Modern plants and sulfur isoscapes - A review, discussion, and construction of a pilot δ³⁴S isoscape for mobility and provenance studies

Rationale: Sulfur isotopes are increasingly used as mobility indicators in humans and animals in biology, archaeology, and forensics. However, there has been a lack of modern sulfur isotope baseline "isoscape" studies using modern plants and animals, largely due to the possibility of contamination of the S isotope values by modern pollution.

Methods: We collected plants from across a 900-km east-west transect of British Columbia Canada and measured their sulfur isotope values. We then used a random forest model to determine which variables best explained the isotope data patterning and produced a sulfur isoscape for the southern region of British Columbia.

Results: We see clear patterning in the plant sulfur isotope values that relate to geographical location and rainfall. Our model also shows that for this study area, it is unlikely that there is a significant influence of anthropogenic pollution on plant δ³⁴S values. We also discuss the use of plants as a substrate for sulfur isoscapes and possible explanations for the often-observed difference between plant and animal δ³⁴S values from the same region, related to differing sources of sulfur in plants compared to amino acids in human and animal tissues.

Conclusions: We found that for areas of the world where sulfur pollution is likely less widespread, it is possible to produce a modern plant S isoscape that should be an accurate baseline for mobility studies. Using random forest modelling, we have produced a baseline sulfur isoscape map of southern British Columbia that can be used for ecology, forensic and archaeological studies.