Glacial Maximum Research Articles

Paleoenvironmental evolution and East Asian monsoon records through three stages of paleochannels since the mid-pleistocene in the Western Bohai Sea, North China

Three stages of paleochannels development, dating back to the Mid-Pleistocene, have been identified in the western Bohai Sea (BS) region. However, the factors controlling their sedimentary formation remain unclear. This study analyzed samples from DZQ01 and adjacent boreholes to establish a chronological framework through AMS 14C and OSL dating, complemented by grain size and geochemical analyses. End-member analysis using the Generalized Weibull method successfully separated three components: EM1, EM2, and EM3. EM3 (<26.28 μm) reflects the influence of the East Asian Summer Monsoon (EASM), while EM2 (26.28–105.1 μm) is indicative of the East Asian Winter Monsoon (EAWM). Geochemical indicators, such as the Rb/Sr ratio, reflect the impact of paleoclimatic changes. This study identified five major glaciation events since the Mid-Pleistocene. The DU6 unit recorded two glacial stages (300–272 cal. ka B.P.), characterized by a weakened EASM and a stronger EAWM. Similarly, the DU4 unit recorded two glacial stages (165–127 cal. ka B.P.), also marked by a subdued EASM and an enhanced EAWM. The DU2 unit reflects a prolonged glacial stages (71–14 cal. ka B.P.), dominated by the EAWM, resulting in cold and dry conditions. Overall, the uplift of the Tibetan Plateau and sea-level fluctuation significantly influenced the formation and evolution of paleochannels, with the sedimentary characteristics closely tied to the intensities of the East Asian Monsoon (EAM).

The last millennium vegetation and environmental history of the Hyrcanian highland region, a pollen record from Shekardasht mire, northern Iran

High-altitude mires are sensitive to environmental change and may provide useful insights into past climate change and human impact. Here we present the results of a palynological study on a mire close to the boundary of a forest-steppe ecotone in western Hyrcanian area of northern Iran. Two major phases of anthropogenic deforestation were detected: one during the period 600–500 cal yr BP and the other over the last decades of the twentieth century. The cold episode of the Little Ice Age was possibly responsible for the abandonment of the original village and expansion of sparse stands of Quercus macranthera-Carpinus orientalis in association with Fagus orientalis, Ulmus carpinifolia/U. glabra, Acer campestre and Vitis vinifera. Our data are crucial for choosing suitable species in reforestation programmes in the highlands of western Alborz Mountains.

Constraints on the continental weathering intensity through the Permian using lithium isotopes of well-preserved brachiopod shells

Lithium isotopic compositions (δ7Li) in marine carbonates provide a powerful means to track continental weathering intensity through geological history and promote our understanding of how and why Earth remained habitable. However, δ7Li values in ancient carbonates can be affected by factors such as primary carbonate mineralogy (e.g., aragonite, high-Mg calcite, low-Mg-calcite) and post-depositional diagenesis, which may alter their primary signals. Modern articulate brachiopod shells have shown a constant δ7Li offset compared to contemporary seawater, suggesting that fossil brachiopod shells could serve as robust archives for this geochemical proxy due to their diagenetically resistant low-Mg calcite composition. Despite this potential, few studies have explored this topic.In this study, we analyze and compare the geochemical compositions of fossil brachiopod shells and their enclosing carbonate rocks from Permian strata in South China to investigate whether brachiopod shells can reliably preserve the δ7Li values of ancient seawater and, if they do, to obtain new constraints on changes in the continental weathering regimes through the Permian. We used several diagenetic screening methods, including scanning electron microscopy, cathodoluminescence microscopy, and elemental concentrations to detect potential diagenetic alteration. Our results indicate that 32 out of 49 shells preserve primary δ7Li values. Notably, we observed that δ7Li offsets between brachiopod shells and their enclosing carbonate rocks vary across the studied sections, likely due to diagenetic alteration of the bulk carbonates. Using the δ7Li values from well-preserved brachiopod shells, we reconstructed δ7Li variations of Permian seawater. We identified three distinct intervals: (1) extremely low δ7Lisw values (as low as 6.7 ± 2.9, n = 12, 1sd) during the Asselian and Sakmarian stages, (2) high δ7Lisw values (32.6 ± 4.5, n = 3, 1sd) similar to modern seawater during the Kungurian stage, and (3) relatively low δ7Lisw values (13.6 ± 3.9‰, n = 14, 1sd) from the Capitanian to the Changhsingian stages. These significant changes in δ7Li values of Permian seawater indicate dramatic changes in continental weathering regimes during the Permian, possibly linked to major climate changes, such as the termination of the Late Paleozoic Ice Age and warming associated with the emplacement of the large igneous provinces.

Microfacies Characteristics of Late Pennsylvanian Cyclothems on the Carbonate Platform Margin in Guizhou, South China

Late Pennsylvanian cyclothems are documented from the carbonate platform margin in Guizhou, South China, providing a unique opportunity to study glacio-eustatic fluctuations and their impact on reef development. This paper focuses on a shallow-water, reef-bearing succession and a deep-water succession in the Houchang area of Guizhou. Fourteen microfacies, grouped into seven associations, represent distinct depositional environments. These microfacies associations exhibit vertical cyclicity, interpreted as cyclothems, similar to those observed globally, which are attributed to the waxing and waning of the Gondwana ice sheet. The cyclothems are primarily composed of sediments below the wave base within a shallow-water platform margin and deep-water settings. Those cyclothems show strong correlations with those observed in South China, Ukraine, and the North American Midcontinent, suggesting a potential connection to global glacio-eustatic processes. A brief and rapid sea-level rise during the late Kasimovian may correspond to a recently recognized global warming event. A microfacies analysis indicates that these cyclothems reflect glacial-type sea-level fluctuations ranging from 15 to 35 m. Notably, the reef-bearing cyclothems correspond to intermediate, major cyclothems identified in South China and the Midcontinent from the late Moscovian to early Kasimovian stages. The global cyclothem correlations and reef development patterns in South China suggest that intermediate, major cycles were the primary controls on reef growth and demise, while minor cycles influenced biostromes and community succession within the reefs. These findings underscore the pivotal role of the Late Paleozoic Ice Age (LPIA) in shaping reef development in far-field regions during the Late Pennsylvanian.

Whether niche changes promote the evolution of species: a case study of Paeonia in Asia and North America

Ecological changes have been observed to promote rates of lineage diversification, yet the precise roles of ecological factors, species evolution, and environmental variability in driving species diversity remain research hot spots. The association between ecological change and lineage diversification, particularly with regard to the size of the time scale, remains poorly understood. To explore whether ecological change facilitates species evolution, we focused on the unique family Paeoniaceae, which encompasses both herbaceous and woody taxa, to investigate the evolutionary rates. As a unique family characterized by a single genus of angiosperms and comprising various climatic types, the ecological niche changes of Paeoniaceae are closely associated with the evolution, making it an ideal model for conducting association analysis. In this study, we integrated the molecular fragments and ecological factors to explore the relationship between species evolution and niche changes in Paeoniaceae. The phylogenetic tree revealed that Paeoniaceae forms a sister relationship with Penthoraceae, Haloragidaceae, Iteaceae, Crassulaceae, and Saxifragaceae, constituting an independent clade based on the positive selection of molecular fragments including two protein-coding genes and eight non-coding regions. The divergence time was estimated to be between 102 and 116 Mya (Million years ago). The phylogenetic tree within Paeonia revealed a clear division into three groups: sections of Paeonia, Moutan, and Onaepia with high support values for each branch based on the ten positive selection of molecular fragments. The rapid rate of evolution observed in Paeonia, about 0-5 Mya. In addition, ecological niche modeling showed that the potential distributions for Paeonia expanded from middle Asia to eastern Asia, and from central North America to the Northern part of North America during the Last Glacial Maximum (LGM) to Mid Holocene (MID) period. This suggests that Paeonia continuously adapted to changing ecological environments over time. Compared to the rate of climatic niche divergence and lineage diversification, the ecological niche of Paeonia underwent significant changes during the period of 3-11 Mya, occurring 5 Mya earlier than the period of evolutionary rate change. These findings offer comprehensive insights into the relationship between niche change and the evolution of species, providing valuable perspectives for further ecological cultivation efforts.

Effect of climate change on the habitat suitability of the relict species Zelkova carpinifolia Spach using ensembled species distribution modelling.

Zelkova carpinifolia is a Tertiary relict tree distributed in Hyrcanian and Colchic forests. Most of its habitat has been destroyed in the last century. This study aimed to model potentially suitable habitat areas for Zelkova carpinifolia from the past to the future. The Last Glacial Maximum (LGM) and Future (2061-2080) models include 19 bioclimatic variables from the CCSM4 global circulation model Pearson correlation coefficient was used to assess collinearity between variables and ten variables were selected for distribution modelling. Habitat suitability was estimated using the Biodiversity Modelling (BIOMOD) ensemble modelling method by combining the results of ten algorithm models using the R package "biomod2". The area under the curve (AUC) of the receiver operating characteristic (ROC) curve and true skills statistics (TSS) were calculated to evaluate the performance of the models. The contributions of the environmental variables were calculated separately for each algorithm model. According to the results obtained, the most effective bioclimatic variable in the distribution of the species is temperature seasonality (Bio4). The modelling results revealed that Zelkova carpinifolia survived in suitable refuge areas in western Asia during the LGM. These distribution areas have remained largely unchanged and even expanded. The future model results predict that the suitable habitats of the species will narrow in the Hyrcanian forests south of Caspian Sea and that more suitable conditions will be found around the Caucasus. Given the increasing destruction of these valuable plant species due to human activities and the expected negative impacts of climate change in the future, it is important to develop policies and strategies for the protection of Zelkova carpinifolia's habitat, the creation of nature reserves, and sustainability.

Dynamic LIA advances hastened the demise of small valley glaciers in central Svalbard

Most small land-terminating glaciers in Svalbard have experienced large recession since the Little Ice Age (LIA) and today are thin, cold, and largely inactive. This likely contrasts to their LIA conditions, but the observational record from that time is sparse. We investigate the evolution of five small glaciers in central Nordenskiöld Land, Svalbard, from the LIA to 2019. Photogrammetric reconstructions and ground penetrating radar are used to reconstruct their geometric changes since 1936, and historical observation, photographs, and geomorphological mapping extend this history to before the 1900s. Our results show that from 1936 to 2019, the study glaciers on average lost 49.6% of their area and 77.4% ± 7.7% of their volume, with the greatest volume loss at Scott Turnerbreen of 91% ± 5%. Four out of these five glaciers strongly indicate a history of surge-like advances near the end of the LIA within one or two decades, and the rate of subsequent mass loss seems connected to their previous dynamics. This apparent switch to high activity during a period of rapid climatic change, could have implications for our understanding of past and future glacier evolution; climate change and highly dynamic glacier responses may be more connected than previously thought.

Secondary contact erodes Pleistocene diversification in a wide-ranging freshwater mussel (Quadrula).

The isolated river drainages of eastern North America serve as a natural laboratory to investigate the roles of allopatry and secondary contact in the evolutionary trajectories of recently diverged lineages. Drainage divides facilitate allopatric speciation, but due to their sensitivity to climatic and geomorphological changes, neighboring rivers frequently coalesce, creating recurrent opportunities of isolation and contact throughout the history of aquatic lineages. The freshwater mussel Quadrula quadrula is widely distributed across isolated rivers of eastern North America and possesses high phenotypic and molecular variation across its range. We integrate sequence data from three genomes, including female- and male-inherited mitochondrial markers and thousands of nuclear encoded SNPs with morphology and geography to illuminate the group's divergence history. Across contemporary isolated rivers, we found continuums of molecular and morphological variation, following a pattern of isolation by distance. In contact zones, hybridization was frequent with no apparent fitness consequences, as advanced hybrids were common. Accordingly, we recognize Q. quadrula as a single cohesive species with subspecific variation (Q. quadrula rumphiana). Demographic modeling and divergence dating supported a divergence history characterized by allopatric vicariance followed by secondary contact, likely driven by river rearrangements and Pleistocene glacial cycles. Despite clinal range-wide variation and hybridization in contact zones, the process-based species delimitation tool delimitR, which considers demographic scenarios like secondary contact, supported the delimitation of the maximum number of species tested. As such, when interpreting species delimitation results, we suggest careful consideration of spatial sampling and subsequent geographic patterns of biological variation, particularly for wide-ranging taxa.

Exploring the impact of deglaciation on fault slip in the Sangre de Cristo Mountains, Colorado, USA

Few natural examples exist where climate’s influence on tectonics is clear. Based on a study of the Sangre de Cristo Mountains in southern Colorado, we argue that climate-driven changes in ice loads affected spatial and temporal slip patterns on the range-front normal fault. Relict glacial features enable the reconstruction of paleoglacier extents and show variable amounts of footwall ice coverage during the Last Glacial Maximum (LGM). Line load models indicate post-LGM ice melting reduced fault clamping stress by ∼20−55 kPa at seismic depths. Flexural isostatic modeling shows several meters of footwall uplift due to ice unloading with spatial patterns and magnitudes consistent with post-LGM fault throw measured from offset Holocene and late Pleistocene alluvial fans. Post-LGM fault throw rates are at least a factor of five higher than middle and early Pleistocene rates. We infer that climate-modulated ice-load changes can pace fault clamping stress and slip patterns on range-bounding normal faults.

Past foraminiferal acclimatization capacity is limited during future warming.

Climate change affects marine organisms, causing migrations, biomass reduction and extinctions1,2. However, the abilities of marine species to adapt to these changes remain poorly constrained on both geological and anthropogenic timescales. Here we combine the fossil record and a global trait-based plankton model to study optimal temperatures of marine calcifying zooplankton (foraminifera, Rhizaria) through time. The results show that spinose foraminifera with algal symbionts acclimatized to deglacial warming at the end of the Last Glacial Maximum (LGM, 19-21 thousand years ago, ka), whereas foraminifera without symbionts (non-spinose or spinose) kept the same thermal preference and migrated polewards. However, when forcing the trait-based plankton model with rapid transient warming over the coming century (1.5 °C, 2 °C, 3 °C and 4 °C relative to pre-industrial baseline), the model suggests that the acclimatization capacities of all ecogroups are limited and insufficient to track warming rates. Therefore, foraminifera are projected to migrate polewards and reduce their global carbon biomass by 5.7-15.1% (depending on the warming) by 2100 relative to 1900-1950. Our study highlights the different challenges posed by anthropogenic and geological warming for marine plankton and their ecosystem functions.

A Whole-Genome Survey and the Mitochondrial Genome of Acanthocepola indica Provide Insights into Its Phylogenetic Relationships in Priacanthiformes

Acanthocepola indica, a deep-sea snake fish, is primarily found in the Indo-west Pacific region, including India, Korea, Japan, and the South China Sea. The taxonomic classification of A. indica based on morphological characteristics remains inaccurate and unclear. In this study, we utilized next-generation sequencing to generate comprehensive genomic data for A. indica. The estimated genome size of A. indica was 422.95 Mb, with a heterozygosity ratio of 1.02% and a sequence repeat ratio of 22.43%. Our analysis suggested that A. indica is diploid, and the draft genome assembly consists of 1,059,784 contigs with a contig N50 of 1942 bp. We identified a total of 444,728 simple sequence repeats in the genome of A. indica. Furthermore, we successfully assembled the complete mitochondrial genome (16,439 bp) of A. indica, which included 13 protein-coding genes, 22 tRNA genes and 2 rRNA genes. Phylogenetic analysis based on mitochondrial genomes revealed that A. indica is closely related to Acanthocepola krusensternii and Cepola schlegelii, providing evidence that the family Cepolidae belongs to the order Priacanthiformes. Population size dynamics analysis indicated that A. indica experienced a bottleneck effect during the Pleistocene Glacial Epoch, likely due to the changes in glacial cycles and sea level fluctuations since ~800 Kya.

Dynamic processes determine precipitation variability in Eastern Central European since the Last Glacial Maximum

AbstractThe response of European precipitation variability to climate change is still poorly understood. Here we present a high-resolution speleothem record of Eastern Central European (ECE) autumn/winter precipitation to study decadal to centennial hydroclimatic variations in the European-Atlantic sector since the Last Glacial Maximum. The Cloşani Cave δ18O record shows that the reorganization of the North Atlantic jet following the demise of the Northern Hemispheric ice sheets lasted until c. 6000 to 5000 years before present. Trace element-derived semi-quantitative autumn/winter precipitation amount reveals that the late Glacial and the early to mid-Holocene experienced about 20–30% higher precipitation than present. During the deglaciation, we detect an increased decadal to centennial precipitation variability decoupled from millennial-scale North Atlantic temperature changes. The findings suggest that dynamic (rather than thermodynamic) processes determine regional precipitation variability and the probability of extreme precipitation events in ECE, highlighting the importance of understanding such dynamics for future predictions.

Species distribution modeling of North American beavers from the late Pliocene into the future

Abstract Beavers have occurred in North America since at least 7 Ma, but relatively little is known about their distribution across the continent. We modeled distributions of beavers in the late Pliocene (3.3 Ma), Pleistocene (130 ka and 21 ka), and recent Holocene (1970 to 2000) to understand their dispersal across North America, predict future distributions and predict their possible response to future climate and habitat changes. Occurrence data for Castor canadensis were derived from the Global Biodiversity Information Facility. Those data were used with both modern (1970 to 2000) and modeled future (EC-Earth-Veg 2081 to 2100) bioclimatic variables from WorldClim as well as past (Pliocene Marine Isotope Stage M2, Pleistocene Last Interglacial, and Pleistocene Last Glacial Maximum) bioclimatic variables from PaleoClim to model beaver distributions through time. Fossil locality points for Castor extracted from the New and Old Worlds Database of Fossil Mammals (NOW), NEOTOMA Paleoecology Database, and Paleobiology Database were overlain on past projection models to use as validation points. Models were run using MaxEnt with post-processing in ArcGIS. Accuracy for the 5 models ranged between 59.6% and 60.2%. Results for the present model (1970 to 2000) showed habitat suitability in areas beavers inhabit today. During the Pliocene MIS M2 cooling event (3.3 Ma) and Pleistocene Last Glacial Maximum (21 ka), habitat suitability shifted further south into Mexico and peninsular Florida and away from more periglacial northern regions. During the Last Interglacial period (130 ka) and modeled future (2081 to 2100) EC-Earth-Veg 2081 to 2100, habitat suitability was higher in coastal and central regions in North America and lower in southern regions compared to their present distribution. Distributions were most affected by precipitation seasonality, isothermality, and mean annual temperature. High variability in seasonal precipitation and temperatures is likely to influence surface water availability, vegetation type, and riparian vegetation composition, which consequently may reduce available food resources and habitat for beavers. Observed shifts during warmer periods may indicate areas in the late Miocene that facilitated dispersal into North America. Future models using other predicted climatic scenarios and shared socioeconomic pathways may provide better resolution of potential future shifts in beaver distribution with best- and worst-case climate scenarios, thereby permitting at-risk areas to be prioritized for conservation in the face of climate change.

The comparative role of physical system processes in Hudson Strait ice stream cycling: a comprehensive model-based test of Heinrich event hypotheses

Abstract. Despite their recognized significance on global climate and extensive research efforts, the mechanism(s) driving Heinrich events remain(s) a subject of debate. Here, we use the 3D thermomechanically coupled glacial systems model (GSM) to examine the Hudson Strait ice stream surge cycling and the role of three factors previously hypothesized to play a critical role in Heinrich events: ice shelves, glacial isostatic adjustment, and sub-surface ocean temperature forcings. In contrast to all previous modeling studies examining HEs, the GSM uses a transient last glacial cycle climate forcing, global viscoelastic glacial isostatic adjustment model, and sub-glacial hydrology model. The results presented here are based on a high-variance sub-ensemble retrieved from North American history matching for the last glacial cycle. Over our comparatively wide sampling of the potential parameter space (52 ensemble parameters for climate forcing and process uncertainties), we find two modes of Hudson Strait ice streaming: classic binge–purge versus near-continuous ice streaming with occasional shutdowns and subsequent surge onset overshoot. Our model results indicate that large ice shelves covering the Labrador Sea during the last glacial cycle only occur when extreme calving restrictions are applied. The otherwise minor ice shelves provide insignificant buttressing for the Hudson Strait ice stream. While sub-surface ocean temperature forcing leads to minor differences regarding surge characteristics, glacial isostatic adjustment does have a significant impact. Given input uncertainties, the strongest controls on ice stream surge cycling are the poorly constrained deep geothermal heat flux under Hudson Bay and Hudson Strait and the basal drag law. Decreasing the geothermal heat flux within available constraints and/or using a Coulomb sliding law instead of a Weertman-type power law leads to a shift from the near-continuous streaming mode to the binge–purge mode.

Slow and diverse: Upslope expansion of tree species since 1854 in the Bavarian Alps (Germany) detected by citizen science

ABSTRACT We motivated mountain hikers to survey high-altitude occurrences of selected tree and shrub species in the Bavarian Alps in a citizen science portal and compared the observations with historical data collected by the botanist Otto Sendtner at the end of the Little Ice Age. For nine of ten species with sufficient numbers of historical and citizen science observations, currently uppermost outpost occurrences were verified after data curation and subsampling. Tree species of montane (Sorbus aucuparia: 342 m, Abies alba: 324 m, Acer pseudoplatanus: 273 m, Fagus sylvatica: 141 m) and of lower subalpine distribution (Betula pubescens: 278 m, Picea abies: 55 m) climbed significantly, whereas species found at the high subalpine tree line ecotone ascended less (Pinus mugo: 21 m, Larix decidua: 13 m, Alnus alnobetula: 6 m) or even appeared to lose elevation (Pinus cembra: −57 m). We explain the gap between realized and potential upslope shift of ca. 500 m suggested by the thermal elevation gradient by the lack of mature soils at high altitudes and low summit heights in the Bavarian Alps.

Predicting trends in atmospheric CO2 across the Mid-Pleistocene Transition using existing climate archives

Abstract. During the Mid-Pleistocene Transition (MPT), ca. 1200–800 000 years ago (ka), the Earth's glacial cycles changed from 41 to 100 kyr periodicity. The emergence of this longer ice age periodicity was accompanied by higher global ice volume in glacial periods and lower global ice volume in interglacial periods. Since there is no known change in external orbital forcing across the MPT, it is generally agreed that the cause of this transition is internal to the Earth system. Resolving the climate, carbon cycle, and cryosphere processes responsible for the MPT remains a major challenge in Earth and palaeoclimate science. To address this challenge, the international ice core community has prioritised recovery of an ice core record spanning the MPT interval. Here we present results from a simple generalised least-squares (GLS) model that predicts atmospheric CO2 out to 1.8 Myr. Our prediction utilises existing records of atmospheric carbon dioxide (CO2) from Antarctic ice cores spanning the past 800 kyr along with the existing LR04 benthic δ18Ocalcite stack (Lisiecki and Raymo, 2005; hereafter “benthic δ18O stack”) from marine sediment cores. Our predictions assume that the relationship between CO2 and benthic δ18O over the past 800 000 years can be extended over the last 1.5 million years. The implicit null hypothesis is that there has been no fundamental change in feedbacks between atmospheric CO2 and the climate parameters represented by benthic δ18O, global ice volume, and ocean temperature. We test the GLS-model-predicted CO2 concentrations against observed blue ice CO2 concentrations, δ11B-based CO2 reconstructions from marine sediment cores, and δ13C of leaf-wax-based CO2 reconstructions (Higgins et al., 2015; Yan et al., 2019; Yamamoto et al., 2022). We show that there is no clear evidence from the existing blue ice or proxy CO2 data to reject our predictions or our associated null hypothesis. A definitive test and/or rejection of the null hypothesis may be provided following recovery and analysis of continuous oldest ice core records from Antarctica, which are still several years away. The record presented here should provide a useful comparison for the oldest ice core records and an opportunity for further constraints on the processes involved in the MPT.

Fauna and landscapes of the Southern Minusinsk basin (southern Siberia) at the end of the late pleistocene

The paleolandscapes of the first half of MIS 2 or the Last Glacial Maximum (LGM) were reconstructed based on the study of the first mammoth fauna locality in the Abakan River valley and comparison with other localities in the South Minusinsk Basin. The sediments in the geological section of Uytag are represented by interlayers of silts, underlain and overlain by sandstone rock slack fragment, interpreted as weakly eroded deluvium. The study of the Uytag locality made it possible to clarify the time and range of distribution in southern Siberia of some mammal species Ovis ammon, Equus ferus, Marmota baibacina, Sicista subtilis, Lagurus lagurus and Microtus gregalis, as well as Pleistocene bird Aquila chrysaetos. The age of the locality is confirmed by radiocarbon dating. The species composition of the fauna is similar to other localities in the region of this age. All known localities and single finds of faunal remains indicate a wide development of open steppe landscapes in the South Minusinsk Basin during the LGM.

Stable isotopic evidence for increased terrestrial productivity through geological time

Marine life on Earth is known back to the Archean Eon, when life on land is assumed to have been less pervasive than now. Precambrian life on land can now be tested with stable isotopes because living soil CO2 is isotopically distinct for both carbon and oxygen from both marine and volcanic CO2. Our novel compilation of previously published oxygen and carbon isotopic compositions of pedogenic and paleokarst carbonate can be compared with the coeval marine record. Long-term enrichment (to heavier isotopic composition) of oxygen, but no significant trend in carbon through time, long apparent from marine carbonate, is now demonstrated also for pedogenic and paleokarst carbonate. Oxygen isotopic enrichment is not due to changing global temperature or hypsometry, but to increased evapotranspiration and photosynthesis on larger continents. Differences in isotopic composition between land and sea have increased in an episodic fashion, peaking at times of major evolutionary innovations for life on land, and also at times of ice ages. The δ13C and δ18O divergences between land and sea correspond to terrestrial productivity spikes including evolution of Neoproterozoic (635 Ma) lichens, middle Ordovician (470 Ma) non-vascular land plants, middle Devonian (385 Ma) forests, early Cretaceous (125 Ma) angiosperms, and middle Miocene (20 Ma) sod grasslands.

Glacial Hydro-Isostatic Adjustment at Mid-Ocean Ridges

Summary Recent studies have suggested a link between ice age sea level fluctuations and variations in magma production and crustal faulting along mid-ocean ridges based on the detection of Milankovitch cycle frequencies in topography off several ridges. These fluctuations have also been connected to variability in hydrothermal metal fluxes near ridges. Ice age sea level calculations have shown that the sea level change across glacial cycles will be characterized by significant geographic variability, that is, departures from eustasy, due to the gravitational, deformation and rotational effects of the glacial isostatic adjustment (GIA) process. Using a state-of-the-art GIA simulation that incorporates 3-D variations in Earth viscoelastic structure, including plate boundaries, and updated constraints on the magnitude and geometry of ice mass fluctuations, we predict global sea level changes from Last Glacial Maximum (LGM, 26 ka) to present and from the Penultimate Glacial Maximum (PGM, 143 ka) to the Last Interglacial (LIG, 128 ka). We focus on the results along three ridges: the Mid-Atlantic Ridge, Juan de Fuca Ridge and East Pacific Rise, which are examples of slow, intermediate and fast spreading ridges, respectively. Sea level change across the Mid-Atlantic Ridge shows the greatest variability, ranging from a sea level fall greater than 200 m in Iceland to a maximum rise of ∼150 m in the South Atlantic, with significant non-monotonicity north of the Equator as the ridge weaves across the field of sea level changes. We also calculate changes in crustal normal stress from LGM to present-day across the Mid-Atlantic and Juan de Fuca Ridges and the East Pacific Rise. These results indicate that the contribution from ice mass changes to the crustal stress field can be significant well away from the location of ancient ice complexes. We conclude that any exploration of the hypothesized links to magma production and crustal faulting must consider both ocean and ice loading effects and, more generally, the profound geographic variability of the GIA process.

Revisiting the mid-Pleistocene transition ocean circulation crisis.

The mid-Pleistocene transition (MPT) [~1.25 to 0.85 million years ago (Ma)] marks a shift in the character of glacial-interglacial climate (1, 2). One prevailing hypothesis for the origin of the MPT is that glacial deep ocean circulation fundamentally changed, marked by a circulation "crisis" at ~0.90 Ma (marine isotope stages 24 to 22) (3). Using high-resolution paired neodymium, carbon, and oxygen isotope data from the South Atlantic Ocean (Cape Basin) across the MPT, we find no evidence of a substantial change in deep ocean circulation. Before and during the early MPT (~1.30 to 1.12 Ma), the glacial deep ocean variability closely resembled that of the most recent glacial cycle. The carbon storage facilitated by developing deep ocean stratification across the MPT required only modest circulation adjustments.