Paleoenvironments in the Nile Valley during the Holocene
Own position between the GEomar (Kiel, Germany) and the Nioz (Texel, the Netherlands) between 2010-2014 (DFG BL1112/1-1)
Sediments deposited on submarine deltas are key archives to reconstruct past changes in fluvial regime, which provide crucial information on changes of the past environments of the hinterland. A sediment core was retrieved at 700 m water-depth on the Nile deep-sea fan (R/V Poseidon cruise P362/2) and has millimeter-scale laminations. Detailed 14C dating shows that the core covers the last 9,500 years, with laminated sediments being deposited at very high rates (up to 600 cm/ka) between 9,500 and 7,500 years ago.
This time interval corresponds to the African Humid Period (AHP), which is known as an interval of enhanced river discharge in northern Africa. The laminated sediments of the core contain high amounts of terrigenous organic matter and minerals (mostly of fluvial origin) and provide the unique opportunity to investigate variations in fluvial regime of the Nile River at sub-seasonal time scales during the AHP. Together with my collaborators, I propose to measure the elemental content at mm-scale (using a X-Ray Fluorescence scanner) for the core mentioned above and other laminated sediments from the region. The records will be analyzed using time series analytic tools (e.g., wavelet analyses) in order to characterize changes in frequency and magnitude of fluvial discharge through time and understand the mechanisms controlling changes in seasonality and hydrology. These unique sediments also allow to investigate the linkages and feedbacks between climate (seasonality), vegetation and erosion dynamics. To achieve this, we will use a combination of inorganic and organic geochemical methods. In order to determine the sources of the sediments and waters transported to the margin, it is proposed to apply neodymium and strontium isotope composition of the lithogenic fraction and foraminifera, respectively. The sources of organic mater and the amount and type of vegetation will be determined using the chemical and isotopic composition of specific lipid biomarkers (branched and isoprenoid tetraethers and plant waxes) originating from terrestrial soils and plants.
Blanchet, Contoux and Leduc, 2015. Runoff and precipitation dynamics in the Blue and White Nile catchments during the mid-Holocene: A data-model comparison. Quaternary Science Reviews 130, 222-230. DOI: 10.1016/j.quascirev.2015.07.014
The 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.
Abstract: The termination of the African Humid Period in northeastern Africa during the early Holocene was marked by the southward migration of the rain belt and the disappearance of the Green Sahara. This interval of drastic environmental changes was also marked by the initiation of food production by North African hunter-gatherer populations and thus provides critical information on human-environment relationships. However, existing records of regional climatic and environmental changes exhibit large differences in timing and modes of the wet/dry transition at the end of the African Humid Period. Here we present independent records of changes in river runoff, vegetation and erosion in the Nile River watershed during the Holocene obtained from a unique sedimentary sequence on the Nile River fan using organic and inorganic proxy data. This high-resolution reconstruction allows to examine the phase relationship between the changes of these three parameters and provides a detailed picture of the environmental conditions during the Paleolithic/Neolithic transition. The data show that river runoff decreased gradually during the wet/arid transition at the end of the AHP whereas rapid shifts of vegetation and erosion occurred earlier between 8.7 and ~6 ka BP. These asynchronous changes are compared to other regional records and provide new insights into the threshold responses of the environment to climatic changes. Our record demonstrates that the degradation of the environment in northeastern Africa was more abrupt and occurred earlier than previously thought and may have accelerated the process of domestication in order to secure sustainable food resources for the Neolithic African populations.
Download for free by clicking on the title! Data available as supplementary on the Plos One website and on the Pangea website.
Blanchet et al., 2013. High- and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan. Earth and Planetary Science Letters 364, 98-110. doi: 10.1016/j.epsl.2013.01.009
Abstract: Sediments deposited on deep-sea fans are an excellent geological archive to reconstruct past changes in fluvial discharge. Here we present a reconstruction of changes in the regime of the Nile River during the Holocene obtained using bulk elemental composition, grain-size analyses and radiogenic strontium (Sr) and neodymium (Nd) isotopes from a sediment core collected on the Nile deep-sea fan. This 6-m long core was retrieved at ~700 m water-depth and is characterized by the presence of a 5-m thick section of finely laminated sediments, which were deposited between 9.5 and 7.3 ka BP and correspond to the African Humid Period (AHP). The data show distinct changes in eolian dust inputs as well as variations in discharge of the Blue Nile and White Nile. Sedimentation was mainly controlled by changes in fluvial discharge during the Holocene, which was predominantly forced by low-latitude summer insolation and by the location of the eastern African Rain Belt. The changes in relative contribution from the Blue Nile and White Nile followed changes in low-latitude spring/autumn insolation, which highlights the role of changes in seasonality of the precipitation on the Nile River regime. The relative intensity of the Blue Nile discharge was enhanced during the early and late Holocene at times of higher spring insolation (with massive erosion and runoff during the AHP at times of high summer insolation), while it was reduced between 8 and 4 ka at times of high autumn insolation. The gradual insolation-paced changes in fluvial regime were interrupted by a short-term arid event at 8.5-7.3 ka BP (also associated with rejuvenation of bottom-water ventilation above the Nile fan), which was likely related to northern hemisphere cooling events. Another arid event at 4.5-3.7 ka BP occurred as the apex of a gradually drier phase in NE Africa and marks the end of the AHP.