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

Abstract

Stable carbon (δ 13 C) and oxygen isotope (δ 18 O) 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 δ 13 C and δ 18 O 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 δ 13 C and δ 18 O values were negatively correlated with AFR or AF-PCR and MCLI. The sampling spots with high Mn and Fe concentrations also exhibited relatively low δ 13 C and δ 18 O. The δ 13 C and δ 18 O 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.

The physical and geochemical character of the post‐depositional alterations in the Early‐Middle Jurassic carbonates on the western margin of the Indian Plate: Implications for cement morphologies and dolomitization

“…The minor element concentrations of the brachiopod shell show values of 3,700 ppm for Mg, 3,000 for Sr, 0–10 for Mn, and 1,100 for Fe. The Mn values correspond with the Mn values of 0–70 (Ullmann, Frei, Korte, & Lüter, 2017), 4–30 (Lee et al, 2004); Fe values of 30–1,100 ppm reported for modern brachiopod shells and Sr values of 730–1,100 ppm (Fujioka et al, 2019) for fossil brachiopod shells. Because the Sr concentration for modern brachiopod shells (810–1,100 ppm) remain fairly constant in their fossilized counterparts, the Sr is not likely to be a good indicator of diagenetic alteration in brachiopod shells (Fujioka et al, 2019).…”

“…Because of more solubility of Mn (Post, 1999), the silicate weathering provides a significant source of Mn to both the surface and ground waters, and in cases of high concentrations of soluble Mn, it can substitute for Ca in authigenic carbonates (Johnson, Webb, Ma, & Fischer, 2016). The higher concentration of Mn in the Shinawari Formation is explained by the near surface dissolution of carbonates by weathering (probably karst), leading to enrichment of Mn oxides and carbonates (with palaeosols) having some siliciclastic input (Fujioka, Takayanagi, Yamamoto, & Lryu, 2019). Alternatively, the Mn could be as a result of the anoxic conditions, as beds are bounded by organic‐rich horizons (Figure 3f), capable to concentrate the soluble Mn (II) to high levels, similar to the Proterozoic carbonates with anoxic platforms (Holland, 1984).…”

Advances and Challenges in Palaeoenvironmental Studies Based on Oxygen Isotope Composition of Skeletal Carbonates and Phosphates

“…Such shell material cannot be fully cleaned during manual preparation and always contains admixtures of calcite cements. The impact of the puncta-filling cement on oxygen isotope composition of the entire brachiopod shells may be considerable, providing that its δ 18 O values are at least one order of magnitude lower that those of the surrounding shell, which is possible in the case of freshwater alteration [107]. Due to the lack of other fossils in certain intervals or locations, terebratulid oxygen isotope data are, however, taken into account in many cases (e.g.…”

Stable isotopes (δ 13 C, δ 18 O) and element ratios (Mg/Ca, Sr/Ca) of Jurassic belemnites, bivalves and brachiopods from the Neuquén Basin (Argentina): challenges and opportunities for palaeoenvironmental reconstructions

“…Instead, iron and manganese contents were analysed for most of the used specimens, which also allow an evaluation of preservational quality (e.g., Veizer, 1980, 1981;Price and Sellwood, 1997;Wierzbowski and Joachimski, 2007;Wierzbowski et al, 2009;Fujioka et al, 2019). Since concentrations of both elements are relatively low in pristine shells, cut-off values can be defined which may be used to separate potentially altered samples from the 7 database.…”