Reports
The shallow sediments of the middle Chincoteague Bay area in Maryland: physical and chemical characteristics
1998, Wells, D.V.; Hill, J.M.; Park, J.; Williams, C.P.
File Reports, Coastal and Estuarine Geology, File Report 1998-01
Abstract
The Maryland Geological Survey, Resource Assessment Service, initiated a multi-year investigation of the character of the shallow sediments of Maryland's Chincoteague and Sinepuxent Bays. This report presents the results of the third year study which focused on the physical and chemical characteristics of the surficial sediments of the lower half of the Maryland portion of Chincoteague Bay (i.e., middle Chincoteague Bay). This year’s study was funded by a grant/agreement from the National Oceanic and Atmospheric Administration, Award #NA67OZ0302.
Five sediment cores and 341 surficial sediment samples were collected in the middle Chincoteague Bay. The core and surficial sediments were analyzed for water content, textural properties, total nitrogen, carbon and sulfur, and for six metals: Cr, Cu, Fe, Mn, Ni, and Zn. Results from these analyses were used to map the distribution of sediment type, nitrogen, carbon and sulfur contents and relative enrichment of the six metals in the surficial sediments.
The five cores were collected in a variety of sedimentary environments, ranging from shallow tidal flats adjacent to Assateague Island to low energy lagoonal deposits within Johnson Bay area. Cores 1 and 5 were analyzed for 210Pb activity, obtaining estimated average sedimentation rates of 0.17±0.08 and 0.25±0.14 cm/yr respectively. Although these rates are low compared to those reported for other areas of the coastal bays, they are probably representative of sedimentation rates within the Johnson Bay area.
Based on the textural analyses of surficial sediment samples, the average textural composition of the bay bottom sediments is 49% sand, 35% silt and 16% clay. Sand is the dominant sediment type found in the eastern half of the study area. Within Johnson Bay and along the western margin of the study area, clayey silt is the predominant sediment type, reflecting low energy conditions. Intermediate sized sediments such as sand-silt-clay, sandy silt, and silty sand are mapped along the central axis of the study area. These sediment represent transitional zones between the high energy sand and low energy clayey silt. Pockets of coarser material are mapped adjacent to the numerous islands, the erosion of which is the primary source of sediment to this area. As sediment is eroded, the coarser fraction stays in place while the fine fraction is winnowed out and redeposited in more sheltered areas.
Nitrogen, carbon, and sulfur contents reported are slightly less than those reported for Newport and Sinepuxent Bay and half of the values reported for Assawoman and Isle of Wight Bays. The lower contents are related to several factors. Overall, the lower values are related to low clay content. In addition, the fairly coarse sediments and shallow water promote aerobic bottom conditions, allowing reactive carbon to be oxidized before it is buried. As a result, a disproportionate amount of non-reactive carbon is left. Nitrogen contents, which mimic the behavior of carbon, show similar trends. Likewise, sulfur is lower as a result of the less reactive carbon being buried and available for anaerobic decay.
Metal data also indicate that the middle Chincoteague Bay is less affected by anthropogenic activities compared to the northern coastal bays. Of the metals measured, Cr, Cu, Fe Mn, and Ni show no significant increase over calculated historical levels. Zn is found to be elevated, with levels higher than those reported for the upper Chincoteague Bay but less than the levels reported for Isle of Wight and Assawoman Bays.
