Identifying Source Soils in Contemporary Estuarine Sediments: A New Compound-Specific Isotope Method |
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Authors: | M M Gibbs |
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Institution: | (1) National Institute of Water and Atmospheric Research Ltd., PO Box 11-115, Hamilton, 3251, New Zealand |
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Abstract: | A new method is proposed for the identification and apportionment of contemporary source soils contributing to estuarine sediments.
The method uses compound-specific isotopic analysis of naturally occurring biomarkers (fatty acids) derived from plants to
link source soils to land use within a single catchment. For identification and apportionment of source soils in the estuarine
samples, the method uses the isotopic mixing model, IsoSource. The feasible proportions obtained from IsoSource are then scaled
to allow for the percent organic carbon in the source soils. With this approach, the estimation of each source soil contribution
to a location in the estuary is independent of any degradation of the biomarkers through microbial or biogeochemical processes.
Identification relies on the evaluation of the sediment sample relative to a “library” of reference source soils from different
land use within the catchment. Selection of potential sources is geographically constrained by the requirement for a natural
linkage between each source soil and the sediment site sampled. A case study, using this method, mapped the distribution of
three main land use source soils (pasture, native forest, and pine forest) across the river delta in a small estuary fringed
with mangroves. Rather than being uniformly distributed, the results indicated that the source soil contributions varied markedly
across the delta, raising concerns about the validity of taking single cores to characterize the sediments of an estuary.
Coupling the source apportionment results with land use data indicated that the mean percent contribution of pine forest soil
in the river delta sediments was almost three times greater than the percent land use area of pine forest in the catchment.
Furthermore, isotopic signatures indicated that most of the pine forest soil came from the much smaller areas exposed to erosion
by clear cut harvesting and that the soil contribution from recently harvested areas of pine forest could be as much as 20
times greater than that land use area in this catchment. This is the first method that can identify and apportion, by land
use on a catchment scale, the sources of soil contributing to the sediment at a location of an estuary. The results are given
as a “best estimate”, within definable limits, of the proportional contribution of each potential source soil. Information
obtained using this method will allow development of management strategies to alter land use practices to reduce the sediment
load to rivers, and thus, the impact on the aquatic ecosystem downstream in estuaries. |
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