WHEN PROXIES DON'T AGREE--COMPLEXITIES IN THE INTERPRETATION OF DROUGHT SIGNALS FROM DIATOM AND OXYGEN-ISOTOPE RECORDS FROM NW MONTANA

Multi-proxy records are considered advantageous in paleoclimate studies, because they restrict the number of potential interpretations. However, proxies from high-resolution (i.e., sub-decadal) records may produce divergent interpretations because of lags, thresholds, different sensitivities, and seasonal influences. We note such factors when comparing changes in d¹⁸O values from lacustrine carbonates with changes in the ratio of benthic to planktonic diatoms from the laminated sediment of Foy Lake, northwestern Montana. Both records were produced from the same contiguous aliquots of sediment spanning the last 2000 years.

Variations in both proxies are usually linked to drought or shifts in effective moisture. Diatoms respond to changes in area of benthic habitat associated with water-level variation; d¹⁸O values respond to evaporation and shifts in the hydrologic balance of the lake. In several instances, including the 1930s drought, the diatoms record a "drought" about five years before the isotopes. This discrepancy may be due to a lag in the isotopic response of the lake or to a greater sensitivity of diatoms to abrupt changes in moisture balance. Alternatively, asynchronous shifts may result from unrelated climatic variables. For example, diatom assemblages may respond to variations in the timing of ice out–a factor that could affect spring-blooming diatoms but that might not affect the isotopes as strongly. Notably, coarser sample resolution would likely miss the offset.

These proxies also occasionally behave independently on longer timescales. From AD 1250 to 1800, small increases in d¹⁸O values are inconsistent with larger and more frequent variations in the diatom ratio. This cannot be attributed to lags and may be indicative of a threshold response. Modeling of the Foy basin indicates that a shift in water level by one meter can increase benthic habitat of the lake basin by 30%, causing a substantial shift in the benthic/planktonic ratio. This small change in volume, however, may be insufficient to significantly alter d¹⁸O values. We conclude that adding more proxies to a high-resolution paleoclimate study enhances interpretation of paleoenvironments only with a full understanding of the system and the environmental factor(s) controlling proxy responses.