Possible expression of the 4.2 kyr event in Madagascar and the south-east African monsoon

Abstract

Abstract. The 4.2 kyr event is regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Drying across the Mediterranean and Middle East is well established and is linked to societal transitions in the Akkadian, Egyptian and Harappan civilizations. Yet the impacts of this regional drought are often extended to other regions and sometimes globally. In particular, the nature and spatial extent of the 4.2 kyr event in the tropics have not been established. Here, we present a new stalagmite stable isotope record from Anjohikely, northwest Madagascar. Growing between 5 and 2 kyr BP, stalagmite AK1 shows a hiatus between 4.32 and 3.83 kyr BP, replicating a hiatus in another stalagmite from nearby Anjohibe, and therefore indicating a significant drought around the time of the 4.2 kyr event. This result is the opposite to wet conditions at 8.2 kyr BP, suggesting fundamentally different forcing mechanisms. Elsewhere in the south-east African monsoon domain dry conditions are also recorded in sediment cores in Lake Malawi and Lake Masoko and the Taros Basin on Mauritius. However, at the peripheries of the monsoon domain, drying is not observed. At the northern (equatorial East Africa) and eastern (Rodrigues) peripheries, no notable event is record. At the southern periphery a wet event is recorded in stalagmites at Cold Air Cave and sediment cores at Lake Muzi and Mkhuze Delta. The spatial pattern is largely consistent with the modern rainfall anomaly pattern associated with weak Mozambique Channel Trough and a northerly austral summer Inter Tropical Convergence Zone position. Within age error, the observed peak climate anomalies are consistent with the 4.2 kyr event. However, outside Madagascar, regional hydrological change is consistently earlier than a 4.26 kyr BP event onset. Gradual hydrological change frequently begins at 4.6 kyr BP, raising doubt as to whether any coherent regional hydrological change is merely coincident with the 4.2 kyr event rather than part of a global climatic anomaly.