Transient, Longitudinal, Sedimentary Furrows in the York River Subestuary, Chesapeake Bay: Furrow Evolution and Effects on Seabed Mixing and Sediment Transport

Abstract

Sedimentary furrows in fine-grained sediments have been observed in a variety of settings ranging from the deep ocean and deep lake bottoms to shallow estuaries and are commonly described as persistent, long-term features of the seabed. A series of 12 sidescan sonar surveys over the course of three years reveal that transient, longitudinal sedimentary furrows regularly form and then occasionally dissipate within the middle portion of the York River. Varying furrow morphologies were observed depending on current conditions, ranging from large regularly space (0.7–7 m) linear furrows during low current conditions to large patches of meandering furrows as the mean current increases or no bed forms during the higher current conditions. Based on210Pb and137Cs profiles of kasten cores, differences in physical mixing depths of ∼25 cm between cores collected <2 m apart indicate a high degree of small-scale spatial heterogeneity within the seabed. By documenting the position of kasten cores using a digital sidescan sonar system, we showed that a core taken within a furrow had a mixing depth 15 cm shallower than an adjacent core taken between furrows. A time-series of mixing depths over the 35 mo of the study reveals that, along with the ∼25 cm scale differences in mixing depths due to the formation and destruction of furrows, there is a longer temporal signal of mixing producing 100-cm-scale changes in mixing depths on the annual to interannual time frame. Although the formation and destruction of the furrows appear to be a significant process contributing to decimeter-scale seabed mixing, there is a longer-term unknown process which is controlling the meter-scale seabed mixing.