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1. Monitoring of Sierra Nevada Caves Reveals the Potential for Stalagmites to Archive Seasonal Variability

   https://doi.org/10.3389/feart.2021.781526  

   Wortham, Barbara E. ; Montañez, Isabel P. ; Bowman, Kimberly ; Kuta, Daphne ; Contreras, Nora Soto ; Brummage, Eleana ; Pang, Allison ; Tinsley, John ; Roemer-Baer, Greg ( December 2021 , Frontiers in Earth Science)

   In the southwestern United States, California (CA) is one of the most climatically sensitive regions given its low (≤250 mm/year) seasonal precipitation and its inherently variable hydroclimate, subject to large magnitude modulation. To reconstruct past climate change in CA, cave calcite deposits (stalagmites) have been utilized as an archive for environmentally sensitive proxies, such as stable isotope compositions (δ¹⁸O, δ¹³C) and trace element concentrations (e.g., Mg, Ba, Sr). Monitoring the cave and associated surface environments, the chemical evolution of cave drip-water, the calcite precipitated from the drip-water, and the response of these systems to seasonal variability in precipitation and temperature is imperative for interpreting stalagmite proxies. Here we present monitored drip-water and physical parameters at Lilburn Cave, Sequoia Kings Canyon National Park (Southern Sierra Nevada), CA, and measured trace element concentrations (Mg, Sr, Ba, Cu, Fe, Mn) and stable isotopic compositions (δ¹⁸O, δ²H) of drip-water and for calcite (δ¹⁸O) precipitated on glass substrates over a two-year period (November 2018 to February 2021) to better understand how chemical variability at this site is influenced by local and regional precipitation and temperature variability. Despite large variability in surface temperatures and precipitation amount and source region (North Pacific vs. subtropical Pacific), Lilburn Cave exhibits a constant cave environment year-round. At two of the three sites within the cave, drip-water δ¹⁸O and δ²H are influenced seasonally by evaporative enrichment. At a third collection site in the cave, the drip-water δ¹⁸O responds solely to precipitation δ¹⁸O variability. The Mg/Ca, Ba/Ca, and Sr/Ca ratios are seasonally responsive to prior calcite precipitation at all sites but minimally to water-rock interaction. Lastly, we examine the potential of trace metals (e.g., Mn²⁺and Cu²⁺as a geochemical proxy of recharge and find that variability in their concentrations has high potential to denote the onset of the rainy season in the study region. The drip-water composition is recorded in the calcite, demonstrating that stalagmites from Lilburn Cave, and potentially more regionally, could record seasonal variability in weather even during periods of substantially reduced rainfall.

    

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2. 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. 

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3. A precisely-dated, composite stalagmite record of Indian summer monsoon variability from Siddha Baba cave, central Nepal, for the last 2700 yr

   Denniston, R.F. ; Sharafuddin, E.K. ; Bransel, L.R. ; Creech, P.D. ; Hughes, E.K. ; Wanamaker, A.D. ; Asmerom, Y. ; Polyak, V.J. ; Ummenhofer, C.C. ; Thatcher, D.L. ; et al ( January 2022 , Transactions American Geophysical Union)

   The Indian summer monsoon (ISM), which today supplies ~75% of annual precipitation to South Asia, has been reconstructed across previous centuries using a variety of hydroclimate-sensitive proxies. In some of these cases, ISM variability far exceeds that observed in the century-and-a-half-long instrumental record. Understanding the origins of these events is best addressed by developing a wide-ranging, multi-proxy network of high-resolution ISM reconstructions. In Nepal, ISM variability has been examined through tree rings, glacial ice, and lake sediments, but no stalagmite isotopic records of ISM rainfall have yet been published. Here we present a sub-decadally-resolved, precisely-dated, composite aragonite stalagmite record of ISM variability from Siddha Baba cave, central Nepal, for the last 2.7 kyr. A rainwater sampling program near the cave site, and a published study from Kathmandu (Adhikari et al., 2020), 150 km to the southeast, reveal that rainfall amount explains little of the observed variance in d18O values. Local hydroclimate is thus reconstructed from stalagmite 13C values, which we interpret as reflecting prior aragonite precipitation driven by changes in effective precipitation above the cave. ISM variability is apparent across a number of time scales, including centennial periods of reduced or enhanced rainfall coincident with societally-relevant precipitation regimes identified at other sites across South Asia. These include the Neo-Assyrian drought in the eastern Mediterranean and Middle East (2.7-2.5 kyr BP; Kathayat et al., 2019), the Mauria Empire (2.1-1.9 kyr BP), and the Guge Kingdom (0.9-0.3 kyr BP) pluvials in India and Tibet (Kathayat et al., 2017). A secular shift toward drier conditions since 0.5 kyr BP in the Siddha Baba record tracks the 18O records from Dasuopu glacier, Nepal Himalaya, and Sahiya cave, North India. Numerous multidecadal oscillations are also evident, including markedly wetter conditions during the 18th century, in the late Little Ice Age, apparent in the Dasuopu and Sahiya records. References Adhikari et al. (2020) Tellus B: Chem. Phys. Meteor., 72, 1-17. Kathayat et al. (2017) Sc. Adv., 7, e1701296. Kathayat et al. (2019) Sci. Adv., 5, eaax6656. 

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4. The Link Between Southern Portuguese Climate and the East Atlantic Mode from Highly Resolved Speleothem Records

   Chormann, A. ( January 2022 , PAGES Open Science Meeting)

   Climate in the Iberian Peninsula is impacted by both internal and external climate modes, which are expected to shift in position and intensity due to anthropogenic climate change. Examples of such modes include the North Atlantic Oscillation (NAO) and the East Atlantic mode (EA). Changes in the behavior in these regional climate modes could significantly alter water availability in the Iberian Peninsula, a region identified by model projections as particularly sensitive to future warming scenarios. There has been extensive research and paleoclimate reconstructions of the NAO and its impacts on Iberian climate. However, to date few paleoclimate records have been developed to evaluate the behavior of the EA over the late Holocene and into the present. The development of highly resolved regional paleoclimate records from Iberia is critical for improving the predictive capability of regional climate models under future warming scenarios and to determine the extent to which different teleconnection patterns are influencing climate. Here we present a near annually resolved stable carbon isotope (δ13C) and oxygen (δ18O) isotope time-series from three stalagmites from the Algarve region of southern Portugal from two caves within 2.3 km of each other. The southern coast of Portugal offers an ideal location to study the behavior of the EA due to the modulation of storm tracks coming across the North Atlantic Ocean into Iberia associated with the EA. U/Th dating indicates that our composite record spans the last millennia continuously through 2018 CE. Two stalagmites (GIA-19-1 and C-18-1) stopped growing around 1600 CE, during a dry interval, and sample GIA-19-2 grew continuously since the 15th century. GIA-19-2, with sub-annual resolution, is compared to modern instrumental records to evaluate the influence of specific environmental controls, including temperature and precipitation amounts. Isotope data from all three stalagmites exhibit substantial multidecadal variability indicating relatively wet and dry intervals. Based on our initial results, it is likely that both temperature and precipitation amount effects are the dominant controls on isotopic variability in these stalagmites. Comparison of the GIA-19-2 oxygen isotope time-series with the instrumental index (1950 to present) and reconstructed index (1650 CE to present) of the EA mode shows strong coherence with both index records. Hence, multidecadal variability observed in our stalagmite isotope time series may provide insight into the historical behavior of the EA mode and its resulting impacts on southern Portuguese climate. 

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5. The Link Between Southern Portuguese Climate and the East Atlantic Mode from Highly Resolved Speleothem Records

   Chormann, A. ; Wanamaker, A.D. ; Gillikin, D.P. ; Asmerom, Y. ; Polyak, V. ; Denniston, R. ; Thatcher, D. ; Ummenhofer, C. ; Regala, F.T. ( April 2022 , PAGES Open Science Meeting 2022, Virtual)

   Climate in the Iberian Peninsula is impacted by both internal and external climate modes, which are expected to shift in position and intensity due to anthropogenic climate change. Examples of such modes include the North Atlantic Oscillation (NAO) and the East Atlantic mode (EA). Changes in the behavior in these regional climate modes could significantly alter water availability in the Iberian Peninsula, a region identified by model projections as particularly sensitive to future warming scenarios. There has been extensive research and paleoclimate reconstructions of the NAO and its impacts on Iberian climate. However, to date few paleoclimate records have been developed to evaluate the behavior of the EA over the late Holocene and into the present. The development of highly resolved regional paleoclimate records from Iberia is critical for improving the predictive capability of regional climate models under future warming scenarios and to determine the extent to which different teleconnection patterns are influencing climate. Here we present a near annually resolved stable carbon isotope (δ13C) and oxygen (δ18O) isotope time-series from three stalagmites from the Algarve region of southern Portugal from two caves within 2.3 km of each other. The southern coast of Portugal offers an ideal location to study the behavior of the EA due to the modulation of storm tracks coming across the North Atlantic Ocean into Iberia associated with the EA. U/Th dating indicates that our composite record spans the last millennia continuously through 2018 CE. Two stalagmites (GIA-19-1 and C-18-1) stopped growing around 1600 CE, during a dry interval, and sample GIA-19-2 grew continuously since the 15th century. GIA-19-2, with sub-annual resolution, is compared to modern instrumental records to evaluate the influence of specific environmental controls, including temperature and precipitation amounts. Isotope data from all three stalagmites exhibit substantial multidecadal variability indicating relatively wet and dry intervals. Based on our initial results, it is likely that both temperature and precipitation amount effects are the dominant controls on isotopic variability in these stalagmites. Comparison of the GIA-19-2 oxygen isotope time-series with the instrumental index (1950 to present) and reconstructed index (1650 CE to present) of the EA mode shows strong coherence with both index records. Hence, multidecadal variability observed in our stalagmite isotope time series may provide insight into the historical behavior of the EA mode and its resulting impacts on southern Portuguese climate. 

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