Geochemistry of ostracode calcite: Part 1. An experimental determination of oxygen isotope fractionation

• Published in: Geochimica et cosmochimica acta 1997, Vol.61 (2), p.377-382
• Main Authors: Xia, J., Ito, E., Engstrom, D.R.
• Format: Article
• Language: English
• Subjects: Candona rawsoni; Freshwater
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(96)00351-1

Late-instar juveniles of the North American ostracode, Candona rawsoni, were cultured to maturity under controlled oxygen isotope composition and temperature (15°C and 25°C) to measure isotopic fractionation during shell calcification. The ostracode shells formed at experimental temperatures are not in isotopic equilibrium with water but had a constant offset from equilibrium based on the oxygen isotope fractionation of inorganic carbonates. The oxygen isotope composition of ostracode shells from 15°C cultures were higher by about 2%o compared to that from 25°C cultures, a difference similar to that expected for inorganic calcite across a 10°C temperature range. The observed fractionations are expressed by the regression equations 25 ° C : δ 18 O s h e l l P D B = − 0.47 + 0.97 δ 18 O w a t e r V S M O W 15 ° C : δ 18 O s h e l l P D B = − 1.12 + 1.078 δ 18 O w a t e r V S M O W The fractionation factors (α) are: 1.0305 at 25°C and 1.0322 at 15°C. The shells of C. rawsoni are elevated by about 0.8–1% in δ 18O compared to inorganic carbonates (O'Neil et al., 1969), but are close to marine biogenic aragonites (Grossman and Ku, 1986) with a slightly different temperature dependence.