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Oxygen isotopes (δ18O) are the most commonly utilized speleothem proxy and have provided many foundational records of paleoclimate. Thus, understanding processes affecting speleothem δ18O is crucial. Yet, prior calcite precipitation (PCP), a process driven by local hydrology, is a widely ignored control of speleothem δ18O. Here we investigate the effects of PCP on a stalagmite δ18O record from central Vietnam, spanning 45 – 4 ka. We employ a geochemical model that utilizes speleothem Mg/Ca and cave monitoring data to correct the δ18O record for PCP effects. The resulting record exhibits improved agreement with regional speleothem δ18O records and climate model simulations, suggesting that the corrected record more accurately reflects precipitation δ18O (δ18Op). Without considering PCP, our interpretations of the δ18O record would have been misleading. To avoid misinterpretations of speleothem δ18O, our results emphasize the necessity of considering PCP as a significant driver of speleothem δ18O. 

This archived Paleoclimatology Study is available from the NOAA National Centers for Environmental Information (NCEI), under the World Data Service (WDS) for Paleoclimatology. The associated NCEI study type is Cave. The data include parameters of speleothems with a geographic location of Mexico. The time period coverage is from 62500 to 5858 in calendar years before present (BP). See metadata information for parameter and study location details. Please cite this study when using the data. 

Most paleoclimate studies of Mainland Southeast Asia hydroclimate focus on the summer monsoon, with few studies investigating rainfall in other seasons. Here we present a multiproxy stalagmite record (45,000 – 4,000 years) from central Vietnam, a region that receives most of its annual rainfall in autumn (September-November). We find evidence of a prolonged dry period spanning the last glacial maximum that is punctuated by an abrupt shift to wetter conditions during the deglaciation at ~14ka. Paired with climate model simulations, we show that sea level change drives autumn monsoon rainfall variability on glacial-orbital timescales. Consistent with the dry signal in the stalagmite record, climate model simulations reveal that lower glacial sea level exposes land in the Gulf of Tonkin and along the South China Shelf, reducing convection and moisture delivery to central Vietnam. When sea level rises and these landmasses flood at ~14ka, moisture delivery to central Vietnam increases causing an abrupt shift from dry to wet conditions. On millennial timescales, we find signatures of well-known Heinrich Stadials (dry conditions) and Dansgaard-Oeschger Events (wet conditions). Model simulations show that during the dry Heinrich Stadials, changes in sea surface temperature related to meltwater forcing cause the formation of an anomalous anticyclone in the Western Pacific, which advects dry air across central Vietnam decreasing autumn rainfall. Notably, sea level modulates the magnitude of millennial-scale dry and wet phases by muting dry events and enhancing wet events during periods of low sea level, highlighting the importance of this mechanism to autumn monsoon variability. 

This archived Paleoclimatology Study is available from the NOAA National Centers for Environmental Information (NCEI), under the World Data Service (WDS) for Paleoclimatology. The associated NCEI study type is Lake. The data include parameters of paleolimnology with a geographic location of California, United States Of America. The time period coverage is from 200000 to 3 in calendar years before present (BP). See metadata information for parameter and study location details. Please cite this study when using the data. 

The relative importance of climate and humans in the disappearance of the Malagasy megafauna remains under debate. Data from southwestern Madagascar imply aridification contributed substantially to the late Holocene decline of the megafauna (the Aridification Hypothesis). Evidence for aridification includes carbon isotopes from tree rings, lacustrine charcoal concentrations and pollen assemblages, and changes in fossil vertebrate assemblages indicative of a local loss of pluvial conditions. In contrast, speleothem records from northwestern Madagascar suggest that megafaunal decline and habitat change resulted primarily from human activity including agropastoralism (the Subsistence Shift Hypothesis). Could there have been contrasting mechanisms of decline in different parts of Madagascar? Or are we lacking the precisely dated, high resolution records needed to fully understand the complex processes behind megafaunal decline? Reconciling these contrasting hypotheses requires additional climate records from southwestern Madagascar. We recovered a stalagmite (AF2) from Asafora Cave in the spiny thicket ecoregion, ~10 km from the southwest coast and just southeast of the Velondriake Marine Reserve. U-series and 14C dating of samples taken from the core of this stalagmite provide a highly precise chronology of the changes in hydroclimate and vegetation in this region over the past 3000 years. Speleothem stable oxygen and carbon isotope analyses provide insight into past rainfall variability and vegetation changes respectively. We compare these records with those for a stalagmite (AB2) from Anjohibe Cave in northwestern Madagascar. Lastly, odds ratio analyses of radiocarbon dates for extinct and extant subfossils allow us to describe and compare the temporal trajectories of megafaunal decline in the southwest and the northwest. Combined, these analyses allow us to test the Aridification Hypothesis for megafaunal extinction. 

This archived Paleoclimatology Study is available from the NOAA National Centers for Environmental Information (NCEI), under the World Data Service (WDS) for Paleoclimatology. The associated NCEI study type is Lake. The data include parameters of paleolimnology with a geographic location of California, United States Of America. The time period coverage is from 200000 to -65 in calendar years before present (BP). See metadata information for parameter and study location details. Please cite this study when using the data. 

The relative importance of climate and humans in the disappearance of the Malagasy megafauna remains under debate. Data from southwestern Madagascar imply aridification contributed substantially to the late Holocene decline of the megafauna (the Aridification Hypothesis). Evidence for aridification includes carbon isotopes from tree rings, lacustrine charcoal concentrations and pollen assemblages, and changes in fossil vertebrate assemblages indicative of a local loss of pluvial conditions. In contrast, speleothem records from northwestern Madagascar suggest that megafaunal decline and habitat change resulted primarily from human activity including agropastoralism (the Subsistence Shift Hypothesis). Could there have been contrasting mechanisms of decline in different parts of Madagascar? Or are we lacking the precisely dated, high resolution records needed to fully understand the complex processes behind megafaunal decline? Reconciling these contrasting hypotheses requires additional climate records from southwestern Madagascar. We recovered a stalagmite (AF2) from Asafora Cave in the spiny thicket ecoregion, ~10 km from the southwest coast and just southeast of the Velondriake Marine Reserve. U-series and 14C dating of samples taken from the core of this stalagmite provide a highly precise chronology of the changes in hydroclimate and vegetation in this region over the past 3000 years. Speleothem stable oxygen and carbon isotope analyses provide insight into past rainfall variability and vegetation changes respectively. We compare these records with those for a stalagmite (AB2) from Anjohibe Cave in northwestern Madagascar. Lastly, odds ratio analyses of radiocarbon dates for extinct and extant subfossils allow us to describe and compare the temporal trajectories of megafaunal decline in the southwest and the northwest. Combined, these analyses allow us to test the Aridification Hypothesis for megafaunal extinction. The trajectories of megafaunal decline differed in northwestern and southwestern Madagascar. In the southwest, unlike the northwest, there is no evidence of decoupling of speleothem stable carbon and oxygen isotopes. Instead, habitat changes in the southwest were largely related to variation in hydroclimate (including a prolonged drought). The megafaunal collapse here occurred in tandem with the drought, and agropastoralism likely contributed to that demise only after the megafauna had already suffered drought-related population reduction. Our results offer some support for the Aridification Hypothesis, but with three caveats: first, that there was no island-wide aridification; second, that aridification likely impacted megafaunal decline only in the driest parts of Madagascar; and third, that aridification was not the sole factor promotingmegafaunal decline even in the dry southwest. A number of megafaunal species survived the prolonged drought of the first millennium, and then likely succumbed to the activities of agropastoralists. 

Saharan dust emissions over the past 240,000 years vary primarily with summer insolation rather than glacial-interglacial changes.