Vegetation and climate changes during the Middle to Upper Palaeolithic transition in the southwestern Mediterranean: What happened to the last Neanderthals during Heinrich stadial 4?

Abstract

During the last glacial period, and particularly Marine Isotope Stage 3 (MIS 3, dated 60–27 ka), various abrupt and arid climate episodes impacted the northern hemisphere. These are known as Heinrich Stadials and are linked with major iceberg discharged in the North Atlantic. Heinrich Stadial 4 (HS4), one of the strongest of these events occurring around 39 ka BP, has raised numerous debates regarding its potential impact on the Middle to Upper Palaeolithic transition (MUPT) and Neanderthal extinction, especially in the Iberian Peninsula where late persistence of Neanderthals has been claimed beyond 40 ka BP. Although palynological studies from the Iberian margins have previously highlighted vegetation changes during this period, these have generally been of low resolution and thus, the environmental conditions during HS4 and the possible impact of this event on the last Neandertal populations are still poorly understood.

This study uses the marine core from Ocean Drilling Program (ODP) Site 976, located in the Alboran Sea, to provide new ultra-high-resolution analyses for the interval between 41 and 34 ka BP, including the Greenland Stadial 9 (GS9)/HS4 and the Greenland Interstadial 8 (GI8). The aim is to reconstruct vegetation and climate changes in southern Mediterranean with a resolution never achieved before and to discuss human occupation in the frame of these changes. Vegetation, climate changes and human occupation are discussed on the basis of 1) pollen analysis, 2) quantitative climate reconstructions based on a multi-method approach, and 3) maps of human occupation based on a synthesis of archaeological sites for western Mediterranean Europe in the 42–34 ka BP time interval. Results show a fast spread at centennial scale of steppe and semi-desert vegetation, with a typical three-phased pattern for the GS9 (HS4). This pattern is compatible with the results of various records from Greenland, southwestern Europe and Brazil. The description of these phases with an unpreceded resolution for the SW Mediterranean region supports the connection between the hydroclimatic changes in Greenland and lower latitudes, while the transition to the following GI8 climate amelioration displays a major vegetation instability that mimics the pattern of the last deglaciation at sub-millennial scale. The climate reconstruction shows cold conditions during HS4 and enhanced aridity mostly linked to the winter rainfall deficit caused by the abrupt disturbance of the Mediterranean seasonal regime of precipitations. The regional comparison with available palynological and paleoclimatic records from the western Mediterranean and the Atlantic sheds light on the spatial expression of the arid event, highlighting a south-westward trend in aridification in the Iberian Peninsula. The Alboran Sea appears as a critical area at the interface of the Atlantic and Mediterranean influences, recording the particular expression of fast climate oscillations in the westernmost part of the Mediterranean. The possible impact of such extreme climate event on the MUPT is discussed based on an updated dataset of archaeological sites and dates for western Mediterranean Europe in the 42–34 ka BP time interval. Maps of human occupation show that Neanderthals may have undergone population contraction during the arid conditions of HS4, but it is still challenging to place the MUPT in a sub-millennial chronological framework due to the uncertainty and limitations of archaeological data.