Coastal seismic wipe-outs: Distribution controlled by pore water salinity

The relationship between seismic wipe-outs, methane accumulation, and pore water salinity was investigated using high-resolution seismic profiles, pore water geochemical measurements, and sediment property data from the White Oak River Estuary, North Carolina. The estuary is a broad, shallow, drowned river valley filled with organic rich sediment where the salinity changes from fresh to sea water within 10 km. Uniboom seismic reflection profiles indicate a distinct pattern in the distribution of seismic wipe-outs. Wipe-outs dominate the upper estuary but are not observed in the lower estuary. Sediment pore water chlorinities decrease upstream but the sharpest gradients occur with depth. At ≤ 50 cm depths in the upper half of the estuary, pore waters typically have less than 100 mmol L−1 chloride even though overlying estuarine waters have 300–400 mmol L−1 chloride. Concave chloride profiles demonstrate that sediments in the upper estuary are actively flushed with fresh water moving upward through the sediments from underlying aquifers. The associated decrease in interstitial sulfate concentrations combined with high organic carbon content provides favorable conditions for methane production and accumulation. Seismic wipe-outs are correlated with areas of high methane concentrations and with sediments containing low pore water salinities.