The effects of meteoric diagenesis on the geochemical composition and microstructure of Pliocene fossil Terebratalia coreanica and Laqueus rubellus brachiopod shells from northeastern Japan

Hiroshi Fujioka,Koshi Yamamoto,Yasufumi Iryu,Hideko Takayanagi

doi:10.1186/s40645-019-0289-7

Hiroshi Fujioka, Koshi Yamamoto + Show 2 more

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https://doi.org/10.1186/s40645-019-0289-7

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Abstract

Stable carbon (δ13C) and oxygen isotope (δ18O) compositions of fossil brachiopod shells can be used to interpret paleoclimatic and paleoceanographic conditions. However, the initial isotopic composition of the living shells might be modified by diagenetic alteration. To assess the degree of this modification, we analyzed δ13C and δ18O and three common indicators of alteration in shells: minor element (manganese (Mn), iron (Fe), and strontium (Sr)) concentrations; cathodoluminescence (CL)/non-luminescence; and the shell microstructure of fossil Terebratalia coreanica and Laqueus rubellus (rhynchonellate brachiopod) shells from an exposure of conglomerates of the Tentokuji Formation (Pliocene, 3.85–2.75 Ma) in northeastern Japan. Two indices were used to quantify the preservation state of shell microstructure: the altered fiber ratio (AFR) and the altered fiber and puncta-filling cement ratio (AF-PCR). We qualified the degree of luminescence by measuring the mean cathodoluminescence index (MCLI), defined as the mean R values in a particular area of a CL image with RGB colors. The δ13C and δ18O values were negatively correlated with AFR or AF-PCR and MCLI. The sampling spots with high Mn and Fe concentrations also exhibited relatively low δ13C and δ18O. The δ13C and δ18O were more strongly correlated with Mn concentration than with Fe concentration. Therefore, of the three minor elements examined in this study, Mn concentration is the most reliable indicator of meteoric diagenesis in the studied shells. Unlike Mn and Fe concentrations, Sr concentrations in the modern and fossil brachiopod shells were similar, with overlapping ranges. This indicates that Sr concentration is not likely to be a good indicator of diagenetic alteration in the studied shells. Our study provides further evidence that multiple criteria should be applied and cross-checked when assessing diagenetic alteration of brachiopod shells.

Highlights

Stable carbon isotope (δ13C) and oxygen isotope (δ18O) compositions of biogenic carbonates are useful for paleoenvironmental analysis
To evaluate the preservation of brachiopod shells, the following three screening criteria have been commonly applied (Ullmann and Korte 2015): (1) minor element (manganese (Mn), iron (Fe), and strontium (Sr)) concentrations, which vary with diagenetic alteration; (2) luminescence/non-luminescence with “cold cathode”; and (3) shell microstructures observed via scanning electron microscope (SEM)
SEM, energy dispersive spectrometer (EDS), and CL observations Modern shells The shells of modern T. coreanica (OTTCo) and L. rubellus (SGLR) consisted of two calcite layers: the thinner outer primary shell layer and a thicker inner secondary shell layer (Fig. 4a, c)

Summary

Introduction

Stable carbon isotope (δ13C) and oxygen isotope (δ18O) compositions of biogenic carbonates are useful for paleoenvironmental analysis. To evaluate the preservation of brachiopod shells, the following three screening criteria have been commonly applied (Ullmann and Korte 2015): (1) minor element (manganese (Mn), iron (Fe), and strontium (Sr)) concentrations, which vary with diagenetic alteration; (2) luminescence/non-luminescence with “cold cathode”; and (3) shell microstructures observed via scanning electron microscope (SEM). Well-preserved shells or shell portions are non-luminescent, having high Sr and low Mn and Fe concentrations and well-preserved microstructure. These techniques do not work effectively in all cases. It has been shown that there might be no significant difference in the isotopic composition between well-preserved and diagenetically altered shells determined using the minor element concentrations (e.g., Bruckschen et al 1999; Korte et al 2005). Applying multiple criteria to assess the diagenetic alteration and cross-checking them are required to distinguish between diagenetically altered and unaltered brachiopod shells (Yamamoto et al 2017)

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