Peat formation in the context of the development of the Mkuze floodplain on the coastal plain of Maputaland, South Africa

Geomorphology 141-142 (2012) 11–20

Contents lists available at SciVerse ScienceDirect

Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Peat formation in the context of the development of the Mkuze floodplain on the coastal plain of Maputaland, South Africa W.N. Ellery a,⁎, S.E. Grenfell b, M.C. Grenfell c, M.S. Humphries d, K. Barnes d, A. Dahlberg e, A. Kindness d a

Department of Environmental Science, Rhodes University, Grahamstown, South Africa SEACAMS, Department of Geography, Swansea University, Singleton Park, Swansea, UK Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, UK d School of Chemistry, University of KwaZulu-Natal, Durban, South Africa e Department of Physical Geography and Quaternary Geology, University of Stockholm, Stockholm 106 91, Sweden b c

a r t i c l e

i n f o

Article history: Received 12 July 2011 Received in revised form 16 November 2011 Accepted 17 November 2011 Available online 13 December 2011 Keywords: Tributary impoundment Peat formation Floodplain development Coastal plain wetlands Blocked-valley lakes Carbon sequestration

a b s t r a c t This paper examines the geomorphological and sedimentological development of blocked-valley lakes in the Mkuze floodplain on the coastal plain of Maputaland, northern KwaZulu-Natal, South Africa. Blocked tributary valley lakes north of the floodplain become progressively shorter, broader, and less linear toward the eastern (downstream) end of the east–west oriented Mkuze floodplain. Clastic sediment forms surface sedimentary fill in tributary valleys in the west, while peat predominates tributary valley fill in the east. Two contrasting adjacent tributary valleys were examined, the more western Yengweni dominated by clastic sediment at the surface, and the more eastern Totweni with peat. The Mkuze floodplain is characterised by silt with a low organic content. Surface sediments fine downstream and with distance from the main channel. Tributary sediment south of the lakes (adjacent to the floodplain) contains little organic material at the surface, but increases with depth. North (upstream) of Yengweni lake, the tributary valley contains peat up to 1.5 m thick, with organic contents up to 30% (generally 10 to 20%). In contrast, north (upstream) of Mpanza lake, peat up to 7 m thick is extensive with high organic contents (typically >60% at the surface but decreasing with depth). The thickness and width of the peat deposits increase longitudinally from the head of the tributary valley toward Mpanza lake. The distribution of clastic and organic sediments illustrates that as aggradation of the Mkuze floodplain progresses, tributary valleys initially fill with sediment from the local tributary catchment, lakes form, there is a phase of peat formation and finally, peat is buried by sediment from the Mkuze floodplain. We hypothesise that peat formation in subtropical and tropical settings through these processes is likely to be an important long-term sink for carbon. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Peat formation occurs where local plant productivity exceeds organic decomposition by microbial activity (Moore, 1987, 1989). Thus, net accumulation of organic material requires high plant productivity, but also sustained impedance of aerobic and anaerobic microbes caused by poor accessibility and palatability of the organic resource (Chimner and Ewel, 2005), low temperature (Montanerella et al., 2006), low pH associated with the presence of humic acids in particular geological settings (Cecil et al., 1985), low nutrient levels in inflowing waters, and most importantly, low oxygen availability (Moore, 1989). Anoxia associated with permanent flooding is considered the principal cause of peat formation globally (Moore, 1987, 1989; Belyea and Clymo, 2001; Dommain et al., 2010), primarily from the observation that humification may progress up to three

⁎ Corresponding author. Tel.: + 27 046 603 7003; fax: + 27 046 622 9319. E-mail address: [email protected] (W.N. Ellery). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.11.009

orders of magnitude more rapidly in the acrotelm (relatively oxygen-enriched upper layer of the peat profile) than in the anoxic catotelm (Ingram, 1987). Given this dependence on sustained waterlogging, peat formation in southern Africa is an enigma. Despite the regional negative water balance associated with low rainfall and high potential evaporation (Ellery et al., 2009), peatlands are present and may be locally abundant. The coastal plain of northern KwaZulu-Natal is an important peat ecoregion in South Africa (Smuts, 1992; Thamm et al., 1996; Grundling et al., 1998) and hosts the greatest area of peatland in the country. The abundance of peat in this area offers an opportunity to further our understanding of the physical controls on peat formation in subtropical and tropical environments, which collectively contain ~ 10% of the total carbon stored in peatlands (Immirzi et al., 1992). Because many subtropical and tropical peatlands occur in floodplain blocked-valley lake environments (Blake and Ollier, 1971; Grenfell et al., 2010), understanding the interplay between clastic and organic sedimentation provides vital context for any effort aimed at quantifying rates of peat formation, and understanding the

12

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

long-term fate of peat and its relationship with landscape settings and development. Although we understand that the ash content (inorganic fraction) of peat is greater in settings with allochthonous sediment input associated with hillslope, channel, or channel-overbank flow (Moore, 1989), little attention has been paid to the geomorphic setting in which peatlands occur or to the geomorphic dynamics that affect peat formation and determine the long-term fate of organic sediments. The aim of this study is therefore to investigate the geomorphic setting in which peat forms in the Mkuze wetland, and to consider its distribution within the context of landscape development. 2. Study area The Mkuze River has a catchment of ~5250 km 2, reaching its highest point at an altitude of 1600 m near the coal mining town of Vryheid. As it flows eastward toward the coast, the gentle undulating hills of the Vryheid area switch to deep, entrenched meanders, confining the Mkuze River as it flows through the Lebombo Mountains. Rivers on the coastal plain were rejuvenated during the Last Glacial Maximum when sea levels were ~ 120 m lower than at present (Ramsay, 1995). The Mkuze floodplain, is bounded to the west by the Lebombo Mountains, a north–south trending belt composed of rhyolitic and basaltic rocks through which the Mkuze River cuts a deep gorge. The Maputaland coastal plain, which lies to the east of the Lebombo Mountains, is an expansive grassland region with a mosaic of lakes and wetlands separated by low sand ridges (Watkeys et al., 1993). The region is characterised by a veneer of Quaternary aeolian dune sands (KwaMbonambi Formation) underlain by a marine sedimentary succession of Cenozoic age. The low dunes with a relief of ~20 m have a general north–south orientation, between which are a number of similarly trending drainage lines that culminate in lakes at their southern boundary, such as the Muzi, Yengweni and Totweni drainage lines (Fig. 1A). The dunes are cut perpendicularly by the east–west trending Mkuze floodplain, a clastic system dominated by silt and clay. The floodplain terminates on its eastern margin at the confluence of the Mkuze River and the north–south orientated

Mbazwana swamp, the combined flow of which enters a northern embayment of Lake St. Lucia via a sinuous, discontinuous channel. The entire complex of floodplain wetlands and lakes, up to the northern margin of Lake St. Lucia, covers an area of some 56 km 2. The aeolian sands that comprise the KwaMbonambi Formation accumulated during multiple dune mobilization events (Porat and Botha, 2008). As a result, the dunes west of Muzi lake are 18.7 ± 2.4 ka old, while toward the east they are slightly younger at 15.4 ± 1.9 ka. At the Mbazwana swamp, the sequence becomes more complex, with multiple phases of dune formation, the oldest occurring 53.2 ± 3.6 ka, and the most recent dune-building event occurring 10.0 ± 1.9 ka. The easternmost southward-flowing drainage lines between the dune fields, notably the Totweni drainage line (drains into Mpanza and Mdlanzi lakes) and the Mbazwana swamp, have become loci of peat formation. Based on radiocarbon dates from peat cores collected throughout this regional wetland complex, Grundling (1996, 2004) suggested that peat formation peaked in the area between 5 and 4 ka. This timing is consistent with the onset of peat formation in a similar setting ~60 km south at Lake Futululu on the Mfolozi floodplain, which began 3.98 ka (Grenfell et al., 2010). Maputaland has a humid, subtropical climate. Noon temperatures average 29 and 23 °C in January and June, respectively, while average temperatures drop at night to 21 °C in January and 12 °C in June. Precipitation is largely restricted to the summer months (November through March) when ~80% of the rainfall occurs (Tyson, 1986), peaking in early summer at the headwaters of the Mkuze River and in late summer toward the coast. Summer rainfall is often associated with easterly low pressure cells that can remain in the region for up to 10 days, as well as infrequent tropical cyclones that may affect coastal regions (Tyson and Preston-Whyte, 2000). During winter, the passage of cold fronts and coastal low pressure systems may also bring rain. Mean annual precipitation is locally variable, averaging 1288 mm.a − 1 at the coastal town of St. Lucia, dropping to 605 mm.a − 1 in the central catchment, and reaching 1262 mm.a − 1 at Hlobane on the westernmost catchment watershed. Mean annual

Fig. 1. The Mkuze floodplain (A) and study area (B) showing locations of topographic surveys and coring (T1 to T4) and tributary valley sampling and the locations of numbered figures.

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

potential evapotranspiration is high in the study area, with atmospheric demands ~ 1700 mm.a − 1 at the coast, increasing to ~ 1800 mm.a − 1 at the head of Mkuze River catchment (Schulze, 1997). The mean annual discharge of the Mkuze River is 4.17 m 3.s − 1, as recorded at the lower Mkuze bridge between Yengweni and Muzi lakes over a period of 17 years between 1970 and 1988 (when measurement stopped because of flood damage). Discharge is highest during the summer months, peaking in February with a mean discharge of 10.47 m 3.s − 1. Lowest discharges are typically recorded during August (mean monthly discharge 0.89 m 3.s − 1). Interannual flow variation on the Mkuze River is high with a coefficient of variation of 68.8%, compared to global norms of between 20 and 45% (Dettinger and Diaz, 2000). Mean daily flow varied from 10.675 m 3.s − 1 in 1975/1976 to a mere 0.014 m 3.s − 1 in 1982/1983. The highest daily discharge recorded was 69.9 m 3.s − 1 on 2 March 1975; while on 8.89% of days gauged, the Mkuze River was dry. At discharges of 17 m 3.s − 1, the Mkuze River flows inundate the floodplain from overtopping of the natural levees. Water flows into tributary valley lakes (Ellery et al., 2003) and sediment is deposited on the floodplain, with most deposition taking place on and adjacent to the levee. Based on data collected on the floodplain, decadal-tocentennial-scale sedimentation rates determined using 137Cs and 210 Pb suggest that the Mkuze floodplain is aggrading vertically at a rate of ~ 2.5 to 5.0 mm.a − 1 (Humphries et al., 2010b). High evapotranspiration rates result in increased solute concentration of local groundwater, causing minerals such as CaCO3 and SiO2 to precipitate. Chemical sedimentation appears to be important in distal reaches of the floodplain (Humphries et al., 2010a, 2011), where accumulation influences sediment properties, hydrological flow and local topography.

3. Methods Topographic surveys of the Mkuze floodplain were conducted at four locations (T1–T4; Fig. 1B) using an automatic level and staff

13

(accuracy of ~ 0.01 to 0.02 m). Coring for particle size distribution and organic content was undertaken using a gouge corer with a diameter of 0.015 m, a soil auger for clastic sediment where gouge coring was not possible owing to the hardness of the sedimentary material, and a Russian peat corer for organic sediment. Samples were taken at a depth interval of 0.5 m or less or where a striking change in stratigraphy was evident. Particle size distribution was measured using a Malvern Mastersizer 2000 that employs laser diffraction to provide percentage frequency data in 100 size classes on a logarithmic scale from 0.02 to 2000 μm. Ash content was measured by combustion in a muffle furnace at 450 °C for a period of at least 6 h. Peat thickness in the Totweni drainage line, other than where peat was sampled by coring, was measured in transects by pushing a galvanized iron pole through the peat until refusal indicated that the underlying KwaMbonambi Formation sands had been reached, an assumption that was tested by coring. Transect starting points were recorded using a differential GPS with a local base station (accuracy of altitude to within 0.1 m and of latitude and longitude to within 0.05 m). Valley width was measured using a tape measure from the starting point, and peat thickness and core positions were recorded along the tape measure.

4. Results 4.1. Mkuze floodplain sedimentology South of Yengweni lake, the Mkuze River possesses a levee that is elevated ~ 2 m above the adjacent floodplain (Fig. 2). Apart from a minor channel regularly used by hippopotami that occupies the central floodplain, the floodplain surface is fairly uniform and elevated ~1 m above the water level in Yengweni lake. At the time of sampling, the Mkuze River was dry; but the elevation of the water table was close to the channel bed. In contrast, the water table was ~ 3 m below the floodplain surface. The organic content of floodplain sediment south of Yengweni lake is very low at depth (2–3%), with a slight increase toward the floodplain surface

Fig. 2. Topography, groundwater elevation, core locations, depths of samples, sediment organic content and particle size distribution of the Mkuze River to Yengweni Lake.

14

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

to ~5% (Fig. 2). The Mkuze floodplain is coarsest on the levee (where fine sand dominates) and finest between the Mkuze River and the hippo trail, as well as immediately adjacent to Yengweni lake, where silt makes up a large fraction of the sediment (Fig. 2). In general, particle size decreases with increasing depth in all floodplain cores south of Yengweni lake. South of the Totweni drainage line, the Mkuze River levee is indistinct and only elevated ~ 1 m above the adjacent floodplain surface. The elevation of the floodplain surface is fairly regular, apart from a broad depression south of the Mpanza lake (Fig. 3). In this case the water table was ~ 1 m below the bed of the Mkuze River and sloped gently upward toward the lake. On the levee and close to the Mkuze River south of the Totweni drainage line (cores F1.1 and F1.2), organic contents are typically 5–10% at a depth >2 m (Fig. 3). At a depth of 2 m, organic content declines to b5% but then increases fairly systematically to ~10% at the surface. In the remaining cores on the floodplain south of the Totweni drainage line, excluding the elevated lake margin (i.e., cores F1.3 to F1.5), organic content is typically b5% at depth and increases to ~10% at the surface. In the slightly elevated lake margin (core BF1), organic contents are variable at 10 to 20% at a depth >2 m. At shallower depths, organic content is b10% but increases systematically upward. The organic content of sediment at the lake margin (core BF2) is nearly 40% at the surface but decreases to b20% at 0.5 m depth. Organic content declines to 10–15% at a depth of 2.5 m, below which it varies from >20% to about 5%. Surface floodplain sediments south of the Totweni drainage line are slightly finer overall than south of Yengweni lake. However, similar trends in particle size are observed at both sites in that sediment is coarsest close to the Mkuze River and becomes progressively finer toward the Mdlanzi lake. A general decrease in particle size with increasing depth in each core was also evident (Fig. 4). The lower floodplain is characterised by a very narrow (b15 m wide) and shallow (b1.5 m deep) channel with a small levee adjacent to a depression on the floodplain surface such that the top of the levee is at the same elevation as the distal floodplain surface (Fig. 5). The water table slopes gently away from the channel, and the particle size of sediment is finer in this transect than that south of Yengweni

Fig. 4. Topography, groundwater elevation, core locations, depths of samples, and particle size distribution of sediments of the Mkuze River to Mdlanzi lake (see Fig. 2 for location of transect).

lake or the Totweni drainage line. Once again sediment is coarsest in the channel levee and fines progressively away from the channel and is typically coarsest at the surface and fines downward in the cores (Fig. 5). 4.2. Tributary valley sedimentology and recent planform changes In the vicinity of the southern margin of Yengweni lake, the organic content increases markedly from 5 to 10% at depth, to >10% near the surface (Fig. 6). North of Yengweni lake, sedimentary fill overlying KwaMbonambi aeolian sands is thin (0.4 to 1.5 m), and organic

Fig. 3. Topography, groundwater elevation, core locations, depths of samples, and sediment organic content of the Mkuze River to Mpanza lake (see Fig. 2 for location of transect).

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

15

Fig. 5. Topography, groundwater elevation, core locations, depths of samples and particle size distribution of floodplain sediments of the eastern Mkuze floodplain from the Mkuze River to the floodplain margin (see Fig. 2 for location of the transect).

content increases from immediately above the contact with the aeolian sand (organic content 2%) to 10–15% at the surface. Organic contents below the bed of Mpanza lake are highly variable, particularly at a depth >3 m (values varied between 5 and 25%) and between 2 m depth and the bed of the lake (values varied between 2 and 45%; Fig. 7). Upstream along the tributary, sediments have organic contents consistently >40% and sometimes as high as 90%. Organic content does not vary consistently with depth, but the overall width and thickness of organic sediment increases systematically downstream along the drainage line. At the head, organic sediment deposits were 40 m wide and 0.8 m thick; while at the toe of the drainage line, they were 500 m wide and greater than 6 m thick (Fig. 7). The valley occupied by the peat deposits is generally deeper in the west and slopes more gently toward the east than to the west.

Based on aerial photography, the southern margin of both Yengweni and Mdlanzi lakes has shifted northward from 1957 to 1990, and the western margin of Mdlanzi lake expanded over the same period (Fig. 8). Over the same 1957–1990 period, the Mpanza lake formed, having been absent in 1937 and 1957 photography. 4.3. Longitudinal trunk river and tributary characteristics South of Yengweni lake, the levee of the Mkuze River is elevated ~2.5 m above the water surface of the lake over a distance of 600 m (gradient of 0.42%; see Fig. 2). In the case of Mpanza lake, there was no difference between the top of the levee and the normal water level in the lake (see Fig. 3), whereas for Mdlanzi lake, the difference in elevation of the levee adjacent to the Mkuze River was ~ 2 m over a

Fig. 6. Organic contents of cores north and south of Yengweni lake. For transects Y1 and Y3, samples were taken at more than a single location across the valley.

16

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

Fig. 7. Organic contents of cores north (MFU1-5) and within (MPAN) Mpanza lake, and peat cross sections across the valley. Triangles on the cross sections show the locations of cores.

distance of ~ 800 m (gradient of 0.25%; see Fig. 4). A general survey of the relative elevations of the Mkuze River bed and adjacent levees in relation to nearby lake levels provides a better understanding of lake and floodplain hydrology. In the upper reaches of the floodplain, the Mkuze River has a fairly uniform but distinctly concave-up longitudinal profile as far as its confluence with the Mbazwana swamp, at which point gradient steepens considerably (Fig. 9). The gradient of the combined streams into Lake St. Lucia declines logarithmically to the point of entry into the lake. The gradients of all tributary streams are relatively steep until the point at which blocked-valley lakes form, whereupon the topography becomes flat and is similar to the floodplain surface. In the case of the Muzi and Yengweni lakes, a distinct levee occurs adjacent to the Mkuze River as shown in 1:10,000 orthophotographs produced by the South African Surveyor General. Furthermore, these two blocked-valley lakes occur at an elevation that is below the levee of the Mkuze River. However, in the case of the Mpanza and Mdlanzi lakes, their elevation is similar to that of the levee adjacent to the Mkuze River.

5. Discussion 5.1. Geomorphic development of the Mkuze floodplain The development of the coastal plain and the Mkuze floodplain has been linked to processes associated with sea level fall and rise during and following the Last Glacial Maximum (LGM; McCarthy and Hancox, 2000). At the LGM sea level fell about 120 m below current sea level (Ramsay, 1995; Ramsay and Cooper, 2002), during which time the Mkuze River and its tributaries on the coastal plain would have incised. Cores to bedrock at the lower Mkuze road bridge, 1 km west of Yengweni lake, indicate that the Mkuze River incised its bed ~42 m at this location (Fig. 9). The sedimentary fill south of Yengweni lake likely extends to a similar depth, and that the bed of tributary streams would also have cut to this depth at their point of confluence with the Mkuze River. With sea level rise, valleys draining to passive margins such as this tend to develop embayments or estuaries (Parker et al., 2008); and in this case, sea level rise and concomitant formation of a barrier beach/dune system drowned and

Fig. 8. Changes in the lake margins based on aerial photography over the period 1957 to 2000 of Yengweni (A) and Mpanza and Mdlanzi lakes (B).

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

Fig. 9. Longitudinal profiles along the Mkuze River and Muzi, Yengweni, Mpanza, Mdlanzi, and Mbazwana drainage lines as mapped from 1:10,000 orthophotographic maps with 2.5 m contour intervals (note pronounced vertical exaggeration). The depth of floodplain sediment to bedrock as measured at the lower Mkuze bridge between Muzi and Yengweni lakes is also indicated.

17

longitudinal slope of the sandy bed of the Totweni drainage line (Fig. 10A), suggests that during aggradation of the Mkuze floodplain, the Totweni drainage line was aggrading with sediment supplied from its own catchment (Fig. 10B). The rate of sedimentation in the Totweni drainage line is controlled by the elevation of the Mkuze River, and as such, the rate of aggradation in the Mkuze floodplain. Based on the extrapolation of the sandy bed of the Totweni drainage line across the Mkuze floodplain, the rate of sedimentation from locally derived sandy material in the Totweni drainage line and silt along the Mkuze floodplain are likely to have been similar in the period during which 20–25 m of sediment accumulated on the floodplain. Following this period of aggradation, the transfer of sandy sediment along the Totweni drainage line is likely to have declined owing to decreasing longitudinal slope. Despite the elevation of the Mkuze floodplain being lower than the bed of the Totweni drainage line, peat began to accumulate in the lower Totweni drainage line from downstream decline in sediment transfer. Currently, the rate of peat accumulation in the lower Totweni drainage line and the rate of sedimentation on the Mkuze floodplain are likely to be similar, resulting in the elevations of the lake and peat in the lower Totweni drainage line and the levee adjacent to the Mkuze River all being similar (Fig. 10C). However, based on

impounded the Mkuze and adjacent Mfolozi River valleys and resulted in the formation of Lake St. Lucia (Orme, 1990). Stillstand at current sea level was reached ~6000 Y BP (Ramsay, 1995); and while the adjacent larger Mfolozi River has largely infilled its valley and maintains a relatively small estuary spatially coincident with the mouth of Lake St. Lucia, infilling of Lake St. Lucia at its northern margin by the smaller, highly variable Mkuze River (Whitfield and Taylor, 2009) is in a premature stage. A key reason for this difference in valley filling is likely to be sediment supply (see, for example, Parker et al., 2008), the outcome in this case of lower discharge and higher flow variability leading to poor conveyance of sediment seaward by the Mkuze relative to the Mfolozi River, which has a much larger and steeper catchment. The upward coarsening nature of sedimentary fill accompanied by downstream fining of sediment on the floodplain suggests that vertical aggradation is accompanied by downstream progradation of a clastic wedge. Presently, sedimentation is expanding eastward into the Mbazwana swamp. Thus, the result of poor sediment conveyance by the Mkuze is a clastic wedge in the Mkuze floodplain undergoing gradual progradation into Lake St. Lucia, progressively blocking tributary valleys in a downstream succession. This is supported by the systematic downstream decrease in levee height and stream cross sectional area, such that the stream is 10 m wide and less that 1 m deep several hundred metres upstream of the confluence of the Mkuze River and the Mbazwana swamp (Ellery et al., 2003). 5.2. Geomorphic development of tributary valleys Despite similarities in their landscape settings, geomorphic development, and the characteristics of floodplain sediments south of the lakes, striking differences in the sedimentary characteristics of the Yengweni and Totweni drainage lines are evident. The accumulation of sediment along the Mkuze River must be accompanied by aggradation of tributary valleys. If the longitudinal slope of the substratum KwaMbonambi Formation sands of the Totweni drainage line is extrapolated using the gradient of 0.5% between the two lowermost survey points (elevation is within 0.1 m for cores MFU3 and MFU5; Figs. 7 and 10), then the depth of sediment accumulated in the centre of the Mkuze floodplain is ~20 m at this point (Fig. 10A). The current

Fig. 10. Conceptual model of tributary valley development from the time of the low sea level during the last glacial maximum (A) to the point of valley filling where sediment derived from the local tributary catchment on the coastal plain delivers sediment longitudinally down the tributary valley (B). Floodplain aggradation then leads to organic sedimentation in the tributary valley (C) and trunk stream sedimentation then starts to fill the tributary valley (D), burying peat deposits.

18

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

observations south of Yengweni lake, continued aggradation along the Mkuze floodplain south of the Totweni drainage line is likely to exceed the rate of peat accumulation in the future. Therefore, clastic sedimentation will gradually bury organic sediment in the Totweni drainage line (Fig. 10D), and it will take on the appearance of the Yengweni drainage line. In the case of Totweni that is fed by two main streams, the two lakes will extend progressively northwards up the valley, become increasingly linear, and have beds of fine clastic sediment. 5.3. A conceptual model of floodplain change Sea level rise initiates increased sedimentation rates along both trunk and tributary valleys. In the case of the Mkuze floodplain, sedimentation along the trunk river controls sedimentation patterns of the tributary streams. Initially, the longitudinal slope along tributary streams is sufficiently high given available discharge to transport clastic sediment down the valley such that sedimentation along the tributary valley is dominated by locally derived sandy sediment. As aggradation along the trunk valley continues, tributaries are no longer able to transport locally derived clastic sediment, at which point continued aggradation along the trunk valley creates storage space for organic sedimentation along tributary valleys. Organic sedimentation seems to happen initially when the elevation of the levee of the trunk stream is lower than the lowermost part of the tributary valley, which prevents the formation of blocked-valley lakes. However, peat may form in the tributary valley at this time, given suitable hydrological conditions and limited clastic sediment supply. As aggradation continues to the point where the elevation of the trunk stream is similar or slightly higher than the elevation of the lowermost part of the tributary valley, a lake may form. Once the elevation of the trunk stream levee greatly exceeds the elevation of the tributary lake and peat surfaces, clastic sediment increasingly fills the tributary valley during flood events, such that peat accumulation is overwhelmed and the lake bed becomes inundated with fine sediment that thickens over time as floodplain aggradation continues. The formation of blocked-valley lakes is associated with gradual encroachment of trunk clastic sediments up the tributary valley, as demonstrated by Grenfell et al. (2010). As such, the downstream margin of the blocked-valley lake gradually moves up the tributary valley (Fig. 11). Furthermore, the lake lengthens as floodplain sediment increasingly fills the valley, causing flattening of the lower part of the tributary valley. Increased upstream valley filling explains the decreasing length of the blocked-valley lakes downstream along the Mkuze floodplain and suggests that the tributary valleys become sinks for very fine sediment (clay) given that the floodplain itself predominantly comprises silt. The formation of peat in tributary valleys requires waterlogged conditions. Water loss from the Mkuze River during periods of low flow does not enter tributary valleys (Humphries et al., 2010a), suggesting that peat formation is largely a consequence of sustained water supply from groundwater on the sandy coastal plain. The role of geological factors that lead to sustained groundwater seepage into drainage lines such as these, is therefore clear. Turner and Plater (2004) dated organic sediment at the base of a core in the upper part of the Totweni drainage line at 1450 ± 40 years old. The model of floodplain development suggests that peat accumulation in tributary valleys starts first at the toe of the drainage line and progresses upstream along the tributaries as floodplain aggradation continues. The model also suggests that tributary valley peat accumulation started first at the head of the Mkuze floodplain (Neshe lake) and was initiated sequentially in the Muzi, Yengweni, Totweni and Mbazwana drainage lines (see Fig. 1A for locations). The study also suggests that peat deposits in Neshe, Muzi and Yengweni drainage lines have been sequentially buried by clastic sediments from the Mkuze floodplain. Sampling and analysis of

Fig. 11. Interactions between trunk valley sediments, organic tributary valley sediments and tributary lake development.

organic sediments in all of the tributary valleys would be required to test these assertions and unravel the record of environmental change within peat deposits. 5.4. Geomorphic controls on peat formation Given that the peat deposits at the toe of the Totweni drainage line are ~8 m thick and are likely to have started forming ~6000 Y BP, peat accumulation rates in the lower part of the drainage line are of the order of 1.3 mm.a − 1. This is higher by a factor of 5 than the rate of peat accumulation in the Mfabeni Mire on the Eastern Shores of Lake St. Lucia during the Holocene, when peat accumulation rates were estimated to be 0.03 mm.a − 1 (Grundling et al., 1998, 2000). However, Grundling et al. (1998) suggested that the Holocene peat accumulation rate in the Mfabeni is lower than for younger (Holocene) peatlands in the region. Turner and Plater (2004) suggested rates between 2 and 3 mm.a − 1 in the upper Totweni drainage line based on radiocarbon dates and pollen from introduced alien plants used in the vicinity for commercial timber production. Thamm et al. (1996) suggested rates of 5 to 10 mm.a − 1 elsewhere in the region. This study illustrates the types of processes that control the formation and rate of peat accumulation in subtropical and tropical environments. The rate of peat accumulation depends largely on the presence of permanently flooded conditions. In a drainage line that does not have a toe that is aggrading, peat accumulation will take place to the point where soils are permanently saturated, such that the aggradation rate may reflect compaction rate and the possible feedback of peat formation on hydrological processes by raising the water table (Clymo, 1984; Belyea and Baird, 2006). However, in systems where the toe of the drainage line is gradually rising owing to clastic sedimentation on the adjacent floodplain, the estimated rate of peat accumulation must be more rapid as peat accumulation integrates compaction rate, feedback of peat formation on the elevation of the water table, and a gradually rising water level from progressive tributary valley impoundment by the trunk floodplain. Of significance in this study, the estimated floodplain sedimentation rate south of the Totweni drainage line based on 210Pb and 137Cs is 2.5 to 5.0 mm.a − 1 (Humphries et al., 2010b). This indicates a minimum rate of peat formation because it ignores the effects of peat compaction and the feedback effect of peat formation on water level. Nevertheless, the

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

estimated rate of peat formation of 1.3 mm.a − 1 estimated in this study is not unreasonable, reinforcing the model of drainage line evolution presented. Assuming a peat accumulation rate of 1.3 mm.a − 1, each hectare would accumulate 13 m 3 of peat per annum. Given an estimated density of 100 kg.m − 3 (see McCarthy et al., 1989 for peat with similar characteristics), this translates to 1.3 t/ha/a of organic matter incorporated into peat deposits annually. Given that the extent of peat deposits in the Totweni drainage line is ~ 390 ha, a total of ~ 500 t of peat is accumulating in the system each year. With a weighted average depth of ~5.1 m (calculated on the basis of Fig. 7), it is estimated that ~ 2 × 10 7 m 3 of peat currently fills the Totweni drainage line, with an approximate mass of 2 × 10 6 t. This serves to draw attention to the value of this system in respect of its ability to sequester carbon and, in the long term, to bury carbon given infilling of the valley with clastic sediment following a lengthy period of peat formation. In the southern African landscape, such carbon sequestration opportunities in wetlands are likely to be limited, emphasizing the particular importance of this system regionally. In peatlands across southern Africa, peat fires are an important process and lead to the combustion of peat deposits such that, even in settings where peat formation is rapid, it is not preserved (Ellery et al., 1989; Tooth and McCarthy, 2007). However, burial of peat deposits by clastic sediment leads to compression and subsidence, shielding peat deposits from fires, impeding microbial activity, and enhancing long-term sequestration. 5.5. Implications for landscape development The present study sheds light on the likely trajectory of change in the Mkuze floodplain over time. Muzi and Yengweni lakes should continue to lengthen over time as a consequence of ongoing clastic sedimentation in these valleys. As aggradation of the Mkuze River continues, the peat in the Totweni drainage line will be buried by clastic sediment. Eventually, aggradation of the trunk stream will block the Mbazwana swamp, resulting in the formation of a blocked-valley lake upstream of the confluence on the Mbazwana swamp (Fig. 12). The lake will move northward and lengthen upstream over time. Initially, peat formation upstream of the lake will continue as the valley slopes down toward the confluence of these

19

streams. However, as aggradation of the Mkuze floodplain continues, clastic sedimentation will likely enter the Mbazwana swamp north of the confluence, burying peat deposits. This will be accompanied by progradation of the Mkuze floodplain toward and into Lake St. Lucia, filling the northern embayment into which it discharges with fine clastic sediment and converting the lake into a floodplain wetland. This study provides a sense of likely changes in the nature and character of the Mkuze wetland system (including the Mbazwana swamp) over much time periods of thousands of years. Simplistically, the Mkuze River has taken 6000 to 10,000 years to fill a valley ~40 km long and 5.5 km wide (0.004 to 0.007 river km of floodplain building per annum). Based on this assumption, a further 3500 to 6000 years are required to fill the Mbazwana swamp from the confluence of the Mkuze River and Mbazwana swamp to Lake St. Lucia, a distance of approximately 25 km. Currently, the Mkuze floodplain is by far the largest natural floodplain wetland system on the coastal plain of KwaZulu-Natal, with agricultural activities mainly characterised by small-scale subsistence farming (Patrick and Ellery, 2006). Large portions of the Mfolozi floodplain have been transformed by commercial agriculture (Grenfell et al., 2009). Furthermore, the Pongolo floodplain has been negatively impacted by the construction of the Pongolapoort dam immediately upstream of the Pongolo floodplain, which has disrupted natural flooding, trapped sediment, and initiated erosion, with negative consequences for floodplain structure and function (Heeg and Breen, 1982; Heath and Plater, 2010). Given the relatively low level of intensive human use and location in the iSimangaliso Greater St. Lucia Wetland Park, the Mkuze floodplain has a remarkably high ecological value, particularly given that it is nutrient rich with fine alluvial soils on a coastal plain dominated by reworked marine sediments that are nutrient poor. Furthermore, the Mkuze River is an important sediment sink for both clastic (McCarthy and Hancox, 2000; Humphries et al., 2010a) and dissolved sediment (Barnes et al., 2002; Humphries et al., 2010b), which together provide considerable benefits to Lake St. Lucia with respect to water quality because the Mkuze River is the main source of fresh water to the lake (Whitfield and Taylor, 2009). Furthermore, as shown in this study, the aggradation on the Mkuze floodplain leads to long-term burial of peat in tributary valleys that sequesters carbon. From an ecological point of view, the Mkuze floodplain functions naturally with natural avulsions operating over timescales of hundreds of years to maintain a heterogeneous suite of habitats in different stages of wetting and drying (Ellery et al., 2003) and in different successional stages such that habitat diversity is high (Patrick and Ellery, 2006).

6. Conclusion

Fig. 12. Hypothesised landscape development of the Mkuze floodplain system given sedimentation patterns described in the present study. The extents of Mpanza, Mdlanzi and Mbazwana lakes have been determined from a digital elevation model of the study area, with the present shorelines of Mpanza and Mdlanzi lakes shown as dotted white lines.

We hypothesise that this model of trunk and tributary change accounts for the formation of peat in the Mkuze wetland system, and that it has relevance to sedimentary processes in other similar floodplain systems on the coastal plain of eastern and southern Africa and in tropical and subtropical areas elsewhere. The relationship between trunk and tributary sedimentation patterns will depend on a range of factors including the relative size of trunk and tributary watersheds, as well as the availability and nature of sediment supplied to the systems. Where material from tributary streams is silt or clay, then the supply of sediment along the tributary may last a longer period of time than in this case, where the material was fine sand. This is because fine sediment may be transported down the tributary valley at lower slopes than sand provided that it is readily available. Therefore, an abundant supply of fine material in the tributary catchment may limit organic sedimentation because, in addition to storage space, peat formation is dependent on a very limited clastic sediment supply. Given this, filling of the tributary may occur initially from a

20

W.N. Ellery et al. / Geomorphology 141-142 (2012) 11–20

local sediment source followed by sediment input from the trunk stream with little peat formation in the tributary valley. An important feature of this model is that the burial of peat in tributary drainage lines constitutes a long term carbon sink. Peat is buried such that it cannot be oxidised or burnt. The burial of organic sediment in this small subtropical floodplain system suggests that large subtropical and tropical floodplains on coastal plains may constitute an important carbon sink globally. This is a subject that requires further investigation. Our understanding of the role of these systems as long-term carbon sinks needs to be improved, particularly given the need for carbon sequestration in the face of climate change associated with greenhouse gas emissions. We should also note that during Ice Ages, when sea level drops and degradation takes place through erosion, these systems may release greenhouse gases and contribute to subsequent warming. Feedback mechanisms such as these are important when considering homeostasis of the global ecosystem over long time periods. Acknowledgements This study was funded by the South African Water Research Commission, the South African National Research Foundation and the Swedish International Development Cooperation Agency (Department for Research Cooperation, SAREC). Several students from South Africa and Sweden assisted with data collection in the field. Dr James Gambiza supported the production of this manuscript. The comments of anonymous reviewers are very gratefully acknowledged. References Barnes, K., Ellery, W.N., Kindness, A., 2002. A preliminary analysis of water chemistry of the Mkuze Wetland System, KwaZulu-Natal: a mass balance approach. Water SA 28, 1–12. Belyea, L.R., Baird, A.J., 2006. Beyond “the limits to peat bog growth”: cross-scale feedback in peatland development. Ecological Monographs 76 (3), 299–322. Belyea, L.R., Clymo, R.S., 2001. Feedback Control of the Rate of Peat Formation. Proceedings of the Royal Society of London 268, 1315–1321. Blake, D.H., Ollier, C.D., 1971. Alluvial plains of the Fly River, Papua. Zeitschrift für Geomorphologie Supplemente Bände 12, 1–17. Cecil, C.B., Stanton, R.W., Neuzil, S.G., Dulong, F.T., Ruppert, L.F., Pierce, B.S., 1985. Paleoclimate controls on Late Paleozoic sedimentation and peat formation in the Central Appalachian Basin (U.S.A.). International Journal of Coal Geology 5, 195–230. Chimner, R.A., Ewel, K.C., 2005. A tropical freshwater wetland: II. Production, decomposition, and peat formation. Wetlands Ecology and Management 13, 671–684. Clymo, R.S., 1984. The limits to peat bog growth. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 303, 605–654. Dettinger, M.D., Diaz, H.F., 2000. Global characteristics of stream flow seasonality and variability. Journal of Hydrometeorology 1, 289–310. Dommain, R., Couwenberg, J., Joosten, H., 2010. Hydrological self-regulation of domed peatlands in south-east Asia and consequences for conservation and restoration. Mires and Peat 6http://mires-and-peat.net Accessed 20 October 2011. Ellery, W.N., Ellery, K., McCarthy, T.S., Cairncross, B., Oelofse, R., 1989. A peat fire in the Okavango Delta, Botswana and its importance as an ecosystem process. African Journal of Ecology 27, 7–21. Ellery, W.N., Dahlberg, A.C., Strydom, R., Neal, M.J., Jackson, J., 2003. Diversion of water flow from a floodplain wetland stream: an analysis of geomorphological setting and hydrological and ecological consequences. Journal of Environmental Management 68, 51–71. Ellery, W.N., Grenfell, M.C., Grenfell, S.E., Kotze, D., McCarthy, T., Tooth, S., Grundling, P.-L., Beckedahl, H., le Maitre, D., Ramsay, L., 2009. WET-origins: controls on the distribution and dynamics of wetlands in South Africa. WRC Report No TT334/09. Grenfell, S.E., Ellery, W.N., Grenfell, M.C., 2009. Geomorphology and dynamics of the Mfolozi River floodplain, KwaZulu-Natal, South Africa. Geomorphology 107, 226–240. Grenfell, S.E., Ellery, W.N., Grenfell, M.C., Ramsay, L.F., 2010. Sedimentary facies and geomorphic evolution of a blocked-valley lake: Lake Futululu, northern KwaZuluNatal, South Africa. Sedimentology 57, 1159–1174. Grundling, P., 1996. The implications of 14C and pollen derived peat ages on the characterization of the peatlands of the Zululand-Mozambique coastal plain. Unpublished Report, Council for Geoscience, 1996–0119, 8 pp.

Grundling, P., 2004. The role of sea level rise in the formation of peatlands in Maputaland. In: Momade, F., Achimo, M., Haldorsen, S. (Eds.), The Impact of Sea-level Change; Past, Present, Future, Vol. 43. Boletim Geológico, pp. 42–48. Grundling, P., Mazus, H., Baartman, L., 1998. Peat resources in northern KwaZulu-Natal wetlands: Maputaland. Department of Environmental Affairs and Tourism Report no. A25/13/2/7. 102 pp. Grundling, P., Baartman, L., Mazus, H., Blackmore, A., 2000. Peat resources of KwaZuluNatal wetlands: Southern Maputaland and the North and South Coast. Council for Geoscience. Report no: 2000–0132. Heath, S.K., Plater, A.J., 2010. Records of pan (floodplain wetland) sedimentation as an approach for post-hoc investigation of the hydrological impacts of dam impoundment: the Pongolo River, KwaZulu-Natal. Water Research 44, 4226–4240. Heeg, J., Breen, C.M., 1982. Man and the Pongolo floodplain. South Africa National Scientific Programme No 40. CSIR, Pretoria. Humphries, M.S., Kindness, A., Ellery, W.N., Hughes, J., 2010a. Sediment geochemistry, mineral precipitation and clay neoformation on the Mkuze River floodplain, South Africa. Geoderma 157, 15–26. Humphries, M.S., Kindness, A., Ellery, W.N., Hughes, J., Benitez-Nelson, C.R., 2010b. 137 Cs and 210Pb derived sediment accumulation rates and their role in the longterm development of the Mkuze River floodplain, South Africa. Geomorphology 119, 88–96. Humphries, M.S., Kindness, A., Ellery, W.N., Hughes, J., 2011. Water chemistry and effect of evapotranspiration on chemical sedimentation on the Mkuze River floodplain, South Africa. Journal of Arid Environments 75, 555–565. Immirzi, C.P., Maltby, E., Clymo, R.S., 1992. The global status of peatlands and their role in carbon cycling. A report for the Friends of the Earth by the Wetlands Ecosystems Research Group, Department of Geography, University of Exeter. Friends of the Earth, London. Ingram, H.A.P., 1987. Ecohydrology of Scottish peatlands. Transactions of the Royal Society of Edinburgh Earth Scientists 78, 287–296. McCarthy, T.S., Hancox, P.J., 2000. Wetlands. In: Partridge, T.C., Maud, R.R. (Eds.), The Cenozoic of Southern Africa. Oxford University Press, Oxford, UK, pp. 218–235. McCarthy, T.S., McIver, J.R., Cairncross, B., Ellery, W.N., Ellery, K., 1989. The inorganic chemistry of peat from the Maunachira channel-swamp system, Okavango Delta, Botswana. Geochimica et Cosmochimica Acta 53, 1077–1089. Montanerella, L., Jones, R.J.A., Hiederer, R., 2006. The distribution of peatland in Europe. Mires and Peat 1http://mires-and-peat.net. Moore, P.D., 1987. Ecological and hydrological aspects of peat formation. Geological Society, London, Special Publications 32, 7–15. Moore, P.D., 1989. The ecology of peat-forming processes: a review. International Journal of Coal Geology 12, 89–103. Orme, A.R., 1990. Wetland morphology, hydrodynamics and sedimentation. In: Williams, M. (Ed.), Wetlands: A Threatened Landscape: Special Publication Series of the Institute of British Geographers, No. 25, pp. 42–95. Parker, G., Muto, T., Akamatsu, Y., Dietrich, W.E., Lauer, J.W., 2008. Unravelling the conundrum of river response to rising sea-level from laboratory to field. Part I: laboratory experiments. Sedimentology 55, 1643–1655. Patrick, M.J., Ellery, W.N., 2006. Plant community and landscape patterns of a floodplain wetland in Maputaland, northern KwaZulu-Natal, South Africa. African Journal of Ecology 45, 175–183. Porat, N., Botha, G., 2008. The luminescence chronology of dune development on the Maputaland coastal plain, South Africa. Quaternary Science Reviews 27, 1024–1046. Ramsay, P.J., 1995. 9000 years of sea-level change along the southern African coastline. Quaternary International 31, 71–75. Ramsay, P.J., Cooper, J.A.G., 2002. Late Quaternary sea-level change in South Africa. Quaternary Research 57, 82–90. Schulze, R.E., 1997. South African atlas of agrohydrology and climatology. WRC Report No. TT 82/96. Water Research Commission, Pretoria, South Africa. Smuts, W.J., 1992. Peatlands of the Natal Mire Complex: geomorphology and characterization. South African Journal of Science 88, 474–483. Thamm, A.G., Grundling, P., Mazus, H., 1996. Holocene and recent peat growth rates on the Zululand coastal plain. Journal of African Earth Sciences 23, 119–124. Tooth, S., McCarthy, T.S., 2007. Wetlands in drylands: geomorphological and sedimentological characteristics, with emphasis on examples from southern Africa. Progress in Physical Geography 31, 3–41. Turner, S., Plater, A., 2004. Palynological evidence for the origin and development of late Holocene wetland sediments: Mdlanza Swamp, KwaZulu-Natal, South Africa. South African Journal of Science 100, 220–229. Tyson, P.D., 1986. Climatic Change and Variability in Southern Africa. Oxford University Press, Cape Town. 220 pp. Tyson, P.D., Preston-Whyte, R.A., 2000. The Weather and Climate of Southern Africa, 2nd Ed. Oxford University Press, Cape Town. 395 pp. Watkeys, M.K., Mason, T.R., Goodman, P.S., 1993. The role of geology in the development of Maputaland, South Africa. Journal of African Earth Sciences 16, 205–221. Whitfield, A.K., Taylor, R.H., 2009. A review of the importance of freshwater inflow to the future conservation of Lake St. Lucia. Aquatic Conservation: Marine and Freshwater Ecosystems 19, 838–848.