Pronounced Northwest African Monsoon Discharge During the Mid- to Late Holocene

Abstract

The mid-to late Holocene aridification pattern of NW Africa remains a matter of controversial debate. While many marine climate reconstructions indicate a relatively abrupt aridification at ∼5.5 ka BP, terrestrial palaeoclimate records rather show a spatially and temporally heterogeneous transition towards the modern arid state. To bridge conflicting evidence, we analysed high-resolution (sub-seasonal) ontogenetic oxygen isotope (δ 18 O) records and bulk 87 Sr/ 86 Sr ratios of bivalve shells and fish otoliths of estuarine-associated species. Samples were excavated from archaeological deposits formed during the mid-to late Holocene in a large palaeo-estuary east of the Banc d'Arguin, Mauritania. The mid-Holocene (∼5.2 ka BP) δ 18 O records indicated unrealistically high water temperatures when assuming a modern value for δ 18 O Seawater , suggesting a substantial input of isotopically lighter water to the study area. Respective salinity estimates consistently indicated persistent monsoonal discharge. Moreover, 87 Sr/ 86 Sr ratios of bivalve shells deviated considerably from the rather stable global seawater composition, further supporting the presence of significant terrestrial runoff between 5.0 and 5.3 ka BP. Altogether, our results support doubts regarding an abrupt termination of the African Humid Period (AHP) in the coastal areas of NW Africa, and show that fully marine conditions were established along the Banc d'Arguin by ∼3.0 ka BP.

Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)

“…During this humid period, increased Northern Hemisphere (NH) summer insolation induced the northward extension and intensification of the tropical rainbelt (Gasse, 2000;Lézine et al, 2011a). Remains of large palaeo-lakes and -estuaries across northern Africa record the substantially altered hydroclimate during the AHP, which transformed the Saharan desert into an open grass savannah and allowed for widespread human occupation of nowadays mostly barren landscapes (Lézine et al, 1990;Kuper and Kröpelin, 2006;Kröpelin et al, 2008;Sereno et al, 2008;Tierney et al, 2017;Höpker et al, 2019). The subsequent return to arid climate is assumed to explain major societal shifts, including the demise of the Egyptian Old Kingdom, attributed to dramatic failure of annual Nile River floods driven by declining rainfall in the Ethiopian highlands (Stanley et al, 2003;Kuper and Kröpelin, 2006).…”

Vegetation Dynamics vs. Sediment Supply During the Late Quaternary: Paradigm of Sea Level Change and Two Distinct Time-Bound Stages of the Niger Delta Coastal Evolution

“…Significant records of hinterland pollen taxa in several other sediment cores in west equatorial Africa, such as T89-16, MD03-2708, and GeoB-4905 (Kim et al , 2010;Marret et al , 2001;Skonieczny et al, 2015) indicate dry conditions. In addition, the observed fluvial sediment supply could be linked to the seasonal latitudinal migration of the Intertropical Convergent Zone (ITCZ) with a mean annual position assumed to be around 13 o N positioned further south (10 o S) (Leroux, 1993;Marret et al , 2001;Shannahan et al, 2015;Adojoh et al, 2017;Hopker et al, 2019;Dai et al, 2021;Boyden et al, 2021).…”

“…During the Early to mid-Holocene (11-6.5 ka), this phase experienced sea level transgression and warm climate based on the higher sedimentation rate, expansion of mangrove vegetation, and marine indicators in the GC depths of ˜202-0 cm (Figures 3-5). This was a setting of both rapid spread of the coastal/littoral vegetation zone associated with sea level rise and linked to the gently sloping shelf where transgressive sedimentation took place, leading to delta plain retreat (Morley, 1995;Rull, 2002;Torricelli et al, 2006;Amorosi et al, 2014;Adojoh et al , , 2017Adeonikpekun and Sowunmi, 2019;Hopker et al, 2019: Dai et al, 2021. This phase also experienced increased marine (tidal) influence when compared to phase I (GC depths of ˜272-202) (Figures 3-5), implying that mangrove pollen abundance in intertidal and tidal settings would be much higher (Oomkens, 1974;Poumot, 1989;Morley, 1995;Rull 2002;Punwong et al , 2013;Joo-Chang et al, 2015;Hopker et al, 2019) (Figures 6-7).…”

“…This was a setting of both rapid spread of the coastal/littoral vegetation zone associated with sea level rise and linked to the gently sloping shelf where transgressive sedimentation took place, leading to delta plain retreat (Morley, 1995;Rull, 2002;Torricelli et al, 2006;Amorosi et al, 2014;Adojoh et al , , 2017Adeonikpekun and Sowunmi, 2019;Hopker et al, 2019: Dai et al, 2021. This phase also experienced increased marine (tidal) influence when compared to phase I (GC depths of ˜272-202) (Figures 3-5), implying that mangrove pollen abundance in intertidal and tidal settings would be much higher (Oomkens, 1974;Poumot, 1989;Morley, 1995;Rull 2002;Punwong et al , 2013;Joo-Chang et al, 2015;Hopker et al, 2019) (Figures 6-7). Consequently, abundance of mangrove pollen type (Rhizophora ) is acknowledged as an indicator of sea level transgression when correlated with the regional sea level curve (e.g., Peltier, 1994;Scourse et al , 2005;Joo-Chang et al, 2015;Adojoh et al, 2017;Chadwick et al, 2020;Boyden et al, 2021) (Figures 3-5).…”

“…During stage 1, the GC data confirm the emergence of the continental shelf in response to a lower than present day sea level (Figures 6-7). The littoral environment (mangrove and coastal swamp) is subaerially exposed, and sedimentation and fluvial transport are effected by mass movement triggered by the weak West African Monsoon (WAM) (Morley, 1995;Reijers, 2011;Zong et al , 2009;Shannahan et al, 2015;Skonieczny et al, 2015;Adojoh et al, 2017;Hopker et al, 2019;Chadwick et al, 2020;Dai et al, 2021). The GC records indicate the predominance of hinterland pollen during stage 1 evolution of the Late Quaternary Niger Delta.…”