ALBERT

Description: Due to their resistance to diagenesis, fossil brachiopods have extensively been used as archives in paleoenviron-mental studies. Most analyses on fossil brachiopods focus on carbon and oxygen stable isotopes, minor and traceelements, and most of the analyses are still done on the bulk of the shell’s low-Mg calcite (LMC). Despite thehuge progress made over the years on the cleaning procedures in order to avoid contaminations, analytical biasescaused by the brachiopod puncta, filled with diagenetic carbonate cement, has not been addressed so far. Here wepresent a preliminary investigation on the chemical differences that this secondary cement might have if comparedto the biogenic calcite nucleated by the brachiopod. Cenozoic specimens of Megerlia truncata, one Gryphus kickxiand one of Terebratula sinuosa were studied from different locations and outcrops. Electron microprobe analysisin wavelength dispersive mode was done in order to measure the Ca, Mg, Sr, Mn, Fe, Ba, Al, P and Si content in avalve’s thin section, targeting both biogenic calcite of the shell and the cement in the puncta. Results show a clearoffset only in G. kickxi, with all the targeted puncta having much higher Fe and Mg contents. M. truncata and T.sinuosa specimens are more homogeneous and with less distinct chemical differences between puncta and shellcalcite. Such differences might be easily explained by different diagenetic stages: e.g. similar in M. truncata andT. sinuosa, and basically different in G. kickxi. The case of G. kickxi points at the possible analytical biases thatpuncta might cause even in well preserved specimens. The differences in chemical composition that we observedare not species specific, although they might be influenced by puncta size, but rather related to the depositionalenvironment and geological history of the depositional basin. A large scale survey is needed to more accuratelyunderstand the influence of infilled puncta across a large variety of geological settings. This will ultimately lead toa better calibration of geochemical proxies when using brachiopod shell geochemistry as environmental archives.

Description: 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 Gyphus 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.

Description: 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 Gyphus 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.

Description: Brachiopods secrete low-Mg calcite shells in near equilibrium with the surrounding sea water, with respect to their secondary and tertiary layers. For this reason, in recent years they have been intensively studied as archives for oceanographic and environmental proxies. The primary layer has been shown not to be deposited in equilibrium with the ambient sea water, leading to a novel cleaning protocol proposed by Zaki et al (2015). In the spite of improving on existing proxies, the shell microstructure and growth has to be taken in to account in their applications. The secretion of the primary layer is known to be external of the shell, but in SEM investigations of Liothyrella uva and L. neozelanica we discovered that the primary layer has its origin within the fibres of the secondary layer. Furthermore, the primary layer calcite is not a continuum but instead it consists of a 'new' band for each major growth increment. There is overlap between the preceding and subsequent 'band' (or shingles) of the primary layer, which may extend into the secondary/tertiary layer. This finding may lead to more comprehensive knowledge of shell microstructure processes in L. uva and L. neozelanica that may be applied and extended to other modern and fossil brachiopods, including age dating of brachiopods. This discovery may make brachiopod archives more reliable and consistent proxies when applied to and interpreting their geological record.