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1. Seasonal Hydroclimate Recorded in High Resolution δ ¹⁸ O Profiles Across Pinus palustris Growth Rings

   https://doi.org/10.1029/2021JG006505  

   Lukens, William E.; Narmour, Robert T.; Schubert, Brian A. (December 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract Rainfall amount and intensity are increasing under anthropogenic climate change, but many instrument records span less than a century. The oxygen isotopic composition of tree‐ring cellulose (δ¹⁸O_cell) reflects local source water, climate, and tree physiology. The patterns of δ¹⁸O_cellwithin tree‐rings has the potential to extend pre‐instrument climate records with subannual resolution, but the influences on intra‐ring δ¹⁸O_cellprofiles are unexplored in many settings. In this study, high‐resolution δ¹⁸O_cellprofiles were analyzed on three longleaf pine trees growing in a native savanna in Louisiana, United States. The time series covers a wide range of rainfall conditions from 2001 to 2008 C.E. with a total of 421 δ¹⁸O_cellanalyses. The δ¹⁸O_cellvalues for individual years are well correlated with each other both within and between trees (r = 0.71–0.78). We used principal components analysis andk‐means clustering to differentiate δ¹⁸O_cellprofiles into two groupings: symmetrical δ¹⁸O_cellprofiles versus asymmetrical profiles that have depressed latewood δ¹⁸O_cellvalues. The slope of latewood δ¹⁸O_cellprofiles and mean δ¹⁸O_cellvalues of latewood tissue correlate with total June‐November precipitation. We hypothesize that poorly drained soils in the study area mediate the influence of any individual storm event: in dry years,¹⁸O‐depleted signals from convective storms are moderated by subsequent evaporative enrichment of standing water, whereas in wet years, increased humidity and frequent re‐supply of¹⁸O‐depleted water overrides evaporative enrichment effects, resulting in low δ¹⁸O_cellof latewood. These results suggest that δ¹⁸O_cellproxies for tropical storm occurrence need to account for soil conditions at the site of tree growth. 

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2. Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part II: δ18Op Response

   https://doi.org/10.1175/JCLI-D-22-0289.1  

   Bao, Yuntao; Liu, Zhengyu; He, Chengfei (July 2023, Journal of Climate)

   Abstract Understanding the hydroclimate representations of precipitationδ¹⁸O (δ¹⁸O_p) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall andδ¹⁸O_p, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of suchδ¹⁸O_pdipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to theδ¹⁸O_pdepletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to theδ¹⁸O_penrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship betweenδ¹⁸O_pand rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennialδ¹⁸O_pchange in the central-western Amazon, whileδ¹⁸O_pdoes not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when usingδ¹⁸O_pas a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennialδ¹⁸O_pdipole response and pose a challenge in reconciling the discrepancy. Significance StatementWe want to comprehensively understand the hydroclimate footprints ofδ¹⁸O_pand the mechanisms of the millennial variability ofδ¹⁸O_pover tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennialδ¹⁸O_pchange in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it. 

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3. Western Pacific Warm Pool δ ¹⁸ O Response to the El Niño‐Southern Oscillation

   https://doi.org/10.1029/2024GL113366  

   Murray, N_K; Conroy, J_L; Colin, P_L; Cobb, K_M; Noone, D_C (February 2025, Geophysical Research Letters)

   Abstract Tropical Pacific seawater and precipitation stable oxygen isotope data aid in understanding modern oceanic and atmospheric interactions, and these data are particularly valuable as they are archived in isotope‐based paleoclimate records. However, the absence of modern seawater isotope time series limits the ability to identify the atmospheric influences on these data, precluding robust paleoclimate interpretations. We present a new 10 year sub‐monthly record of seawater and precipitation stable oxygen isotope values (δ¹⁸O_swand δ¹⁸O_p) from Koror, Palau. Our dataset indicates that temporally, δ¹⁸O_swis strongly influenced by local δ¹⁸O_p.Both monthly δ¹⁸O_swand δ¹⁸O_pare highly correlated with outgoing longwave radiation across the tropical Pacific, reflecting a Walker Circulation imprint on the surface ocean. Changes in the Palau δ¹⁸O_sw—salinity relationship correspond to NINO3.4 variability, indicating a difference in how these variables record El Niño Southern Oscillation (ENSO) information, but demonstrating the utility of δ¹⁸O_swto reconstruct ENSO variability in the western tropical Pacific. 

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4. On the Interpretation of the ENSO Signal Embedded in the Stable Isotopic Composition of Quelccaya Ice Cap, Peru

   https://doi.org/10.1029/2018JD029064  

   Hurley, J. V.; Vuille, Mathias; Hardy, Douglas R. (January 2019, Journal of Geophysical Research: Atmospheres)

   Abstract Theδ¹⁸O signal in ice cores from the Quelccaya Ice Cap (QIC), Peru, corresponds with and has been used to reconstruct Niño region sea surface temperatures (SSTs), but the physical mechanisms that tie El Niño–Southern Oscillation (ENSO)‐related equatorial Pacific SSTs to snowδ¹⁸O at 5,680 m in the Andes have not been fully established. We use a proxy system model to simulate how QIC snowδ¹⁸O varies by ENSO phase. The model accurately simulates higher and lowerδ¹⁸O values during El Niño and La Niña, respectively. We then explore the relative roles of ENSO forcing on different components of the forward model: (i) the seasonality and amount of snow gain and loss at the QIC, (ii) the initial water vaporδ¹⁸O values, and (iii) regional temperature. Most (more than two thirds) of the ENSO‐related variability in the QICδ¹⁸O can be accounted for by ENSO's influence on South American summer monsoon (SASM) activity and the resulting change in the initial water vapor isotopic composition. The initial water vaporδ¹⁸O values are affected by the strength of upstream convection associated with the SASM. Since convection over the Amazon is enhanced during La Niña, the water vapor over the western Amazon Basin—which serves as moisture source for snowfall on QIC—is characterized by more negativeδ¹⁸O values. In the forward model, higher initial water vaporδ‐values during El Niño yield higher snowδ¹⁸O at the QIC. Our results clarify that the ENSO‐related isotope signal on Quelccaya should not be interpreted as a simple temperature response. 

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5. Variability and Controls on δ ¹⁸ O, d‐excess, and ∆′ ¹⁷ O in Southern Peruvian Precipitation

   https://doi.org/10.1029/2020JD034009  

   Aron, Phoebe G.; Poulsen, Christopher J.; Fiorella, Richard P.; Levin, Naomi E.; Acosta, Rene P.; Yanites, Brian J.; Cassel, Elizabeth J. (December 2021, Journal of Geophysical Research: Atmospheres)

   Abstract The isotopic composition of precipitation is used to trace water cycling and climate change, but interpretations of the environmental information recorded in central Andean precipitation isotope ratios are hindered by a lack of multi‐year records, poor spatial distribution of observations, and a predominant focus on Rayleigh distillation. To better understand isotopic variability in central Andean precipitation, we present a three‐year record of semimonthly δ¹⁸O_pand δ²H_pvalues from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′¹⁷O_p, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ¹⁸O_pand that seasonal δ¹⁸O_pvariations are related to upstream rainout and local convection. Spatial δ¹⁸O_pvariations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excess_pcycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excess_pand ∆′¹⁷O_pclearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′¹⁷O_pvalues in the central Andes and set the foundation for future paleoclimate and paleoaltimetry studies in the region. This study highlights the hydrologic understanding that comes from a combination of δ¹⁸O_p, d‐excess_p, and ∆′¹⁷O_pdata and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes. 

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