Climate transitions on recent and geologic timescales are linked to perturbations in atmospheric carbon dioxide concentrations (pCO2). Records of ocean carbonate chemistry allow us to investigate the role of CO2 during past climate events but are limited by the availability of paleo-proxies. This thesis presents the development and calibration of Boron/Calcium (B/Ca) in planktonic foraminifera as a proxy for surface ocean carbonate chemistry from sediment traps and modern surface sediments. The B/Ca proxy is then used to reconstruct surface ocean acidification across the Paleocene-Eocene boundary (~55.8 Myr). Observations of B/Ca in the surface dwelling planktonic foraminifer Globigerinoides ruber white from the Oceanic Flux Program (OFP) sediment trap time-series located near Bermuda are used to suggest that the photosynthetic activity of symbiotic algae within the living foraminifer modify the internal pH relative to the ambient seawater, thereby influencing the B/Ca recorded in the calcitic test (Chapter Two). I hypothesize that the apparent covariance between G. ruber B/Ca and the temperature at the OFP site is due to the seasonal change in incident light affecting the symbiont activity, which can increase the internal pH during calcification from seawater by ~0.2-0.3 units. Measurements of B/Ca and δ11B in different species of planktonic foraminifera from globally distributed core-top sediments reveal that symbiotic foraminifera are offset from the theoretically predicted equilibrium with seawater (Chapter Three). I find no significant temperature effect on B/Ca and the departure from equilibrium for symbiont-bearing species is attributed to biological effects. I provide empirical calibrations for thermocline and deep-dwelling planktonic foraminifera as being primarily controlled by seawater [(〖"B(OH)" 〗_"4" ^"-" )⁄(〖"HCO" 〗_"3" ^"-" )]. Paired isotopic (δ13C, δ18O) and elemental (Mg/Ca, B/Ca) measurements are applied to reconstruct the relative timing and magnitude of environmental changes across the Paleocene-Eocene boundary, occurring ~55.8 Myr using sections from ODP Leg 174AX sites at Bass River and Ancora. Reconstructions of ocean temperature (Mg/Ca) and carbonate chemistry (δ13C and B/Ca) from planktonic foraminifera document an abrupt and significant decrease in B/Ca ratios, coincident with δ13C records and the concomitant ~6-8°C warming. The synchronous changes in all three proxies do not support the occurrence of significant precursor warming or carbon release argued elsewhere.