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Between 1992 and 2018, the breeding population of Adélie penguins around Anvers Island, Antarctica declined by 98%. In this region, natural climate variability drives five‐year cycling in marine phytoplankton productivity, leading to phase‐offset five‐year cycling in the size of the krill population. We demonstrate that the rate of change of the Adélie breeding population also shows five‐year cycling. We link this population response to cyclical krill scarcity, a phenomenon which appears to have arisen from the interaction between climate variability and climate change trends. Modeling suggests that, since at least 1980, natural climate variability has driven cycling in this marine system. However, anthropogenic climate change has shifted conditions so that fewer years in each cycle now prompt strong krill recruitment, triggering intervals of krill scarcity that result in drastic declines in Adélie penguins. Our results imply that climate change can amplify the impacts of natural climate oscillations across trophic levels, driving cycling across species and disrupting food webs. The findings indicate that climate variability plays an integral role in driving ecosystem dynamics under climate change. [ABSTRACT FROM AUTHOR]

Faunal analog reconstructions suggest that Last Interglacial (MIS 5e) sea surface temperatures were cooler around Bermuda and in the Caribbean than modern climate. Here we describe new and revised clumped isotope measurements of Cittarium pica fossil shells supporting previous findings of cooler than modern temperatures in Bermuda during the Last Interglacial. We resolve temperature and δ18Ow differences between two closely located and apparently coeval sites described in Winkelstern et al. (2017), https://doi.org/10.1002/2016pa003014 through reprocessing raw isotopic data with the updated Brand/IUPAC parameters. New subannual‐resolution clumped isotope data reveal large variations in δ18Ow out of phase with seasonal temperature changes (i.e., lower δ18Ow values in winter). Supported by modern δ18Ow measurements identifying similar processes occurring today, we suggest past variations in coastal δ18Ow were driven by seasonally variable freshwater discharge from a subterranean aquifer beneath the island. Taken together, our results emphasize the importance of δ18Ow in controlling carbonate δ18O, and suggest that typical assumptions of constant δ18Ow should be made cautiously in nearshore settings and can contribute to less accurate reconstructions of paleotemperature. Key Points: Bermudan climate during the Last Interglacial period (MIS 5e) was colder than todaySubannual clumped isotope analyses capture seasonal‐scale changes in temperature and water compositionVariable discharge from an underground aquifer affects oxygen isotopic composition of coastal water during MIS 5e and today [ABSTRACT FROM AUTHOR]

3He and 4He concentrations in excess of those in water in solubility equilibrium with the atmosphere by up to two and three orders of magnitude are observed in the shallow Glacial Drift and Saginaw aquifers in the Michigan Basin. A simplified He transport model shows that in situ production is negligible and that most He excesses have a source external to the aquifer. Simulated results show that 3He and 4He fluxes entering the bottom of the Saginaw aquifer are 7.5 × 10−14 and 6.1 × 10−7 cm3 STPcm−2 yr−1, both of which are lower than fluxes entering the underlying Marshall aquifer, 1.0 × 10−13 and 1.6 × 10−6 cm3 STPcm−2 yr−1 for 3He and 4He, respectively. In contrast, He fluxes entering the Saginaw aquifer are higher than fluxes entering the overlying Glacial Drift aquifer of 5.2 × 10−14 and 1.5 × 10−7 cm3 STPcm−2 yr−1 for 3He and 4He, respectively. The unusually high He fluxes and their decreasing values from the lower Marshall to the upper Glacial Drift aquifer strongly suggest the presence of an upward cross-formational flow, with increasing He dilution toward the surface by recharge water. These fluxes are either comparable to or far greater than He fluxes in deeper aquifers around the world. Model simulations also suggest an exponential decrease in the horizontal groundwater velocity with recharge distance. Horizontal velocities vary from 13 to 2 myr−1 for the Saginaw aquifer and from 18 to 6 myr−1 for the Marshall aquifer. The highly permeable Glacial Drift aquifer displays a greater velocity range, from 250 to 5 myr−1. While Saginaw 4He ages estimated based on the simulated velocity field display an overall agreement with 14C ages, 14C and 4He ages in the Glacial Drift and Marshall aquifers deviate significantly, possibly due to simplifications introduced in the He transport model leading to calculation of first-order approximation He ages and high uncertainties in Glacial Drift 14C ages. [ABSTRACT FROM AUTHOR]

Abstract: Aragonite is easily altered during diagenesis, therefore presumed pristine when present. In effect, beyond polymorphic transformation to calcite, alteration paths of aragonite remain poorly understood despite heavy reliance on such material to produce palaeoenvironmental and chronostratigraphic interpretations. Previous work on core material from Southern McMurdo Sound, Antarctica, showed that unlike their calcitic counterparts, seemingly unaltered aragonite shell fragments invariably produced older than expected 87Sr/86Sr ages. In this study, we pursued additional analyses of these aragonite shells and of the porewater of the core to understand this discrepancy. Aragonite mineralogy was reconfirmed and elemental mapping of shell fragments revealed growth lines within the middle layer suggestive of good preservation. The outer layer, however, showed anomalously high Sr concentrations (average 4·5 ± 0·6 mole% SrCO3; ca 25 mmol mol−1 Sr/Ca) and was depleted in 18O and 13C compared to the middle layer, both features inconsistent with pristine material. The δ18O values and Sr concentrations of the porewater were used to model outer layer compositions reasonably well. Coincidentally, porewater Sr isotope composition was in general agreement with the age model of the core only at the aragonite‐bearing interval suggesting that Sr‐isotopic disequilibrium between porewater and the carbonates was the rule rather than the exception in the core. The Sr isotope compositions of the aragonite shells are most likely the result of early diagenesis as suggested by the inconsistent O and C isotope compositions between shell layers and the anomalously high Sr concentrations. We conclude that knowledge of Sr concentration and distribution in shells is critical to determine the viability of Sr stratigraphy and the scale at which it may be applied. Reliance on traditional indicators of lack of alteration, such as cathodoluminescence, Mn‐Fe concentration, and the presence of labile mineralogies to assert chronostratigraphic and palaeoenvironmental questions may produce erroneous conclusions due to obscurely altered material. [ABSTRACT FROM AUTHOR]

RATIONALE: Mass-47 CO2 clumped isotope thermometry requires relatively large (∼20 mg) samples of carbonate minerals due to detection limits and shot noise in gas source isotope ratio mass spectrometry (IRMS). However, it is unreasonable to assume that natural geologic materials are homogenous on the scale required for sampling. We show that sample heterogeneities can cause offsets from equilibrium Δ47 values that are controlled solely by end member mixing and are independent of equilibrium temperatures. METHODS: A numerical model was built to simulate and quantify the effects of end member mixing on Δ47. The model was run in multiple possible configurations to produce a dataset of mixing effects. We verified that the model accurately simulated real phenomena by comparing two artificial laboratory mixtures measured using IRMS to model output. RESULTS: Mixing effects were found to be dependent on end member isotopic composition in δ13C and δ18O values, and independent of end member Δ47 values. Both positive and negative offsets from equilibrium Δ47 can occur, and the sign is dependent on the interaction between end member isotopic compositions. The overall magnitude of mixing offsets is controlled by the amount of variability within a sample; the larger the disparity between end member compositions, the larger the mixing offset. CONCLUSIONS: Samples varying by less than 2 ‰ in both δ13C and δ18O values have mixing offsets below current IRMS detection limits. We recommend the use of isotopic subsampling for δ13C and δ18O values to determine sample heterogeneity, and to evaluate any potential mixing effects in samples suspected of being heterogonous. [ABSTRACT FROM AUTHOR]

We report the results of a yearlong noble gas study conducted in 2008-2009 together with continuous physical and chemical measurements collected in a monitoring well in an aquifer in southern Michigan. Conditions near the water table are correlated with noble gas concentrations, corresponding noble gas temperatures (NGTs), and precipitation events. This yearlong study is the first noble gas field test that has employed natural recharge and in situ monitored conditions, with minimal disturbance of the unsaturated zone. This detailed study demonstrates that significant changes in conditions near the water table can occur over a year that can greatly affect NGTs. Results show that precipitation events are detected within hours at the water table, but a lag in pressure response argues for a long time constant for gas transport within the unsaturated zone. There is strong evidence for the depletion of oxygen near the water table, which affects the noble gas air-saturated water component. During reducing conditions there is evidence for significant noble gas degassing. Rain from the passage of Hurricane Ike caused a significant shift in stable isotope ratios and injection of a large quantity of excess air and likely led to a much more oxygen-rich environment in the soil gas. Although individual models can account for NGTs over portions of the record, no single NGT model can account for all features observed over the entire study. It is likely that the NGT temperature proxy must be viewed as an average of recharge conditions over several years. [ABSTRACT FROM AUTHOR]

A sectioned and polished specimen of the coral Archohelia vicksburgensis from the early Oligocene Byram Formation (∼30 Ma) near Vicksburg, Mississippi, reveals 12 prominent annual growth bands. Stable oxygen isotopic compositions of 77 growth-band-parallel microsamples of original aragonite exhibit well-constrained fluctuations that range between −2.0 and −4.8. Variation in δ18O of coral carbonate reflects seasonal variation in temperature ranging from 12 to 24 °C about a mean of 18 °C. These values are consistent with those derived from a bivalve and a fish otolith from the same unit, each using independently derived palaeotemperature equations. Mg/Ca and Sr/Ca ratios were determined for 40 additional samples spanning five of the 12 annual bands. Palaeotemperatures calculated using elemental-ratio thermometers calibrated on modern corals are consistently lower; mean temperature from Mg/Ca ratios are 12.5 ± 1 °C while those from Sr/Ca are 5.8 ± 2.2 °C. Assuming that δ18O-derived temperatures are correct, relationships between temperature and elemental ratio for corals growing in today's ocean can be used to estimate Oligocene palaeoseawater Mg/Ca and Sr/Ca ratios. Calculations indicate that early Oligocene seawater Mg/Ca was ∼81% (4.2 mol mol−1) and Sr/Ca ∼109% (9.9 mmol mol−1) of modern values. Oligocene seawater with this degree of Mg depletion and Sr enrichment is in good agreement with that expected during the Palaeogene transition from ‘calcite’ to ‘aragonite’ seas. Lower Oligocene Mg/Ca probably reflects a decrease toward the present day in sea-floor hydrothermal activity and concomitant decrease in scavenging of magnesium from seawater. Elevated Sr/Ca ratio may record lesser amounts of Oligocene aragonite precipitation and a correspondingly lower flux of strontium into the sedimentary carbonate reservoir than today. [ABSTRACT FROM AUTHOR]

Examines the carbon and oxygen isotopic composition of the Austin Chalk in order to document the effects of burial diagenesis on carbon and oxygen-isotopic composition. Analysis of low magnesium calcite oysters to estimate the starting composition of Cretaceous marine sediment; Compositions of micrite, intergranular and fracture cement; Minor variation in composition of micrite.

Tectono-stratigraphic models of foredeep sedimentation have generally presumed a direct link between changing rates of tectonism and concomitant sedimentological response as manifested by change in thickness, composition or depositional environment of sediment accumulating in adjacent basins. Lacustrine limestone units within the early Cretaceous fluvial/lacustrine Gannett Group of western Wyoming exhibit systematic variation in several geochemical proxies of relative rates of precipitation and evaporation, indicating that lakewater chemistry was controlled by variation in regional climate. Change in proportion of allochthonous terrigenous clastic vs. autochthonous carbonate deposition, as well as carbonate Ma/Ca ratio and stable isotopic composition, occurs at two scales. Metre-scale alternation of micritic limestone and argillaceous marl is accompanied by mineralogical and isotopic variation within individual beds, indicating preferential carbonate accumulation during intervals of decreased regional meteoric precipitation relative to lake-surface evaporation. Limestone deposition began during intervals of maximum aridity, and decreased as increased meteoric precipitation-driven flux of terrigenous clastic sediment overwhelmed sites of carbonate accumulation. Similar upsection variation in limestone mineralogy and isotopic composition at a scale of tens of metres reflects the multiple processes of long-term increase in meteoric precipitation and lakewater freshening prior to influx of terrigenons sediment, across-basin fluvial--deltaic progradation, and renewed accumulation of riverine terrigenous units. Such trends suggest that formation-scale alternation between fluvial clastic and lacustrine carbonate deposition was controlled by climate change. [ABSTRACT FROM AUTHOR]

Sclerosponges have great potential as temperature (T) recorders; they precipitate carbon and oxygen isotopes in apparent equilibrium with seawater. These animals lack photosynthetic symbionts, which simplifies interpretation of their isotopic record. It also allows them to live in subphotic depths, recording T below the thermocline and as deep as the carbonate compensation depth. The coralline sponge Acanthocaetetes wellsi is widespread in caves in the western Tropical Pacific. The West Pacific warm pool accumulates here before moving eastward to the Americas during El Niño events. A. wellsi has distinct growth bands averaging ∼1 mm/yr. Skeletal ∂13C and ∂18O show millimeter-scale cyclicity, apparently due to annual T variation, and large amplitude swings that occur every four to seven cycles, likely indicating El Niño events. Because individual sponges live for several centuries, they can provide high-resolution records of pre- and post-industrial El Niños. Additionally, the caves contain both live and dead A. wellsi. Cross-correlation of successively older specimens should yield a 1000–2000-year record. The area, depth, and T distribution of prior warm pools will be defined. The velocity of their eastward movement may be determinable from East Pacific A. wellsi and American Mytilus. From these data, estimates of heat transfer rates can be derived, allowing determination of past El Niño intensities. By correlating past intensities with known impacts, refined prediction of El Niño effects will be possible. [ABSTRACT FROM AUTHOR]