CARBON AND NITROGEN ISOTOPIC COMPOSITIONS IN UINTA MOUNTAIN (UTAH, USA) LAKE SEDIMENTS: POTENTIAL PROXIES OF ENVIRONMENTAL CHANGE

High-elevation alpine lakes are remote, often with little human activity in their catchments. These lakes are also nutrient-poor, so are particularly sensitive to additions of nutrients as a result of atmospheric pollution or climate change. Carbon (C) and nitrogen (N) isotopes have been used to determine lake response to additions of nutrients; however, isotopic values can also be affected by degradation processes, and shifts in organic matter sources. An improved understanding of: (i) the isotopic signatures of possible OM sources, and (ii) the processes affecting OM isotopic compositions prior to and after deposition, is needed to develop our understanding of the C and N isotopic records preserved in lacustrine sediments.

We have initiated such a project to better understand controls on C and N isotopic compositions and C/N ratios in high-elevation Uinta Mountain (Utah, USA) lakes. As a first step, we are characterizing potential sources of OM to the sediments. Our current baseline for potential terrestrial and semi-aquatic OM sources shows that they share similar C/N ratios and C isotopic compositions, but have more distinctive N isotopic characteristics. We have compared these source compositions to surface sediments from the littoral zone of Uinta Mountain lakes. We have found little overlap in N isotopic compositions and C/N ratios between sediment OM and terrestrial plants, confirming the dominance of aquatic OM in these deposits. We are expanding our baseline dataset to better represent algal samples. We are also initiating compound-specific isotopic analysis of lake sediment OM and its potential sources. Isotopic measurements of n-alkanes, in particular, should help to quantify how degradation can modify original bulk C isotopic signals of an OM source. This database will help us to interpret the environmental signals provided by the C and N isotopic compositions of OM from Uinta Mountain lake sediments with much greater confidence.