Sharp Decrease in Summer Monsoon Strength 4000–3500 cal yr B.P. in the Central Higher Himalaya of India Based on Pollen Evidence from Alpine Peat

Abstract

Age-constrained pollen data and magnetic susceptibility of an alpine peat profile from the Garhwal Higher Himalaya display a continuous record of climate and monsoon trends for the past 7800 yr. About 7800 cal yr B.P., dominance of evergreen oak (Quercus semecarpifolia), alder (Alnus), and grasses in the pollen record reflect a cold, wet climate with moderate monsoon precipitation. From 7800 to 5000 cal yr B.P., vegetation was progressively dominated by conifers, indicating ameliorated climate with a stronger monsoon. A warm, humid climate, with highest monsoon intensity, from 6000–4500 cal yr B.P. represents the mid-Holocene climatic optimum. Between 4000 and 3500 cal yr B.P., the abundance of conifers sharply decreased, with the greatest increase in evergreen oak. This trend suggests progressive cooling, with a decrease in the monsoon to its minimum about 3500 cal yr B.P. Two relatively minor cold/dry events at ca. 3000 and 2000 cal yr B.P. marked step-wise strengthening of the monsoon until ca. 1000 cal yr B.P. After a cold/dry episode that culminated ca. 800 cal yr B.P., the monsoon again strengthened and continued until today. A sharp decrease in temperature and rainfall at 4000–3500 cal yr B.P. represents the weakest monsoon event of the Holocene record. This cold/dry event correlates with proxy data from other localities of the Indian subcontinent, Arabian Sea, and western Tibet.