Originally developed to provide information on the content of hydrocarbons and the kerogen type in sedimentary rocks, as well as determine the maturity of the kerogen , Rock-Eval analysis has become an attractive and fast alternative for the analysis of organic matter in soils and sediments compared to conventional methods due to the lack of sample preparation . Rock-Eval analysis provides not only the quantity of organic carbon but also information on the elemental composition of the OM. In particular, the hydrogen index (HI; milligrams of HC per gram of TOC) and oxygen index (OI; milligrams of O2 per gram of TOC), which correlate with the classical elemental parameters of the H/C and O/C ratios known from van Krevelen diagrams , not only provide insights into the degree of degradation but also provide indications of OM origin. The representation of the HI against TOC allows the estimation of the soil OM quality, which is usually determined by the mineralization of the soil OM and the dilution by the mineral matrix. Fresh and fragmented litter usually shows high TOC levels (10 %–40 %) and HI values greater than 300 . This results from the material consisting primarily of unconverted biopolymers such as lignin, cellulose, and hemicellulose. During degradation (oxidation) of the biopolymers, the HI decreases and the OI increases. Material consisting of a distinct composition of the dominant biopolymers, such as aquatic inputs, likewise shows distinct ranges of its HI values, which provide a rough source indication.

In addition, Rock-Eval provides information on the thermal stability of the immature OM . The so-called S2 peak integrates the quantity of pyrolyzed emission products between 200 and 650 ∘C. Utilizing mathematical deconvolution determines individual peaks within the S2 peak. Fractions of OM differing in thermal stability are assignable to the resulting temperature intervals: (A1) 200–340 ∘C, thermally unstable biological macromolecules, like saccharides; (A2) 340–400 ∘C, stable biological macromolecules, like cellulose and/or lignin or polypeptides; (A3) 400–460 ∘C, immature geological macromolecules, like humic substances; (A4) 460–520 ∘C, refractory geological macromolecules; and (A5) 520–650 ∘C, highly refractory pool . Investigations of composts using 13C nuclear magnetic resonance (NMR) showed that the thermal stability separating the mentioned groups also correlates with the chemical stability . Based on this, and developed indices based on the aforementioned temperature intervals, whereby the indices are defined as follows: R index (R is recalcitrant OM) = (A3 + A4 + A5)/100, and I index (I is immature OM) = log10((A1 + A2)/A3). However, point out that due to their mathematical derivation, these indices cannot exactly indicate the quantity or identity of the chemical components in the OM, unlike the classic Rock-Eval parameters. We acknowledge that the four different temperature intervals cannot differentiate the entire spectrum of organic compounds and therefore refer to the thermal rather than chemical stability of OM . Nevertheless, this sufficiently reflects the changes in the organic components during their transformation. On this basis, the R index and the I index describe the contribution of refractory and persistent fractions by pedogenic processes, as well as the degree of transformation of unstable components via decompositional processes . proposed the use of the I/R diagram (I index vs. R index), which resembles the representation of the van Krevelen diagram (OI vs. HI). One of the most notable features of the I/R diagram is the linear relationship between the two parameters (“decomposition regression line” or “humic trend”). During the decomposition of fresh OM, labile organic compounds are converted into their respective more stable counterparts, which are converted into either stable humic acids or components complexed by the mineral matrix. Samples are plotted along the linear relationship when the stabilization of OM from the progressive degradation of organic components is based on their biochemical stability. Samples are not plotted along this linear relationship in situations with OM mixture from different sources.

In general, lipid biomarkers reflect only a small portion of the total OM but overcome problems of source specificity by having a specific origin. Plants, for instance, produce waxes to control their water balance and counteract external stressors, such as UV radiation and bacterial and fungus-based attacks . These waxes enter the environment through physical processes, such as wind, precipitation, abrasion by particles, and when plants die or plant parts detach. The waxes contain a variety of molecules, including long-chain n-alkanes, which are very resistant to degradation . Plant-wax-derived n-alkanes show homologous series with a strong odd-over-even chain-length dominance due to their biosynthesis causing a high carbon preference index (CPI) for fresh plant waxes. Besides, different plant types show characteristic features in their n-alkane distribution patterns, such as aquatic plants showing dominant contributions of the n-alkanes C23 and C25 , whereas grasses show distribution patterns dominated by the very long-chain n-alkanes C33 and C35 as well as C31 . The differing abundances and dominance of the “higher-plant” n-alkanes are generally resembled in the ACL (average chain length) parameter . The homologues C29 and C27 seem to be mainly derived from tree-like vegetation . However, both the C29 and the C31 n-alkane can reflect a mixed signal from trees and grasses. The distribution patterns of the contributing plant types are closely mirrored in topsoils . However, some studies have identified limitations of this approach and question the validity of distinctions by distribution patterns alone e.g.,. Therefore, it is recommended to combine the information derived from n-alkane distribution patterns with additional proxies, such as compound-specific isotopic compositions e.g.,.

The carbon and hydrogen in plant-wax n-alkanes derive from the carbon and hydrogen source the plants utilize for biomass metabolism, i.e., inorganic carbon and water. The isotopic composition of n-alkanes thus allows interpretation of either the metabolic process during biosynthesis or effects influencing the isotopic composition of the respective carbon and hydrogen sources .

In the case of carbon, the effect of different photosynthetic metabolic pathways of different types of vegetation has the greatest influence on the compound-specific stable carbon (δ13C) of n-alkanes. Information on contributing photosynthetic plant types can be obtained from their compound-specific δ13C isotope compositions . Waxes from C4 vegetation are isotopically enriched in 13C relative to waxes from C3 plants due to more effective photosynthetic carbon (CO2) fixation . Most tropical grasses are of C4 type, while all trees and shrubs are C3 plants. The occurrence of C4, i.e., 13C-enriched n-alkanes, in sediments has thus been attributed to contributions from grassy environments. However, plants from specific environments, such as specific Cyperaceae or Poaceae in swamps or salt-tolerant plants in saline settings, can also be of C4 type, complicating a simple interpretation of δ13C compositions . A complication arises when CAM plants are present. CAM plants produce variable and intermediate δ13C compositions but occur only in specific environments, such as in the Succulent Karoo Biome along the west coast of South Africa . The δ13C compositions of plant waxes also depend on environmental stress, such as water shortage. Under drought conditions, plants close their stomata to increase their water use efficiency, leading to 13C enrichment . δ13C compositions of plant waxes should thus only be interpreted in conjunction with other parameters.

The hydrogen isotope composition (δD) of plant waxes mainly depends on environmental controlling factors rather than biosynthetic mechanisms. The hydrogen that plants incorporate into their biomass originates from the water they absorb. The hydrogen isotope composition of this water mainly depends on the hydrogen isotope composition of precipitation and the hydrological status of the environment. Regarding the hydrogen isotope composition of rainfall, different effects can be distinguished . The continental and altitude effects describe the isotopic depletion with progressive rainout when moisture travels inland or uphill. The temperature effect describes the isotopic depletion with lower condensation temperatures. For the Mkhuze system, these effects result in plant waxes being isotopically depleted in hydrogen when derived from more inland or mountainous regions, whereas temperature variations play a negligible role. In contrast, the amount effect has a large influence leading to depleted hydrogen isotope compositions of rainfall under high-precipitation regimes . Wetter areas are thus characterized by more depleted hydrogen isotope compositions of plant waxes. The latter effect is amplified by secondary isotope effects due to evaporation and transpiration. Water in soils, lakes, rivers, and wetlands may be isotopically enriched due to evaporation while also leaf water used for biosynthesis of waters may become isotopically enriched due to transpiration . Such isotopic enrichment due to evapotranspiration has, for instance, been observed in dry environments of South Africa . Additionally, a slight dependency of δD composition of waxes occurs for different plant types . Disentangling all processes affecting the δD of plant waxes may not always be possible in specific environments. Nevertheless, in combination these primary and secondary effects give an indication of the hydrological status of the contributing vegetation, i.e., about the general humidity of environments, especially when considered in conjunction with other parameters, as is done in this study.

The Mkhuze Wetland System (27.8∘ S, 32.5∘ E; Fig. ), South Africa's largest freshwater wetland system (∼450 km2), is located in the northeastern coastal region of South Africa in the KwaZulu-Natal province. It is bordered by the Lebombo Mountain Range to the west and the Indian Ocean to the east and forms a mosaic of wetland types, including swamp forests, grassy swamps, and open water (pans) . This ecological heterogeneity and biotic diversity are of international conservation importance, and therefore the area is listed as a wetland of international importance by the Ramsar Convention.

Collection of samples took place during a single field campaign in November–December 2018. Ten plant samples were collected. If possible, replicate plant species were sampled at various sampling sites within the system. The different species were selected based on the occurrence of large cohorts in the field or based on high reported occurrences in previous studies (; see also Fig. ), but not all major plant communities mentioned could successfully be sampled during the field campaign. Sampled species include aquatic plants from the Nymphaeaceae family (n=2) as well as the aquatic plant Phragmites australis (n=2) growing on both dry and flooded soils; two species of wetland grasses, namely Vossia cuspidata (n=2) and Cynodon dactylon (n=2); and two representatives of the Cyperaceae family, namely Cyperus papyrus (n=1) and Cyperus alternifolius (n=1). A total of 41 surface sediment samples (uppermost 10 cm) were collected along the course of the Mkhuze River and a transect extending into North Lake (northern part of Lake St Lucia, Fig. ). The collection of sediment samples was conducted under permit from Ezemvelo KZN Wildlife and iSimangaliso Wetland Park Authority. Because of the limited number of samples due to the difficult accessibility of the sampling area, the results should be considered qualitatively rather than quantitatively.

All surface sediments were freeze-dried. A sieved (<212 µm) sub-sample of approximately 100 mg was removed for classical bulk parameter analysis, and the residual sample was ground and sub-sampled for analyses. All glassware was combusted at 450 ∘C for 5 h prior to use.

The carbon preference index (CPI) and the average chain length (ACL) were adapted from and , respectively. The following calculations were made: Carbon preference index. 1CPI=0.5×∑2333Codd∑2232Ceven+∑2535Codd∑2434Ceven. Average chain length. 2ACL=∑2335i×Ci, odd∑2335Ci, odd. Relative concentration (contribution) [%]. 3Cx=Cx∑2335Ceven, odd.

All statistical analyses were performed using R software (R version 4.0.3 and RStudio version 1.4.1103). To test whether statistically significant differences occurred between the sub-environments, the Kruskal–Wallis test was performed using the stat_cor_mean() function of the “ggpubr” package (version 0.4.0). When the p value indicated that differences (p<0.05) between sub-environments were evident, the test was supplemented with a pairwise Wilcoxon rank sum test (base package “stats”) to determine which sub-environments had significant differences between them. The Benjamini and Hochberg “BH” method was used to adjust p values. Correlation coefficients were calculated by using the stat_cor function with the default Pearson method of the ggpubr package (version 0.4.0). Box-and-whisker plots were generated using the “ggplot2” system (version 2.3.3.3) of the “tidyverse” package (version 1.3.0) and ggpubr (version 0.4.0) and the geom_boxplot() function implemented here. Here, the median of the respective data is shown as a solid line and the lower and upper hinges correspond to the first (25th percentile) and third (75th percentile) quartiles. The lower/upper whisker extends from the lower/upper hinge to the smallest/largest value if it is not smaller/larger than 1.5 times the interquartile range from the hinge. Otherwise, the respective data point is drawn as a single point representing an outlier. For the principal component analysis (PCA) performed, missing values of the data set were first imputed using the impute.PCA() function and applying the regularized iterative PCA algorithm of the “missMDA” package (version 1.18). Subsequently, the actual PCA calculations were performed by using the prcomp() function (base package stats).

Total organic carbon (TOC), as a measure of organic matter (OM) content, increased in surface sediments from the upper reach to the swamp sub-environment, where samples contained the highest amounts of TOC, ranging from 0.9 % to 8.1 % (Table  and Fig. ). Higher variance is observed for the upper reach, floodplain, and swamp sub-environment, compared to very narrow ranges of values in the delta (1.4 %–1.8 %) and lake sub-environment (1.4 %–1.7 %). Samples from the floodplain and swamp sub-environments show significantly higher C/N ratios compared to the other sub-environments (p<0.05, Table ). To estimate the total amount of plant-wax input to the sub-environments, the sum of the concentrations of all long-chain odd-numbered n-alkanes was considered (Table  and Fig. ). When normalized to sample mass, delta and lake samples contained significantly lower amounts of plant-wax-derived lipids compared to swamp samples (p<0.005). However, when normalized to organic carbon, no significant differences were detected between any of the sub-environments investigated (p>0.05). Bulk OM δ13C shows more depleted values in the samples from the upper reach and the swamp sub-environments than in the other sub-environments, but statistical evidence is present only for the distinction between the swamp sub-environment on the one hand and the delta and lake sub-environments on the other (p<0.05).

The HI values show a decreasing trend from the upstream (ca. 115 mg HC per gram of TOC for upper reach and 100 mg HC per gram of TOC for floodplain; Fig. a) to the downstream sub-environments (ca. 75 mg HC per gram of TOC for swamp and delta; ca. 60 mg HC per gram of TOC for lake; Fig. a). The range around the mean value is generally small (ca. 25 mg HC per gram of TOC), except for floodplain samples which displayed a high variability, ranging between ca. 50 and 135 mg HC per gram of TOC (Fig. a). The R-index values show a comparable pattern with decreasing mean values from upstream (>0.60 for upper reach and floodplain; Fig. b) to downstream sub-environments (<0.60 for swamp, delta, and lake; Fig. b). The highest values were measured in floodplain samples (>0.65, Fig. b), while swamp samples displayed the highest variance, splitting into two groups (ca. 0.55 and 0.65, Fig. b). The I-index values revealed an inverse pattern with low values in upstream sub-environments (ca. 0.1 for upper reach and floodplain, Fig. c), high values for the downstream sub-environments (ca. 0.3 for delta and lake, Fig. c), and swamp samples divided into two groups (ca. 0.06 and 0.22, Fig. c).

In the I/R diagram (Fig. ) the studied samples are projected onto the decomposition line describing the linear relationship between R and I when the gradual decomposition of the most labile constituents controls the OM transformation (i.e., stabilization). Lake and delta samples contain the most labile OM (R<0.57 and I>0.25, Fig. ) while upper reach and floodplain samples contain the most stable OM (R>0.62 and I<0.25, Fig. ) with swamp samples falling in between these extremes (Fig. ). It is important to highlight that floodplain and swamp samples display high dispersion around the decomposition line in comparison with the strong correlation (R2>0.9) usually observed for composts, litters, and topsoils .

All surface sediment samples analyzed contained long-chain, odd-numbered n-alkanes (C23 to C35). The carbon preference index (CPI) has average values of 6.8 ± 1.5 for all surface sediments, confirming that n-alkanes originated from plant waxes due to the characteristic odd-over-even dominance. The CPI of the collected plant samples was 11.4 ± 6.4. The average chain length (ACL) for surface sediment samples was 30.2 ± 0.5, reflecting the high proportion of longer-chain n-alkanes, and 29.6 ± 1.9 for plant samples.

Differences in n-alkane distribution patterns between plant species have previously been observed in numerous studies . We are aware that the use of n-alkane distribution patterns to distinguish plant species and chemotaxonomic fingerprinting approaches is more controversial when transferring findings from one area to another, as it has been shown that variations can also occur within specific species and even between plant parts of the same species . Because all investigated plants, however, are from the same system and, when possible, multiple plants of the same species were sampled in different sub-environments of the wetland system, we believe that influences such as variability due to different climatic growing conditions and intra-species variability are small. Although this study investigates a rather small plant sample set, the obtained data show very good agreement with the literature (as discussed in more detail in the following text). For this reason, we are confident that the assignment of specific n-alkanes as marker compounds provides realistic indicators of inputs of dominantly occurring plants when performing intra-system comparisons especially when combined with information provided by compound-specific isotope analyses.

Aquatic plant species such as the floating Nymphaea spp. (common water lily) and the emergent wetland sedge P. australis (common reed) are primarily found in stagnant or slow-flowing waters. These two C3 plants (Fig. a and b) show elevated concentrations of the medium-chain n-alkanes (C23 and C25), while all other investigated plants show no or negligible concentrations (Fig. c to f). The occurrence of the medium-chain n-alkanes, although in previous studies reported as predominant alkanes of the respective distribution patterns , therefore seems to be explicitly indicative of aquatic plant types. The riparian zone along the Mkhuze River is dominated by typical woody plants of riparian forests, such as A. xanthophloea (fever tree) and F. sycomorus (sycamore fig) . While no representative of these species was sampled in this study, woody plants, such as trees, are generally well studied . Tropical C3 trees are typically characterized by high C29 and C31 n-alkane contributions (in sum >75 %), while the adjacent homologues show concentration mainly below 5 %. Especially the C29 n-alkane was recently described as “a responsive sensor” of tropical tree vegetation , which further corroborates its earlier observed tree bias . However, we pursue identifying alkane homologues which are present in certain plant types and absent or only very low concentrated in others so that they can serve as marker compounds for system-specific vegetation types. For this reason and given the high contributions of this alkane to nearly all displayed distribution patterns, we decided to regard the C29 and C31 n-alkane as more of an integrated signal which possibly originates from a diverse range of plants.

The wetland sedges C. papyrus (Papyrus; C4) and C. alternifolius (Umbrella papyrus; C3) (Fig. c and d) predominantly produce C31 n-alkanes, a finding consistent with the limited data available . Their natural occurrence is restricted to permanently flooded soils, whereas C4 wetland grasses such as V. cuspidata (hippopotamus grass) or Cynodon dactylon (Bermuda grass) (Fig. e and f) are mostly tolerant of intermittent soil flooding and other disturbances , so they generally occur widely. Like many (sub)tropical grasses these plant types are characterized by the presence of the very long-chain n-alkanes (C33 and C35) , which cannot be found in relevant concentrations in the other investigated plants (Fig. ).

Clear differences between the individual sub-environments of the Mkhuze Wetland System become evident by comparing the organic matter characteristics with respect to stability, degree of degradation, primary contributing vegetation, and its hydrological growth conditions. That these differences sometimes appear small in absolute values is attributable to the system itself. The discussed sub-environments (see Sect. ) cannot be separated from each other by sharp boundaries, but rather the transitions are shown to be gradual in their ecological manifestations. That the characteristics of the OM differ despite this gradual transformation underscores the importance of the individual environments to the system.

Characterization of organic matter in surface soils and sediments in terms of its stability, degree of decomposition, and source vegetation, as well as the hydrological conditions in the sub-environments of the Mkhuze Wetland System, allows us to evaluate transport pathways. Plant-wax lipids are hydrophobic and associated with the mineral component of sediments . Thus, the transport and identification of sources and sinks are not exclusive to the organic component of the sediment. In the Mkhuze Wetland System, sedimentary organic matter derived from the Mkhuze River catchment (hinterland) is primarily deposited on the floodplain but also reaches the Mkhuze Swamps. Under present conditions, Lake St Lucia does not receive significant quantities of sedimentary material from the hinterland or material exported from the Mkhuze Swamps. Cores retrieved from Lake St Lucia and the Mkhuze River bayhead delta indicate that sedimentary infilling commenced 6000–7000 years ago when the main oceanic inlets at Lake St Lucia sealed in response to rising sea levels during the Holocene transgression. This deposited sediment gave rise to the establishment of the current Mkhuze Swamps which currently act as an effective trap, preventing the rapid siltation of the lake. An exception might be large disruptive events (severe droughts or strong floods) which occur occasionally and could induce erosion of the swamp or bypassing of its filter capacity. The high concentration of plant waxes in the upper reach of the Mkhuze River is due to low-flow conditions during the sampling campaign. During the spring season, the river typically has low or even no flow , resulting in the deposition of suspended sediment in the riverbed. The upper reach samples collected in this study thus likely reflect the “undiluted” hinterland signal (highly degraded, stable, C3, most likely of woody origin). During periods of high flow, transported fluvial OM is deposited along the river channel and on the floodplain by bank overtopping . The even greater stability of the organic matter in the floodplain illustrates this depositional process (Fig. b). Proportions of the organic matter found in the Mkhuze Swamps are characteristically similar to that of the floodplain, so we assume that some suspended material is transported into and deposited in this sub-environment. This is clearly shown in the R index vs. I index diagram (Fig. ), which shows several clusters of samples: (i) high R index, low I index (upper reach, floodplain, swamp subset); (ii) low R index, medium I index (swamp subset); and (iii) low R index, high I index (lake and delta samples). Surface sedimentary OM from the delta and, in particular, the lake is significantly different from the upstream sub-environments. As discussed in the previous section, not only does Lake St Lucia experience a significantly lower input of sedimentary organic matter and plant waxes, but also the deposited material does not originate from the upstream sub-environments through which the Mkhuze River flows but most likely from the shoreline around the lake.

Analyzed signals from the lake area are thus local in origin and reflect locally occurring eco(hydro)logical conditions. Characteristics of transported hinterland signals predominate only in the upstream areas (upper reach, floodplain, swamp), although in the floodplain and the Mkhuze Swamps slight overprinting by locally introduced signals can be observed. Based on these findings, we suggest that the Mkhuze Swamps ultimately capture material transported by the Mkhuze River and consequently the integrated signals encompassing the river catchment, at least under current climatic and environmental conditions.

The identification of the Mkhuze Swamps as the ultimate sink for suspended OM from the Mkhuze River confirms previous studies that the Mkhuze Wetland System, specifically the floodplain and swamp, acts as an efficient filter upstream of Lake St Lucia . The current active filtering and trapping function of high sediment loads, including organic sedimentary organic load from the Mkhuze River, may prevent the otherwise rapid siltation of Lake St Lucia. OM in the surface sediments of Lake St Lucia originates primarily from lakeshore vegetation, as indicated by the lake transect data shown in comparison with upstream areas of the system. However, some studies assume that the Mkhuze Swamps, in their function as freshwater reservoirs (“sponges”) , are also responsible for input of OM, serving as a potential energy source in Lake St Lucia. Our data on particulate organic matter contradict this assumption, showing that sedimentary particulate OM is presently not transported from the hinterland nor exported directly from the swamp. In contrast, OM export from wetlands occurs primarily through the export of dissolved organic matter DOM;, which accounts for about 90 % of total OM . In saline waters, like Lake St Lucia, DOM is likely to flocculate . Assuming that OM is exported in dissolved form from the Mkhuze Swamps, it should thus be detectable in the lake transect surface samples, but this was not observed in this study. In part, this could be due to the fact that a significant proportion of DOC may be removed by sorption onto precipitating oxides when sediments contain substantial amounts of aluminum and iron metal oxides , as is the case in upstream sub-environments. With the employed methods we cannot confirm either any particulate OC or DOC export from the Mkhuze Wetland System into Lake St Lucia. The present study is the first examining sedimentary OM transport within the Mkhuze Wetland System, revealing that OM is indeed transported even to the Mkhuze Swamps and is not just deposited near the river channel and on the floodplain. Three processes might play a role for transport of material into the swamps: (i) the ongoing eastward progression of the floodplain , (ii) transport during severe flood events caused by cyclones and cutoff lows, or (iii) channelization having an impact on the transport efficiency. The transport path of the Mkhuze River has been dramatically shortened by channelization (see Sect. ). As a result, most of the Mkhuze River water now flows through the Tshanetshe–Demazane Canal System , which may have altered sediment transport. A shift in the area of deposition of material transported by the Mkhuze River is likely to affect the local vegetation distribution, i.e., causing a shift in the ecological zones by altered substrate conditions. In general, however, the Mkhuze Wetland System overall appears to exhibit high resilience against natural and/or anthropogenically induced changes. The severe drought of 2016, which led to the drying of large parts of Lake St Lucia, does not seem to have had any lasting impact on the filtering function of the swamps. Likewise, the establishment of the canal system also does not appear to have caused lasting damage to the filtering function of the Mkhuze Swamps.

In comparison with humid-region (tropical and temperate) wetlands, the Mkhuze Wetland System exhibits distinctive characteristics that reflect the low ratio between precipitation and potential evapotranspiration characterizing the region (Fig. 2). High evaporative demand and transmission losses from the river to the surrounding floodplain result in marked declines in both channel width and channel depth downstream . Downstream decreases in discharge and stream power result in a gradual decline in the ability of the Mkhuze River to transport particulate material, ultimately terminating in the Mkhuze Swamps. Although typically unusual, such downstream changes appear to be distinctive features of wetlands found in sub-humid and semi-arid regions of the world and are likely an important reason why the Mkhuze Wetland System acts as such an efficient trap for organic material. Most large tropical and temperate river systems are associated with wetlands along their river courses. For example, studies conducted on the Cuvette Congolaise , a large wetland system traversed by the Congo River, indicate that fluvially transported particulate OM signals from upstream sources are seasonally overprinted by storage and release of particulates in and from the wetlands . Depending on variable climatic conditions throughout the year, the Cuvette Congolaise wetlands may thus show a similar trapping function of transported material to the Mkhuze Wetland System under relatively dry conditions while showing increased export from material at higher water flows . Wetlands may thus switch from a trapping function to export of carbon depending on hydrological conditions and seasonal climatic changes. These differential trapping and export functions of wetlands need to be considered when reconstructing climatic changes based on sedimentary archives recovered from terminal lakes and offshore archives. Depending on the activity and efficiency of wetlands, material from more upstream areas will be effectively masked by wetlands depending on their hydrologic state and can even be overprinted be wetland export of OM or OM input from downstream areas. Such a process has been suggested for the transport of OM in the Amazon River system, where Andean material is effectively overprinted by lowland sources from rainforests, floodplains, and wetlands . In such cases, reconstructing environmental changes in the integrated watershed using offshore archives may thus not be possible. Wetland systems with an active trapping function effectively change the transported OM, so signals detected in offshore archives instead reflect specific sections of the river catchment. Other sediment-related proxies may also be affected by wetland trapping, so such geomorphological settings could have a much stronger influence than is often assumed. Combining environmental analyses with specific markers released from wetlands, on the other hand, allows an assessment of the hydrologic changes that lead to inefficient OM trapping and degradation of wetlands e.g., . In addition, carbon sequestration and the ability of wetlands to act as carbon sinks are considered to play an important role in the global carbon budget. There is concern that global warming may alter the hydrological balance of wetlands, releasing significant amounts of carbon to the atmosphere through direct oxidation processes or to adjacent water bodies through erosion of wetland soils, as has been observed for the Cuvette Congolaise and elsewhere . In certain cases, extreme weather events, which are expected to become more frequent as the global climate changes, have also been shown to promote the release of DOC from wetlands . It is likely that particulate material would also be exported by excessive flooding, as is similarly observed by increased flushing of terrestrial carbon into river systems . Thus, the trapping function of wetlands, overridden by overloading, would just as likely contribute to increased carbon dioxide emissions to the atmosphere through turnover of exported OM in adjacent waters.