The geographic distribution of strontium isotopes in Danish surface waters - A base for provenance studies in archaeology, hydrology and agriculture

摘要

In this paper Sr isotope signatures are reported for 192 surface water (lakes/ponds and rivers/creeks) samples from within Denmark and an isotope distribution map is presented that may serve as a base for provenance applications, including archaeological migration studies, ground water - surface water - seawater interaction/contamination monitoring, and potentially for agricultural applications, including cases of authenticity proof for particular food products. The Sr isotopic compositions of surface waters range from ~(87)Sr/~(86)Sr=0.7078 to 0.7125 (average 0.7096±0.0016; 2σ). This average value lies above the range of ~(87)Sr/~(86)Sr values between 0.7078 and 0.7082 expected from Late Cretaceous to Early Tertiary (Oligocene) limestones which form the dominant bedrock type in a NW-SE trending belt in Denmark. The elevated ~(87)Sr/~(86)Sr signatures >～0.7095 are explained by additions to the surface waters of radiogenic Sr predominantly derived from the near-surface weathering and wash-out of Quarternary glaciogenic tills and soils deposited and formed during and after the last two ice age stages (Saale and Weichsel). The Sr isotopic compositions and concentrations of the surface waters can, therefore, best be modeled by a two-component mixing involving carbonaceous bedrock and glaciogenic cover sediments as the two predominant Sr sources. A feasibility study for using Sr isotopic compositions of surface waters as a proxy for bio-available Sr signatures was conducted in a representative test area on Zealand (Land of Legends, Lejre) where there is no use and application of commercial fertilizers. It is demonstrated that the Sr isotopic signatures of lake waters from within this area are slightly higher (but statistically still indistinguishable) from the average value defined by snail shells and soil leachates considered to characterize the true bio-available Sr. In combination with results from other studies, this is interpreted to reflect the wash-out of Sr with a higher ~(87)Sr/~(86)Sr signature released by weathering in the topsoils into the saturated water tables, a component which is consequently not readily transferred into the shallow-rooting plants and into small herbivores feeding on them. Since drinking water is a likely important source of Sr uptake of humans and larger animals, the contention is that a surface water isotopic composition can potentially characterize the bio-available component relevant for human and carnivorous large animals. Spline functions and ordinary linear kriging were used for modeling the geographic distribution of bio-available Sr isotopes over territorial Denmark. As expected, based on the two-source mixing scenario, the water ~(87)Sr/~(86)Sr ratio contour maps (with some notable exceptions) neither mirror the pre-Quarternary bedrock geology nor a (Pleistocene) soil-type distribution map of Denmark. As a conservative suggestion, the use of the average ~(87)Sr/~(86)Sr ratio of 0.7096±0.0015 (2σ) is recommended as an average for bio-available Sr from within Denmark (Bornholm excluded) for human and larger animals and as a confidence band for distinguishing "local" from "non-local" signatures in archaeological provenance studies. The study also reveals that the average ~(87)Sr/~(86)Sr ratio of ～0.7088 defined from soil extracts and small herbivores lies approximately 0.15% lower than that defined by the surface waters. The authors recommend using this lower value as a "local" -" non-local" discriminator for food and plant authenticity control in agricultural applications.