The bulge appears to be a chemically distinct component of the Galaxy ; at b = - 4 ^ { \circ } the average [ Fe/H ] and [ Mg/H ] values are + 0.06 and + 0.17 dex respectively , roughly 0.2 dex higher than the solar neighborhood thin disk , and \sim 0.7 dex greater than the local thick disk .
This high average metallicity suggests a larger effective yield for the bulge compared to the solar neighborhood , perhaps due to more efficient retention of supernova ejecta .

A vertical metallicity gradient in the bulge , at \sim 0.5 dex/kpc , is attributed to the changing mixture of metal-rich and metal-poor sub-populations ( at [ Fe/H ] + 0.3 and - 0.3 dex from Hill et al .
2011 ; but + 0.15 , - 0.25 and - 0.7 dex from Ness et al .
2013 ) , where the metal-poor sub-populations have a larger scale height than the metal-rich population .

Abundances of O , Mg , Si , Ca , Ti , and Al are enhanced in the bulge compared to solar composition , with [ \alpha /Fe ] = + 0.15 dex at solar [ Fe/H ] ; below [ Fe/H ] \sim - 0.5 dex , the bulge and local thick disk [ \alpha /Fe ] ratios are very similar .
Small enhancements in [ Mg/Fe ] and possibly [ < SiCaTi > /Fe ] relative to the thick disk trends are apparent , suggesting slightly higher SFR in the bulge .
This is supported by low [ s-/r- ] process ratios , as measured by [ La/Eu ] , and dramatically enhanced [ Cu/Fe ] ratios compared to the thick disk .
However , the differences between thick disk and bulge composition trends could , conceivably , be due to measurement errors and non-LTE effects .
Unfortunately , the comparison of bulge with solar neighborhood thick disk composition may be confused by uncertainties in the identification of local thick disk stars ; in particular , the local thick disk [ \alpha /Fe ] trend is not well defined above [ Fe/H ] \sim - 0.3 dex .

The unusual zig-zag abundance trends of [ Cu/Fe ] and [ Na/Fe ] are qualitatively consistent with the Type Ia supernova time-delay scenario of Tinsley ( 1979 ) and Matteucci & Brocato ( 1990 ) for elements made principally by core-collapse supernovae , but with metallicity-dependent yields .

The metallicities , [ \alpha /Fe ] ratios and kinematics of the metal-poor and metal-rich bulge sub-populations resemble the solar neighborhood thick and thin disks , respectively , but with higher [ Fe/H ] than at the solar circle .
If these sub-populations really represent the inner thin and thick disks , but at higher [ Fe/H ] , then both the thin and thick disks possess a radial [ Fe/H ] gradient , within the solar circle , near \sim - 0.04 to - 0.05 dex/kpc .

In the secular bulge scenario , the bulge was built from entrained inner disk stars driven by a stellar bar .
Thus , it appears that the inner thin and thick disk stars retained vertical scale heights characteristic of their kinematic origin , resulting in the vertical [ Fe/H ] gradient seen today .