Transient, longitudinal, sedimentary furrows in the York River subestuary,Chesapeake Bay: Furrow evolution and effects on seabed mixing and sedimenttransport

Sedimentary furrows in fine-grained sediments have been observed in a varie ty 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 for m and then occasionally dissipate within the middle portion of the York Riv er. Varying furrow morphologies were observed depending on current conditio ns, ranging from large regularly spaced (0.7-7 m) linear furrows during low current conditions to large patches of meandering furrows as the mean curr ent increases or no bed forms during the higher current conditions. Based o n Pb-210 and Cs-137 profiles of kasten cores, differences in physical mixin g depths of similar to 25 cm between cores collected < 2 m apart indicate a high degree of small-scale spatial heterogeneity within the seabed. By doc umenting the position of kasten cores using a digital sidescan sonar system , we shelved that a core taken within a furrow had a mixing depth 15 cm sha llower 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 <similar t o>25 cm scale differences in mixing depths due to the formation and destruc tion 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 signif icant process contributing to decimeter-scale seabed mixing, there is a lon ger-term unknown process which is controlling the meter-scale seabed mixing .