An official website of the United States government
Abstract The Arctic hydrological cycle is predicted to intensify as the Arctic warms, due to increased poleward moisture transport during summer and increased evaporation from seas once ice‐covered during winter. Records of past Arctic precipitation seasonality are important because they provide a context for these ongoing changes. In some Arctic lakes, stable isotopes of oxygen and hydrogen (δ18O and δ2H, respectively) vary seasonally, due to seasonal changes in precipitation δ18O and δ2H. We reconstruct precipitation seasonality from Lake N3, a well‐dated lake sediment archive in Disko Bugt, western Greenland, by generating Holocene records of two proxies that are produced at different times of the year, and therefore record different lake water seasonal isotopic compositions. Aquatic plants synthesize waxes throughout the summer, and their δ2H reflects winter‐biased precipitation δ2H at Lake N3, whereas chironomids synthesize their head capsules between late summer and winter, and their δ18O reflects summer‐biased precipitation δ18O at Lake N3. During the middle Holocene at Lake N3, aquatic plant leaf wax was strongly2H‐depleted, while chironomid chitin was18O‐enriched. We guide interpretations of these records using sensitivity tests of a lake water and energy balance model, where we change precipitation amount and isotope seasonality inputs. The sensitivity tests suggest that the contrasting trends between proxies were likely caused by an increase in precipitation amount during all seasons and an increase in precipitation isotope seasonality, in addition to proxy‐specific mechanisms, highlighting the importance of understanding lake‐ and proxy‐specific systematics when interpreting records from sediment archives.
more »
« less
Variability and Controls on δ 18 O, d‐excess, and ∆′ 17 O in Southern Peruvian Precipitation
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 δ18Opand δ2Hpvalues from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′17Op, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ18Opand that seasonal δ18Opvariations are related to upstream rainout and local convection. Spatial δ18Opvariations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excesspcycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excesspand ∆′17Opclearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′17Opvalues 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 δ18Op, d‐excessp, and ∆′17Opdata and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes.
more »
« less
- Award ID(s):
- 1954660
- PAR ID:
- 10445801
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 126
- Issue:
- 23
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Stable oxygen isotopic ratios in corals (δ18Ocoral) are commonly utilized to reconstruct climate variability beyond the limit of instrumental observations. These measurements provide constraints on past seawater temperature, due to the thermodynamics of isotopic fractionation, but also past salinity, as both salinity and seawater δ18O (δ18Osw) are similarly affected by precipitation/evaporation, advection, and other processes. We use historical observations, isotope‐enabled model simulations, and the PAGES Iso2k database to assess the potential of δ18Ocoralto provide information on past salinity. Using ‘‘pseudocorals’’ to represent δ18Ocoralas a function of observed or simulated temperature and salinity/δ18Osw, we find that δ18Oswcontributes up to 89% of δ18Ocoralvariability in the Western Pacific Warm Pool. Although uncertainty in the δ18Osw‐salinity relationship influences the inferred salinity variability, corals from these sites could provide valuable δ18Oswreconstructions. Coordinated in situ monitoring of salinity and δ18Oswis vital for improving estimates of hydroclimatic change.more » « less
-
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 (δ18Oswand δ18Op) from Koror, Palau. Our dataset indicates that temporally, δ18Oswis strongly influenced by local δ18Op.Both monthly δ18Oswand δ18Opare highly correlated with outgoing longwave radiation across the tropical Pacific, reflecting a Walker Circulation imprint on the surface ocean. Changes in the Palau δ18Osw—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 δ18Oswto reconstruct ENSO variability in the western tropical Pacific.more » « less
-
Abstract Understanding the hydroclimate representations of precipitationδ18O (δ18Op) 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 andδ18Op, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of suchδ18Opdipole 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 theδ18Opdepletion 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 theδ18Openrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship betweenδ18Opand rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennialδ18Opchange in the central-western Amazon, whileδ18Opdoes not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when usingδ18Opas a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennialδ18Opdipole response and pose a challenge in reconciling the discrepancy. Significance StatementWe want to comprehensively understand the hydroclimate footprints ofδ18Opand the mechanisms of the millennial variability ofδ18Opover tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennialδ18Opchange 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.more » « less
-
Speleothem δ18O records from central southern China have long been regarded as a key benchmark for Asian summer monsoon intensity. However, the similar δ18O minima observed among precession minima and their link to seasonal precipitation mixing remains unclear. Here, we present a 400,000-y record of summer precipitation δ18O from loess microcodium, which captures distinct precession cycles similar to those seen in speleothem δ18O records, particularly during glacial periods. Notably, our microcodium δ18O record reveals very low-δ18O values during precession minima at peak interglacials, a feature absent in speleothem δ18O records from central southern China. This discrepancy suggests that the mixed summer and nonsummer climatic signals substantially influence the speleothem δ18O records from central southern China. Proxy-model comparisons indicate that the lack of very low-δ18O values in speleothem δ18O records is due to an attenuated summer signal contribution, resulting from a lower summer-to-annual precipitation ratio in southern China at strong monsoon intervals. Our findings offer a potential explanation for the long-standing puzzle of the absence of 100- and 41-kyr cycles in speleothem δ18O records and underscore the critical role of seasonality in interpreting paleoclimatic proxies in central southern China. These insights also have broader implications for interpreting speleothem δ18O records globally, advocating for a more multiseason interpretive framework.more » « less
