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El Niño–Southern Oscillation dynamics affect global weather patterns, with regionally diverse hydrological responses posing critical societal challenges. The lack of seasonally resolved hydrological proxy reconstructions beyond the observational era limits our understanding of boundary conditions that drive and/or adjust El Niño–Southern Oscillation variability. Detailed reconstructions of past El Niño–Southern Oscillation dynamics can help modelling efforts, highlight impacts on disparate ecosystems and link to extreme events that affect populations from the tropics to high latitudes. Here, mid‐Holocene El Niño–Southern Oscillation and hydrological changes are reconstructed in the south‐west Pacific using a stalagmite from Niue Island, which represents the period 6.4–5.4 ka BP. Stable oxygen and carbon isotope ratios, trace elements and greyscale data from a U/Th‐dated and layer counted stalagmite profile are combined to infer changes in local hydrology at sub‐annual to multi‐decadal timescales. Principal component analysis reveals seasonal‐scale hydrological changes expressed as variations in stalagmite growth patterns and geochemical characteristics. Higher levels of host rock‐derived elements (Sr/Ca and U/Ca) and higher δ18O and δ13C values are observed in dark, dense calcite laminae deposited during the dry season, whereas during the wet season, higher concentrations of soil‐derived elements (Zn/Ca and Mn/Ca) and lower δ18O and δ13C values are recorded in pale, porous calcite laminae. The multi‐proxy record from Niue shows seasonal cycles associated with hydrological changes controlled by the positioning and strength of the South Pacific Convergence Zone. Wavelet analysis of the greyscale record reveals that El Niño–Southern Oscillation was continuously active during the mid‐Holocene, with two weaker intervals at 6–5.9 and 5.6–5.5 ka BP. El Niño–Southern Oscillation especially affects dry season rainfall dynamics, with increased cyclone activity that reduces hydrological seasonality during El Niño years. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Chemical indicators from stalagmites are useful in reconstructing changes in monsoon systems. However, the chemical measurements typically made on stalagmites do not provide direct reconstructions of rainfall amount. We developed a record of uranium isotope variability over the last 60 years in a stalagmite from Northeast India to evaluate this potential metric of past monsoon rainfall. We use a geochemical model to evaluate the climatic and environmental controls on uranium isotopes in cave drip waters and stalagmites. Comparison of our record with measured rainfall indicates that the relationship between rainfall amount and uranium isotope ratios is opposite to that observed in arid environments. Further, a large linear increase in uranium isotopes between ∼1985 and 2000 CE does not correspond to a change in rainfall. Rather, our model suggests that large changes in uranium isotopes likely result from the opening up of new water flow paths and the dissolution of fresh hostrocks. Investigation of previously analyzed stalagmites from the cave reveals a correspondence between uranium isotopes and insolation, suggesting that sustained changes in rainfall amount, coupled with opening of new flow paths, may be recorded in stalagmite uranium isotopes over the past ∼100,000 years. Key Points: Modern record of speleothem initial uranium isotope ratios from NE India shows a linear increase that does not correspond to rainfallGeochemical model shows that dripwater U isotopes respond to changing infiltration and changing U characteristics of weathering rockLong‐term variations in speleothem U isotope ratios from NE India record infiltration with additional control of changes in flow path [ABSTRACT FROM AUTHOR]

To date Indian summer monsoon (ISM) dynamics have been assessed by changes in stalagmite δ18O. However, stalagmite δ18O is influenced by multiple environmental factors (e.g., atmospheric moisture transport, rainfall amount at the study site, and ISM seasonality), precluding simple and clear reconstructions of rainfall amount or variability. This study aims to disentangle these environmental factors by combining δ18O, δ44Ca, and elemental data from a stalagmite covering Termination II and the last interglacial from Mawmluh Cave, NE India, to produce a semiquantitative reconstruction of past ISM rainfall. We interpret δ18O as a mixed signal of rainfall source dynamics and rainfall amount and coupled δ44Ca and X/Ca ratios as indicators of local infiltration rate and prior calcite precipitation in the karst zone. The wettest conditions in our studied interval (135 and 100 kyrs BP; BP = before present, with the present being 1950 CE) occurred during Marine Isotope Stage 5e. Our multiproxy data set suggests a likely change in seasonal distribution of Marine Isotope Stage 5e rainfall compared to the Holocene; the wet season was longer with higher‐than‐modern dry season rainfall. Using the last interglacial as an analogue for future anthropogenic warming, our data suggest a more erratic ISM behavior in a warmer world. Key Points: This study represents a first multiproxy stalagmite record from the Indian summer monsoon domain covering Termination II and Marine Isotope Stage 5eCombining δ18O, δ44Ca, and elemental data helps to discriminate between rainfall source and amount and local infiltrationThe rainy season during Marine Isotope Stage 5e was wetter, likely with more extreme rainfall events, compared to the Holocene [ABSTRACT FROM AUTHOR]

Mountainous depocentres are often the single available archives in intracontinental areas. As such, the investigation of their sedimentary dynamics is the only way to reconstruct palaeoenvironmental evolution in these peculiar regions. Here, the basin‐scale sedimentary dynamics of the Agadir‐Tissint Feija (i.e. feija is the local name for lowlands) are discussed together with new palaeoenvironmental information on the late Quaternary in the north‐western Sahara margin. The succession consists of up to ca 40 m thick fluvio‐lacustrine deposits subdivided into three depositional sequences (S1–S3) that were deposited between ca 75 ka and ca 10 ka. S1 and S2 reflect the evolution of a palaeolake intercalated by carbonate‐rich palustrine to alluvial plain environments, while S3 shows the transition to an entirely clastic alluvial plain environment. The three sequences are genetically associated with three pulses of tufa buildups at the outlet of the depocentre. Each pulse of tufa buildup is attributed to wet conditions, while each interruption is associated with drier conditions. The wet–dry alternation at the origin of S1 fits with summer insolation fluctuation suggesting that precession parameters may have paced long‐term climate variability in the area. Superimposed on wet–dry cycles, submillennial‐scale (750–900 year) shorter‐term lake fluctuations are identified in S1 and S2; their origins may be related to North Atlantic Oscillation mechanisms. Furthermore, the sudden increase in clastic sediment supply in S3 evidences a change in sediment production in the catchment area attributed to lower temperatures established during the last glacial period. Finally, this newly identified continental archive is unique at the northern Sahara margin; it enables reconstructions of the local and regional hydrological conditions during the last glacial cycle. Even if a more systematic investigation of palaeoclimate forcing is still necessary, the Agadir‐Tissint Feija will potentially bring important information to understand past and anticipate future regional climate change. [ABSTRACT FROM AUTHOR]

About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve long‐standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long‐standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories. Key Points: Standardization and correction procedures for carbonate clumped isotopes are reviewedWe propose a carbonate‐based standardization scheme to improve reproducibilityWe demonstrate improved interlaboratory comparability of clumped isotope measurements [ABSTRACT FROM AUTHOR]

Abstract: Transient sediment storage and mixing of deposits of various ages during transport across alluvial piedmonts alter the clastic sedimentary record. We quantify buffering and mixing during cycles of aggradation–incision in the north piedmont of the Eastern Tian Shan. We complement existing chronologic data with 20 new luminescence ages and one cosmogenic radionuclide age of terrace abandonment and alluvial aggradation. Over the last 0.5 Myr, the piedmont deeply incised and aggraded many times per 100 kyr. Aggradation is driven by an increased flux of glacial sediment accumulated in the high range and flushed onto the piedmont by greater water discharge at stadial–interstadial transitions. After this sediment is evacuated from the high range, the reduced input sediment flux results in fluvial incision of the piedmont as fast as 9 cm year−1 and to depths up to 330 m. The timing of incision onset is different in each river and does not directly reflect climate forcing but the necessary time for the evacuation of glacial sediment from the high range. A significant fraction of sediments evacuated from the high range is temporarily stored on the piedmont before a later incision phase delivers it to the basin. Coarse sediments arrive in the basin with a lag of at least 7–14 kyrs between the first evacuation from the mountain and later basinward transport. The modern output flux of coarse sediments from the piedmont contains a significant amount of recycled material that was deposited on the piedmont as early as the Middle Pleistocene. Variations in temperature and moisture delivered by the Westerlies are the likely cause of repeated aggradation–incision cycles in the north piedmont instead of monsoonal precipitation. The arrival of the gravel front into the proximal basin is delayed relative to the fine‐grained load and both are separated by a hiatus. This work shows, based on field observations and data, how sedimentary systems respond to climatic perturbations, and how sediment recycling and mixing can ensue. [ABSTRACT FROM AUTHOR]

For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre-Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18O-depleted fluids for durations between two and twenty weeks. Alteration was documented using X-ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ18O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple-blue luminescence with orange bands. The δ18O and Δ47 signatures reveal exchange with the aqueous fluid, whereas δ13C remains unaltered in all experiments, indicating a carbonate-buffered system. Clumped isotope temperatures in high-temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow-up experiments. [ABSTRACT FROM AUTHOR]

RATIONALE Carbonate clumped isotope thermometry is a powerful new technique increasingly used in many fields in earth science. Recently, it has been shown that clumped isotope measurements can be performed with a Kiel carbonate preparation device and micro-volume analyses, allowing measurements of small (1.5-2 mg) carbonate samples. However, common data correction schemes rely on measurements of gases prepared offline, potentially leading to unrecognized biases in the results. METHODS We propose a new correction scheme for the Kiel device method including: (1) A pressure-sensitive baseline correction (PBL) of the raw beam signals; (2) Transfer of data to the absolute reference frame; (3) Correction for acid fractionation; (4) Correction for average standard offsets; (5) When necessary, correction for Δ47 scale compression based on offsets among standards with different ordering state. The long-term performance of the new scheme was tested with a large set of standard measurements (N = 432) obtained over the course of 15 months. RESULTS The PBL correction reliably removes composition-dependent artifacts, which are commonly corrected for with gas measurements, and offsets observed in micro-volume measurements when ion beams are imbalanced. We show that the shape of the PBL can vary strongly and needs to be properly characterized. Combined PBL and standard correction resulted in long-term stability with standard deviations in Δ47 of 0.012-0.016 ‰ for the five standards over the whole period, close to the average error of 0.011 ‰ observed for individual measurements consisting of 10 replicate analyses. CONCLUSIONS Our correction scheme eliminates the need for routine gas measurements, allowing for equal treatment of samples and standards with the Kiel device setup. While the PBL and standard data obtained over 15 months reveal variable mass spectrometer behavior, they provide a robust means of correction, yielding reproducible results from small carbonate samples in the long term. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RATIONALE The measurement of the abundances of minor isotopologues by mass spectrometry requires correction of subtle non-linearities in the mass spectrometer that cause deviations in the relationship between actual and measured isotope ratios. Here we show that negative backgrounds on the Faraday cups recording the minor ion beams are the cause of the observed non-linearities in the measurement of CO2 isotopologues, and propose a new correction procedure for clumped isotope measurements. METHODS We carefully investigated the cause of non-linearity effects in the measurement of the abundance of 13C18O16O, a minor isotopologue of CO2 with m/z 47, on two different mass spectrometers. By using gases of different composition with close to stochastic and with non-random distribution of isotopes we demonstrate that the apparent dependence of the excess abundance of the isotopologue of m/z 47 on the bulk isotopic composition of CO2 is due to a background interference that is linearly dependent on the partial pressure of the gas in the source of the mass spectrometer. CONCLUSIONS Background determination with gas flowing into the source of the mass spectrometer is necessary for accurate clumped isotope measurements of CO2. Background corrections can be performed accurately if the slit width of the m/z 44 Faraday cup significantly exceeds that of the one for m/z 47, using a correlation between m/z 44 signal intensity and the corresponding minimum in m/z 47 background. We propose two new correction schemes that reduce the time-consuming measurement of gases of different bulk isotopic compositions. These findings may also be relevant for the measurement of other rare isotopologues by mass spectrometry. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]