Climate variability in the SW Indian Ocean from an 8000-yr long multi-proxy record in the Mauritian lowlands shows a middle to late Holocene shift from negative IOD-state to ENSO-state

Abstract

A multi-proxy reconstruction of a sediment core from the Tatos basin in the Mauritian lowlands reveals a dynamic environmental history during the last 8000 years. Under influence of sea level rise, the basin progressed from a wetland to a shallow lake between 8000 and 2500cal yr BP and it slowly changed back into a wetland after sea level reached its highest position at around 2500cal yr BP. The groundwater level in the basin was strongly affected by sea level rise and precipitation-forced runoff through the porous volcanic bedrock.Millennial-scale precipitation changes in the Mauritian lowlands were derived from the pollen records of semi-dry forest and palm woodland. Salinity and environmental reconstructions based on diatoms, ostracods, stable isotopes and sediment composition showed numerous decadal and centennial droughts and wet events. Mauritius experienced wet conditions between ~8000 and ~6800cal yr BP, followed by decreasing humidity from 6800 to 6000cal yr BP. Dry conditions persisted until ~1200cal yr BP, after which wetter conditions have prevailed as recorded from Mauritian lowland and upland records. Climate dynamics reflects northern hemisphere monsoon activity and suggest that Mauritian rainfall and the Indian and Asian summer monsoons are linked, as both receive moisture from the southern equatorial Indian Ocean.The anti-phased relationship of climate dynamics between the Mauritian lowlands and western tropical Australia during the middle Holocene is interpreted as a prolonged configuration of a negative mode of the Indian Ocean Dipole (IOD). A negative IOD-like state is supported by decreased Asian summer monsoon rainfall, higher Austral-Indonesian summer monsoon rainfall and lower temperatures in the Kilimanjaro record. Conversely, repeated decadal-scale wet events in the Mauritian lowlands occurring every ~350 years reflect short positive IOD-like events.The onset of ENSO climate variability followed an anomalously strong negative IOD-like event and shifted teleconnections from the tropical Indian Ocean to the Pacific Ocean. A shift in ENSO activity around ~2600cal yr BP signifies the decoupling of ENSO from the Atlantic ITCZ. Subsequently, the influence of ENSO on climate in the western Indian Ocean is indicated by increased storm frequency and drought events after 2660cal yr BP in Mauritius and reduced monsoon activity in the western and eastern Indian Ocean.