How Far North Did the African Monsoon Fringe Expand During the African Humid Period? Insights From Southwest Moroccan Speleothems

Abstract

We present new high-resolution oxygen isotope (δ 18 O) records from three NW African speleothems located at~31°N. The present-day rainfall patterns at 31°N in NW Africa are linked to negative winter North Atlantic Oscillation phases. However, on multimillennial time scales, our δ 18 O records, together with other hydroclimate records, provide new evidence of humid conditions during the mid-Holocene, a period that was presumably characterized by arid climate. Thus, the apparent increase in moisture during the mid-Holocene is interpreted better as an increase in summer rainfall. This is most likely linked to the expansion of the West African summer monsoon fringe during the African Humid Period, which terminated in our record abruptly around 4 Kyr BP. The temporospatial difference with speleothem records from N Morocco suggests that the High-Atlas Mountains might have been a topographic barrier to further expansion of the West African summer monsoon fringe into higher latitudes. Plain Language SummaryThe Holocene African Humid Period in North Africa, characterized by the expansion of vegetation into the Green Sahara, has been linked to the intensification of the West African summer monsoon (WASM). However, the temporospatial pattern of the African Humid Period, especially the northernmost expansion of the WASM, remain a matter of controversy, largely owing to the lack of precisely dated and high-resolution paleoclimatic records. This study presents new highresolution paleoclimate data based on speleothem oxygen isotope records from a key site at~31°N in NW Africa. Our data suggest that the WASM expanded to 31°N in NW Africa during the mid-Holocene and terminated abruptly at 4 Kyr BP.

Early Holocene greening of the Sahara requires Mediterranean winter rainfall

“…When all surface feedbacks are prescribed, simulated precipitation in the northern Sahara is still too low compared to paleoclimatic evidence for substantially increased moisture at 31° N ( 11 , 13 ) or too high in the 15 to 20° N range ( 20 ), creating incompatibility with prescribed vegetation ( 22 ). Additional sources of moisture ( 25 , 26 ) may have contributed to an AHP that extended toward the Mediterranean borderlands through different mechanisms.…”

Technical note: Characterising and comparing different palaeoclimates with dynamical systems theory

“…Most of the precipitation in north-western Africa is associated with the West African Monsoon (WAM), which reaches to around 16-17 • N (e.g. Sultan and Janicot, 2003) and effectively sets the boundary between the semiarid Sahel and the Sahara. However, the region has repeatedly experienced momentous hydroclimatic shifts in the past.…”

“…Palaeo-archives suggest that during the MH AHP, summer precipitation reached the northern parts of the present-day desert (e.g. Sha et al, 2019) and that tropical vegetation may have extended as far as 24 • N (Hély et al, 2014).…”

Migration of Afro-Asian Monsoon Fringe Since Last Glacial Maximum

“…To address these questions, the change of monsoon fringe, the limit of the monsoon domain, may be the critical factor. Besides the general monsoon precipitation which widely used to represent the intensity of monsoon (Shi and Yan, 2019), the fringe of Afro-Asian monsoon varied largely as documented in geological records (Winkler and Wang, 1993;Yan and Petit-Maire, 1994;Adams, 1997;Jiang and Liu, 2007;Schneider et al, 2014;Skonieczny et al, 2015;Yang et al, 2015;Goldsmith et al, 2017;Tierney et al, 2017;Sha et al, 2019), and simulated in climate models (Doherty et al, 2000;Jiang et al, 2015a,b). Reconstructions and simulations consistently indicated monsoon fringe swung northward during the last deglaciation, afterward swung southward during the Holocene.…”