Weak monsoon event at 4.2Â ka recorded in sediment from Lake Rara, Himalayas

The strength of the Indian summer monsoon varied significantly on centennial- to millennial timescales during the Holocene. Among these variations, a weak summer monsoon event occurred approximately at 4 ka, the so called “4.2 ka event”. This event is considered to have been a relatively sustained and widespread phenomenon at regional scales across southern Asia to eastern Africa. However, little is known about the impact of such variations in the Himalayas, a key location for understanding the monsoon system. Here we present a record of changes in summer monsoon strength during the Holocene from Lake Rara, western Nepal, using redox sensitive elements (Mn/Ti and Mn/Fe) measured by X-ray fluorescence (XRF) core-scanner. In addition, organic matter properties (TOC, δ13C, and C/N) and changes in the 14C age offset between bulk organic matter and leaf matter - namely the old carbon offset - are used to augment the environmental reconstruction. Collectively, the data indicate a period of prolonged weak monsoon interval at around 4 ka. The timing of onset and recovery from the event observed at Lake Rara exhibits particular similarity with changes in the Indus River discharge. This is the first record of the “4.2 ka event” from the Himalayas based on a relatively well-constructed age model using 14C dates that are not influenced by reservoir effects. These findings provide an important step towards better understanding the spatial and temporal variability in the Holocene Indian Monsoon.