J. Gleason | USGS Science

USGS2017

Isotopic evidence of Holocene climatic change in the San Juan Mountains, Colorado

The ??D of cellulose from 14C-dated wood, collected in the San Juan Mountains of southwestern Colorado, decreased by about 45??? from 9600 to 3100 yr B.P. and an additional 25??? to the present. The wood samples are from trees that grew above present-day tree line and reflect a time of warmer average summer temperatures. These changes in ??D are interpreted to indicate a major change during the Holocene in the sources of moisture, in the seasonality of precipitation, or in both. ?? 1988.

Quaternary Research↗

USGS1990

Natural occurrence of silicon carbide in a diamondiferous kimberlite from Fuxian

Considerable debate surrounds the existence of silicon carbide in nature, mostly owing to the problem of possible contamination by man-made SiC. Recently, Gurney 1 reviewed reports of rare SiC inclusions in diamonds, and noted that SiC can only be regarded as a probable rather than proven cogenetic mineral. Here we report our observation of clusters of SiC coexisting with diamond in a kimberlite from Fuxian, China. Macrocrysts of α-SiC are overgrown epitaxially by β-SiC, and both polymorphs are structurally well ordered. We have also measured the carbon isotope compositions of SiC and diamonds from Fuxian. We find that SiC is more enriched in 12 C than diamond by 20‰, relative to the PDB standard. Isotope fractionation might have occurred through an isotope exchange reaction in a common carbon reservoir. Silicon carbide may thus ultimately provide information on carbon cycling in the Earth's mantle.

Nature↗