Echinoderms and Oligo‐Miocene Carbonate Systems: Potential Applications in Sedimentology and Environmental Reconstruction

Abstract

Echinoderms represent a major ecological component and contribute considerably to Oligocene-Miocene carbonate sediments, both as macrofossils and as skeletal grains. The skeletal morphology of all five extant echinoderm classes (echinoids, asteroids, ophiuroids, crinoids, holothuroids) is reviewed. Disarticulated skeletal elements are much more common in sediments than articulated specimens for all echinoderm classes except for echinoids; studies relying on complete specimens alone may be severely biased. The reproduction and growth of echinoderms, the composition of the skeleton, and the crystallography and diagenesis of echinoderm ossicles are reviewed. The echinoderm skeleton consists of high-Mg calcite with 3-18.5 wt% Mg. The skeleton exhibits strong interlacing of microcrystalline calcite with organic material and nonrandom orientation of crystals, achieving considerable hardness and durability. Echinoderm biostratinomy and the identification of disarticulated material are considered. The echinoderm origin of sediment particles can usually be recognised by their characteristic microstructure. Due to the high degree of specialisation, disarticulated remains can often be identified to family or genus level, leading to a more accurate picture of spatial and temporal echinoderm distributions.Echinoderm geochemistry is reviewed with respect to the Mg-content of the skeleton as a palaeotemperature proxy, and the Mg/Ca ratio as a monitor of ancient seawater composition; Sr/Ca ratios and carbon and oxygen stable-isotopes are considered. The echinoderm skeleton is altered during diagenesis and is transformed to low-Mg calcite. The microstructure of the skeleton is largely unaffected by this process, but changes in the isotopic signature and minor/trace-element contents may occur. These factors, together with physiological effects of isotope intake, hamper geochemical applications. However, echinoderms have been used successfully in studies of Phanerozoic seawater chemistry: the Mg and Sr contents of echinoderm skeletons apparently strongly correlate with temperature. Asteroids and ophiuroids are probably best suited for palaeotemperature reconstructions because of the lack of known fractionation within the skeleton and because genetic effects are less pronounced than in echinoids. Controlled laboratory experiments are needed to establish calibrations.Echinoderm remains may account for 5-30 % of the particles within specific Oligocene and Miocene carbonate facies. They seem to be more abundant in temperate shelf carbonates than in tropical settings. Diagenetic changes associated with echinoderm ossicles strongly affect the embedding sediment and promote lithification. Bioerosion by grazing echinoids is important for carbonate budgets in coral reefs and influences the modal size-distribution of sediments by the production of carbonate mud. Burrowing echinoderms may cause intensive bioturbation and reworking of sediments. Echinoderms provide valuable evidence for palaeoenvironmental reconstructions. Ecological ...

Patterns of magnesium content in Arctic bryozoan skeletons along a depth gradient

“…The results from bryozoans contrast with those from crinoids summarized by Kroh and Nebelsick (2010), one of the few published studies dealing with skeletal Mg content in relation to depth. Using data from two literature sources, Kroh and Nebelsick found a linear relationship between skeletal Mg and depth (see also Roux et al 1995).…”

“…The contrast between the bryozoan and crinoid patterns may reflect a different degree of biological control (Lowenstam and Weiner 1989) of skeletal chemistry in these two groups. In addition, our bryozoan data were gathered only from the Arctic, whereas the crinoid data of Kroh and Nebelsick (2010) came from all around the world and included tropical sites and fewer polar sites. Borremans et al (2009) among others have noted that patterns of ocean chemistry are spatially homogenous, and geographical factors can be ruled out as the cause of the observed differences.…”

“…Among echinoderms, a bathymetric gradient has been recognized in crinoids that matched the prediction of declining Mg with depth (Roux et al 1995;Kroh and Nebelsick 2010, fig. 9), while Sewell and Hofmann (2011) examined the effect of depth-related processes on the skeletal magnesium content of echinoids and their test thickness.…”

The first record record of the comatulid crinoid Sievertsella Radwańska, 2003 and its implications for the Miocene of the Bragança-Viseu Basin, Brazil

“…On the other hand, the stalkless forms (the comatulids) occur mainly in reef environments (e.g. Meyer and Macurda 1997;Kroh and Nebelsick 2010). The comatulid from the Pirabas Formation, reported herein, would have been cryptic, set inside a rock cavity or coral in a more coastal tropical environment, obtaining its food through the movements of its arms mainly during the night, similar to recent forms.…”

“…Finally, they change to a free way of life (Hyman 1955). According to Kroh and Nebelsick (2010), the Oligocene-Miocene crinoids have had an auxiliary role in defining the palaeoenvironment. The crinoids that have columns are rare in shallow coastal palaeoecosystems but are more common in deep water.…”