A library of ancient climate
Along the coast of central Chile, near communities like Cobquecura, the shifting dunes conceal a remarkable scientific record. Here, erosion exposes dense layers of discarded shells, bones, and charcoal known as shell middens, or conchales. These are ancient garbage dumps; they are stratified archives of human life and environmental conditions stretching back at least 6,000 years to the Middle Archaic period.
Archaeological investigations of sites in this region, such as those near the mouth of the Itata River, provide evidence of long-term occupation by coastal hunter-gatherer groups. Radiocarbon dating of materials within these middens confirms a human presence from at least 6,200 years Before Present. The layers record history. Deeper, older strata contain the waste of the earliest inhabitants, while layers closer to the surface represent more recent activity. The volume of these deposits is large, with some middens in Chile extending over several hectares and reaching heights of more than four meters.
Inside these layers, scientists find a detailed account of the local marine ecosystem. The deposits are dominated by the remains of shellfish, but also contain fish bones, remnants of marine mammals like sea lions, and bones from birds. Stone tools, including various types of projectile points, and bone implements also appear, showing the technology and subsistence strategies of these early coastal peoples.
Reading the shells
The species of shellfish found in each distinct layer of the middens are a direct proxy for past ocean temperatures. As sea surface temperatures fluctuated over millennia due to climatic shifts, the dominant mollusk populations available to shoreline foragers also changed. A layer rich in species that prefer warmer waters, followed by a stratum containing cold-water species, gives a clear signal of environmental change.
Analysis of these faunal remains allows scientists to reconstruct the paleoenvironment with surprising detail. For example, the presence and abundance of the large gastropod Concholepas concholepas (known locally as "loco") can indicate periods of intense exploitation or specific oceanic conditions. These changes in diet, preserved in the garbage heaps of ancient people, correlate with larger climatic patterns, including the frequency and intensity of the El Niño-Southern Oscillation (ENSO), which has long affected this coastline.
The middens record diet and provide a long-term dataset on the interaction between human adaptation and climate change. By studying the contents of each layer, from the types of fish caught to the tools used to process them, researchers can understand how human populations responded to a constantly changing marine environment over thousands of years.