[MIS08-P26] Southward Migration of Inter-Tropical Convergence Zone (ITCZ) during Mid-Late Holocene: New Evidence from Sri Lanka
キーワード：Paleoclimate, ITCZ, Monsoon, Sri Lanka, Holocene
Southward migration of the mean latitudinal position of the Inter-Tropical Convergence Zone (ITCZ) as a response to decreasing solar insolation in the Northern hemisphere during the Mid and Late Holocene is discussed widely (Haug et al. 2001; Broccoli et al. 2006; Fleitmann et al. 2007). This hypothesis is supported by the millennial-scale continuous weakening trend of the Indian summer monsoon (ISM) and Asian summer monsoon (ASM) within the core monsoon domains in the Indian sub-continent (Gupta et al. 2003), East Asia (Wang and Tsonis 2008), Middle East (Fleitmann 2003) and South America (Maslin and Burns 2000). However, in contrast to this hypothesis, some studies suggest an abrupt weakening of the ISM and ASM between ~ 4.5 and 5.0 ka BP (Morrill et al. 2003 and references therein). This forms a question of whether the ISM and ASM weakening occurred gradually related to orbital forcing or abruptly due to variations in other components of the climatic system, that is important to know for developing general and regional climate models that help to predict future climate trends.
The South Asian region is directly impacted by the ISM and retreating Indian winter monsoon (IWM) and the agriculture-based economy in this region is strongly dependent on the seasonal precipitation. However, very limited well-dated and continuous paleoclimate archives, especially in the southern parts of the Indian sub-continent that is fed by both summer and winter monsoon rainfalls creates a significant knowledge gap to understand the behavior of Indian monsoon system. So far, our understanding of millennial-scale ITCZ fluctuations depends on the ISM records and evidence from the retreating IWM are still scarce.
The geographical location of Sri Lanka, lying in the center of the seasonal ITCZ migration path and the rain shadow effect due to the central highlands, allows us to study the summer and winter monsoon rainfall variations separately. The position of ITCZ is directly impacting the duration of both monsoon seasons (Fig 1). Therefore, a multi-proxy analysis was conducted on three sediment cores retrieved from the summer monsoon-fed wet zone (South Bolgoda Lake and Horton Plains) and winter monsoon-fed dry zone (Panama lagoon) of Sri Lanka. Together with 14C based age models, rainfall variations in the regions were traced based on compound-specific carbon and hydrogen isotopes of leaf wax n-alkane (n-C33 and C31) and detrital elemental ratios (Zr/Rb and Zr/Al) related to grain size variation.
The increasing trend of δD and δ13C values of leaf wax n-alkane of ISM influenced South Bolgoda Lake and Horton Plains showed a gradually decreasing trend of the rainfall over last ~6 ka BP despite few short-term abrupt events. This trend is also supported by the gradual decreasing trend of detrital element ratios (Zr/Rb and Zr/Al) supporting less continental erosion from declining rainfall. In contrast to these trends observed in the wet zone, retreating IWM influenced Panama lagoon located in the dry zone showed inverse results indicating a more gradual increase in winter precipitation during the same period. This inverse relationship between ISM and IWM rainfall can be explained by the concept of southward migration of the mean latitudinal position of ITCZ during the mid and late Holocene that occurred as a response of solar insolation variations in the Northern and Southern
Hemispheres. When the southern and northernmost positions of ITCZ gradually shift southward, the duration of IWM rainfall can increase resulting in relatively higher rainfall in the dry zone of Sri Lanka. Meantime, the inverse effect on the ISM can be expected in the wet zone. Our results indicate that the annual precipitation of Sri Lanka shifting towards a winter monsoon dominant system from summer monsoon dominant system over last ~ 6 ka BP, as a result of the solar-induced shift of ITCZ.
The South Asian region is directly impacted by the ISM and retreating Indian winter monsoon (IWM) and the agriculture-based economy in this region is strongly dependent on the seasonal precipitation. However, very limited well-dated and continuous paleoclimate archives, especially in the southern parts of the Indian sub-continent that is fed by both summer and winter monsoon rainfalls creates a significant knowledge gap to understand the behavior of Indian monsoon system. So far, our understanding of millennial-scale ITCZ fluctuations depends on the ISM records and evidence from the retreating IWM are still scarce.
The geographical location of Sri Lanka, lying in the center of the seasonal ITCZ migration path and the rain shadow effect due to the central highlands, allows us to study the summer and winter monsoon rainfall variations separately. The position of ITCZ is directly impacting the duration of both monsoon seasons (Fig 1). Therefore, a multi-proxy analysis was conducted on three sediment cores retrieved from the summer monsoon-fed wet zone (South Bolgoda Lake and Horton Plains) and winter monsoon-fed dry zone (Panama lagoon) of Sri Lanka. Together with 14C based age models, rainfall variations in the regions were traced based on compound-specific carbon and hydrogen isotopes of leaf wax n-alkane (n-C33 and C31) and detrital elemental ratios (Zr/Rb and Zr/Al) related to grain size variation.
The increasing trend of δD and δ13C values of leaf wax n-alkane of ISM influenced South Bolgoda Lake and Horton Plains showed a gradually decreasing trend of the rainfall over last ~6 ka BP despite few short-term abrupt events. This trend is also supported by the gradual decreasing trend of detrital element ratios (Zr/Rb and Zr/Al) supporting less continental erosion from declining rainfall. In contrast to these trends observed in the wet zone, retreating IWM influenced Panama lagoon located in the dry zone showed inverse results indicating a more gradual increase in winter precipitation during the same period. This inverse relationship between ISM and IWM rainfall can be explained by the concept of southward migration of the mean latitudinal position of ITCZ during the mid and late Holocene that occurred as a response of solar insolation variations in the Northern and Southern
Hemispheres. When the southern and northernmost positions of ITCZ gradually shift southward, the duration of IWM rainfall can increase resulting in relatively higher rainfall in the dry zone of Sri Lanka. Meantime, the inverse effect on the ISM can be expected in the wet zone. Our results indicate that the annual precipitation of Sri Lanka shifting towards a winter monsoon dominant system from summer monsoon dominant system over last ~ 6 ka BP, as a result of the solar-induced shift of ITCZ.