Measurement of attogram quantities of 231Pa in dissolved and particulate fractions of seawater by isotope dilution thermal ionization mass spectroscopy

Links between climatic histories and the rise and fall of a Pacific chiefdom

Sea level rise and climate change are shaping present societies, particularly those on oceanic islands. Few historical examples could serve as references for these changes. One such potential model is the Saudeleur Dynasty with its capital Nan Madol on the Pacific Island of Pohnpei. However, the timing of its construction, as well as the dynasty's fluctuations and potential environmental influences, has remained unresolved. Through the analyses of 230Th ages on 171 dates on corals fragments used as building materials and charcoal 14C ages from excavations, 2 major construction phases spanning from the 10th to the 15th century CE can be discerned. The results show that the first phase of the site's construction, spanning the 10th-12th century, marked the dynasty's rise. The second period, spanning from the late 12th to the early 15th century, provides the most substantial evidence for the demise of the island-scale chiefdom and a significant societal reorganization. The phases are centuries earlier than previously believed. With this new evidence, we propose the hypothesis that variations in the El Niño-Southern Oscillation and subsidence-related sea level rise presented major challenges for building and maintaining Nan Madol, and thus, influenced the course of the island's history. This case serves as a compelling example of how adverse climatic conditions can spur investments-in this case, in seawater defense under high sea levels-yet ultimately may contribute to abandonment. It offers lessons for island nations, showcasing coastal resilience in the face of worsening catastrophic events that unfolded over generations.

Tracking westerly wind directions over Europe since the middle Holocene

The variability of the northern westerlies has been considered as one of the key elements for modern and past climate evolution. Their multiscale behavior and underlying control mechanisms, however, are incompletely understood, owing to the complex dynamics of Atlantic sea-level pressures. Here, we present a multi-annually resolved record of the westerly drift over the past 6,500 years from northern Italy. In combination with more than 20 other westerly-sensitive records, our results depict the non-stationary westerly-affected regions over mainland Europe on multi-decadal to multi-centennial time scales, showing that the direction of the westerlies has changed with respect to the migrations of the North Atlantic centers of action since the middle Holocene. Our findings suggest the crucial role of the migrations of the North Atlantic dipole in modulating the westerly-affected domain over Europe, possibly modulated by Atlantic Ocean variability.

Stratigraphic reassessment of Grotta Romanelli sheds light on Middle-Late Pleistocene palaeoenvironments and human settling in the Mediterranean

During the last century, Grotta Romanelli (Southern Italy) has been a reference site for the European Late Pleistocene stratigraphy, due to its geomorphological setting and archaeological and palaeontological content. The beginning of the sedimentation inside the cave was attributed to the Last Interglacial (MISs 5e) and the oldest unearthed evidence of human occupation, including remains of hearths, was therefore referred to the Middle Palaeolithic. Recent surveys and excavations produced new U/Th dates, palaeoenvironmental interpretation and a litho-, morpho- and chrono-stratigraphical reassessment, placing the oldest human frequentation of the cave between MIS 9 and MIS 7, therefore embracing Glacial and Interglacial cycles. These new data provide evidence that the sea reached the cave during the Middle Pleistocene and human occupation occurred long before MISs 5e and persisted beyond the Pleistocene- Holocene boundary.

Stalagmite-Inferred Climate in the Western Mediterranean during the Roman Warm Period

The circum-Mediterranean region is the cradle of ancient civilizations that had their roots in the Holocene. Climate change has been considered a key element that contributed to their rise or fall. The Roman Warm Period (RWP), 200 B.C. to 400 A.D., was the warmest period in Europe during the last two thousand years. Hydroclimatic change at the end of the RWP has been suggested as a possible influence on the stability of the Roman political regime and the eventual collapse of the Roman Empire in 476 A.D. A lack of precise proxy records hampers our understanding of hydroclimatic variability over the RWP. Here we present a stalagmite-based climate record from 550 ± 10 B.C. to 950 ± 7 A.D. (2σ) from northern Italy, which reveals a climatic trend of warming and increased humidity throughout the RWP. By comparison with other proxy records in Europe and the circum-Mediterranean region, we argue that the warm, humid climate in southern Europe could be linked to the multi-centennial warming of the Mediterranean Sea. Our record further suggests a century-long rapid drying trend from the early-4th to early-5th century, followed by a 100-year-long drought event, which could have influenced the fall of the Roman Empire.

Insights from barium variability in a Siderastrea siderea coral in the northwestern Gulf of Mexico

Coral Ba/Ca is a proxy for seawater barium concentration that varies with upwelling, terrigenous input, and marine productivity whereas coral Sr/Ca varies with temperature. We examine monthly coral Ba/Ca and Sr/Ca before and during offshore oil exploration in a Siderastrea siderea coral from West Flower Garden Bank located on the continental shelf edge in the Gulf of Mexico. Coral Ba/Ca variations lack pulses driven by upwelling or river outflow and are not in sync with coral Sr/Ca that exhibit a different seasonal pattern. Seasonal variations in chlorophyll-a concentration negatively correlate with coral Ba/Ca explaining 25% of that variability. A significant increase in mean coral Ba/Ca of 1.76 μmol/mol between 1931-1944 and 1976-2004 corresponds to the increase in the United States barite production and consumption primarily used in offshore oil drilling, which escalated in the 1970s, suggesting oil drilling operations are increasing seawater Ba concentration in the Gulf of Mexico.

Last glacial temperature reconstructions using coupled isotopic analyses of fossil snails and stalagmites from archaeological caves in Okinawa, Japan

We applied a new geoarchaeological method with two carbonate archives, which are fossil snails from Sakitari Cave and stalagmites from Gyokusen Cave, on Okinawa Island, Japan, to reconstruct surface air temperature changes over the northwestern Pacific since the last glacial period. Oxygen isotope ratios (δ¹⁸O) of modern and fossil freshwater snail shells were determined to infer seasonal temperature variations. The observational and analytical data confirm that δ¹⁸O values of fluid inclusion waters in the stalagmite can be regarded as those of spring waters at the sites where snails lived. Our results indicate that the annual mean, summer, and winter air temperatures were lower by 6–7 °C at ca. 23 thousand years ago (ka) and 4–5 °C at ca. 16–13 ka than those of the present day. Our reconstruction implies that surface air cooling was possibly two times greater than that of seawater around the Ryukyu Islands during the Last Glacial Maximum, which potentially enhanced the development of the East Asian summer monsoon during the last deglaciation. Considering the potential uncertainties in the temperature estimations, the climatic interpretations of this study are not necessarily definitive due to the limited number of samples. Nevertheless, our new geoarchaeological approach using coupled δ¹⁸O determinations of fossil snails and stalagmite fluid inclusion waters will be useful for reconstructing snapshots of seasonally resolved time series of air temperatures during the Quaternary.

Geochemical and U-Th isotopic insights on uranium enrichment in reservoir sediments

Uranium (U) geochemistry and its isotopic compositions of reservoir sediments in U mine area were poorly understood. Herein, U and Th isotopic compositions were employed to investigate source apportionment and geochemical behavior of U in 41 reservoir sediments from a U mining area, Guangdong, China. The remarkably high contents of both total U (207.3-1117.7 mg/kg) and acid-leachable U (90.3-638.5 mg/kg) in the sediments exhibit a severe U contamination and mobilization-release risk. The U/Th activity ratios (ARs) indicate that all sediments have been contaminated apparently by U as a result of discharge of U containing wastewater, especially uranium mill tailings (UMT) leachate, while the variations of U/Th ARs are dominated by U geochemical behaviors (mainly redox process and adsorption). The U isotopic compositions (δ²³⁸U) showed a large variance through the sediment profile, varying from - 0.62 to - 0.04‰. The relation between δ²³⁸U and acid-leachable U fraction demonstrates that the U isotopic fractionation in sediments can be controlled by bedrock weathering (natural activity), UMT leachate (anthropogenic activity) and subsequent biogeochemical processes. The findings suggest that U-Th isotopes are a powerful tool to better understand U geochemical processes and enrichment mechanism in sediments that were affected by combined sources and driving forces.

236U/238U Analysis of Femtograms of 236U by MC-ICPMS

A new analytical method has been developed to determine atomic ²³⁶U/²³⁸U ratios in samples with only femtograms of ²³⁶U using a secondary electron multiplier (SEM) on a multicollector high-resolution inductively coupled plasma mass spectrometer (MC-ICPMS). The abundance sensitivity of the ²³⁸U tail at 236 atomic mass unit is reduced from 10^-6 to 10^-10 with the deployment of a retarding potential quadrupole lens. This method features the reduction of polyatomic interferences from hydride, nitride, lead, and plutonium and the evaluation of nonlinear SEM behavior. The instrument sensitivity is 1-2%, and the estimated methodological detection limit of the ²³⁶U/²³⁸U atomic ratio is as low as 2 × 10^-10. Measurements on reference materials with ²³⁶U/²³⁸U ratios of 10^-7-10^-9, including the IRMM-075 series and the ETH Zurich in-house standard ZUTRI, demonstrate the accuracy of our MC-ICPMS technique. The analytical precisions (2σ) are ±4% for 5 fg of ²³⁶U at a ²³⁶U/²³⁸U of 1 × 10^-8 and ±8% for 2 fg of ²³⁶U at a ²³⁶U/²³⁸U of 4 × 10^-9 level. Compared to state-of-the-art accelerator mass spectrometry techniques and triple quadrupole-based ICPMS, our detection limit is not as low, but the required sample size is 3-40 times lower, and the throughput is as high as 3-4 samples per hour. The new MC-ICPMS-SEM technique is sensitive enough for determining ²³⁶U/²³⁸U in various small natural samples, such as marine carbonates and seawater.

Half-life determination and comparison of activity standards of 231Pa

The results of an international comparison of activity measurements of a solution of ²³¹Pa are reported and analysed. Prior to this, no known standardisation of ²³¹Pa by activity measurement had been carried out. The comparison was run in 2017-2018 involving eight laboratories, and returned results with no identifiable inconsistencies between methods or laboratories. The results, including one mass determination, gave a²³¹Pa activity concentration of 41.461(48) kBq g^-1 and a²³¹Pa atom concentration of 61.48(23) × 10¹⁵ atoms g^-1, from which a half-life value of 32 570(130) years was derived.

Multidecadally resolved polarity oscillations during a geomagnetic excursion

Polarity reversals of the geomagnetic field have occurred through billions of years of Earth history and were first revealed in the early 20th century. Almost a century later, details of transitional field behavior during geomagnetic reversals and excursions remain poorly known. Here, we present a multidecadally resolved geomagnetic excursion record from a radioisotopically dated Chinese stalagmite at 107-91 thousand years before present with age precision of several decades. The duration of geomagnetic directional oscillations ranged from several centuries at 106-103 thousand years before present to millennia at 98-92 thousand years before present, with one abrupt reversal transition occurring in one to two centuries when the field was weakest. These features indicate prolonged geodynamo instability. Repeated asymmetrical interhemispheric polarity drifts associated with weak dipole fields likely originated in Earth's deep interior. If such rapid polarity changes occurred in future, they could severely affect satellites and human society.

Separation of Protactinium Employing Sulfur-Based Extraction Chromatographic Resins

Protactinium-230 ( t_1/2 = 17.4 d) is the parent isotope of ²³⁰U ( t_1/2 = 20.8 d), a radionuclide of interest for targeted alpha therapy (TAT). Column chromatographic methods have been developed to separate no-carrier-added ²³⁰Pa from proton irradiated thorium targets and accompanying fission products. Results reported within demonstrate the use of novel sulfur bearing chromatographic extraction resins for the selective separation of protactinium. The recovery yield of ²³⁰Pa was 93 ± 4% employing a R₃P═S type commercially available resin and 88 ± 4% employing a DGTA (diglycothioamide) containing custom synthesized extraction chromatographic resin. The radiochemical purity of the recovered ²³⁰Pa was measured via high purity germanium γ-ray spectroscopy to be >99.5% with the remaining radioactive contaminant being ⁹⁵Nb due to its similar chemistry to protactinium. Measured equilibrium distribution coefficients for protactinium, thorium, uranium, niobium, radium, and actinium on both the R₃P═S type and the DGTA resin in hydrochloric acid media are reported, to the best of our knowledge, for the first time.

Improved Separation of Pa from Th and U in Marine Sediments with TK400 Resin

Protactinium-231 is a radionuclide of broad interest in paleoceanography and paleoclimatology. This study describes an improved method for the purification and separation of Pa from marine sediment samples using the new TK400 resin by TrisKem International. The focus lies on the improvement of the separation of the abundant ²³²Th from the Pa fraction of the sample, which would reduce the peak tailing from ²³²Th on masses 231 and 233 during ICP-MS measurement. Furthermore, the reusability of TK400 has been tested. For this purpose, the conventional method using Dowex AG1X8 for the separation and purification of Pa has been compared to methods using the TK400 resin. A combination of a Dowex AG1X8 prior to a TK400 column has shown most convincing results. Based on our results we suggest a new efficient procedural method of analyzing ²³¹Pa from marine sediment samples using TK400. Chemical Pa yields for a Dowex-TK400 combination are highest compared to the Dowex only method. Furthermore, the ²³²Th/²³¹Pa ratio of the Pa-fractions has been reduced by 1 order of magnitude compared to conventional methods with Dowex AG1X8. Additionally, the reusability of TK400 resin up to nine times has been proven. The usage of TK400 is only limited in the presence of samples with a high matrix load (e.g., Fe). Therefore, matrix from sediment samples needs to be removed (here using Dowex resin) before samples are loaded onto TK400. We also report a series of concentration measurements from standard reference materials (UREM-11, NIST 2702), which have been used for ²³³Pa calibration.

Synchronous precipitation reduction in the American Tropics associated with Heinrich 2

During the last ice age temperature in the North Atlantic oscillated in cycles known as Dansgaard-Oeschger (D-O) events. The magnitude of Caribbean hydroclimate change associated with D-O variability and particularly with stadial intervals, remains poorly constrained by paleoclimate records. We present a 3.3 thousand-year long stalagmite δ¹⁸O record from the Yucatan Peninsula (YP) that spans the interval between 26.5 and 23.2 thousand years before present. We estimate quantitative precipitation variability and the high resolution and dating accuracy of this record allow us to investigate how rainfall in the region responds to D-O events. Quantitative precipitation estimates are based on observed regional amount effect variability, last glacial paleotemperature records, and estimates of the last glacial oxygen isotopic composition of precipitation based on global circulation models (GCMs). The new precipitation record suggests significant low latitude hydrological responses to internal modes of climate variability and supports a role of Caribbean hydroclimate in helping Atlantic Meridional Overturning Circulation recovery during D-O events. Significant in-phase precipitation reduction across the equator in the tropical Americas associated with Heinrich event 2 is suggested by available speleothem oxygen isotope records.

New dating evidence of the early presence of hominins in Southern Europe

The first “Out of Africa” migrations represent a seminal event in the history of humankind. At the gates of Europe, the first appearance of Hominins is recorded in Georgia, 1.8 million years ago (Ma); however, the picture of migration across the continent remains incomplete. Vallonnet Cave (France) is a Lower Paleolithic prehistoric site with traces of hominin activities including lithic remains and cut-marks on mammal bones. Here, we apply the uranium-lead (U-Pb) methods to two flowstones to date the intervening archaeological levels. The U-Pb data, coupled with paleomagnetic constraints, provide an age range from 1.2 to 1.1 Ma. The results conclusively demonstrate that Vallonnet Cave is one of the oldest European prehistoric sites in France with early hominin occupations associated with an Epivillafranchian fauna. Combined with data from other archaeological sites, the new precise chronology suggests a widespread occupation the Northern Mediterranean to Southwestern Europe at ~1.2 Ma.

Gradual onset and recovery of the Younger Dryas abrupt climate event in the tropics

Proxy records of temperature from the Atlantic clearly show that the Younger Dryas was an abrupt climate change event during the last deglaciation, but records of hydroclimate are underutilized in defining the event. Here we combine a new hydroclimate record from Palawan, Philippines, in the tropical Pacific, with previously published records to highlight a difference between hydroclimate and temperature responses to the Younger Dryas. Although the onset and termination are synchronous across the records, tropical hydroclimate changes are more gradual (>100 years) than the abrupt (10-100 years) temperature changes in the northern Atlantic Ocean. The abrupt recovery of Greenland temperatures likely reflects changes in regional sea ice extent. Proxy data and transient climate model simulations support the hypothesis that freshwater forced a reduction in the Atlantic meridional overturning circulation, thereby causing the Younger Dryas. However, changes in ocean overturning may not produce the same effects globally as in Greenland.

New precise dates for the ancient and sacred coral pyramidal tombs of Leluh (Kosrae, Micronesia)

Monumental tombs within ancient civilizations worldwide hold precious clues for deciphering the architectural skill, acumen, and industry of prehistoric cultures. Most tombs were constructed from abiotic materials-stone, soil, and/or clay, predominately-and were built to permanently inter royalty or high-status individuals. On the island of Kosrae in the central Pacific, monumental tombs were constructed with scleractinian coral and were confined to the prehistoric island capital of Leluh, where they served as temporary mortuary processing points. Like other prehistoric tombs, the Leluh tombs were dated by association-from the remnants of the temporarily interred. We present new dates for three sacred tombs using high-precision U-Th dates from 24 corals collected directly from the structural materials. The results suggest that the tombs were built about 700 years ago during the 14th century, about three centuries earlier than previously reported. The new dates redefine the peak occupation of Leluh and place its ruling paramountcy at the leading edge of the developing trans-oceanic political hierarchies, as well as the social and economic systems that dominated the civilizations in this part of the world.

Application of U/Th and 40Ar/39Ar dating to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in Ardèche, France

Refined radio-isotopic dating techniques have been applied to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in France. Evidence of Levallois core technology appeared in level 4b in the middle of the sequence, became predominant in the upper horizons, and was best represented in uppermost level 1, making the site one of the oldest examples of Levallois technology. In our dating study, fourteen speleothem samples from levels 7, 6 and 5b, were U/Th-dated. Four pure calcite samples from the speleothem PL1 (levels 5b, 6) yield ages between 265 ± 4 (PL1-3) and 312 ± 15 (PL1-6) thousand years ago (ka). Three samples from the top of a second stalagmite, PL2, yield dates ranging from 288 ± 10 ka (PL2-1) to 298 ± 17 ka (PL2-3). Three samples from the base of PL2 (level 7) yield much younger U/Th dates between 267 and 283 ka. These dates show that the speleothems PL1 and PL2 are contemporaneous and formed during marine isotope stage (MIS) 9 and MIS 8. Volcanic minerals in level 2, the upper sequence, were dated by the ⁴⁰Ar/³⁹Ar method, giving a weighted mean of 302.9 ± 2.5 ka (2σ) and an inverse isochron age of 302.9 ± 5.9 ka (2σ). Both ⁴⁰Ar/³⁹Ar dating of volcanic sanidines and U/Th dating of relatively pure and dense cave calcites are known to be well established. The first parallel application of the two geochronometers to Orgnac 3 yields generally consistent results, which point to the reliability of the two methods. The difference between their age results is discussed.

Testing the annual nature of speleothem banding

Speleothem laminae have been postulated to form annually, and this lamina-chronology is widely applied to high-resolution modern and past climate reconstructions. However, this argument has not been directly supported by high resolution dating methods. Here we present contemporary single-lamina (230)Th dating techniques with 2σ precision as good as ±0.5 yr on a laminated stalagmite with density couplets from Xianren Cave, China, that covers the last 300 years. We find that the layers do not always deposit annually. Annual bands can be under- or over-counted by several years during different multi-decadal intervals. The irregular formation of missing and false bands in this example indicates that the assumption of annual speleothem laminae in a climate reconstruction should be approached carefully without a robust absolute-dated chronology.