Design and application of a proxy system model for the quantitative reconstruction of hydroclimate variabiility recorded by oxygen isotopes in lacustrine carbonate sediment

Abstract

Oxygen isotope analyses of lacustrine sediment, which are widely used as proxies of past climatic variability, have become increasingly reliant on computational modeling approaches that allow for quantitative interpretations of past hydroclimate, constraining of water resources’ sensitivities to changes in climate, and direct comparisons of proxy data with climate models. In this study, we present the development, structure and application of a Proxy System Model (PSM) designed for Castor Lake (Washington, U.S.A): a well-understood, highly monitored small lake system. The principal goal is to improve upon the understanding of the relationships between climate and the stable oxygen isotope (18O) proxy system in the context of lake sediments, by addressing the impacts that a variety of climate variables, as well as non-climate relate factors such as basin morphology, vegetation, hydrologic setting and lake mixing, have on the isotopic signatures of resulting sediments in the lake, as well as to provide a quantitative basis from which well-informed reconstructions of past climate can be made. Following a calibration process based on over a century of compiled daily weather data as well as approximately 14 years of in-situ continuous measurements of lake level, temperature and water oxygen isotope samples, the PSM was shown to accurately reproduce seasonal, interannual and century-scale trends of sediment oxygen isotope values and water balance, with varying degrees of accuracy for different timescales. Model-based reconstructions of hydroclimate variables for an early Holocene (~10000 years B.P.) δ18O maximum in the Castor Lake sediment record show that cold-season (i.e. winter) precipitation and relative humidity must have been lower by 21% ± 5% and 14% ± 7%,respectively, in order for the observed sediment δ18O signature to be produced and recorded. Furthermore, air temperature and warm-season precipitations seem to be negligible controls on sediment δ18O signatures, opposite to what was expected following the temperature dependence of carbonate sediment formation and isotopic fractionation. These results showcase the advantages of the application of PSMs to the analysis of paleoclimate proxy records as a way to make well-informed quantitative interpretations of past climate change through the constraining of physical, chemical and biological processes that impact the formation of the sedimentary archive.