Measurement of boron isotope compositions in species of planktonic foraminifera that calcified their tests at different depths in the water column are used to reconstruct the pH profile of the upper water column of the tropical ocean. Results for five time windows from the middle Miocene to the late Pleistocene indicate pH-depth profiles similar to that of the modern ocean in this area, which suggests that this method may greatly aid in our understanding of the global carbon cycle.

Depth assignments: Planktonic foraminifera have complex life cycles and different species calcify at different depths in the water column [C. Hemleben, M. Spindler, O. R. Anderson, Modern Planktonic Foraminifera (Springer-Verlag, Berlin, 1989). Tropical areas, like site 871, with a strong thermocline and thick mixed layer tend to have the most diverse and well-stratified assemblages. Many species (such as in the genus Globigerinoides) that spend their entire life cycle in the mixed layer (upper 100 m) possess obligate photosymbionts and are omnivorous. Other species stratify through intermediate, thermocline, and deep planktonic habitats (such as Globorotalia species) and feed on algae and sinking phytodetritus. Calcification depths for modern and extinct plankton species were estimated based on previous plankton tow and oxygen isotope studies [Shackleton and Vincent, 1978, doi:10.1016/0377-8398(78)90008-7; Erez and Honjo 1981, doi:10.1016/0031-0182(81)90035-3; Fairbanks et al., 1982, doi:10.1038/298841a0; Keller, Geol. Soc. Am. Mem. 163, 177 (1985); Deuser, 1987, doi:10.2113/gsjfr.17.1.14; Gasperi and Kennett, 1993, doi:10.1016/0377-8398(93)90046-Z; Pearson and Shackleton, 1995, doi:10.2973/odp.proc.sr.144.054.1995; Watkins et al., 1998, doi:10.1016/S0377-8398(97)00036-4]. Specimens of G. tumida with and without a thick gametogenic crust were picked separately and are interpreted as representing thermocline and deep planktonic habitats, respectively. Designation of Dentoglobigerina altispira as a thermocline calcifier differs from some earlier suggestions but follows Pearson and Shackleton, as well as Opdyke and Pearson (1995, doi:10.2973/odp.proc.sr.144.052.1995).

Supplement to: Palmer, Martin R; Pearson, Paul N; Cobb, S J (1998): Reconstructing Past Ocean pH-Depth Profiles. Science, 282(5393), 1468-1471