Impact of fires, earthquakes, and climate on catchment response since 600 CE, Pallett Creek, San Gabriel Mountains, Southern California USA

<div> <p>In tectonically active regions, sedimentary records are overprinted by landscape response to climate, fire, and local earthquakes.&#160; We explore this issue using a new paleoclimate record developed at the Pallett Creek paleoseismic site in southern California USA, a recently incised distal fan located along the San Andreas Fault at the base of a 35 km² catchment in the San Gabriel Mountains.&#160; To date, we have analyzed 6 m of section, spanning the last 1300 yr, for grain size, total organic material (TOM), carbon/nitrogen (C/N) ratios, magnetic susceptibility, and charcoal count. Existing C-14 dates (Scharer et al., 2011) inform rates of sediment deposition and charcoal accumulation (CHAR). Additional dating and macrofossil analysis is ongoing.&#160; Sedimentological variability within the section is dominated by two general units. Unit 1 is characterized by high % clay, % silt, and % TOM, while Unit 2 is distinctly coarser with higher % sand and lower % TOM.&#160; Pulses of high CHAR occur from 1150-1260 yr BP and during the Little Ice Age (100-500 yr BP) and are associated with high sedimentation rates (0.3-2 cm/yr), while only a few relatively weak fire episodes are recorded in the Medieval Climate Anomaly (700-1000 yr BP), despite similarly high sedimentation rates (0.6 cm/yr).&#160; Ten earthquakes documented at the site (Sieh et al., 1989) occurred about every 135 years and impart no obvious short-term impact on sedimentation rates, perhaps reflecting the distance between the site and steeper portions of the drainage network (>4 km) likely to produce mass wasting.&#160; Overall, the landscape response of this large, integrated catchment appears to reflect a stronger influence of fire and climate than earthquakes. Future work will focus on the impact of the fire episodes on sediment delivery and resultant paleoearthquake ages.</p> </div>