Widespread Evidence of Terminated Marine Transgressive Sand Supply and Failing Longshore Sand Transport to Eroding Coastal Eolian Sand Ramps during the Latest Holocene Time in Oregon and California (Pacific Coast, USA)

Holocene perched dunes and wave-truncated sand ramps were used to project paleoramp geometries and to estimate sand ramp erosion during the late Holocene in the U.S. Pacific Coast study area (1500 km in length). Both recently accreted sand ramp slopes (n = 24) and wave-truncated sand ramps (n = 59) in 11 sand ramp complexes (0.3-11.4 km in alongshore length) were profiled from areas of previously abundant dune sand supply. Modern sand ramp slopes (shore normal) ranged from 2 to 26 degrees, with slope steepening resulting from upslope eolian deflation (10-20 degrees) and downslope mass wasting by landslide (>20 degrees). Paleoramp surface slopes at 10, 15, and 208 were projected seaward from truncated sand ramp tops (11-97 m in elevation) to intersect with projected basal ramp horizons (4-m elevation). Differences between the 10 degrees slope paleoramps and the corresponding modern sea cliffs yield eroded ramp cross-sectional areas (78-16,770 m(2)). The eroded cross-sectional areas were multiplied by corresponding alongshore lengths to estimate truncated ramp complex erosion (1.29 x10(5)-6.41 x 10(7) m(3), mean 12.62 x 10(6) m(3)). Modern beach sand volumes in shorelines fronting the 11 truncated ramp complexes are estimated to range from 1.05 x 10(5) to 1.47 x 10(6) m(3). The average modern beach sand volume of 4.25 x 10(5) m(3) represents only 3.4% of the average eroded ramp complex volume, as estimated from differencing the 108 projected slopes and the modern sea cliff profiles. Near-surface C-14 ages of buried archaeological materials in the 11 truncated ramp complexes (932 +/- 672 cal BP 1 sigma, n = 20) demonstrate terminal ramp accretion and associated ramp truncation by beach retreat during the latest Holocene time. The widespread sand ramp erosion, following 3.0 m of sea-level rise during the last 3.0 ka, serves as a warning for the U.S. Pacific Coast and other high-wave energy cliff-backed shorelines for potential beach response to near-future sea-level rise (1-3 m) predicted to occur from ongoing global warming.