San Nicholas Island Twin Rivers Marsh. Davis, O.K., 1997.
Tilia II ASCII file loscertos.txt

Davis, O.K., 1997.
Pollen Analysis of San Nicholas Island Twin Rivers Marsh. Report submitted
to Donn R. Grenda, Statistical Research, Inc., Redlands, CA 92373, 17 p.

Routine pollen analysis was performed for 22 sediment samples and 2 contemporary samples from San Nicholas Island off the west coast of California, 33o 15' N 119o 30' W. Pollen preservation is very good (5 - 15% deteriorated) above 30 cm in the core, but is generally poor (20 - 40% deteriorated) from 40 cm to the base of the cored sediments at 165 cm. The pollen concentration ranges from 2000 - 12,000 grains cm-3 in the upper core, but is generally less than 500 grains cm-3 below 40 cm depth. However, the interval 105 - 115 cm in the lower core is has good preservation and high pollen concentration. Three radiocarbon dates indicate a maximum age of ca. 1500 year for the core, but the chronology is uncertain below 100 cm. The pollen spectrum is dominated by moderate percentages (20-40 %) of sunflower (Other Compositae) pollen throughout most of the core. Above 40 cm (ca. A.D. 1480), percentages of ragweed (Ambrosia) pollen increase to 20%, percentages of pine, oak, ceanothus Cruciferae, and Frankenia increase too. This probably results both from climate change and from human disturbance following the Medieval Warm Period. Greater effective moisture produced vegetation changes similar to those recorded in coastal sites, and permitted increased human population density on the island, which impacted island vegetation. Historic vegetation disturbance is apparent above 15 cm in the dramatic decline of oak, ceanothus, Liguliflorae and grass, and the increase in Chenopodiaceae-Amaranthus percentages. These results of historic disturbance are similar to those recorded at coastal sites. Contemporary samples from the marsh surface, 91 m, and 183 m elevation show a steady decrease of Chenopodiaceae-Amaranthus pollen with elevation, similar to patterns recorded in nearby coastal settings.