More Like this

1. Determining the source of speleothem δ18O variability from in situ measurements of seasonal and inter-annual isotope trends in precipitation, cave drip water and modern calcite from sites in the Central Peruvian Andes

   Olson, E.J. (December 2022, American Geophysical Union Conference. Chicago, IL)

   In the past decade, Huagapo and Pacupahuain Caves in the Central Peruvian Andes have become sources of speleothem oxygen isotope (δ18O) paleoclimate records. These studies identify the South American Summer Monsoon (SASM) as the main climate system controlling δ18O variability. While this interpretation is verified through inter-proxy record comparisons on millennial scales, interpretation of the high-resolution variability within these records is limited by a lack of modern proxy calibration studies at these sites. Here we present results from a modern cave monitoring study undertaken to address the controls on the δ18O values of precipitation at these sites and how surface and in-cave processes affect the δ18O value of speleothem calcite. Speleothem calcite δ18O values reflect an integrated signal of atmospheric processes (e.g., rainout, Raleigh distillation, upstream moisture recycling, changes in moisture source), evaporation and mixing during infiltration in the soil and epikarst, and in-cave processes such as degassing and evaporation. In consideration of these factors, we compare isotope trends in precipitation, cave drip water and modern farmed calcite from the two cave sites. We find that precipitationδ18O values during peak monsoon activity (January -February) shows considerable inter-annual variation with averages of -16.7‰ for 2020, -18.5‰ for 2021 and -13.8‰ in 2022. We investigate the source of this variability in regional atmospheric circulation patterns using weather station data and back trajectories. The mean annual precipitation (MAP) from outside Huagapo Cave is δ18O = -15.5+/- 6‰, while seasonal samples of drip water δ18O = -14.5+/- 1‰, are offset from MAP possibly due to evaporation during infiltration. Cave drip waterδ18O has low variability over inter-annual and seasonal timescales indicating homogenization in the epikarst. Using geochemical and sensor data (e.g. cave relative humidity, temperature, and drip rate) as inputs for a karst based forward model, we simulate modern speleothem δ18O to quantitatively assess the combined effects of hydroclimate processes integration to the isotope record. 

   more » « less
2. Greenland (inter)stadial events in a stalagmite oxygen and carbon isotope record from Nepal

   Johnson, R; Denniston, R; Ummenhofer, C; Thatcher, D; Wanamaker, A; Asmerom, Y; Polyak, V; Gurung, A; Magar, S (April 2025, Geological Society of America)

   Greenland stadials and interstadials (GS/GI) were millennial climate oscillations during the last glacial period that were originally identified in Greenland ice cores but that have been correlated with environmental change around much of the globe, including in monsoon regimes, with enhanced monsoon rainfall coincident with North Atlantic warming. Hydroclimate variability associated with GS/GI have been investigated in detail using terrestrial (primarily oxygen isotopes in stalagmites) and marine records, particularly for the Southeast Asian monsoon. However, a considerably smaller number of terrestrial records preserve these events in the Indian summer monsoon (ISM), the primary water source for ~2 billion people across South Asia. Here we present the first glacial-age speleothem stable isotope time series from Nepal, located in the ISM regime. UK-1 is a 187 mm tall aragonite stalagmite from the Pokhara Valley of central Nepal, ~150 km west of Kathmandu. The chronology of UK-1, which was established by 8 U/Th dates, all of which fall in stratigraphic order (within the errors), reveals continuous growth from 34,350-31,500 yr BP (Marine Isotope Stage 3); age uncertainties average ±84 yr. Stable isotope samples were measured every 1 mm, corresponding to a temporal resolution of 18 yr. Oxygen isotope ratios range from -5.6‰ to -7.6‰, and share the same timing and structure as Greenland (inter)stadials GS/GI 6 and 5.2 in the NGRIP record. We interpret this as reflecting an amount effect response to a strengthened ISM driven by more (less) poleward migration of the intertropical convergence zone during periods of northern hemisphere warming (cooling). This clear millennial signal in UK-1 is a somewhat unexpected result given that amount effects in oxygen isotopes in precipitation are weak (R^2=0.1) in this area today. UK-1 carbon isotope ratios range from -3‰ to -6‰ (excluding a small number of negative spikes) and exhibit variability coarsely similar to the NGRIP record, with lower (higher) values generally corresponding to GI (GS), possibly due to prior calcite precipitation in voids above the cave concomitant with changes in precipitation. Some periods of antiphasing between carbon and oxygen are also apparent and may reflect flushing of soil carbon dioxide during particularly wet phases. 

   more » « less
3. Stable isotope composition of surface waters across the Pamir, Central Asia: Implications of precipitation seasonality

   https://doi.org/10.1016/j.jhydrol.2025.132815  

   Chen, Shenqiang; Rugenstein, Jeremy KC; Mulch, Andreas (June 2025, Journal of Hydrology)

   The Pamir range, located in Central Asia, mainly receives moisture from the mid-latitude westerlies, but its western side (i.e., Tajikistan Pamir) receives much of its precipitation in the winter and spring and its eastern side (i.e., Chinese Pamir) in the summer. Thus, the Pamir provides a natural laboratory to study the distribution of surface water stable isotopes across a large mountain range that ultimately receives moisture from one single source but has different precipitation seasonality regimes between its two sides. In this study, we present stable oxygen (δ18O) and hydrogen (δ2H) isotope data for 113 surface water samples from the Chinese Pamir. Our new data, along with previously published stable isotope data, show that the slope of the Chinese Pamir local meteoric water line is higher than that of the Global Meteoric Water Line (GMWL), and almost all of the data plot above the GMWL, implying that the Chinese Pamir surface waters have not experienced significant isotopic modification by evaporation. The Chinese Pamir surface waters have substantially higher δ18O and d-excess values and a steeper apparent δ18O lapse rate than surface water samples collected from the Tajikistan Pamir. We suggest that this contrast results from the shift in precipitation seasonality across the Pamir, with dominantly winter and springtime precipitation on the Tajikistan side and summertime precipitation on the Chinese side of the Pamir. This predominant summertime precipitation regime results in surface waters with high δ18O values in the Chinese Pamir. Further, this summertime moisture is dominantly convectively recycled moisture, resulting in high d-excess values in surface waters. The percentage of summertime moisture, which has high δ18O values, decreases west and with elevation in the Chinese Pamir, resulting in a steep apparent δ18O lapse rate of − 3.2 ‰/km. The importance of precipitation seasonality in modulating δ18O values across the Pamir suggests that proxy-derived records of past environments in the region must consider the mechanisms that today cause the seasonality contrast. 

   more » « less
4. Late Holocene Shifts in Indian Summer Monsoon Rainfall from Central Nepal Stalagmites and Coupled Climate Model Simulations

   Wiggins, C.C.; Ummenhofer, C.C.; Denniston, R.F.; Wanamaker, A.D.; Asmerom, Y.; Polyak, V.J.; Whitney, N.M.; Basnet, K.; Poudel, K.; Baral, S. (April 2022, American Geophysical Union Ocean Sciences Meeting)

   The Indian Summer Monsoon [ISM] provides approximately 80% of South Asia’s annual average precipitation. Nepal represents a particularly important sector of the ISM because of its location at the base of the Himalayas, Asia’s water tower, and in the zone of influence of the mid-latitude westerlies. Late Holocene ISM variability has previously been examined using high resolution resolved stable isotope records of stalagmites from northern, northeastern, and central India, but as of yet, no such records have been published from Nepal. We present high resolution stable isotopic time series from two precisely-dated and partially overlapping stalagmites spanning the last 2400 years from Siddha Baba Cave, central Nepal, as well as a year of isotopic data from rainwater collected near the cave. It has been suggested that the amount effect has only a minor effect on the oxygen isotope variability in precipitation in this area. As a result, we couple oxygen and carbon isotopes from these stalagmites to examine both regional and local-scale ISM dynamics. The Siddha Baba record reveals two periods suggestive of changes in the ISM: an apparent increase in rainfall during approximately CE 1350-1550 and a reduction in rainfall characterizing the last two centuries. We investigate these intervals using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. A primary focus is on links between Indo-Pacific ocean-atmosphere interactions and subsequent changes in the monsoon circulation over the Indian subcontinent, as well as regional moisture transport into Nepal between these periods. 

   more » « less
5. Late Holocene Shifts in Indian Summer Monsoon Rainfall from Central Nepal Stalagmites and Coupled Climate Model Simulations

   Wiggins, C.C.; Ummenhofer, C.C.; Denniston, R.F.; Wanamaker, A.D.; Asmerom, Y.; Polyak, V.J.; Whitney, N.M.; Basnet, K.; Poudel, K.; Baral, S. (April 2022, American Geophysical Union Ocean Sciences Meeting)

   The Indian Summer Monsoon [ISM] provides approximately 80% of South Asia’s annual average precipitation. Nepal represents a particularly important sector of the ISM because of its location at the base of the Himalayas, Asia’s water tower, and in the zone of influence of the mid-latitude westerlies. Late Holocene ISM variability has previously been examined using high resolution resolved stable isotope records of stalagmites from northern, northeastern, and central India, but as of yet, no such records have been published from Nepal. We present high resolution stable isotopic time series from two precisely-dated and partially overlapping stalagmites spanning the last 2400 years from Siddha Baba Cave, central Nepal, as well as a year of isotopic data from rainwater collected near the cave. It has been suggested that the amount effect has only a minor effect on the oxygen isotope variability in precipitation in this area. As a result, we couple oxygen and carbon isotopes from these stalagmites to examine both regional and local-scale ISM dynamics. The Siddha Baba record reveals two periods suggestive of changes in the ISM: an apparent increase in rainfall during approximately CE 1350-1550 and a reduction in rainfall characterizing the last two centuries. We investigate these intervals using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. A primary focus is on links between Indo-Pacific ocean-atmosphere interactions and subsequent changes in the monsoon circulation over the Indian subcontinent, as well as regional moisture transport into Nepal between these periods. 

   more » « less