230Th Age Research Articles

Dual pathways of aragonite-to-calcite transformation in stalagmites: Implications for paleoclimate reconstructions

Multiple geochemical proxies (e.g., trace elements, δ18O and δ13C) and precise 230Th ages in speleothems are crucial for conducting robust paleoclimatic reconstructions, but the impact of diagenesis on these proxies and 230Th ages is not well-established. In this study, we investigate the petrography and geochemical composition of six aragonite-calcite stalagmites from Shennong Cave in southeastern China to analyze the transformation from aragonite to calcite (primary versus secondary) and its effects on geochemical proxies and 230Th ages. No aragonite relics are observed in columnar calcite in four aragonite-calcite stalagmites (SN55, SN49, SN104 and SNB1), although traces of aragonite exist between the calcite crystals or grow parallel at the aragonite-calcite borders. This is different from the obvious aragonite relics preserved in secondary mosaic calcite in the other two aragonite-calcite stalagmites (SN3 and SN15). Both mosaic and columnar calcite show various changes in geochemical proxies and 230Th ages compared with primary aragonite. The trend in the geochemical proxies analyzed (trace elements, δ18O and δ13C) of columnar calcite remains consistent with those of aragonite, despite significant differences in their absolute values within the overlapping periods. These differences are due to the different fractionation coefficients of isotopes and trace elements in aragonite and calcite formed under the same solution conditions rather than late diagenesis. The 230Th ages of columnar calcite are similar to those of primary aragonite, and the values of subsamples from the same calcite layer are consistent. In contrast, the trace elements and δ13C values of mosaic calcite are similar to those of primary aragonite, while its 230Th age significantly differs from that of primary aragonite, exhibiting no consistent patterns. Therefore, these geochemical characteristics of columnar calcite are significantly different from secondary mosaic calcite, indicating that the columnar calcite in these four aragonite-calcite stalagmites is primary. A comparison of trace elements and δ13C records between aragonite-calcite and pure aragonite stalagmites during the overlapping period further confirms that these columnar calcites are primary. Their geochemical proxies can be used for paleoclimate reconstruction after careful evaluation, and the transformation from aragonite to columnar calcite is caused by increased precipitation. However, due to age and geochemical proxies change, secondary mosaic calcite is unsuitable for paleoclimate reconstruction, unlike columnar calcite. This study provides competent evidence to identify whether calcite in aragonite-calcite stalagmites is primary versus secondary and whether it can be used for paleoclimatic reconstruction.

Testing the initial 230Th/232Th for “Known Age Carbonate” and its significance for 230Th dating and paleoclimate research

The 230Th dating (238U–234U–230Th dating method) has become one of the most important dating methods for Quaternary research. However, the 230Th/232Th ratio of 4.4 ± 2.2ⅹ10−6 is widely used for age correction in 230Th dating method. Here, we collected active speleothems with exactly known age and zero-year-old calcium carbonate from caves and dated them by 230Th dating. Our data indicated that the initial 230Th/232Th ratio can be one order of magnitude higher than the ratio of 4.4 ± 2.2 × 10−6. High initial 230Th/232Th ratio may result in false and comparatively old calculated age than the real age of young samples with low 238U concentration. So, the accurate initial 230Th/232Th ratio of active speleothem deposited in the same cave, instead of the general initial 230Th/232Th ratio, is essential for the precise age correction of young speleothem samples of low 238U concentration. Finally, we proposed a procedure to correlate modern observation and speleothem based paleoclimate reconstruction, on the basis of precise 230Th age which corrected by the true initial 230Th/232Th ratio.

Late Quaternary sporadic development of Desmophyllum dianthus deep-coral populations in the southern Labrador Sea with specific attention to their 14C- and 230Th-dating

About 70 fossil and live-collected specimens of Desmophyllum dianthus were collected from steep rocky surfaces and time-averaged assemblages at their base (“coral graveyards”) in the southern Labrador Sea at depths ranging from ~1700 m (Orphan Knoll) to ~2200 m (Flemish Cap). Aside from stable isotope and mineralogical analysis, U-series and 14C age determinations were performed on the samples. Fossil corals from the coral graveyards display two principal age clusters corresponding respectively to the Holocene and Marine Isotope Stage (MIS) 5c intervals. Three samples lay outside these clusters, at ~13.5 ka (Bølling-Allerød), ~ 64 ka (MIS 3) and at ~181 ka (MIS 7a). The clusters are thought to record intervals with both i) high food availability, either through pelagic primary production in the overlying water column or more effectively in relation with particulate and dissolved organic carbon transport via an active Western Boundary Undercurrent (WBUC), and ii) high carbonate micrite advection by this current from more open North Atlantic coccolithophorid production areas, maintaining an aragonite saturation horizon (now at 2300 m) slightly deeper than the coral habitat (~ 2200 m). Attempts at estimating ventilation ages from 230Th vs 14C age offsets in Holocene samples failed. This is due to uncertainties about the initial 14C-apparent age of coral skeletons and to their high initial excesses in 230Th. This initial excess relates mostly to the scavenging of 230Th produced by the dissolved marine U by the coral-feeding organic matter carried along the WBUC trajectory. In addition, discrete diagenetic U-fluxes can be documented in fossil samples. They are well illustrated in specimens from MIS 5c age, which depict variable excesses in 234U vs 238U, but similar 230Th/238U activity ratios, pointing to a recent but fractionating redistribution of uranium in the samples. These are also documented by the development of FeMn coatings of specimens exposed at the sea floor, due to the winnowing of embedding slope sediments, with the mid- to late Holocene intensified WBUC. Such diagenetic processes are to be notably taken into consideration for the dating of pre-Holocene samples. Thus, both the initial isotopic properties of corals from this WBUC-influenced, mid-continental slope, and subsequent diagenetic effects impact their calculated ages, and blur estimates of “ventilation ages” based on their 14C vs 230Th-ages offsets.

Age and sedimentary characteristics of beach sediments from Yongle Atoll, South China Sea: Implications for sediments supply in a coral reef system

Low-lying reef islands constructed from unconsolidated sediments are of growing interest due to concerns over the future stability of island landforms in response to climatic change and sea-level rise. Sediment characteristics, combined with the constituents and physical processes, are fundamental to understand the links between sediment production and their supply to an island’s beach reservoir. The grain size, constituents, and 230Th age of beach sediments from five islands at Yongle Atoll in the South China Sea were analyzed. Data reveal that coral grains are the dominant constituents of the beach sediments, followed by mollusks, coralline algae, foraminifera, Halimeda, and bryozoans. The mean grain sizes of the beach sediment were within a range of 226–4215 μm, and the coarsest sands were located near the passages in the atoll rim. Transportation by tropical cyclone–generated waves and winnowing by tides are potentially the main factors that influenced the grain-size characteristics. Calibrated 230Th age dates for bulk sand samples date the beach sediment between 424 yr BP and 5953 yr BP. Dating of coral sand samples from the beach sediment provides ages of 686–5780 yr BP; the coral sands from a vertical section (1.2 m depth) located on the beach at Jinqing Island were dated at 1154–3824 yr BP. The age reversal on the vertical section, with similar ages of beach sediment on any single island and different age ranges on different islands in an atoll, indicates that the coral sediment supply processes occurred within a relatively close time period and that different islands had different sediment supply rates. The temporal link between the coral-dominated sediment production and the beach reservoir provides an important insight into understanding the sediments supply mechanism of atoll islands in the northwestern Pacific Ocean.

Timing and structure of the weak Asian Monsoon event about 73,000 years ago

Abstract Oxygen isotope (δ18O) records of cave speleothem have played an important role in the past two decades in characterizing the Asian Monsoon (AM) variability and correlating the monsoonal events with other global climate events on millennial timescales. Of a series of millennial events occurred during the last glacial period, the Chinese Stadial-20 (CS-20, corresponding to the Greenland Stadial-20, GS-20) event around ∼73 kyr BP (thousand years before present, where present = 1950 AD) is distinctive, since it is the weakest AM event during the last glacial period and is likely linked to the Toba volcanic super-eruption. While Greenland ice core records are commonly used to correlate the last glacial millennial events, yet their absolute age uncertainties around the GS-20 is larger than 1000 years. This prohibits precise correlations of the event between global climate archives from different climate systems to investigate the underlying climatic dynamics. Here, we present three Chinese cave stalagmite δ18O records from the AM region, covering a period from 76 to 71 kyr BP, across the CS-20. All stalagmites have high uranium contents and relatively fast growth rates, allowing acquisition of high-resolution (∼10 years) δ18O records with precise 230Th age controls (≤200 years, 2σ) to precisely characterize the structure and timing of the CS-20. Our results demonstrate that the onset and termination of the CS-20 are more gradual in Chinese cave records relative to the GS-20 event in Greenland ice core records. As such, we suggest a new ‘break-point’ approach to correlate CS-20 with GS-20, at either the initial onset or initial termination shifts of the events, instead of the conventional ‘mid-point’ match. We dated the initial onset and initial termination of CS-20 to ~74.0 ± 0.2 and 72.5 ± 0.2 kyr BP, respectively, confirming the Greenland ice core chronology well within the quoted uncertainty. The ‘break-point’ correlation at the GS-20/CS-20 initial termination suggests a lagged onset of CS-20 relative to the onset of GS-20. The lagged onset is in line with a northern high-latitude forcing mechanism triggering the event and a central role of oceanic reorganizations in the propagation of the climate signal. An alternative ‘break-point’ correlation at the GS-20/CS-20 initial onset suggests that the CS-20 initial termination leads the GS-20 initial termination by a few hundred years. This apparent paradox thus calls for further empirical and theoretical studies to better understand the underlying climatic dynamics and in turn the correlation strategy.

Evolution of <sup>231</sup>Pa and <sup>230</sup>Th in overflow waters of the North Atlantic

Abstract. Many palaeoceanographic studies have sought to use the 231Pa∕230Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As of yet, however, no study has fully assessed the concentration of, or controls on, 230Th and 231Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231Pa and 230Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. 230Th and 231Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 µBq kg−1 and between 0.37 and 4.80 µBq kg−1, respectively. Both 230Th and 231Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Assessing this dataset using extended optimum multi-parameter (eOMP) analysis and CFC-based water mass age indicates that the low values of 230Th and 231Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. The importance of water age is confirmed for 230Th by a strong correlation between 230Th and water mass age (though this relationship with age is less clear for 231Pa and the 231Pa∕230Th ratio). Scavenged 231Pa and 230Th were estimated and compared to their potential concentrations in the water column due to ingrowth. This calculation indicates that more 230Th is scavenged (∼80 %) than 231Pa (∼40 %), consistent with the relatively higher particle reactivity of 230Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young overflow waters. Calculation of the meridional transport of 230Th and 231Pa with this new GEOVIDE dataset enables a complete budget for 230Th and 231Pa for the North Atlantic. Results suggest that net transport southward of 230Th and 231Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicate that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of 230Th and 74 % of the production for 231Pa. This result confirms a significantly higher advective loss of 231Pa to the south relative to 230Th and supports the use of 231Pa∕230Th to assess meridional transport at a basin scale.

Magnetostratigraphy and 230Th dating of a drill core from the southeastern Qaidam Basin: Salt lake evolution and tectonic implications

The Qarhan Salt Lake area is the Quaternary depocenter of the Qaidam Basin, and carries thick lacustrine sediments, as well as rich potassium and magnesium salt deposits. The abundant resources and thick sediments in this lake provide an ideal place for the study of biogas formation and preservation, salt lake evolution, and the uplift of the Tibetan Plateau. In this study, we attempt to construct a paleomagnetic and 230Th age model and to obtain information on tectonic activity and salt lake evolution through detailed studies on a 1300-m-long drill core (15DZK01) from the northwestern margin of the Qarhan Salt Lake area (Dongling Lake). Based on gypsum 230Th dating, the age of the uppermost clastic deposit was calculated to be around 0.052 Ma. The polarity sequence consist of 13 pairs of normal and reversed zones, which can be correlated with subchrons C2r.1r-C1n of the geomagnetic polarity timescale (GPTS 2012) (from ∼2.070 Ma to ∼0.052 Ma). Sedimentary characteristics indicate that Dongling Lake witnessed freshwater environment between ∼2.070 Ma and 1.546 Ma. During this period, the sedimentary record reflects primarily lakeshore, shallow-water and swamp environments, representing favourable conditions for the formation of hydrocarbon source rocks. Between 1.546 Ma and ∼0.052 Ma, the Dongling Lake was in sulphate deposition stage, which contrasts with the central Qarhan Salt Lake area, where this stage did not occur in the meantime. During this stage, Dongling Lake was in a shallow saltwater lake environment, but several periods of reduced salinity occurred during this stage. During the late Pleistocene at ∼0.052 Ma, the Dongling Lake experienced uplift due to tectonic activity, and saltwater migrated through the Sanhu Fault to the central Qarhan Salt Lake area, resulting in the absence of halite deposition stage. The residual saline water was concentrated into magnesium-rich brine due to the lack of freshwater, and few potassium salt deposits occur in the Dongling Lake area.

Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand

Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.

Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U–230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238U–230Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in 176Hf/177Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.

Improvements in 230Th dating, 230Th and 234U half-life values, and U–Th isotopic measurements by multi-collector inductively coupled plasma mass spectrometry

We have developed techniques for measuring 234U and 230Th on Faraday cups with precisions of 1–3 epsilon units (1 ε-unit=1 part in 104) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Using a Thermo-Scientific Neptune with desolvation nebulization, we obtained ionization/transmission efficiencies of 1–2% for both U and Th. We set up protocols to correct for tailing, prepared U and Th gravimetric standards, tested a Th mass fractionation correction procedure based on U isotopes, and identified natural calcite samples likely to be in U–Th isotopic secular equilibrium. The measured atomic ratios, 234U/238U=54.970 (±0.019)×10−6 and 230Th/238U=16.916 (±0.018)×10−6, for these calcite samples were identical within errors (quoted 2σ uncertainties calculated combining all sources of error). Half-life values calculated from these ratios are consistent with previous values, but have much smaller errors: 245,620±260a for 234U and 75,584±110a for 230Th (quoted 2σ uncertainties calculated using all sources of error). In calculating a 230Th age, some of the systematic errors included in estimating the full error in the half-lives effectively cancel. Removing these uncertainties (uncertainty in the 238U half-life value, uncertainty in our gravimetric uranium and thorium standards, and uncertainty in the absolute isotopic composition of the uranium standard), yields effective uncertainties for the purposes of 230Th dating of ±70a for the 234U half-life value and ±30a for the 230Th half-life value. Under ideal circumstances, with our methods, the 2σ uncertainty in age, including uncertainty in half-life values is ±10a at 10ka, ±100a at 130ka, ±300a at 200ka, ±1ka at 300ka, ±2ka at 400ka, ±6ka at 500ka, and ±12ka at 600ka. The isotopic composition of a sample with an age <800ka can clearly be resolved from the isotopic composition of a sample in secular equilibrium, assuming closed system behavior. Using these techniques, we analyzed a Sanbao Cave (Hubei, China) stalagmite that formed between 510 and 640ka ago. As the half-life values were determined independent of the Sanbao Cave ages, the observed co-variation between stalagmite δ18O and Northern Hemisphere summer insolation is consistent with accurate ages and half-life values.

Constraints on the nature of the subvolcanic reservoir at South Sister volcano, Oregon from U-series dating combined with sub-crystal trace-element analysis of plagioclase and zircon

We present sub-crystal-scale 238U– 230Th zircon ages and 238U– 230Th– 226Ra plagioclase ages of bulk mineral separates from the Holocene (2.0–2.3 ka) eruptions of the Rock Mesa (RM) and Devil's Hills (DH) rhyolites at South Sister volcano, Oregon. We link these age data with sub-crystal trace-element analyses of zircon and plagioclase to provide insight into the subvolcanic system at South Sister, as an example of a small-volume continental arc volcano. Our results document the presence of coeval yet physically-distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed. Zircons from the RM and DH dominantly record ages from 20 to 80 ka, with some grains recording ages > 350 ka, whereas plagioclase records 230Th– 226Ra ages of 2.3–6.8 ka (RM) and 4.0–9.6 ka (DH-3) and a 238U– 230Th age of 10 ± 34 ka (DH-3). We interpret zircons with ages < 350 ka as antecrysts inherited from a longer lived upper-crustal magma reservoir from which the rhyolites were generated, based on crystallization ages coeval with earlier periods of silicic volcanism at South Sister, the undersaturated nature of the RM and DH magmas with respect to zircon, and Ti-in-zircon temperatures consistent with low-temperature (< 815 °C) crystallization. In contrast, plagioclase ages are near the eruption age and dominantly preserve information about the recent (< 10 ka), higher-temperature evolution of the host magmas. Although zircon and plagioclase record different crystallization ages, each phase crystallized over the same time period in the RM compared to DH rhyolites. Linking these crystal age data with sub-crystal trace-element analyses demonstrates that zircon and plagioclase have distinct trace-element characteristics between eruptions, which require that the RM and DH crystals (and therefore magmas) were derived from distinct regions that had evolved independently for > 50 ka within a heterogeneous magmatic system and coexisted as physically-distinct, dominantly-liquid bodies prior to eruption. Thus, we favor a model where rhyolites are generated in independent batches by accumulation of evolved liquids in a heterogeneous, largely crystalline reservoir. Similarities in crystal age and chemical data to that at other young silicic systems (e.g., Mount St. Helens, Okataina Caldera Complex) suggest that this model may be more generally applicable to silicic magmas.

Imprint of solar activity on Nanjing stalagmite annual layer thickness sequence during the Last Glacial Maximum

A 3000-year-long stalagmite chronology from Hulu Cave near Nanjing was established by counting annual layers under microscope. Based on the 230Th age, this chronology covers the period 24-21 kaBP, within the Last Glacial Maximum (LGM). Two proxies, annual layer thickness and gray level were measured along the growth axis of the stalagmite profile in order to establish a high-resolution East Asian monsoon history during the LGM. The high correlation coefficient (r = 0.55) between the two proxies suggests that both of them were controlled by a common factor, possibly reflecting changes in the strength of summer monsoon circulation and its precipitation. Low frequency variations of the annual layer thickness, ranging from centennial to millennial scales, are approximately in agreement with the 10Be-flux recorded in the Greenland ice core, indicating that changes in East Asian monsoon strength might be forced by solar outputs during the LGM periods. In support of this, Fourier power spectrum analysis of the annual layer thickness showed certain decadal to centennial-scale cycles that agree well with the periodicities of solar activity.

Petrogenesis and magma residence time of lavas from Tengchong volcanic field (China): Evidence from U series disequilibria and 40Ar/39Ar dating

We present U‐Th disequilibria measurements as well as radiogenic and trace element data for volcanic rocks erupted over the past ∼200,000 years at Tengchong volcano field (TVF), near the collisional belt of India‐Asia, southwestern China. Mineral separates, matrix, and whole rock were analyzed separately by mass spectrometry for 230Th‐238U disequilibria. Whole rocks are characterized by either 230Th enrichment or depletion relative to 238U. Three mineral internal 230Th age isochrons and a CO2 laser 40Ar/39Ar age were determined. A difference of ∼40 ka between the 230Th age and the 40Ar/39Ar age can be interpreted as the residence time of the associated magma. Large variations in Sr, Nd, and Pb isotopic compositions and trace element ratios suggest more than one source for the TVF lavas. High Th contents, Th/U ratios and LILE element abundances, as well as their large variations can be attributed to partial melting of mantle previously metasomatized by subduction‐derived components. Excesses of 238U in some lavas could be the feature of another end‐member, an arc‐type melt, in the source. Fast extraction of the melts may result in the observed chemical disequilibrium.

U-Th dating of Lake Lisan (late Pleistocene dead sea) aragonite and implications for glacial east Mediterranean climate change

U-series dating can be an effective means to obtain accurate and precise ages on Quaternary carbonates. However, most samples require a correction for U and Th in admixed detritus. This complication is often addressed through generation of U-Th isochrons, requiring analyses of several coeval samples. In addition, presence of water-derived (hydrogenous) Th in the carbonate can cause inaccuracies in isochron ages. This study reports a high-resolution U-series chronology of sediments deposited by Lake Lisan, the last glacial precursor of the Dead Sea. The strategy employed combines multiple measurements from a few stratigraphic heights and fewer analyses from many heights in a single described and measured section. The resulting chronology is based on ages at 22 heights in a ∼40-m-thick section covering the interval of ∼70–14 calendar ka BP. The effects of admixed detritus are evaluated using trace elements. Nearly pure aragonite samples, indicated by very low abundances of insoluble elements such as Nb and Zr, were found to contain hydrogenous Th, which causes the uncorrected U- 230Th age of a modern sample to be ∼2.5 ka. Nevertheless, accurate ages have been obtained by correcting for the detrital and aqueous interferences. The resulting ages are in stratigraphic order, and their accuracy is evidenced by consistency of Lisan Formation U-series and 14C ages with the coral-based calendar-radiocarbon age calibration. The U-Th ages provide a context to unravel the limnological history of Lake Lisan. Boundaries between the Lower, Middle, and Upper stratigraphic units correspond to the MIS 4/3 and 3/2 transitions, respectively. During MIS 2 and 4 the lake generally showed a stable two-layer configuration and a positive fresh-water balance, reflected by deposition of laminated aragonite-detritus. Dry intervals during MIS 2 and 4 are indicated by thick gypsum layers and an inferred depositional hiatus, which are temporally associated with Heinrich events H1 at ∼17 ka and H6 at ∼65 ka, respectively. During MIS 3 the lake level was unstable with intermittent dry periods indicated by abundant clastic layers and a significant hiatus between ∼43–49 ka. Clastic layers are associated with Dansgaard-Oeschger events during MIS 3, and indicate lake level declines during abrupt Northern Hemisphere warmings. Overall, the climate of the Eastern Mediterranean region shows a strong linkage to the Northern Hemisphere climate, with increasing lake size and stability during cold periods, and fluctuations and dessication during warmings and Heinrich events.

Dissolved and particulate 231Pa and 230Th in the Atlantic Ocean: constraints on intermediate/deep water age, boundary scavenging, and 231Pa/230Th fractionation

231Pa and 230Th concentrations were determined in filtered seawater and suspended particulate matter collected from the Labrador Sea and the Equatorial and South Atlantic to constrain their application as tracers of intermediate/deep water age and Atlantic thermohaline circulation. Distributions of total 231Pa and 230Th indicate the influence of recently formed North Atlantic Deep Water, as evidenced by nearly invariant concentrations below ∼1000 m in the Labrador Sea and increasing 231Pa and 230Th concentrations as deep waters progress southward from northern source regions. Application of a scavenging–mixing model to both tracer distributions indicates an intermediate/deep water age of 12 yr in the Labrador Sea and a ∼30–140 yr transit time to the low-latitude stations. We attribute a striking increase in total 230Th in the Labrador Sea from 1993 to 1999 to aging of intermediate waters as a consequence of the cessation of deep convection in the Labrador Sea since 1993. The temporal change in the 230Th age of these waters is consistent with the 6 yr time interval between the observations. The average particulate 231Pa/230Th activity ratio in the Labrador Sea and low-latitude deep waters is 0.057±0.003, significantly below the 231Pa/230Th production ratio (0.093) and in agreement with excess 231Pa/230Th ratios in Holocene sediments (0.060±0.004) and sediment trap material (0.034±0.012) from the Atlantic and model simulations. This observation is consistent with the southward transport of deep water strongly attenuating boundary scavenging in the Atlantic. A latitudinal dependence in particle fractionation of these tracers is also evident, with elevated fractionation factors (FTh/Pa) observed near the Equator and South Atlantic gyre (∼11) compared to low values in the Labrador Sea (∼3) and Southern Ocean (∼2). There also exists a depth dependence in FTh/Pa, characterized by low values in surface waters, a broad mid-depth maximum, and decreasing values towards the sea-floor. The latitudinal and depth variations in FTh/Pa are suggested to reflect differences in the chemical composition of marine particles.

Ion microprobe U–Pb dating of Quaternary zircon: implication for magma cooling and residence time

An ion microprobe U–Pb dating method of Quaternary zircon is developed using the sensitive high resolution ion microprobe (SHRIMP) and applied to igneous rocks from Japan. Total Pb/U isochron age of 1.36±0.23 Ma (95% confidence limit (CL); MSWD=1.03) was obtained for zircons extracted from Takidani granodiorite in central Japan. This may provide a constraint on magma cooling when coupled with fission-track ages of the zircon as well as Rb–Sr and K–Ar ages on co-existing minerals. The closure temperature determined by each method has lead to exponentially declining cooling history. The history is consistent with a simple conductive cooling model of a small magma body (10 2–10 3 km 3). A Pb/U isochron age of 271±58 ka (95% CL; MSWD=0.85) has been determined for volcanic zircons from Torihama dacite, SW Japan. The age is significantly older than the 238U– 230Th (Ionium) age of 145±33 ka and fission-track age of 163±8 ka found in the literature, suggesting either the magma residence time of ∼100 kyr or a rapid cooling rate of about 6°C/kyr. In contrast a tephrochronological estimate of 240±10 ka agrees well with the observed Pb/U age, implying that the Ionium and fission-track ages are incorrect. The Pb/U isochron age of 90±29 ka (95% CL; MSWD=1.04) is obtained for zircons in pyroclastic rocks from the Unzen eruption in 1991, SW Japan. The age is again older than the eruptive age, which suggests a magma residence time of ∼90 kyr in a shallow reservoir.

High-precision in situ 238U– 234U– 230Th isotopic analysis using laser ablation multiple-collector ICPMS

We have developed a method for the rapid, in situ measurement of U–Th isotopic compositions at the semimicro scale using laser ablation sampling, combined with multiple collector ICP magnetic sector mass spectrometry (MC-ICPMS). The system uses a Q-switched and frequency quadrupled 266 nm Nd:YAG laser to ablate samples containing 100 ppm levels of U at 150 μm scale resolution, corresponding to 1–4 ng 238U, ∼70–200 fg of 234U and 20–60 fg of 230Th consumed per analysis. Synthetic glass standards and naturally occurring samples of zircon and opal, with U contents of 460, 260, and ∼200 ppm, respectively, were used to assess the precision and accuracy of our laser ablation technique. Our initial experiments used argon as the plasma support gas. Thirty-seven laser analyses on the glass and 29 on the zircon give respective mean [ 234U/ 238U] act of 0.17114 ± 0.00022 and 1.0018 ± 0.0014 (2σ M), indistinguishable from the MC-ICPMS solution nebulization values of 0.17094 ± 0.00006 and 1.0011 ± 0.0009 (2σ M), respectively. The usual within-run precision obtained for both glass and zircon is ±3‰ at the 2σ M level. An additional 12 laser analyses on the opal give a mean [ 234U/ 238U] act of 0.9997 ± 0.0034, in excellent agreement with the expected secular equilibrium value of unity and a typical within-run precision of ±8‰ (2σ M). Our Nd:YAG laser, coupled with an all Ar gas system, produces large elemental fractionation effects between U and Th. Both 238U/ 232Th and [ 230Th/ 238U] act can be measured at the per mill level, but Th ion beams are suppressed relative to U. As a result, the Th/U ratios are systematically lower, and the apparent 238U– 234U– 230Th ages are systematically younger than the true values. The U–Th fractionation is primarily controlled by ionization conditions in the plasma, transportation efficiency of ablated particles, and the composition of the sample matrix. The use of helium instead of Ar in the ablation cell significantly improves the relative sensitivity of Th, and entirely eliminates the elemental fractionation between U and Th, while retaining accuracy and precision in U isotope measurement. With He, mean values for [ 230Th/ 238U] act of 0.996 ± 0.013 and 238U/ 232Th of 1.625 ± 0.092 were determined for the zircon standard, in excellent agreement with the solution nebulization values of 1.0042 ± 0.0016 and 1.6288 ± 0.0006 (2σ M), respectively. In an unknown sample, it is possible to determine correct values for [ 230Th/ 238U] act and 238U/ 232Th, with respective within-run uncertainties as good as 7‰ and 2‰, by monitoring the isotopic composition of a well characterized, matrix-matched standard. For high-U material, the combined uncertainties in [ 234U/ 238U] act and [ 230Th/ 238U] act routinely translate to 2σ M errors in the 238U– 234U– 230Th age of better than ±2,500 years in 100,000-year-old samples.

U-Pb dating of a speleothem of Quaternary age

We demonstrate that U-Pb dating is a promising method for secondary carbonate materials of Quaternary age and older by obtaining a 206Pb∗/ 238U age for a speleothem with high U (>10 μg g −1) and very low Pb (<10 ng g −1) that is supported by an independent 230Th age. Thermal ionisation mass-spectrometry was used to determine the U and Pb isotopic ratios and concentrations for subsamples of a stalactite from Winnats Head Cave, Peak District, UK. We obtained 206Pb/ 204Pb ratios up to 50, and determined a 206Pb∗/ 238U age of 248 ± 10 ka, which is within error of the 207Pb∗/ 235U age of 333 ± 79 ka and a-spectrometric U-Th age of ∼255 ka. For samples of Tertiary and Quaternary age, the initial state of U-series disequilibrium is an important consideration and, as with most radiometric dating techniques, the mineral must have remained closed to U, Th, Pb, and all intermediate daughters. We show that dense calcite speleothems are ideal in this respect and that no loss of Rn has occurred. Unlike U-series disequilibrium methods, U-Pb dating has no upper limit and, hence, materials of Quaternary age older than 0.6 Ma can be analysed to investigate landscape development, paleoclimate, hominid evolution or hydrogeochemistry in carbonate terrains.

Uranium-series dating of fossil corals from marine sediments of southeastern United States Atlantic Coastal Plain

Extensive low-lying marine deposits border the southeastern United States Atlantic Coastal Plain. Some units are fossiliferous and contain corals as isolated fragments in sediments of a detrital character. These corals are subject to alteration processes such that suites of related samples must be examined to determine the suitability of these coral samples for reliable uranium-series dating. With the exception of those from one location, most samples appear to have remained closed systems with respect to the isotopes of uranium and thorium throughout their geologic history. Extraneous 230Th has been detected in some of the corals due to incorporation of some detrital materials into their skeletons. For these samples, different methods are applied to correct for the initial 230Th contamination. Continued sampling and analyses have resulted in 55 individual uranium-thorium determinations. The average 230Th ages of samples from the Norfolk Formation, and from later- and earlier-deposited sediments of the Wando Formation are ∼71,000, 87,000, and 129,000 yr, and they appear to correlate with oxygen isotope substages 5a, 5c, and 5e, respectively. The average 230Th age of samples from beds of the Rappahannock River, Ponzer, and Ten Mile Hill localities is ∼212,000 yr, and they correlate with oxygen isotope stage 7. The sediment of the Canepatch Formation is ∼460,000, yr old, and it is tentatively correlated with oxygen isotope stage 11. There is general agreement between uranium-series and uranium-trend dates and between the quantitative trends of the amino acid data and uranium-series dates. The amino acid values, however, ure unacceptably high in at least two groups of samples, those from localities near Charleston, South Carolina, and from central Virginia.