Geology | 2019
Oman corals suggest that a stronger winter shamal season caused the Akkadian Empire (Mesopotamia) collapse
Abstract
The Akkadian Empire was the first united empire in Mesopotamia and was established at 4.6 kyr B.P. (where present is A.D. 1950). The empire abruptly collapsed in 4.2 ± 0.2 kyr B.P. Seasonal-scale climatic dynamics behind this collapse have not yet been resolved. Here, we present monthly climatic parameters (temperature and hydrology) inferred from fossil Omani corals that lived between 4.5 and 2.9 kyr B.P. Winter temperatures derived from a modern Omani coral correlate with winter shamal (western Asian dust storm) frequency. A fossil coral from 4.1 kyr B.P. shows a prolonged winter shamal season with frequent shamal days. This likely caused agricultural failures in Mesopotamia and contributed to the Akkadian Empire collapse, as this region depends on winter rainfall. INTRODUCTION Mesopotamian civilizations thrived following the development of irrigation and rain-fed agriculture between the Euphrates and Tigris Rivers (Jacobsen and Adams, 1958; Wilkinson et al., 1994; Macklin and Lewin, 2015) (Fig. 1A). The first united empire in the Mesopotamian region, the Akkadian Empire, was established around the metropolis Tell-Leilan under the rule of Sargon of Akkad in ca. 4.6 kyr B.P. (where present is A.D. 1950) (Weiss et al., 1993; Ristvet and Weiss, 2005). The Akkadian Empire linked the remote rain-fed agricultural lands of northern Mesopotamia with the irrigation-based southern Mesopotamian city-states. Archaeological ruins dated by radiocarbon isotopes show that the Akkadian Empire abruptly collapsed at ca. 4.2 kyr B.P., and its settlements were abandoned (Weiss et al., 1993, 2012; Ristvet and Weiss, 2005). Three-hundred (300) years after the collapse, new populations resettled in the region of the former Akkadian Empire. Soil morphological investigations at TellLeilan suggest a sudden shift toward drier, more arid conditions at ca. 4.2 kyr B.P. (Weiss et al., 1993), while a sediment core from the Gulf of Oman, taken directly downwind of the Akkadian Empire, shows an abrupt increase in aeolian dust from the Mesopotamian region (Cullen et al., 2000). Taken together, these results suggest that the collapse of the Akkadian Empire was caused by an abrupt drought (Weiss, 2017), which involved strong surface winds blowing from western Asia toward the Gulf of Oman– Arabian Sea. However, the climatic processes behind the 4.2 kyr B.P. event still need to be resolved (Staubwasser et al., 2003; Walker et al., 2012; Kathayat et al., 2017). The agriculture of the Akkadian Empire depended on winter rainfall in the headwaters of the Euphrates and Tigris Rivers and on their seasonally varying river discharge (Fig. 1; Macklin and Lewin, 2015). To better constrain the climatic processes that caused the abrupt aridification at 4.2 kyr B.P. in the Mesopotamian basin and the collapse of the Akkadian Empire, paleoclimate reconstructions are required (deMenocal, 2001). Here we reconstruct wintertime temperatures and hydrological changes from six fossil Porites corals sampled in the Gulf of Oman. The fossil corals are from 4.5 to 2.9 kyr B.P.; i.e., they provide time windows of seasonal climate variations before, during, and after the collapse of the Akkadian Empire. MATERIALS AND METHOD We collected fossil Porites colonies from coastal tsunami deposits on the northeastern coast of Oman (city of Fins: 22°54.08′N, 59°13.37′E; Hoffmann et al., 2013; Fig. 1). The coral samples were sliced into 5-mm-thick slabs, which were X-rayed (Fig. DR1 in the GSA Data Repository1). We collected powder samples for geochemical analysis along the maximum growth axes of the corals at 0.2–0.9 mm intervals (Table DR1 in the Data Repository). All fossil corals were screened for diagenetic alteration using scanning electron microscope images and X-ray diffraction analysis (Fig. DR2; Table DR1). For geochemical analysis, we selected fossil corals that did not show any signs of diagenetic alteration. The ages of fossil corals were determined using a radiocarbon technique at the Accelerated Mass Spectrometry Center of Yamagata University (Yamagata, Japan). Ages were corrected for a local reservoir effect following Cullen et al. (2000) and then calibrated using the Marine13 program (Reimer et al., 2013; Table DR2). We used six fossil Porites corals from 4.5 to 2.9 kyr B.P., bracketing the