The grain-size characteristics and selected aspects of the geochemistry of sediment cores in the vicinity of a mangrove tidal channel meander bend in Malaysia highlight an abrupt change in channel hydrodynamic and sedimentation regimes following a meander cut-off. The straight, nearly 1 km-long cut-off channel (Fig. 1) points out to a rather abrupt hydrological change, rather than being the product of the slow progressive rapprochement of two opposite channel segments that eventually coalesced to form a cut-off. A scrutiny of available aerial photographs and Google Earth images shows that the Kerteh cut-off has been extant over at least the last 13 years. The cut-off may well have occurred during the big flood event that hit Terengganu nearly a century ago from 21 to 29 December, 1926, accompanied by exceptional rainfall (1944 mm compared to the annual mean of ~2800 mm), and resulting in severe environmental damage and the destruction of thousand hectares of forests (Chan, 2012; Williamson, 2016).
Whatever the true age of the cut-off, the change engendered by this event is recorded in the grain-size and TOC trends. Sediment particle characteristics have been shown to provide information on the relationship between hydrodynamics and transport or deposition (e.g., Droppo, 2015). The dominantly fine-grained nature of the sediments found in KR1 and KR2, composed essentially of silts and, to a lesser degree, clays (Figs. 2, 3), reflects both the high degree of weathering of tropical soils and the effect of mangrove swamps on dampening water flows and favouring the accumulation of mud. Notwithstanding, differences in grain size have been found between the two cores in the Kerteh swamp. Core KR2 shows, in particular, a very sharp transition from coarse, through fine to medium sand, at a depth of about 80 cm, to relatively homogeneous silt and clay in the rest of the upper part of the core. We attribute this sharp change to the transition from an active channel meander to meander cut-off. Meander cut-offs in tidal environments have been shown to exhibit sharp upward changes in facies following abandonment (e.g., Anthony et al., 1996). The coarse basal deposits correspond to sand deposited in an active channel bed. Once the cut-off occurred, the diversion of flows through the straight new channel was accompanied by a shift to fine-grained deposits associated with settling in a low-energy, abandoned channel environment. The grain size in KR1 is relatively more homogeneous, though the upper part of the core (above 70 cm) shows a clearly overall fining trend. This characteristic may reflect a shift from relatively more coarser-grained sedimentation at the base to more continuously finer-grained point-bar sedimentation. The differences between the two cores are less manifest in sorting, with the exception of basal sediments in the point bar which show poor sorting, probably reflecting mixing of sediment in an energetic channel environment. It may be surmised that the abrupt change in sorting in this core may pinpoint the time of the meander cut-off. Fleming (2017) identified a tendency for sorting to be homogeneous in sediment composed of a similar range of size, whereas poor sorting implies sediment mixing with different sizes of sediment.
The abrupt variation in grain-size and sedimentation rate in the meander cut-off is also consistent with an increase in OC, thus further reflecting the consequences of this event on local sedimentation. The grain size influence on channel meander evolution is embedded in these changes in OC but also in heavy metals. Both particle size distribution and OC concentrations changed significantly at ~70 cm, pointing to a change in the depositional environment. Meander cut-offs in tidal flats have been shown to be depocentres of organic matter following the instauration of quiescent hydrodynamic conditions, as flow becomes diverted through the cut-off, and the abandoned meander belt may also become progressively covered by vegetation (Anthony et al., 1996). This situation also concerns the Kerteh meander belt which appears to be progressively invaded by mangroves (Fig. 1). This increase in OC has also been shown to go with a decrease in grain size (Sutherland, 1999). Generally, OC binds more easily with fine sediment, clay or silt.
Previous studies have shown that the distribution of heavy metals in sediment is closely related to organic matter and mostly depends on the type of sediment (Karbassi et al., 2005; Abdul Razak et al., 2018). The metal concentration in the point bar is almost uniform from the bottom to the top of the core, whereas the concentration in the cut-off shows a clear transition phase at a depth of 70 cm, thus further providing evidence for a morpho-sedimentary change in the Kerteh channel associated with meander cut-off. The abandonment of the meander created conditions for the active accumulation of elements and OC in the cut-off, compared to the point bar. Louma (1990) has shown this relationship between the concentration of elements and sedimentation processes. The variability in metal concentration has also been shown to depend on the size and texture of sediments (Jicknell and Kump 1984; Ramos et al., 1994), with larger mean grain size being associated with a larger variation of metal concentration at the same area (Morse et al., 1993).
In order to further clarify the relationships in the grain size and geochemistry of these deposits, we used a correlation coefficient matrix for each core (KR1: Table 2; KR2: Table 3). The correlation of metals with mean grain size and TOC in cut-off was relatively stronger compared to the point bar. The highest correlation value of the latter is 0.76 (Fe-Mg), followed by Fe-Cu (r = 0.75), Cu-Sr (r = 0.74), Mg-Mn (r = 0.73), Mean Sediment-TOC (r = 0.71) and TOC-Fe (r = 0.71). The highest correlation value of the cut-off is r = 0.98 (Mg-Mn), whereas the lowest correlations occur between Ba-Fe (𝑟 = 0.59) and Ba-TOC (r = 0.53), although both are still considered as strongly correlated to each other. The strong positive correlation between all parameters in the cut-off suggests that grain size is an important criterion for the attachment of metals and TOC under the conditions of reduced flow energy that ensued following meander abandonment. Nguyen et al. (2005) have shown that highly correlated metals exhibited a similar behavior in the study area. Generally, the contents decreased as grain size increased.
Principal components analysis (PCA) highlighted two significant compounds (eigenvalue >1) that explained 71.2% (KR1) and 93.0% (KR2) of the total variance of the datasets (Table 4). For KR1, varifactor VF1 explained 28.3% of the total variance in the datasets with positive loadings on metal content such as Mg (0.89), Mn (0.79), Ba (0.75), and Sr (0.86). Varifactor VF2 accounted for 32.8% of the total variance with positive loadings on mean grain size (0.75) and total organic content (0.90). Meanwhile, for KR2, varifactor VF1 explained 54.7% of the total variance with strong loadings on MGS (0.83), total organic carbon (0.90) and metal, namely Fe (0.90) and Na (0.80), whereas varifactor VF2 accounted for 38.3% with negative loadings exhibited by the concentrations of Ba and Sr. The PCA shows that only Fe and Na are influenced by changes in MGS and organic content. In this study, communalities of the variance explained a higher value for KR2 (0.88 – 0.97) than KR1 (0.50 – 0.93), revealing that the extracted factor fitted well with factor solution.
It is noteworthy that a distribution pattern of metal content versus sample depth was clearly observed in KR2 (Fig. 5b) compared to KR1 (Fig. 5a). Two clusters were formed in KR2 in which metal accumulation in the upper depths had a higher concentration. The relationship between metal (Fe, Na) and MGS or organic content was much stronger for KR2 than KR1. This was highlighted by a straight line with a similar direction on the biplot graph (not shown). We deduce from this the relationship that the accumulation of finer-grained sediments following meander cut-off is favourable to binding between metals and organic matter.
The long-term (multi-decadal) mobility of mangrove tidal channels has been shown to depend essentially on sediment inputs into the system, which alter the morphodynamics of meanders, thus eventually generating, over even longer timescales (secular to millennial) the gradual reworking of mangrove tidal flats (Anthony, 2004). Active meander belt reworking in mangrove swamps occurs where high sediment loads in channels, notably bedload, induces instability in flow conditions. This has probably been the case in the Kerteh channel, given the ‘abrupt’ morphology of the meander cut-off (Fig. 1). The foregoing study of sediments associated with the tidal channel in the Kerteh Sungai mangrove swamp has shown that channel dynamics can be important in generating variability in the sedimentology and geochemistry of mangrove sediments. The reasons for the formation of a meander cut-off in this swamp are not clear. However, since its formation, the Kerteh channel and its meander cut-off have exhibited apparent stability at the timescale of available satellite images and aerial photographs covering the study area (13 years). This stability suggests that the swamp and its channel network may now be largely in equilibrium with flow conditions.