman et al., 2016). The REY in thrombolites might provide direct linkage between thrombolitic mesostructures and depositional environments because the authigenic carbonates precipitating in equilibrium with seawater can potentially capture and preserve the REY characteristics of contemporaneous seawater (Webb and Kamber, 2000). However, further evidence suggests that REY in ancient microbialites is likely affected by terrigenous detritus inputs caused by ambient sedimentary environments (Van Kranendonk et al., 2003, Kamber et al., 2004, Nothdurft et al., 2004, Olivier and Boyet, 2006, Della Porta et al., 2015, Chen et al., under review) and diagenetic fluids (Della Porta et al., 2015, Zwicker et al., 2018). Both effects can overprint original signature inherited from seawater. It is therefore essential to evaluate influences of possible controlling factors on the distributions of REYs in ancient microbialites before interpreting the paleoenvironments.

In this study, we focus on REY concentrations and patterns in four types of mesostructures of thrombolites in the Changhia Formation (Miaolingian, Cambrian) at the Jiulongshan section, Shandong Province, China. The aims of this contribution are to (1) study how terrestrial detritus and diagenesis affect the REY composition of thrombolites; (2) investigate the relationship between thrombolitic mesostructures and depositional environments; and (3) verify whether the thrombolites REY composition can be an effective proxy for the paleoenvironment reconstruction.