Marsh Accretion

Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates (Armentano and Woodwell, 1975 Krishnaswami et al., 1980 Church et al., 1981 Kuehl et al., 1982 Olsen et al., 1985 Paez-Osuna and Mandelli, 1985 McKee et al., 1986 Lynch et al., 1989 Bricker-Urso, 1989 Moore, 1992 Smoak et al., 1996 Dellapenna et al., 1998, 2001 Benoit, 2001 Corbett et al., 2003). Lead-210 is considered to be a reliable method for dating sediments deposited over the last 100 to 110 y (Krishnaswami et al., 1971). In the absence of bioturbation/mixing the activity gradient of excess 210Pb in sediments, which is the net result of accumulation and radioactive decay, can provide information on the sedimentation rate of recent sediments. Unfortunately, in many... 

Figure 7.17 Estimated accretion rates of intertidal salt marshes on the coast of Rhode Island (USA). Despite having well-developed exponential decay curves of excess 210Pb in most marsh sediment cores, indicating a relatively a constant rate of sediment accretion, some variations in the log excess 210Pb indicate that accretion may not have been constant through time. R = correlation coefficient, I = inventories of excess 210Pb (dpm cm-2) (Benninger, 1979 Olsen et al., 1985). (Modified from Bricker-Urso et al., 1989.)...

Table 7.4 Marsh accretion rates and sea-level rise rates.

Location (USA) Marsh type3/ reference13 Accretion rate (cm y 1) Method SLRC (20)... 

Lead-210 is produced by radioactive decay of 222Rn and can enter estuaries as a dissolved/complexed ion or in association with particles from the ocean, river, and atmosphere. Much of the work to date in estuaries and adjacent marsh/shelf environments using 210Pb has been to determine sediment accumulation and accretion rates. 

Bricker-Urso, S., Nixon, S.W., Cochran, J.K., Hirschberg, D.J., and Hunt, C. (1989) Accretion rates and sediment accumulation in Rhode Island salt marshes. Estuaries 12, 300-317. 

Delaune, R.D., Patrick, W.H., and Buresh, R.J. (1978) Sedimentation rates determined by 137Cs dating in a rapidly accreting salt marsh. Nature 275, 532-533. 

Delaune, M.L., Reddy, C.N., and Patrick, W.H. (1981) Accumulation of plant nutrients and heavy metals through sedimentation processes and accretion in a Louisiana salt marsh. Estuaries 4, 328-334. 

Delaune, R., Smith, C.J., and Patrick, W.H., and Roberts, H.H. (1987) Rejuvenated marsh and bay-bottom accretion on the rapidly subsiding coastal plain of the U.S. Gulf Coast A second-order effect of the emerging Atchafalaya delta. Estuar. Coastal Shelf Sci. 25, 381-389. 

Hatton, R.S., Delaune, R.D., and Patrick, W.H. (1983) Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana. Limnol. Oceanogr. 28, 494-502. 

Richard, G.A. (1978) Seasonal and environmental variations in sediment accretion in a Long island salt marsh. Estuaries 1, 29-35. 

Ward, L.G, Kearney, M.S., and Stevenson, J.C. (1986) Accretion rates and recent changes in sediment composition of estuarine marshes. Ches. Bay Sci. EOS 67, 998. 

Although the data on N burial is somewhat sparse, Chmura et al. (2003) compiled an extensive data set on rates of C sequestration in tidal sahne wetlands, which included more than 110 salt marsh sites. They concluded that on average salt marshes store 210 g C m year. Sediment C/N data for salt marshes has been compiled by VaHela (1983) and Craft et al, 1991 and while this data also comprises a wide range of values the majority of the sites have C/N values that fall between 14 and 30. Combining estimates of C accretion and C/N ratios, yields an overall... 

Table 22.5 Burial rates of nitrogen Rates were calculated using several methods, assuming accretion has kept up with relative sea level rise (SLR), a budget of organic matter inputs and losses (OM budget), mass accretion rates using radiometric dating techniques ( Cs or Pb), or net particulate nitrogen (net PN) budgets to the marsh or in tidal creeks. GSW stands for Great Sippewissett Marsh...

Gaboon, D. R. (1994). Recent accretion in two managed marsh impoundments in coastal Louisiana. Ecol. Appl. 4, 166-176. 

Craft, C. B., Seneca, E. D., and Broome, S. W. (1993). Vertical accretion in microtidal regularly and irregularly flooded estuarine marshes. Estuar. Coastal Shelf Sd. 37, 371—386. 

Warren, R. S., andNiering, W. A. (1993). Vegetation change on a northeast tidal marsh Interaction of sea-level rise and marsh accretion. Ecology 74, 96-103. 

In comparing the biomarker signatures of the Upper Marsh and Lower Marsh, the most distinguishing feature is the different rate of decline of total viable microbial biomass with depth in the upper part of the profiles. In the Upper Marsh an order of magnitude decrease in the viable biomass occurred in the upper 20cm while in the Lower Marsh profile a similar order of magnitude decrease occurred over a vertical distance of c. 40 cm. This difference reflects the varying rates of sediment accretion on the two marshes. 

To test whether or not the salt marsh has accreted at a rate comparable... 

Using the smoothed sea-level curve, as plotted in Fig. 12, the agreement between the rate of salt-marsh accretion and tide-gauge record is reasonably good, except in that part of the record where temporary reversals in the sea-level curve appear. 

One of the earliest and most succinct statements of the peat-formation hypothesis of salt-marsh accretion was made by Mudge (1862), after he examined a section of the Romney Marsh, near Lynn, Massachusetts. He said ... 

Mudge s concept of salt-marsh accretion clearly incorporates the notion that the grasses of the high marsh maintain their position during a gradual change in relative sea level, by upward growth. However, the special... 

---