How well do sediment indicators record past climate? An evaluation using annually laminated sediments

The reliability of paleoclimatic inferences from lake-sediment records rests on the understanding of how various sediment indicators respond to environmental changes. Despite the recent proliferation of paleoclimatic records, only a limited number of studies have rigorously evaluated potential indicators by comparing lake-sediment records with instrumental weather data. We analyzed annually laminated sediments of the past 100 years from a lake in north-central Minnesota for a suite of variables commonly used for climatic reconstructions. Results were compared with time series of climatic or climate-derived variables, as well as with indices of climate modes thought to influence the regional climate of the midwestern United States. The oxygen-isotopic composition of calcite (delta O-18(c)) shows trends similar to those of effective moisture (as measured by precipitation [P] minus actual evapotranspiration [AET], P-AET, and the Palmer Drought Severity Index [PDSI]), with high delta O-18(c) values generally corresponding to low P-AET and low PDSI. delta O-18(c) also exhibits striking correspondence with the index of Pacific Decadal Oscillation (PDO) but it lags behind PDO by 3 years. Elevated delta O-18(c) values during the warm PDO phase probably reflect warm and dry climatic conditions in the midwestern US, especially during winter months. The carbon-isotopic composition of calcite (delta C-13(c)) shows some similarity with delta O-18(c) but also displays stratigraphic patterns resembling those of lake-productivity indicators, including biogenic silica, inverse of inorganic carbon, ratio of organic carbon to nitrogen, and to a lesser extent, organic carbon. delta C-13(c) is correlated with mean annual air temperature (MAAT) and the index of Atlantic Multidecadal Oscillation (AMO), probably because elevated MAAT stimulates the rate of algal carbon uptake, lengthens growing season, and/or enhances CO2 degassing. Varves are typically thicker during periods of lower delta O-18(c) and higher P-AET (or PDSI) values, because wet climatic conditions probably increase nutrient availability and lake productivity. Gray-scale intensity does not co-vary with any of the above climatic variables or climate-mode indices. These results demonstrate the utility of lake-sediment analyses for reconstructing temperature, drought, and large-scale climatic modes at Steel Lake. However, application to down-core reconstructions may be compromised by a number of factors, including the site specificity and non-stationarity of such relationships.