This discussion paper, by a Working Group of INTIMATE (Integration of ice‐core, marine and terrestrial records) and the Subcommision on Quaternary Stratigraphy (SQS) of the International Commission on Stratigraphy (ICS), considers the prospects for a formal subdivision of the Holocene Series/Epoch. Although previous attempts to subdivide the Holocene have proved inconclusive, recent developments in Quaternary stratigraphy, notably the definition of the Pleistocene–Holocene boundary and the emergence of formal subdivisions of the Pleistocene Series/Epoch, mean that it may be timely to revisit this matter. The Quaternary literature reveals a widespread but variable informal usage of a tripartite division of the Holocene (‘early’, ‘middle’ or ‘mid’, and ‘late’), and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology. We propose an Early–Middle Holocene Boundary at 8200 a BP and a Middle–Late Holocene Boundary at 4200 a BP, each of which is linked to a Global Stratotype Section and Point (GSSP). Should the proposal find a broad measure of support from the Quaternary community, a submission will be made to the International Union of Geological Sciences (IUGS), via the SQS and the ICS, for formal ratification of this subdivision of the Holocene Series/Epoch. Copyright © 2012 John Wiley & Sons, Ltd.

The emergence of high-resolution proxy records from the Asian monsoon region suggests 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 (δ 18 O p) are documented. The most dramatic of these events occurred 4000 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 North America. We hypothesize that the excursion could represent a shift toward an earlier Indian Summer Monsoon withdrawal 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 Asia during this period when a number of major societal changes occurred. While evidence of a causal relationship between climate and the reorganization 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 deurbanization of the Indus Valley at 3.9 kyr B.P. occurred after multiple decades of a shift in the monsoon's character but before the monsoon returned to its previous mid-Holocene state.

The “internally” generated intraseasonal variability of the Indian Summer Monsoon is characterized by intermittent periods of enhanced (“active”) and deficient (“break”) precipitation, which produce a quasi east‐west precipitation dipole over the Indian subcontinent. Here we present multicentennial‐length and near annually‐resolved reconstructions of monsoon precipitation, inferred from absolute‐dated and instrumentally calibrated speleothem oxygen isotope records from regions (central and northeast India) that have diametric responses to active‐break monsoon circulation patterns. On centennial timescales (AD 1400–2008), precipitation variability from these two regions exhibit opposing behavior, oscillating between periods with a persistently “active‐dominated” (AD ∼1700 to 2007) and a “break‐dominated” (AD 1400 to ∼1700) regime. The switch between these regimes occurs abruptly (within decades) at a time (AD ∼ 1650–1700) when a proxy record of upwelling intensity from the Arabian Sea suggest an abrupt increase in the monsoon winds. On the basis of these observations, we hypothesize that the frequency distribution of active‐break periods varies on centennial timescales, implying a leading role of internal dynamics in governing the ISM response to slowly‐evolving changes in the external boundary conditions.

Observations show that summer rainfall over large parts of South Asia has declined over the past five to six decades. It remains unclear, however, whether this trend is due to natural variability or increased anthropogenic aerosol loading over South Asia. Here we use stable oxygen isotopes in speleothems from northern India to reconstruct variations in Indian monsoon rainfall over the last two millennia. We find that within the long-term context of our record, the current drying trend is not outside the envelope of monsoon's oscillatory variability, albeit at the lower edge of this variance. Furthermore, the magnitude of multi-decadal oscillatory variability in monsoon rainfall inferred from our proxy record is comparable to model estimates of anthropogenic-forced trends of mean monsoon rainfall in the 21st century under various emission scenarios. Our results suggest that anthropogenicforced changes in monsoon rainfall will remain difficult to detect against a backdrop of large natural variability.