Mechanisms of rapid lithification of anthropogenically reworked legacy slag deposits

Anthropogenically deposited clastic sediment is a major part of the Earth’s land surface but its long term post-depositional fate is poorly understood. In anthropogenic sediment deposits dominated by ferrous slag (the by-product of iron and steel making), lithification can occur rapidly. Here, we analyse two case study slag heaps in West Cumbria, UK, and demonstrate that they have been anthropogenically reworked since their original formation. Clast angularity determined from clast analysis indicates anthropogenic, rather than water-based natural reworking. Geomorphological and historic map evidence indicate anthropogenic reworking occurred post-1969 at both case study sites, and thus the material has undergone lithification in a period of ca. 40-60 years. This is orders of magnitude quicker than lithification in natural clastic sediment systems. Microstructural and chemical analysis reveals that a calcite cement is binding the clasts of slag together. The lithification of anthropogenically reworked ferrous slag deposits can have a wide range of implications from improving geotechnical properties to increasing the likelihood of surface flooding. This understanding of the lithification process highlights the importance of planning where anthropogenic sediment is deposited, especially as lithification increases the permanence of the landform.