Diagenesis of echinoderm skeletons: Constraints on paleoseawater Mg/Ca reconstructions

Abstract

One of the most profound environmental changes thought to be reflected in chemical composition of numerous geological archives is Mg/Ca ratio of the seawater, which has varied dramatically throughout the Phanerozoic. Echinoderms that today typically form high magnesium calcite skeletons are increasingly being utilized as a proxy for interpreting secular changes in seawater chemistry. However, accurate characterization of the diagenetic changes of their metastable high magnesium calcite skeletons is a prerequisite for assessing their original, major-element geochemical composition. Here we expand the existing models of diagenesis of echinoderm skeleton by integration of various analytical methods that up to now rarely have been used to assess the diagenetic changes of fossil echinoderms. We validated the preservation of a suite of differently preserved echinoderm ossicles, mostly crinoids, ranging in age from the Cambrian through Recent. In 13 of 99 fossil echinoderm ossicles we found well-preserved porous microstructure (stereom), non-luminescent behaviour or blotchy dark color in cathodoluminescence, and distinct nanostructural features (layered and nanocomposite structure). Moreover, in representatives of such preserved samples,

Sea urchin growth dynamics at microstructural length scale revealed by Mn-labeling and cathodoluminescence imaging

“…Owing to this fact, labeling with enhanced Mn 2+ concentration in seawater proved to be a useful method in studying growth dynamics of some invertebrates, in particular some species of molluscs [ 19 – 21 ]. With respect to echinoderms, it has been argued that most of them are non-luminescent or reveal light blue zones of intrinsic luminescence [ 12 , 18 , 23 , 27 ] although it has been shown that test plates of some field collected echinoids from offshore environments may also display orange luminescence zoning [ 28 ].…”

Effects of seawater chemistry (Mg2+/Ca2+ ratio) and diet on the skeletal Mg/Ca ratio in the common sea urchin Paracentrotus lividus

“…Mg/Ca ratio may be significantly enriched or depleted due to various diagenetic factors (e.g. Dickson, 1995;Gorzelak et al, 2016) High-magnesium calcite of echinoderm biomineral is metastable and tends to transform into low-magnesium polymorph during diagenesis. Only the samples that meet all structural and geochemical criteria of good preservation can be used in further interpretation but even then, a precise assessment of the extent of diagenetic alteration is often difficult to evaluate (Gorzelak et.…”

“…al., 2016). For instance, sub-microscale removal of intrastereom organic matrix and internal dissolution-precipitation with some transfer of ions may occur without change of stereom architecture (Gorzelak and Zamora, 2013;Gorzelak et al, 2016).…”

Jurassic shift from abiotic to biotic control on marine ecological success

“…Among existing proxies, echinoderm skeletal remains have perhaps the highest potential to eventually provide a densely sampled continuous Phanerozoic record of a single proxy. However, the suitability of echinoderms skeletal remains for reconstructing seawater Mg/Ca ratios has been questioned as magnesium concentrations significantly vary within individual skeletons 83 , are dependent on the food source 84 , and do not always mirror expected secular Mg/Ca trends even in very well-preserved specimens 85 .…”