DOI: 10.5281/zenodo.11278343
title: Decoupled hydroclimate of Central and Southwestern Iran controlled by the strength of southerly-westerly jets during Marine Isotope Stage 3

description: Abstract

The regional impact of abrupt glacial climate variability remains poorly constrained for arid southwestern Asia, particularly winter dynamics during Marine Isotope Stage 3, due to limited paleoarchives in the Middle East. Here, we present novel continuous speleothem records of δ18O and δ13C for Southwestern and Central Iran, spanning the interval ~50-30 ka. Stable-isotope signals in the two stalagmites are generally uncorrelated and do not exhibit a consistent response to Greenland stadials or interstadials; however, both show a positive δ18O excursion that coincides with Heinrich event 4. We explore the potential mechanisms for intermittent coupling of speleothem δ18O across Iran through isotope-enabled atmospheric modelling outputs, from which we utilize the spatial δ18O gradient as a proxy for wintertime westerly vs. southerly jet strength. Our results suggest that during Heinrich event 4 and several Greenland stadials, stronger westerly winds enhanced Mediterranean moisture contributions to both sites and reduced aridity in southern Iran.
description: Samples were collected in February (sample SIB-7) and July (sample KAH-3) 2019 in southwestern and central Iran (southwestern Asia) from Sibaki and Kahak Caves, respectively. Stalagmites SIB-7 and KAH-3 were cut along their growth axes and the polished slabs were inspected for possible hiatuses. A total of 24 and 12 samples were drilled for 230Th dating along individual growth lamina on SIB-7 and KAH-3, respectively. Samples were chemically processed at the Department of Earth Sciences, University of Oxford. The process included spiking with a 229Th-236U spike, dissolving in HNO3, and refluxing overnight on hot plates to reach the spike-sample equilibrium. U and Th chemical separation were performed following the procedure of Edwards et al. (1987) and the samples were analyzed on a Nu Plasma multi-collector inductively coupled plasma mass spectrometer following the procedure described in Carolin et al. (2019). The 230Th ages and their 95% confidence intervals were calculated by an in-house Monte Carlo script that incorporates the analytical, chemical blanks and initial 230Th/232Th uncertainties. Initial 230Th is corrected using an initial 230Th/232Th atomic ratio of 5.38*10-06. Corresponding U/Th data are presented in Dataset 1.

 Sampling for oxygen and carbon isotope was performed at 0.15 and 0.3 mm intervals on SIB-7 and at 0.2 mm on KAH-3 using a micromill. These subsamples were measured using a semi-automated carbonate preparation device (Gasbench II) linked to a ThermoFisher Delta V Plus isotope ratio mass spectrometer at the University of Innsbruck, following the method of Spötl and Vennemann (2003). Isotope values are reported relative to the VPDB standard. Long-term precision was better than 0.1‰ (1σ) for both δ13C and δ18O (Spötl, 2011). Stable isotope data are presented in Dataset 2.

 Age models were produced using OxCal version 4.4. A depositional model with k0= 0.1 mm-1 and log10(k/k0) = U (-2, 2) was used to create the age models (Bronk Ramsey, 2008, 2009; Bronk Ramsey and Lee, 2013). Oxcal input used to create the SIB-7 and KAH-3 age model are presented in Dataset 3.