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1. A Nepal stalagmite record reveals east-west antiphasing of Indian Summer Monsoon rainfall during the 4.2 ka event

   Wimmer, Ethan (April 2024, Cornell College)

   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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2. A precipitation dipole between central Nepal and eastern India during the 4.2 ka event

   https://doi.org/10.5194/egusphere-egu24-4596  

   Denniston, R F; Tiger, B H; Wimmer, E; Ummenhofer, C C; Asmerom, Y; Wanamaker, A D; Polyak, V J; Thatcher, D L; Gurung, A; Magar, S T (May 2024, European Geosciences Union)

   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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3. Hydroclimatic variations in southeastern China during the 4.2 ka event reflected by stalagmite records

   https://doi.org/10.5194/cp-14-1805-2018  

   Zhang, Haiwei; Cheng, Hai; Cai, Yanjun; Spötl, Christoph; Kathayat, Gayatri; Sinha, Ashish; Edwards, R. Lawrence; Tan, Liangcheng (January 2018, Climate of the Past)

   Abstract. Although the collapses of several Neolithic cultures in China areconsidered to have been associated with abrupt climate change during the4.2kaBP event (4.2–3.9kaBP), the timing and nature of this event andthe spatial distribution of precipitation between northern and southern Chinaare still controversial. The hydroclimate of this event insoutheastern China is still poorly known, except for a few published recordsfrom the lower reaches of the Yangtze River. In this study, a high-resolutionrecord of monsoon precipitation between 5.3 and 3.57kaBP based on astalagmite from Shennong Cave, Jiangxi Province, southeast China, ispresented. Coherent variations in δ¹⁸O and δ¹³Creveal that the climate in this part of China was dominantly wet between 5.3and 4.5kaBP and mostly dry between 4.5 and 3.57kaBP, interrupted by awet interval (4.2–3.9kaBP). A comparison with other records frommonsoonal China suggests that summer monsoon precipitation decreased innorthern China but increased in southern China during the 4.2kaBP event.We propose that the weakened East Asian summer monsoon controlled by thereduced Atlantic Meridional Overturning Circulation resulted in thiscontrasting distribution of monsoon precipitation between northern andsouthern China. During the 4.2kaBP event the rain belt remained longer atits southern position, giving rise to a pronounced humidity gradient betweennorthern and southern China. 

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4. Evaluating the timing and structure of the 4.2 ka event in the Indian summer monsoon domain from an annually resolved speleothem record from Northeast India

   https://doi.org/10.5194/cp-14-1869-2018  

   Kathayat, Gayatri; Cheng, Hai; Sinha, Ashish; Berkelhammer, Max; Zhang, Haiwei; Duan, Pengzhen; Li, Hanying; Li, Xianglei; Ning, Youfeng; Edwards, R. Lawrence (January 2018, Climate of the Past)

   Abstract. A large array of proxy recordssuggests that the “4.2ka event” marks an approximately300-year long period (∼3.9 to 4.2ka) ofmajor climate change across the globe. However, the climatic manifestation ofthis event, including its onset, duration, and termination, remains lessclear in the Indian summer monsoon (ISM) domain. Here, we present new oxygenisotope (δ¹⁸O) data from a pair of speleothems (ML.1 and ML.2)from Mawmluh Cave, Meghalaya, India, that provide a high-resolution record ofISM variability during a period (∼3.78 and 4.44ka) that fullyencompasses the 4.2ka event. The sub-annually to annually resolved ML.1δ¹⁸O record is constrained by 18 ²³⁰Th dates with anaverage dating error of ±13 years (2σ) and a resolution of ∼40 years, which allows us to characterize the ISM variability withunprecedented detail. The inferred pattern of ISM variability during theperiod contemporaneous with the 4.2ka event shares broad similarities andkey differences with the previous reconstructions of ISM from the MawmluhCave and other proxy records from the region. Our data suggest that the ISMintensity, in the context of the length of our record, abruptly decreased at∼4.0ka ( $\sim \pm 13$ years), marking the onset of a multi-centennialperiod of relatively reduced ISM, which was punctuated by at least twomulti-decadal droughts between ∼3.9 and 4.0ka. The latter stands outin contrast with some previous proxy reconstructions of the ISM, in which the4.2ka event has been depicted as a singular multi-centennial drought. 

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5. Expression of the “4.2 ka event” in the southern Rocky Mountains, USA

   https://doi.org/10.5194/cp-18-1109-2022  

   Liefert, David T.; Shuman, Bryan N. (January 2022, Climate of the Past)

   Abstract. The use of the climatic anomaly known as the “4.2 ka event” as thestratigraphic division between the middle and late Holocene has prompteddebate over its impact, geographic pattern, and significance. The anomalyhas primarily been described as abrupt drying in the Northern Hemisphere atca. 4 ka (ka, thousands of years before present), but evidence of thehydroclimate change is inconsistent among sites both globally and withinNorth America. Climate records from the southern Rocky Mountains demonstratethe challenge with diagnosing the extent and severity of the anomaly.Dune-field chronologies and a pollen record in southeastern Wyoming revealseveral centuries of low moisture at around 4.2 ka, and prominent low standsin lakes in Colorado suggest the drought was unique amid Holocenevariability, but detailed carbonate oxygen isotope (δ18Ocarb) records from Colorado do not record drought at the sametime. We find new evidence from δ18Ocarb in a smallmountain lake in southeastern Wyoming of an abrupt reduction in effectivemoisture or snowpack from approximately 4.2–4 ka, which coincides in timewith the other evidence of regional drying from the southern Rocky Mountainsand the western Great Plains. We find that the δ18Ocarb inour record may reflect cool-season inputs into the lake, which do not appearto track the strong enrichment of heavy oxygen by evaporation during summermonths today. The modern relationship differs from some widely appliedconceptual models of lake–isotope systems and may indicate reduced winterprecipitation rather than enhanced evaporation at ca. 4.2 ka.Inconsistencies among the North American records, particularly in δ18Ocarb trends, thus show that site-specific factors can preventidentification of the patterns of multi-century drought. However, theprominence of the drought at ca. 4 ka among a growing number of sites in theNorth American interior suggests it was a regionally substantial climateevent amid other Holocene variability. 

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