Isotopic composition and morphology of living Globorotalia scitula:: a new proxy of sub-intermediate ocean carbonate chemistry?

Abundance, isotopic composition and morphological imprints of the planktonic foraminifera Globorotalia scitula (Brady) were closely examined for possible use as a novel reconstruction tool of chemical environments in sub-intermediate depth seawater in the past. Based on the MOCNES plankton tow observation of dwelling depths of G. scitula and the isotopic compositions together with hydrochemistry data, the empirical relations between isotopic disequilibria in carbon (Delta delta C-13 = delta C-13(G. scitula) - delta C-13(DIC)) and oxygen (Delta delta O-18 = delta O-18(G.scitula) - delta O-18(w)) isotopes in the carbonate tests and the seawater delta O-18 and delta C-13 of dissolved inorganic carbon (DIC), respectively, are introduced. The morphological information such as pore density and porosity is also examined for significant relations to carbonate chemistry. Shell porosity is strongly correlated saturation state of calcite. The dissolution of living G, scitula tests may promote the observed isotopic differences as well as the increases in porosity. Delta delta O-18 of G. scitula is found effectively to be linear function of both water temperature and calcite saturation state (Omega), and thereby temperature equation for G. scitula is provided, while Delta delta C-13 of G. scitula is a linear function of only calcite saturation state. The equation was validated by using Globorotalia scitula collected by a sediment trap in intermediate water depths. Satisfactory agreements were found between observed and calculated Delta delta O-18 from the empirical equations based on temperature and hydrochemistry data at sediment trap deployment site, indicating that the equation may be useful in paleo-environmental reconstruction of sub-intermediate water. The sediment trap observation further suggests that the abundance of G. scitula does not necessarily correspond to surface water productivity and to POC flux, but instead, it correlates well with the supply of fine organic matter, which appears to be a result of water convection. Thus, G. scitula may be an unambiguous and excellent paleo-environmental recorder for carbonate chemistry and for fine organic matter transport to the depths, if isotopic and morphological observations are combined. (C) 2001 Elsevier Science B.V. All rights reserved.