City-Strata of the Anthropocene

The fabric of a city represents a transformation of raw geological materials into a complex assemblage of new, human-made minerals and rocks such as steel, glass, plastics, concrete, brick, and ceramics. This activity has been considered in terms of an “urban metabolism,” with day-to-day inflows and outflows of people, food, water, and waste materials. Here we adopt a longer time-scale spanning years to millennia, related to geological time-scales but still meaningful for present and future generations of humans, and consider cities as sedimentary systems. In natural sedimentary systems, flows of materials are governed by natural forces such as climate and gravity, and leave physical records in, for instance, river-strata. In cities, the flows of geological materials needed for construction and reconstruction are directed by humans, and are largely powered by the fossil energy stored in hydrocarbons rather than by gravity or the sun. The resultant assemblages of anthropogenic rocks and minerals may be thought of as sedimentary (and/or trace-fossil) systems that can undergo fossilization and now exist on a planetary scale. Far more diverse than natural geological strata, they are also evolving much more rapidly, not least in terms of their growing waste products. Considering cities through such a perspective may become increasingly useful as they come to be influenced by, and need to adapt to, the changing conditions of the emerging Anthropocene epoch.