An official website of the United States government
We compare hydrogen isotopic measurements of long-chain leaf-wax n-alkanes (2Hw; C27, C29, and C31) from Martin Lake, IN, United States of America (USA), with a calcite-based reconstruction of the oxygen isotopic composition of precipitation (18Op) from the same lake. We observe stable and high 2Hw during the Common Era (last 2000 years), which we interpret as growing-season precipitation originating mainly from the Gulf of Mexico and Atlantic. During the Little Ice Age (LIA; 1200-1850 CE), 2Hw values increased by 3-8 ‰, concomitant with a significant decrease in 18Op values by up to 12.5 ‰. Multiple proxy records for this time indicate persistent growing-season drought. We interpret these relatively high 2Hw values, as compared to the 18Op values, as a signal of low relative humidity that resulted in an 2H enrichment in plant source water resulting in high 2H values through enhanced plant water and/or soil evaporation. These results support the occurrence of low humidity conditions during the LIA in the midcontinental USA that also contributed to the marked decline of regional pre-Columbian Mississippian populations.
more »
« less
Effects of aridity and vegetation on plant-wax δD in modern lake sediments
We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (εa) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of 2H and 18O associated with evaporation of lake waters. The most negative εa values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum εa values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0–2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ18O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.
more »
« less
- Award ID(s):
- 0741400
- PAR ID:
- 10616631
- Publisher / Repository:
- Geochimica et Cosmochimica Acta
- Date Published:
- Journal Name:
- Geochimica et Cosmochimica Acta
- Volume:
- 74
- Issue:
- 20
- ISSN:
- 0016-7037
- Page Range / eLocation ID:
- 5785-5797
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The past elevation of the land surface provides a unique constraint on the underlying lithospheric structure during mountain and plateau formation. Development of new paleoaltimetry techniques that can be applied to a wide variety of sample types is therefore of continuing importance. This study evaluates organic substrates that preserve the δD ratio of surface waters as a new approach to reconstruct paleoaltimetry. We measured the hydrogen isotope composition of n-alkanes from epicuticular plant waxes preserved in lacustrine deposits to reconstruct the δD of precipitation in Cenozoic basins that have been elevated as part of the Tibetan Plateau. n-Alkane δD- and carbonate δ18O-inferred water compositions from the Eocene–Miocene Lunpola Basin and Miocene Hoh-Xil Basin plot near or at enriched values relative to the global meteoric water line, as expected for evaporative lakewater and leafwater systems that have the same precipitation source. n-Alkane δD-based water compositions are nearly identical to the minimum carbonate δ18O-based values, demonstrating that plant-wax δD is minimally affected by evaporation compared to lacustrine calcite δ18O. This agreement strongly supports the presence of similar precipitation isotopic compositions in both archives despite different isotope systems, source water reservoirs, archive materials, modes of incorporation, and diagenetic processes. Paleoelevations for each basin and time period were calculated from precipitation isotope ratios using the isotope–altitude relationship derived from both a simple thermodynamic model and modern precipitation sampling from the Plateau region. Our new results from the Hoh-Xil Basin suggest 1700 to 2600 m of uplift may have occurred some time between the late Eocene and early Miocene. The timing of this uplift is consistent with late-Oligocene compressional deformation of the Hoh-Xil Basin and northward growth of the Tibetan Plateau however, the calculated uplift is not a unique solution from the paleoisotope data because of uncertainties in Eocene and Miocene moisture sources and isotope gradients for the northern plateau. Our results demonstrate the utility of lipid-based estimates of paleoelevation and expand the types of deposits amenable to paleoaltimetry analysis.more » « less
-
Uncertainties surround the relationship between salinity and the stable isotopic composition of seawater, largely due to a dearth of modern seawater isotope data. Here we report 191 new, paired measurements of salinity and seawater oxygen isotopes (δ18Osw) taken from the central tropical Pacific in May 2012, from the surface to 4600 m depth. We observe significant correlations between δ18Osw and salinity across the study region, with slopes ranging from 0.23 to 0.31‰/psu for the mixed layer, and 0.35–0.42‰/psu for waters between the mixed layer and 500 m depth. When considering δ18Osw–salinity across averages of individual water masses in the region, slopes range from 0.21 to 0.40‰/psu, albeit with appreciable scatter. Surface salinity and δ18Osw data corresponding to the North Equatorial Countercurrent are significantly higher than previously observed, which we attribute to a weak westerly current and dry conditions in the region during the May 2012 cruise. Subsurface (80–500 m) salinity values from 2012 are significantly lower than corresponding values from pre-existing regional data, highlighting a different latitudinal sampling distribution, while subsurface δ18Osw is not significantly different. Thus, in May 2012, δ18Osw in this region could not be used to distinguish between subsurface water masses of different salinities. Unlike other regions where the surface ‘freshwater endmember’ is close to the δ18O value of regional precipitation, the freshwater endmember implied by our dataset (− 10.38‰) is consistent with a strong evaporative influence. Paired δ18O–δD values of precipitation and surface seawaters have similar slopes (5.0, 5.1), and relatively low intercepts (1.4, 0.8) indicating isotopic variability in both reservoirs is also partly controlled by evaporation.more » « less
-
Hydroclimate interpretations of stalagmite δ18O records from tropical regions requires an understanding of the temporal integration of rainfall amount and its isotopic composition by drip waters that form stalagmite deposits. This study presents oxygen (δ18O) and hydrogen (δD) isotopic results from over 1200 groundwater, rainfall and drip water samples, collected at ~weekly time intervals, over three hydrological years at Río Secreto Cave, in the Yucatán Peninsula, Mexico. Cave environmental conditions and the isotopic composition of drip water were monitored in three chambers with different degrees of air ventilation, along with temperature and relative humidity conditions at the surface. We examined 16 drips and observed that annual δD and δ18O variability reflects the isotopic variability of rainfall to varying degrees. The observed annual amplitude of drip water isotopic variability represents between 5% and 95% of that of rainfall, reflecting epikarst water reservoir size and the complexity of flow paths. Drips that closely reflect the isotopic variability of rainfall and best preserve the isotopic signal of individual rainfall events are observed, but they are uncommon. Only two drips out of 16 were found to have potential to record rainfall isotopic shifts associated with tropical cyclones if sampled at weekly resolution. The relationship between δD and δ18O in drip water suggests that recharge is biased toward the rainy season (June to November), which represents up to 80% of total annual precipitation. We find that over the course of a year most drips reflect the annual δ18O composition of rainfall, in support of quantitative precipitation estimates from stalagmite δ18O records. We find evidence that the effective recharge in this cave system is controlled by precipitation amount and that recharge is not limited to the months when precipitation exceeds evaporation.more » « less
-
Abstract Measurements of oxygen and hydrogen stable isotope ratios (δ18O and δD) in meteoric waters provide insight to overlapping effects of evaporation, precipitation, and mixing on basin scale hydrology. This study of waters collected between 2016 and 2021 in the Turkana Basin, northern Kenya, uses δ18O and δD to understand water balance in Lake Turkana, a large, low‐latitude, alkaline desert lake. The Omo River, a major river system in the Ethiopian Highlands, is historically understood to provide approximately 90% of the water input to Lake Turkana. Discharge of the Omo is prohibitively difficult to measure, but stable isotope ratios in the lake may provide a meaningful method for monitoring the lake's response to changes in input. Precipitation in the Turkana Basin is low (<200 mm/year) with negligible rainfall on the lake's surface, and all water loss from the lake is evaporative. We compare new measurements with previous data from the region and records of lake height and precipitation from the same time period. We show that a Bayesian approach to modeling evaporation using atmospheric conditions and river δ18O and δD yields results consistent with published water balance models. Continued sampling of lake and meteoric waters in the Turkana Basin will be a useful way to monitor the lake's response to regional and global climate change.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.gca.2010.06.018
