Variations in sedimentary environments and accretionary patterns in estuarine marshes undergoing rapid submerge, Chesapeake Bay
Major processes controlling sedimentological characteristics and accretionary rates and patterns were evaluated in two of the most common tidal marsh settings (estuarine tributaries and estuarine embayments) in Chesapeake Bay, one of the largest microtidal estuaries in the world. Textural analyses of 29 vibracores show spatial and temporal trends in grain size and organic matter which reflect differences in depositional settings within the marshes and changes in the controlling processes, as well as broader events in the surrounding watersheds. Spatially, the surficial sediments normally follow expected patterns, with coarsest sediments and lowest organic matter contents found in bay margin or channel margin locations, while the finest, organic-rich sediments occur in interior and submerged upland marshes. However, temporally, the marshes do not always show the expected increasing dominance of organic input over time (indicated by increasing % loss on ignition with decreasing depth), indicative of the marsh building process. Sea-level rise, tidal channel migrations, and anthropogenic effects likely contribute to decreases in organic matter contents in the upper sediment column. Furthermore, accretion rates and stratigraphic characteristics can be affected by these same processes, illustrating the importance of understanding temporal changes in marsh depositional environments over various time scales before sedimentologic and accretionary patterns are evaluated. Examination of the vibracores reveals that the marsh is composed of a complex stratigraphy with at least four major stratigraphic sequences occurring: (1) emerging or developing marsh sequences characteristic of channel margin and interior marshes; (2) submerging or mineral matter enriched marshes, also characteristic of channel and interior marsh areas; (3) high-energy marsh sequences characteristic of bay margin environments; and (4) submerged upland marsh sequences. The sedimentological properties of the stratigraphic sequences are controlled by overwash processes, tidal channel migrations, duration of tidal flooding, various anthropogenic effects, and sea-level rise. Accretion rates and patterns over the last ∼200 years, determined from pollen histories, differ between the marshes along the estuarine tributary (Nanticoke River) and those found in the tidal embayment (Monie Bay). Marshes in the upper-estuarine tributary have a larger riverine sediment input, and trapping of the high sediment loads in the upper estuary often yields marsh accretion rates (maximum of 0.74 cm/yr) which are greater than relative sea-level rise (∼0.4 cm/yr), resulting in stable marshes. By comparison, estuarine processes limit penetration of high sediment loads to the lower reaches of marshes along the tributary and, as a consequence, accretion rates are generally lower than the rate of relative sea-level rise, and marsh loss is prevalent. Overall, accretion rates in the marshes in the embayment are close to or less than the local sea-level rise and do not have as distinctive spatial patterns as the tributary marshes. Although marsh loss has yet to be identified as a significant process here, decreases in organic content of the upper sediment column may signal the early stages of submergence due to sea-level rise.
Center for Coastal and Ocean Mapping, Jackson Estuarine Laboratory
151, Issues 1-4
New York, NY, USA
Copyright © 1998 Elsevier Science B.V. All rights reserved.
Digital Object Identifier (DOI)
Larry G Ward, Michael S Kearney, J.Court Stevenson, Variations in sedimentary environments and accretionary patterns in estuarine marshes undergoing rapid submergence, Chesapeake Bay, Marine Geology, Volume 151, Issues 1–4, October 1998, Pages 111-134, ISSN 0025-3227, http://dx.doi.org/10.1016/S0025-3227(98)00056-5.