Oxygen isotope ratios of organic matter in arctic lakes as a paleoclimate proxy: field and laboratory investigations

Paleoclimate research based on the stable isotopic composition of lake sedi ments is often hampered by the lack of preservation of suitable material fo r isotopic analysis. We examined organic material as a proxy for past water isotopic composition in a series of experiments. First, we cultured aquati c moss under constant illumination, temperature, and water delta O-18, and show that new cellulose records source water delta O-18 precisely (r(2) = 0 .9997). Second, we analyzed paired lakewater and vegetation samples collect ed from sites spanning strong climatic gradients. In field conditions, the relationship between organic delta O-18 and water delta O-18 is more variab le, though it is still controlled by environmental water isotopic compositi on. However, terrestrial mosses in the arctic are often significantly enric hed in delta O-18 relative to aquatic mosses in nearby lakes due to their u se of different water sources. Third, we measured delta O-18 of cellulose e xtracted from disseminated sedimentary organic material. In the majority of the middle- to high-arctic lakes in this study, the delta O-18 of dissemin ated sediment cellulose is greatly enriched relative to the expected values based on lakewater delta O-18, suggesting a significant component of terre strial cellulose. This interpretation is supported by radiocarbon dates fro m a Holocene sediment core in which C-14 ages of sediment cellulose are 700 -5000 yrs older than the enclosing sediments. We conclude that aquatic cell ulose can be used as a reliable tracer of lakewater isotope ratios, but ter restrial cellulose often dominates the sedimentary cellulose pool in places such as Baffin Island where sedimentation rates are low enough to allow th e degradation of aquatic cellulose. Care must be taken when interpreting se diment cellulose delta O-18 records where diagenesis has played a role, bec ause terrestrial cellulose is more resistant to degradation, and therefore can predominate in environments with low organic carbon burial.