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CONTEXT:The detailed chemical abundances of extremely metal-poor (EMP) starsare key guides to understanding the early chemical evolution of the Galaxy.Most existing data are, however, for giant stars which may have experiencedinternal mixing later. AIMS: We aim to compare the results for giants with new,accurate abundances for all observable elements in 18 EMP turnoff stars.METHODS:VLT/UVES spectra at R ~45,000 and S/N~ 130 per pixel (330-1000 nm) areanalysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to deriveabundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. RESULTS:For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sampleof EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr,Mn and Co are ~0.2 dex larger than in giants of similar metallicity. Mg and Siabundances are ~0.2 dex lower (the giant [Mg/Fe] values are slightly revised),while Zn is again ~0.4 dex higher than in giants of similar [Fe/H] (6 starsonly). CONCLUSIONS:For C, the dwarf/giant discrepancy could possibly have anastrophysical cause, but for the other elements it must arise from shortcomingsin the analysis. Approximate computations of granulation (3D) effects yieldsmaller corrections for giants than for dwarfs, but suggest that this is anunlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations forNa and Al provide consistent abundances between dwarfs and giants, unlike theLTE results, and would be highly desirable for the other discrepant elements aswell. Meanwhile, we recommend using the giant abundances as reference data forGalactic chemical evolution models.

First stars XII. Abundances in extremely metal-poor turnoff stars,and comparison with the giants