Wetland soil formation in the rapidly subsiding Mississippi River Deltaic Plain: Mineral and organic matter relationships

Abstract

The elevation of submerging coastal marshes is maintained by vertical accretion of mineral and organic matter. Submergence rates currently exceed 1·0 cm year −1 in the Mississippi Deltaic Plain and are expected to increase. Mineral matter-organic matter relationships were examined in surface profiles of Mississippi Deltaic Plain soil from both Active Delta Zone marsh (which receives freshwater and mineral sediment from the Atchafalaya or Mississippi Rivers) and Inactive Delta Zone marsh (which relies on rainfall for freshwater and on reworked sediments for mineral matter) to gain insights into marsh soil structure and formation. Mineral and organic matter accounted for 4–14% of soil volume. The remainder was pore space and was occupied by water and entrapped gases. Organic matter occupied more volume than mineral matter in all but saline marsh soil. The regular influx of mineral matter to active fresh marsh resulted in active fresh marsh soil containing twice as much mineral and organic matter as inactive fresh marsh soil. Within the Inactive Delta Zone, the volume of mineral and organic matter increased from fresh (inland) to saline (seaward) marshes. Saline marsh soil required 1·7 times as much mineral matter as brackish marsh soil to vertically accrete at similar rates, possibly as a result of soil bulk density requirements of the dominant saline marsh plant, Spartina alterniflora. Vertical accretion rates were highest in the Active Delta Zone, probably as a result of increased mineral matter availability and delivery. Current, best estimates of the combination of mineral and organic matter required (g m −2 year −1) to maintain marsh surface-water level relationship are fresh marsh: organic matter = 1700 + 269 x, mineral matter = 424 x; brackish marsh: organic matter = 553 + 583 x, mineral matter = 1052 x; saline marsh: organic matter = 923 + 601 x, mineral matter = 1798 x, where x= the rate of submergence (cm year −1).