Jeffrey A. Dorale

Uranium and thorium isotopic and concentration measurements by magnetic sector inductively coupled plasma mass spectrometry

We have developed techniques by sector-field inductively coupled plasma mass spectrometry (ICP-MS) for measuring the isotopic composition and concentration of uranium and thorium, focusing on the rare isotopes, 230Th and 234U. These isotopes have been widely used as tracers in earth sciences, e.g., chronology, paleoclimatology, archeology, hydrology, geochemistry, and oceanography. Measurements made on reference materials demonstrate that the analytical precision approximates counting statistics and that the accuracy of the measurement is within error of accepted values. Routine measurement times are 20 min for U and 10 min for Th. The sensitivities (ions counted/atoms introduced) are 2–3‰ for U and 1.5–2‰ for Th. Samples of 10–40 ng of 238U (0.5–2.0 pg of 234U) give measurement precisions of 1–2‰ (2σ) for δ234U and U concentration ([U]). Only 0.4 pg of 230Th are needed to achieve [230Th] and 230Th/232Th data with errors less than 5‰ even for cases where 230Th/232Th is 10−5 or less. Our ICP-MS data, including uranium standards, thorium standards, 238U–234U–230Th–232Th dating of speleothems and 230Th–232Th in oceanic particulates, replicates measurements made by thermal ionization mass spectrometry (TIMS). Compared to TIMS, the ICP-MS method allows smaller sample size and higher sample throughput due to higher sensitivity, fewer sample preparation steps and shorter measurement times. However, mass biases, intensity biases, spectral interferences and instrumental blanks are significant and must be addressed.

A high-resolution record of holocene climate change in speleothem calcite from cold water cave, northeast iowa.

High-precision uranium-thorium mass spectrometric chronology and 18O-13C isotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in †18O and †13C isotopic values coincide with well-documented Holocene vegetation changes. Temperature estimates based on 18O/16O ratios suggest that the climate warmed rapidly by about 3�C at 5900 years before present and then cooled by 4�C at 3600 years before present. Initiation of a gradual increase in †13C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.

Sea-Level Highstand 81,000 Years Ago in Mallorca

Standing High Sea-level rises and falls as Earths giant ice sheets shrink and grow. It has been thought that sea level around 81,000 years ago—well into the last glacial period—was 15 to 20 meters below that of today and, thus, that the ice sheets were more extensive. Dorale et al. (p. 860; see the Perspective by Edwards) now challenge this view. A speleothem that has been intermittently submerged in a cave on the island of Mallorca was dated to show that, historically, sea level was more than a meter above its present height. This data implies that temperatures were as high as or higher than now, even though the concentration of CO2 in the atmosphere was much lower. Measurements from the island of Mallorca indicate that past sea levels were much higher than had been assumed. Global sea level and Earth’s climate are closely linked. Using speleothem encrustations from coastal caves on the island of Mallorca, we determined that western Mediterranean relative sea level was ~1 meter above modern sea level ~81,000 years ago during marine isotope stage (MIS) 5a. Although our findings seemingly conflict with the eustatic sea-level curve of far-field sites, they corroborate an alternative view that MIS 5a was at least as ice-free as the present, and they challenge the prevailing view of MIS 5 sea-level history and certain facets of ice-age theory.

Hydroclimate changes across the Amazon lowlands over the past 45,000 years

Reconstructing the history of tropical hydroclimates has been difficult, particularly for the Amazon basin—one of Earth’s major centres of deep atmospheric convection. For example, whether the Amazon basin was substantially drier or remained wet during glacial times has been controversial, largely because most study sites have been located on the periphery of the basin, and because interpretations can be complicated by sediment preservation, uncertainties in chronology, and topographical setting. Here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide. Our results are based on a decadally resolved, uranium/thorium-dated, oxygen isotopic record for much of the past 45,000 years, obtained using speleothems from Paraíso Cave in eastern Amazonia; we interpret the record as being broadly related to precipitation. Relative to modern levels, precipitation in the region was about 58% during the Last Glacial Maximum (around 21,000 years ago) and 142% during the mid-Holocene epoch (about 6,000 years ago). We find that, as compared with cave records from the western edge of the lowlands, the Amazon was widely drier during the last glacial period, with much less recycling of water and probably reduced plant transpiration, although the rainforest persisted throughout this time.

Comparison of multiple proxy records of Holocene environments in the midwestern United States

We compare four emerging approaches to reconstructing Holocene vegetation and climate from south of the glacial border in northeastern Iowa, United States. Pollen, plant macrofossils, carbon isotopic (δ 13 C) values from alluvial organic matter, and carbon isotopic values in stalagmites from a nearby cave all show similar paleovegetational and paleoclimatic trends during the Holocene. Pollen and plant macrofossils show a rapid change from forest to prairie about 6000 cal. yr B.P., followed by a return of oaks to a presumably savanna-like community about 3500 cal. yr B.P. The δ 13 C values in alluvial organic matter and the percentage of C 4 plants both increase ca. 6300 cal. yr B.P., and then decrease in the last 3500 years. In the cave, δ 13 C values rise beginning at 6000 cal. yr B.P. to a broad peak ca. 4500 to 3000 cal. yr B.P., and decrease thereafter. Pollen and plant macrofossils record the composition of the vegetation that produced the isotopic signals, and verify C 3 -C 4 interpretations based on the isotopic records. We demonstrate that these methods are complementary, but that any single method will provide an accurate reconstruction of past environments.

Uranium-series dating of speleothems: Current techniques, limits & applications

Carbonate speleothems that contain ppb-ppm levels of uranium can be dated by the 238U-234U-230Th and 235U-231Pa disequilibrium techniques. Accurate ages are possible if the initial concentrations of 230Th and 231Pa are well constrained and if the system has remained closed to post-depositional exchange of uranium, thorium, and protactinium. An estimate of the plausible range of initial 230Th/232xh values combined with the measured 232Th/238U ratio reveal, to first order, whether initial 230Th is significant. If significant, initial 230Th can be constrained by monitoring 232Th and employing isochron techniques to closely estimate the value of the contaminant 230Th/232Th ratio. The sensitivity of the age error to uncertainties in the initial 230Th/232Th ratio decreases with increasing U concentration, increasing age, and decreasing detrital contamination. For many speleothems, particularly those composed of nearly pure calcite, initial 230Th may be trivial. Because Pa is more soluble than Th, 232Th is a poor analog for 231Pa. Therefore, initial 231Pa tends to be more significant than initial 230Th for young samples, although this problem becomes less significant or even insignificant with increasing age. Closed-system behavior can be judged by petrographic considerations, stratigraphic ordering of ages, and 230Th-231Pa concordance. Thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS) offer significant improvements in counting efficiency and sample throughput compared to traditional decay-counting techniques. For ∼ 100 mg samples containing ∼ 1.0 ppm 238U, bestscenario 230Th age errors, based on the analytical errors (2a) of current TIMS and ICP-MS techniques, are approximately: 500 ± 6 yrs, 10,000 ± 40 yrs, 50,000 ± 180 yrs, 120,000 ± 500 yrs, and 500,000 ± 15,000 yrs. Materials as young as tens of years and older than 600,000 years are potentially dateable by the 238U-234U-230Th method.

Synchronous millennial-scale climatic changes in the Great Basin and the North Atlantic during the last interglacial

Stalagmites from Goshute Cave, located in the Great Basin of the western United States, preserve ~20,000 yr of millennial-scale oxygen isotopic variability during marine isotope stages 5c and 5b, similar in timing and structure to Dansgaard-Oeschger (D-O) events 23–21 from the Greenland Ice Sheet Project 2 record. That D-O interstades 23–21 were of longer duration than many of the later D-O events, coupled with the asymmetric shape of the D-O oxygen isotope curve, and the direct U-Th dating of the Goshute Cave stalagmites, allows for an improved understanding of the synchroneity of climatic changes between the western continental United States and the North Atlantic. Eastern Pacifi c–atmosphere interactions are a likely mechanism for transmission of millennial-scale climate variability into the Great Basin.

Measurements of natural uranium concentration and isotopic composition with permil‐level precision by inductively coupled plasma–quadrupole mass spectrometry

A new analytical technique using inductively coupled plasma–quadrupole mass spectrometry (ICP-QMS) has been developed that produces permil-level precision in the measurement of uranium concentration ([U]) and isotopic composition (δ234U) in natural materials. A 233U-236U double spike method was used to correct for mass fractionation during analysis. To correct for ratio drifting, samples were bracketed by uranium standard measurements. A sensitivity of 6–7 × 108 cps/ppm was generated with a sample solution uptake rate of 30 μL/min. With a measurement time of 15–20 min, standards of 30-ng uranium produced a within-run precision better than 3‰ (±2 R.S.D.) for δ234U and better than 2‰ for [U]. Replicate measurements made on standards show that a between-run reproducibility of 3.5‰ for δ234U and 2‰ for [U] can be achieved. ICP-QMS data of δ234U and [U] in seawater, coral, and speleothem materials are consistent with the data measured by other ICP-MS and TIMS techniques. Advantages of the ICP-QMS method include low cost, easy maintenance, simple instrumental operation, and few sample preparation steps. Sample size requirements are small, such as 10–14 mg of coral material. The results demonstrate that this technique can be applied to natural samples with various matrices.

Age of the Laschamp excursion determined by U-Th dating of a speleothem geomagnetic record from North America

This project was funded by NSF-EAR grant 1316385, a University of Minnesota McKnight Land Grant Professorship to JMF, and ERC grant 320750. Confocal microscopy was performed at the University of Minnesota Imaging Centers. We are grateful to John Geissman, Brad Singer, and James Channell for their constructive reviews. This is Institute for Rock Magnetism contribution 1506.

Isotopic evidence for Younger Dryas aridity in the North American midcontinent

Determining the impact of the Younger Dryas (YD) climate event on the unglaciated North American midcontinent has proved difficult due to a scarcity of suitable paleoclimate proxies. Here we present a well-dated carbon isotope (δ13C) record from a buried soil sequence in southwestern Missouri, which reveals a large isotopic excursion during the YD chronozone. In this region of the modern prairie-forest border, the δ13C signature of soil organic matter is a reliable indicator of past climatic change because δ13C values are controlled primarily by the relative abundance of C3 and C4 plants, which is tied to the environmental setting. Between ca. 13,200 and 11,900 yr ago, the abundance of C4 grasses increased by upwards of 50% of the total biomass, indicating expansion of grassland most likely driven by increased aridity during this period. Environmental gradients in the midcontinent must have been very steep, because at the same time that a C4-rich prairie existed in southwestern Missouri, spruce forests grew in Iowa, Illinois, and Ohio.