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.