Geochemistry of modern brachiopods: applications and implications for oceanography and paleoceanography

Abstract

Modern Rhynchonellida, Terebratulida, Thecideida and a few inarticulated brachiopods were collected live or soon after death at 30 locations from depths of 2–3940 m, from the equator to high latitudes covering the Atlantic, Pacific, Indian, Caribbean and Mediterranean Oceans/Seas. Well-preserved fibers of the secondary layer combined with Sr (450–1928 mg/kg), Mn (1–199 mg/kg) and Fe (1–610 mg/kg) contents confirm, with few exceptions, the excellent preservation of the modern brachiopod material. Evaluation of the stable isotope compositions was conducted on 276 specimens and on 374 published samples for a total database of 650 samples from 80 locations. It revealed that oxygen isotope incorporation into shell calcite (secondary layer) of modern articulated brachiopods, in most cases, occurs in equilibrium with the ambient water and as such reflects surrounding environmental conditions. Thecideida as a group (shallow water and low latitude) incorporate oxygen isotopes (−3.04‰ to +0.35‰) into shell calcite in equilibrium with their ambient water. A large majority of genera and species of the Terebratulida (shallow water and low-mid latitude) also incorporate oxygen (−2.30‰ to +3.48‰) into shell calcite in isotopic equilibrium with their surrounding water. In contrast, equilibrium incorporation of oxygen isotopes into shell calcite by the Rhynchonellida is uncertain, but a vital effect probably applies to the Craniida. The question of equilibrium/disequilibrium incorporation of carbon isotopes into shell calcite of modern brachiopods remains unresolved at this time. Isotope compositions range up to 4.5‰, 5.6‰ and 1.5‰ for δ 13C, and up to 5.5‰, 5.9‰ and 0.3‰ for δ 18O, for shallow-water low, mid, and high latitude brachiopods, respectively. Strontium isotopes of modern brachiopods, as expected, show no significant variation with location, latitude, water depth, temperature, salinity and general water conditions. Once local habitat and ecological influences of fossil brachiopods from similar latitudes are considered, their isotopic/elemental values should be valuable diagenetic indicators and important oceanographic/chemostratigraphic proxies.