Recent changes in the composition of sedimentary organic material (OM)
from a well-oxygenated lake (Lake Neuchatel, Switzerland) were
investigated using the new generation Rock-Eval pyrolysis (Re6).
Although Re6 offers a range of new analytical parameters, the classical
hydrogen and oxygen indices (HI and OI) prove to be most useful for the
kind of material analysed. For a rapid screening of lake sediment cores
we propose an additional parameter, POI=S3/(S1 + S2a). which reflects
most of the observed changes in the nature of the OM, but can be
calculated from the pyrolysis step alone.
In the Lake Neuchatel sediment cores the HI values and total organic
carbon (TOC) are low in samples older than ca. 1880 (HI: 150-200; TOC:
0.7%). and thereafter gradually increase. The HI and TOC values
stabilize in the 1960's (HI: 300-350; TOC: 1.7 %). This latter age is
inferred from the location of a layer enriched in talc, originating from
a paper mill. This local time marker horizon is dated to 1952. The OI
curve is by and large inversely correlated with the HL Together, the HI,
OI and TOC curves trace the change from an oligotrophic lake, where the
relative contribution of terrestrial OM is important, towards a more
eutrophic system with steadily increasing autochthonous OM with an
important diatom component.
A horizon (ca. 1960's) containing allochthonous material introduced by
dumping of excavation material was identified based on its mineralogy
and on the soil-type HI-OI values of the OM present.