Pseudo dissolution of marine calcite

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-CO2-rich core-top mixed layer and there redissolve. We term this phenomenon pseudo dissolution.