The Nile River and the fertile sediments deposited along its
I
banks have long been recognized as a cradle of civilization. Nekhen, the ancient Egyptian town whose
m e means “sacred city of the falcon:’ and its surroundings
are an ideal place to study the role of the Nile River and other environmental
factors in the development of the earliest Egyptian civilization.
572A * ANALYTICAL CHEMISTRY, VOL. 58. NO. 4, APRIL 1986
0003-2700/86/0358-572A$01.50/0 @ 1986 American Chemical Soclety
Ralph 0. Alien Hany Hamroush Department of Chemistry University of Virginia Charlonesvilie,Va. 22901 Michael A. Hoffman Earth Sciences and Resources Institute University of South Carolina Columbia, S.C. 29208
Nekhen, later referred to by the Greeks as Hierakonpolis, was the legendary home of the first pharaohs of Egypt (ca. 3100 B.c.). Substantial evidence indicates that the site was also occupied during the Predynastic period (5000-3200 B.C.).Many extensive cemetery and habitation sites of the Predynastic cultural periods are located far from the recent Nile flood plain in one of the two wadis. Now desertlike, these wadis were formed during less arid times by erosion of the older Pleistocene Nile sediment deposits (40,000-20,OOO B.c.) and the even older Cretaceous sedimentary deposits of the Nuhian formation. Later the inhabited area shifted to a more urban settlement area nearer to the Nile. In the middle of what had once been the water course from the larger or Great Wadi, English archeologists discovered the buried remains of an ancient temple. Near the ancient temple a t Nekhen they recovered a ceremonial palette commemorating the unification of Upper and Lower Egypt under Narmer, the first pharaoh. Knowing why this particular site was occupied during the late Predynastic period (ca. 3500-3100 B.C.) through the earliest Dynastic period (3100-2700 B.C) and why the population center shifted may help archeologists understand why Nekhen became the capital city of the first nation state. The area surrounding the temple at Nekhen, today called Kom El-Ahmar, is difficult to study because it is within the modern flood plain and covered by thick sedimentary deposits that have accumulated over the past several thousand years. These deposits are difficult to distinguish from one another; they can be affected by natural processes (erosion and deposition) and hy the activities of the local inhahitBlack-topped Plum Red Ware (opposite) manufactured a t Hierakonpolis. Most often it held artifacts placed in the tombs of important people during the Predynastic period
L
CHKS
L
-
Older sediments
F W e 1. Relatbnship among the sediment layers found in the vicinity of the ancient temple of Nekhen with the approximate dates of deposition of the sediments CHKS and CHK14 are two 01 me cwes, and THKI is me of the trenches dug during the sxcBvBtim
ants because they are within the present cultivation zone. Reliance on purely stratigraphic methods can lead to misinterpretations because of human use over the thousands of years that the area was occupied. Investigation of deposits at Kom ElAhmar is further complicated by the high water table, which prevents extensive excavation to the earliest occupation levels. However, during field work in 1984, special techniques were introduced that allowed excavations to be made helow the water table along a series of trenches and cores. The results of these efforts revealed a complex pattern of sediments deposit. ed during the extensive period the site was occupied. The sequence of pottery artifacts deposited in the sediments gives a reliable method of dating the sediments. Such pottery provides evidence for the continuous occupation of the site a t the very time the nation state was developing, beginning in the Predynastic period and extending through the early Dynastic period. Figure 1 illustrates several discontinuities in the sedimentary layers that were recognized in the field. However, it was difficult to distinguish the sediments on the basis of their physical appearance and correlate them with ANALYTICAL
different cores and with other sediments in the area. When these materials were brought hack to the laboratory for analysis, it was clear that determining the relationships among these sediments and other sedimentary units would require an analytical approach combining geochemical and sedimentological knowledge with detailed analysis of the sediments.
River sediments River sediments are a complex mixture of mineral fragments weathered by the rocks in the drainage basin of the river. The chemical altering of minerals to form clays and the tumbling of mineral grains, which grinds them into smaller particles, are both processes that tend to homogenize the sediments. Therefore, the chemical composition of the sediments is usually taken to represent an average of the composition of rocks in the drainage region. Because the homogenization process also affects the trace elements, trace analysis is not normally an effective way of distinguishing the various deposits of sediments from the same river. However, over the c o m e of time the sediments deposited a t Hierakonpolis have had contributions from the two geologically distinct drainage ha-
CHEMISTRY, VOL. 58, NO. 4. APRIL 1986 * 573A
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Flaure 2. Chondrite-normalizedREE Datterns are averages of sediments from layers exkvated and found near Nekhen
sins of the Blue Nile and the White Nile. If the relative contributions from these sources to the total Nile sediment load have changed as a function of time, then the sediments should reflect such changes. A comparison of the geological terrains suggests that the biggest differences between the sediments carried from the two sources of the Nile would be in the more dense, heavier minerals. Sediments from the Blue Nile should contain more pyroxenes yhereas the White Nile would contribute amphiholes. Among the trace elements, the Cr levels are appreciably higher in the rocks from the drainage hasin of the Blue Nile; Th and Hf concentrations are higher in the rocks drained by the White Nile.
Trace element analysts The heavy minerals and the clay minerals contain most of the trace elements, whereas quartz, a hard mineral of larger, sand-sized grains, contains very small concentrations of trace element impurities. The relative distributions of geochemically similar trace elements, such as lanthanides or rareearth elements (REE), depend on the minerals present. Figure 2 shows the relative concentrations of the REE in a form in which the measured concentrations are normalized relative to the concentrations measured in chondritic meteorites. The results, plotted as a function of atomic number (inversely related to ionic radii), show the effects of the natural geochemical processes, which separate the elements relative to each other. It is assumed that chondritic meteorites are our best approxi-
ANALYTICAL CHEMISTRY, VOC. 58, NO. 4, APRIL 1986
mation of the relative REE concentrations prior to any differentiation by geochemical processes. The most reliable method of measuring REE concentrations is instrumental neutron activation analysis (INAA). Not all of the REE can be determined with suitable accuracy (f3%),hut enough of them are measurable to show the relatively smooth differentiation patterns. The notable exception to a smooth variation with respect to ionic size (atomic number) is Eu, which can OCcur as Eu*+and is partitioned into minerals differently than the other trivalent REE ions. In the Nile sediments the clay minerals predominate in all of the layers. Because the clays contain most of the REE in the sediments, any differences among the sediment units due to heavy minerals could be obscured. In fact, when bulk samples of the sediments from the different units (N, C, B) shown in Figure 1were analyzed by INAA, the REE contents were nearly indistinguishable (Figure 2). Based on the pottery sherds found within the sediments a t Kom El-Ahmar, Units N, C, and B were deposited between 5000 B.C. and 200 B.C. A much older Nile sediment (the Sahaha formation, deposited ahout 20,ooO years ago and found farther from the Nile on the wadi) could he distinguished from the younger Nile sediments on the basis of the REE distrihution patterns. However, to distinguish among the younger Nile sediment units around Kom El-Ahmar, we examined some samples in greater detail. After we analyzed the bulk samples by INAA, they were leached with di-
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lute HCI to remove soluble minerals (e.g., carbonates) and the exchangeable ions were adsorbed onto clay. When the samples were reanalyzed the results showed that Unit C, which was deposited during the period from ca. 3200 B.C. to ca. 2500 B.C., could be distinguished from those sediments deposited before (Unit N deposited from before 5000 B.C. to 3200 B.C.) and after (Unit B deposited from ca. 2500 B.C. to ca. 200 B.C.). Next, the sediments were sieved into coarser grained (>63 pm) sand-sized fractions and finer grained (
