A LARGE FLOOD EVENT AT ~1410 C.E. RECORDED IN PETALUMA MARSH, CALIFORNIA

Petaluma Marsh is the largest remaining extant salt marsh in San Pablo Bay, California. The marsh lies within the Petaluma River watershed which drains approximately 380 km² of land in southern Sonoma county and portions of the northeastern Marin county. The marsh formation and continued development is connected to the fluvial system as well as sea level. Sediment cores retrieved from the marsh therefore have the potential to reveal long-term trends in climate change as well as extreme weather events, particularly floods and droughts.

We present data from several cores retrieved from Petaluma Marsh. The cores were collected approximately 700 m due south of the Petaluma River, approximately 100m away from Tule tidal slough. The dominant vegetation at the site is pickleweed (Sarcocornia pacifica) with dodder (Cuscuta salina) and gum plant (Grindelia stricta) at slightly higher elevations.

Sedimentary and biological proxy analysis of the core includes magnetic susceptibility, density, loss on ignition, carbon and sulphur isotopes, grain size and macro botanical analyses. The chronology of the core was determined by 8 radiocarbon dates. The core shows classic sedimentary facies changes indicating the progression of marsh formation from mud flat to incipient marsh to fully developed peat marsh. However, on three occasions, fully developed marsh peat is abruptly overlain by thick packages of inorganic muds. The abrupt change in stratigraphy suggests a dynamic event occurred which interrupted the deposition of organic-rich sediment.

The initial analysis has focused on the youngest inorganic package, which is characterized by high magnetic susceptibility and density levels and sand, which is absent from the peat deposits. Further, it contains poorly preserved detrital organic material unlike the peat section, which is composed of pickleweed roots. We hypothesize that the deposit, which dates to ~ 1410 C.E., formed as the result of intense flooding in the watershed. Floods of this magnitude in California are typically caused by frequent and intense atmospheric river events. We place this finding in the context of regional climate records and evidence for extreme precipitation events in central and northern California.