We report in this paper the processes and rates of vertical accretion as determined 

 by Cs dating and by the use of artificial marker horizons, and relate them to apparent 

 sea-level rise and marsh deterioration at two sites along the Louisiana coast. The two 

 sites were independently studied with slightly different objectives, but the results 

 pertaining to wetland loss were similar. 



STUDY AREA DESCRIPTIONS 



The two study areas are representative of the two coastal regions of Louisiana: the 

 chenier and Mississippi deltaic plains. The site within the chenier plain is a brackish to 

 saline Spartina patens marsh known locally as the East Cove marsh, located on the south 

 shore of Calcasieu Lake within the Sabine National Wildlife Refuge (Figure I). The 

 deltaic plain site is the saline Spartina alterniflora marsh surrounding Barataria Bay 

 (Figure 2). 



Both sites have been experiencing above average land loss rates of over 1%/yr since 

 the mid-1950's. A major difference between the two sites is their respective geologic 

 foundation. Underlying the East Cove marsh is a I to 6 m sequence of Recent sediments 

 (Gosselink et al. 1979) whereas in the lower Barataria basin, the Pleistocene surface lies 

 30 to 100 m below the marsh surface (Kolb and Van Lopik 1966). This difference in 

 sediment thickness suggests that the Barataria site has an inherently greater subsidence 

 potential. If all other factors were equal we would expect land loss rates to be 

 comparatively greater at the Barataria site. 



METHODS 



Details of sampling design, laboratory procedures, materials used, and statistical 

 analyses are provided in previous reports (DeLaune et al. 1978; DeLaune et al. in review; 

 Baumann 1980). Discussion here will be limited to a general application of various 

 methods and techniques as they pertain to monitoring sedimentation in Louisiana's 

 marshes. 



The numerous techniques employed to monitor sediment accretion can be divided 

 into five broad categories: (I) surveys through time based on benchmarks or other 

 datums; (2) calculations based on sediment budgets; (3) simple mechanical devices such as 

 calibrated rods; (4) radiometric dating; and (5) natural and artificial marker horizons. 

 Categories one through three are generally unacceptable for work in Louisiana marshes 

 for many reasons, some of which have been discussed by Letzsch and Frey ( 1 980). 



Radiometric dating can provide accurate sedimentation rate information provided 

 the substance being dated has been deposited in situ and the sedimentary sequence has 

 not been subsequently disturbed. '~^'Cs was the radioactive element used in the case 

 studies discussed in this report. It was first introduced into the biosphere as a product of 

 atmospheric nuclear testing with significant fallout levels first appearing in I95|f and 

 peaking in 1963 (Pennington et al. 1973). By obtaining cores and measuring the Cs 

 activity at regular intervals throughout the core, the average sedimentation rate from 

 1954 to 1963 and from 1963 to the present can be determined. 



Artificial marker horizons have been extensively used in monitoring studies 

 involving a few years or less. Various substances have been employed, but most are not 



