Climate and surface water acidity: development and application of a generalized predictive model

Abstract

A simple generalized geochemical model is presented that predicts long-term mean lake water acidity from rock type, annual precipitation and temperature, aeolian dust supply and catchment to lake area ratio; all of which can be estimated from mapped data. The model provides a geochemical tool to aid palaeoenvironmental interpretation of diatom inferred lake pH records, and shows how the sensitivity of lake water acidity to climate change varies with the physical and chemical properties of its catchment. Analysis of the model suggests that direct temperature effects on surface water alkalinity are generally minor, while variations in runoff rate caused by changing precipitation or evaporation can have a substantial effect. It is argued that variations in aeolian dust flux are more important than climate in regulating surface water alkalinity in areas of the world subject to above-average dust deposition, an effect that must be taken into account when considering possible direct climate impacts. Applying the model to early Holocene diatom inferred pH records, climatic factors are predicted to be minor compared with the effect of mineral depletion. The model is sensitive to assumptions about dissolved organic carbon production and lake water pCO2, the two factors that are least well understood. The lack of a quantitative model linking pCO2 with climate seriously limits our ability to quantify general environmental controls over lake-water pH. These issues require urgent attention if future climate change impacts on surface water acidity and pH are to be predicted reliably.