Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 116, Issue 1, Pages 67-72Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1808103115
Keywords
4.2 ka event; stalagmite; drought; Mesopotamia; dust
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Funding
- Leverhulme Trust
- Royal Geographical Society Thesiger-Oman Fellowship
- British Cave Research Association Ghar Parau Foundation Expedition Grant
- [RPG-2013-235]
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The extent to which climate change causes significant societal disruption remains controversial. An important example is the decline of the Akkadian Empire in northern Mesopotamia similar to 4.2 ka, for which the existence of a coincident climate event is still uncertain. Here we present an Iranian stalagmite record spanning 5.2 ka to 3.7 ka, dated with 25 U/Th ages that provide an average age uncertainty of 31 y (1 sigma). We find two periods of increased Mg/Ca, beginning abruptly at 4.51 and 4.26 ka, and lasting 110 and 290 y, respectively. Each of these periods coincides with slower vertical stalagmite growth and a gradual increase in stable oxygen isotope ratios. The periods of high Mg/Ca are explained by periods of increased dust flux sourced from the Mesopotamia region, and the abrupt onset of this dustiness indicates threshold behavior in response to aridity. This interpretation is consistent with existing marine and terrestrial records from the broad region, which also suggest that the later, longer event beginning at 4.26 ka is of greater regional extent and/or amplitude. The chronological precision and high resolution of our record indicates that there is no significant difference, at decadal level, between the start date of the second, larger dust event and the timing of North Mesopotamia settlement abandonment, and furthermore reveals striking similarity between the total duration of the second dust event and settlement abandonment. The Iranian record demonstrates this region's threshold behavior in dust production, and its ability to maintain this climate state for multiple centuries naturally.
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