Brigitte Coque-Delhuille
Pr.Universite de Paris VII et Laboratoire de Geographie Physique, CNRS Meudon, France

In the Yemeni desert, SE of Marib, between wadi Bayhin and wadi Markha (14-15°N), recent geomorphological field research gives informations about the Holocene environment. The data are mainly based on the anthrosoils of the ancient irrigation perimeters, developed during the south-arabian kingdoms (8th c. BC-3rd c. AD) and at least since 3500 years BP (al-Hinwa), with sedimentological analyses, 14C and OSL-IRSL dating.

The present state of environment is characterised by two main original features in Yemen. On the morphostructural aspect, this mountainous area is shaped in the outcrops of the crystalline Precambrian basement, between the volcanic high plateaus to the west and the limestone plateaus of the Hadramawt to the east. These mountains, which altitudes range between 1500-2000m up to 2600m, are characterized by bare, steep-sloped and deeply incised reliefs. On their northern rim, wide endoreic valleys NNE-SSW to NE-SW, situated at about 1000-1100m in altitude, trend towards the Ramlat as-Sabatayn and the Arabian desert to the North.

Most of the main landforms are inherited from the Tertiary period, with a very intensive tectonic phase of the Red Sea rift and the Aden Gulf rift, with the uplift of the marginal ridge to the west and to the south and the volcanic activity on the high western plateaus. The immunity of the rocky slopes is testified by the black ferro-manganese desert varnish spread over all the slopes of the crystalline basement. Some estimations of the age of this varnish on some slopes, with the (K+ + Ca2+)/Ti4+ ratio, give ages between 30 000 and 200 000 years BP. The thick alluvium in the valleys (over 100m) were deposited during the Pliocene and the Quaternary periods, and the erg of the Ramlat as-Sabatayn was edified during different very arid phases along the Quaternary era. So, the main features of the geomorphological environment during the south-arabian kingdoms were edified long time before and was similar to the present one.

On the climatic aspect, these valleys are marked by a strong aridity, close to hyperaridity (value of the FAO-UNESCO aridity index: 0.03-0.04). At Nuqub (wadi Bayhan), the mean annual rainfall is about 60mm, with a very high interannual variability). But the semi-arid mountainous impluvium with strong monsoon-type rainfalls (at least 300-400mm/y.) produce allogeneous pluriannual flash- floods in the lower parts of the valleys, now and in ancient times, which allowed the functionning of irrigation perimeters in these valleys.

During the Holocene, two different main periods can be distinguished in the arid Yemen. First, the last more humid period which took place at the beginning of the Holocene, between about 9000 and 5600 years BP, with the development of paleosoils as in the wadi al-Jubah dated between 6595+ 95y.BP and 5750+ 110y.BP. We found the same paleosoil in coarse alluvium in the wadi Dhalimayn. The youngest remnants of paleolake deposits, in the south of the Rub al-Khali, are dated also of this climatic optimum (9000-5700y.BP). More over, an exoreic hydrographic system seems to have been functionning in the Jawf-Marîb-Shabwa graben, during this period, the water of the inside basin flowing out to the Aden gulf through the Hadramawt and Masila wadis. This more humid period seems also to be the last one of desert varnish formation. Now, the varnish is more or less destroyed, and the engraved south-arabian inscriptions on rock faces are not revarnished.

After this period, a strong aridity was established, at least since 3500 years BP (al-Hinwa anthrosoils, and core-samples from the Aden Gulf and the Oman Sea showing the installation of the present aridity since 3500-3000y.BP). The ancient anthrosoils give informations about this arid climate and some minor fluctuations of the degree of aridity.

The allogeneous flash-floods or sayl, which number and intensity vary considerably in time and in space, were perfectly controlled during the Antiquity by means of various hydraulic equipment permitted the derivation of part of the water, with largely decreased flow speed and competency. So, only the fine particulate matter was deposited at the place of the ancient irrigation perimeters, and human controlled terraces made of anthrosoils were built. The average thickness is mostly less than 10m, but they can reach 15m upstream, as at al-Hinwa (wadi Dura').The characteristics and the age of these anthrosoils give informations about the Holocene environment. On aerial photographs, they are easily identified by their light-color and the shape of the ancient fields, arranged in an almost geometrical manner.

They show specific sedimentological characteristics:

• a mainly silty texture, mixed with fine sand and clay, showing in a given perimeter, the decrease in grain size from the inlet to the outlet and from the base to the top;
• the disappearance of the primitive microbedding because of disturbance by ploughing;
• high degree of carbonatation (9 to 16% carbonate content), partly composed of carbonated silts of a loessic type;
• the absence of paleosoils and, sometimes, the intercalation of aeolian sands. These two characteristics indicate a strong aridity, similar to the present time.

In the anthrosoils, two irrigation levels can be recognized in some places, as at Hajar am-Dhaibiyya and al-Hinwa (wadi Dura'). The al-Hinwa perimeter upstream, displays the thickest anthrosoils (15m), and reveals two successive irrigation systems, the first using the wâdî, the second one using a naqab (cut in the moutain). It represents the longest irrigation period known in this area, from about 3500 years BP (+ 400y.) at the bottom to 1500 years BP (+200y.) at the top, so about 2000 years of irrigation. These data were obtained by applying the new optically stimulated luminescence dating method (OSL-IRSL) for the first time on irrigation sediments. The interruption is not significant in terms of changes of the natural environment, but is due to technical reasons.

Nevertheless, the OSL-IRSL ages obtained on other anthrosoils in the wâdî Bayhân and the periods of functionning of the irrigation perimeters, indicate a regression of the floods downstream during the south-arabian period (end of Timna' anthrosoils about 2200-2300 years BP, where now there is less than one flood every three years, beginning of the al-Haraja perimeter, close to the gorge, around 2100-2200y.BP, end of al-Hinwa irrigation sequences about 1500 years BP), and so, fluctuations to more aridity since 2 or 3 centuries before the common era.

But the abandoning of the irrigation system was due to historical features and not to important variations in the natural environment.