Autocompaction of shallow silty salt marsh clay
Introduction
Sedimentary deposits in estuarine environments are related to sea level, and to the morphodynamic framework which controls deposition in the accommodation space left between the actual sediment surface and the highest high water level. As a consequence, it is natural (and also often most simple) to measure salt marsh deposition in the form of accretion rates. For a wide range of salt marshes this is reported to vary between a few and up to a few tens of mm year− 1 (e.g. Richard, 1978, Letzsch and Frey, 1980, Bartholdy and Madsen, 1985, Stevenson et al., 1986, Bricker-Urso et al., 1989, Cahoon and Turner, 1989, Oertel et al., 1989, Stoddart et al., 1989, French, 1993, Shi, 1993, Cahoon et al., 1995, Allen, 2000b, Pedersen and Bartholdy, 2006, Proosdij et al., 2006a, Goodman et al., 2007). Evaluation of salt marsh deposition from accretion data alone, however, involves problems because of autocompaction. This is apparent when accretion based on marker horizons and absolute levels are compared (e.g. Cahoon et al., 1995, Cahoon et al., 2000a, Cahoon et al., 2000b).
Richards, 1934, Nielsen, 1935 published (independent of each other) the first time series of salt marsh accretion based on direct measured accretion rates. They both used marker horizons consisting of coloured sand. Richards worked on the salt marsh of the Dovey (Dyfi) Estuary (west coast of Wales) and Nielsen on the backbarrier peninsula of Skallingen, Denmark. Shortly hereafter other researchers in both the UK and the USA engaged in salt marsh deposition (e.g. Steers, 1936, Steers, 1938, Chapman, 1938). Since these first studies based on direct measurements of salt marsh accretion, many studies using marker horizons have been published (e.g. Letzsch and Frey, 1980, Cahoon and Turner, 1989, Stoddart et al., 1989, French and Spencer, 1993, Shi, 1993, Bartholdy et al., 2004, Proosdij et al., 2006a, Goodman et al., 2007). Often these datasets indicate a relatively high initial deposition which decreases with time. This signature is expected when autocompaction creates subsidence of the marsh sediment, and should not be confused with a real change in the rate of deposition. This is important when e.g. analyzing the ability of salt marshes to keep pace with sea level rise on the basis of accretion rates measured by means of marker horizons.
The continuity equation related to vertical salt marsh accretion has been formulated by Allen (1990). It relates elevation change to the sum of net accretion due to clastic material and indigenous plant detritus minus autocompaction. The role of organic matter is hard to model, and consists of both positive and negative components (e.g. Callaway et al., 1996). For temperate primarily minerogenic salt marshes, as those considered in this paper, it is often regarded as insignificant and sometimes described by a small constant accumulation term (e.g. Allen, 1990, French, 1993). Any interpretation of relations between dynamics and the rate of deposition is restricted to deal with the amount of sediment in the form of weight concentration in the water flooding the salt marsh surface in question. When evaluating sedimentation as a result of salt marsh dynamics, the most appropriate measure, therefore, is weight per unit area and time, as achieved from sediment traps e.g. in French and Spencer, 1993, Proosdij et al., 2006b. In order to translate accretion rates measured in length per unit time to such results and vice versa, the bulk dry density and its variation in the uppermost layers become essential. The bulk dry density depends on grain size, organic content and level of compaction. For the same location, the sediment type might be regarded as constant. In such cases the level of autocompaction is usually related to the sediment deposited above the observed layer. Models relating this overburden to autocompaction are described in e.g. Skempton (1969), Temmerman et al. (2003), Allen, 1999, Allen, 2000a. The latter two are referring to the work of Skempton (1969). None of these attempts to describe the natural compacting behaviour of silty salt marsh sediments, however, manages to make a comprehensive description of this effect for the uppermost ≈ 0.5 m. This is a serious lack as many marker horizon studies and studies in general dealing with recent sedimentation in salt marsh environments derive inferences from this part of the sediment column.
This paper deals with an examination of results from field studies related to salt marsh compaction in the Danish Wadden Sea. Empirically founded algorithms are analyzed and combined in order to give an overview of the consequences related to autocompaction and to stress the necessity of relating accretion measurements to this, before such datasets are used to evaluate the sediment accumulation. The bulk dry density of the uppermost 5 cm salt marsh is related to its content of sand and organic matter. Furthermore, this measure of the surface sediments' density is used to describe the most likely density variation with depth under the salt marsh surface.
Section snippets
Study area
The data analyzed relates to salt marsh sediments in the Danish Wadden Sea. Samples of all major salt marsh areas from the northern part of the Wadden Sea and down to the Rømø Dam, connecting the island Rømø with the main land (Fig. 1) have been incorporated. Special interests have been paid to the large salt marsh area of Skallingen (the northernmost barrier spit) which comprises the largest natural salt marsh area in the Wadden Sea. Results from here are regarded to be comparable to results
Methods
Results derived from experiences and measurements carried out during about the last two decades (e.g. Bartholdy and Madsen, 1985, Bartholdy, 1997, Pedersen and Bartholdy, 2006, Pedersen et al., 2007) form the core of experimental data used (Dataset 1). In addition 37 samples (Dataset 2) of the topmost 5 cm across the backbarrier of the Skallingen Peninsula (Fig. 1.) have been analyzed for bulk dry density and loss on ignition. 11 of these samples have in addition been analyzed for grain-size
Measured bulk dry density
As the bulk dry density (BDD, kg m− 3) increases with depth beneath the salt marsh surface (Fig. 5), it depends on the layer depth. Thus, if the variation of BDD is to be related to sediment properties like organic matter (here described as the percentage weight loss on ignition, LOI), and sand content (here described as the weight percent sand, S), it has to be done for surface samples covering the same depth. A sample depth of 0.05 m was chosen as a tradeoff between accuracy and sample size. As
Example
This example is meant to describe the use of the suggested algorithms, and to illustrate their validity based on an independent dataset.
The surface elevation change at a test field on the Skallingen backbarrier salt marsh area has been monitored using a 1.5 m long aluminium rider mounted on two 1.5 m long aluminium sticks driven about 1.3 m into the surface. This consists of about 0.2 m salt marsh clay overlaying firm sand. Each record consisted of the mean of 29 measurements spaced 0.05 m apart
Summary and conclusions
This paper suggests empirically derived algorithms by means of which it is possible to evaluate relations between physical properties in the upper parts (≈ 0.5 m) of homogeneous silty salt marsh clay.
The following relations were found between the bulk dry density in the topmost 0.05 m (BDD0–0.05) and the weight percentage of loss on ignition (LOI) and sand content (S):
The latter equation represents a combination and gives
Acknowledgement
This paper was supported by the Danish Agency for Science, Technology and Innovation, Grant #: 272-06-0225. Kirsten Simonsen at the Skalling Laboratory is thanked for her steady and always helpful hand in performing grain-size analysis and solving logistic problems. Dan Olsen is thanked for running the gamma ray analysis. Martin Hermansen and Rasmus Ringaard offered invaluable help during the field work.
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