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1. Holocene Monsoon Change and Abrupt Events on the Western Chinese Loess Plateau as Revealed by Accurately Dated Stalagmites

   https://doi.org/10.1029/2020GL090273  

   Tan, Liangcheng; Li, Yanzhen; Wang, Xiqian; Cai, Yanjun; Lin, Fangyuan; Cheng, Hai; Ma, Le; Sinha, Ashish; Edwards, R. Lawrence (November 2020, Geophysical Research Letters)

   Abstract Here we present, to date, the highest‐resolved (~5 years) and most precisely dated Holocene monsoon climate reconstruction for the western Chinese Loess Plateau based on five replicated stalagmite δ¹⁸O records from Wuya Cave, eastern Gansu, China. Our record suggests the wettest period occurred between 10,500 and 6,600 a BP in this region. After this period, the amplitude of Asian summer monsoon decadal‐scale variability progressively increased likely in response to increasing ENSO frequency since the middle Holocene. Our study reveals similar asymmetric centennial‐scale double‐plunging structures of the 8.2, 5.5, and 2.8 ka events in the western Chinese Loess Plateau, suggesting a possible role of solar activity whose impact was amplified around 8.2 ka BP by the meltwater flood. In contrast, the 4.2 ka event exhibit gradually declining monsoon rainfall with centennial‐ to decadal‐scale fluctuations. 

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2. NOAA/WDS Paleoclimatology - Tham Doun Mai Cave, Northern Laos Trace Metal and Stable Isotope Data from 7.6 to 9 ka

   https://doi.org/10.25921/seqw-yh76  

   Wood, CT; Johnson, KR; Lewis, LE; Wright, KT; Wang, JK; Borsato, A; Griffiths, ML; Mason, AJ; Henderson, GM; Setera, JB; et al (January 2023, NOAA National Centers for Environmental Information)

   The 8.2 ka event is the most significant global climate anomaly of the Holocene epoch, but a lack of records from Mainland Southeast Asia (MSEA) currently limits our understanding of the spatial and temporal extent of the climate response. A newly developed speleothem record from Tham Doun Mai Cave, Northern Laos provides the first high resolution record of this event in MSEA. Our multiproxy record (d18O, d13C, Mg/Ca, Sr/Ca, and petrographic data), anchored in time by 9 U-Th ages, reveals a significant reduction in local rainfall amount and weakening of the monsoon at the event onset at ~8.29 +/- 0.03 ka BP. This response lasts for a minimum of ~170 years, similar to event length estimates from other speleothem d18O monsoon records. Interestingly, however, our d13C and Mg/Ca data, proxies for local hydrology, show that abrupt changes to local rainfall amounts began decades earlier (~70 years) than registered in the d18O. Moreover, the d13C and Mg/Ca also show that reductions in rainfall continued for at least ~200 years longer than the weakening of the monsoon inferred from the d18O. Our interpretations suggest that drier conditions brought on by the 8.2 ka event in MSEA were felt beyond the temporal boundaries defined by d18O-inferred monsoon intensity, and an initial wet period (or precursor event) may have preceded the local drying. Most existing Asian Monsoon proxy records of the 8.2 ka event may lack the resolution and/or multiproxy information necessary to establish local and regional hydrological sensitivity to abrupt climate change. 

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3. 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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4. High‐Resolution, Multiproxy Speleothem Record of the 8.2 ka Event From Mainland Southeast Asia

   https://doi.org/10.1029/2023PA004675  

   Wood, Christopher T.; Johnson, Kathleen R.; Lewis, Lindsey. E.; Wright, Kevin; Wang, Jessica K.; Borsato, Andrea; Griffiths, Michael L.; Mason, Andrew; Henderson, Gideon M.; Setera, Jacob B.; et al (December 2023, Paleoceanography and Paleoclimatology)

   Abstract The 8.2 ka event is the most significant global climate anomaly of the Holocene epoch, but a lack of records from Mainland Southeast Asia (MSEA) currently limits our understanding of the spatial and temporal extent of the climate response. A newly developed speleothem record from Tham Doun Mai Cave, Northern Laos provides the first high‐resolution record of this event in MSEA. Our multiproxy record (δ¹⁸O, δ¹³C, Mg/Ca, Sr/Ca, and petrographic data), anchored in time by 9 U‐Th ages, reveals a significant reduction in local rainfall amount and weakening of the monsoon at the event onset at ∼8.29 ± 0.03 ka BP. This response lasts for a minimum of ∼170 years, similar to event length estimates from other speleothem δ¹⁸O monsoon records. Interestingly, however, our δ¹³C and Mg/Ca data, proxies for local hydrology, show that abrupt changes to local rainfall amounts began decades earlier (∼70 years) than registered in the δ¹⁸O. Moreover, the δ¹³C and Mg/Ca also show that reductions in rainfall continued for at least ∼200 years longer than the weakening of the monsoon inferred from the δ¹⁸O. Our interpretations suggest that drier conditions brought on by the 8.2 ka event in MSEA were felt beyond the temporal boundaries defined by δ¹⁸O‐inferred monsoon intensity, and an initial wet period (or precursor event) may have preceded the local drying. Most existing Asian Monsoon proxy records of the 8.2 ka event may lack the resolution and/or multiproxy information necessary to establish local and regional hydrological sensitivity to abrupt climate change. 

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5. Last glacial hydroclimate variability in the Yucatán Peninsula not just driven by ITCZ shifts

   https://doi.org/10.1038/s41598-023-40108-6  

   Travis-Taylor, Leah; Medina-Elizalde, Martín; Karmalkar, Ambarish V; Polanco-Martinez, Josué; Serrato_Marks, Gabriela; Burns, Stephen; Lases-Hernández, Fernanda; McGee, David (December 2023, Scientific Reports)

   Abstract We reconstructed hydroclimate variability in the Yucatán Peninsula (YP) based on stalagmite oxygen and carbon isotope records from a well-studied cave system located in the northeastern YP, a region strongly influenced by Caribbean climate dynamics. The new stalagmite isotopic records span the time interval between 43 and 26.6 ka BP, extending a previously published record from the same cave system covering the interval between 26.5 and 23.2 ka BP. Stalagmite stable isotope records show dominant decadal and multidecadal variability, and weaker variability on millennial timescales. These records suggest significant precipitation declines in the broader Caribbean region during Heinrich events 4 and 3 of ice-rafted discharge into the North Atlantic, in agreement with the antiphase pattern of precipitation variability across the equator suggested by previous studies. On millennial timescales, the stalagmite isotope records do not show the distinctive saw-tooth pattern of climate variability observed in Greenland during Dansgaard–Oeschger (DO) events, but a pattern similar to North Atlantic sea surface temperature (SST) variability. We propose that shifts in the mean position of the Intertropical Convergence Zone (ITCZ), per se, are not the dominant driver of last glacial hydroclimate variability in the YP on millennial timescales but instead that North Atlantic SSTs played a dominant role. Our results support a negative climate feedback mechanism whereby large low latitude precipitation deficits resulting from AMOC slowdown would lead to elevated salinity in the Caribbean and ultimately help reactivate AMOC and Caribbean precipitation. However, because of the unique drivers of future climate in the region, predicted twenty-first century YP precipitation reductions are unlikely to be modulated by this negative feedback mechanism. 

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