Gaia Crippa

How cold were the Early Permian glacial tropics? Testing sea-surface temperature using the oxygen isotope composition of rigorously screened brachiopod shells

Abstract: Brachiopod carbonate from Early Permian brachiopod shells from low palaeolatitude north Iran and higher palaeolatitude Pakistan Karakorum were screened for diagenesis and analysed for oxygen isotope ratios to derive seawater palaeotemperatures. Screening techniques employed included SEM ultrastructural analysis, cathodoluminescence (CL), image analysis of CL images, trace-element (Sr, Mn, Fe) determinations, and carbon and oxygen stable-isotope determinations. The Karakorum shells were found to be diagenetically altered, but those from north Iran were judged to be pristine. Using data from pristine material, two distinct time slices were analysed: the early and middle Asselian. Two contrasting δ18O values for seawater (0‰ and +1.0‰ V-SMOW) were used to account for different extensions of the Gondwanan ice caps. The δ18O data from north Iran indicate a range of seawater temperatures from +24.3 to +30.3 °C (for δ18Oseawater = 0‰ V-SMOW) or from +30.3 to +35.4 °C (for δ18Oseawater = +1.0‰ V-SMOW) for the early Asselian. Results for δ18O from the middle Asselian indicate seawater temperatures of +24.4 to +28.0 °C (for δ18Oseawater = 0‰) or +29.2 to +32.8 °C (for δ18Oseawater = +1.0‰). The maximum calculated temperatures in the middle Asselian are about 2 °C lower than those for the early Asselian. The average temperature for both time slices is similar to modern tropical sea-surface temperatures, indicating that low-latitude Early Permian ocean waters in Iran did not undergo significant cooling during the final Glacial III episode of Gondwanan glaciation. This confirms other evidence based on biotic provinces, which suggests that during the Permo-Carboniferous glaciation, the low-latitude warm belt became narrower and confined to the western Tethys and Cathaysian provinces, and was not subject to a reduction in temperature, but rather a reduction in size.

Mapping of recent brachiopod microstructure: A tool for environmental studies

Shells of brachiopods are excellent archives for environmental reconstructions in the recent and distant past as their microstructure and geochemistry respond to climate and environmental forcings. We studied the morphology and size of the basic structural unit, the secondary layer fibre, of the shells of several extant brachiopod taxa to derive a model correlating microstructural patterns to environmental conditions. Twenty-one adult specimens of six recent brachiopod species adapted to different environmental conditions, from Antarctica, to New Zealand, to the Mediterranean Sea, were chosen for microstructural analysis using SEM, TEM and EBSD. We conclude that: 1) there is no significant difference in the shape and size of the fibres between ventral and dorsal valves, 2) there is an ontogenetic trend in the shape and size of the fibres, as they become larger, wider, and flatter with increasing age. This indicates that the fibrous layer produced in the later stages of growth, which is recommended by the literature to be the best material for geochemical analyses, has a different morphostructure and probably a lower organic content than that produced earlier in life. In two species of the same genus living in seawater with different temperature and carbonate saturation state, a relationship emerged between the microstructure and environmental conditions. Fibres of the polar Liothyrella uva tend to be smaller, rounder and less convex than those of the temperate Liothyrella neozelanica, suggesting a relationship between microstructural size, shell organic matter content, ambient seawater temperature and calcite saturation state.

Microstructural data of six recent brachiopod species: SEM, EBSD, morphometric and statistical analyses

Here, we provide the dataset associated with the research article “Mapping of recent brachiopod microstructure: A tool for environmental studies” [1]. We present original data relative to morphometric and statistical analyses performed on the basic shell structural units (the secondary layer fibres) of brachiopod shells belonging to six extant species adapted to different environmental conditions. Based on SEM micrographs of the secondary layer, fibres from ventral and dorsal valves, and from different shell positions, showing regular and symmetrical cross sectional outlines, were chosen for morphometric measurements using Adobe Photoshop CS6, Image-Pro Plus 6.0 and ImageJ. To work out the reliability of the measurements, the most significant parameters were tested for their probability density by distribution plots; for data visualization and dimension reduction, principal component analysis (PCA) was performed using R 3.3.0 [2] and independent-samples t-tests were performed using SPSS Statistics (IBM Version 22.0. Armonk, NY). Besides a quantitative analysis, a qualitative description of the shell microstructure is provided by detailed SEM imaging and EBSD measurements.

A sampling strategy for recent and fossil brachiopods : selecting the optimal shell segment for geochemical analyses

Recent and fossil brachiopod shells have a long record as biomineral archives for (palaeo)climatic and (palaeo)environmental reconstructions, as they lack or exhibit limited vital effects in their calcite shell and generally are quite resistant to diagenetic alteration. Despite this, only few studies address the issue of identifying the best or optimal part of the shell for geochemical analyses. We investigated the link between ontogeny and geochemical signatures recorded in different parts of the shell. To reach this aim, we analysed the elemental (Ca, Mg, Sr, Na) and stable isotope (δ18O, δ13C) compositions of five recent brachiopod species (Magellania venosa, Liothyrella uva, Aerothyris kerguelensis, Liothyrella neozelanica and Gryphus vitreus), spanning broad geographical and environmental ranges (Chile, Antarctica, Indian Ocean, New Zealand and Italy) and having different shell layer successions (two-layer and three-layer shells). We observed similar patterns in the ventral and dorsal valves of these two groups, but different ontogenetic trends by the two- and three-layer shells in their trace element and stable isotope records. Our investigation led us to conclude that the optimal region to sample for geochemical and isotope analyses is the middle part of the mid-section of the shell, avoiding the primary layer, posterior and anterior parts as well as the outermost part of the secondary layer in recent brachiopods. Also, the outermost and innermost rims of shells should be avoided due to diagenetic impacts on fossil brachiopods.