Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia

Abstract

Funder: NERC RONO: NE/D012996/1The African Humid Period of the early to mid-Holocene (12,000?5000 years ago) had dramatic ecological and societal consequences, including the expansion of vegetation and civilization into the ?green Sahara.? While the humid period itself is well documented throughout northern and equatorial Africa, mechanisms behind observed regional variability in the timing and magnitude of the humid period remain disputed. This paper presents a new hydrogen isotope record from leaf waxes (?Dwax) in a 15,000-year sediment core from Lake Tana, Ethiopia (12?N, 37?E) to provide insight into the timing, duration, and intensity of the African Humid Period over northeastern Africa. ?Dwax at Lake Tana ranges between ?80? and ?170?, with an abrupt transition from D-enriched to D-depleted waxes between 13,000?11,500 years before present (13?11.5 ka). A similarly abrupt transition from D-depleted to D-enriched waxes occurs ca 8.5?8 ka and is followed by generally D-enriched waxes throughout the late Holocene. Trends in ?Dwax covary with changes in Northern Hemisphere summer insolation and reflect increased precipitation at Lake Tana during the AHP; however, the transition from D-depleted to D-enriched waxes occurs earlier at Lake Tana (ca 8 ka, vs 5 ka) than in many other regional records, and the amplitude of D-depletion during the AHP is larger at Lake Tana as well. We attribute this early enrichment to a reduction of moisture derived from westerly sources (the Congo Basin and Atlantic Ocean) which we suggest are D-depleted relative to moisture sourced from the east (Indian Ocean) and the north (Red Sea and Mediterranean Sea). Our new record highlights the importance of both the northward migration of the tropical rain belt as well as east-west migration of the Congo Air Boundary to precipitation source and amount during the African Humid Period.authorsversionPeer reviewe

Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya

“…Between 8,500 and 7,800 years BP, there are numerous proxy records across northern and equatorial Africa indicating low lake levels; however, most of the lakes are recharged at the end of this event (Gasse 2000). The exception to this appears to be in the northern portion of the Horn of Africa, where the termination of the AHP is earlier than in the southerly locations (Costa et al 2014). This is likely the result of the CAB gradually migrating further west (Costa et al 2014) which correlated with the progressively more southerly displacement of the ITCZ in the Middle Holocene (Barker et al 2002;Thevenon et al 2002).…”

“…In another isotopic measure of moisture (δD wax ) from the same lake core, the Early Holocene pluvial period ended ca. 8,500 years BP as conditions transitioned to a permanent xeric state (Costa et al 2014). Maximum aridity is recorded ca.…”

“…Estimated northernmost positions of the ITCZ during the AHP and present day adapted from Junginger et al (2014). a Gulf of Aden sediment core (Tierney and deMenocal 2013), b Indian Ocean sediment core (Bard et al 1997), c Paleolake Suguta (Garcin et al 2009;Junginger et al 2014), d Lake Chew Bahir (Foerster et al 2012), e Lake Ashenge (Marshall et al 2009), f Lake Tilo (Lamb et al 2000), g Lakes Ziway and Shala (Gillespie et al 1983), h Lake Tana (Costa et al 2014;Marshall et al 2011), i Lake Abbe (Gasse and Van Campo 1994), j Blue Nile sediment sources (Krom et al 2002) and k Lake Hayq (Lamb et al 2007) (image in full color online) and a permanent lowstand at Lake Abbe (a.k.a. Abhé; Fig.…”

“…Fluctuating precipitation regimes across space and time in the paleoclimatic proxy record on the Ethiopian Plateau have been attributed to the eastern limit and precipitation delivery rates of the CAB during the Holocene (Bloszies et al in press;Costa et al 2014;Forman et al 2014;Levin et al 2009;Liu et al 2003;Tierney and deMenocal 2013;Tierney et al 2010Tierney et al , 2011. During the Early Holocene, the CAB has been inferred (Bloszies et al in press;Costa et al 2014;Forman et al 2014;Junginger et al 2014;Levin et al 2009;Tierney and deMenocal 2013) to extend east of the Rift Valley from present-day Branched and Isoprenoidal Tetraether (BIT) isotopic data, providing a significant amount of summer rainfall (Tierney et al 2011).…”

“…During the Early Holocene, the CAB has been inferred (Bloszies et al in press;Costa et al 2014;Forman et al 2014;Junginger et al 2014;Levin et al 2009;Tierney and deMenocal 2013) to extend east of the Rift Valley from present-day Branched and Isoprenoidal Tetraether (BIT) isotopic data, providing a significant amount of summer rainfall (Tierney et al 2011). Enhanced monsoon strength in the Gulf of Guinea (West Africa) and positive state of the IOD are attributed to a more easterly positioning of the CAB and higher rainfall across Lake Turkana, specifically (Morrissey and Scholz 2014), and eastern Africa, more generally (Tierney et al 2011).…”