(Table S1) Age determination of ODP Site 202-1240

Interannual-decadal variability in the equatorial Pacific El Niño-Southern Oscillation (ENSO) induces climate changes at global scale, but its potential influence during past global climate change is not yet well constrained. New high-resolution eastern equatorial Pacific proxy records of thermocline conditions present new evidence of strong orbital control in ENSO-like variability over the last 275,000 years. Recurrent intervals of saltier thermocline waters are associated with the dominance of La Niña-like conditions during glacial terminations, coinciding with periods of low precession and high obliquity. The parallel dominance of d13C-depleted waters supports the advection of Antarctic origin waters toward the tropical thermocline. This "oceanic tunneling" is proposed to have reinforced orbitally induced changes in ENSO-like variability, composing a complex high- and low-latitude feedback during glacial terminations.

(Table 1) Boron concentration and d11B ratios of ODP Hole 130-803D

The pH of the surface ocean is a sensitive function of its alkalinity and total inorganic carbon concentration, properties which also control the partial pressure of atmospheric carbon dioxide (Broecker and Peng, 1982). Thus, an accurate proxy for past ocean pH could yield information about variations in atmospheric CO2. Recently, it has been suggested that the boron isotopic composition of foraminiferal tests depends on the pH of sea water as well as its isotopic composition (Vengosh et al., 1991, doi:10.1016/0016-7037(91)90139-V; Hemming and Hanson, 1992, doi:10.1016/0016-7037(92)90151-8). Here we present boron isotope and elemental data for sedimentary pore fluids and isotope data for bulk foraminiferal samples from a deep-sea sediment core. The composition of the pore waters implies that sea water boron concentrations and isotopic composition have been constant during the past 21 Myr, allowing us to reconstruct past ocean pH directly from the foraminiferal isotope data. We find that 21 Myr ago, surface ocean pH was only 7.4 ±0.2, but it then increased to 8.2 ±0.2 (roughly the present value) about 7.5 Myr ago. This is consistent with suggestions (Popp et al., 1989; Cerling, 1991; Arthur et al., 1991) that atmospheric CO2 concentrations may have been much higher 21 Myr ago than today.