Student: Matthew Box
Supervisor: Michael Krom (Earth Sciences, Leeds)
Project start: 2004
Figure 1:: Map of the River Nile from its headwaters in the equatorial rift lakes to the Medoterranean Sea. The location of core 9509 is shown on the top inset picture
Introduction
Sr isotopic composition of Nile sediment is controlled by provenance and the relative amount of Blue Nile (BN) (87Sr/86Sr ~ 0.7055) to White Nile (WN) (87Sr/86Sr ~ 0.7105) detritus. These are, in turn, controlled by climate change and monsoon intensity over the Nile catchment. Thus Sr isotope analysis of River Nile sediment is potentially a good proxy for relative climate change over the Nile catchment area (Krom et al. 2002).The River Nile stretches 2,700 km across 30° of latitude (Figure 1). Its headwaters are located in two climate belts: the BN sources from sub-tropical Ethiopia, whereas the WN sources from the equatorial rift lakes. The annual monsoon rains (July-Oct) brought by the migration of the ITCZ - fall upon the headwaters of the BN in the Ethiopian Highlands. It is this combination of geography and geochemistry that makes the River Nile unique and an important resource for looking at past global climate change.
Core 9509 (32°02’N, 34°17’E) was raised from 884 m water depth off the coast of Israel (Figure 1) and is located under the pre-Aswan Nile plume. The studied section provides a continuous record of Nile sediment over the past 25 ka (see Figure 1).
Why Study core 9509 and Nile sediments?
- The River Nile is the major hydrological conduit in Northeast Africa and is very susceptible to changes in sediment provinance brought about by changes in precipitation over its headwaters.
- The sediment of core 9509 represents an unbroken proxy record of palaeoclimate change over equatorial and sub-tropical Africa over the past 25 ka.
- The d18OG.ruber record from core 9509 provides complimentary information on palaeoclimate change in the Mediterranean region.
- The core is well dated by mapping its d18OG.ruber record to that of well dated speleothem records from the moutains of Judea.
Why do the Sr isotopes work as a proxy record?
The 87Sr/86Sr ratio from River Nile sediment can be thought of as a mixture from two ‘end-member’ sediment sources: The Blue and White Niles. During a ‘wet’ climate phase when the African Summer Monsoon is more intense the growing season is also extended and vegetal cover over the Ethiopian Highlands is increased spatially and temporaly. This reduces the availability of sediment from the Blue Nile (which supplies the River Nile with the bulk of its sedimentary load) and therefore increases the relative amount of White Nile sediment. This alters the bulk 87Sr/86Sr value of River Nile sediment toward the White Nile ‘end-member’ and vice versa.Some Recent Results
- d18O and 87Sr/86Sr values display a strong inverse correlation over the entire 16.0 ka BP (Figure 2) including through the sapropel (S1), Younger Dryas (YD) and Heinrich 1 (H1) event. Before the H1 event at ca. 16.0 ka BP the inverse correlation breaks down and Nile 87Sr/86Sr curve does not mirror the EMS d18OG. ruber profile.
- The base of the S1 sapropel at ca. 9.25 ka BP displays a major transition in d18OG. ruber and 87Sr/86Sr values simultaneously.
- Both TOC and Baexcess peaks increase and decrease in coincidence through the sapropel at ca. 9.25 - 6.55 ka BP indicating that the S1 sapropel has suffered little or no ‘burn-back’ at this location.
- Core 9509 records the influence of several Holocene climate events: 1. The 8.2 ka BP event within the Sr data (single datum). This event saw a return of drier conditions over the Nile catchment and coincides with the period of reoxygenation within S1. 2. The general trend throughout the Holocene is one of drying since a climate maximum at the base of the sapropel ca. 9.25 ka BP. 3. Two small (single datum) drying trends: ca. 3.8 ka BP and ca. 2.0 ka BP.
Figure 2: 87Sr/86Sr and d18O versus core depth for core 9509. Below are some major element profiles, normailised to Al, that highlight the S1 sapropel. (Click on picture for a larger version)
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Discussion
87Sr/86Sr profiles as palaeoclimate indicator:The inverse correlation between the two independent climate proxies, 87Sr/86Sr and d18O, is direct evidence that both equatorial and Mediterranean climate zones moved in tandem in response to global climate change events. Inverse correlation between the two proxies over the past 16.0 ka BP is indication that the Sr isotope ratio of Nile sediment is controlled by climate change over the headwaters and that in the case of the Nile Sr isotope profiles are a good climate proxy.
S1 Sapropel:
The major transition in 87Sr/86Sr and d18O values observed at the base of the sapropel indicates that an increase of monsoon intensity over the Nile catchment occurs synchronously with changes in the evaporation/precipitation in the EMS. The Sr data suggests that Nile outflow of was greatly increased during the start of sapropel conditions in the EMS ca. 9.2 ka BP and that a larger relative proportion of sediment was derived from the White Nile.
Younger Dryas (YD):
The YD event is documented in Atlantic sediment cores as well as the GISP, GISP2 and GRIP cores. The YD is given as lasting from ca. 12.8 - 11.6 ka BP (Alley et al. 1993; Stuvier et al. 1995). Core 9509 87Sr/86Sr data appears compatible with this, the YD event culminates ca. 12.3 - 11.3 ka BP after a rapid aridification trend starting at ca. 13.6 ka BP.
Heinrich Event 1:
The breakdown of the correlation between the 87Sr/86Sr and d18O data occurs at a distinct time ca. 16.0 ka BP. This coincides with peak conditions during the H1 event (ca. 17.0-15.0 ka BP) a period of iceberg calving which slowed oceanic THC.
Evidence points to a drying of both Lake Tana by 17.0 ka BP (Lamb et al. submitted) and Lake Victoria at 16.0 ka BP (Talbot and Livingstone, 1989; Talbot and Lærdal, 2000; Stager et al. 2002). Other evidence of severe aridification of equatorial Africa during H1 is observed in Lake Albert which has a desiccation surface dated from ca. 17.5 - 15.2 ka BP.
References:
Alley, R., et al. (1993). Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event. Nature, vol. 362, pp. 527-529.Almogi-Labin, A., et al. (2002). The relationships between the marine and the terrestrial (speleothems) climatic record during the last 80 ka in the eastern Mediterranean. The 12thAnnual Goldscmidt Conference, Davos, Switzerland, Geochemica et Cosmochimica Acta, special abstract volume, pp. A14.
Bar-Matthews, M., et al. (2003). Sea-land oxygen isotope relationships from planktonic foraminifera and speleothems in the East Mediterranean region and their implication for palaeorainfall during interglacial intervals. Geochim. et Cosmochim. Acta, vol. 67, pp. 3181-3199.
Beuning, et al. (1997). A revised 30,000 yr palaeoclimate and palaeohydrologic history of Lake Albert, east Africa. Palaeogeog., Palaeoclimat., Palaeoecol., vol. 136, pp. 259-279.
Fairbanks, R. (1989). A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature, vol. 342, pp. 637-642.
Gasse, F., et al. (1989). Water-level fluctuations of lake Tanganyika in phase with oceanic changes during the last glaciation and deglaciation
Krom, M. D., et al. (2002). River Nile fluctuations over the past 7000 yr and their key role in sapropel development. Geol. Soc. Am., vol. 30, pp. 71-74.
Lamb, H. et al. (submitted). North Atlantic Heinrich events linked to desiccation of Lake Tana, the source of the Blue Nile.
Stager, J., et al. (2002). Cooling cycles, Heinrich event 1, and the desiccation of Lake Victoria. Palaeogeog., Palaeoclimat., Palaeoecol., vol. 183, pp. 169-178.
Stuvier, M., Grootes, P. and Braziunas, T. (1995). The GISP2 d18O Climate Record of the Past 16,500 years and the Role of the Sun, Ocean, and Volcanoes. Quaternary Research, vol. 44, pp. 341-354.
Talbot, M. R., and Lærdal, T. (2000). The Late Pleistocene - Holocene palaeolimnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter. Journal of Paleolimnology, vol. 23, pp. 141-164.
Talbot, M. R. and Livingstone, D. (1989). Hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators. . Palaeogeog., Palaeoclimat., Palaeoecol., vol. 70, pp. 121-137.