Archaeological shell middens from the coastal Atacama Desert as regional proxies for Holocene upwelling and environmental history

Shell middens are human refuse mounds often interspersed with dwelling structures and even burials. Such deposits are found throughout coastal northern Chile, and span more than 9000 years. These deposits contain abundant terrestrial plants and shellfish remains and accumulate very quickly (one midden accumulated 5 m in c. 2000 years). They can be dated to estimate local radiocarbon comparisons of marine and atmospheric ages (R) along a given stratigraphic horizon. Such estimates can then be used to estimate local (or regional) marine reservoir departures from the global marine ¹⁴C calibration curve (ΔR) and constitutes a local record of upwelling conditions. Here, we present a well-replicated ΔR chronology spanning more than 9,000 years obtained from several different shell middens located c. 100 km apart. Our replicated ΔR estimates show that shell middens are proxies for regional upwelling and not just limited to local geographical effects. Our ΔR record shows remarkably stable values during the early to mid-Holocene (9.2 to 5.4 ka), which averaged 270 ¹⁴C yrs, implying stable upwelling. This stability was coeval with the largest increase in local coastal hunter gatherer populations during their entire history. In contrast, ΔR values became highly variable between 5.4 - 4.7 ka, with oscillations in ΔR exceeding 1000 yrs. This implies strong environmental fluctuations associated with variable upwelling conditions and the onset of El Niño-Southern Oscillation (ENSO). These variations were also coeval with one of the largest prehistoric coastal population collapses in northern Chile. Conditions stabilized until 2.4 - 2.0 ka when large excursions again occur in our regional ΔR record. More recent ΔR estimates of c. 200 ¹⁴C yrs occurred c. 500 yrs ago, comparable to modern (pre-bomb) estimates from northern Chile (which range from 141 to 196 ¹⁴C yrs). In summary, our coastal shell midden record shows that most of the Holocene had a stable upwelling regime, but was punctuated by strong changes in upwelling conditions in the mid and late Holocene. Such changes where likely driven by large-scale Holocene climate change (such as the onset and intensification of ENSO) and may be one of key factors behind the large-scale demographic changes of coastal hunter-gatherer societies.

Funding: FONDECYT 1150763 & AFB 170008