An official website of the United States government
SIMS Bias on Isotope Ratios in Ca-Mg-Fe Carbonates (Part III): δ 18 O and δ 13 C Matrix Effects Along the Magnesite-Siderite Solid-Solution Series
More Like this
-
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
-
Abstract Freshwater lakes are vital water resources, especially in the context of a changing climate. Supplementing existing hydrological methods to monitor lake levels may greatly improve resource management, particularly in drought‐prone regions. In this study, we performed dual‐isotope (δ18O and δ2H) calculations to model the hydrological balance of Bear Lake, Utah‐Idaho. The lake is a critical water resource and site for paleoclimate studies of the latest Pleistocene. Using the Craig‐Gordon isotopic mass balance model, we simultaneously constrained unknown fluxes, including groundwater discharge and particularly evaporation, which is typically under‐constrained due to inconsistencies across existing methods. Data from community databases and sampling campaigns in 2022 and 2023 were utilized to derive an evaporation rate of 2.18 × 108 m3/yr (±4.94 × 106 m3/yr, 1σ using δ18O; ±3.47 × 106 m3/yr, 1σ using δ2H) at a calculated relative humidity of 0.62 above the lake. Detailed analysis of the sensitivity of the model revealed that parameters related to atmospheric moisture, particularly humidity and its isotopic composition, significantly influence evaporation estimates. Using carbonate‐based isotope data, we leveraged this sensitivity to provide insights in the evaporation and humidity at Bear Lake during different time periods. This study shows the potential of using modern water isotopic composition to aid with interpreting carbonate‐based paleoclimate data sets and informing current and future water resource management practices.more » « less
-
ABSTRACT RationaleStable oxygen isotope measurements in silicate clays, such as smectite and kaolinite, provide crucial information for understanding Earth's climate history and environmental changes. Despite a growing interest in the oxygen isotope analysis of silicate clays and clay‐rich sediments, there lacks a consensus on the preparation and standardization of clay mineral samples. To improve the accuracy and interlaboratory comparisons of clay isotope measurements, especially those involving laser fluorination techniques, newly established kaolinite and smectite oxygen isotope standards are much needed. MethodsWe employed conventional nickel bomb fluorination combined with dual‐inlet isotope ratio mass spectrometry to establish precise δ18O and Δ′17O values for leached clay reference materials KGa‐1b and SHCa‐1, a kaolinite and a hectorite/smectite, respectively. We further measured leached KGa‐1b and SHCa‐1 pressed into pellets with a lithium fluoride as a binding agent for the laser fluorination method, allowing us to test the reproducibility between methods and utilize a standard laser chamber drift correction scheme. ResultsThe laser fluorination technique yielded highly precise and reproducible δ18O and Δ′17O measurements for the KGa‐1b and SHCa‐1, aligning with bomb values of δ18O. This confirms the method's reliability and comparability to conventional isotope measurement techniques while also stressing the importance of proper sample preparation and laser chamber drift corrections. ConclusionsThis study demonstrates that laser fluorination is an effective method for accurately measuring the stable oxygen isotope composition of silicate clays or clay‐rich sediments when corrected with known silicate clay standards. These methods offer a valuable methodology for future research and applications that will significantly improve our understanding of past climate and environmental conditions.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/ggr.12194
