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

Abstract

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.

Local-scale environmental gradients in ‘snail-shell’ stable isotopes from Holocene Jordanian archaeological sites

“…All other symbols are terrestrial snail shell values; orange dots are G. granum (W-80), black dots are G. granum (SS-1) and open triangles are P. coenopictus (M7 SS-1). Green stippled box shows field for comparative ~8.2 ka BP C 3 terrestrial snail data from Italy (Colonese et al, 2010, 8.8 to 6.7 ka BP), Algeria (Padgett et al, 2019, ~8.0 ka BP), and Spain (Yanes et al, 2012 8.4 ka BP).…”

Chronology, time averaging, and oxygen isotope composition of harvested marine mollusk assemblages from Ifri Oudadane, northeast Morocco

“…The present environmental conditions at Ifri Oudadane and northwest Africa are dictated by several climate mechanisms, including the Intertropical Convergence Zone (ITCZ), the westerly winds, and the North Atlantic Oscillation (NAO) (Bazzicalupo et al, 2018; Padgett et al, 2019). The seasonal migration of the ITCZ affects wind intensity in the area as well as the African monsoon rain (Bazzicalupo et al, 2018).…”

“…The westerly winds bring rainfall to northern Africa during the winter months (Bazzicalupo et al, 2018). The sea level pressure gradient from Iceland to Azores, characterized by the NAO index, causes variations in storm intensity and the westerly winds (NOAA, 2012; Bazzicalupo et al, 2018; Padgett et al, 2019). The mode of the NAO switches between positive and negative according to variations of pressure at sea level between the Azores high-pressure system and the Icelandic low-pressure system (NOAA, 2012; Parker et al, 2020).…”

Evaluation of Aqueous Gastropod Shells as Groundwater Radiocarbon Proxies Across Species and Sites

“…Calcareous material (either calcite or aragonite) is fixed in layers with an organic matrix to extend or expand the shell whorls (Wilbur 1964). We note that lung-breathing gastropods may use more metabolic C in their shells, making them less ideal representatives of water chemistry (Goodfriend 1992;Goodfriend and Ellis 2002;Stott 2002;Copeland et al 2012;Hill et al 2017;Padgett et al 2019).…”