A geostatistical framework for predicting variations in strontium concentrations and isotope ratios in Alaskan rivers

• We develop a GIS-based approach to predict 87Sr/86Sr in rocks and rivers of Alaska.
• Lithological proportion, permafrost and alpine glacial cover, and slope are the main controls of Sr flux to Alaskan rivers.
• The predicted 87Sr/86Sr from Alaskan runoff differs from the 87Sr/86Sr value used for Alaska in the global Sr budget.

Bataille and Bowen (2012) developed models to predict variations in the ratio of 87-strontium to 86-strontium (87Sr/86Sr) in rocks (bedrock model) and rivers (catchment water model) for regional provenance studies. Here, we revisit those models' formulation and calibration and apply them to predict Sr concentrations ([Sr]) and 87Sr/86Sr of Alaskan rivers. In a first step, we add several new components and/or improvements to resolve limitations of the model, including: 1) an independent siliciclastic sediment sub-model, 2) an explicit consideration of 87Sr/86Sr variability at the local scale, and 3) a fully-coupled assessment of prediction uncertainty. Tested against a compilation of 885 87Sr/86Sr rock analyses across Alaska, the new bedrock model significantly improves 87Sr/86Sr prediction accuracy in both igneous and sedimentary settings. In a second step, we develop a fully independent Sr chemical weathering model calibrated using a database of 339 [Sr] analyses from rivers of Northern Hemisphere high-latitude and predicting spatial variations in the rate of Sr release from rocks as a function of lithology, permafrost cover and slope. We combine the bedrock and Sr chemical weathering models to predict [Sr] and 87Sr/86Sr in Alaskan rivers. Tested on a dataset of 61 water samples, the resulting catchment water model explains 82% of 87Sr/86Sr variations in Alaskan rivers. We compare the average [Sr] and 87Sr/86Sr of Alaskan runoff estimated with the catchment water model to observed data of the Yukon River. The estimated average [Sr] and 87Sr/86Sr of Alaskan surface runoff – 104.3 μg/L and 0.7098 respectively – differ significantly from those of the Yukon River — 139.3 μg/L and 0.7137 respectively. This result calls into question the assumption that [Sr] and 87Sr/86Sr values estimated only from large rivers are representative of the Sr weathering flux from the entire Earth surface. The data products from this work provide an alternative basis for estimating 87Sr/86Sr values in rocks and rivers for regional provenance and chemical weathering studies across Alaska.