The removal of exchangeable oxygen from diatom opal prior to diatom δ¹⁸O analysis is a crucial first step before analyzing frustule oxygen isotopes for paleoceanographic applications. We present the results of experiments that quantify the temperature dependent reactivity of biogenic silica with water under laboratory conditions. We quantified the equilibration between exchangeable oxygen in the diatom opal lattice by immersing samples from sediment traps (STS-1) and a sediment core (BC-43, sample from 11cm core depth) in two different equilibration waters with δ¹⁸O = -8.2‰ (laboratory DI at UC Davis), and +94.4‰ (enriched water used for experiments) at a laboratory temperature of 21.2°C (±0.3°C) for 70 hours We demonstrate that controlled exchange between rinse water and diatom opal at room temperature results in predictable alteration of diatom δ¹⁸O, after vacuum dehydroxylation. All remaining fossil diatom samples from BC-43 core intervals were soaked in the δ¹⁸O solution of +94.4‰ prior to vacuum dehydroxylation and IRMS analysis in order to produce diatom δ¹⁸O data that could be directly applied to previously established diatom δ¹⁸O vs temperature relationships. All sediment trap samples besides STS-1 were only rinsed with our laboratory DI. We use the BC-43 sediment core samples to present an ~ 220 year record of diatom δ¹⁸O data from an Eastern Pacific Guaymas Basin (GB) boxcore compiled by the microfluorination technique. Our STS diatom δ¹⁸O data span an entire year's worth of accumulation, and include the fall diatom bloom, cool winter, and warm summer conditions from 1996 to 1997.

Supplement to: Menicucci, Anthony J; Thunell, Robert C; Spero, Howard J (2019): 220 year diatom δ¹⁸O reconstruction of the Guaymas Basin thermocline using microfluorination. Paleoceanography and Paleoclimatology