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The observed radial and vertical metallicity distribution of old stars in theMilky Way disk provides a powerful constraint on the chemical enrichment anddynamical history of the disk. We present the radial metallicity gradient,\Delta[Fe/H]/\Delta R, as a function of height above the plane, |Z|, using 7010main sequence turnoff stars observed by the Sloan Extension for GalacticUnderstanding and Exploration (SEGUE) survey. The sample consists of mostly oldthin and thick disk stars, with a minimal contribution from the stellar halo,in the region 6 < R < 16 kpc, 0.15 < |Z| < 1.5 kpc. The data reveal that theradial metallicity gradient becomes flat at heights |Z| > 1 kpc. The medianmetallicity at large |Z| is consistent with the metallicities seen in outerdisk open clusters, which exhibit a flat radial gradient at [Fe/H] ~ -0.5. Wenote that the outer disk clusters are also located at large |Z|; because theflat gradient extends to small R for our sample, there is some ambiguity inwhether the observed trends for clusters are due to a change in R or |Z|. Wetherefore stress the importance of considering both the radial and verticaldirections when measuring spatial abundance trends in the disk. The flatteningof the gradient at high |Z| also has implications on thick disk formationscenarios, which predict different metallicity patterns in the thick disk. Aflat gradient, such as we observe, is predicted by a turbulent disk at highredshift, but may also be consistent with radial migration, as long as mixingis strong. We test our analysis methods using a mock catalog based on the modelof Sch\"onrich & Binney, and we estimate our distance errors to be ~25%. Wealso show that we can properly correct for selection biases by assigningweights to our targets.

Metallicity Gradients in the Milky Way Disk as Observed by the SEGUE Survey