2:15 PM - 2:30 PM
[BPT08-03] Influence of cold seep methane on the forming of echinoderm skeletons
Keywords:cold seep, echinoderms, chemosynthetic community, biomineralization
The fossil echinoderms have been collected and studied from two cold seep areas; the Upper Cretaceous (Campanian) in South Dakota, USA and the coeval formation in Hokkaido, Japan. The both crinoids from South Dakota and Hokkaido have clearly lower δ13C values than normal echinoderms, suggesting their skeletons were influenced by cold seep methane which has extremely low δ13C values (Kato and Oji, 2015). However, it was still unknown how these crinoids formed their skeletons with such low δ13C values.
A lot of marine organisms such as many mollusks, are considered to precipitate their skeletons in equilibrium with the ambient sea water in regards to their carbon isotope (Epstein et al., 1951). For example, bivalves shells from a chemosynthetic community show ±5‰ δ13C values (e.g. Mae et al., 2007). However, echinoderm skeletons, even from a normal (non-seep) environment, are known to be formed not in isotopic equilibrium, and the δ13C values of the echinoderms skeletons are different depending on the classes or species (e.g. Weber, 1968). Thus, in echinoderm biomineralization, an isotope fractionation due to vital effect or some carbon sources other than from sea water should be possible.
In order to clarify the cause of inequilibrium of the δ13C value in the skeletons of echinoderms and to estimate degree of isotope fractionation in forming skeletons, specimens of modern echinoid (Strongylocentrotus intermedius) have been raised in two experiments; 1) the echinoids has been raised under three types of sea water with differently controlled δ13C value of DIC (dissolved inorganic carbon), and 2) with two different types diets (a kelp (Saccharina longissima) and a land plant (Fallopia sachalinensis)) with different δ13C value.
Compared with the controlled case of culture with normal sea water with seaweed diet, δ13C value of the echinoid test changed in both cases (in waters with high δ13C value, and with different diets. Thus, it suggests that skeletons δ13C of this echinoid, are influenced by isotope ratio of both sea water and foods. This result clearly shows that the process of isotope fractionation and forming skeletons of echinoderms should be different from those of most of mollusks. It is thought that the echinoderms in cold seep environment depend on bacteria mat for their diet, or detritus that have low δ13C.