Holocene stable isotope record of insolation and rapid climate change in a stalagmite from the Zagros of Iran

We explore Holocene climatic changes recorded by geochemical proxies in a single, well-dated, stalagmite from the northern Zagros Mountains of Iran, a region where stalagmite records have so far only provided short glimpses of Holocene climatic changes. Stalagmite KT-3 from Katalekhor Cave began growing ~9.5 ka under wet early Holocene conditions coincident with the timing of Sapropel 1. At this time d18O values were at or below 9.0‰, stalagmite growth diameter was at its maximum, 234U/238U0 activity values were low and trace element contents indicate flushing of the epikarst. Progressive reduction in winter precipitation amount after 7.0 ka, is recorded by increasing d18O and 234U/238U0 activity values and reduction in trace element contents and growth diameter until ~2.0 ka. These trends follow the reduction in summer insolation and agree with model-simulated changes in total rainfall and rainwater isotopes. Sub-centennial-scale variability is not a feature of the d18O or trace element records; this suggests a stable winter recharge regime, a feature that might militate against KT-3 d18O recording changes in the seasonality of rainfall. KT-3 d13C compositions are enriched relative to lower altitude stalagmites in the Levant, implying low soil CO2 contribution (thin montane soils). However, a broadly decreasing millennial-scale trend in d13C suggests that soil carbon contributions increased with time despite the progressive reduction in winter precipitation amount. The d13C changes probably reflect decreasing summer temperatures and less extreme wintertime coldness, driven by reducing summer and increasing winter-insolation respectively. These combined effects decreased effective evaporation and improved soil-moisture availability favouring vegetation development during the growth season.
The d13C values also show ~1.5‰ centennial-scale variability with higher d13C values between 9.0 and 8.7 ka, 8.3 - 7.7 ka, 6.5 - 5.5 ka, 5.4 - 4.5 ka and ~4.3- 2.0 ka: three of these correspond with Rapid Climate Change (RCC) events based on non-seasalt potassium (Kþ) in Greenland ice cores. Higher d13C values
indicate poor soil development caused by aridity. The centennial-scale d13C anomaly (8.3-7.7 ka) is in part overprinted by the ~160 year-long, 8.2 ka cold/dry event, but culmination ~7.7 ka corresponds with other records suggesting an intensified Siberian High Pressure system affecting regional climate. The centennial-scale d13C anomaly between 4.3 and 2.0 ka overlaps the 2.65 to 2.50 ka ‘Assyrian megadrought’ evident in stalagmite stable isotope records in northern Iraq. The KT-3 record is key in better understanding Holocene climate change in the central Zagros region, representative of montane ‘fertile crescent’ environments.