Implications for sea-level research of salt marsh and mudflat accretionary processes along paraglacial barrier coasts

MARINE GmwGY Marine

Geology

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Implications for sea-level research of salt marsh and mudflat accretionary processes along paraglacial barrier coasts SC. Jennings a, R.W.G. Carter b,l, J.D. Orford ’ a School of Geography and Environmental Studies, University of North London, UK b Department of Environmental Studies, University of Ulster, UK ’ School of Geosciences, Queen’s University, Belfast, UK

Received 8 March 1994; revision accepted 16 September 1994

Abstract Paraglacial coastlines with dynamic, coarse elastic barrier beaches are common in mid- to high-latitudes today, and have been during the Holocene. Sediments that have been deposited along these coastlines, often in back-barrier locations, have been used to reconstruct relative sea-level (RSL) movements and tendencies, usually through the use of sea-level index points, derived frequently from salt marsh and mudflat deposits. However, the asymptotic relationship between minerogenic marsh/mudflat accretion rates and time will produce regressive stratigraphies with a negative sea-level tendency signature, even within regimes of rising RSL. The influence of gravel barriers on sedimentation will also tend to encourage regressive stratigraphies. Furthermore, organogenic sediment production may release marsh accretion rates from the control of tidal inundation and result in succession to reed swamp despite RSL rise. Episodic minerogenic sediment supply to the marsh can trigger switches between marsh emergence and submergence, which may result in stratigraphic sequences corresponding to negative and positive tendencies of RSL respectively, although submergence may be prevented by organogenic sediment supply. Forcing by RSL rise may result in marsh emergence following initial submergence, as a result of renewed accretion on the submerged marsh.

1. Introduction and aims Stratigraphic examination of low-energy deposits within estuaries, lagoons and tidal marshes has played an important role in the reconstruction of past RSL movements. Central to this research is the use of stratigraphic data from (former) salt marshes and mudflats, usually through the identification of sea-level index points which are utilised to provide a temporal and spatial framework for RSL change and coastline position. This paper discusses some of the problems of using such stratigraphic data for RSL research, specifically ‘Deceased.

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from gravel-based, barrier coastlines in paraglacial settings, under conditions of longer-term (mid- to late-Holocene) RSL rise. This coastal type is common in mid- to high-latitudes (Forbes and Taylor, 1987). The paper concentrates on accretionary marshes; erosional marshes and fen and reedswamp stratigraphies are not considered. 2. Barrier+tuwsh relationships In mid- to high-latitudes, paraglacial sediment supply has resulted in gravel-barrier coastlines which provide at least partial enclosure of lowenergy mudllat and marshes. The barriers and the mudflats/marshes can be viewed as two systems

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coupled through an energy gradient and through sediment transport networks. In the absence of catchment-derived material, sediment is brought into the coastal system via glacial material eroded from unconsolidated cliffs or reworked from the shelf, which is then effectively partitioned with the coarser fraction (mostly gravel) forming the barrier beaches, while a proportion of the fines (clay, silt and sand) is transported into the mudflat/marsh areas where it may be augmented by organogenic material. The result is a highly complex pattern of sedimentation, within both the barrier and marsh systems. A good example of this situation is provided by Chezzetcook Inlet, on the Eastern Seaboard of Nova Scotia. Chezzetcook Inlet is an infilling, glacially scoured valley. Late-Holocene RSL has risen at a rate of 2.0-3.0 mm/yr (Scott, 1980; Scott et al., 1987; Shaw and Forbes, 1990), although data from the Halifax (Nova Scotia) tide gauge show a 70-year trend averaging 3.8 mm/yr, but with substantial inter-annual variations. Sediment supply to the inlet is predominantly local and glacigenic, originating from drumlin headlands (Carter et al., 1992), and the sediment-transport system is dynamic as evidenced during this century by barrier-beach retreat rates of up to 8 m/yr, while within the inlet, flood-tidal deposits have accreted rapidly (Scott, 1980; Carter et al., 1990, 1992; Forbes et al., 1991; Orford et al., 1991, 1995; Stea et al., 1992). Cat&rent-derived sources of sediment have been important only over the last 2OOG300years, and confined largely to the head of the inlet (Scott, 1980). Carter et al. (1992) and Jennings et al. (1993) reported on the considerable differences in intraestuarine sedimentation patterns during the lateHolocene within Chezzetcook Inlet, and cautioned against the use of stratigraphic data from such environments to reconstruct Holocene RSL movements, because the estuarine system appeared to be driven by local processes of sedimentation rather than by vertical oscillations of RSL. Jennings et al. (1993) suggested that the complexity of the sedimentation within Chezzetcook Inlet, particularly in the outer estuary, resulted from a depositional form of equifinality, exemplified by the factors that could cause a switch from

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minerogenic to organogenic salt marsh sedimentation. They argued that this equifinality had two components, namely a sea-level and a barrierdynamics component. The sea-level component could effect a switch to an organogenic marsh through either a short-term (storm-induced) sealevel rise that, by the transfer of sediment onto marshes, could elevate marsh surfaces to a critical elevation when organogenic deposition may dominate (see Stumpf, 1983); or by a longer-term stillstand or fall in RSL when continuing marsh accretion is likely to be organogenic, and may lead to the development of reedswamp or fen carr. The barrier-dynamics component could encourage organogenic marsh formation by the breakdown of estuary-mouth barriers that could inject a sediment pulse onto the marshes, either directly via overwash or by contributing sediment to the development of flood-tidal structures. Conversely, barrier stability and progradation would encourage back-barrier deposition by creating prolonged periods of low-energy conditions, when catchmentderived and flood-tidal sediment could accumulate, elevating the marsh surface and allowing the potential for an organogenic marsh to form. The deposition associated with this form of equifinality would encourage the formation of regressive stratigraphies, even with a rising RSL. This conceptual framework, based upon the stratigraphic examination of sediments within Chezzetcook Inlet, will be developed further, with specific reference to salt marsh and mudflat accretion, and the implications of this for RSL research.

3. Salt marsh and mudflat sedimentation 3. I. Asymptotic marsh accretion rates The main factors which govern salt marsh and mudflat sedimentation are: the age of the marsh and mudflat; the rate and direction of RSL movement and the associated tidal range, which are likely to have changed through the Holocene; and the rate and type of sediment influx to the marsh and mudflat, which may be influenced strongly by the morphodynamic nature of an enclosing barrier or spit.

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An asymptotic relationship has been observed between minerogenic marshand mudflataccretion rates and time (Kestner, 1975; Pethick, 1981; Allen, 1990a,b; French, 1993; French and Spencer, 1993). This is shown schematically on Fig. la. There are several points of interest to RSL research arising from this relationship: (1) Point B on the graph marks the attainment of marsh maturity when accretion rates are in equilibrium with RSL rise. Maturity is likely to take 10’ to lo3 years, and represents a marshsurface elevation from between approximately MHW to below HAT levels (Pethick, 1981; Allen 1990a,b; French, 1993). Maturity is usually synonymous with the establishment of high marsh (eg. Redfield, 1972; Kidson and Heyworth, 1979), and follows a period of maturation time when accretion rates, typically on mudtlats and low marsh, are faster than RSL rise, although Letzsch and Frey (1980) found marsh maturation arrested in a submature stage, implying that immature, low marshes may remain in balance with RSL rise. (2) The establishment of an asymptotic relationship between marsh/mudflat-accretion rates and time will produce regressive marsh stratigraphies, even though RSL is rising. The graph of Fig. la plots an initial mudflat, then an immature (low) marsh surface which is rising within the tidal frame, and culminates with the establishment of a high, mature marsh. The resultant lithostratigraphy is likely to show a transition from an inorganic silty clay or silty sand of the mudflat and low marsh stages, to a clay to silty clay high marsh, which will contain an increasing amount of organic material. A transition to an organogenic marsh may then occur, as found for example, along parts of the eastern seaboard of North America, provided the rate of organogenic production permits the elevation of the marsh surface to equal or exceed RSL rise (see Allen, 1990a, b). The biostratigraphy will reflect this regressive transition from mudflat to high marsh, although there are problems of using biostratigraphic data from these environments (Kidson and Heyworth, 1979; Van de Plassche, 1986). Increased minerogenic supplies to a marsh may trigger rapid accretion, possibly resulting in the establishment of mature, high marsh, overriding

MAR = MARSH ACCRETION RATE RSLR - RELATIVE SEA-LEVEL RISE .

-THRESHOLD

lb MARSHEMERGENCE MARSH

MATURIN

MARSH

SUBMERGENCE

Fig. 1. (a) Schematic representation of the asymptotic relationship between minerogenic salt marsh surface elevation (i.e. accretion rates) and time. A RSL rise is assumed. A = initiation of mudflat (or low marsh). B=attainment of marsh maturity. C= present day or termination of mature marsh (after Pethick, 1981; Allen, 1990a). An accretion rate and marsh surfaceelevation threshold separates the immature from the mature marsh. (b) Schematic modification to Fig. la to allow for episodic sediment influx, achieved through input of minerogenic and organogenic material, leading to phases of marsh surface emergence and submergence. The resultant stratigraphy would depict alternations between negative and positive RSL tendencies.

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the RSL signal. This may occur catastrophically through the deposition of washover material onto the marsh surface (Jennings et al., 1993), in which case the stratigraphic evidence for this (local ) negative sea-level tendency will be clear, but more subtle increases in minerogenic input may be difficult to recognise in a stratigraphy. This is another facet of the complexity of mudflat/marsh accretion, and stresses the need to establish detailed lithostratigraphic controls, so that the spatial extent and relationship of lithostratigraphic horizons may be identified. This may then allow the recognition of structures such as flood-tidal deltas (e.g. Carter et al., 1992) which can provide a platform for subsequent marsh growth. These structures will constitute areas of negative sea-level tendency within an estuary as a result of increased supplies of mostly minerogenic sediment (3) The establishment of maturity is a critical phase in the development of a marsh as it represents a threshold separating two domains of marsh dynamics (Fig. la); one (the immature domain) when accretionary processes drive the system independent from longer-term RSL rise, the other (the mature domain) when marsh accretion rates are in equilibrium with RSL rise. It is therefore, only on mature marshes that reliable stratigraphic data on rates of RSL rise can be obtained, although changes in the vertical direction of RSL movement will not be recorded reliably (Allen, 1990a). However, the broad altitudinal range over which a high, mature marsh can become established (MHW to below HAT) will impart an error term on the water level represented by a sea-level index point from a mature marsh (see Heyworth and Kidson, 1982; Shennan, 1982, 1986). Also, the elevation of the mature marsh may be increased incrementally by storm-surge deposits, usually sand lenses, so that accretion rates on high marshes may be punctuated by sudden influxes of sediment (see French and Spencer, 1993). 3.2. ModiJication to the asymptotic

growth pattern

The time-dependent nature of accretion rates on minerogenic marshes may be modified by a number of factors, for example by compaction of the marsh sediments or by rapid vertical movements of RSL.

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Also of importance will be variations in the amount of sediment supplied to the marsh, the relative importance of minerogenic and organogenie supplies, and the balance between supply and RSL rise (see Allen, 1990a). An increase in organogenic sediment input, mostly through in situ production but including allochtonous material, allows marsh accretion rates a degree of independence from tidal inundation. Succession from high marsh to reedswamp may take place if organogenic production can outpace RSL rise. These emergent, organic marshes were recognised in the Chezzetcook Inlet stratigraphies from pollen data that showed the development of reedswamp and fen carr following the establishment of high marsh (Jennings et al., 1993). As discussed below, these successional trends were not thought to be a response to a fall or slower rise in RSL, so the rate of vertical marsh growth (and allowing for compaction) must have exceeded the late-Holocene RSL rise rate of up to 3.0 mm/yr, or possibly 3.8 mm/yr if tide gauge records of the last 70 years give rates of rise analogous to the late-Holocene. With RSL rise, the termination of a minerogenic pulse of sediment may induce a stochastic response by the marsh. If the original sediment pulse has promoted the development of an organogenic marsh, then marsh maturity may be sustained provided organic productivity can keep pace with or exceed RSL rise (see Allen, 1990a,b). Alternatively, the marsh may submerge following a reduction in minerogenic sediment supply, particularly if there is no compensatory organogenic production. Under these circumstances, the marsh is likely to switch between three domains: emergence, maturity and submergence, with thresholds in sediment supply and RSL rise rates separating these domains (Fig. lb), resulting in episodes of negative and positive RSL tendency. Emergence and submergence episodes may be recognised from lithostratigraphic and biostratigraphic analysis (e.g. pollen, diatoms and forams). However, lowmagnitude submergence and emergence following the establishment of marsh maturity may not be manifest in the lithostratigraphy, occurring within a uniform high marsh peat or clay (see discussion below).

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3.3. Lags in the marsh system The oscillatory nature of the graph in Fig. lb schematically represents the consequences to a marsh of a (mostly) minerogenic sediment supply flux, given rising RSL. The question arises, however, as to whether RSL oscillations would produce the same marsh response. One approach is to assume that the marsh emergence and submergence phases on Fig. lb are synonymous with falls and rises in RSL respectively. Allen (1990a) has discussed this situation, but observed that this type of marsh response is in effect a lag. For example, submergence of a minerogenic marsh due to RSL rise is likely to trigger accelerated accretion (assuming the availability of sediment) as a result of increased tidal inundation. The point at which accretion leads to renewed emergence will occur when the rate of sediment supply exceeds the rate of RSL rise so that the asymptotic marsh-growth pattern is re-established. Therefore, marsh emergence following submergence may reflect this accretionary response to a RSL rise, rather than signal a subsequent fall in RSL. A simple correlation between marsh submergence and RSL rise, or marsh emergence and RSL fall is unfounded unless supported by evidence for regionally synchronous movements of RSL (Shennan, 1986). An episode of (increased) RSL rise should produce a coupled positive then negative tendency signal in the marsh stratigraphy. This assumes that the RSL rise has an impact on the marsh such that the initial submergence phase induces a shift within the high to low marsh spectrum. If this is correct, then forcing by RSL rise will produce a stratigraphic signal indistinguishable from that shown in Fig. lb for sediment fluxes, with the submergence (or positive tendency) episode representing a lag in the system (see also Shennan, 1989, in Allen, 1990a). Kestner (1975) reported that with the development of salt marsh from mudflat, most of the accretion occurred during the mudflat phase, before colonisation by halophytic plants. Once established, the marsh plants subsequently increased accretion rates faster than would have been expected on an unvegetated mudflat of equivalent elevation. Kestner’s examples of marsh

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accretion, from the Wash, were associated with embankments and artificial barriers, but provide further evidence of important lags in the mudflat/marsh system. The implications of Kestner’s observations for RSL research lie in the recognition that regressive sedimentary sequences are initiated before any change to biogenic deposition. As a consequence, regressive overlaps will lag the inception of negative RSL tendencies (horizontal and/or vertical RSL movements), and there is likely to be a lag in the biostratigraphic response to a negative RSL tendency if tidal flats and channels are maintained during the early stages of accretion, as implied by Kestner’s work. As Long (1992) has discussed, transgressive and regressive contacts are likely to lag the biostratigraphic record, necessitating careful consideration when dating RSL tendencies. The implication here is that lags within the mudflat/marsh system fall within the resolution of radiocarbon dating.

4. Discussion Heyworth and Kidson (1982, p. 96) in a summary statement of an earlier paper, commented on “the generally unsuitable nature of saltmarsh deposits for accurate sea-level determinations”. One element of this general unsuitability is the probability that marsh development can be driven by accretionary processes rather than by RSL movements. We have attempted to illustrate this by suggesting that because salt marshes can develop irrespective of sea-level rise, and because of the asymptotic nature of this development, regressive stratigraphies will tend to be produced, a trend enhanced by organogenic marsh formation. Work from Chezzetcook Inlet has shown that regressive sequences can form despite a RSL rise rate of 3.0-3.8 mm/yr, and that even under these rates of rise, organogenic marshes and reedswamp can also develop (Jennings et al., 1993). However, the pattern of accretion was more complex than just the establishment of regressive stratigraphies. Episodes of marsh surface emergence and submergence were recognised by biostratigraphic (pollen) evidence, mostly from within organogenic marsh deposits, indicating a complex, oscillatory relation-

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ship between marsh accretion rates and RSL movement. Examination of the cause of this relationship exemplifies a critical issue within RSL research, namely the viability of distinguishing between sediment-driven and RSL-driven changes in marsh stratigraphies, as discussed above. If this relationship between the Chezzetcook marshes and RSL has been driven by RSL movements, then a correlation between emergencesubmergence episodes and RSL movements should be apparent. Van de Plassche ( 1991) using stratigraphic data from the eastern seaboard of North America (specifically, a Connecticut salt marsh), has suggested a sequence of global RSL oscillations, comprising phases of ‘accelerated’ and ‘slow’ MHW rise, during the last 2000 radiocarbon years, a period covered by the Chezzetcook Inlet stratigraphies. There is no clear correspondence between the marsh emergence and submergence episodes identified in Chezzetcook Inlet with the proposed oscillations in MHW rise rate from Connecticut, although a greater number of dated sea-level index points from Chezzetcook Inlet would be needed to confirm this. However, two sites within Chezzetcook Inlet had an emergent marsh episode around 500 radiocarbon yrs B.P. (Jennings et al., 1993), which correlates with a period of ‘accelerated’ MHW rise on the Connecticut marsh (Van de Plassche, 1991). This correlation may not be a paradox because, as discussed above, a marsh’s response to RSL rise may be first to submerge followed by emergence, and such a pattern is noticeable at the two sites in Chezzetcook Inlet at around that time suggesting that changes in marsh stratigraphies were driven by changes in the rate of RSL rise. However, it is also possible that marsh emergence was triggered in Chezzetcook Inlet at this time by enhanced accretion due to storm activity associated with the Little Ice Age (see Stumpf, 1983; DeLaune et al., 1986; French and Spencer, 1993). The paraglacial setting of the inlet, with drumlin headlands, provides scope for increased minerogenic sediment supplies during periods of greater storminess. At the two sites where this marsh emergence phase was recorded, the associated lithostratigraphy was a sandy peat. At one of these sites, the sandy peat was soon buried by

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washover gravel from a barrier, while at the other, a sandy-organic high marsh developed upon washover material at this time. Subsequent submergenceeemergence episodes, identified by fluctuations in the amount of aquatic salt marsh, high marsh and reedswamp taxa pollen, were probably due to organic productivity being out of balance with RSL rise, which includes the possibility of variations in the rate of rise. The formation of lagoons due to barrier migration was also an important factor. It would appear that with marsh stratigraphies like those from Chezzetcook Inlet, it is probably impossible to distinguish with any certainty, stratigraphies that reflect a response to RSL change from those responding to sediment supply variations, the latter possibly linked to the impact of storm-surge events. However, mineral magnetic studies may identify episodes of influx of catchment-derived sediment, for example due to anthropogenic disturbances, which may correlate with regressive marsh stratigraphies. Carter et al. (1992) used this technique in Chezzetcook Inlet, and the results supported Scott’s conclusion that high marsh has developed at the head of the inlet over the last 300 years due to the inwash of sediment from catchment sources following the arrival of European settlers to the area, and subsequent road-building activity (Scott, 1980).

5. Conclusion The asymptotic growth pattern of minerogenic marshes will tend to produce negative sea-level tendency stratigraphies, irrespective of RSL rise. A switch from minerogenic (low) marsh to organogenie (high) marsh, due to organic productivity being at least equal to RSL rise, will enhance this tendency. When a RSL rise induces initial marsh submergence (a positive sea-level tendency), a subsequent negative tendency in marsh stratigraphies will be produced, as renewed accretion begins to raise the marsh surface relative to the new tidal frame. Such a stratigraphy may mistakenly be interpreted as a vertical oscillation in RSL, rather than as a single RSL rise event. Morphodynamic changes to gravel barriers will

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also tend to favour negative sea-level tendencies in associated marsh stratigraphies through washover sediments, and by contributing sediment to flood-tidal structures on barrier breakdown. Stationary or prograding barriers will also encourage areas of negative tendency in their energy lee, unless they form seepage lagoons which, in the absence of catchment-derived sediment, will be areas of sediment starvation and marsh submergence, unless offset by organic productivity. One consequence of the complexity of marsh accretionary processes is the difficulty of distinguishing between marsh response to forcing by RSL and by sediment supply, which stresses the importance of establishing detailed stratigraphic controls. Allied to this is the need to recognise linkages between sea-level change, sediment supply and marsh accretion, both on short time scales (ie. storm-surge events) and on longer (Holocene) time scales. A clearer understanding of the processes of marsh accretion (and erosion), within a Holocene context, is of paramount importance for sea-level research.

Acknowledgements

We thank Don Forbes, Dave Frobel, John Shaw and Bob Taylor of the Atlantic Geoscience Centre (Bedford Institute of Oceanography, Halifax, Nova Scotia) for fieldwork support; the Geological Survey of Canada and the Natural Environment Research Council (NERC), (UK) for financing the radiocarbon dating; and John Gibbs for drawing the diagrams. We gratefully acknowledge the Canadian High Commission (London) and NERC (grant GR 3/7711) for financial backing. This is a contribution to the International Geological Correlation Programme (IGCP) Project 274, Coastal Evolution in the Quaternary.

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