Holocene climate change in southern Oman deciphered by speleothem records and climate model simulations

Southern Oman speleothem oxygen isotope and multi-proxy data reveal diverse changes in the Afro-Indian summer monsoon circulations and local hydroclimate conditions during the Holocene, confirming climate model simulations.
Holocene climate change in southern Oman deciphered by speleothem records and climate model simulations

In the Indian monsoon region, both speleothem δ18Oc and Arabian Sea marine records are believed to reflect the intensity of the Indian summer monsoon, but there is a significant phase difference in the precession band between these two kinds of records, coined as the “land-sea precession phase paradox”. This call for further investigations to figure out how the Indian summer monsoon varied during the Holocene and more studies need to improve the understanding of the climate meaning of speleothem δ18Oc

This study provides stalagmite Q5 multi-proxy records from Qunf Cave, southern Oman, focusing on critical issues: (1) What is the hydroclimate significance of Qunf δ18Oc variation? (2) What is the local hydroclimate variation pattern across the Holocene? (3) What is the relation between the Qunf δ18Oc record and North Hemisphere summer insolation (NHSI)?   (4) Is precipitation at Qunf Cave sourced from single or multiple moisture sources? The study used the EC-Earth climate model to conduct climate simulation experiments of 8 ka BP (8K) and pre-industrial period (PI) respectively and compare their differences. 

Moisture Source Change

Q5 δDfi, Δ17O and the relative humidity of moisture source area reconstructed by Δ17O indicate that the moisture sources or their climate characteristics changed at ~6 ka BP. By comparing 8K and PI simulation results, we found that while the moisture from the Arabian Sea prevails in southern Oman, the North African summer monsoon also contributes water vapor to the area to some extent during the early to middle Holocene.

 EC-Earth simulation results of JuneJulyAugust (JJA) hydroclimate difference between 8 ka BP (8 K) and preindustrial period (PI). a Precipitation (shadings) and 850hPa wind (vectors) difference. b Near-surface relative humidity difference. The red asterisk shows the location of Qunf Cave. All the results have passed the significance test of the mean difference

Afro-Asian monsoon system

The monsoons over the African continent and the Indian Ocean exhibit remarkably similar hydroclimate trends during the middle to late Holocene. This suggests that they were sensitive to, or driven by, a common forcing mechanism, i.e., Northern Hemisphere summer insolation, on suborbital timescales. More broadly, the entire North African summer monsoon and Asian summer monsoon (Indian summer monsoon and East Asian summer monsoon) may be viewed as a large unified summer monsoon system. Therefore, the two monsoon systems can be regarded as a “super monsoon” system, that is, the Afro-Asian monsoon system.

 Qunf δ18Oc record shows the Holocene hydroclimate variation of the Afro-Asian monsoon regime. During the early to middle Holocene, Qunf Cave received more remote moisture brought by the North African summer monsoon, and the precipitation δ18Op was lighter, causing negative speleothem δ18Oc values. From ~6 ka BP to the late Holocene, only the Indian summer monsoon provided near-source moisture to Qunf Cave, and the precipitation amount decreased, resulting in heavy speleothem δ18Oc values.

 Q5 multi-proxy records. a δDfi, error bars are the standard deviation (1 SD) of the reproducibility of crushed speleothem samples; b Δ'17O, error bars are the standard deviation (1 SD); c Relative humidity calculated by Q5 Δ'17O data, error bars are the standard deviation (1 SD); d δ18Oc; e δ13C; f (234U/238U)0; g Mg/Ca; h Sr/Ca; and i Ba/Ca. The top yellow and blue banners show the monsoon systems prevailed at the cave site, with ISM stands for Indian summer monsoon, and NASM stands for North African summer monsoon. The Holocene Humid Period is from 10.5 to 6 ka BP, coinciding with the intensified NASM and ISM.

Local Hydroclimate change

Qunf (234U/238U)0, δ13C and trace element ratios (Mg/Ca, Sr/Ca, Ba/Ca) reflect the local hydroclimate conditions. They show a slightly increased or unchanged effective precipitation (precipitation-evaporation) at Qunf Cave over the Holocene, consistent with our model simulation results.

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