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 planktoni c foraminifera Globorotalia scitula (Brady) were closely examined for possi ble use as a novel reconstruction tool of chemical environments in sub-inte rmediate depth seawater in the past. Based on the MOCNES plankton tow obser vation of dwelling depths of G. scitula and the isotopic compositions toget her with hydrochemistry data, the empirical relations between isotopic dise quilibria 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-1 3 of dissolved inorganic carbon (DIC), respectively, are introduced. The mo rphological information such as pore density and porosity is also examined for significant relations to carbonate chemistry. Shell porosity is strongl y correlated saturation state of calcite. The dissolution of living G, scit ula tests may promote the observed isotopic differences as well as the incr eases in porosity. Delta delta O-18 of G. scitula is found effectively to b e linear function of both water temperature and calcite saturation state (O mega), and thereby temperature equation for G. scitula is provided, while D elta delta C-13 of G. scitula is a linear function of only calcite saturati on state. The equation was validated by using Globorotalia scitula collected by a sed iment trap in intermediate water depths. Satisfactory agreements were found between observed and calculated Delta delta O-18 from the empirical equati ons based on temperature and hydrochemistry data at sediment trap deploymen t site, indicating that the equation may be useful in paleo-environmental r econstruction of sub-intermediate water. The sediment trap observation furt her suggests that the abundance of G. scitula does not necessarily correspo nd to surface water productivity and to POC flux, but instead, it correlate s 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 m atter transport to the depths, if isotopic and morphological observations a re combined. (C) 2001 Elsevier Science B.V. All rights reserved.