Abstract

In this article, geologically recent sedimentation rates across a high wave- and wind-energy inner-shelf are used to constrain depths of closure, which are needed to estimate potential offshore displacements of littoral (beach) sand that could result from future sea level rise (SLR). Seven shallow vibracores (1.0–2.3 m subsurface depth) were analyzed from a single transect across the inner-shelf (19–72 m water depth) at the South PacWave study site in the high-wave energy (peak wave H_s= 10–15 m) coast of Central Oregon. The vibracores were ¹⁴C dated to establish 1) near-modern mixing depths and 2) net sedimentation rates that equaled or exceeded rates of coeval SLR (10 cm century^-1) during very latest-Holocene time (≤1.0 ka). Sedimentation rates of 17 cm century^-1 and 31 cm century^-1 for vibracore P1-2A22 in 34 m water depth do exceed coeval SLR, even after accounting for apparent mixing depths of 60–100 cm. But sedimentation rates in deeper core sites (47–53 m water depth) do not approach coeval SLR rates. These results support a proposed near future (one century) 30 m depth of closure in the innermost-shelf of northern Oregon. Vibracore sand grain sizes (mean 0.20±0.03 mm 1σ, n=18) are similar across the inner-shelf, but heavy-mineral sand tracers confirmed that the latest-Holocene inner-shelf sand accumulations were supplied from seaward transport of littoral sand. The net loss of littoral sand to the inner-shelf sand sink accounts for the narrowing and thinning of beach deposits in northern Oregon during latest-Holocene time. A 1.0 m thickness of littoral sand displaced across the innermost-shelf (5–30 m water depth) following a 1.0 m SLR, or equivalent increase in offshore accommodation space, would yield a cross-sectional area of 1.5x10³ m². That value is three times larger than the mean cross-sectional area of the modern adjacent beaches (mean 4.8x10² m²) in the South PacWave study area. Following a possible near future SLR of 1.0 m, the popular sandy beaches of northern Oregon could be converted to intertidal gravel/algae covered bedrock platforms.

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