Reconstruction of ancient land-use in the Orkhon Valley, Central Mongolia, by waste markers: phosphorous, sterol and bile acid analyses

The Mongol Empire (13^th - 14^th century CE) was the largest contiguous land empire in world history. However, it is not yet known how an urban lifestyle in its first capital Karakorum was sustained in the heart of the Mongolian steppe. One aspect of this is how food supply and subsequent delivery of raw materials was secured.

Therefore, we aimed at characterizing soil resources in Sarlag Tolgoi, an ancient settlement about 50 km northwest of Karakorum. We took samples of a transect through the settlement and reference samples of undisturbed soil, serving as control. We (i) analyzed these samples regarding their phosphorus concentration as waste marker and (ii) the sterol and bile acid concentration in cases where phosphorus levels were elevated, in order to reconstruct past settlement structures and the fingerprint they left on the surrounding environment.

We found a tenfold increase of phosphorus concentrations from 2 to 20 mg P kg^-1 in the topsoil from the surrounding area compared to the soil within the settlement itself. This clearly supports the hypothesis of anthropogenic influence at this site. A closer examination of those samples with increased phosphorus concentration by sterol and bile acid analysis revealed hotspots of an ancient faecal input by grazing animals - mainly cattle and sheep - within the settlement. Furthermore, the applied ratio (epi-5β-stigmastanol/5β-stigmastanol + epicroprostanol/coprostanol) revealed no indication of faecal input from horses, whereas low proportions of coprostanol suggested limited human faecal input. Therefore, we suppose that horses and ditches for human waste were outside of the settlement area.

In conclusion, our results demonstrate that Mongolian steppe soils preserve ancient fingerprints of human settlement associated with the Mongol Empire, expressed through changes in both, their morphology, and chemical signature. These findings highlight the considerable potential of basic soil science approaches to refine and strengthen archaeological interpretations in this region. Moreover, complementary chemical analyses provide valuable insights into past lifeways and human-environmental interactions. While the unambiguous attribution of signals to specific historical periods remains challenging, future integration of compound-specific biomarker dating in the vicinity of archaeological findings holds strong promise for achieving more robust chronological resolution.