Rare earth elements (REEs) in vertebrate microremains from the upper Pridoli Ohesaare beds of Saaremaa Island, Estonia: geochemical clues to palaeoenvironment c

A Fadel,A Pérez-Huerta,H Blom,P E Ahlberg,T Jeffries,T Märss,Ž Žigaitė

doi:10.3176/earth.2015.21

Abstract

Rare earth element (REE) compositions of Nostolepis sp. scales, spines, plates and tesserae from Ohesaare bone beds were measured by in situ microsampling using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The obtained REE concentrations, normalized to Post-Archean Australian Shale concentrations, were evaluated using basic geochemical calculations and quantifications. The REE compositions were nearly identical across all the morphotypes and histologies of Nostolepis microremains, showing flat REE patterns with slight depletion in heavy REEs. There was no visible enrichment in middle REEs, indicating good geochemical preservation of bioapatite and absence of any pronounced fractionated REE incorporation during later stages of diagenesis. The shale-normalized (La/Yb)SN and (La/Sm)SN REE ratio compilations indicated adsorption as the dominating REE uptake mechanism across all datapoints. The absence of well-defined Ce anomaly suggested oxic palaeoseawater conditions, which agrees with the existing interpretations of the Ohesaare sequence as high-energy shoal and regressive open ocean sedimentary environments.

Highlights

IntroductionRare earth element (REE) concentrations in biogenic apatites, including fossil bones, teeth and fish scales, have been used to infer palaeoseawater chemistry and redox conditions (e.g., Wright et al 1987), to detect reworking of fossils (e.g., Trueman 1999; Suarez et al 2010), and to accomplish or test palaeoenvironmental and palaeogeographic interpretations (e.g., Reynard et al 1999; Trueman & Tuross 2002; Kemp & Trueman 2003; LÈcuyer et al 2004; Patrick et al 2004; Ounis et al 2008)
This late diagenetic effect is characterized by a strong middle Rare earth element (REE) (MREE) enrichment relative to light REEs (LREEs) and heavy REEs (HREEs)
REE concentrations can serve as a record of depositional or early diagenetic conditions only as long as it is demonstrated that the later contribution of REEs such as from diagenetic pore fluids, is relatively minor (e.g., Reynard et al 1999; Trueman 1999; Patrick et al 2004) and the histology and crystallinity of fossil remains are taken into account

Summary

Introduction

Rare earth element (REE) concentrations in biogenic apatites, including fossil bones, teeth and fish scales, have been used to infer palaeoseawater chemistry and redox conditions (e.g., Wright et al 1987), to detect reworking of fossils (e.g., Trueman 1999; Suarez et al 2010), and to accomplish or test palaeoenvironmental and palaeogeographic interpretations (e.g., Reynard et al 1999; Trueman & Tuross 2002; Kemp & Trueman 2003; LÈcuyer et al 2004; Patrick et al 2004; Ounis et al 2008). The preservation of this palaeoceanographic signal in marine vertebrate remains depends on the presence and extent of later, fractionated REE uptake during post-depositional recrystallization, or ëextensive diagenesisí (Reynard et al 1999; LÈcuyer et al 2004) This late diagenetic effect is characterized by a strong middle REE (MREE) enrichment relative to light REEs (LREEs) and heavy REEs (HREEs). REE concentrations can serve as a record of depositional or early diagenetic conditions only as long as it is demonstrated that the later contribution of REEs such as from diagenetic pore fluids, is relatively minor (e.g., Reynard et al 1999; Trueman 1999; Patrick et al 2004) and the histology and crystallinity of fossil remains are taken into account (éigaitė et al 2013; Fadel et al 2015)

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Conclusion