Methane (CH4) emissions from Arctic lakes are of global concern in a warming world. Past Holocene warming provides an opportunity to examine carbon-climate feedbacks that develop over hundreds to thousands of years, but thus far records of past long-term changes in lake CH4 dynamics are rare. Here, we demonstrate for the first time that the hydrogen stable isotopic composition of aquatic plant wax records incorporation of CH4 in plant (aquatic moss) biomass. Trends in δ2H and δ13C values of aquatic plant derived leaf waxes point to widespread and sustained middle Holocene shifts in CH4 cycling at climatically diverse sites across Greenland during millennia of elevated summer temperatures. Independent proxies indicate concurrent increases in local primary productivity and decreases in hypolimnetic oxygen. These data portend that ongoing warming may promote an enduring shift towards conditions that enhance methanogenesis in many Arctic lakes, including in lakes where these conditions do not exist today. This work highlights a previously unrecognized factor influencing δ2H values of aquatic leaf waxes in some high-latitude lakes, and also draws attention to the role of common aquatic mosses as a potentially important sink of lake CH4 across the Arctic that has yet to be quantified.
Accurate dating of marine sediments is essential to reconstruct past changes in oceanography and climate. Benthic foraminiferal oxygen isotope series from such sediments record long-term changes in global ice volume and deep-water temperature. They are commonly used in the Plio-Pleistocene to correlate deep ocean records and to construct age models. However, continental margin settings often display much higher sedimentation rates due to variations in regional depositional setting and local input of sediment. Here, it is necessary to create a regional multi-site framework to allow precise dating of strata. We create such a high-resolution regional framework to determine the ages of events for the Northwest Shelf (NWS) of Australia, which was cored by International Ocean Discovery Program (IODP) Expedition 356. We employ benthic foraminiferal oxygen and carbon isotopes to construct an astronomically-tuned age model for IODP Site U1463. The age model is applied to the IODP Site U1463 downhole-logging natural gamma radiation (NGR) depth-series, which was then correlated to the NGR of other IODP sites and several industry wells in the area. The IODP Site U1463 age-depth model provides geologic time anchors for numerous sedimentary archives on the NWS. This approach allows assigning ages to regional seismic reflectors and the timing of key climate-related siliciclastic phases in a predominantly carbonate-rich sequence like the Bare Formation. Finally, this age model is used to chronologically calibrate planktonic foraminiferal biostratigraphic datums showing that the Indonesian Throughflow had shoaled enough during the early Pliocene to act as biogeographical barrier between the Pacific and Indian Ocean.
Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere’s composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth’s radiative energy budget. Paleoclimate records at high-altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper-tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped-isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three-dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped-isotope composition of atmospheric oxygen to infer that the upper troposphere (5 – 15 km altitude, effective mean 10 – 11 km) was 4 – 10°C cooler during the LGM than during the late preindustrial Holocene. These results support a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy-model disagreements with other high-altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land-atmosphere feedbacks.
The Eocene–Oligocene transition (~34 Ma), is marked by the rapid development of a semi-permanent Antarctic ice-sheet, as indicated by ice-rafted debris. Proxy reconstructions indicate a drop in atmospheric CO₂ and global cooling. How these changes affected sea surface temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped-isotope thermometry to well-preserved planktic foraminifera, that are associated with mixed-layer and thermocline dwelling depths from the drift sediments at IODP Site 1411, Newfoundland, across four intervals bracketing the EOT. The mixed-layer dwelling foraminifera record a cooling of 2.2 ± 2.4 °C (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous SST reconstructions, absolute temperatures (Eocene 20.0 ± 2.7 °C, Oligocene 18.0 ± 2.1 °C) appear colder than what is expected for this location based on previously reconstructed SSTs for the northernmost Atlantic. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Thermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.4 °C, Oligocene 14.0 ± 3.1 °C, cooling of 5.2 ± 3.2 °C), than foraminifera from the mixed layer, consistent with an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation.
Reconstructions of aeolian dust flux to West African margin sediments can be used to explore changing atmospheric circulation and hydroclimate over North Africa on millennial to orbital timescales. Here, we extend West African margin dust flux records back to 35 ka in a transect of core sites from 19°N to 27°N, and back to 67 ka at Ocean Drilling Program (ODP) Hole 658C, in order to explore the interplay of orbital and high-latitude forcings on North African climate and make quantitative estimates of dust flux during the core of the Last Glacial Maximum (LGM). The ODP 658C record shows a “Green Sahara” interval from 60 to 50 ka during a time of high Northern Hemisphere summer insolation, with dust fluxes similar to levels during the early Holocene African Humid Period, and an abrupt peak in flux during Heinrich event 5a (H5a). Dust fluxes increase from 60 to 35 ka while the high-latitude Northern Hemisphere cools, with peaks in dust flux associated with North Atlantic cool events. From 35 ka through the LGM dust deposition decreases in all cores, and little response is observed to low-latitude insolation changes. Dust fluxes at sites north of 20°N were near late Holocene levels during the LGM time slice, suggesting a more muted LGM response than observed in mid-latitude dust sources. Records along the northwest African margin suggest important differences in wind responses during different stadials, with maximum dust flux anomalies centered south of 20°N during H1 and north of 20°N during the Younger Dryas.
Projected changes in climate are likely to affect not only its mean state but also its variability. As such, improving our understanding of the spectrum of climate variability and how different feedbacks in the climate system influence it is of vital importance. We perform a process-based examination of variability with respect to changing orbital insulation, ice coverage, and land/sea distribution during the Last Glacial Maximum and the Holocene. To this end, we adapt a two-dimensional energy balance model [Zhuang, North & Stevens, 2017] to run transient simulations. The model is forced by carbon dioxide and solar insolation changes for the last Glacial cycle. We evaluate the model’s ability to reproduce changes in local to global, seasonal to millennial temperature distributions during the Last Glacial Maximum and the Holocene. We compare the simulated states and the transient evolution to those obtained by comprehensive coupled climate models. Finally, we test the mean-state dependence of temperature variability over a large range of model configurations and discuss implications for future climate.
Sedimentary Ice Rafted Debris (IRD) provides critical information about the climate sensitivity and dynamics of ice sheets. In recent decades, high-resolution investigations have revelated ice rafting events in response to rapid warming: such reconstructions help us constrain the near-future stability of our planet‘s fast-changing cryosphere. However, similar efforts require laborious and destructive analytical procedures to separate and count IRD. Computed Tomography (CT) holds great promise to overcome these impediments to progress by enabling the micrometer scale visualization of individual IRD grains. This study demonstrates the potential of this emerging approach by 1) validating CT counts in synthetic sediment archives (phantoms) spiked with a known number of grains, 2) replicating published IRD stratigraphies, and 3) improving sampling resolution. Our results show that semi-automated CT counting of grains in the common 150-500 µm size fraction reproduces actual particle numbers and tracks manually counted trends. We also find that differences between manual and CT-counted data are explained by image processing artifacts, offsets in sampling resolution and bioturbation. By acquiring these promising results using basic image processing tools, we argue that our work advances and broadens the applicability of ultra-high resolution IRD counting with CT to deepen our understanding of ice sheet-climate interactions on human-relevant timescales.
Sedimentary specimens of the planktonic foraminifera Globorotalia inflata can provide much needed information on subsurface conditions of past oceans. However, interpretation of its geochemical signal is complicated by possible effects of cryptic diversity and encrustation. Here we address these issues using plankton tow and sediment samples from the western South Atlantic, where the two genotypes of G. inflata meet at the Brazil-Malvinas Confluence Zone. The 18O and δ13C of encrusted specimens from both genotypes from a core within the confluence zone are indistinguishable. However, we do find a large influence of encrustation on δ18O and Mg/Ca. Whereas crust Mg/Ca ratios are at all locations lower than lamellar calcite, the crust effect on δ18O is less consistent in space. Plankton tows show that encrusted specimens occur at any depth and that even close to the surface crust Mg/Ca ratios are lower than in lamellar calcite. This is inconsistent with formation of the crust at lower temperature at greater depth. Instead we suggest that the difference between the crust and lamellar calcite Mg/Ca ratio is temperature-independent and due to the presence of high Mg/Ca bands only in the lamellar calcite. The variable crust effect on δ18O is more difficult to explain, but the higher incidence of crust free specimens in warmer waters and the observation that a crust effect is clearest in the confluence zone, hint at the possibility that the difference reflects advective mixing of specimens from warmer and colder areas, rather than vertical migration.
Large alluvial rivers transport water and sediment across continents and shape lowland landscapes. Repeated glacial cycles have dominated Earth's recent climate, but it is unclear whether these rivers are sensitive to such rapid changes. The Amazon River system, the largest and highest-discharge in the world, features extensive young terraces that demonstrate geologically rapid change temporally correlated with changes in runoff from Quaternary climate cycles. To test the plausibility of a causal relationship, we use a simple model to estimate from empirical measurements how quickly a river profile responds to changes in discharge or sediment supply. Applying this model to data from 30 gauging stations along alluvial rivers throughout the Brazilian Amazon, we find that many rivers of the Amazon basin can respond faster than glacially induced changes in runoff or sediment flux. The Amazon basin is unusually responsive compared to other large river systems due to its high discharge and sediment flux, narrow floodplains, and low slopes. As a result, we predict that the Amazon basin has been highly dynamic during Quaternary glacial cycles, with cyclical aggradation and incision of lowland rivers driving repeated habitat and environmental change throughout the region. This dynamic landscape may have contributed to the exceptional biodiversity of the region and patterns of ancient human settlement.
The mid-Piacenzian (Pliocene) climate represents the most geologically recent interval of long-term average warmth, relative to the last million years, sharing similarities with the climate projected for the end of the 21st century. Therefore, this period has been studied by both geoscientists and climate modelers for many years. A better understanding of regional late Pliocene conditions can provide insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Previous work comparing climate model results with geologic data highlighted key regional and dynamic situations where there was discord between mean annual SST estimates generated by climate model simulations and paleoenvironmental reconstructions. One key area identified was the mid- to high-latitude North Atlantic. Here, we present a comparison between alkenone-based North Atlantic PRISM4 (Pliocene Research, Interpretation and Synoptic Mapping Project, Phase 4) mean annual SST estimates and an ensemble of ten climate model simulations produced as part of PlioMIP2 (Pliocene Model Intercomparison Project, Phase 2). Our latest research demonstrates that improved experimental design incorporating temporal refinement of the paleoenvironmental reconstruction, and inclusion of new PRISM4 boundary condition data sets, significantly reduces discord between data and models.
The Fram Strait is the only deep gateway between the Arctic Ocean and the Nordic Seas and thus is a key area to study past changes in ocean circulation and the marine carbon cycle. Here, we study deep ocean temperature, δ18O, carbonate chemistry (i.e., carbonate ion concentration, [CO32-]), and nutrient content in the Fram Strait during the late glacial (35,000–19,000 years BP) and the Holocene based on benthic foraminiferal geochemistry and carbon cycle modelling. Our results indicate a thickening of Atlantic water penetrating into the northern Nordic Seas, forming a subsurface Atlantic intermediate water layer reaching to at least ~2600 m water depth during most of the late glacial period. The recirculating Atlantic layer was characterized by relatively high [CO32-] and low δ13C during the late glacial, and provides evidence for a Nordic Seas source to the glacial North Atlantic intermediate water flowing at 2000–3000 m water depth, most likely via the Denmark Strait. In addition, we discuss evidence for enhanced terrestrial carbon input to the Nordic Seas at ~23.5 ka. Comparing our δ13C and qualitative [CO32-] records with results of carbon cycle box modelling suggests that the total terrestrial CO2 release during this carbon input event was low, slow, or directly to the atmosphere.
Continued global warming is expected to result in drying of Central America, with projections suggesting a decrease in precipitation. Poor hindcasting of precipitation, however, due in part to spatial and temporal limitations in instrumental data, subjects these projections to considerable uncertainty. Paleoclimate proxy data are therefore critical for understanding regional climate responses during times of global climate reorganization. Here we present two lake-sediment based records of precipitation variability in Guatemala along with a synthesis of Central American hydroclimate records spanning the last millennium (800-2000 CE). The synthesis reveals that regional climate responses have been strikingly heterogeneous, even over relatively short distances. Our analysis further suggests that shifts in the mean position of the Intertropical Convergence Zone, which have been invoked by numerous studies to explain variability in Central American and circum-Caribbean proxy records, cannot alone explain the observed pattern of hydroclimate variability. Instead, interactions between several ocean-atmosphere processes and their disparate influences across variable topography have resulted in complex precipitation responses. These complexities highlight the difficulty of reconstructing past precipitation changes across Central America and point to the need for additional paleo-record development and analysis before the relationships between external forcing and hydroclimate change can be robustly determined. Such efforts should help anchor model-based predictions of future responses to continued global warming.
Few of the large Southern peri-alpine lakes have been studied with a sedimentological approach in their deep basin to understand the dynamics of their long-term sedimentation due, among other factors, to the high complexity of the coring in such deep lakes. In 2018, a 15.5 m-long sediment section was retrieved from the deep basin of Lake Iseo (Italy) at 251 m of water depth. Seismic survey associated to a multi-proxy approach with sedimentological and geochemical analyses, reveals a high number of event layers that corresponds to 61.4 % of the total sedimentation during the last 2000 years. The great heterogeneity of textures, colours, and grain-size distribution between the different types of event layers can be explained by the high number of potential sources of sediment inputs in this large lake system. By combining proxies for sediment source with transport processes, we were able to distinguish: i) flood events, and ii) destabilisations of slopes and deltas due to an increase of the sediment load and/or to seismic shaking. From a thorough comparison with both, the regional climatic fluctuations, and the human activity in the watershed, it appears that periods of high sediment remobilization can be linked to a previous increase in Critical Zone erosion in the watershed mainly under human forcing. Hence, even in large catchments, human activities play a key role on erosion processes and on sediment availability, disrupting the recording of the Critical Zone functioning in such lacustrine archive.
Estimates of climate sensitivity rely in part on the magnitude of global cooling during the Last Glacial Maximum (LGM). While ice cores provide reliable LGM temperatures in high-latitude regions, proxy records of sea-surface temperature (SST) disagree substantially in the low latitudes (1-3), and quantitative low-elevation paleotemperature records on land are scarce. Filling this gap, noble gases in groundwater record land surface temperatures via their temperature-dependent solubility in water (4), a direct physical relationship uncomplicated by biological and chemical processes (5-6). Individual groundwater noble gas studies (e.g. 7-8) have shown 5-7 °C LGM cooling, in line with some proxy data (e.g. tropical snowline reconstructions) but larger than notable low-latitude SST reconstructions considering land-sea cooling ratios. To date, limited spatial coverage and the use of different physical frameworks to determine temperature from noble gas data has prevented a comprehensive estimate of low-latitude LGM cooling from noble gases in groundwater. Here we compile four decades of groundwater noble gas data from six continents, all interpreted using a consistent physical framework (9). We evaluate the accuracy of the “noble gas paleothermometer” by comparing noble gas derived temperatures in late Holocene groundwater with modern observations. From LGM noble gas data, we find that the low-elevation, low-to-mid-latitude land surface cooled by 5.8 ± 0.6 °C during the LGM (9). The ratio of our land cooling estimate to a recent SST reconstruction (1) that found 4.0 °C cooling over the same low latitude band is consistent with the inter-model mean land-sea cooling ratio of 1.45 °C from PMIP4 simulations (10). Together, these recent land- and sea-surface LGM temperature reconstructions indicate greater low-latitude cooling and thus climate sensitivity than prior studies, with implications for projections of future climate. 1) Tierney et al. (2020). Nature. 2) CLIMAP Project Members (1976). Science. 3) MARGO Project Members (2009). Nat. Geosci. 4) Jenkins et al. (2019). Mar. Chem. 5) Aeschbach et al. (2000). Nature. 6) Kipfer et al. (2002). Rev. Mineral. Geochem. 7) Stute et al. (1995). Science. 8) Weyhenmeyer et al. (2000). Science. 9) Seltzer et al. (2021). Nature. 10) Kageyama et al. (2021). Clim. Past.
The Community Earth System Model version 2 (CESM2) simulates a high equilibrium climate sensitivity (ECS > 5 degC) and a Last Glacial Maximum (LGM) that is substantially colder than proxy temperatures. In this study, we use the LGM global temperature from geological proxies as a benchmark to examine the role of cloud parameterizations in simulating the LGM cooling in CESM2. Through substituting different versions of cloud schemes in the atmosphere model, we attribute the excessive LGM cooling to the new schemes of cloud microphysics and ice nucleation. Further exploration suggests that removing an inappropriate limiter on cloud ice number (NoNimax) and decreasing the time-step size (substepping) in cloud microphysics largely eliminate the excessive LGM cooling. NoNimax produces a more physically consistent treatment of mixed-phase clouds, which leads to more cloud ice content and a weaker shortwave cloud feedback over mid-to-high latitudes and the Southern Hemisphere subtropics. Microphysical substepping further weakens the shortwave cloud feedback. Based on NoNimax and microphysical substepping, we have developed a paleoclimate-calibrated CESM2 (PaleoCalibr), which simulates well the observed 20th century warming and spatial characteristics of key cloud and climate variables. PaleoCalibr has a lower ECS (~4 degC) and a 20% weaker aerosol-cloud interaction than CESM2. PaleoCalibr represents a physically and numerically better treatment of cloud microphysics and, we believe, is a more appropriate tool than CESM2 in climate change studies, especially when a large climate forcing is involved. Our study highlights the unique value of paleoclimate constraints in informing the cloud parameterizations and ultimately the future climate projection.
Coral skeletal growth is sensitive to environmental change and may be adversely impacted by an acidifying ocean. However, physiological processes can also buffer biomineralization from external conditions, providing apparent resilience to acidification in some species. These same physiological processes affect skeletal composition and can impact paleoenvironmental proxies. Understanding the mechanisms of coral calcification is thus crucial for predicting the vulnerability of different corals to ocean acidification and for accurately interpreting coral-based climate records. Here, using boron isotope (δ11B) measurements on cultured cold-water corals, we explain fundamental features of coral calcification and its sensitivity to environmental change. Boron isotopes are one of the most widely used proxies for past seawater pH, and we observe the expected sensitivity between δ11B and pH. Surprisingly, we also discover that coral δ11B is independently sensitive to seawater dissolved inorganic carbon (DIC). We can explain this new DIC effect if we introduce boric acid diffusion across cell membranes as a new flux within a geochemical model of biomineralization. This model independently predicts the sensitivity of the δ11B-pH proxy, without being trained to these data, even though calcifying fluid pH (pHCF) is constant. Boric acid diffusion resolves why δ11B is a useful proxy across a range of calcifiers, including foraminifera, even when calcifying fluid pH differs from seawater. Our modeling shows that δ11B cannot be interpreted unequivocally as a direct tracer of pHCF. Constant pHCF implies similar calcification rates as seawater pH decreases, which can explain the resilience of some corals to ocean acidification. However, we show that this resilience has a hidden energetic cost such that calcification becomes less efficient in an acidifying ocean
Long, continuous palaeoclimate records provide an opportunity to extend knowledge of decadal to multi-decadal scale climate variability beyond the limit of instrumental records. In this study, quality-controlled proxy records from southeastern Australia are examined for coherent variability during the Common Era, with age uncertainty for each record estimated using iterative age modeling. Site-level empirical orthogonal functions (EOFs) are derived from multivariate records for the purpose of objective comparison of climate signals between sites without selection bias. A regional Monte Carlo EOF (MCEOF) analysis is conducted on combined time-uncertain single-proxy records and site-level EOFs. The analysis identifies two robust vectors, which are inferred to represent hydroclimate changes. The first regional MCEOF suggests an increase in effective moisture between 900 – 1750 CE. Agreement between regional MCEOF1 and Australian temperature reconstructions suggests suppressed evaporation was a significant influence on regional effective moisture during this time. Regional MCEOF2 exhibits shorter, centennial-scale oscillations that show some similarity with rainfall reconstructions based on remote high-resolution proxies. We interpret MCEOF2 to represent regional-scale rainfall patterns driven by changes in seasonal rainfall and the influence of the Southern Annular Mode over southern Australian rainfall. This study presents the first quantitative regional synthesis of southeastern Australian hydroclimate reconstructions from multivariate sedimentary archives covering the last 1200 years. The resulting MCEOFs demonstrate the utility of low-resolution climate records from this region, but also highlight the limitations of the existing data network, which must be resolved through the generation of new records.
Carbonate clumped isotope thermometry has been calibrated for a wide variety of carbonates, including calcite, aragonite, dolomite, siderite, and many of their biogenic forms. The clumped isotope composition of the carbonate group substituting for phosphate or hydroxyl in bioapatite (Ca(PO4,CO3)(OH,F)) has also been temperature calibrated using vertebrate tooth enamel from a range of endothermic body temperatures. We apply this method to other bioapatite-bearing taxa and the calibrated temperature range is extended to lower paleoclimatologically relevant temperatures. Furthermore, because relatively large bioapatite samples are required for carbonate clumped isotope measurements (Δ47), replicate sampling of thin tooth enamel may not be feasible in many situations. Here, we use gar fish (Lepisosteus sp.) scales to extend the calibration. These fish are unique in that they are entirely covered in ganoine scales, which are >95% hydroxyapatite. Their enamel structure also makes them resistant to diagenesis. Additionally, gar fossils are common in lacustrine, fluvial, and near-shore facies, and have a wide distribution in time (Cretaceous to modern) and location (North America, South America, Europe, India, and Africa). We have developed a reliable lab protocol for measuring Δ47 in gar bioapatite. We estimate the standard error (SE) for a single measurement as 0.027‰, which is based on replicate analyses and Student T-distribution to account for sample size. We report results for modern gar scales from seven North American localities with mean annual water temperatures (MAWT) ranging from 9 to 26 °C. These data give a temperature calibration curve for gar scales of Δ47 = (0.1095 ± 0.0159) x 106/T2 – (0.5941 ± 0.0548) (R2 = 0.74) and a curve for pooled bioapatite of Δ47 = (0.1003 ± 0.0144) x 106/T2 – (0.4873 ± 0.0495) (R2 = 0.76).