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Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited
MARINE GEOLOGY ELSEVIER
Marine Geology 128 (1995) l-9
Letter Section
Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited Donald
R. Cahoon
a, Denise J. Reed b, John W. Day, Jr. ’
a National Biological Service, Southern Science Center, 700 Cajundome Boulevard, Lafayette, LA 70506, USA ’ Department
b Louisiana Universities Marine Consortium, 8124 Hwy 56, Chauvin, LA 70344, USA of Oceanography and Coastal Sciences, Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA 70803, USA
Received 16 February 1995; revision accepted 3 May 1995
Abstract Simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow (3-5 m) subsurface datum were made in selected coastal salt marshes of Louisiana, Florida, and North Carolina to quantitatively test Kaye and Barghoorn’s contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate. Rates of subsidence of the upper 3-5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation-erosion table. Surface elevation change was significantly lower than vertical accretion at each site after 2 years, indicating a significant amount of shallow subsidence had occurred, ranging from 0.45 to 4.90 cm. The highest rate of shallow subsidence occurred in the Mississippi delta. Results confirm Kaye and Barghoom’s contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate, including not only compaction but also apparently shrink-swell from water storage and/or plant production-decomposition at some sites. Indeed, surface elevation change was completely decoupled from vertical accretion at the Florida site. The assumption of a 1:l relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes. Consequently, the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits. These findings also indicate the need for greater understanding of the influence of subsurface and small-scale hydrologic processes on marsh surface elevation.
1. Introduction More than 30 years ago Kaye and Barghoorn (1964) described the process of autocompaction of the marsh substrate (what they called peat, but which includes both organic and inorganic material) and its important implications for the relationship between vertical accretion and surface SSDZ 0025-3227(95)00087-9
elevation change. Compaction of the substrate results in a change of level of soil particles, what Kaye and Barghoorn (1964) called settlement, with thicker substrates having a greater rate of settlement because of greater potential for postdepositional compaction of peats and clays. Since the compaction of the upper 5-10 m of the substrate contributes a shallow component to the total
2
D. R. Cuhoon et dIMurine
subsidence of a coastal marsh soil, we use the term shallow subsidence to describe this component of surface settlement. The thickness of new material (organic and inorganic) added to the marsh surface must equal the total subsidence rate (shallow plus deep) in order for the marsh surface to remain at the same level, and the thickness of material added must exceed the total subsidence rate for a building up or aggradation of the marsh surface to occur. Such an increase would be required to maintain position relative to a rising mean sea level. Kaye and Barghoorn (1964, p. 69) recognized that “...most accretion studies have paid scant attention to settlement of marsh surfaces, assuming instead that accretion raised the level of the marshes...“. Indeed, vertical accretion measured from artificial soil marker horizons laid on the marsh surface likely overestimates the rate of marsh surface elevation increase because it does not incorporate subsurface processes occurring below the marker horizon. Techniques for dating marsh soils developed since Kaye and Barghoorn’s work such as 210Pb (Armentano and Woodwell, 1975)’ and 13’Cs (DeLaune et al., 1978), incorporate compaction of the upper -30-50 cm of substrate in their measurements, more than marker horizons located near the surface. Compaction occurring below these depths, however, may be considerable, particularly in thick marsh deposits such as occur in deltas (e.g., 0.26 cm/yr over a 0.9-6.5 m depth in the Mississippi delta; Penland et al., 1994). On the other hand, direct measures of surface elevation change also do not provide an estimate of shallow subsidence because these data integrate the net result of all processes contributing to both shallow subsidence and vertical accretion. Shallow subsidence can be calculated, however, by subtracting measures of surface elevation change from vertical accretion, thereby directly testing the validity of using vertical accretion as a surrogate for elevation change. The purpose of this study was to calculate rates of shallow subsidence (O-5 m) for selected salt marshes in Louisiana, Florida, and North Carolina from direct and simultaneous measurements of vertical accretion and surface elevation change. Data were collected to test the null hypothesis: Marsh vertical accretion and surface elevation
Geology 128 (1995) I-9
change rates are equal in coastal salt marshes (i.e., shallow subsidence =zero). We present in this letter: (1) illustrative examples of the types and range of data that were obtained; (2) a discussion of the factors influencing surface elevation change; and, (3) the implications for evaluating the potential for marsh submergence. 1.1. Terminology and study sites For the purposes of this study, we employed the following terms and definitions, the conceptual relationships of which are illustrated in Fig. 1. Marsh vertical accretion is the vertical dimension of marsh soil development determined from a marker horizon, and integrates the sedimentologic and biologic processes occurring on and within the uppermost part of the marsh substrate above the marker horizon (Reed and Cahoon, 1993). These processes initially include sediment deposition and sediment erosion, but within a short period of time (weeks to months) also include
I
Sedimentation Erosion Table Marsh Surface
I
Fig. 1. Conceptual diagram (not to scale) showing those portions of the soil profile being measured by the Sedimentation-Erosion Table (SET) and marker horizon techniques. The boundary separating the shallow and deep subsidence zones is defined operationally by the bottom of the SET pipe.
D.R. Cahoon et al.IMarine
plant production (above ground and below ground), and plant decomposition as the horizon becomes buried, roots grow above it, and plant litter accumulates. Surface elevation change is the change in elevation relative to a subsurface datum, the depth of which is determined by the technique used. In our study, the depth is 3-5 m below the surface. The change in elevation can be due to vertical accretion (above the marker horizon) or changes in the volume of the soil related to subsurface processes occurring below the marker horizon shrink-swell, and decomposi(e.g., compaction, tion). Shallow subsidence is calculated as the difference between vertical accretion and surface elevation change. Subsidence which occurs below the subsurface datum (bottom of the pipe, Fig. 1) is termed deep subsidence and includes additional compaction and isostatic processes (e.g., tectonic activity; Penland et al., 1989). Shallow and deep subsidence combined equal total subsidence. Comparison of vertical accretion and surface elevation change rates to the rate of relative sea-level rise (RSLR, eustatic sea-level rise plus subsidence) allows for calculation of an accretion deficit and an elevation dejicit, respectively. Bayou Chitigue and Old Oyster Bayou study sites are Spartina alternzyora salt marshes within the Terrebonne basin of the Mississippi delta plain of thick Holocene (Fig. 2) where subsidence deposits estimated from tide gauge data is -0.7 cm/yr for Old Oyster Bayou (Boesch et al.,
Fig. 2. Location
map of field sites.
Geology 128 (1995) 1-9
3
1983) and 1.17-l.l9cm/yr for Bayou Chitigue (Penland et al., 1989). The marsh at Bayou Chitigue is remote from an external sediment source and rapidly deteriorating. Old Oyster Bayou, located adjacent to Atchafalaya Bay, receives high inputs of riverine sediments and the marsh has not undergone any substantial breakup. The marsh surface at Bayou Chitigue lies approximately at mean sea level, which is lo-15 cm lower than that at Old Oyster Bayou (J. Suhayda, 1994, unpubl. data, Dep. Civil Eng., Louisiana State Univ.), resulting in 4-5 fold increases in duration and depth of flooding at Bayou Chitigue during 1992-1994 (Reed, 1994, unpublished data). The study sites at St. Marks NWR in the big bend area of Florida and Cedar Island NWR behind the Outer Banks in North Carolina are both Juncus roemerianus salt marshes on relatively stable (i.e., low subsidence), low-energy, sediment-poor coasts compared to the Mississippi delta plain.
2. Methods Vertical accretion was measured as the buildup of organic and inorganic material above feldspar marker horizons laid upon the marsh surface at the start of the study (Cahoon and Turner, 1989). Cryogenic cores (Knaus and Cahoon, 1990) were taken twice yearly through the marker horizons, in the spring and fall, and the depth of the marker below the surface was measured to the nearest millimeter. Elevation change was measured with a sedimentation-erosion table (SET) (Boumans and Day, 1993). SET stations were established adjacent to the markers and SET readings were taken when the markers were laid and at each accretion sampling interval. The SET is a levelling device attached to a benchmark pipe (6.1 m long) driven into the marsh surface. The pipe is assumed to be a stable datum for the period of study (Childers et al., 1993). Nine pins located at the end of an accurately levelled horizontal arm are lowered to the marsh surface to measure elevation with an accuracy of + 2 mm (Fig. 1). Pin readings were taken at 4 fixed positions of the arm at each pipe. The reference datum for the estimate of shallow subsidence is the bottom of the SET pipe, usually
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D. R. Cahoon et aLlMarine Geology 128 ( 1995) l-9
3. Results and discussion
3-5 m below the marsh surface. The SET pipes were driven with a sledge hammer as far as possible (3.5-4 m) into the upper portion of the Holocene deposits at Bayou Chitigue and Old Oyster Bayou. The SET pipes were vibra-cored down to limestone bedrock -3 m below the marsh surface at St. Marks and 5 m down at Cedar Island, a depth well below the Pleistocene contact at 1.8 m (R. Young, 1993, unpubl. data, Duke Univ., Dep. Geol.). Hence, our estimates of shallow subsidence at St. Marks and Cedar Island reflect total subsidence of Holocene sediments. At the Louisiana sites with thick Holocene deposits, any deep subsidence (e.g., compaction) occurring below the base of the pipe was not included in our measurements. Measures were taken in interior marsh areas at all sites, at least 15 m from the nearest marsh channel. At each site, seven small sampling platforms were constructed to allow access to the marsh surface while minimizing disturbance. The platforms were randomly located within an area of the marsh selected for uniformity of vegetative cover and species composition. Three feldspar marker horizons were laid at each of the platforms (n = 21) and 3 of the 7 platforms were randomly selected as SET stations (n= 108 pins, 36 at each of three platforms; see Boumans and Day, 1993; Childers et al., 1993). Statistical comparisons of 2-year means were made by using a 2-sample t-test (SAS Institute Inc., 1989) and comparisons over time were made using a repeated measures design under the split-plot framework (SAS Institute Inc., 1991).
Table 1 Two-year
totals (cm) of vertical
accretion,
marsh
surface
3.1. Vertical accretion and elevation relationships Vertical accretion significantly overestimated elevation increase at all sites by factors ranging from 1.5 to more than an order of magnitude (Table 1). We attribute these differences to shallow subsidence. The marker horizon technique has an inherent bias toward measuring accreting marsh surfaces to the exclusion of eroding surfaces (because those markers are lost), and therefore can overestimate accretion rates on a rapidly eroding marsh surface, but this was not the case at our sites. The marker horizons at Old Oyster Bayou, Cedar Island, and St. Marks showed no signs of erosion and recovery of marker horizons at these sites was 100%. The surface of the deteriorating marsh at Bayou Chitigue showed some signs of erosion and disturbance, but even there the recovery rate over 2 years of sampling was 98% (100% during the last sampling interval). Hence, we believe the data in Table 1 indicate that although material accumulated on the marsh surface, the surface moved downward because of shallow subsidence so that elevation increase was less than vertical accretion. This is illustrated at Bayou Chitigue (Fig. 3). The highest rate of shallow subsidence occurred at this deteriorating marsh, even though this site had the highest vertical accretion rate, indicating how misleading vertical accretion data can be when used as a surrogate for elevation increase.
elevation
change,
and shallow
subsidence
Site
Vertical accretiona
Elevation change”
Shallow subsidence”
Depth of pipe (m)
Bayou Chitigue Old Oyster Bayou St. Marks NWR Cedar Island NWR
5.19kO.32 2.07kO.10 0.89+0.06 0.77 f 0.09
0.29+0.15b 1.30 * 0.09b -0.14+0.04b 0.32+0.11b
4.90 0.77 1.03 0.45
4 4 3 5
‘Data are means + 1 SE. b Indicates that accretion and elevation means are significantly ’ Shallow subsidence = (vertical accretion) - (elevation change).
different
(p = 0.001)
D.R
Cahoon et al./Marine
Geology 128 (1995) 1-9
5
Interpretation (A) Start of Study Vertical Accretion
Vertical Ah%tion and Surface Elevation Change (O-4 m)
T
1 T
VUlb4l
ka&ion &lQcm_
CadtrIsland,NC
Fig. 3. Conceptual diagram showing two interpretations of actual data (A and B) that demonstrate the relationship of marsh surface elevation change to vertical accretion of the marsh surface over 2 years at Bayou Chitigue. The dashed line indicates the initial position (interpreted) of the marsh surface at Time,. TI represents the marsh surface 2 years. Interpretation (A) measures vertical accretion data only (i.e., does not include shallow subsidence) because it assumes a 1:l ratio of vertical accretion (i.e., burial of the horizon) and surface elevation change. Interpretation (B) includes simultaneous evaluation of separate vertical accretion and surface elevation datasets and indicates that the TO horizon has moved downward due to shallow subsidence and has been buried.
There appear to be several factors that influenced elevation change at the four sites. Although vertical accretion certainly influences elevation change, other factors appear to be as or more important. Changes over time in vertical accretion and surface elevation (Fig. 4) suggest that compression of the substrate during major storms and seasonal variations in water level and/or plant production-decomposition may strongly influence marsh surface elevation. A significant divergence of accretion and elevation change rates occurred at both Bayou Chitigue (p = 0.00001) and Cedar Island (p = 0.0006) during sampling intervals which included a major storm event: Hurricane Andrew in Louisiana and Hurricane Emily in North Carolina (Fig. 4). We observed a thick layer of mud on the marsh surface at Bayou Chitigue 6 days after Hurricane Andrew, which likely accounts for the majority of the 3 cm of accretion measured in December 1992. Yet there was a loss of marsh surface elevation during the 6-month sampling interval which included the storm. The loss of elevation was not due to scour-
1
“I_-/ -._._..... -._...._.._. -._...._.._. -._./
Fig. 4. Marsh surface elevation change and vertical accretion at four sites in the southeastern U.S.A. Dashed line=elevation change, solid line = vertical accretion. Separation between the two lines represents shallow subsidence. Arrows indicate the data of a major storm event. Note difference in the scales between top and bottom graphs.
ing by the storm surge as we had 100% recovery of the marker horizons at the end of that sampling interval. The difference between accretion and elevation persisted during subsequent samplings. Hence, most of the observed shallow subsidence occurred in the sampling interval that included the storm. Similarly, there was a loss of elevation at Cedar Island marsh during the 6-month sampling interval which included the storm, and the difference persisted during subsequent samplings (Fig. 4). Hurricane Andrew did not have the same impact on elevation at Old Oyster Bayou marsh probably because the cohesive shear strength of the soil was l-2 orders of magnitude greater at this site than Bayou Chitigue (Day et al., 1994). The substrates at both Bayou Chitigue and Cedar Island marshes show high potential for compression. At Cedar Island the substrate is highly organic (> 60% by weight) and has a low bulk density (co.1 g/cm”). Knott et al. (1987) showed that compressibility of salt marsh peat is
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D. R. Cahoon et al.iMarine Geologic 128 (1995) I-9
directly related to organic matter content, with an order of magnitude more compressibility at 60% versus 10% organic matter content. At Bayou Chitigue, marsh surface elevation appears to have been influenced by structural collapse of the living root network caused by flooding stress (Delaune et al., 1994) and a high potential for degasification of the substrate (McGinnis et al., 1991). We hypothesize that the weight of the mud (storm deposits) compressed the marsh substrate. There was ample opportunity at both sites for compression caused by sediment overburden to occur through expulsion of pore waters during low water events. Our measurements at Bayou Chitigue were made in December 1992, four months after the storm and one month after the first prolonged low water stand (7 days in November 1992, Cahoon et al., 1995) following the storm, while our measurements at Cedar Island were made 4.5 months after passage of Hurricane Emily. The fact that the elevation did not rebound in subsequent samplings suggests that elevation loss was due to compaction of organic matter and/or loss of porosity. Elevation of the Juncus marsh at St. Marks showed a strong seasonal trend (Fig. 4). There was a significant (p = 0.0001) increase during each summer followed by a significant decrease (p= 0.0001) each winter. After 2 years, marsh surface elevation had not changed significantly compared with initial elevation even though material continuously accreted on the marsh surface. The changes in elevation were not related to daily tidal flooding and associated water storage, as reported by Nuttle et al. (1990) for two New England salt marshes. We measured marsh surface elevation at both low and high tide in May 1994. There was no significant difference in elevation between low and high tide (32.40 + 0.09 vs 32.43 k 0.08 cm, respectively, n= 36), although water level measured in shallow wells varied from > 33 cm below the marsh surface to 6 cm above it. We propose, therefore, two possible explanations for this elevation pattern: (1) changes in water storage on a seasonal basis, and (2) changes in the volume of the root zone related to a seasonal pattern of plant production (summer) and decomposition (winter). The seasonal trend in elevation could be directly
related to the seasonal variation in Gulf mean water level and/or an increase in depth of water flooding the marsh at high tide resulting in increased water storage. A seasonal pattern in mean water levels exists in the northern Gulf of Mexico with water level an average 25 cm higher in summer (Marmer, 1954). There was also a 16-cm difference in mean daily tide range between summer (90 cm) and winter (74 cm) measured by a water level gauge at the St. Marks study site (D.J. Reed, 1994, unpubl. data). The seasonal variations in water levels may also influence plant production and belowground decomposition processes. Increased daily tidal range during the summer creates more oxidized substrate conditions which are well suited for plant production (Steever et al., 1976; Howes et al., 1986). In addition, evapotranspiration during periods of active photosynthesis in salt marsh plants can remove significant amounts of soil water (Dacey and Howes, 1984; Morris and Whiting, 1985). During the winter, aerobic plant decomposition processes are enhanced by lower mean water levels and plant senescence and death (Hackney and De la Cruz, 1980). The combined influence of seasonal variations in both water level and plant production-decomposition may be responsible for the pattern of elevation change shown in Fig. 4. In contrast, the seasonal trend was not observed in the Juncus marsh at Cedar Island because there was no strong seasonal pattern in daily tidal range ( 19 cm in summer vs. 15 cm in winter). In addition, Cedar Island marsh is irregularly flooded, winddominated and characterized by long shallow flooding events (average flooding event is 19 h long and 8.9 cm deep vs. 3.4 h and 19.3 cm at St. Marks; Reed, 1994, unpublished data). 3.2. Implications dejicits
jbr marsh submergence:
elevation
Understanding the accretionary and elevational status of marshes is becoming increasingly important given the recent predictions of acceleration in the rate of eustatic sea-level rise in the next century (4-5 times faster than past 100 years; Wigley and Raper, 1992) and current interest in maintenance and restoration of coastal wetlands (Boesch et al.,
D. R Cahoon et aLlMarine
1994). Major river deltas and coastal wetlands will be especially vulnerable to submergence if these projections are true (Gornitz, 1991). Accurate estimates of the potential for submergence of coastal wetlands are critical in developing effective coastal management strategies. The accuracy of such estimates is influenced not only by projections of sea-level rise but also by the accuracy of current accretion-deficit estimates. The vulnerability of a coastal marsh to submergence is typically determined by a comparison of vertical accretion rates with the rate of RSLR and reported as an accretion deficit when RSLR is greater than vertical accretion (Baumann et al., 1984; Stevenson et al., 1986). Implicit in the accretion deficit concept is the assumption that a 1: 1 relationship exists between vertical accretion and surface elevation change (Kaye and Barghoorn, 1964; Reed and Cahoon, 1993). Separate comparisons of annual mean vertical accretion and elevation data to local rates of RSLR yield important differences with serious management implications (Fig. 5). The concept of accretion deficit based on near-surface marker 3
2.5 -
Bayou Chiiigw,
n
m
0
/
LA
old oyster
0
Bayou. LA
Cedar Island. NC / /_.
~%St. 0.5
Marks, FL
, . . . . , . . . . , . . . . 2
2.5
Fig. 5. Relationship of vertical accretion and marsh surface elevation change with local relative sea-level rise. Diagonal line indicates parity between accretion or elevation change and sealevel rise. Sea-level rise rates are from Stevenson et al. (1986) (North Carolina and Florida), Baumann et al. (1984) (Old Oyster Bayou), and Nyman et al. (1993) (Bayou Chitigue).
Geology 128 (199.5) 1-9
I
horizons is inadequate to describe the potential for submergence in these marshes, particularly for the marsh at Bayou Chit&e where regional RSLR estimates are relatively high (1.26-l .3 1 cm/yr, Penland et al., 1989). There was no accretion deficit for this site but RSLR was an order of magnitude greater than elevation change. In this rapidly deteriorating marsh, plants grew more robustly following the storm (Cahoon, 1992, personal observation) probably because of nutrients associated with the sediment deposits (Nyman et al., in press). Without any knowledge of marsh surface elevation change, it would be easy to conclude that the hurricane had a generally positive effect on the marsh because of the thick sediment deposits and associated nutrients. The potential for submergence was not reduced at this site, however, because of the storm’s apparent effect on compaction of weak soils coupled with the high deep subsidence rate in this region. In contrast, there was no compaction of the stronger soils at Old Oyster Bayou during the sampling interval that included the storm. Compared to Bayou Chitigue, the other sites have a lower potential for submergence (Fig. 5). They are similar to Bayou Chitigue, however, in that the accretion data indicate little potential for submergence while the elevation data indicate that an elevation deficit may exist at Old Oyster Bayou and St. Marks. The implications for long-term marsh stability may not be severe for Old Oyster Bayou and St. Marks, however, given the small elevation deficits and the big difference in the lengths of the data sets used to make the comparisons (2-yr elevation rate versus a >20-yr sea-level rise rate). The decoupling of elevation change from accretion rates at St. Marks and the healthy state of the marsh at both St. Marks and Old Oyster Bayou (high productivity and low wetland loss rate) imply that these marshes are in long-term equilibrium with sea-level rise, and current deficits may be due to short-term annual fluctuations in sea-level rise or elevation change. In any event, our data indicate that elevation deficits, not accretion deficits based on near-surface marker horizons, should be used to determine current submergence potentials. The elevation change data from the Mississippi
8
D. R. Cahoon et al. IMarine Geology 128 (1995) 1-9
delta plain also has implications for current estimates of total subsidence determined from tide gauge data (Penland et al., 1989). Estimates of total subsidence based on tide gauge records may be underestimated by the amount of shallow subsidence shown at some sites in this study because shallow subsidence occurs at depths between the tide gauge base (similar to the SET base, Fig. 1) and the marsh surface. As such, total subsidence may be underestimated by as much as 2.5 cm/yr (e.g. Bayou Chitigue). Acknowledgements
J. Lynch assisted in collection of the accretion data, processing of soil cores, and preparation of figures. D. McNally collected the elevation data. R. Boustany, S. Fournet, M. Griffis, P. Hensel, R. Herbert, K. Landau, E. Pendleton, M. Radford, A. Stroy, M.A. Townson, and R. Young also assisted in field data collection and processing of soil cores. We are grateful to Louisiana Land and Exploration Company and Fina LaTerre, Inc. for allowing access to their properties in Louisiana. Personnel from Cedar Island NWR, the University of North Carolina, Marine Sciences Institute, and the National Marine Fisheries Service, Beaufort Laboratory provided essential logistical support of our sampling efforts at Cedar Island NWR. Personnel from St. Marks NWR, Florida Geological Survey (R. Hoenstine), Florida State University (J. Donoghue) and C.S. Giddens provided support of our sampling efforts at St. Marks NWR. D. Fuller and B. Pugesek provided advice on sampling design, and D. Johnson assisted in statistical analyses. A.L. Foote, P. Kemp, and B. Lock provided critical technical reviews and B. Vairin edited early drafts. This research was supported by the National Biological Service’s Southern Science Center with funds from the Department of the Interior, Global Climate Research Program. References Armentano, T.V. and Woodwell, G.M., 1975. Sedimentation rates in a Long Island marsh determined by ‘iOPb dating. Limnol. Occanogr., 20: 4522456.
Baumann, R.H., Day, J.W., Jr. and Miller, C.A., 1984. Mississippi deltaic wetland survival: sedimentation versus coastal submergence. Science, 224: 1093-1095. Boesch, D.F., Levin, D., Nummedal, D. and Bowles, K., 1983. Subsidence in coastal Louisiana: causes, rates, and effects on wetlands. U.S. Fish and Wildlife Service, Div. Biol. Serv., Washington, DC, FWS/OBS-83/26, 30 pp. Boesch, D.F., Josselyn, M.N., Mehta, A.J., Morris, J.T., Nuttle, W.K., Simenstad, C.A. and Swift, D.J.P., 1994. Scientific assessment of coastal wetland loss, restoration and management in Louisiana. J. Coastal Res., Spec. Issue, 20, 103 pp. Boumans, R.M.J. and Day, J.W., Jr., 1993. High precision measurements of sediment elevation in shallow coastal areas using a sedimentation-erosion table. Estuaries, 16: 375-380. Cahoon, D.R. and Turner, R.E., 1989. Accretion and canal impacts in a rapidly subsiding wetland II. feldspar marker horizon technique. Estuaries, 12: 260-268. Cahoon, D.R., Reed, D.J., Day, Jr., J.W., Steyer, G.D., Boumans, R.M., Lynch, J.C., McNally, D. and Latif, N. 1995. The influence of Hurricane Andrew on sediment distribution in Louisiana coastal marshes. J. Coastal Res., Spec. Issue, 18: 280-294. Childers, D.L., Sklar, F.H., Drake, B. and Jordan, T., 1993. Seasonal measurements of sediment elevation in three midAtlantic estuaries. J. Coastal Res., 9: 98661003. Dacey, J.W. and Howes, B.L., 1984. Water uptake by roots controls water movement and sediment oxidation in short Spartina marsh. Science, 224: 487-489. Day, J.W., Jr., Reed, D., Suhayda, J., Kemp, G.P., Cahoon, D., Boumans, R.M., and Latif, N., 1994. Physical processes of marsh deterioration. In: H.H. Roberts (Editor), Critical Physical Processes of Wetland Loss. U.S. Geol. Surv., Open File Rep., pp. 5.1-5.40. DeLaune, R.D., Patrick, W.H., Jr. and Buresh, R.J., 1978. Sedimentation rates determined by ‘37Cs dating in a rapidly accreting salt marsh. Nature, 275: 532-533. DeLaune, R.D., Nyman, J.A. and Patrick, W.H., Jr., 1994. Peat collapse, ponding and wetland loss in a rapidly submerging coastal marsh. J. Coastal Res., 10: 1021-1030. Gornitz, V., 1991. Global coastal hazards from future sea level rise. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 89: 379-398. Hackney, CT. and De la Cruz, A.A., 1980. In situ decomposition of roots and rhizomes of two tidal marsh plants. Ecology, 61: 226-23 1. Howes, B.L., Dacey, J.W.H. and Goehringer, D.D., 1986. Factors controlling the growth form of Spartina alternipbra: feedbacks between above-ground production, sediment oxidation, nitrogen and salinity. J. Ecol., 74: 881-898. Kaye, C.A. and Barghoorn, E.S., 1964. Late Quaternary sealevel change and crustal rise at Boston, Massachusetts, with notes on the autocompaction of peat. Geol. Sot. Am. Bull., 75: 63-80. Knaus, R.M. and Cahoon, D.R., 1990. Improved cryogenic coring device for measuring soil accretion and bulk density. J. Sediment. Petrol., 60: 622-623.
D. R. Cahoon et al. JMarine Geology 128 (I 995) I-9
Knott, J.F., Nuttle, W.K. and Hemond, H.F., 1987. Hydrologic parameters of salt marsh peat. Hydrol. Process., 1: 21 l-220. Manner, H.A., 1954. Tides and sea level in the Gulf of Mexico. In: Gulf of Mexico, its origin, waters and marine life. U.S. Fish Wildlife Serv., Fisheries Bull., 55: 101-l 18. McGinnis, L.D., Johnson, D.O., Zimmerman, E.Z., Isaacson, H.R., Penland, S. and Connor, P.F., 1991. Velocity depression and degasification subsidence in Louisiana wetlands. In: Coastal Depositional Systems in the Gulf of Mexico: Quatemary Framework and Environmental Issues. 12th Annu. Res. Conf., Gulf Coast Sect., SEPM Found., Houston, TX, pp. 128-133. Morris, J.T. and Whiting, G.J., 1985. Gas advection in sediments of a South Carolina salt marsh. Mar. Ecol. Prog. Ser., 27: 187-194. Nuttle, W.K., Hemond, H.F. and Stolzenbach, K.D., 1990. Mechanisms of water storage in salt marsh sediments: the importance of dilation. Hydrol. Process., 4: 1-13. Nyman, J.A., DeLaune, R.D., Roberts, H.H. and Patrick, W.H., Jr., 1993. Relationship between vegetation and soil formation in a rapidly submerging coastal marsh. Mar. Ecol. Prog. Ser., 96: 269-279. Nyman, J.A., Crozier, C.R., and DeLaune, R.D., in press. Roles and patterns of hurricane sedimentation in an estuarine marsh landscape. Estuarine Coastal Shelf Sci. Penland, S., Ramsey, K.E., McBride, R.A., Moslow, T.F., and Westphal, K.A., 1989. Relative sea level rise and subsidence
9
in Louisiana and the Gulf of Mexico. Coastal Geol. Tech. Rep., 3, Louisiana Geol. Surv., Baton Rouge, 65 pp. Penland, S., Roberts, H.H., Bailey, A., Kuescher, G.J., Suhayda, J.N., Connor, P.C. and Ramsey, K.E., 1994. Geologic framework, processes, and rates of subsidence in the Mississippi River delta plain. In: H.H. Roberts (Editor), Critical Physical Processes of Wetland Loss. U.S. Geol. Surv., Open File Rep., pp. 7.1.1-7.1.51. Reed, D.J. and Cahoon, D.R., 1993. Marsh submergence vs marsh accretion: interpreting accretion deficit data in coastal Louisiana. In: O.T. Magoon, W.S. Wilson, H. Converse, and L.T. Tobin (Editors), Proc. 8th Symp. Coastal and Ocean Management, Coastal Zone ‘93. (New Orleans, LA.) ASCE, New York, NY, pp. 243-257. SAS Institute Inc., 1989. SAS/STATR User’s Guide. Version 6, 4th ed., 2, Cary, NC. SAS Institute Inc., 1991. SAS’ System for Linear Models. 3rd ed., Cary, NC, pp. 272-274. Steever, E.Z., Warren, R.S. and Niering, W.A., 1976. Tidal energy subsidy and standing crop production of Spartina alternipora. Estuarine Coastal Mar. Sci., 4: 473-478. Stevenson, J.C., Ward, L.G. and Keamey, MS., 1986. Vertical accretion in marshes with varying rates of sea level rise. In: D.A. Wolfe (Editor), Estuarine Variability. Academic Press, Orlando, FL, pp. 241-259. Wigley, T.M.L. and Raper, S.C.B., 1992. Implications for climate and sea level of revised IPCC emissions scenarios. Nature, 357: 293-300.
Marine Geology 128 (1995) l-9
Letter Section
Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited Donald
R. Cahoon
a, Denise J. Reed b, John W. Day, Jr. ’
a National Biological Service, Southern Science Center, 700 Cajundome Boulevard, Lafayette, LA 70506, USA ’ Department
b Louisiana Universities Marine Consortium, 8124 Hwy 56, Chauvin, LA 70344, USA of Oceanography and Coastal Sciences, Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA 70803, USA
Received 16 February 1995; revision accepted 3 May 1995
Abstract Simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow (3-5 m) subsurface datum were made in selected coastal salt marshes of Louisiana, Florida, and North Carolina to quantitatively test Kaye and Barghoorn’s contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate. Rates of subsidence of the upper 3-5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation-erosion table. Surface elevation change was significantly lower than vertical accretion at each site after 2 years, indicating a significant amount of shallow subsidence had occurred, ranging from 0.45 to 4.90 cm. The highest rate of shallow subsidence occurred in the Mississippi delta. Results confirm Kaye and Barghoom’s contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate, including not only compaction but also apparently shrink-swell from water storage and/or plant production-decomposition at some sites. Indeed, surface elevation change was completely decoupled from vertical accretion at the Florida site. The assumption of a 1:l relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes. Consequently, the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits. These findings also indicate the need for greater understanding of the influence of subsurface and small-scale hydrologic processes on marsh surface elevation.
1. Introduction More than 30 years ago Kaye and Barghoorn (1964) described the process of autocompaction of the marsh substrate (what they called peat, but which includes both organic and inorganic material) and its important implications for the relationship between vertical accretion and surface SSDZ 0025-3227(95)00087-9
elevation change. Compaction of the substrate results in a change of level of soil particles, what Kaye and Barghoorn (1964) called settlement, with thicker substrates having a greater rate of settlement because of greater potential for postdepositional compaction of peats and clays. Since the compaction of the upper 5-10 m of the substrate contributes a shallow component to the total
2
D. R. Cuhoon et dIMurine
subsidence of a coastal marsh soil, we use the term shallow subsidence to describe this component of surface settlement. The thickness of new material (organic and inorganic) added to the marsh surface must equal the total subsidence rate (shallow plus deep) in order for the marsh surface to remain at the same level, and the thickness of material added must exceed the total subsidence rate for a building up or aggradation of the marsh surface to occur. Such an increase would be required to maintain position relative to a rising mean sea level. Kaye and Barghoorn (1964, p. 69) recognized that “...most accretion studies have paid scant attention to settlement of marsh surfaces, assuming instead that accretion raised the level of the marshes...“. Indeed, vertical accretion measured from artificial soil marker horizons laid on the marsh surface likely overestimates the rate of marsh surface elevation increase because it does not incorporate subsurface processes occurring below the marker horizon. Techniques for dating marsh soils developed since Kaye and Barghoorn’s work such as 210Pb (Armentano and Woodwell, 1975)’ and 13’Cs (DeLaune et al., 1978), incorporate compaction of the upper -30-50 cm of substrate in their measurements, more than marker horizons located near the surface. Compaction occurring below these depths, however, may be considerable, particularly in thick marsh deposits such as occur in deltas (e.g., 0.26 cm/yr over a 0.9-6.5 m depth in the Mississippi delta; Penland et al., 1994). On the other hand, direct measures of surface elevation change also do not provide an estimate of shallow subsidence because these data integrate the net result of all processes contributing to both shallow subsidence and vertical accretion. Shallow subsidence can be calculated, however, by subtracting measures of surface elevation change from vertical accretion, thereby directly testing the validity of using vertical accretion as a surrogate for elevation change. The purpose of this study was to calculate rates of shallow subsidence (O-5 m) for selected salt marshes in Louisiana, Florida, and North Carolina from direct and simultaneous measurements of vertical accretion and surface elevation change. Data were collected to test the null hypothesis: Marsh vertical accretion and surface elevation
Geology 128 (1995) I-9
change rates are equal in coastal salt marshes (i.e., shallow subsidence =zero). We present in this letter: (1) illustrative examples of the types and range of data that were obtained; (2) a discussion of the factors influencing surface elevation change; and, (3) the implications for evaluating the potential for marsh submergence. 1.1. Terminology and study sites For the purposes of this study, we employed the following terms and definitions, the conceptual relationships of which are illustrated in Fig. 1. Marsh vertical accretion is the vertical dimension of marsh soil development determined from a marker horizon, and integrates the sedimentologic and biologic processes occurring on and within the uppermost part of the marsh substrate above the marker horizon (Reed and Cahoon, 1993). These processes initially include sediment deposition and sediment erosion, but within a short period of time (weeks to months) also include
I
Sedimentation Erosion Table Marsh Surface
I
Fig. 1. Conceptual diagram (not to scale) showing those portions of the soil profile being measured by the Sedimentation-Erosion Table (SET) and marker horizon techniques. The boundary separating the shallow and deep subsidence zones is defined operationally by the bottom of the SET pipe.
D.R. Cahoon et al.IMarine
plant production (above ground and below ground), and plant decomposition as the horizon becomes buried, roots grow above it, and plant litter accumulates. Surface elevation change is the change in elevation relative to a subsurface datum, the depth of which is determined by the technique used. In our study, the depth is 3-5 m below the surface. The change in elevation can be due to vertical accretion (above the marker horizon) or changes in the volume of the soil related to subsurface processes occurring below the marker horizon shrink-swell, and decomposi(e.g., compaction, tion). Shallow subsidence is calculated as the difference between vertical accretion and surface elevation change. Subsidence which occurs below the subsurface datum (bottom of the pipe, Fig. 1) is termed deep subsidence and includes additional compaction and isostatic processes (e.g., tectonic activity; Penland et al., 1989). Shallow and deep subsidence combined equal total subsidence. Comparison of vertical accretion and surface elevation change rates to the rate of relative sea-level rise (RSLR, eustatic sea-level rise plus subsidence) allows for calculation of an accretion deficit and an elevation dejicit, respectively. Bayou Chitigue and Old Oyster Bayou study sites are Spartina alternzyora salt marshes within the Terrebonne basin of the Mississippi delta plain of thick Holocene (Fig. 2) where subsidence deposits estimated from tide gauge data is -0.7 cm/yr for Old Oyster Bayou (Boesch et al.,
Fig. 2. Location
map of field sites.
Geology 128 (1995) 1-9
3
1983) and 1.17-l.l9cm/yr for Bayou Chitigue (Penland et al., 1989). The marsh at Bayou Chitigue is remote from an external sediment source and rapidly deteriorating. Old Oyster Bayou, located adjacent to Atchafalaya Bay, receives high inputs of riverine sediments and the marsh has not undergone any substantial breakup. The marsh surface at Bayou Chitigue lies approximately at mean sea level, which is lo-15 cm lower than that at Old Oyster Bayou (J. Suhayda, 1994, unpubl. data, Dep. Civil Eng., Louisiana State Univ.), resulting in 4-5 fold increases in duration and depth of flooding at Bayou Chitigue during 1992-1994 (Reed, 1994, unpublished data). The study sites at St. Marks NWR in the big bend area of Florida and Cedar Island NWR behind the Outer Banks in North Carolina are both Juncus roemerianus salt marshes on relatively stable (i.e., low subsidence), low-energy, sediment-poor coasts compared to the Mississippi delta plain.
2. Methods Vertical accretion was measured as the buildup of organic and inorganic material above feldspar marker horizons laid upon the marsh surface at the start of the study (Cahoon and Turner, 1989). Cryogenic cores (Knaus and Cahoon, 1990) were taken twice yearly through the marker horizons, in the spring and fall, and the depth of the marker below the surface was measured to the nearest millimeter. Elevation change was measured with a sedimentation-erosion table (SET) (Boumans and Day, 1993). SET stations were established adjacent to the markers and SET readings were taken when the markers were laid and at each accretion sampling interval. The SET is a levelling device attached to a benchmark pipe (6.1 m long) driven into the marsh surface. The pipe is assumed to be a stable datum for the period of study (Childers et al., 1993). Nine pins located at the end of an accurately levelled horizontal arm are lowered to the marsh surface to measure elevation with an accuracy of + 2 mm (Fig. 1). Pin readings were taken at 4 fixed positions of the arm at each pipe. The reference datum for the estimate of shallow subsidence is the bottom of the SET pipe, usually
4
D. R. Cahoon et aLlMarine Geology 128 ( 1995) l-9
3. Results and discussion
3-5 m below the marsh surface. The SET pipes were driven with a sledge hammer as far as possible (3.5-4 m) into the upper portion of the Holocene deposits at Bayou Chitigue and Old Oyster Bayou. The SET pipes were vibra-cored down to limestone bedrock -3 m below the marsh surface at St. Marks and 5 m down at Cedar Island, a depth well below the Pleistocene contact at 1.8 m (R. Young, 1993, unpubl. data, Duke Univ., Dep. Geol.). Hence, our estimates of shallow subsidence at St. Marks and Cedar Island reflect total subsidence of Holocene sediments. At the Louisiana sites with thick Holocene deposits, any deep subsidence (e.g., compaction) occurring below the base of the pipe was not included in our measurements. Measures were taken in interior marsh areas at all sites, at least 15 m from the nearest marsh channel. At each site, seven small sampling platforms were constructed to allow access to the marsh surface while minimizing disturbance. The platforms were randomly located within an area of the marsh selected for uniformity of vegetative cover and species composition. Three feldspar marker horizons were laid at each of the platforms (n = 21) and 3 of the 7 platforms were randomly selected as SET stations (n= 108 pins, 36 at each of three platforms; see Boumans and Day, 1993; Childers et al., 1993). Statistical comparisons of 2-year means were made by using a 2-sample t-test (SAS Institute Inc., 1989) and comparisons over time were made using a repeated measures design under the split-plot framework (SAS Institute Inc., 1991).
Table 1 Two-year
totals (cm) of vertical
accretion,
marsh
surface
3.1. Vertical accretion and elevation relationships Vertical accretion significantly overestimated elevation increase at all sites by factors ranging from 1.5 to more than an order of magnitude (Table 1). We attribute these differences to shallow subsidence. The marker horizon technique has an inherent bias toward measuring accreting marsh surfaces to the exclusion of eroding surfaces (because those markers are lost), and therefore can overestimate accretion rates on a rapidly eroding marsh surface, but this was not the case at our sites. The marker horizons at Old Oyster Bayou, Cedar Island, and St. Marks showed no signs of erosion and recovery of marker horizons at these sites was 100%. The surface of the deteriorating marsh at Bayou Chitigue showed some signs of erosion and disturbance, but even there the recovery rate over 2 years of sampling was 98% (100% during the last sampling interval). Hence, we believe the data in Table 1 indicate that although material accumulated on the marsh surface, the surface moved downward because of shallow subsidence so that elevation increase was less than vertical accretion. This is illustrated at Bayou Chitigue (Fig. 3). The highest rate of shallow subsidence occurred at this deteriorating marsh, even though this site had the highest vertical accretion rate, indicating how misleading vertical accretion data can be when used as a surrogate for elevation increase.
elevation
change,
and shallow
subsidence
Site
Vertical accretiona
Elevation change”
Shallow subsidence”
Depth of pipe (m)
Bayou Chitigue Old Oyster Bayou St. Marks NWR Cedar Island NWR
5.19kO.32 2.07kO.10 0.89+0.06 0.77 f 0.09
0.29+0.15b 1.30 * 0.09b -0.14+0.04b 0.32+0.11b
4.90 0.77 1.03 0.45
4 4 3 5
‘Data are means + 1 SE. b Indicates that accretion and elevation means are significantly ’ Shallow subsidence = (vertical accretion) - (elevation change).
different
(p = 0.001)
D.R
Cahoon et al./Marine
Geology 128 (1995) 1-9
5
Interpretation (A) Start of Study Vertical Accretion
Vertical Ah%tion and Surface Elevation Change (O-4 m)
T
1 T
VUlb4l
ka&ion &lQcm_
CadtrIsland,NC
Fig. 3. Conceptual diagram showing two interpretations of actual data (A and B) that demonstrate the relationship of marsh surface elevation change to vertical accretion of the marsh surface over 2 years at Bayou Chitigue. The dashed line indicates the initial position (interpreted) of the marsh surface at Time,. TI represents the marsh surface 2 years. Interpretation (A) measures vertical accretion data only (i.e., does not include shallow subsidence) because it assumes a 1:l ratio of vertical accretion (i.e., burial of the horizon) and surface elevation change. Interpretation (B) includes simultaneous evaluation of separate vertical accretion and surface elevation datasets and indicates that the TO horizon has moved downward due to shallow subsidence and has been buried.
There appear to be several factors that influenced elevation change at the four sites. Although vertical accretion certainly influences elevation change, other factors appear to be as or more important. Changes over time in vertical accretion and surface elevation (Fig. 4) suggest that compression of the substrate during major storms and seasonal variations in water level and/or plant production-decomposition may strongly influence marsh surface elevation. A significant divergence of accretion and elevation change rates occurred at both Bayou Chitigue (p = 0.00001) and Cedar Island (p = 0.0006) during sampling intervals which included a major storm event: Hurricane Andrew in Louisiana and Hurricane Emily in North Carolina (Fig. 4). We observed a thick layer of mud on the marsh surface at Bayou Chitigue 6 days after Hurricane Andrew, which likely accounts for the majority of the 3 cm of accretion measured in December 1992. Yet there was a loss of marsh surface elevation during the 6-month sampling interval which included the storm. The loss of elevation was not due to scour-
1
“I_-/ -._._..... -._...._.._. -._...._.._. -._./
Fig. 4. Marsh surface elevation change and vertical accretion at four sites in the southeastern U.S.A. Dashed line=elevation change, solid line = vertical accretion. Separation between the two lines represents shallow subsidence. Arrows indicate the data of a major storm event. Note difference in the scales between top and bottom graphs.
ing by the storm surge as we had 100% recovery of the marker horizons at the end of that sampling interval. The difference between accretion and elevation persisted during subsequent samplings. Hence, most of the observed shallow subsidence occurred in the sampling interval that included the storm. Similarly, there was a loss of elevation at Cedar Island marsh during the 6-month sampling interval which included the storm, and the difference persisted during subsequent samplings (Fig. 4). Hurricane Andrew did not have the same impact on elevation at Old Oyster Bayou marsh probably because the cohesive shear strength of the soil was l-2 orders of magnitude greater at this site than Bayou Chitigue (Day et al., 1994). The substrates at both Bayou Chitigue and Cedar Island marshes show high potential for compression. At Cedar Island the substrate is highly organic (> 60% by weight) and has a low bulk density (co.1 g/cm”). Knott et al. (1987) showed that compressibility of salt marsh peat is
6
D. R. Cahoon et al.iMarine Geologic 128 (1995) I-9
directly related to organic matter content, with an order of magnitude more compressibility at 60% versus 10% organic matter content. At Bayou Chitigue, marsh surface elevation appears to have been influenced by structural collapse of the living root network caused by flooding stress (Delaune et al., 1994) and a high potential for degasification of the substrate (McGinnis et al., 1991). We hypothesize that the weight of the mud (storm deposits) compressed the marsh substrate. There was ample opportunity at both sites for compression caused by sediment overburden to occur through expulsion of pore waters during low water events. Our measurements at Bayou Chitigue were made in December 1992, four months after the storm and one month after the first prolonged low water stand (7 days in November 1992, Cahoon et al., 1995) following the storm, while our measurements at Cedar Island were made 4.5 months after passage of Hurricane Emily. The fact that the elevation did not rebound in subsequent samplings suggests that elevation loss was due to compaction of organic matter and/or loss of porosity. Elevation of the Juncus marsh at St. Marks showed a strong seasonal trend (Fig. 4). There was a significant (p = 0.0001) increase during each summer followed by a significant decrease (p= 0.0001) each winter. After 2 years, marsh surface elevation had not changed significantly compared with initial elevation even though material continuously accreted on the marsh surface. The changes in elevation were not related to daily tidal flooding and associated water storage, as reported by Nuttle et al. (1990) for two New England salt marshes. We measured marsh surface elevation at both low and high tide in May 1994. There was no significant difference in elevation between low and high tide (32.40 + 0.09 vs 32.43 k 0.08 cm, respectively, n= 36), although water level measured in shallow wells varied from > 33 cm below the marsh surface to 6 cm above it. We propose, therefore, two possible explanations for this elevation pattern: (1) changes in water storage on a seasonal basis, and (2) changes in the volume of the root zone related to a seasonal pattern of plant production (summer) and decomposition (winter). The seasonal trend in elevation could be directly
related to the seasonal variation in Gulf mean water level and/or an increase in depth of water flooding the marsh at high tide resulting in increased water storage. A seasonal pattern in mean water levels exists in the northern Gulf of Mexico with water level an average 25 cm higher in summer (Marmer, 1954). There was also a 16-cm difference in mean daily tide range between summer (90 cm) and winter (74 cm) measured by a water level gauge at the St. Marks study site (D.J. Reed, 1994, unpubl. data). The seasonal variations in water levels may also influence plant production and belowground decomposition processes. Increased daily tidal range during the summer creates more oxidized substrate conditions which are well suited for plant production (Steever et al., 1976; Howes et al., 1986). In addition, evapotranspiration during periods of active photosynthesis in salt marsh plants can remove significant amounts of soil water (Dacey and Howes, 1984; Morris and Whiting, 1985). During the winter, aerobic plant decomposition processes are enhanced by lower mean water levels and plant senescence and death (Hackney and De la Cruz, 1980). The combined influence of seasonal variations in both water level and plant production-decomposition may be responsible for the pattern of elevation change shown in Fig. 4. In contrast, the seasonal trend was not observed in the Juncus marsh at Cedar Island because there was no strong seasonal pattern in daily tidal range ( 19 cm in summer vs. 15 cm in winter). In addition, Cedar Island marsh is irregularly flooded, winddominated and characterized by long shallow flooding events (average flooding event is 19 h long and 8.9 cm deep vs. 3.4 h and 19.3 cm at St. Marks; Reed, 1994, unpublished data). 3.2. Implications dejicits
jbr marsh submergence:
elevation
Understanding the accretionary and elevational status of marshes is becoming increasingly important given the recent predictions of acceleration in the rate of eustatic sea-level rise in the next century (4-5 times faster than past 100 years; Wigley and Raper, 1992) and current interest in maintenance and restoration of coastal wetlands (Boesch et al.,
D. R Cahoon et aLlMarine
1994). Major river deltas and coastal wetlands will be especially vulnerable to submergence if these projections are true (Gornitz, 1991). Accurate estimates of the potential for submergence of coastal wetlands are critical in developing effective coastal management strategies. The accuracy of such estimates is influenced not only by projections of sea-level rise but also by the accuracy of current accretion-deficit estimates. The vulnerability of a coastal marsh to submergence is typically determined by a comparison of vertical accretion rates with the rate of RSLR and reported as an accretion deficit when RSLR is greater than vertical accretion (Baumann et al., 1984; Stevenson et al., 1986). Implicit in the accretion deficit concept is the assumption that a 1: 1 relationship exists between vertical accretion and surface elevation change (Kaye and Barghoorn, 1964; Reed and Cahoon, 1993). Separate comparisons of annual mean vertical accretion and elevation data to local rates of RSLR yield important differences with serious management implications (Fig. 5). The concept of accretion deficit based on near-surface marker 3
2.5 -
Bayou Chiiigw,
n
m
0
/
LA
old oyster
0
Bayou. LA
Cedar Island. NC / /_.
~%St. 0.5
Marks, FL
, . . . . , . . . . , . . . . 2
2.5
Fig. 5. Relationship of vertical accretion and marsh surface elevation change with local relative sea-level rise. Diagonal line indicates parity between accretion or elevation change and sealevel rise. Sea-level rise rates are from Stevenson et al. (1986) (North Carolina and Florida), Baumann et al. (1984) (Old Oyster Bayou), and Nyman et al. (1993) (Bayou Chitigue).
Geology 128 (199.5) 1-9
I
horizons is inadequate to describe the potential for submergence in these marshes, particularly for the marsh at Bayou Chit&e where regional RSLR estimates are relatively high (1.26-l .3 1 cm/yr, Penland et al., 1989). There was no accretion deficit for this site but RSLR was an order of magnitude greater than elevation change. In this rapidly deteriorating marsh, plants grew more robustly following the storm (Cahoon, 1992, personal observation) probably because of nutrients associated with the sediment deposits (Nyman et al., in press). Without any knowledge of marsh surface elevation change, it would be easy to conclude that the hurricane had a generally positive effect on the marsh because of the thick sediment deposits and associated nutrients. The potential for submergence was not reduced at this site, however, because of the storm’s apparent effect on compaction of weak soils coupled with the high deep subsidence rate in this region. In contrast, there was no compaction of the stronger soils at Old Oyster Bayou during the sampling interval that included the storm. Compared to Bayou Chitigue, the other sites have a lower potential for submergence (Fig. 5). They are similar to Bayou Chitigue, however, in that the accretion data indicate little potential for submergence while the elevation data indicate that an elevation deficit may exist at Old Oyster Bayou and St. Marks. The implications for long-term marsh stability may not be severe for Old Oyster Bayou and St. Marks, however, given the small elevation deficits and the big difference in the lengths of the data sets used to make the comparisons (2-yr elevation rate versus a >20-yr sea-level rise rate). The decoupling of elevation change from accretion rates at St. Marks and the healthy state of the marsh at both St. Marks and Old Oyster Bayou (high productivity and low wetland loss rate) imply that these marshes are in long-term equilibrium with sea-level rise, and current deficits may be due to short-term annual fluctuations in sea-level rise or elevation change. In any event, our data indicate that elevation deficits, not accretion deficits based on near-surface marker horizons, should be used to determine current submergence potentials. The elevation change data from the Mississippi
8
D. R. Cahoon et al. IMarine Geology 128 (1995) 1-9
delta plain also has implications for current estimates of total subsidence determined from tide gauge data (Penland et al., 1989). Estimates of total subsidence based on tide gauge records may be underestimated by the amount of shallow subsidence shown at some sites in this study because shallow subsidence occurs at depths between the tide gauge base (similar to the SET base, Fig. 1) and the marsh surface. As such, total subsidence may be underestimated by as much as 2.5 cm/yr (e.g. Bayou Chitigue). Acknowledgements
J. Lynch assisted in collection of the accretion data, processing of soil cores, and preparation of figures. D. McNally collected the elevation data. R. Boustany, S. Fournet, M. Griffis, P. Hensel, R. Herbert, K. Landau, E. Pendleton, M. Radford, A. Stroy, M.A. Townson, and R. Young also assisted in field data collection and processing of soil cores. We are grateful to Louisiana Land and Exploration Company and Fina LaTerre, Inc. for allowing access to their properties in Louisiana. Personnel from Cedar Island NWR, the University of North Carolina, Marine Sciences Institute, and the National Marine Fisheries Service, Beaufort Laboratory provided essential logistical support of our sampling efforts at Cedar Island NWR. Personnel from St. Marks NWR, Florida Geological Survey (R. Hoenstine), Florida State University (J. Donoghue) and C.S. Giddens provided support of our sampling efforts at St. Marks NWR. D. Fuller and B. Pugesek provided advice on sampling design, and D. Johnson assisted in statistical analyses. A.L. Foote, P. Kemp, and B. Lock provided critical technical reviews and B. Vairin edited early drafts. This research was supported by the National Biological Service’s Southern Science Center with funds from the Department of the Interior, Global Climate Research Program. References Armentano, T.V. and Woodwell, G.M., 1975. Sedimentation rates in a Long Island marsh determined by ‘iOPb dating. Limnol. Occanogr., 20: 4522456.
Baumann, R.H., Day, J.W., Jr. and Miller, C.A., 1984. Mississippi deltaic wetland survival: sedimentation versus coastal submergence. Science, 224: 1093-1095. Boesch, D.F., Levin, D., Nummedal, D. and Bowles, K., 1983. Subsidence in coastal Louisiana: causes, rates, and effects on wetlands. U.S. Fish and Wildlife Service, Div. Biol. Serv., Washington, DC, FWS/OBS-83/26, 30 pp. Boesch, D.F., Josselyn, M.N., Mehta, A.J., Morris, J.T., Nuttle, W.K., Simenstad, C.A. and Swift, D.J.P., 1994. Scientific assessment of coastal wetland loss, restoration and management in Louisiana. J. Coastal Res., Spec. Issue, 20, 103 pp. Boumans, R.M.J. and Day, J.W., Jr., 1993. High precision measurements of sediment elevation in shallow coastal areas using a sedimentation-erosion table. Estuaries, 16: 375-380. Cahoon, D.R. and Turner, R.E., 1989. Accretion and canal impacts in a rapidly subsiding wetland II. feldspar marker horizon technique. Estuaries, 12: 260-268. Cahoon, D.R., Reed, D.J., Day, Jr., J.W., Steyer, G.D., Boumans, R.M., Lynch, J.C., McNally, D. and Latif, N. 1995. The influence of Hurricane Andrew on sediment distribution in Louisiana coastal marshes. J. Coastal Res., Spec. Issue, 18: 280-294. Childers, D.L., Sklar, F.H., Drake, B. and Jordan, T., 1993. Seasonal measurements of sediment elevation in three midAtlantic estuaries. J. Coastal Res., 9: 98661003. Dacey, J.W. and Howes, B.L., 1984. Water uptake by roots controls water movement and sediment oxidation in short Spartina marsh. Science, 224: 487-489. Day, J.W., Jr., Reed, D., Suhayda, J., Kemp, G.P., Cahoon, D., Boumans, R.M., and Latif, N., 1994. Physical processes of marsh deterioration. In: H.H. Roberts (Editor), Critical Physical Processes of Wetland Loss. U.S. Geol. Surv., Open File Rep., pp. 5.1-5.40. DeLaune, R.D., Patrick, W.H., Jr. and Buresh, R.J., 1978. Sedimentation rates determined by ‘37Cs dating in a rapidly accreting salt marsh. Nature, 275: 532-533. DeLaune, R.D., Nyman, J.A. and Patrick, W.H., Jr., 1994. Peat collapse, ponding and wetland loss in a rapidly submerging coastal marsh. J. Coastal Res., 10: 1021-1030. Gornitz, V., 1991. Global coastal hazards from future sea level rise. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 89: 379-398. Hackney, CT. and De la Cruz, A.A., 1980. In situ decomposition of roots and rhizomes of two tidal marsh plants. Ecology, 61: 226-23 1. Howes, B.L., Dacey, J.W.H. and Goehringer, D.D., 1986. Factors controlling the growth form of Spartina alternipbra: feedbacks between above-ground production, sediment oxidation, nitrogen and salinity. J. Ecol., 74: 881-898. Kaye, C.A. and Barghoorn, E.S., 1964. Late Quaternary sealevel change and crustal rise at Boston, Massachusetts, with notes on the autocompaction of peat. Geol. Sot. Am. Bull., 75: 63-80. Knaus, R.M. and Cahoon, D.R., 1990. Improved cryogenic coring device for measuring soil accretion and bulk density. J. Sediment. Petrol., 60: 622-623.
D. R. Cahoon et al. JMarine Geology 128 (I 995) I-9
Knott, J.F., Nuttle, W.K. and Hemond, H.F., 1987. Hydrologic parameters of salt marsh peat. Hydrol. Process., 1: 21 l-220. Manner, H.A., 1954. Tides and sea level in the Gulf of Mexico. In: Gulf of Mexico, its origin, waters and marine life. U.S. Fish Wildlife Serv., Fisheries Bull., 55: 101-l 18. McGinnis, L.D., Johnson, D.O., Zimmerman, E.Z., Isaacson, H.R., Penland, S. and Connor, P.F., 1991. Velocity depression and degasification subsidence in Louisiana wetlands. In: Coastal Depositional Systems in the Gulf of Mexico: Quatemary Framework and Environmental Issues. 12th Annu. Res. Conf., Gulf Coast Sect., SEPM Found., Houston, TX, pp. 128-133. Morris, J.T. and Whiting, G.J., 1985. Gas advection in sediments of a South Carolina salt marsh. Mar. Ecol. Prog. Ser., 27: 187-194. Nuttle, W.K., Hemond, H.F. and Stolzenbach, K.D., 1990. Mechanisms of water storage in salt marsh sediments: the importance of dilation. Hydrol. Process., 4: 1-13. Nyman, J.A., DeLaune, R.D., Roberts, H.H. and Patrick, W.H., Jr., 1993. Relationship between vegetation and soil formation in a rapidly submerging coastal marsh. Mar. Ecol. Prog. Ser., 96: 269-279. Nyman, J.A., Crozier, C.R., and DeLaune, R.D., in press. Roles and patterns of hurricane sedimentation in an estuarine marsh landscape. Estuarine Coastal Shelf Sci. Penland, S., Ramsey, K.E., McBride, R.A., Moslow, T.F., and Westphal, K.A., 1989. Relative sea level rise and subsidence
9
in Louisiana and the Gulf of Mexico. Coastal Geol. Tech. Rep., 3, Louisiana Geol. Surv., Baton Rouge, 65 pp. Penland, S., Roberts, H.H., Bailey, A., Kuescher, G.J., Suhayda, J.N., Connor, P.C. and Ramsey, K.E., 1994. Geologic framework, processes, and rates of subsidence in the Mississippi River delta plain. In: H.H. Roberts (Editor), Critical Physical Processes of Wetland Loss. U.S. Geol. Surv., Open File Rep., pp. 7.1.1-7.1.51. Reed, D.J. and Cahoon, D.R., 1993. Marsh submergence vs marsh accretion: interpreting accretion deficit data in coastal Louisiana. In: O.T. Magoon, W.S. Wilson, H. Converse, and L.T. Tobin (Editors), Proc. 8th Symp. Coastal and Ocean Management, Coastal Zone ‘93. (New Orleans, LA.) ASCE, New York, NY, pp. 243-257. SAS Institute Inc., 1989. SAS/STATR User’s Guide. Version 6, 4th ed., 2, Cary, NC. SAS Institute Inc., 1991. SAS’ System for Linear Models. 3rd ed., Cary, NC, pp. 272-274. Steever, E.Z., Warren, R.S. and Niering, W.A., 1976. Tidal energy subsidy and standing crop production of Spartina alternipora. Estuarine Coastal Mar. Sci., 4: 473-478. Stevenson, J.C., Ward, L.G. and Keamey, MS., 1986. Vertical accretion in marshes with varying rates of sea level rise. In: D.A. Wolfe (Editor), Estuarine Variability. Academic Press, Orlando, FL, pp. 241-259. Wigley, T.M.L. and Raper, S.C.B., 1992. Implications for climate and sea level of revised IPCC emissions scenarios. Nature, 357: 293-300.