Habitability’s planetary signature takes many forms. Large scale impacts can be found in the biodiversity of “natural” communities (see Ingo’s post on invasive species), global geochemical signatures, and the shape or composition of landforms. As discussed by Lynn Soreghan in a readings post, one marker or “golden spike” delimiting the Anthropocene as a new geological epoch may be plastic. Not only is this human-generated material found in great quantities in the terrestrial biome, but it is found distributed throughout the marine water column, and has been implicated in the formation of a new kind of rock: plastiglomerate (Corcoran et al. 2014). This situation is not plastic’s fault, as noted by Lynn. The kinds of relationships people have with plastic, and the discard practices used to remove it from daily life, have resulted in plastic’s global reach.

Plastiglomerate may very well serve as a future reference point for the start of the Anthropocene. The age of plastic is just one example, however, of a pervasive history of human landscape modifications.  Archaeologists have identified numerous sediments and soils that have been created by human activity. So-called anthrosols form in a variety of contexts due to different social practices and environmental processes. Perhaps the best known are associated with agro-pastoral land use: the Amazonian dark earths (ADE, or “terra preta”), and plaggen soils. Broadly speaking, these soils form as carbon and other nutrients are concentrated in certain localities, often through burning, manuring, or disposal of food waste. While they may have started incidentally (although there is ongoing debate on this point), anthrosols contributed to long-term changes in biodiversity and landscape utility, and are even sought out today as sources of fertile soil.

Erlandson introduces shell middens as another widespread anthrosol. Broadly speaking, shell middens are anthropogenic soils found in marine, lacustrine, or riverine settings. They were created by hunter-gatherer communities (more often than not), through the deposition of invertebrate organisms’ hard shells and other substances. Shell middens occur in many different shapes and scales, including small scatters with a handful of shells only, linear features stretching for kilometers, or mounds well over 10 meters high and hundreds of meters long. Archaeologists have long been interested in shell middens due to their visibility, and for what they contain. Because of their chemistry, shell middens often have remarkable preservation of organic matter (i.e. animal bones and plant matter). At a larger scale, shell midden accumulation promotes “distinctive soil chemistry conditions (e.g., highly elevated phosphate, calcium, and organic levels) that can alter soil hydrology and support unique plant communities” (Erlandson, p. 26-27). Erlandson argues that shell middens provide a useful, widespread stratigraphic marker for an early anthropocene (or paleoanthropocene) start date. Although humans likely exploited aquatic resources during the Pleistocene, it is only during the Holocene that there is widespread evidence for the intensive exploitation of aquatic resources and subsequent deposition of shells in particular locations. There are likely several reasons for the increased frequency of shell middens, including widespread human colonization, and deceleration of sea level, particularly after 8,000 years ago.

Beyond their status as an indicator of past human activity, shell middens evoke the same kind of ambiguity in values and significance that the broader notion of the Anthropocene elicits.  Are shell mounds “trash” or are they valued cultural constructions?  The etymology of the word “midden” is revealing. When shell-bearing sites were first described by archaeologists (in early 19^th-century Denmark), they were referred to as kjökkenmödding, the basis for the English word “midden.”  Kjökkenmödding literally translates into “kitchen refuse,” with the implication that shell middens are simply trash heaps. Yet closer examination demonstrates that there were a variety of reasons why shell middens were created: some were trash piles, others were cemeteries, and in many cases shell was used as a medium to construct monuments or ceremonial architecture (see Bailey et al. 2013). Often shell middens underwent several transformations in social values from community refuse piles to sacred venues, all the while influencing local biodiversity. Thus, far from resulting from indifference, anthrosols such as shell middens could be created intentionally, often with long-term, unintended consequences.

Mollusk shells are certainly not plastic fragments. Just like plastiglomerate, however, shell middens should challenge how we think about human discard practices and their effects through time and space. That is, the formation of these deposits, how the materials arrived in particular places, the subsequent reworking or recycling of materials in situ, and the various significance or values attached to these golden spikes provide insight into the complexities of habitability. We should not assume, for example, that the presence of anthrosols is indicative of a cross-cultural predilection for wasteful behavior.