Three distinct Holocene intervals revealed in NW Madagascar: evidence from two stalagmites from two caves, and implications for ITCZ dynamics

Abstract. Petrographic features, mineralogy, and stable isotopes from two stalagmites collected from Anjohibe and Anjokipoty Cave allow distinction of three intervals of the Holocene in northwestern Madagascar. The Malagasy early Holocene interval (between ca. 9.8 and 7.8 ka) was wet, and vegetation changes seem to have been controlled by changes in climate. The Malagasy late Holocene interval (after ca. 1.6 ka) also records evidence of wet conditions, but changes in vegetation were influenced by anthropogenic effects, as suggested by the stalagmite δ13C shift. The Malagasy middle Holocene interval seems to be characterized by drier conditions, relative to the early and late Holocene. The alternating wet/dry/wet conditions in northwestern Madagascar during each of these Holocene intervals could be linked to the long-term migration of the Inter-Tropical Convergence Zone (ITCZ). Higher southern hemisphere (SH) insolation and globally colder conditions drove the ITCZ's mean position further south, bringing more rainfall to northwestern Madagascar. This condition was favorable for stalagmite deposition. In contrast, higher northern hemisphere (NH) insolation and globally warmer conditions displaced the ITCZ further north, bringing less rainfall to northwestern Madagascar. This condition was not favorable for stalagmite deposition. The linkage between global cooling and wet conditions in regions of the SH, in response to the southward migration of the ITCZ, is further exemplified at centennial scale by the negative δ18O and δ13C values in northwestern Madagascar during the 8.2 ka cold event when the Atlantic Meridional Overturning Circulation (AMOC) weakened. Weakening of the AMOC led to an enhanced temperature gradient between the two hemispheres, i.e. cold NH and warm SH, shifting the mean position of the ITCZ further south. This brought wet conditions in the SH monsoon regions, such as northwestern Madagascar, and dry conditions in the NH monsoon regions, including the Asian Monsoon and the East Asian Summer Monsoon. This climatic relationship is useful to test for climate models that are used to predict changes in future climate.