Water exchange in near-surface marsh strata of Barataria Basin, Louisiana and the implications for subsidence
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Description
Louisiana's deltaic wetlands are threatened in the 21st century by rapid sediment subsidence that compounds climate-driven rising sea levels. Much of the coastal wetlands that fringe the Mississippi Delta are underlain by 1-2 m of an organic-rich and highly compressible surficial marsh mat substrate, where shallow subsidence is controlled by the interrelated factors of mineral and organic loading-induced compaction and the vertical accretion rate, as well as subsidence occurring deeper in the sediment column through a variety of drivers. The supply of organic and mineral sediment varies dependent on local elevation, marsh type, and hydrologic connectivity with riverine and turbid estuarine water bodies that supply mineral sediment. Shallow subsidence is thought to be a major factor driving coastal wetland land loss in the Mississippi Delta compounding other factors such as disconnection with riverine sediment sources, prolonged inundation resulting in marsh drowning, and marsh fragmentation through canal dredging. An analysis of delta-wide Coastwide Reference Monitoring Systems (CRMS), rod- surface elevation table (R-SET) data illustrates that there is also a relationship between mineral and organic loading to subsidence. Continuously recording surface elevation tables (CR-SET) were deployed at four different marsh ecotypes within Barataria Basin, Louisiana to examine mechanisms that influence marsh surface elevation change. The CR-SET’s are located adjacent to long-term CRMS stations that record vertical accretion and surface elevation change. This study examines (1) the impact of seasonal and daily meteorological changes on water levels and marsh surface elevation, aims to (2) directly measure subsidence in the wetland layer over a water year, and (3) examine the interrelationship of these factors with marsh stratigraphy. Results show that marsh elevation fluctuations are correlated with subsurface groundwater levels controlled by water level in nearby canals. The most organic-rich site showed a higher correlation with water levels and larger fluctuations of the surface elevation on diel and monthly timescales. Surface fluctuations are a direct response to lateral draining and filling of the marsh vadose zone, with frequency and degree of water exchange and surface elevation variation controlled by distance from the nearest canal edge and subsurface stratigraphy. Shallow subsidence rates calculated by previous studies from CRMS vertical accretion and total elevation change measurements, are revised for the fresh, intermediate, brackish, and saline marsh study sites using a normalization for a mean water level—the revised subsidence rates are not greatly influenced by the marsh expansion-contraction process. Marsh fragmentation through canal cutting and interior ponding increases linear extent of marsh edge, creating increased vadose spatial extent and frequency of exposure to air-filled pores in a given marsh. A hypothesis is explored herein that suggest that this increase in marsh edge in Barataria and other degrading Mississippi Delta sub-basins can result in increased peat oxidation that may have increased compactional subsidence in recent decades.