The Decapolis region (Northern Jordan) as historical example of desertification. Mutual dependencies of land use, climate, soil genesis and settlement history.
by
Lucke, Bernhard
Brandenburg University of Technology Cottbus, Germany
Coauthors: Michael Schmidt (Brandenburg University of Technology Cottbus, Germany), Ziad al-Saad (Yarmouk University, Irbid, Jordan), Rupert Bäumler (University of Technology Munich, Germany)

There are many attempts to reconstruct the way of climate during the past 10.000 years in the Near East. As it is situated close to the Saharo-Arabian desert belt, fluctuations of the atmospheric circulation have great impact on precipitation, flora and fauna. Apart from the areas with great rivers like Egypt or Mesopotamia, rain-fed agriculture was the general food production method. It was the economic basis of the cities of the Levant, which flourished during the Roman, Byzantine and Ummayyad period. But from the 10th until the early 20th century, the Levant lost much of its wealth and economic importance. Especially the cities in areas close to the desert - like the Decapolis region in Northern Jordan - were deserted and only some farmers and bedou tribes continued to live there.

Huntington (1911) suggested that climate change was responsible for this development, but his ideas were rejected when Lowdermilk (1944) found erosion indicators. Lowdermilk postulated that bad land use after the Islamic conquest caused the desertion of the Decapolis region, and his findings were supported by the climatic investigations of Butzer (1955, 1961), who estimated that rainfall was even increased in the time of the abandonment. However, other climatic studies found very different results (Nützel 1976; Issar 1998, Bar-Matthews et al. 1998). Summarising, it could have been moister, as today or drier in the time of abandonment. Bar-Matthews et al. (1998) conducted a rainfall calculation: It seems that the collapse of the Decapolis cities took place when there was a sudden drop in annual precipitation by 50 mm, lasting ca. 100 years. But this coincidence alone does not prove causality. Are 50 mm less precipitation already climate change? And what changes? The early rains, the late rains, the rainstorms or is every rainfall just a little fewer?

Other authors maintain that there was no change of environment, but solely political and economic reasons led to the abandonment of the region (Walmsley 1992). And in fact it is not clear whether a reduction of the annual rainfall by 50 mm would have had influence in local food production sufficient enough to enforce the desertion of a whole region. The area is very heterogeneous: The Jordan valley enjoys a different climate than the mountains east of the river, or the highlands further east, close to the desert. Bar-Matthews et al. (1998) conducted their investigations at Soreq cave in Israel, which is west of the Jordan river and again a different climatic frame is present. So it can be questioned whether the results of Soreq cave are applicable to the Decapolis region, and maybe the mentioned differences between the climate investigations root from different research locations.

Additionally, land use influences the local climate. For example, forest coverage of vast areas is not only advantageous due to keeping the soil, reducing wind blow, and supplying the adjacent fields with nutrients, but also due to leading to increased and better distributed rainfall (Seth 1978). In the Decapolis area, the total amount of rainfall is not as important as its distribution. Especially early and late rains are necessary for the seeding and harvest success, while heavy rainstorms can destroy crops and carry away the soils, although providing a lot of water (Lucke 2002). Therefore climate must not be seen independent from the land use. Local climate, land use and soil genesis are important interrelated site factors.

Today, a lot of climatic proxy data about the climatic history of the Decapolis region is available, but this information is not applicable to the settlement history, as the results lack linking under each other as well as to the general environmental history. Thanks to the work of Issar (1998), it seems meanwhile clear that the majority of climate investigations point to a drought period in the time of the Decapolis' abandonment, but it cannot be said how the consequences for agriculture looked like. To predict further environmental scenarios, and to work out strategies to cope with them, it is necessary to know exactly the mutual dependencies of climate and land use.

The link between climate research and settlement history can be provided by soil science. The soil is like a memory for past environments. The methodology is not new, but since 1980 applied in the middle european archaeology (there the findings are often only soil structures; Born 1980, Fries 1995). It was transferred and adapted to Jordan in a first attempt in a master thesis 2001 (Lucke 2002). But to reconstruct the environmental history of Jordan, it is not only necessary to analyse the soil, but also to know the general circumstances, like land use, geology, hydrology and the possible climatic framework. In opposition to the classical methods of climate research and archaeology, investigation in the soil and its genesis does not provide a static picture of past environments (although this is possible if relic land surfaces are available), but a process which can be described like an equation with several variables. These variables are the soil qualities, the bedrock, land use, climate conditions and relief, and the equation can only be solved by iteration, requiring close cooperation of several disciplines.

To answer the question of the environmental history of the Decapolis region, initial research was carried out at the site of Abila (ca. 15 km north of the modern city of Irbid). The results indicated that climate could in fact have played a decisive role in the abandonment of the region, but the exact circumstances and mutual dependencies of climate - land use - soil development - settlement activity remained unclear (Lucke 2002, 2003a). Therefore further research was carried out at Tell Zera'a in the Wadi el-Arab (Lucke 2003b), and in the framework of a planned joint project, the Jordan valley will also be investigated in. The results of the 2003 campaign showed again that conditions in the region are very heterogeneous, varying strongly in small areas.

The prevailing red clayey soil (Terra rossa) should be very old, as the colouring clay mineral haematite only comes into being after long weathering processes. The distribution of these sometimes very deep profiles (up to 3 m), makes clear that there was no general devastating erosion as Lowdermilk (1944) postulated. On the other hand, erosion gullies in the fields show that there is steady erosion, especially after strong rains. The distribution of substrates indicates that the area saw long and repeated soil movement, dependent on the relief. Wind erosion should have played no greater role. This is also indicated by the hard crust which is formed by the summer heat (Lucke 2002, 2003a).

The drought crust plays an important role for agriculture. It was found extending up to 30 cm deep, while drought fissures carried sherds up to 1,50 m deep. The fields are usually ploughed before the first rains at the end of October, but the simple wooden plough, reported by Schumacher (1889) for the traditional Arabs, is hardly able to break the crust. Such a plough only opens the surface of the soil, breaking capillarity and storing moisture in the underground. In general it was thought that this ancient plough design is of advantage in semi-arid areas. But if the drought crust is not fully broken, it dissolves only slowly under the first rains and most of the water runs off (Lucke 2002). Long fields observed in ancient land use systems in Israel could point to the usage of the heavy plough, which is usually expected only in the northern part of the Roman empire (Kuhnen 1989). If the heavy plough was in use in he Decapolis, this would explain why there are no field divisions in the Decapolis region which refer to the classical rectangular system of the simple wooden plough (Lucke 2002, 2003a).

A different ploughing and field distribution technique would have had different influence in the soil genesis. One field seemed to have experienced a very long cultivation, and maybe was never abandoned, as Ayyubid-Mamluk sherds were found. It varies with a brighter colour from the surrounding fields, seemingly because of different humus contents. According to the travel reports of Seetzen (1854) and Schumacher (1890), most of the area experienced natural reforestation (with oak trees) in the medieval period, but this possible old field seemingly did not. It was characterised by enrichment of nutrients and strong weathering of the bedrock. The prevailing bedrock in the region is soft limestone.

Some soils on the limestone are very shallow (30 cm), but have the red colour. Their shallowness could root from intensive erosion in recent times, but the lack of gullies at the shallow places and the change with deep substrates just some meters away makes this conclusion not very probable. It merely seems that very different weathering speeds are present, seemingly influenced by the source rock. The source rock could also play a decisive role in the colouring of the soil, as it contains many iron oxides. It is possible that the red colour is influenced by the bedrock, questioning whether the prevailing soils are really very old.

At some places, the sedimentation history can be reconstructed. A Roman bridge in Wadi Queilbeh stopped material carried by the river, and a Roman theatre was seemingly reused as cistern, later filling with clearly separated sedimentation layers. Each represents a single event of soil movement due to rainwater. The bridge and the theatre are well-suited to reconstruct the precipitation as well as the sedimentation history, and ash bands allow dating.

Summarising, the soil scientific methodology is well suited to reconstruct the environmental history, although the presented results are only preliminary. For now it can be said that the shallowness of soils and richness in calcium carbonate point to a high vulnerability of agriculture to drought. Additionally, the region shows very different conditions and a high influence of the relief, which points to soil movement processes and strongly changing environments during the past 10.000 years. More interdisciplinary research is necessary to confirm these conclusions.

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Date received: November 17, 2003