Terrestrial hydroclimate reconstructions of ‘fossil rainwater’ from Arabian arid-zone speleothems

The global development and modern distribution of arid zones in subtropical regions (drylands) are likely associated with the global cooling marking the dawn of the Quaternary (~2.6 Ma). This coincides with global shifts in ocean circulation patterns, the intensification of the Walker-Hadley circulation, declining atmospheric CO2, the initiation of glacial–interglacial cycles and the intensification of Northern Hemisphere glaciation, the combination of which has led to the current global distribution of non-polar deserts at ~30° latitude. Although modern drylands represent Earth’s largest terrestrial biome, covering ~46% of global land surfaces and supporting a global population of ~3 billion people, it is currently unclear how drylands will change under future climate change scenarios. Speleothems, preserved in arid-zone caves, are particularly useful terrestrial climate archives as they act as underground rain gauges, which require a minimum of ~300 mm a-1 precipitation, pedogenesis and vegetation cover to form. Moreover, they can be accurately and precisely dated and are subsequently a valuable tool in identifying past large-scale hydrological and vegetation changes in ancient drylands. Here, we present new data from speleothems during past ‘warm periods’ from the Arabian hyper-arid zone. We apply a novel technique of extracting speleothem fluid inclusions waters to reconstruct the isotopic composition (d18O and d2H) of ‘fossil rainwater’ preserved in the speleothem fabric pristinely over millions of years. Speleothem evidence suggests that during the last 7 million years, the Sahara-Arabian desert experienced numerous intermittent humid phases, typically occurring with periods of low global ice-volume and warmer global temperatures. We further explore tropical push-pull mechanisms driving heating of the deep tropics and subsequent expansion of the tropical zone and synchronicity of humid phases regionally. These results have significant implications for understanding the drivers of dryland aridity in non-polar deserts globally.