Exact dating of the Meghalayan lower boundary based on high-latitude tree-ring isotope chronology

Abstract

Subdivision of the Holocene Series/Epoch into the Greenlandian, Northgrippian and Meghalayan Stages/Ages has recently been ratified based on stable isotope records from ice-core and speleothem archives. The base of the most recent chronostratigraphic unit, corresponding to the Northgrippian−Meghalayan boundary, coincides with the ‘4.2 ka event’ recognised as a low-latitude drought anomaly. The Global Stratotype Section and Point (GSSP) for the boundary, the oxygen isotope record from the Mawmluh Cave speleothem (India), demonstrates this event with markedly weakened Asian summer monsoon. Here we contribute to the assessments of the geological time scale and the global characteristics of this event by detailing an isotopic excursion in tree-ring carbon isotopes from high-latitude/subarctic Europe. The δ13C chronology demonstrates extremely overcast (wet) conditions, especially between 2190 and 2100 BCE, with anomalous conditions sustaining until 1990 BCE. In addition to demonstrating its exact dating and duration, the δ13C data also illustrate the two-stage nature of the event and highlight the greater magnitude of the earlier stage. This reinforces the characterisation of this ‘Meghalayan anomaly’ in the context of other proxy-sites around the world. The North Atlantic forcing, previously associated with weakened Asian summer monsoon, accords with the suggested roles of North Atlantic Oscillation and/or southwards shift of Inter-Tropical Convergence Zone in producing the hydroclimatic anomaly. In fact, it would be clarifying to separate the hydroclimatic ‘Meghalayan anomaly’ from the broadly cited and potentially longer lasting ‘4.2 ka event’, to analyse their respective forcing mechanisms. We conclude by dating the Meghalayan lower boundary to 2190 BCE.