Leaf wax δD inferring variable medieval hydroclimate and early initiation of Little Ice Age (LIA) dryness in southern Mozambique

A sediment sequence from a coastal, hydrologically isolated lake in southern Mozambique was analysed for leaf wax δD, n-alkane abundance (ACL) and bulk organic geochemistry (δ13C, TOC, %N), providing a record of past rainfall variability and savanna dynamics over the last 1500 years. The δDwax rainfall reconstruction reveals a stable hydroclimate between 500-700 CE, while ACL and δ13C together with previous pollen data suggest savanna vegetation was characterized by a relatively dense woody cover. Highly variable hydroclimate conditions are inferred by δDwax between 800-1350 CE, with repeated centennial scale intervals of extreme dry and wet conditions overlapping the Medieval Climate Anomaly (MCA; 950-1250 CE). Savanna tree cover stayed relatively intact over this phase. After ca 1250 CE, a progressive change towards drier conditions was initiated, leading up to maximum aridity during the AD 1700s, a period associated with the Little Ice age (LIA; 1500-1850 CE). Tree cover was now replaced by a more grass-dominated savanna. The clear antiphase rainfall patterns between Nhaucati and equatorial East African proxy records gives support to the notion that Indian Ocean sea surface temperature (SST) gradients act as modulator of southern African climate on a multi-decadal time scale, possibly forced by long-term El Niño Southern Oscillation (ENSO) variability. We suggest that strong ENSO variability and greater occurrence of La Niña events triggered the generally wet and unstable MCA in southern Africa. From around 1250 CE, a shift towards a predominance of El Niño induced drier conditions in south-east Africa during the LIA. Our study of vegetation and hydroclimate proxies in parallel suggests that savanna tree and shrub cover was relatively resilient to the abrupt shifts in hydroclimate over the MCA, but more sensitive to the long-term progressive drying over the LIA.