SEDIMENTATION IN THE LAKES AND MARSHES (AHWAR) OF THE TIGRIS-EUPHRATES DELTA, SOUTHERN MESOPOTAMIA

The fluvial plains of lower Mesopotamia crossed by the Tigris and Euphrates rivers are occupied by fresh-brackish water lakes and extensive reed-marshes dominated by Phragmites sp. and Typha sp. (together locally called Ahwar). Allochthonous sediment is mostly supplied by the Tigris and Euphrates seasonal floodwaters, and by winds which bring aeolian dust from the north-west and sand during storms from the western deserts in summer. Other contributions to the sediment budget are made by in situ biological activity and by biochemical-chemical processes within the sediments. The sediments are dominantly silts, with silt comprising more than 60% of the sediment. The remainder of the sediment consists of varying quantities of clay and sand. Clay is more abundant in the lower parts of most of the analysed cores (30-80 cm in length), indicating a change in sediment supply and/or sedimentary environment. Well developed stratification is found only on the river levees and in some clay-rich lake and marsh sediments. The remainder of the sediments are generally poorly stratified with irregular stratification and considerable bioturbation. Mineralogically, the sediments consist mainly of calcite, quartz, dolomite and feldspar (particularly albite). Aragonite is present as a result of in situ production by macrofauna. The clay fraction (<2 mu m) consists of smectite, illite-smectite, illite, palygorskite, kaolinite and chlorite. The total organic carbon (TOC) content is usually <5% except for some peaty surface layers of the freshwater lakes and marshes where TOC may rise to 10-20%. The sediments have a relatively high calcium carbonate content, 20-80%, the highest values only being found in mollusc-rich horizons. The sediments contain high amounts of Ca, Mg and Sr and are generally low in Al, Ti, K and V. Values for Fe, Co, Pb, Zn and Mn are close to the average for mudstones, whereas Cu and P are relatively low. The high Ni and Cr contents of the sediments are related to high contents in the source material of the major rivers and aeolian deposits. Multivariate statistical analysis shows that elemental components of surface and source materials correlate well with other sedimentary properties. Aragonitic molluscs of various species dominate the macrofauna, but a microfauna of foraminifers, ostracods and diatoms is also present in the sediments.