One line of research, which looked at grain sizes, rock magnetic properties, and bulk geochemistry, among other factors, suggested that the early Miocene sediments were transported to the Tianshui Basin by the wind. The existence of wind-borne sediment, known as loess, would support the 22-million-year desertification hypothesis. Other researchers, however, suggest that though many of the sediment properties are similar to loess, they also show a good match for lake bed or wetland material.
To discriminate between the two hypotheses, Peng et al. measured the chain length distributions of n-alkanes-a type of hydrocarbon found in waxy plant material-from preserved organic material found in Tianshui Basin sediment samples. Long n-alkane chains, those with 27-31 carbon atoms, mainly stem from terrestrial plants, while midsized chains, with 23-25 carbon atoms, derive from aquatic plants or wetland flora. The authors find an abundance of the shorter carbon chains in the Tianshui sediments, an important difference from nearby loess samples. Supported by the observation of preserved pollen and algae, the authors suggest that the Miocene Tianshui Basin was a lake, mudflat, or floodplain region and not the arid loess hypothesized by some researchers.
|Contact: Mary Catherine Adams|
American Geophysical Union