Abstract

The late-Holocene (5–0 ka) record of accommodation space controls of tidal channel and tidal flat deposition in the shallow mesotidal wave-dominated Grays Harbor estuary (236 km² surface area) was investigated in previously reported drill cores (n=15) and new vibracores (n=20), reaching 3–10 m depth subsurface. Continuous vibracore facies sequences (3–4 m depth subsurface) discriminate between tidal channel and tidal flat deposition and demonstrate responses of both depositional settings to interseismic uplift and coseismic subsidence (1±0.5 m vertical) from cyclic neotectonic forcing (200–800 yr recurrence intervals) in the Cascadia subduction zone. Vibracore channel samples, at 0.5 m or 1.0 m depth intervals, were analyzed for sediment grain size (sample n=124) and sand source mineralogy (sample n=67). The mean and standard deviation of sand size in the sand fraction is 175±x34 1σ µm. Sediment ¹⁴C dates (n=29) range from 376 to 6,579 median calyrBP and establish long-term sedimentation rates in subtidal channel accretionary banks (average 4.2 m ka^-1), intertidal channel accretionary banks (average 3.7 m ka^-1), and tidal flats (average 1.1 m ka^-1). Tidal channel accretionary bank deposition largely reflects reworking of pre-existing estuary deposits. Long-term total basin sediment accumulation rates (232x10⁶ m³ ka^-1) are tied to rates of net sea level rise (1.0 m ka^-1) or increasing basin accommodation space. In latest Holocene time (3–0 ka) littoral sand import (117x10⁶ m3 ka^-1) was about twice as large as the retention of river sand and mud in the estuary. The selective export of winnowed mud from the estuary provided the necessary accommodation space for the import of littoral sand in latest-Holocene time. Shallow intertidal settings in Grays Harbor (60% by surface area) are maintained by self-regulating conditions of channelized sediment import, wind-wave erosion of tidal flats, and tidal prism forcing of tidal channel discharge. Hind-casted wind-wave bottom orbital velocities (>20 cm sec^-1) are sufficient to truncate tidal flat elevations to lower-intertidal levels, which maintain substantial tidal prism volumes (modern MLLW-MHHW ~6.1 x 10⁸ m³) and associated tidal channel discharge in the shallow estuary. Net sediment deposition in the estuary is controlled by the interaction of limiting accommodation space controls in the tidal flats and tidal channels. The balance between sediment supply, energy of sediment transport/resuspension, and sediment export has survived small changes in relative sea level (1±0.5 m) from cyclic neotectonic forcing. However, the prehistoric (natural) balance could be altered by future anthropogenic impacts from sustained global sea level rise (> 1.5 m during the next century) or diminished wind-wave fetch distances, which could result from tidal flat diking/filling or uncontrolled spread of non-native invasive stabilizing sea grass (Spartina). In this regard, the susceptibilities of prehistorically-balanced sediment dynamics in Grays Harbor serve as warning for other similar mesotidal wave-dominated estuaries that could be impacted by future global sea level rise, changing sediment inputs, and/or tidal flat diking/filling, which could reduce intertidal habitat and associated ecosystem functions. 

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