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We combine 131 new medium-resolution (R~2000) J-band spectra of M, L, and Tdwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97previously published BDSS spectra to study surface-gravity-sensitive indicesfor 228 low-mass stars and brown dwarfs spanning spectral types M5-T9.Specifically, we use an established set of spectral indices to determinesurface gravity classifications for all M6-L7 objects in our sample bymeasuring equivalent widths (EW) of the K I lines at 1.1692, 1.1778, 1.2529 um,and the 1.2 um FeHJ absorption index. Our results are consistent with previoussurface gravity measurements, showing a distinct double peak - at ~L5 and T5 -in K I EW as a function of spectral type. We analyze K I EWs of 73 objects ofknown ages and find a linear trend between log(Age) and EW. From thisrelationship, we assign age ranges to the very low gravity, intermediategravity, and field gravity designations for spectral types M6-L0.Interestingly, the ages probed by these designations remain broad, change withspectral type, and depend on the gravity sensitive index used. Gravitydesignations are useful indicators of the possibility of youth, but currentdatasets cannot be used to provide a precise age estimate.

Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey