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The Paleoenvironment of Petaluma Marsh, Northern California, USA
Ingram, B.L., De Deckker, P., Chivas, A.R., Conrad, M.E. and Byrne, A.R. 1998. Stable isotopes, Sr/Ca, and Mg/Ca in biogenic carbonates from Petaluma Marsh, northern California, USA. Geochemica et Cosmochimica Acta 62: 3229-3237.

What was done
The authors conducted isotopic (18O/16O and 13C/12C) and elemental chemical analyses (Sr/Ca and Mg/Ca ratios) of sediment cores taken from Petaluma Marsh, San Francisco Bay, Northern California, USA, in an effort to develop a record of paleoenvironmental change in this region over the past 700 years.

What was learned
High frequency variations in δ18O, δ13C, Mg/Ca and Sr/Ca were noted throughout the 700-yr record, indicating the presence of oscillations in freshwater inflow, temperature and evaporation at periods of 35-115 years. Between 150 and 400 cal yr BP, however, δ18O and Mg/Ca were relatively low, indicative of a period of cold and wet climatic conditions associated with the Little Ice Age. Prior to that, δ18O and Mg/Ca were higher from 480 to 650 cal yr BP, indicating, in the words of Ingram et al., "drier and warmer conditions during the end of the Medieval Warm Period." In addition, they note that the record "suggests that the duration of wet and dry periods was greater over the past 700 years than in the twentieth century instrumental record."

What it means
On the basis of what was discovered, there appears to be nothing unusual about the 20th-century climate of the San Francisco Bay area. Periods of both drought and wetness over the last hundred years of the past millennium were less extreme than similar periods of the preceding six centuries. As for temperature, the results of Ingram et al.'s work support the findings of Graumlich (1990), who found tree-ring evidence in the nearby Sierra Nevada Mountains that the period from 510 to 420 cal yr BP was warmer and wetter than any part of the twentieth century.

Graumlich, L.J. 1990. Interaction between variables controlling subalpine tree growth: Implications for the climatic history of the Sierra Nevada. Proc. Sixth Annu. Pacific Climate (PACLIM) Workshop, pp. 115-118.

Reviewed 17 May 2006