Stratigraphic signatures of climatic change during the Holocene evolution of the Tigris–Euphrates delta, lower Mesopotamia

Fluctuations in climate, sea level and sedimentation rates, in addition to the neotectonic activity, during the geological evolution of the Tigris–Euphrates delta (in the last 10,000 years) had resulted in the deposition of various sedimentary units. Previously, five main stratigraphic units, with other sub-units, have been identified by the author during the study of the Holocene deltaic successions of Lower Mesopotamia and as based upon the results of petrological, geochemical, palaeontological and radiometric analyses of his PhD dissertation. Each unit has been produced through various depositional and diagenetic processes in addition to the dominant climate. Such processes together have been clearly recorded in the forms of either the authigenic minerals occurring in each sequence, particularly the Ca–Mg carbonates, evaporites and clay minerals, the biological activities represented by shell remains of molluscs, foraminifers and ostracods, or the preservation of organic matters within organic-rich layers. This review discusses the impact climatic changes had on the accumulated sedimentary facies during the Holocene evolution of the Tigris–Euphrates delta. Arid climate dominated the study area in the early Holocene after a long period of the wetter conditions of Pleistocene. Such a climatic change has resulted in the formation of gypcretes rich in palygorskite and dolomite occurring within the calcareous fluvial-plain muds, similar to the modern fluvial plain deposits. However, the sediments were highly admixed with coarser sandy deposits of playa and aeolian sources in the western desertic margins, and with older reworked sands of Zagros foothills to the Northeast of Lower Mesopotamia. During the mid-Holocene marine invasion, when the climate became wetter as well, brackish-water/marine sedimentary sub-units were deposited, overlying the previous fluvial plain deposits. The deposition started with a transitional sub-unit flourishing over the older early Holocene gypcrete deposits signaling the marine transgression. The best preservation of organic matters occurred beneath this sub-unit. This sub-unit, which is also characterised by microcrystalline mixing zone dolomite, was followed by the deposition of another sub-unit rich in molluscs and foraminifera that collected during the transgressive period. After about 2000 years of maximum flooding (i.e. high stand period), a regressive tidal flat unit was deposited ending the marine/brackish-water deposition and signaling another climatic change towards a more arid setting around 4000 years ago. These climatic conditions are still continuous in the area and reflect the petrology of the modern salt-covered fluvial plain deposits of these rivers. However, marsh/lacustrine deposits and environments remained, covering some vast lowland parts of the fluvial plains, but further inland to the north of the present-day northern Gulf coasts. Complex implications of neotectonic activities, sea-level fluctuations and differential sedimentation rates in addition to the climatic changes during Holocene have resulted in the formation and preservation of these unique marshlands, which are still covering most parts of the ancient Tigris–Euphrates–Karun delta.