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The so-called “4.2 ka event” is a dramatic climate oscillation that impacted many areas of the mid-to-low latitudes spanning roughly 4.2-3.9 ka (ka = thousands of years ago). Records of this event have been identified on every continent except Antarctica, with clear evidence of precipitation being affected on a large scale. Subtropical and tropical regions of Africa and Asia experienced drought, while mid-latitude areas of Africa and Europe saw anomalously wet conditions. The 4.2 ka event is argued to have had a substantial cultural impact, including the collapse of numerous dynasties and cultures such as in the Indus valley and south-central China, as well as parts of Mesopotamia, northeastern Africa, and across parts of southeast Asia. However, despite its wide geographic extent and societal importance, a great deal remains unknown about the 4.2 ka event, its global effects, and its origins. The apparent lack of a climate anomaly in the polar regions at 4.2 ka suggests it may have originated in the tropics, possibly through the El Niño-Southern Oscillation (ENSO). I analyzed a stalagmite (SB-18) from Siddha Cave, located in the Pokhara Valley of central Nepal (28.0N, 84.0E elev.~600 meters), a region that receives 80% of its annual 1500 mm of rainfall from the Indian Summer Monsoon (ISM). In contrast to many tropical stalagmite records, which use oxygen isotopes to track past monsoon rainfall, I focused on carbon isotopes because at Siddha Cave, oxygen isotopes in rainfall do not have a strong correlation to rainfall amount (the so-called “amount effect”). Carbon isotopes respond to hydroclimate variability through prior aragonite precipitation (PAP), which reflects out-gassing of carbon dioxide and precipitation of aragonite in voids in the bedrock above the cave. This process preferentially removes 12C from the infiltrating water that subsequently migrates downward into the cave. During periods with less rainfall, open spaces in the bedrock are more likely to be dewatered, thereby allowing for more prior aragonite precipitation. In order to ensure that carbon isotopes accurately capture ISM rainfall variability, I also examined uranium abundances in the same stalagmite. Changes in the concentration of uranium are also driven by PAP: uranium is incorporated into aragonite preferentially over dripwater and thus PAP reduces the amount of uranium in dripwater, thereby decreasing uranium in the underlying stalagmite. Carbon isotopes and U abundances in SB-18 suggest that central Nepal experienced anomalously high rainfall during the 4.2 ka event, in contrast with the majority of lower latitude sites around the globe, including a cave record from northeastern India, that record a reduction in rainfall at this time. This rainfall dipole provides an important climatic fingerprint that allows us to investigate the origins of the 4.2 ka event through analysis of modern climate data, including rainfall anomalies associated with ENSO.
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A precipitation dipole between central Nepal and eastern India during the 4.2 ka event
Over the late Holocene, a variety of hydroclimate-sensitive proxies have identified substantial, multidecadal changes in Indian summer monsoon (ISM) precipitation, the most prominent of which is the “4.2 ka event”. This interval, dated to ~4.2-3.9 ka, is associated with severe droughts across South Asia that are linked to societal change. Given the absence of the 4.2 ka event in polar records, the 4.2 ka event is generally associated with low latitude forcings, but no clear consensus on its origins has been reached. We investigated the ISM response to the 4.2 ka event through analysis of aragonite stalagmites from Siddha cave, formed in the lower Paleozoic Dhading dolomite in the Pokhara Valley of central Nepal (28.0˚N, 84.1˚E; ~850 m.a.s.l.). The climate of this region is dominated by small monthly variations in air temperature (21±5˚C) but strong precipitation seasonality associated with the ISM: ~80% of the annual 3900 mm of rainfall occurs between June and September. High uranium and low detrital thorium abundances in these stalagmites yield precise U/Th ages that all fall within stratigraphic order. These dates reveal continuous growth from 4.30-2.26 ka, interrupted only by a hiatus from 3.27-3.10 ka. Overlap with coeval aragonite stalagmites reveals generally consistent trends in carbon and oxygen isotope ratios, suggesting that these stalagmites reflect environmental variability and not secondary (e.g., kinetic) effects. Many stalagmite-based paleomonsoon reconstructions rely on oxygen isotope ratios, which track amount effects in regional rainfall. However, our on-going rainwater collection and analysis program, as well as a previous study conducted in Kathmandu, 120 km the east of Siddha cave, reveals that amount effects in precipitation are weak in this region, particularly during the monsoon season, and thus we rely instead on carbon isotope ratios, which have been demonstrated to track site-specific effective precipitation. Siddha cave stalagmite carbon isotopes, in contrast to other South Asian proxy records, indicate that ISM rainfall increased at Siddha cave from 4.13-3.91 ka. As a further test of this result, we analyzed uranium abundances in the section spanning 4.3-3.4 ka. Uranium serves as an indicator of prior aragonite precipitation and thus of hydroclimate, and like carbon isotopes, suggests increased ISM rainfall coincident with the 4.2 ka event. This precipitation anomaly is nearly identical in timing and structure but anti-phased with stalagmites from Mawmluh cave, northeastern India. We investigated the climatic origins of this precipitation dipole using observational data from the Global Precipitation Climatology Centre (GPCC) and Hadley Center Sea Ice and Sea Surface Temperature (HadISST) products. Preliminary spatial composites suggest that large precipitation differences between Mawmluh and Siddha caves are associated with SST anomalies in the equatorial Pacific. Additionally, superposed Epoch Analysis shows relatively rapid eastern Indian Ocean cooling during the summer monsoon season coeval with large precipitation differences between these sites. Our findings lend support to a tropical Indo-Pacific origin of the 4.2 ka event.
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- Award ID(s):
- 2102864
- PAR ID:
- 10504859
- Publisher / Repository:
- European Geosciences Union
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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