Origin, transport, and retention of fluvial sedimentary organic matter in South Africa's largest freshwater wetland, Mkhuze Wetland System

Sedimentary organic matter (OM) analyses along a 130 km long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St Lucia, Africa's most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. Combining bulk OM analyses, such as Rock-Eval (R), and source-specific biomarker analyses of plant-wax n-alkanes and their stable carbon (delta C-13) and hydrogen (delta D) isotopic composition showed that fluvial sedimentary OM originating from inland areas is mainly deposited in the floodplain and swamp area of the wetland system but not in the downstream lake area. A distinctly less degraded OM signature, i.e., a considerably lower degree of transformation of unstable components (higher I index) and lower contribution of refractory and persistent fractions (lower R index) as well as recognizably higher delta D values compared to samples from upstream sub-environments, characterizes surface sediments of Lake St Lucia. The offset in delta D indicates that the contributing vegetation, although similar to upstream vegetation inputs in terms of photosynthetic pathway (delta C-13) and alkane distribution pattern, experienced different hydrological growth conditions. The results suggest that under current conditions hinterland sedimentary OM is deposited throughout the wetland system up to the Mkhuze Swamps, which ultimately captures the transported OM. Consequently, samples from the downstream located Lake St Lucia show locally derived signals instead of integrated signals encompassing the river catchment. This finding raises important constraints for future environmental studies as the assumption of watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore might not hold true in certain settings.

Automated summary

The study reveals that sedimentary organic matter from inland areas is mainly deposited in the floodplain and swamp area of the wetland system, rather than the downstream lake area. Surface sediments in the downstream Lake St Lucia show distinct characteristics of less degraded organic matter, indicating different hydrological growth conditions for the contributing vegetation. This finding has important implications for future environmental studies.