3.2 Climatic and Environmental Conditions During SAHP 4
It has been shown that fluctuations in δ18Oca from Mukalla Cave speleothems are related to changes in the amount of ASM precipitation in Yemen (Fleitmann et al., 2011; Nicholson et al., 2020). This is confirmed by isotope measurements (δD and δ18O) of stalagmite fluid inclusion water, showing that the ASM was the dominant moisture source at Mukalla Cave during MIS 5e (Nicholson et al., 2020). The δ18Oca profile of stalagmite Y99 shows three distinct features. Firstly, δ18Oca values are lowest at onset and during the first phase of SAHP 4, indicating that ASM rainfall increased rapidly at the onset of SAHP 4, most likely within a few centuries and similar to ISM monsoon records (Fleitmann et al., 2003a). Secondly, ASM rainfall is highest until ~124 ka BP and decreases following summer insolation. Thirdly, the abrupt positive shift in δ18Oca at 121.170 ± 0.500 ka BP indicates an abrupt termination of SAHP 4, most likely within a few decades (Burns et al., 2001). This is a common feature of SAHPs (Nicholson et al., 2020) and related to the geographical position of the cave in relation to the position of the Intertropical Convergence Zone and monsoonal rainfall belt respectively (Fleitmann et al., 2007). The abrupt termination of speleothem growth and positive shift indicates a rapid retraction of the Intertropical Convergence Zone and associated monsoonal rainfall southwards of Mukalla Cave. In addition, Y99 SAHP 4 δ18Oca values show that monsoon precipitation was substantially higher during MIS 5e (SAHP 4) compared with subsequent SAHPs (Fig. 2B). This isotopic difference is also observed at Hoti Cave (Fleitmann et al., 2011; Nicholson et al., 2020) (Fig. 2B). Overall, Y99 δ18Oca values indicate that MIS 5e saw the most substantial enhancement of monsoon precipitation during the Late Pleistocene.
Stalagmite Y99 δ13Cca values are influenced by numerous factors, including vegetation type and density, and soil thickness and moisture above the cave (Nicholson et al., 2020). However, the various, and sometimes counteracting, controls means that stalagmite δ13Cca values can be difficult to interpret and that the principal factors controlling δ13Cca values may change over time. Y99 δ13Cca values vary between -4.6 and -9.0 ‰ and thus fall into a mixed C3/C4 vegetation signal (Clark and Fritz, 1997), suggesting that grasslands with some woody cover were present above Mukalla Cave during SAHP 4. This is consistent with palaeontological records across Arabia and phytolith records from Jebel Faya (MIS 5e) and Mundafan (MIS 5c/5a), indicating that now arid areas of Arabia were characterised by grasslands and some woody cover during wetter periods (Rosenberg et al., 2011, 2013; Bretzke et al., 2013; Groucutt et al., 2015c; Stewart et al., 2020a, 2020b). Similar to the Y99 δ18Oca profile, the termination of stalagmite growth is characterised by an abrupt increase in δ13Cca (Fig. 2A) as rainfall, drip-rate and vegetation density decreased rapidly above Mukalla Cave. Overall, the Mukalla Cave δ13Ccaprofile indicates that increased rainfall was associated with the formation of herbaceous grasslands, with some woody cover, in the now arid interior of Yemen during MIS 5e.