Indian Ocean Monsoon Dynamics recorded in a Speleothem from Socotra, Yemen

The arid tropical island of Socotra is located in the northwestern part of the Indian Ocean, between the horn of Africa and the Arabic peninsula. Rainfall has a strong bimodal distribution on Socotra, associated with the migration of the Intertropical Convergence Zone (ITCZ). Precipitation is delivered as the northward migrating ITCZ passes over the island in May-June and returns during its southward migration from September to December. Little or no rain falls on Socotra during the summer (southwestern) and winter (northeastern) monsoons (Shakun et al., 2007). The 1540m-high SW/NE oriented Haggeher Mountains create a watershed on the island forcing precipitation derived from the ITCZ passages, to fall mainly on the windward side of the mountain range. As a consequence thepr ecipitation associated with the northward migrating ITCZ falls mainly on southwestern part of the island whereas precipitation related to the autumn passage of the ITCZ falls mainly on thenortheastern part of the island (Scholte and De Geest, 2010).
To study the evolution in the latitudinal position of the ITCZ and coupled changes in hydrological cycle (sources, directions, amounts), two stalagmites were recovered from two different caves in the eastern part of the island. Stalagmite STM1 sampled in Hoq Cave covers the last 6000 years; stalagmite STM5 from Casecas Cave, 6 km away, spans the last 1000 years (TIMS U/Th-dating).
Multi-proxy analyses (d18O, d13C and greyscale analyses) were carried out at an exceptionally high resolution of 500µm, corresponding to a time-scale of ~3 years. Although modern speleothems grow under fragile equilibrium conditions, the similar isotopic signals in both speleothems over the last 1000 years demonstrate the reproducibility of the time-series. All proxies can be interpreted as indicators for dry or wet conditions on Socotra and all show the same trends over the studied period.
Consequently, the high-resolution isotopic datasets allow for a detailed insight into the hydrological cycle on Socotra. Fleitmann et al. (2007) presented the isotopic record of a stalagmite, recovered from the western part of Socotra, on the other side of the watershed, only 65 km away from Hoq and Casecas Caves. However, clear correlation of the isotopic records can be established. This lack of correlation emphasizes the important role of the Haggeher Mountains in the formation of two distinct climate-regimes on either side of the island. Our record reflects precipitation-changes during the southward passage of the ITCZ, whereas the record of Fleitmann et al. (2007) reflects variations in the intensity of the May-June rainy season. When our isotopic records are compared to a speleothem isotopic record from Southern Oman (Fleitmann et al., 2007), again no distinct correlation can be observed. On the other hand, STM1 and a stalagmite from Northern Oman exhibit a similar isotopic pattern over the last 6000 years. This finding seems to suggest that, over the last 6000 years, both Northern Oman's and eastern Socotra's rainy seasons are influenced by the same monsoonal dynamics.

The exceptional high-resolution and continuity of this paleoclimate record allow for the detection of cyclical components in the climate signal on a decennial and centennial scale. Spectral analysis reveals an important ~205-years component in the STM1 isotopic records. Moreover, by comparing the oxygen and carbon isotopic records with a reconstruction of solar activity (Steinhilber et al., 2009), an obvious and statistical significant correlation becomes clear. This correlation suggests that for most of the last 6000 years, in Socotra, periods of lower precipitation and less vegetation can be associated with periods of high solar activity. Periods of low solar activity induce more precipitations and higher vegetation abundance. However, several authors reported that solar activity is positively related with the intensity of the rainy seasons in Oman (Fleitmann et al., 2003; Neff et al., 2001), which is the exact opposite of our findings in Socotra. This teleconnection in the Indian Ocean Monsoon dynamics was explained by Fleitmann et al. (2007) and attributed to changes in the latitudinal position of the ITCZ and the convective activity above Oman. Our results independently confirm this hypothesis and refine it by identifying the 205-years "De Vries / Suess" sunspot cycle as the dominant forcing cycle for the Indian Monsoon Dynamics on a centennial scale.

References:
Fleitmann et al. 2007. Quaternary Science Reviews 26, 170-188; Fleitmann et al., 2003. Science 300,
1737-1739. Neff et al., 2001. Nature 411, 290-293.; Scholte, P., De Geest, P., 2010, Journal of Arid
Environments 74, 1507-1515.; Shakun et al., 2007, Earth and Planetary Science Letters 259, 442-456.;
Steinhilber, et al., 2009. Geophysical Research Letters 36, 1-5.