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Holocene cultural and climate shifts in NW Africa as inferred from stable isotopes of archeological land snail shells

Padgett, Abigail, Yanes, Yurena, Lubell, David, Faber, Meredith L
TheHolocene 2019 v.29 no.6 pp. 1078-1093
C3 plants, Gastropoda, Melanostoma, Sahel, carbon, climate change, cold, drought, humans, models, ontogeny, oxygen, paleoclimatology, radiocarbon dating, relative humidity, shell (molluscs), snails, stable isotopes, time series analysis, Algeria
Cultural transitions or even societal collapses have often been associated with long-term drought events. Linkages between humans and the environment are best documented in well-constrained archeological records that offer multi-centennial to multi-millennial cultural and paleoclimate data. Two Holocene Capsian sites, Kef Zoura D and Aïn Misteheyia, from NE Algeria document a marked change in subsistence strategies near 8200 cal. yr BP. Radiocarbon dated archeological shells (from 10,300 to 6700 cal. yr BP) of the terrestrial gastropod Helix melanostoma were studied to examine the role of climate change on cultural shifts. Oxygen (δ¹⁸O) and carbon (δ¹³C) stable isotopes were measured from whole shells and time-series profiles along shell ontogeny to assess average annual paleoclimate and degree of seasonality, respectively. Shell δ¹⁸O values illustrate that conditions were wetter between 10,300 and 9000 cal. yr BP, coinciding with the ‘African Humid Period’, whereas the environment turned drier at 8000–7600 cal. yr BP, immediately after the 8.2-ka climate event, feasibly comparable in magnitude to the drought episode initiated in 1968 in the Sahel and N Africa. A snail evaporative steady-state flux balance-mixing model suggests that snails at around 8000 years ago precipitated shells under notably lower relative humidity conditions than previous wetter scenarios of the earlier Holocene. The well-known 8.2 ka cold event of the Northern Hemisphere was detected as a drought event that seems to have lasted several centuries in Algeria. The δ¹³C values indicate that snails only consumed and assimilated C₃ plants without noticeable shifts. This study points to multi-millennial humidity fluctuations in NE Algeria, which likely affected the economy and subsistence strategies of prehistoric human groups in the area.