NOAA/WDS Paleoclimatology - Mawmluh Cave, India Holocene Stalagmite d18O Data

The emergence of high-resolution proxy records from the Asian Monsoon region suggest that the monsoon system is bistable and can abruptly transition between a suppressed and active state. This observation is critical in considering how the monsoon system may have influenced the development of societies across South and East Asia during the Holocene. Using a new high-resolution (~5 years/sample) speleothem stable isotope record from Northeast India that spans the early and mid-Holocene, a number of abrupt changes in the oxygen isotopic composition of precipitation (d18O) are documented. The most dramatic of these events occurred ~4,000 years ago when, over the course of approximately a decade, isotopic values abruptly rose above any seen during the early to mid Holocene and remained at this anomalous state for almost two centuries. This event occurs nearly synchronously with climatic changes documented in a number of proxy records across North Africa, the Middle East, the Tibetan Plateau, Southern Europe and perhaps as far as North America. The changes in d18O were likely driven by a dynamic response of the Indian Summer Monsoon (ISM) to changes in surface heating on the Tibetan Plateau and/or to large-scale ocean dynamics in the Indian Ocean Basin. At this point, a quantitative characterization of the change in the ISM is hindered by unconstrained aspects of the regional response of d18O to climate, but we hypothesize the excursion could represent a shift towards an earlier monsoon withdrawal, a more northerly vapor source region or a general decline in the total amount of monsoon precipitation. The new record provides a very significant advance with respect to age control and sample resolution of terrestrial climate change over South Asian during this period when a number of major societal changes occurred. While evidence of a causal relationship between climate and the re-organization of the Indus Valley and Old Kingdom Nile civilizations is beyond the scope of this study, the tight age constraints of the record show with a high degree of certainty that much of the documented de-urbanization of the Indus Valley at 3.9k yr BP occurred after multiple decades of a shift in the monsoon's character but before the monsoon returned to its previous mid-Holocene state.