EVIDENCE FOR THE PLEISTOCENE-HOLOCENE TRANSITION FROM A 20.8M CORE OBTAINED FROM THE VEDANTA MARSH, OLEMA CA

Over 11,000 years of accumulated sediments were collected in a 20.8m core (VM1B) at the Vedanta marsh 70km north of San Francisco.  The core was obtained in order to distinguish styles of sedimentation as climatic or coseismic.  A detailed visual description was made of the core and samples were collected, at 5cm intervals, for a variety of diagnostic tests including total organic matter content and radiocarbon dating.  Additional samples were collected for the identification of pollen from peat and clay layers.  One radiocarbon date from the base of the core gave an age of approximately 11,300 yrs B.P.  The total core sequence shows a large sedimentary cycle from coarse-grained channel deposits in the lower sequence to fine-grained marsh deposits in the upper sequence.  There is a distinct boundary at 11.8m between the channel and marsh deposits.  Using radiocarbon dates, sedimentation rates for the channel deposits were determined to be 3.1 to 5.5 mm/yr while marsh deposits decreased to 1.5 to 1.8 mm/yr.  Total organic matter was determined using the Loss on Ignition method.  Organic matter below the boundary (channel deposits) ranges from 3% to 5% and above the boundary (marsh deposits) ranges from 20% to 70%.  A reconnaissance of fossil pollen was conducted using the standard HCL-KOH-HF-Acetolysis procedure.  Pollen was identified from four depths (0.70m, 8.2m, 15.7m, 20.7m).  Pollen counts revealed a vegetation shift between 20.7m to 15.7m.  Pseudostuga, Salix, and Cyperaceae decreased while Alnus and Pinus increased.  Abies (Douglas Fir), which grew in the area during the Pleistocene, was identified only at 20.7m.  This shift probably marks the disappearance of the closed canopy forest between 9,400 to 10,000 yrs B.P.  Another shift was identified between 15.7m and 9m.  Salix, Pinus and Monolete spores increased while Alnus decreased.  This vegetation shift and the alternating layers of clay, silt and peat may reflect the stabilization of the marsh around 7,800 yrs B.P.  as local vegetation adjusted to the change in climate.  Diagnostic tests and pollen analysis performed on core VM1B from the Vedanta marsh has provided evidence of the transition from the cooler conditions of the Pleistocene to the modern climate regime of the Holocene.  Further investigation of marsh evolution, coseismic events and human impact on the marsh is planned.