Runoff and precipitation dynamics in the Blue and White Nile catchments during the mid-Holocene: A data-model comparison

Abstract

International audienceThe Blue Nile is the major contributor of freshwater and sediments to the modern-day main Nile River and exerts a key control on seasonal flooding in the Nile valley. Recent studies have postulated that the relative contribution from the Blue Nile to the main Nile runoff might have been reduced during the mid-Holocene, at a time when higher boreal summer insolation stimulated enhanced precipitation in North Africa. Whether the decrease in the relative contribution from the Blue Nile resulted from a decrease in precipitation over the catchment, from an increase in White Nile runoff or from a combination of both is still a matter of debate. By comparing regional proxy-records with the output from a global atmospheric model zoomed on Africa, we propose that the reduced contribution from the Blue Nile at 6 ka originated from both a higher White Nile runoff and a lower Blue Nile runoff. Enhanced African and Indian monsoons at 6 ka induced a northern shift of the Intertropical Convergence Zone and an eastward shift of the Congo Air Boundary. Such an atmospheric configuration led to a negative anomaly of summer precipitation over the Blue Nile catchment that likely resulted in a reduction in the Blue Nile runoff. By contrast, a sustained positive anomaly of precipitation over the White Nile catchment during both summer and autumn most likely induced a higher main Nile runoff during the mid-Holocene. Using the model output, we propose a first synoptic view on regional rainfall dynamics that permits to reconcile contrasting proxy records

High-resolution Holocene climate and hydrological variability from two major Mediterranean deltas (Nile and Rhone)

“…These results suggest that enhanced precipitation allowed vegetation to expand in the Green Sahara till about 8400 yr BP. High and variable concentrations of the freshwater algae Pediastrum (Figure 4e) combined with high log(Ti:Ca) values are consistent with a high Nile discharge (Blanchet et al, 2013(Blanchet et al, , 2015Costa et al, 2014;Marshall et al, 2011). Interestingly, Pediastrum show low values during the ~9200 yr BP but high ones between 8800 and 8400 yr BP, a result that reinforces the idea of a drier climate at 9200 yr BP as opposed to the subsequent wet interval (8800-8400 yr BP).…”

“…This reduction in the Blue Nile flow would be caused by lower rainfall as the ITCZ starts migrating south toward the end of the AHP. Besides monsoon rainfall, the Mediterranean source would have also been important in the reduction in the Nile discharge (Blanchet et al, 2015).…”

Reconstruction of former channel systems in the northwestern Nile Delta (Egypt) based on corings and electrical resistivity tomography (ERT)

“…The visible lamination of the sand section with silty strata highlights changing hydrodynamics (Vött et al, 2002;Goiran et al, 2014). The Ca/Fe and Ca/Ti ratios show peaks towards Ca, indicating predominantly eolian input at certain intervals (Woronko, 2012;Blanchet et al, 2015;Pint et al, 2015;Pennington et al, 2019). Partially present high S concentrations reflect the existence of reducing conditions, detectable within silty sediments corresponding to phases of low-energy flow or stagnant water (Goiran et al, 2014;Martinez-Ruiz et al, 2015).…”

“…Vött et al (2002),Revel et al (2010),Eckert (2014),Pint et al (2015),Pennington et al relative contributions of eolian (in favor of Ca) vs. fluvial (in favor of Ti) inputVött et al (2002),Blanchet et al (2015),Pint et al (2015),Castañeda et al (2016), Pennington et al (2019) Cu/Zn nature of river sediments (natural watercourses vs. human-constructed canals) Ginau et al (2019) 4.1 Coring M006 M006 (31.22794 • N, 30.773801 • E; 2.178 m a.s.l., depth: 11 m) is situated approx. 135 m north of the northern edge of the present-day kom area (Figs.…”

Geochemical composition of Tanzanian shelf sediments indicates Holocene climatic and sea-level changes

“…The pace of the AHP over East Africa, the East African Humid Period (EAHP), is inconsistent in both timing and abruptness, according to the existing paleoclimatic records and is still a matter of debate (Castañeda et al, 2016). Though the expression of the EAHP appears highly variable in space and time, a general pattern can be deduced: a significant humid period from the early Holocene to ~8 ka (EAHP I; Costa et al, 2014) and a second moderate humid phase between 8 and 5 ka (EAHP II) during the mid-Holocene (Blanchet et al, 2015). The second humid phase ended with an abrupt or gradual termination (Revel et al, 2014; Castañeda et al, 2016; Liu et al, 2017), followed by an arid late Holocene (Fig.…”