Regime Shifts in Holocene Paleohydrology as Recorded by Asian Speleothems.

Speleothem oxygen isotope records offer unique insights into Asian Monsoon evolution, with their precise chronologies used to identify abrupt climatic events. However, individual records are sometimes used to draw broad conclusions about global climate, without considering the dynamical context in which they exist. We present a robust framework for assessing the regional significance, and hence the potential global significance, of paleoclimate events, using the proposed Meghalayan age onset (associated with the "4.2 ka event") as a case study. Analyzing 14 well‐dated speleothem oxygen isotope records from the SISAL v3 database and recent literature, we investigate the regional coherency of rapid shifts in Asian paleohydrology, which is the regional center of action for the proposed event, over the Holocene. Three robust methods fail to detect spatially coherent variability consistent with a 4.2 ka event across Asia, either because none exists or because it is of insufficient magnitude. In contrast, the 8.2 ka event is expressed in most records that resolve it. The absence of a clear isotopic excursion across this data set suggests that the "4.2 ka megadrought" was not global, with important implications for archeology and geochronology. This casts doubt on the proposal that the 4.2 ka event marks the onset of a new geologic age. We do, however, observe support for a gradual isotopic enrichment between 3.9 and 3.6 ka, followed by partial recovery—consistent with the "Double Drying" hypothesis and possibly related to changes in El Niño‐Southern Oscillation variability. Plain Language Summary: Cave formations, known as speleothems, preserve valuable records of past climate. We analyzed 14 of these records to examine a proposed global drought that occurred about 4,200 years ago, centered in South Asia. This event, called the "4.2 ka event," is thought to have contributed to the fall of several ancient civilizations and currently marks the beginning of our most recent geologic age. However, our study found little evidence of a widespread drought across Asia during this time. We developed a robust framework to assess the regional/global importance of past climate events, addressing concerns about using limited data to draw broad conclusions. While we didn't find strong support for the 4.2 ka event, we observed clear signs of an earlier, globally significant climate shift around 8,200 years ago. Interestingly, we discovered a consistent pattern of gradual drying over thousands of years, with a more pronounced dry period between 3,900 and 3,600 years ago. This drying, possibly linked to changes in Pacific Ocean surface temperature patterns, affected areas influenced by both the Indian and East Asian monsoons. Our findings challenge the global significance of the 4.2 ka event and highlight the importance of querying multiple records when studying past climate changes. Key Points: A pronounced isotopic enrichment trend is observed from 3.9 to 3.6 ka in a network of Asian speleothem recordsThis trend is observed in records influenced by both the Indian Summer Monsoon and the East Asian Summer MonsoonThis network fails to show a spatially coherent isotopic signal around the 4.2 ka event interval [ABSTRACT FROM AUTHOR]