Pseudo dissolution of marine calcite

Abstract

An enigma exists regarding observations aimed at determining the extent of calcite dissolution on the Ceara Rise in the western equatorial Atlantic. Based on pore-water gradients for Ca, alkalinity, pH, 13C and 14C, a strong case can be constructed that at a depth of 3.3 km about 30% of the calcite rain dissolves [Martin and Sayles, Geochim. Cosmochim. Acta 60 (1996) 243–263; Hales and Emerson, Geochim. Cosmochim. Acta 61 (1997) 501–514; Martin et al., Geochim. Cosmochim. Acta 61 (2000) 1391–1404]. However, benthic chambers deployed at this site record no Ca or alkalinity flux [Jahnke and Jahnke, Geochim. Cosmochim. Acta (2002) in press]. Further, there is no evidence for shell weight loss or shell breakup between 2.8 and 4.1 km water depth on the Ceara Rise [Broecker and Clark, Global Geochem. Cycles (2002) in press]. A possible scenario which accounts for all but the 14C observations [Martin et al., Geochim. Cosmochim. Acta 61 (2000) 1391–1404] is that coatings formed on the calcite are stirred downward into the respiration-CO 2-rich core-top mixed layer and there redissolve. We term this phenomenon pseudo dissolution.