| Summary: | Lakes play an important role in the global carbon (C) cycle as they process carbon from terrestrial (allochthonous) and within lake (autochthonous) sources and may store C over long periods of time. Some arctic lakes contain high concentrations of dissolved organic carbon (DOC) that does not absorb light and thus remains colourless. The origin of this DOC remains unknown, but the sediment of these lakes have been suggested to accumulate primarily autochthonous (algal) C. I developed an experimental chamber for hydrogen (H) isotope pre-treatments and applied a novel H isotope tracing approach to determine the origin of the DOC and sediment C of a lake on SW Greenland known to contain colourless DOC. I hypothesized that autochthonous C was the prime source of DOC and sediment C, in line with previous theories. Analyses of algae and soil samples from the catchment revealed that local allochthonous and autochthonous C sources had a δ2H composition of -139 ‰ and -209 ‰, respectively. In contrast to my hypothesis, the analysed DOC had a mean δ2H isotopic composition of -147 ‰ indicating a dominance (ca 80-90 %) of allochthonous C. Similarly, the sediment had a mean δ2H isotopic composition of -155 ‰, suggesting that about 84 % of the C accumulating in the sediment was derived from terrestrial sources. The terrestrial origin was supported by field observations of high DOC seepage water (up to 70 mg L-1) with uncharacteristically low light absorption values entering the lake during high precipitation events. My results indicate that terrestrial processes are fundamental C sources for arctic lakes, even in regions with very low precipitation.
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