Mid- to Late Holocene Indian Ocean Monsoon variability recorded in four speleothems from Socotra Island, Yemen

Since the Mid-Holocene, the summer position of the Intertropical Convergence Zone (ITCZ) is gradually moving south due to the diminishing boreal summer insolation (Wanner et al., 2006). Understanding this behavior for the Indian Ocean Monsoon (IOM) and its northeast and southwest subsystems is of major importance, especially since further drying is predicted (Fleitmann et al., 2007). To investigate how precipitation from the northeast IOM subsystem is evolving since the mid Holocene, we sampled four stalagmites on Socotra, an island in the northern Indian Ocean. On Socotra, rain is delivered at the start of the southwest IOM in May-June and at the start of the northeast IOM from September to December. The Haggeher Mountains, reaching up to 3000m altitude in the middle of the island, act as a barrier forcing precipitation to fall preferentially on the windward side of the mountain range. Consequently, rain delivered by northeast winds at the start of the northeast IOM, falls on the eastern side of the island, where the studied caves are located. ?18O and ?13C and Mg/Ca and Sr/Ca signals in the stalagmites are interpreted as indicators of wetter or drier conditions created by the northeast IOM. The stalagmite records suggest a long-term weakening of the northeast IOM since 7 ka confirming a link between the Holocene decreasing boreal summer insolation and the diminishing rainfall of the IOM. A similar ?18O record to that of eastern Socotra occurs in Northern Oman stalagmites, after 6.2 ka (Fleitmann et al., 2007). At this time, the summer ITCZ moved south of Northern Oman making precipitation from northeast winds the only moisture source available. A drying around 6 ka is also seen in sedimentary records from the Arabian Peninsula (Lezine et al., 2010; Parker et al., 2006), which nowadays are located outside the migration pathway of the ITCZ. Records on the Arabian Peninsula that today are still within the ITCZ migration belt, and thus receive rain by both the northeast as the southwest IOM, display a gradual drying after the wet Holocene optimum at 8.0 ka. Socotra, a region still affected by both monsoons, display a drying on the eastern side but a wettening on the western side (Fleitmann et al, 2007). Due to southward retreat of the summer ITCZ, the southwesterly winds that were previously blowing over dry Somalia, are forced into a more southerly path over the Indian Ocean leading to more precipitation at the beginning of the southwest IOM on western Socotra. This local effect shows that in a generally drying pattern, smaller-scale regional changes towards wetter conditions can occur.