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1. 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 intervalsmore »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.« less
2. 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 intervalsmore »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.« less
3. Enhancing understanding of the hydrological cycle via pairing of process‐oriented and isotope ratio tracers

   https://doi.org/10.5061/dryad.8w9ghx3mr  

   Fiorella, Richard ; Siler, Nicholas ; Nusbaumer, Jesse ; Noone, David ( January 2021 )

   Abstract

   This dataset contains monthly average output files from the iCAM6 simulations used in the manuscript "Enhancing understanding of the hydrological cycle via pairing of process-oriented and isotope ratio tracers," in review at the Journal of Advances in Modeling Earth Systems. A file corresponding to each of the tagged and isotopic variables used in this manuscript is included. Files are at 0.9° latitude x 1.25° longitude, and are in NetCDF format. Data from two simulations are included: 1) a simulation where the atmospheric model was "nudged" to ERA5 wind and surface pressure fields, by adding an additional tendency (see section 3.1 of associated manuscript), and 2) a simulation where the atmospheric state was allowed to freely evolve, using only boundary conditions imposed at the surface and top of atmosphere. Specific information about each of the variables provided is located in the "usage notes" section below. Associated article abstract: The hydrologic cycle couples the Earth's energy and carbon budgets through evaporation, moisture transport, and precipitation. Despite a wealth of observations and models, fundamental limitations remain in our capacity to deduce even the most basic properties of the hydrological cycle, including the spatial pattern of the residence time (RT) of water in the atmosphere and the mean distance traveled from evaporation sources to precipitation sinks. Meanwhile, geochemical tracers such as stable water isotope ratios provide a tool to probe hydrological processes, yet their interpretation remains equivocal despite several decades of use. As a result, there is a need for new mechanistic tools that link variations in water isotope ratios to underlying hydrological processes. Here we present a new suite of “process-oriented tags,” which we use to explicitly trace hydrological processes within the isotopically enabled Community Atmosphere Model, version 6 (iCAM6). Using these tags, we test the hypotheses that precipitation isotope ratios respond to parcel rainout, variations in atmospheric RT, and preserve information regarding meteorological conditions during evaporation. We present results for a historical simulation from 1980 to 2004, forced with winds from the ERA5 reanalysis. We find strong evidence that precipitation isotope ratios record information about atmospheric rainout and meteorological conditions during evaporation, but little evidence that precipitation isotope ratios vary with water vapor RT. These new tracer methods will enable more robust linkages between observations of isotope ratios in the modern hydrologic cycle or proxies of past terrestrial environments and the environmental processes underlying these observations.  
   

   Methods

   Details about the simulation setup can be found in section 3 of the associated open-source manuscript, "Enhancing understanding of the hydrological cycle via pairing of process‐oriented and isotope ratio tracers." In brief, we conducted two simulations of the atmosphere from 1980-2004 using the isotope-enabled version of the Community Atmosphere Model 6 (iCAM6) at 0.9x1.25° horizontal resolution, and with 30 vertical hybrid layers spanning from the surface to ~3 hPa. In the first simulation, wind and surface pressure fields were "nudged" toward the ERA5 reanalysis dataset by adding a nudging tendency, preventing the model from diverging from observed/reanalysis wind fields. In the second simulation, no additional nudging tendency was included, and the model was allowed to evolve 'freely' with only boundary conditions provided at the top (e.g., incoming solar radiation) and bottom (e.g., observed sea surface temperatures) of the model. In addition to the isotopic variables, our simulation included a suite of 'process-oriented tracers,' which we describe in section 2 of the manuscript. These variables are meant to track a property of water associated with evaporation, condensation, or atmospheric transport.
   

   Other

   Metadata are provided about each of the files below; moreover, since the attached files are NetCDF data - this information is also provided with the data files. NetCDF metadata can be accessed using standard tools (e.g., ncdump). Each file has 4 variables: the tagged quantity, and the associated coordinate variables (time, latitude, longitude). The latter three are identical across all files, only the tagged quantity changes. Twelve files are provided for the nudged simulation, and an additional three are provided for the free simulations: Nudged simulation files iCAM6_nudged_1980-2004_mon_RHevap: Mass-weighted mean evaporation source property: RH (%) with respect to surface temperature. iCAM6_nudged_1980-2004_mon_Tevap: Mass-weighted mean evaporation source property: surface temperature in Kelvin iCAM6_nudged_1980-2004_mon_Tcond: Mass-weighted mean condensation property: temperature (K) iCAM6_nudged_1980-2004_mon_columnQ: Total (vertically integrated) precipitable water (kg/m2).  Not a tagged quantity, but necessary to calculate depletion times in section 4.3 (e.g., Fig. 11 and 12). iCAM6_nudged_1980-2004_mon_d18O: Precipitation d18O (‰ VSMOW) iCAM6_nudged_1980-2004_mon_d18Oevap_0: Mass-weighted mean evaporation source property - d18O of the evaporative flux (e.g., the 'initial' isotope ratio prior to condensation), (‰ VSMOW) iCAM6_nudged_1980-2004_mon_dxs: Precipitation deuterium excess (‰ VSMOW) - note that precipitation d2H can be calculated from this file and the precipitation d18O as d2H = d-excess - 8*d18O. iCAM6_nudged_1980-2004_mon_dexevap_0: Mass-weighted mean evaporation source property - deuterium excess of the evaporative flux iCAM6_nudged_1980-2004_mon_lnf: Integrated property - ln(f) calculated from the constant-fractionation d18O tracer (see section 3.2). iCAM6_nudged_1980-2004_mon_precip: Total precipitation rate in m/s. Note there is an error in the metadata in this file - it is total precipitation, not just convective precipitation. iCAM6_nudged_1980-2004_mon_residencetime: Mean atmospheric water residence time (in days). iCAM6_nudged_1980-2004_mon_transportdistance: Mean atmospheric water transport distance (in km). Free simulation files iCAM6_free_1980-2004_mon_d18O: Precipitation d18O (‰ VSMOW) iCAM6_free_1980-2004_mon_dxs: Precipitation deuterium excess (‰ VSMOW) - note that precipitation d2H can be calculated from this file and the precipitation d18O as d2H = d-excess - 8*d18O. iCAM6_free_1980-2004_mon_precip: Total precipitation rate in m/s. Note there is an error in the metadata in this file - it is total precipitation, not just convective precipitation.
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4. Investigating temperature and origin of fluids that circulated within a brittle fault array in the West Antarctic Rift System

   Pollatsek, Emory ; Siddoway, Christine ; and Gevedon, Michelle ( September 2022 , 29th West Antarctic Ice Sheet Workshop)

   Outcrops of brittle faults are rare in Marie Byrd Land, West Antarctica, because fault damage zones commonly undergo enhanced erosion and form bedrock troughs occupied by glacier ice. Where exposures do exist, faults yield information about regional strain in the West Antarctic Rift System (WARS) and may host minerals that contain a record of the temperature and chemistry of fluids during regional-scale faulting. In MBL’s southern Ford Ranges, bordering Ross Sea, a distinctive fault array was sampled that hosts tourmaline and quartz, a mineral-pair that can provide temperature and composition of fault-associated fluids, using 18O. Host rocks are tourmaline-free. At three separate sites, fault surfaces display strongly aligned tourmaline, suggesting that mineralization occurred during tectonism. One site features highly polished, or mirrored, surfaces, a characteristic that may indicate tourmaline precipitation during seismic slip. The orientation and kinematics of the high angle faults are NNW-striking: normal-slip, and WNW-ESE striking: right-lateral strike-slip. The timing of mineralization is yet to be determined, but viable possibilities are that the faults formed during broad intracontinental extension during formation of Ross Embayment in the Cretaceous, or during development of deep, narrow basins beneath the RIS grounding zone, in the Neogene (newly detected, see Tankersley et al.,more »this meeting). Once formed, tourmaline is resistant to chemical and isotopic re-equilibration, and therefore can retain a record of its conditions during formation. We used oxygen isotope compositions of tourmaline and quartz pairs to investigate temperatures, fluid-rock ratios, and fluid sources, with bearing on fault-localized flux of fluids and geothermal heat. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces, as well as quartz separated from host rock in the same hand samples. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. ∆Qtz-Trm values fall between 1.3 and 2.0, and 18O of quartz in faults closely resembles 18O of host rock quartz. We tentatively determine the water oxygen isotope ratio as greater than ~7.7 ‰. Plutonic-metamorphic associations in the immediate region, and comparisons with similar faults elsewhere (i.e. Isola d’Elba, Italy), suggest temperatures as high as 500°C for the fluids that circulated into the faults. The data are interpreted to show that brittle faults provided pathways for hot fluids derived from mid-crustal processes to make their way to shallow crustal depths. 18O values indicate magmatic and/or metamorphic fluid sources, with minor to no introduction of meteoric fluids. Tourmaline-quartz pairs did not attain equilibrium, likely due to tourmaline’s rapid crystallization. On-going investigation includes analysis of H and B isotopes in tourmaline, which will better characterize the relationship between fault-hosted and mid-crustal fluids.« less
5. Hydroclimatic Controls on the Isotopic (δ18 O, δ2 H, d-excess) Traits of Pan-Arctic Summer Rainfall Events

   https://doi.org/10.3389/feart.2021.651731  

   Mellat, Moein ; Bailey, Hannah ; Mustonen, Kaisa-Riikka ; Marttila, Hannu ; Klein, Eric S. ; Gribanov, Konstantin ; Bret-Harte, M. Syndonia ; Chupakov, Artem V. ; Divine, Dmitry V. ; Else, Brent ; et al ( May 2021 , Frontiers in Earth Science)

   Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ 18 O, δ 2 H, d-excess ) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 ( n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ 2 H = 7.6⋅δ 18 O–1.8 ( r 2 = 0.96, p < 0.01). Mean amount-weighted δ 18 O, δ 2 H, and d-excess values were −12.3, −93.5, and 4.9‰, respectively, with the lowest summer mean δ 18 O value observed in northwest Greenland (−19.9‰) and the highest in Iceland (−7.3‰). Southern Alaska recordedmore »the lowest mean d-excess (−8.2%) and northern Russia the highest (9.9‰). We identify a range of δ 18 O-temperature coefficients from 0.31‰/°C (Alaska) to 0.93‰/°C (Russia). The steepest regression slopes (>0.75‰/°C) were observed at continental sites, while statistically significant temperature relations were generally absent at coastal stations. Model outputs indicate that 68% of the summer precipitating air masses were transported into the Arctic from mid-latitudes and were characterized by relatively high δ 18 O values. Yet 32% of precipitation events, characterized by lower δ 18 O and high d-excess values, derived from northerly air masses transported from the Arctic Ocean and/or its marginal seas, highlighting key emergent oceanic moisture sources as sea ice cover declines. Resolving these processes across broader spatial-temporal scales is an ongoing research priority, and will be key to quantifying the past, present, and future feedbacks of an amplified Arctic water cycle on the global climate system.« less