Proxies recorded in speleothems offer insight into past patterns of rainfall, wildfires, vegetation, and other components of the terrestrial climate system. For these records to be effective they must be accurately and precisely dated. Uranium-thorium (U-Th) dating is commonly applied to create chronological models for speleothems, including stalagmites, from present-day to similar to 650 ka. Making accurate and precise corrections for initial thorium (i.e., Th-230 that is not a result of in-situ decay of uranium within the sample) can be the limiting factor in building robust speleothem age models. Two methods are broadly used to correct for initial thorium: (1) direct estimation using isochron techniques; or (2) assuming a constant, model value of the initial Th-230/Th-232 ratio. However, isochron approaches are resource-intensive, while corrections made using a single, assumed initial thorium value with arbitrary uncertainty may fail to characterize significant variations in initial thorium, leading to inflated age uncertainties and potentially inaccurate ages, thereby limiting speleothems that can be utilized for climate reconstruction. IBIS is a Bayesian model that uses a probabilistic approach to constrain the initial thorium isotope composition of each sample, thereby facilitating the derivation of accurate U-Th ages and errors and robust age-depth models for speleothems. The Bayesian model proceeds in two stages - first estimating U-Th ages and unique initial thorium compositions under a stratigraphic order constraint (Part 1), then fitting those ages with an age-depth model (Part 2). To test the validity of the approach, we show the efficacy of IBIS when applied to case studies from the published literature that exhibit rapidly changing growth rates, uranium loss, growth hiatuses, and very young (<1 ka) samples with heterogeneous initial thorium. The ability of the IBIS modeling framework to make accurate, precise, and unique corrections of the initial thorium for each sample within a speleothem has the potential to extend the application of U-Th dating to more complex speleothem systems (e.g., variable initial thorium) and provide more accurate and precise age information for samples where initial Th corrections are significant and variable.

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 (δ 18 O and δ 2 H) 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 × 10 8 m 3 /yr (±4.94 × 10 6 m 3 /yr, 1σ using δ 18 O; ±3.47 × 10 6 m 3 /yr, 1σ using δ 2 H) 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.

Speleothem initial uranium isotope ratios (( 234 U/ 238 U) i ) can be influenced by processes along the seepage water flow‐path including alpha‐recoil into porewater during 238 U decay and hostrock weathering, the balance of which can reflect the infiltration rate. Thus, speleothem ( 234 U/ 238 U) i may provide information about past changes in rainfall amounts. However, the utility of ( 234 U/ 238 U) i as a paleo‐infiltration proxy has only been explored in a limited set of rainfall regimes. We present a speleothem ( 234 U/ 238 U) i record from Mawmluh Cave in northeast India, an area influenced by the Indian Summer Monsoon, covering 1964–2012 CE. Speleothem ( 234 U/ 238 U) i was relatively constant from 1964 to 1984 but then linearly increased by 0.05 over ∼15 years, a trend that does not correspond with observed rainfall changes. To evaluate potential drivers of ( 234 U/ 238 U) i variability, we model the evolution of water ( 234 U/ 238 U) in a simple karst system using an advection‐reaction model parameterized by Mawmluh Cave variables. Although varying infiltration influences modeled water ( 234 U/ 238 U), the larger, sustained change observed in the speleothem record can only be modeled by varying the U concentration and ( 234 U/ 238 U) of the weathering hostrock. This suggests that larger shifts in speleothem ( 234 U/ 238 U) i may result from flow path changes, bringing waters in contact with hostrock of variable U characteristics. Consideration of published Mawmluh Cave records suggests that these mechanisms may also explain variability in stalagmite ( 234 U/ 238 U) i on precessional timescales. Further examination of speleothems ( 234 U/ 238 U) i from climates characterized by high rainfall and extensive weathering is warranted to better constrain the controls on ( 234 U/ 238 U) i in these dynamic environments.

Abstract Speleothem initial uranium isotope ratios ((234U/238U)i) can be influenced by processes along the seepage water flow‐path including alpha‐recoil into porewater during 238U decay and hostrock weathering, the balance of which can reflect the infiltration rate. Thus, speleothem (234U/238U)i may provide information about past changes in rainfall amounts. However, the utility of (234U/238U)i as a paleo‐infiltration proxy has only been explored in a limited set of rainfall regimes. We present a speleothem (234U/238U)i record from Mawmluh Cave in northeast India, an area influenced by the Indian Summer Monsoon, covering 1964–2012 CE. Speleothem (234U/238U)i was relatively constant from 1964 to 1984 but then linearly increased by 0.05 over ∼15 years, a trend that does not correspond with observed rainfall changes. To evaluate potential drivers of (234U/238U)i variability, we model the evolution of water (234U/238U) in a simple karst system using an advection‐reaction model parameterized by Mawmluh Cave variables. Although varying infiltration influences modeled water (234U/238U), the larger, sustained change observed in the speleothem record can only be modeled by varying the U concentration and (234U/238U) of the weathering hostrock. This suggests that larger shifts in speleothem (234U/238U)i may result from flow path changes, bringing waters in contact with hostrock of variable U characteristics. Consideration of published Mawmluh Cave records suggests that these mechanisms may also explain variability in stalagmite (234U/238U)i on precessional timescales. Further examination of speleothems (234U/238U)i from climates characterized by high rainfall and extensive weathering is warranted to better constrain the controls on (234U/238U)i in these dynamic environments.

Tree ring records show cool-season droughts in the western US have been characterized by three spatial patterns over the past 500 years: “western-wide drought”, “wet north/dry south”, and “dry north/wet south”. Previous work shows that these drought patterns can persist on timescales of decades to centuries and are driven by internal climate variability, with secondary influence by sea surface temperatures. However very few high-resolution records of western US precipitation extend beyond the tree ring record (~1400 CE), limiting our understanding of the occurrence and persistence of these patterns of natural climate variability on longer timescales and further in the past. Here we use trace element (Mg/Ca, Sr/Ca, Ba/Ca, P/Ca) and stable isotope (δ18O, δ13C) data from two coeval stalagmites to construct a Holocene paleoclimate record for Titan Cave (TC), northern Wyoming, extending the hydroclimate record of the northern Rockies and providing the opportunity to assess longer-term natural climate variability in the northern Rockies. The TC-2 and TC-7 speleothems grew over the past ~5.7 ka and ~3.1 ka, respectively. Proxies from both stalagmites exhibit strong correlations with several coeval climate records, including regional snowpack as recorded by tree rings during the past ~600 years, suggesting the speleothems record winter precipitation patterns in the northern Rockies. Decreased snowpack and dry conditions at TC correlate to warmer sea surface temperatures in the Gulf of Alaska and the positive phase of the Pacific Decadal Oscillation. Comparison of speleothem δ18O with the δ18O of Bison Lake sediments from central Colorado suggests all three patterns of western US drought occurred during intervals of the late Holocene. Specifically, the wet north/dry south and dry north/wet south winter precipitation dipoles in the Rocky Mountains with a boundary near 40° N latitude, were established by approximately 3 ka. Multiple, centuries-long “western-wide” droughts occurred throughout the record, most notably from 2.2 – 2 ka during the Roman Warm Period. Other dated TC stalagmites grew during the mid-Holocene and Last Interglacial, providing records of winter precipitation in the northern Rockies during past warm intervals, which may serve as analogs for future anthropogenically-warmed climate states.

We have analyzed the 16S rDNA sequence and the phylogenetic position of an uncultivated spirochete from the hindgut contents of the Australian termite Mastotermes darwiniensis Froggatt. The 16S rRNA genes of bacteria from the hindgut contents of Mastotermes darwiniensis were amplified by polymerase chain reaction. The amplification products were cloned and sequenced. The sequences were compared to known homologous primary structures. Two of the clones (MDS1 and MDS3) had an insert of 1498 nucleotides showing typical signatures of spirochete 16S rRNA sequences. The sequences of the two clones were most similar to the 16S rRNA sequence of Spirochaeta stenostrepta (89.8%) and Treponema sp. strain H1 (90.7%). Phylogenetical analysis positioned the hindgut spirochete sequence with that of the free-living anaerobic Spirochaeta stenostrepta and Treponema sp. strain H1 as its nearest relatives within the cluster of the spirochetes. We conclude that the analyzed SSU rDNA sequences originate from a spirochete related to the genus Treponema. It is possibly one of the uncultivated unique spirochetes symbiotic in termite hindguts.

The Halibee member of the Upper Dawaitoli Formation of Ethiopia's Middle Awash study area features a wealth of Middle and Later Stone Age (MSA and LSA) paleoanthropological resources in a succession of Pleistocene sediments. We introduce these artifacts and fossils, and determine their chronostratigraphic placement via a combination of established radioisotopic methods and a recently developed dating method applied to ostrich eggshell (OES). We apply the recently developed 230Th/U burial dating of OES to bridge the temporal gap between radiocarbon (14C) and 40Ar/39Ar ages for the MSA and provide 14C ages to constrain the younger LSA archaeology and fauna to ∼24 to 21.4 ka. Paired 14C and 230Th/U burial ages of OES agree at ∼31 ka for an older LSA locality, validating the newer method, and in turn supporting its application to stratigraphically underlying MSA occurrences previously constrained only by a maximum 40Ar/39Ar age. Associated fauna, flora, and Homo sapiens fossils are thereby now fixed between 106 ± 20 ka and 96.4 ± 1.6 ka (all errors 2σ). Additional 40Ar/39 results on an underlying tuff refine its age to 158.1 ± 11.0 ka, providing a more precise minimum age for MSA lithic artifacts, fauna, and H. sapiens fossils recovered ∼9 m below it. These results demonstrate how chronological control can be obtained in tectonically active and stratigraphically complex settings to precisely calibrate crucial evidence of technological, environmental, and evolutionary changes during the African Middle and Late Pleistocene.

Evidence for complex behaviours appears sporadically through the Middle Stone Age of Africa, leaving unclear the major factors shaping the evolution of human behaviour. Here we present evidence for a novel suite of adaptations in the arid Knersvlake region of southern Africa that were deployed during a specific set of environmental conditions dating 90-80 000 years before present, at the archaeological site Varsche Rivier 003 (VR003). This includes the earliest production of artefacts from ostrich eggshell, long-distance transportation of economic marine molluscs, and stone tool technologies focussed on manufacture of small flakes and blades from deliberately heat fractured blocks of silcrete. Similar evidence is absent from coeval assemblages at sites in less arid areas immediately to the south, and subsequently disappears from VR003 with the onset of more humid conditions. The results indicate complex cultural dynamics in southern Africa during the Middle Stone Age, and suggest that marginal subsistence conditions fostered distinctive technological and subsistence innovations.

Modern human behavioral innovations from the Middle Stone Age (MSA) include the earliest indicators of full coastal adaptation evidenced by shell middens, yet many MSA middens remain poorly dated. We apply 230Th/U burial dating to ostrich eggshells (OES) from Ysterfontein 1 (YFT1, Western Cape, South Africa), a stratified MSA shell midden. 230Th/U burial ages of YFT1 OES are relatively precise (median ± 2.7%), consistent with other age constraints, and preserve stratigraphic principles. Bayesian age-depth modeling indicates YFT1 was deposited between 119.9 to 113.1 thousand years ago (ka) (95% CI of model ages), and the entire 3.8 m thick midden may have accumulated within ∼2,300 y. Stable carbon, nitrogen, and oxygen isotopes of OES indicate that during occupation the local environment was dominated by C3 vegetation and was initially significantly wetter than at present but became drier and cooler with time. Integrating archaeological evidence with OES 230Th/U ages and stable isotopes shows the following: 1) YFT1 is the oldest shell midden known, providing minimum constraints on full coastal adaptation by ∼120 ka; 2) despite rapid sea-level drop and other climatic changes during occupation, relative shellfish proportions and sizes remain similar, suggesting adaptive foraging along a changing coastline; 3) the YFT1 lithic technocomplex is similar to other west coast assemblages but distinct from potentially synchronous industries along the southern African coast, suggesting human populations were fragmented between seasonal rainfall zones; and 4) accumulation rates (up to 1.8 m/ka) are much higher than previously observed for dated, stratified MSA middens, implying more intense site occupation akin to Later Stone Age middens.

A multiproxy record from a fast‐growing stalagmite reveals variable hydroclimate on the California coast across the 8.2 kyr event and a precursor event likely caused by initial drainage of proglacial Lake Agassiz. Using speleothem δ 44 Ca, we develop the first semiquantitative estimates of paleorainfall variability for California through calibration with measurements of the modern climate and cave environment. We find that the magnitude of rainfall variability during the 8.2 kyr event approached the multiyear variability observable in the recent past (1950–2019) and the magnitude of variability during the precursor event likely exceeded this range. Additionally, we observe other instances of multidecadal variability comparable in magnitude to the precursor event during the record. Our work suggests that speleothem calcium isotope ratios are a powerful semiquantitative means to reconstruct paleorainfall, although numerous factors must be assessed in each cave system before applying this approach.

Speleothem deposits can provide a wealth of critical, detailed paleoclimate information from low and mid-latitude terrestrial environments. A key strength of these archives is that they may be dated precisely using U-Th techniques. Yet, depending on the cave environment, overlying geology, seepage water flow characteristics, and speleothem growth habit, accurate, precise dating of speleothems can be challenging. For example, contamination by Th-bearing detritus degrades precision and accuracy when model-based corrections for initial 230Th are applied, and partial dissolution or secondary infilling of porosity can also lead to inaccurate ages, thereby confounding interpretations of paleoclimatic change. Here we present a new chronology from a Holocene stalagmite, WMC2, from White Moon Cave in the Santa Cruz Mountains of California, USA, that exhibits multiple challenges. Stalagmite WMC2 was not active at the time of collection, but it was in situ, with a top age of 3267 ± 28 yrs BP 1950. WMC2 calcite has relatively high U (3-7 ppm), however, the stalagmite contains sporadically distributed sub-millimeter pockets of silicate detritus, leading to 100-fold differences in common Th (232Th) concentrations in dating samples (i.e., >80 to <1ppb). Additionally, ages that appear to be anomalously young are associated with zones containing high densities of fluid inclusions, suggesting possible secondary calcite growth. We overcome these challenges using a combination of micro-CT imaging, transmitted-light microscopy and assessing replicate samples. Micro-CT provides a non-destructive method for imaging the internal structure of the stalagmite, allowing for the sampling of dense, pure calcite. Using this approach, we are able to avoid sub-millimeter pockets of silicate detritus that are not visible from the cut surface of the sample, thereby reducing 232Th concentrations and associated initial 230Th corrections, and obtaining more precise and accurate ages. Dating replicate samples from individual growth bands can confirm or refute whether diagenesis suspected from petrographic study has measurably affected U-Th ages since corrupt ages should scatter more than expected from analytical errors alone. We use our carefully screened ages for WMC2 to evaluate various age modeling approaches typically used for stalagmite proxy records, including those that apply Monte Carlo methods and Bayesian approaches. By employing multiple techniques to optimize stalagmite dating samples, reliable, precise U-Th ages (median 2s ~30 yr) may be obtained from stalagmites previously deemed too flawed for accurate dating, thereby broadening our ability to develop accurately dated speleothem paleoclimate records.

Klein (1) challenges two interpretations in Niespolo et al. (2). Regarding his first point, we maintain that Ysterfontein 1 (YFT1) does provide the oldest known example of full coastal adaptation as indicated by the presence of shell middens (cf. ref. 3). Klein inaccurately characterizes the age of the deepest shell midden layers at Klasies River Main (KRM) given in ref. 4. While other workers identify even the oldest light brown sand (LBS) layer at KRM as similar in age or younger than the deepest deposits at YFT1 (cf. refs. 5 and 6), the reference cited by Klein (4) and subsequent dating efforts show that the shell midden lenses in beach-sand … [↵][1]2To whom correspondence may be addressed. Email: niespolo{at}caltech.edu. [1]: #xref-corresp-1-1

Establishing the timing of human colonization of the eastern Pacific and developing cultural chronologies within the island groups of Eastern Polynesia has relied primarily on C-14 dating. Despite advancements in C-14 dating, however, uncertainties introduced during calibration to calendar ages remain large relative to the tempo of human settlement of the eastern Pacific and ensuing Polynesian cultural development. Th-230 dating of coral abraders, a common artifact in Polynesian archaeological sites, can potentially provide more precise ages. We report a high-precision chronology for the Kitchen Cave rockshelter on Kamaka Island in the Mangareva (Gambier) Islands, based on parallel series of 13 C-14 AMS dates on short-lived plant materials and 19 Th-230 dates on Acropora coral abraders and non-utilized Acropora coral branches. The Th-230 coral dating results are highly consistent with ages from C-14 dating, except in two cases where corals younger than expected occupied what are most likely intrusive contexts. Moreover, because the C-14 and Th-230 dating techniques are largely independent, obtaining consistent results via the two techniques increases confidence in the resulting chronology. A reliable Th-230 date of 860 +/- 5 CE for a coral from the basal layer of the cultural sequence, whose deposition cannot readily be explained by natural processes, raises the possibility of an early Polynesian visit to Kamaka Island some centuries prior to initiation of permanent occupation in the 11th to 13th centuries. These results confirm that Th-230 dating of Acropora branch coral abraders can be applied to other sites in the Pacific with a high degree of confidence.

Paleoclimate and vegetation reconstruction of Abric Romani (Capellades, Spain) during MIS-3, 4, and MIS-5 (a-d) Demet Biltekin1,2, Francesc Burjachs1,3,4, Josep Vallverdú1,4, Warren D. Sharp5, Regina Mertz-Kraus6, M. Gema Chacón1,4, Palmira Saladié1,4, James L. Bischoff5, Eudald Carbonell1,4 1Institut Català de Paleoecologia Humana i Evolucio Social (IPHES), Zona Educacional 4, Campus Sescelades URV, edifici W3, 43007 Tarragona, Spain. 2Istanbul Technical University, Eurasia Institute of Earth Sciences, Ayazağa Campus, Maslak, Sarıyer, 34469, Istanbul/Turkey 3ICREA, Barcelona, Catalonia, Spain. 4URV, Universitat Rovira i Virgili, Facultat de Lletres, Avinguda Catalunya, 35, 43002 Tarragona, Catalonia, Spain. 5Berkeley Geochronology Center, Berkeley, CA 94709, United States. 6Institute for Geosciences, Johannes Gutenberg University, Mainz, Germany. This new pollen data provides the vegetation and climate history during ca. 110 ka-55 ka BP from Abric Romaní archaeological site using pollen analysis of a 30 m-long sedimentary sequence. The beginning of the MIS 3 starts an abundance in steppes and herbs, indicating cold and dry climate in the region. However, this was replaced by a slight increase in deciduous Quercus and Mediterranean trees. During the MIS 4, the pollen records reflect a predominance of Artemisia steppes and herbaceous communities (Poaceae and Asteraceae families), indicating dry and cold conditions in Abric Romaní. The MIS 5 was well recorded with its substages, including 5a, 5b, 5c and 5d. The MIS 5d is characterized by Pinus and Artemisia steppes with herbaceous assemblages. The higher abundance of Artemisia during the second part of the MIS 5b, reflecting cold and dry climate, while temperate forest and Mediterranean trees decline. Mélisey II stadial was marked by an increase in Artemisia and herbs. This suggests that cold climatic conditions existed during this time period. The abundance of oaks during the MIS 5c indicate warmer and humid climate in the region. Other deciduous and broadleaved forest developed as well, including Ulmus, Viburnum, Juglans and Castanea. A short cooling Montaigu event was also recorded within this interstadial, which is dominated by a high percentage of Ericaceae with Artemisia. The first part of the MIS 5a is characterized by Corylus, Carpinus, Hedera, Ulmus, Betula, pointing to warmer climatic conditions. In contrast, the high amount of Artemisia steppes may indicate an enhanced degree of continentality during the second half of the MIS 5a in the north-eastern Iberian Peninsula. Keywords: paleovegetation, climate, pollen analysis, Late Pleistocene, Spain