SEDIMENTATION AND PEAT FORMATION IN SMALL POOLS OF THE SACRAMENTO-SAN JOAQUIN DELTA (CALIFORNIA, USA) IN THE HOLOCENE: SIGNIFICANT RESPONSE OF A SYSTEM TO MILD CLIMATIC CHANGE

Vegetation resists change in climate, unless that change remains too long or occurs too abruptly. The wetlands of the Sacramento-San Joaquin Delta of California are a very sensitive environment even to minor climatic change, which leaves its mark on the sediment record and subsequent formation of plant communities. Thus, the formation of peat in the Delta provides unique insights into climatic processes.

Peat accretion in the Delta occurred in response to climatic-driven postglacial sealevel change during the last 6,500 years. In this study of three peat cores from different Delta locations, we use palynological, diatoms, and paleomagnetic methods to test the hypothesis that two factors influence peat deposition: tidal and riverine.

Based on the close correlation of bulk density of the peat with organic carbon content and peat accretion (Drexler et al., 2009), we found that:

1) Pollen concentration is highest prior to intervals with high carbon content, and the warmest interval, as determined from pollen analysis, precedes the period with the highest peat accretion rate. Freshwater diatoms are found in these intervals.

2) Organic content is inversely related to the lithic content as determined from paleomagnetic measurements of isothermal remanent magnetization (IRM);

3) A salinity index based on pollen criteria is highest during the highest stands of sea level and generally corresponds to an increase in Poaceae pollen, which might indicate the formation of a peat type different from tidal marsh peat. Brackish water diatoms characterize these layers.

A visible increase in IRM intensity at about 4,000 cal yr B.P. suggests a strong terrigenous input, which could be related to climatic warming and an increased riverine contribution to the lithic component. The coincidence of the IRM peak with the lowest value of the salinity index and with the presence of freshwater diatoms supports the hypothesis that the riverine contribution was the cause of the increase in the inorganic component in the peat and provided the substrate for the growth of vegetation: this is reflected in the subsequent increase in total pollen concentration. Similar events appear to have occurred in the late Holocene but on a much shorter time scale, indicating low grade warming.