Over the late Holocene, a variety of hydroclimate-sensitive proxies have identified substantial, multidecadal changes in Indian summer monsoon (ISM) precipitation, the most prominent of which is the “4.2 ka event”. This interval, dated to ~4.2-3.9 ka, is associated with severe droughts across South Asia that are linked to societal change. Given the absence of the 4.2 ka event in polar records, the 4.2 ka event is generally associated with low latitude forcings, but no clear consensus on its origins has been reached.We investigated the ISM response to the 4.2 ka event through analysis of aragonite stalagmites from Siddha cave, formed in the lower Paleozoic Dhading dolomite in the Pokhara Valley of central Nepal (28.0˚N, 84.1˚E; ~850 m.a.s.l.). The climate of this region is dominated by small monthly variations in air temperature (21±5˚C) but strong precipitation seasonality associated with the ISM: ~80% of the annual 3900 mm of rainfall occurs between June and September. High uranium and low detrital thorium abundances in these stalagmites yield precise U/Th ages that all fall within stratigraphic order. These dates reveal continuous growth from 4.30-2.26 ka, interrupted only by a hiatus from 3.27-3.10 ka. Overlap with coeval aragonite stalagmites reveals generally consistent trends in carbon and oxygen isotope ratios, suggesting that these stalagmites reflect environmental variability and not secondary (e.g., kinetic) effects.Many stalagmite-based paleomonsoon reconstructions rely on oxygen isotope ratios, which track amount effects in regional rainfall. However, our on-going rainwater collection and analysis program, as well as a previous study conducted in Kathmandu, 120 km the east of Siddha cave, reveals that amount effects in precipitation are weak in this region, particularly during the monsoon season, and thus we rely instead on carbon isotope ratios, which have been demonstrated to track site-specific effective precipitation. Siddha cave stalagmite carbon isotopes, in contrast to other South Asian proxy records, indicate that ISM rainfall increased at Siddha cave from 4.13-3.91 ka. As a further test of this result, we analyzed uranium abundances in the section spanning 4.3-3.4 ka. Uranium serves as an indicator of prior aragonite precipitation and thus of hydroclimate, and like carbon isotopes, suggests increased ISM rainfall coincident with the 4.2 ka event.This precipitation anomaly is nearly identical in timing and structure but anti-phased with stalagmites from Mawmluh cave, northeastern India. We investigated the climatic origins of this precipitation dipole using observational data from the Global Precipitation Climatology Centre (GPCC) and Hadley Center Sea Ice and Sea Surface Temperature (HadISST) products. Preliminary spatial composites suggest that large precipitation differences between Mawmluh and Siddha caves are associated with SST anomalies in the equatorial Pacific. Additionally, superposed Epoch Analysis shows relatively rapid eastern Indian Ocean cooling during the summer monsoon season coeval with large precipitation differences between these sites. Our findings lend support to a tropical Indo-Pacific origin of the 4.2 ka event.

The Gulf of Maine holds significant ecological and economic value for fisheries and communities in north-eastern North America. However, there is apprehension regarding its vulnerability to the effects of increasing atmospheric CO2. Substantial recent warming and the inflow of low alkalinity waters into the Gulf of Maine have raised concerns about the impact of ocean acidification on resident marine calcifiers (e.g. oysters, clams, mussels). With limited seawater pH records, the natural variability and drivers of pH in this region remain unclear. To address this, we present coastal water pH proxy records using boron isotope (δ11B) measurements in long-lived, annually banded, crustose coralline algae (1920–2018 CE). These records indicate seawater pH was low (~ 7.9) for much of the last century. Contrary to expectation, we also find that pH has increased (+ 0.2 pH units) over the past 40 years, despite concurrent rising atmospheric CO2. This increase is attributed to an increased input of high alkalinity waters derived from the Gulf Stream. This delayed onset of ocean acidification is cause for concern. Once ocean circulation-driven buffering effects reach their limit, seawater pH decline may occur swiftly. This would profoundly harm shellfisheries and the broader Gulf of Maine ecosystem.

The monarch butterfly (Danaus plexippus) is a vagile species that undertakes an annual, multi-generational migration across North America. The abundance of this species at both eastern and western overwintering sites in Central Mexico and California indicates a population decline. Success of continental-scale conservation programs for a migratory species depends on providing, maintaining, and protecting habitats at appropriate temporal and spatial scales. Here, dynamics of monarch continental-scale migration and gene flow were obtained by combined stable isotope, morphological, and genetic analyses. These analyses were applied to temporal monarch samples collected from May to September during 2016-2021 at locations in Iowa, USA and spatial collections from Pennsylvania, Delaware, Iowa, Ohio, Nevada, Idaho, Hawaii, 3 Australian locations during July and August 2016, and Texas in April 2021. Evidence of seasonal multi-generational migration was obtained through δ2H analyses of spatial collections, which was corroborated by decreased wing hue (a morphological marker for non-migratory individuals). In Iowa, 10-15% of monarchs represented migrants from southern areas throughout the breeding season and 6% were migrants from the North in midsummer. Limited sequence variation detected across the mitochondrial genome impacted the capability to detect significant population genetic variation in our North American samples. However, 2 novel substitutions were identified and predicted to be fixed among Australia samples, contributing to intercontinental differentiation from counterparts in North America. Our assessment of temporal and spatial population dynamics across the North American monarch breeding range provides insight into continental-scale migration and previously undetected mitochondrial DNA variation among globally distributed populations.

Modern calcifying marine organisms face numerous environmental stressors, including overfishing, deoxygenation, increasing ocean temperatures, and ocean acidification (OA). Coastal marine settings are predicted to become warmer and more acidic in coming decades, heightening the risks of extreme events such as marine heat waves. Given these threats, it is important to understand the vulnerabilities of marine organisms that construct their shells from calcium carbonate, which are particularly susceptible to warming and decreasing pH levels. To investigate the response of four commercially relevant bivalve species to OA and differing temperatures, juvenile Mercenaria mercenaria (hard shell clams), juvenile Mya arenaria (soft shell clams), adult and juvenile Arctica islandica (ocean quahog), and juvenile Placopecten magellanicus (Atlantic sea scallops) were grown in varying pH and temperature conditions. Species were exposed to four controlled pH conditions (7.4, 7.6, 7.8, and ambient/8.0) and three controlled temperature conditions (6, 9, and 12 degrees C) for 20.5 weeks and then shell growth and coloration were analyzed. This research marks the first direct comparison of these species' biological responses to both temperature and OA conditions within the same experiment. The four species exhibited varying responses to temperature and OA conditions. Mortality rates were not significantly associated with pH or temperature conditions for any of the species studied. Growth (measured as change in maximum shell height) was observed to be higher in warmer tanks for all species and was not significantly impacted by pH. Two groups (juvenile M. arenaria and juvenile M. mercenaria) exhibited lightening in the color of their shells at lower pH levels at all temperatures, attributed to a loss of shell periostracum. The variable responses of the studied bivalve species, despite belonging to the same phylogenetic class and geographic region, highlights the need for further study into implications for health and management of bivalves in the face of variable stressors.

A collection of 37 near-annually-resolved proxy records is used to evaluate the main modes of marine variability in the North Atlantic for the past 600 years. The temporal gaps in the proxy data are first filled in using a nesting procedure, and the 19 longest proxy time series are used to derive the modes of variability linked to the instrumental sea surface temperature (SST) data. By comparing with other indices and reconstructions, the four main modes of variability in the North Atlantic appear most closely related to 1) the North Atlantic subtropical/subpolar SST dipole (often used as an indicator for Atlantic Meridional Overturning Circulation [AMOC] variability), 2) the Atlantic Multidecadal Variability (AMV), 3) the North Atlantic Tripole/Oscillation, and 4) the East Atlantic Pattern. Modes 1 and 2 show a significant change in variability during the historical period, whereas modes 3 and 4 show more high frequency variability. This change is also observed in the correlations between modes during the historical and pre-industrial periods. The AMV mode published here is the first to be reconstructed exclusively using multi-proxy marine data from across the North Atlantic. The suitability of the spatial fingerprint representative of AMOC dynamics is investigated from simulations of a large climate model ensemble, and results show that the derived AMOC trends are overestimated. Our analysis suggests that multiple dynamical/thermodynamical processes influence the SST-dipole mode commonly used to reconstruct AMOC behavior. The implications of this result may provide avenues for future research and should act as a cautionary note.

The nature of Late Glacial millennial-scale climate variability in the central United States is not well understood owing to a paucity of high-resolution, precisely-dated records in this region. Here we present a discontinuous, composite stalagmite record from Cosmic Cavern (36.4˚N, 93.5˚W), located on the southwestern margin of the Ozark Plateau in northwestern Arkansas, that preserves evidence of environmental change coincident with Greenland (inter)stadials and Heinrich stadials from 54-14 ka (marine isotope stages 3-2). Among the four stalagmites that comprise this record, carbon isotopes replicate well. Oxygen isotope ratios in three of the four stalagmites also exhibit robust replication, but in the fourth, oxygen isotope ratios are offset from coeval stalagmites by 1.5 permil, and thus this stalagmite was excluded from the composite record. Variability in stalagmite carbon and oxygen isotopes are broadly coincident with, and characterized by the same asymmetry as, the Greenland Ice Sheet Project Two (GISP2) oxygen isotope record; pronounced positive carbon and oxygen isotope anomalies also mark Heinrich stadials. Elevated carbon isotope values likely reflect reductions in soil respiration rates and increased CO2 out-gassing in voids above the cave during colder, and perhaps drier, stadials. Shifts in stalagmite oxygen isotope ratios are interpreted as reflecting variations in air temperature, consistent with modern temperature effects on precipitation oxygen isotope values documented over the course of a multi-year (and still on-going) precipitation monitoring program. We investigate the impacts of precipitation seasonality and moisture source on oxygen isotopic variability using Coupled Model Intercomparison Project phase 5 (CMIP5) simulations, namely the historical, pre-industrial, and Last Glacial Maximum simulations of the MIROC Earth System Model. Particular focus is placed on quantifying changes in moisture source and transport in the model simulations and the roles played by the North Atlantic subtropical high pressure system to the south and east, and precipitation derived from a Pacific moisture source to the west.

ABSTRACT Quantifying the marine radiocarbon reservoir effect, offsets (ΔR), and ΔR variability over time is critical to improving dating estimates of marine samples while also providing a proxy of water mass dynamics. In the northeastern Pacific, where no high-resolution time series of ΔR has yet been established, we sampled radiocarbon ( 14 C) from exactly dated growth increments in a multicentennial chronology of the long-lived bivalve, Pacific geoduck ( Paneopea generosa ) at the Tree Nob site, coastal British Columbia, Canada. Samples were taken at approximately decadal time intervals from 1725 CE to 1920 CE and indicate average ΔR values of 256 ± 22 years (1σ) consistent with existing discrete estimates. Temporal variability in ΔR is small relative to analogous Atlantic records except for an unusually old-water event, 1802–1812. The correlation between ΔR and sea surface temperature (SST) reconstructed from geoduck increment width is weakly significant (r 2 = .29, p = .03), indicating warm water is generally old, when the 1802–1812 interval is excluded. This interval contains the oldest (–2.1σ) anomaly, and that is coincident with the coldest (–2.7σ) anomalies of the temperature reconstruction. An additional 32 14 C values spanning 1952–1980 were detrended using a northeastern Pacific bomb pulse curve. Significant positive correlations were identified between the detrended 14 C data and annual El Niño Southern Oscillation (ENSO) and summer SST such that cooler conditions are associated with older water. Thus, 14 C is generally relatively stable with weak, potentially inconsistent associations to climate variables, but capable of infrequent excursions as illustrated by the unusually cold, old-water 1802–1812 interval.

Ocean acidification (OA) directly impacts marine calcifying organisms including ecologically and commercially important shellfish species such as Arctica islandica (A. islandica). To test whether documented growth resilience of A. islandica to OA is a general response across ages and populations or a function of adaptation to local habitat, we cultured juvenile and adult clams collected from an environment with little pH variation under four pH levels (7.5, 7.7, 7.9, and 8.1) for three months and integrated our understanding with relevant literature. The average shell growth over the experiment among all (69) individuals was 57 +/- 55 ism, and there were no statistically significant differences in growth among pH treatments, including the control treatment, despite the general growth rate differences between juveniles and adults. Our results show that A. islandica can maintain its shell growth even in aragonite undersaturated (omega < 1) conditions (0.65 and 0.83 for pH 7.5 and 7.7 treatments, respectively), supporting the hypothesis that resistance to OA conditions is likely a generalized response across populations. Although the present results show A. islandica can maintain their shell growth under short-term OA, long-term impacts of OA on A. islandica shell growth and other physical parameters including shell density and microstructure are still needed to better assess the sustainability of A. islandica in a more acidified future and to provide guidance on managing this important shellfish stock.

A collection of 37 near-annually-resolved proxy records is used to evaluate the main modes of marine variability in the North Atlantic for the past 600 years. The temporal gaps in the proxy data are first filled in using a nesting procedure, and the 19 longest proxy time series are used to derive the modes of variability linked to the instrumental sea surface temperature (SST) data. By comparing with other indices and reconstructions, the four main modes of variability in the North Atlantic appear most closely related to 1) the North Atlantic subtropical/subpolar SST dipole (often used as an indicator for Atlantic Meridional Overturning Circulation [AMOC] variability), 2) the Atlantic Multidecadal Variability (AMV), 3) the North Atlantic Tripole/Oscillation, and 4) the East Atlantic Pattern. Modes 1 and 2 show a significant change in variability during the historical period, whereas modes 3 and 4 show more high frequency variability. This change is also observed in the correlations between modes during the historical and pre-industrial periods. The AMV mode published here is the first to be reconstructed exclusively using multi-proxy marine data from across the North Atlantic. The suitability of the spatial fingerprint representative of AMOC dynamics is investigated from simulations of a large climate model ensemble, and results show that the derived AMOC trends are overestimated. Our analysis suggests that multiple dynamical/thermodynamical processes influence the SST-dipole mode commonly used to reconstruct AMOC behavior. The implications of this result may provide avenues for future research and should act as a cautionary note.

Tropical cyclones (TCs) regularly form in association with the intertropical convergence zone (ITCZ), and thus, its positioning has implications for global TC activity. While the poleward extent of the ITCZ has varied markedly over past centuries, the sensitivity with which TCs responded remains poorly understood from the proxy record, particularly in the Southern Hemisphere. Here, we present a high-resolution, composite stalagmite record of ITCZ migrations over tropical Australia for the past 1500 years. When integrated with a TC reconstruction from the Australian subtropics, this time series, along with downscaled climate model simulations, provides an unprecedented examination of the dependence of subtropical TC activity on meridional shifts in the ITCZ. TCs tracked the ITCZ at multidecadal to centennial scales, with a more southward position enhancing TC-derived rainfall in the subtropics. TCs may play an increasingly important role inWestern Australia's moisture budgets as subtropical aridity increases due to anthropogenic warming.

The state of the atmospheric circulation and the associated hydroclimate in the North Atlantic during the last millennium remain the subject of considerable debate in both proxy- and model-based studies. Of particular interest in the Iberian region is the Azores High (AH) system, the southern node of the North Atlantic Oscillation (NAO), an atmospheric dipole closely tied to regional hydroclimate. Hydroclimate-sensitive proxy reconstructions from this region offer some insights into atmospheric dynamics, but large spatiotemporal gaps in these data inhibit a robust evaluation of hydroclimate variability. In this study, we present a continuous, sub-decadally-resolved composite stalagmite carbon isotopic record from three partially overlapping stalagmites from Buraca Gloriosa (BG) cave, western Portugal, situated within the center of the AH, that preserves evidence of regional hydroclimate variability from approximately 800 CE to the present. Chronologies are derived from U/Th dating and annual laminae. Stalagmite carbon isotopic values primarily reflect the amount of effective moisture and reveal generally dry conditions during the Medieval Climate Anomaly (MCA; ~ 850–1250 CE) and Modern Climate/Industrial Era (1850 CE-present), and wetter conditions during the Little Ice Age (LIA; ~ 1400–1850 CE). Multidecadal to centennial variability in the BG record and state-of-the-art last millennium climate model simulations show considerable coherence with precipitation-sensitive records from Spain and Morocco that, like BG, are strongly influenced by the intensity, size, and location of the AH. Model-proxy synthesis suggests that western Portugal was persistently dry during much of the MCA consistent with other NAO reconstructions; however, even considering age uncertainties, the apparent timing in the transition from a relatively dry MCA to a wetter LIA is spatially variable and confirms the non-stationary behavior of the AH system indicated by model output.

Shells of bivalve mollusks serve as archives for past climates and ecosystems, and human-environmental interactions as well as life history traits and physiology of the animals. Amongst other proxies, data can be recorded in the shells in the form of element chemical properties. As demonstrated here with measured chemical data (10 elements) from 12 Arctica islandica specimens complemented by numerical simulations, mistakes during sclerochronological data processing can introduce significant bias, adding a further source of error to paleoenvironmental or biological reconstructions. Specifically, signal extraction from noisy LA-ICP-MS (Laser Ablation—Inductively Coupled Plasma—Mass Spectrometry) data generated in line scan mode with circular LA spots requires a weighted rather than an arithmetic moving average. Otherwise, results can be in error by more than 41%. Furthermore, if variations of seasonal shell growth rate remain unconsidered, arithmetic annual averages of intra-annual data will be biased toward the fast-growing season of the year. Actual chemical data differed by between 3.7 and 33.7% from weighted averages. Numerical simulations not only corroborated these findings, but indicated that arithmetic annual means can overestimate or underestimate the actual environmental variable by nearly 40% relative to its seasonal range. The magnitude and direction of the error depends on the timing and rate of both seasonal shell growth and environmental change. With appropriate spatial sampling resolution, weighting can reduce this bias to almost zero. On average, the error reduction attains 80% at a sample depth of 10, 92% when 20 samples were analyzed and nearly 100% when 100 samples were taken from an annual increment. Under some exceptional, though unrealistic circumstances, arithmetic means can be superior to weighted means. To identify the presence of such cases, a numerical simulation is advised based on the shape, amplitude and phase relationships of both curves, i.e., seasonal shell growth and the environmental quantity. To assess the error of the offset induced by arithmetic averaging, Monte Carlo simulations should be employed and seasonal shell growth curves randomly generated based on observed variations.

The Azores High is a persistent atmospheric high-pressure ridge over the North Atlantic surrounded by anticyclonic winds that steer rain-bearing weather systems and modulate the oceanic moisture transport to Europe. The areal extent of the Azores High thereby affects precipitation across western Europe, especially during winter. Here we use observations and ensemble climate model simulations to show that winters with an extremely large Azores High are significantly more common in the industrial era (since ce 1850) than in pre-industrial times, resulting in anomalously dry conditions across the western Mediterranean, including the Iberian Peninsula. Simulations of the past millennium indicate that the industrial-era expansion of the Azores High is unprecedented throughout the past millennium (since ce 850), consistent with precipitation proxy evidence from Portugal. Azores High expansion emerges after ce 1850 and strengthens into the twentieth century, consistent with anthropogenically driven warming.