Climate during the Upper Pleistocene and Holocene

Between 20ka and 12.5kaBP, the Nile was a highly seasonal, braided river. Towards the end of this period, from 14.5 ka to

12.5kaBP, the amount of water flowing through the Nile began to increase and several high sub-stages can be recognized, as well as moderate activity in the wadis (Adamson et al., 1980; Butzer, 1980). Around 12.5 kaBP, there was a huge increase in the discharge of the Nile, partly caused by overflowing east African lakes, which expanded the river's catchment area. The overflow from Lake Victoria and other lakes, as well as higher rainfall in Ethiopia, sent extraordinary floods down the main Nile. This marked a revolutionary change from an ephemeral to a continuous flow, with a superimposed flood peak. As a result, the main Nile and its tributaries formed more stable channels of higher sinuosity, from which suspended Ethiopian silt and clay was deposited on the floodplains (Adamson et al., 1980). The "wild" Nile regime, characterized by floodplain sedimentation, lasted until c. 11.5kaBP and was followed by a period of strong dissection and down-cutting in the Nile and associated wadis of at least 20 m (Butzer, 1980; Fairbridge, 1962).

Later on, two periods of high Nile levels and aggradation can be recognized in Egypt: 11.2 ka to 7.7 kaBP and 7.3 ka to 6 kaBP. These two periods are separated by a regression (Butzer, 1980), apparently caused by a more arid climate in the entire region of northern Africa, as recognized by falling lake levels c. 7.5 kaBP (Street-Perrot et al., 1985).

Butzer (1980) summarized the main features of upper Holocene history of the Nile valley in Egypt and lower Nubia.

1. 3000 to 2800 BC. Flood levels declined significantly, representing an overall reduction in volume of 25-30%. The concomitant down-cutting appears to have initiated the modern flood plain downstream of Wadi Halfa (Bell, 1971).

2. 2250 to 1950 BC. A period of catastrophically low floods.

3. 1950 to 1840 BC. Improved floods.

4. 1840 to 1770 BC. Excessive floods are documented, reoc-curring every 2 to 5 years, with peak discharge three times that of the ten greatest floods of the nineteenth century AD (Bell, 1971).

5. 1770 to 1180BC. Average levels remained high.

6. 1180 to 1130 BC. Strong decline in levels.

7. 1130 BC to 600 AD. "Normal" levels.

8. 600 to 1000 AD. Generally high levels.

9. 1000 AD to present. "Normal" levels.

The present level of the Nile was reached about 5 ka BP. Archae-ologically dated high-water marks on Egyptian temples and associated Christian constructions near Wadi Halfa suggest that the last important high Nile periods were c. 500 AD and 800 AD (Fairbridge, 1962).

Flohn andNicholson (1980) observed two separate wet periods: the first, c. 9.5 ka BP, coinciding with the warmest Holocene period in southern latitudes and the second, c. 6 ka BP, coinciding with the thermal maximum in Europe (Atlantic). The very dry periods in the Sahara were between 7 ka and 6 ka BP and from 1 ka BP to the present. According to Nicholson (1980), the level of the Nile during the Roman period was low.

Fairbridge (1984) analyzed the flood levels of the Nile, by the Fourier and maximum entropy statistical analysis. He found cyclic fluctuations, which he attributed to various factors: first, an 18.6 year cycle that is connected to the lunar nodal notations; second, a 78 year cycle that, in his opinion, is a solar cycle. He found only a weak reflection of the 11-22 year solar cycle.

Sneh et al. (1986) reconstructed the evolution of the northeastern corner of the Nile delta by investigating a column of sediments obtained from a 48 m drill hole divided into five units. They found that only a minor part of the sediment derived from the Nile in the early Holocene (unit 1, 42-48 m), the main part being marine littoral sands. The shoreline at that time was more than 20 km south of that of the present. The 14C age of the solitary corals found in this layer is 8480 ± 280 BP. The minor influence of the Nile, even though it was at high levels at this time, may be explained by the fact that it had not yet developed any eastern branch. Overlying the marine sands was a layer of black clays (unit 2, 42-30.5 m), which is evidence of an influx of sediments derived from land soils, yet no evidence of the typically tropical material supplied by the Nile was to be found. The lower part of this section was of marine character, containing foraminifers, ostracodes and molluscs. In the upper part of the unit, the influence of the Nile was more pronounced, with freshwater diatoms, but there was still a relative lack of tropical pollen, showing that the outlet of the Nile was still in the west and, therefore, no sediments could have been brought by the sea currents which move from the west to the east. What pollen there was in the core was typical of desert vegetation. The nearby seashore retreated away to the south of the borehole site. Sneh et al. (1986) related this terrigenous extension to high floods in the upper part of the Nile and correlated it with the upper sapropel (i.e., deposits rich in organic material), which reflects a major influx of freshwater into the Mediterranean sea (Luz, 1979; Rossignol-Strick et al., 1982).

In unit 3 (30.5-17.5 m), there were layers of clayey silt, also deposited in a marine environment, the lower part of which showed a marked increase in tropical fern spores and pollen of freshwater plants, while the amount of desert plants sharply decreased. In the upper part of unit 3, although the deposition of silts continued, there was a marked change in the flora to few tropical elements, less freshwater pollen but a relative abundance of pollen of sabkha and saline marsh vegetation. Sneh et al. (1986) suggested that this was a regressional phase.

In unit 4 (17.5-8.5 m), the sediments were of a typically prodelta character, with alternating silt and clay laminae. There was also an increase in the tropical elements. Salinity, determined by the "sieve pore" shapes of the ostracoda, decreased. The pollen spectra suggest that a warm and dry climate dominated lower Egypt.

Unit 5 comprised the uppermost 8.5 m of the section. A 14C age of 2890 ± 220 years was found at the base of this unit, indicating that it was deposited during the last 3000 years. The Pelusian channel, which is the most eastern tributary of the Nile, determined the character of the layers. The sediments were mainly those of coarse silts and sands. The pollen assemblage showed an increase in tropical spores and a relative decrease of delta plain pollen. There was also some increase in Sabkha vegetation. The fauna suggested deposition in a saline lagoonar environment. Consequently, this section represents the stage of the seaward build-up of the eastern part of the delta as a function of the outlet of the Pelusian channel.

Foucault and Stanley (1989) also investigated the Quaternary paleo-climatic oscillations in east Africa as recorded by the heavy minerals in the Nile delta. The minerals are supplied by the White Nile tributary, which drains the tropical region and which contributes about a third of the Nile's discharge. It has a relatively low sediment load, because of the dumping effect in the Sudd swamps. As the White Nile comes from a region constructed of metamor-phic rocks, the proportion of amphiboles in its sediments is high. While the Blue Nile supplies more than half of the Nile discharge, it contributes about three quarters of its sediment supply. As it drains the volcanic Ethiopian highlands, the proportion of pyroxenes to amphiboles is high. The Atabara contributes one quarter of the Nile's sediments, which are also rich in amphiboles. The percentage ratio of amphiboles to amphiboles plus pyroxenes, which is called the amphibole index, was investigated in core holes in the Nile delta in order to elucidate variations in the paleo-climates that had affected its tributaries. At present, there is a high pyroxene load in the summer floods because of the high rainfall on the rather arid Ethiopian highlands. Foucault and Stanley reasoned that a more humid climate over this region would increase the vegetation cover, and thus reduce the sediment load, as well as the quantity of pyroxenes derived from this region. They found that high amphibole ratios c. 40ka to 20kaBP corresponded to periods with high levels of water in the lakes east of the Ethiopian plateau as well as in lakes in the Ethiopian rift. A low amphibole ratio c. 20kato 12ka-10kaBP corresponded to low lake levels.

Stanley and Warne (1993) analyzed mineralogical, textural, fau-nal and floral content and trace element geochemistry of the sequence of layers found in more than 80 core holes in the northern part of the Nile delta. They found three distinct lithofacies sequences. The lower sequence, dated from c. 35 ka to 12kaBP, was one of non-marine alluvial sandy deposits. Interleaved with these sands were variegated mud layers. These layers were deposited on a low relief, partially vegetated plain and on sabkhas. The layers were separated by an unconformity from the overlying layers composed of near-shore marine to coastal sands. The age of these sands ranged from c. 11.5 ka to 8 kaBP. A hiatus separated these transgressive sands from the overlying layers, which were younger than 7.5 ka BP and which were deposited in variable environments, from an inner shelf to a lower deltaic alluvial plain.

The paleo-geographical interpretation of this sequence of deposits by Stanley and Warne (1993) is as follows.

1. The lowest unit of alluvial and sabkha sands (35 ka to 18kaBP) was deposited on an alluvial plain across which seasonally active braided channels flowed. At that time, the sea had a low level, its coast being located about 50 km further north than at present. Floodplain mud accumulated in ephemeral seasonally dry depressions. Carbonate-rich desert sand and sabkha mud were deposited in the west. The climate during this period was arid.

2. From 15 ka to 8 ka BP, the sea advanced southward, reworking the alluvial deposits. The modern Nile delta began to form c. 7.5 kaBP. About 6.5 kaBP, the sea level was some 9 or 10 mbelow its present level. The river gradient was steeper and the climate was more humid.

3. By c. 4 ka BP, the sea level continued to rise, but more slowly. Climate became more arid, flood levels subsided and more distributary channels carried a less-coarse bed load.

4. By c. 2 ka BP, sea level had risen to about 2 m below the present level and the delta took on its present configuration. Stanley and Warne explained the fivefold thickening of the Holocene deltaic sequence from west to east by differential subsidence, which resulted in a northeast tilting, connected to a major system of regional faulting.

From the three sets of sections (Foucault and Stanley, 1989; Sneh et al., 1986; Stanley and Warne, 1993), it can be concluded that the lower non-marine sequence of the Last Glacial Period was deposited in an area from which the sea had retreated because of glaciation. At this time, the Nile was at its lowest levels as a consequence of the weak monsoon system. This period was followed, from 15 ka to 8 ka BP, by a rise in sea level caused by global deglaciation. The hiatus between 12 ka and 11 ka, separating nonmarine deposits from the overlying near-shore marine deposits, may represent the Younger Dryas regression. The following hiatus (i.e., before 7.5 kaBP) may be attributed to the mid-Neolithic cold phase. The failure to deposit sediments was probably caused by the retreat of the sea, so marine layers were not deposited, and weak monsoons, so supplies of Nile sediments was also reduced. The modern delta started to form at c. 7.5 ka, which can be correlated with the Upper Neolithic of the Levant, a warm period. Although sea-level rise was relatively slow, the strengthening of the monsoons caused the influx of flood sediments to dominate the reworking of the marine sediments by the rising sea. No Nile deposits evidencing a regressional phase during the Chalcolithic period and EB were observed. Again, this may have been because regression was accompanied by a low Nile, owing to a weak monsoonal system, and, therefore, no deposits reached the lower stretches of the delta. The rise in sea level c. 4 ka BP can be correlated with the transgressional phase of the MB warm period. I believe the rise in sea level c. 2 ka BP, to about 2 m below its present level, represented the sea rise caused by the short warm period from c. 2.3 ka to 2.4 ka BP, which was also observed along the shores of England (Thompson, 1980).

Bell (1971), analyzing Egyptian historical literary sources, found evidence for two drought waves, one between 4.18 ka and 4.15kaBP and the other between 4ka and 3.9kaBP. According to Bell (1971), these severe droughts were responsible for catastrophic famine, especially in upper Egypt. He also claimed that the abandonment of the EB culture in the Mediterranean areas, the collapse of the Acadian empire and the fall of the Samarian Ur III kingdom were all caused by the 4 ka BP climatic crises. Nicholson (1980) mentioned that the Sahara was considerably moister between 6 ka and 5 ka BP than it is now, and a major change towards aridization had appeared by 4 kaBP. Rognon (1987a) found proof of a rapid aridization in north Africa in general between 4.5 ka and 4kaBP. Butzer (1966) claimed that the beginning of the fifth milleniumBP was the last humid period in north Africa.

Stanley etal. (2001) have discovered in Egypt's Abu Qir Bay the submerged ruins of two Hellenistic-Byzantine cities, Herakleion and Canopus. The ruins are below 6-7 m of water and are located on the Canopic branch of the Nile delta. These authors believe that the increased supply of sands, which started in the eighth century AD, led to the submergence of the cities. This was the period of the Moslem-Arab warm climate change in the Levant, which brought the strengthening of the monsoonal system over the catchment basin of the Nile, as already discussed.

A recent mean sea-level rise of 8.4 cm was observed at Alexandria from 1944 to 1973 (this was 2.9 mm/year at Port Said).

From 1926 to 1970, there was a mean sea-level rise of 10.1 cm (2.2 mm/year). Part of the rise that has taken place on the Nile delta coast can be explained by the thermal expansion of the upper layer of the oceans, resulting from the observed warming of 0.4 °C in the past 100 years. The other part of the rise may be related to subsidence. It is predicted that the sea level will rise by 37.8 cm and 28.6 cm by 2100 AD over the 1970 level at Alexandria and Port Said, respectively (El-Fishawi and Fanos, 1989).

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