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We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search forlow-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption isdetected on a pixel-by-pixel basis by its correlation with Lyman-alpha forestabsorption. We have developed a novel Locally Calibrated Pixel (LCP) searchmethod that uses adjacent regions of the spectrum to calibrate interlopers andspectral artifacts, which would otherwise limit the measurement of OVIabsorption. Despite the challenges presented by searching for weak OVI withinthe Lyman-alpha forest in spectra of moderate resolution and signal-to-noise,we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2(the null hypothesis has a chi^2=80 for 9 data points).  We interpret our results using synthetic spectra generated from a lognormaldensity field assuming a mixed quasar-galaxy photoionizing background (Haardt &Madau 2001) and that it dominates the ionization fraction of detected OVI. TheLCP search data can be fit by a constant metallicity model with [O/H] =-2.15_(-0.09)^(+0.07), but also by models in which low-density regions areunenriched and higher density regions have a higher metallicity. Thedensity-dependent enrichment model by Aguirre et al. (2008) is also anacceptable fit. All our successful models have similar mass-weighted oxygenabundance, corresponding to [<O/H>_MW] = -2.45+-0.06. This result can be usedto find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) =1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on themass-weighted mean oxygen abundance and the cosmic oxygen density in theLyman-alpha forest to date and indicates that it contains approximately 16% ofmetals produced by star formation (Bouch\'e et al. 2008) up to z = 3.

A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the Sloan Digital Sky Survey