Glacial-interglacial Cycles Research Articles

Evolutionary response of cold‐adapted chasmophytic plants to Quaternary climatic oscillations in the Mountains of Central Asia (a world hotspot of biodiversity)

AbstractAimPast climatic oscillations are the main driving force of evolutionary changes in alpine species. Species' response to paleoclimatic oscillations is crucial in forecasting their future response in face of climate warming. The aim of this research is to explore the effect of climatic fluctuations on the evolutionary history, demography, and distribution of high‐mountain bellflowers (Campanula lehmannianacomplex), the flagship and taxonomically problematic members of chasmophytic vegetation within an underexplored biodiversity hotspot, the Mountains of Central Asia.LocationCentral Asia (Tian Shan, Alai and Zeravshan‐Hissar Mountains).MethodsWe used molecular data (ITS, cpDNA, DArTseq‐based SNPs) of 262 individuals (70 for the phylogeny reconstruction, and 247 from 31 localities for population studies). We analysed the data using phylogenetic and molecular clock reconstructions, coalescent simulations, and ecological niche modelling.ResultsTertiary isolation between the Tian‐Shanian and Pamir‐Alaian populations led to the differentiation of the two main lineages (~5–6 Mya) corresponding toC. eugeniaeandC. lehmanniana, whereas further Quaternary isolation into subregions led to intraspecific genetic differentiation, which starts almost simultaneously for both species (~2.7–1.5 Mya). The relatively small genetic admixture among populations indicates rare historic events of connectivity. In response to Holocene warming, the analysed species experienced a substantial decline in effective population size. Currently, the distribution of both taxa is highly influenced by precipitation in the coldest and driest quarters.Main ConclusionsOur results highlight a general principle that glacial–interglacial cycles and contemporary island‐like habitats distribution, shape the genomic variation of high‐mountain species. The similar declining demographic trend of examined taxa may suggest the overall response to ongoing climate warming. The results underline also the urgent need for conservation action in alpine regions to preserve their biodiversity.

The importance of C3 and C4 grasses and CAM shrubs in the Greater Cape Floristic Region under contemporary and Last Glacial Maximum climates

Southern Africa’s Greater Cape Floristic Region (GCFR) – a hotspot of biological diversity and palaeo-anthropological evidence for modern human evolution – is a climatically complex region where broad climatic gradients influence the relative importance of plant photosynthetic types in local vegetation. This mix of photosynthetic pathways imprint an isotopic signature on palaeo-records, thereby allowing for the reconstruction of historical climate and vegetation. Here, we use vegetation plot data, coupled with the photosynthetic-type affinities of plant genera, to relate their relative cover to bioclimatic drivers and use these to create spatial distribution models of C3 and C4 grasses and CAM shrubs across the contemporary GCFR. We then use palaeodistribution modelling, drawing on downscaled climate models for the Last Glacial Maximum (LGM), to hindcast the LGM distributions of photosynthetic types for the GCFR and its then-exposed offshore areas. In addition, we construct a model to account for the effect of lower atmospheric CO2 concentration ([CO2]) during the LGM on the competitive interactions between C3 and C4 grasses and use this model to hindcast relative C4-grass cover across the glacial-era GCFR. Our palaeodistribution models suggest that, even though there were some reductions in CAM cover and shifts in C3-/C4-dominated grass layers between the LGM and Holocene, these were mostly localised, with changes at the regional scale being relatively muted. For C3/C4 grasses, distributional shifts were even more subtle once sub-ambient glacial [CO2] was accounted for. Our hindcasts of glacial, [CO2]-adjusted grass cover show agreements and inconsistencies with various sedimentary and herbivore-diet records in the palaeo-archive: both lines of evidence suggest the dominance of C3 species in the strongly winter-rainfall western GCFR; in the eastern GCFR, with muted rainfall seasonality, hindcasts of relative C3:C4 cover and δ13C signals from palaeo-records vary, with greater disparities between them. The lack of significant upheavals in the distribution of photosynthetic types suggests that the GCFR presented a relatively stable and dependable resourcescape for hunter-gatherer populations through the glacial–interglacial cycle of the LGM–Holocene transition. Our maps of relative C3-/C4-grass and CAM-shrub cover provide a reasonable surrogate for relative biomass of different photosynthetic types in local vegetation at a landscape scale and depict an independent estimate of carbon isoscapes across the GCFR – under both an interglacial (Holocene) and glacial (terminal Pleistocene) climate – against which palaeo-records can be assessed.

Glacial–Interglacial Cycles and Early Human Evolution in China

China is a crucial region for investigating the relationship between climate change and hominin evolution across diverse terrestrial ecosystems. With the continuous development of palaeoclimatology, chronology, and archaeology, the environmental and hominin record of the Early and Middle Pleistocene in China is steadily accumulating, shedding light on the effects of climate change on the distribution of early human settlements and population dynamics. However, the migration and dispersal of these early humans within long-term climate fluctuations and their underlying mechanisms remain to be clarified. Based on the spatial-temporal distribution of 95 Early to Middle Pleistocene archaeological sites in China, we found that intensified hominin activities gradually shifted southward under the influence of multiple glacial–interglacial cycles. The frequent bidirectional movements of these early humans between north and south were assumed as follows. During glacial periods, hominins living in North China migrated to southern areas, while inter-glacial periods witnessed the northward expansion of hominins inhabiting South China. Among all the potential driving mechanisms, we suggest that the available resources in terrestrial ecosystems may be the most fundamental factor. Combined with paleoenvironmental and archaeological records, we provide an Asian perspective for a better understanding of how the glacial–interglacial cycles shaped early human evolution.

BrGDGTs-based temperature and hydrological reconstruction from loess-paleosol deposits in the Eastern European Plain since 200 ka

Knowledge of glacial-interglacial changes in the temperature-humidity configuration of Eastern Europe can provide insights into the regional climate changes in response to ongoing global warming. Here, we present temperature and humidity (soil pH) reconstructions since 200 ka based on records of branched glycerol dialkyl glycerol tetraethers (brGDGTs) for the Strelitsa and Beglitsa loess sections located in the central and southern Eastern European Plain (EEP), respectively. The reconstructed temperatures ranged from 10 to 17 °C in the Strelitsa section and varied between 8 and 19 °C in the Beglitsa section over glacial-interglacial cycles. The reconstructed soil pH in the Strelitsa and Beglitsa sections ranged from 6.1 to 7.8 and from 6.6 to 8.6, respectively, indicating wet conditions during MIS 7 and MIS 1, relatively humid conditions during the interstadials of MIS 6 and MIS 5d–5a, and dry conditions during MIS 5e and the last glacial period. By correlating with climatic records from Eastern Europe, we find that the spatial-temporal distribution of humidity in the central and southern EEP was in line with the meridional shift of the northern westerlies on the orbital time scale. We suggest that the northern westerlies maximum was located over the central and southern EEP, which brought abundant moisture during the interstadials (MIS 6 and MIS 5d–5a) and relatively cool interglacials (MIS 7 and MIS 1), whereas it shifted southward and northward during cold glacial periods and warm MIS 5e, respectively, thereby leading to dry conditions in the central and southern EEP. In this context, the central and southern EEP will become dry in response to the ongoing global warming.

Extreme glacial cooling likely led to hominin depopulation of Europe in the Early Pleistocene.

The oldest known hominin remains in Europe [~1.5 to ~1.1 million years ago (Ma)] have been recovered from Iberia, where paleoenvironmental reconstructions have indicated warm and wet interglacials and mild glacials, supporting the view that once established, hominin populations persisted continuously. We report analyses of marine and terrestrial proxies from a deep-sea core on the Portugese margin that show the presence of pronounced millennial-scale climate variability during a glacial period ~1.154 to ~1.123 Ma, culminating in a terminal stadial cooling comparable to the most extreme events of the last 400,000 years. Climate envelope-model simulations reveal a drastic decrease in early hominin habitat suitability around the Mediterranean during the terminal stadial. We suggest that these extreme conditions led to the depopulation of Europe, perhaps lasting for several successive glacial-interglacial cycles.

Palynological implications for the paleoclimate and paleoceanographic reconstruction of the East Sea since the early Pleistocene at IODP site U1430

Palynomorphs, including pollen and dinoflagellate cysts from marine sediments, can provide a history of paleoclimate and paleoceanography. We present new palynological profiles based on a core from the Integrated Ocean Drilling Program Site U1430, which was drilled in the Ulleung Plateau East Sea. This core covers the past 2 Ma and spans the Mid-Pleistocene Transition (MPT, 1.2–0.8 Ma) and Mid-Brunhes Event (MBE, ∼0.43 Ma). Terrestrial and marine palynomorph records indicate orbital-scale variations in paleoclimate and paleoceanography, reflecting changes in the interconnections between vegetation, climate, and the ocean. Before the MPT, the high pollen taxa ratio and abundant temperate pollen suggest a relatively warm temperature associated with astronomical forcing-induced insolation changes. At ca. 1.3 Ma, boreal coniferous pollen increased, suggesting intensification of the East Asian winter monsoon (EAWM). The long-term trend of boreal coniferous pollen concentration was similar to that of benthic δ18O. This orbital scale boreal conifer pollen assemblage variation indicates Quaternary EAWM evolution controlled by changes in global ice volume and climatic cooling. The marine palynological data reflect the ocean circulation history of the study area, where sea level fluctuations and tectonic activity played important roles around the marginal sea. The abundance of warm-water dinoflagellate species after ca. 1.9 Ma, including Tuberculodinium vancampoae, indicates Tsushima Warm Current intrusion related to the expansion of the Korea (Tsushima) Strait. Nematosphaeropsis labyrinthus and other cold-water species increased markedly during the MPT, suggesting a low sea surface temperature and weakened vertical mixing of the East Sea due to the cooled climate and sea ice during the MPT. The marked sea level fluctuation associated with the glacial–interglacial cycling after the MBE resulted in an assemblage composition shift from cold to warm water species. Tsuga and Betula thrived during mild winters, primarily influenced by winter insolation. These genera were particularly prosperous in the early Pleistocene, coinciding with a strong intrusion of the Tsushima Warm Current (TWC). Our findings should be considered in developing a comprehensive understanding of the paleoclimate and paleoceanographic history of northern East Asia across the major climate transitions of the Pleistocene.

ENSO-like evolution of the tropical Pacific climate mean state and its potential causes since 300ka

The tropical Pacific Ocean plays a significant role in climate change, and the El Niño-Southern Oscillation (ENSO) is considered to be closely related to extreme climate phenomenon worldwide. However, the evolution of the ENSO-like patterns in the tropical Pacific during the Pleistocene glacial cycles remains controversial. In this study, we present geochemical indices and a transient model simulation to explore the ENSO-like evolution during the glacial-interglacial cycles and its driving mechanisms. Our results indicate that during the interglacial periods, the Western Pacific Warm Pool (WPWP) is characterized by the modern El Niño-like conditions with relatively decreased precipitation and weakened East Asian Summer Monsoon (EASM). By contrast, the mean state of WPWP during glacial periods is more similar to the modern La Niña-like conditions, with increased precipitation and the intensified of EASM. Spectral analyses showed that the WPWP hydrodynamics were controlled by the earth's orbital parameters through their impacts on ocean circulations. Combined with published data, we find that expansion of water masses formed in the Southern Ocean would change the deep-water redox conditions, subsurface water upwelling and surface productivity in the WPWP, and ultimately influence the ENSO-like conditions through CO2-related Walker Circulation on glacial-interglacial cycles.

Southward displacement of the glacial westerly jet over Asia driven by enhanced Arctic amplification after the Mid-Brunhes Event

The climate and paleoenvironmental evolution of the East Asian continent and adjacent Northwest Pacific Ocean is governed by the interplay of the region's dominant features in oceanic and atmospheric circulation, which control the redistribution of heat and moisture. In the atmospheric realm, the subtropical westerly jet stream and East Asian monsoon are crucial components of climate evolution over East Asia, an area that is inhabited by almost half of the world's population. Reconstruction of long-term changes in atmospheric circulation in this region is to date scarce, especially on timescales predating the last glacial-interglacial cycle. Here, we reconstruct dust fluxes, marine productivity and water temperatures in the Northwest Pacific Ocean using lipid biomarkers both from terrestrial and marine organisms and elemental scanning data. Land-plant derived wax lipids in the studied sediments were used as indicators for dust fluxes and hence are representative of past changes in wind strength and pathways. We evaluate changes both in the zonal westerlies and monsoonal winds over the past 1 Ma and across major climatic events. Prior to the Mid-Brunhes Event (MBE; ~ 430 ka ago), concentrations of these compounds showed a distinct glacial-interglacial pattern, which we relate to past changes in the position of the westerly jet stream. We demonstrate substantial changes in wind patterns across the MBE, resulting in reduced dust fluxes to the Northwest Pacific in ensuing glacials. We propose that the westerly jet was permanently located south of the Himalaya-Tibetan Plateau through peak glacials, fostered by amplified Arctic cooling.

High-resolution luminescence chronology of loess-paleosols reveals East Asian summer monsoon and winter monsoon variation in Hexi Corridor during the past 25 kyr

The variability and interaction of the East Asian summer monsoon (EASM), East Asian winter monsoon (EAWM) and Westerlies dominate mid-latitude Asian hydroclimate and influence over a billion lives in Asia. However, how variation in the EASM, EAWM and Westerlies influences climate change in their transition zones in NW China remain unclear. Specifically, how these atmospheric systems interact during strong-weak phases variation over different timescales is still hotly debated. This study investigates four loess-paleosol sequences in the transition zone between EASM and Westerlies-influenced areas along the central Hexi Corridor of NW China, and presents 63 quartz optically stimulated luminescence (OSL) ages to generate the most detailed chronology for any loess record within the Hexi Corridor. In combination with sedimentology, proxy indexes of magnetic parameters (χlf, χfd, χARM/SIRM) and grain size of these loess-paleosol sequences, and TraCE-21 ka climatic modelling, the loess deposition and climatic changes at central Hexi Corridor regions over the past 25 ka are reconstructed. Our results indicate that: (1) loess deposition initiated by at least 25 ka at Hexi Corridor. Loess depositional rates decrease from late last glacial into the Holocene, with maximum accumulation rates (140–180 g/cm3/ka) occurring during the Last Glacial Maximum. Depositional hiatuses up to 8 ka are recognized in late Quaternary Hexi Corridor loess, which suggests that high-resolution chronologies are necessary for robust paleoclimatic reconstructions from loess records in the region. (2) Over glacial-interglacial cycles, climate in the Hexi Corridor demonstrates a cold-dry glacial and a warm-moist interglacial, consistent with moisture changes observed in the monsoonal Chinese Central Loess Plateau and Westerlies-dominated Central Asia. However, moisture in central Hexi Corridor increased from the early Holocene to mid-Holocene, with a wettest period at 8–7 ka, and followed by a relatively wet late Holocene. These changes are most consistent with the EASM-dominated Chinese Central Loess Plateau and suggests that Westerlies influence in the central Hexi Corridor is minor over at least the past 25 kyr. (3) EAWM intensities demonstrate an anti-phase relationship to EASM intensities over the past 25 kyr in the Hexi Corridor, with decreasing EAWM intensity into the early-mid Holocene and an increase into the late Holocene. (4). EASM precipitation variability in the Hexi Corridor appears to not to be a direct nor singular response to changes in 30°N summer insolation. EASM lags insolation by 3 kyr during the Holocene, supporting the idea that high-latitude forcing through changes in ice volume, greenhouse gases, and/or meltwater input to North Atlantic and Southern oceans can effectively modulate EASM intensity and critically influence Hexi Corridor paleoclimatic conditions.

Asymmetric glaciation, divide migration, and postglacial fluvial response times in the Qilian Shan

Abstract Glacial-interglacial cycles have repeatedly perturbed climate and topography in many midlatitude mountain ranges during the Quaternary. Glacial erosion can move drainage divides and induce fluvial adjustments downstream, yet the time scale over which these adjustments occur remains unclear. We examined landscape evolution in the northwest-southeast–trending Qilian Shan, where the contrast in solar insolation between north- and south-facing slopes has generated larger glaciers on the northern range crest. Our analyses suggest that this asymmetric glaciation has caused southward migration of the main drainage divide, prompting river channels below the extents of ice on north-facing slopes to become oversteepened for their drainage area and channels on south-facing slopes to become analogously understeepened. These changes in steepness should accelerate or slow down postglacial fluvial incision, even in the regions where topography has not been directly modified by glacial erosion. Numerical modeling suggests these discrepancies persist for millions of years, much longer than the duration of recent glacial-interglacial cycles, implying a widespread and enduring influence of intermittent glaciations on landscape evolution in glaciated mountain ranges during the Quaternary.

Comment on “Luminescence chronology of the Sankosh river terraces in the Assam–Buthan foothills of the Himalayas: Implications to climate and tectonics” by Quaternary Geochronology 72, 101364

In a study of which the main objective was to assess the impact of climate change and tectonics on the formation of river terraces along the Sankosh River, the eastern foreland of the Himalayas, the authors obtained geochronological data using luminescence technique. Four strath river terraces (T4–T1) were distinguished within the valley bottom, and alluvial sediments from three terraces (T4, T2 and T1) were dated to the age range from 143 to 14 ka. The alluvial mantels of river terraces were then linked to the scheme of glacial-interglacial cycle. The paper, however, suffers from many inconsistent and missing information, and the assessment on the landscape evolution of the river valley is incomprehensive. For instance, the authors relied on feldspars and therefore the IRSL method was used, but “OSL ages” are discussed at the end. They state that (i) tectonics creates space for sediment accommodation and (ii) the luminescence ages flank deposition and incision phases. Despite the fact that the statements are questionable, the ways these happen are not elucidated by the authors. Due to the lack of geochronological data for T3 terrace, its formation is very enigmatic. It is even more mysterious in the light of the data for the other terraces, but the authors made no attempt to explain this riddle. However, that certain inability of reconstructing the history of T3 would shed a shadow on the robustness of the ages obtained.

Delayed valley incision due to karst capture (Demänová Cave System, Western Carpathians, Slovakia)

Karst systems can control the amount of water flowing on the surface and thus the fluvial erosional efficiency. Here, we used the ages of speleothems from the active cave system to determine the middle Pleistocene history of the entrenchment of the Demänová Valley. By referencing the vertical position of fluvially active and inactive cave passages to the valley bottom, we estimated the deceleration magnitude of the valley incision due to karst drainage through the Demänová Cave System. The well-developed karst system captures a significant volume of surface water and reduces surface erosion. This, in turn, causes a delay in the incision of the valley drained by the caves in comparison to the downstream positions (below the springs of sinking waters), where river-driven erosion dominates. Karst drainage has reduced the erosional efficiency in the inflow part of the Demänová Valley due to a hydraulic gradient among inputs and outputs of allochthonous waters, mostly during the middle and late Pleistocene. The cave level that contains the active underground segment of the Demänovka River, previously dated to ~350 ka, definitely existed earlier than 600 ka. The period from approximately 600 to 395 ± 5 kyr ago was characterized by relatively stable conditions with continuous deposition of flowstones, regardless of the climate episodes, including several glacial–interglacial cycles.

Evaluation of heterocyte glycolipids with a hexose sugar moiety for tracing terrestrial organic matter in the South China Sea

Heterocyte glycolipids (HGs) are biomarkers for N2-fixing heterocytous cyanobacteria (HCB). The hexose (hereafter C6) sugar moiety of HGs is produced by free-living HCB dominant in fresh and brackish waters, while the pentose moiety (hereafter C5) of HGs is found in endosymbiotic HCB associated with diatoms in tropical-subtropical marine waters. Therefore, C6 HGs detected in marine environments may indicate terrestrial organic matter (TOM) input, but this may be complicated by the fact that free-living HCB may be active over a wide salinity range. In this study, we evaluate C6 HGs and their fraction in total HGs, referred to as FHG, to trace TOM in the South China Sea (SCS) by analyzing a series of surface sediments from the Pearl River Estuary (PRE) to the open SCS, as well as suspended particular matters (SPM) in the euphotic zone of central SCS and a sediment core 17950-2 in the deep SCS. Our results showed that C6 HGs were abundant in the PRE sediments but almost absent in sediments or SPM from the northern shelf to the central SCS, leading us to conclude that free-living HCB producing C6 HGs are inactive in the modern ocean where salinity is >30. In the sediment core, C6 HGs and FHG showed a variation pattern similar to sea level changes during glacial-interglacial cycles, and the reconstructed sea surface salinity was above 32 throughout the cycles. Such a pattern was also evident in the records of branched to isoprenoid tetraether (BIT) index and detrital content from the same core. Thus, the modern distribution of C6 HGs and FHG, together with their downcore records in this study, demonstrates that C6 HGs and FHG are excellent for tracking TOM in tropical-subtropical marine environments.

Large obliquity-paced Antarctic ice-volume fluctuations suggest melting by atmospheric and ocean warming during late Oligocene

The late Oligocene (~27.8–23 My ago) offers an opportunity to study past climate variability under high-CO2, warmer-than-present and the unipolar (Antarctic) glaciated state. Here, we present new high-resolution geochemical records from exquisitely well-preserved benthic foraminifera for the late Oligocene, an interval for which Antarctic ice-sheet size and stability are debated. Our records indicate four obliquity-paced glacial-interglacial cycles with ice-volume changes of up to ~70% of the modern Antarctic ice-sheet. The amplitude of ice-volume change during these late Oligocene glacial-interglacial cycles is comparable to that of the late Pliocene and early Pleistocene. Ice-volume estimates for interglacials are small enough to be accommodated by a land-based Antarctic ice-sheet but, for three of the four glacials studied, our calculations imply that ice sheets likely advanced beyond the Antarctic coastline onto the shelves. Our findings suggest an Antarctic ice-sheet vulnerable to melting driven by both bottom-up (ocean) and top-down (atmospheric) warming under late Oligocene warmer-than-present climate conditions.

Intermediate water warming caused methane hydrate instability in South China Sea during past interglacials

Methane hydrates are widely distributed along continental margins, representing a potential source of methane to the ocean and atmosphere, possibly influencing Earth’s climate. Yet, little is known about the response of methane hydrates to global climate change, especially at the timescale of glacial-interglacial cycles. Here we present a chronology of methane seepage from seep carbonates derived from a series of tens to hundreds of meters long hydrate-bearing sediment records from the South China Sea, drilled at water depths of 664−871 m. We find that six out of seven episodes of intense methane seepage during the last 440,000 years were related to hydrate dissociation, all coinciding with major interglacials, the so-called Marine Isotope Stages 1, 5e, 7c, 9c, and 11c. Using numerical modeling, we show that these events of methane hydrate instability were possibly triggered by the rapid warming of intermediate waters by ∼2.5−3.5 °C in the South China Sea. This finding provides direct evidence for the sensitivity of the deep marine methane hydrate reservoir to glacial-interglacial climatic and oceanographic cyclicity.

Instability of Atlantic Meridional Overturning Circulation: Observations, Modelling and Relevance to Present and Future

The Atlantic Meridional Overturning Circulation (AMOC) has changed dramatically during the glacial–interglacial cycle. One leading hypothesis for these abrupt changes is thermohaline instability. Here, I review recent progress towards understanding thermohaline instability in both observations and modelling. Proxy records available seem to favor thermohaline instability as the cause of the abrupt climate changes during the glacial–deglacial period because the deep North Atlantic water mass and AMOC seemed to have changed before the North Atlantic climate. However, most fully Coupled General Circulation Models (CGCMs) so far seem to exhibit monostable AMOC, because (1) these models have failed to simulate abrupt AMOC changes unless they are forced by an abrupt change of external forcing and, (2) these models have shown opposite freshwater convergence from the current observations. This potential model bias in the AMOC stability leaves the model projection of the future AMOC change uncertain.

Gzhelian cyclothem development in the western North China cratonic basin and its glacioeustatic, tectonic, climatic and autogenic implications

The Gzhelian Ximing Sandstone–to–No. 8 Coal succession of the Taiyuan Formation in the Linxing gas field and adjacent areas records the complex nested architecture of the Palougou Cyclothem and component cyclothems 2–10 developed in the western North China underfilled cratonic basin. Integrated facies and sequence stratigraphic analysis of core, well log, outcrop, and seismic data reveals architectural variation of the Palougou Cyclothem in the study area from southwestward retrograding incised-valley estuary and lagoon (TST) through upward and northeastward prograding coastal plain to epeiric shelf (HST) into downward and eastward prograding open-coast tidal flat and epeiric shelf with aggrading distributive fluvial system (FSST). The component cyclothems vary in the internal architecture through the Palougou Cyclothem from mixed retrogradational–progradational (R–P; R/P > 1) cyclothems 2–4 in the early TST to retrogradation-dominated (R/P ≫1) cyclothems 5–7 in the late TST, progradation-dominated (R/P ≪1) Cyclothem 8 in the HST, and fluvial aggradation-dominated cyclothems 9 and 10 in the FSST. The Palougou Cyclothem and component cyclothems constrained by conodont-bearing limestone marker beds and radiometric dating have been observed to be substantially synchronized with the Gzhelian interglacial–glacial cyclothem and nested 400-kyr cyclothems in the North American Midcontinent and Donets Basin. The cyclic change in accommodation at the scale of the Palougou Cyclothem and component cyclothems has been ascribed to the third-order interglacial–glacial cycle and fourth-order, long eccentricity (ca. 400 kyr)-driven glacioeustatic fluctuations coupled with the tectonically driven source–to–sink reorganization. The progressively decreasing siliciclastic flux and increasing vegetation through both the Palougou Cyclothem and individual component cyclothems have been ascribed to the third- and fourth-order equatorial seasonally dry–wet climatic cycles coupled with the tectonically driven source–to–sink reorganization, in concert with the Gzhelian third- and fourth-order interglacial–glacial eustatic cycles. Both the transgressive fluvial degradation and the falling-stage fluvial aggradation have been ascribed to a higher-gradient fluvial profile relative to the gradient of the receiving North China epeiric shelf. The autogenic shoreline retreat, advance and channel migration modified the depositional response to the third- and fourth-order sea level rise through time. The termination of the falling-stage fluvial channel belts in the lateral and distal floodplain mires has been ascribed to the sustained fluvial aggradation and autodetachment of the fluvial fan from the syn-depositional shoreline in response to steady fall of relative sea level. This study contributes to a proper understanding of the Gzhelian North China intracratonic cyclothem development in response to the coupled glacioeustatic, tectonic, climatic, and autogenic processes at the third- and fourth-order scales, but also provides a context within which to interpret the symbiotic relationship, and predict the distribution, of superimposed coal–sandstone–shale gas reservoirs, and to investigate the impact of depositional heterogeneities on hydrocarbon generation and migration pathways.

Milankovitch-paced South Asian monsoons during Marine Isotope Stage 5

The orbital-scale variations and associated driving mechanisms of the South Asian summer monsoon (SASM) and South Asian winter monsoon (SAWM) remain poorly understood. In this study, we present the first record of a 4.3-m loess-paleosol sequence from the southern Tibetan Plateau, which provides insight into the changes of these two monsoons over most of the last interglacial-glacial cycle. The age framework of this sequence was established using twenty-one samples with absolute luminescence techniques. The SASM was represented by frequency-dependent susceptibility in percentage and redness, while the SAWM was reflected by the recently proposed sorting coefficient of grain size. The results suggest an anti-phase relationship between the SASM and SAWM. During marine isotope stage (MIS) 5, variations of the SASM and SAWM showed a strong correlation with the Northern Hemisphere summer (July) insolation (NHSI) at 65°N, in terms of both phase and amplitude. Stronger SASM (SAWM) corresponded to higher (lower) July NHSI at 65°N, supporting the Milankovitch theory. The variation amplitude of the SASM record in this study significantly differs from the that of stalagmite δ18O from the nearby Tianmen Cave, which presented no distinct difference between MIS 5e, MIS 5c, and MIS 5a. During MIS 4 and MIS 3, the larger global ice volume and lower atmospheric CO2 could have enhanced the SAWM but weakened the SASM, which might have restrained the response amplitude of the South Asian monsoons to the July NHSI at 65°N.

Paleoclimatic controls on natural tracer profiles in biogenic sedimentary formations of the Horonobe area, Japan

The experience gained in modeling the evolution, from past to present, of natural tracer profiles in geologic media can help support safety assessments of disposal concepts for radioactive wastes in deep geologic repositories because the assessments will be based on predictions using similar models of conditions that could evolve, from the present to the future, in the repository's near-field and far-field environments. Solute-transport models were developed in the present study using a forward modeling approach constrained by boundary conditions inferred from the paleo-hydrogeological evolution of the Horonobe area in Hokkaido, Japan and ranges in model parameter values determined in laboratory and field-based investigations at two deep borehole locations in the area. The models were calibrated by adjusting individual parameter values within limits imposed by the site-characterization data to optimize agreement between model predictions and observations. Results from the calibrated models were consistent with results from other studies suggesting that observed variations with sampling depth in Cl− concentrations and δ18O and δD values resulted primarily from an advective transport regime that existed for a relatively brief period during the most recent glacial-interglacial cycle. Advective transport was possible during this time because thawing of a discontinuous permafrost zone starting about 18 ka allowed meteoric water to recharge the flow system and hydraulic gradients were larger than at present because sea levels were significantly lower than the current levels reached at the beginning of the Holocene (i.e., since about 12 ka). Similar climatic controls on the generation and duration of advective/diffusive transport regimes likely existed during nine previous glacial-interglacial cycles that occurred in the Horonobe area since regional uplift began about 1 Ma. If so, alternative versions of the transport models suggest the tracer profiles observed in this area may have evolved over a much longer cumulative period of advective transport and at lower Darcy velocities than assumed in the calibrated models but in a manner that is still compatible with ranges in model parameter values determined in laboratory and field investigations. Apparent differences in transport behavior at the two borehole locations considered in this study, which were situated only about 1 km apart, appear to have resulted from relatively small differences in accessible porosity and hydraulic conductivity, which in turn may have been controlled by local differences in fracture density and fracture connectivity.

Global biome patterns of the Middle and Late Pleistocene

AbstractOur primary aim was to assess the hypothesis that distinctive features of the patterns of vegetation change during successive Quaternary glacial–interglacial cycles reflect climatic differences arising from forcing differences. We addressed this hypothesis using 207 half‐degree resolution global biome pattern simulations, for time slices between 800 and 2 ka, made using the LPJ‐GUESS dynamic global vegetation model. Simulations were driven using ice‐core atmospheric CO2 concentrations, Earth's obliquity, and outputs from a pre‐industrial and 206 palaeoclimate experiments; four additional simulations were driven using projected future CO2 concentrations. Climate experiments were run using HadCM3. Using a rule‐based approach, above‐ground biomass and leaf area index of LPJ‐GUESS plant functional types were used to infer each grid cell's biome. The hypothesis is supported by the palaeobiome simulations. To enable comparisons with the climatic forcing, multivariate analyses were performed of global vegetation pattern dissimilarities between simulations. Results showed generally similar responses to glacial–interglacial climatic variations during each cycle, although no two interglacials or glacials had identical biome patterns. Atmospheric CO2 concentration was the strongest driver of the dissimilarity patterns. Dissimilarities relative to the time slice with the lowest atmospheric CO2 concentration show the log‐linear relationship to atmospheric CO2 concentration expected of an index of ecocarbon sensitivity. For each simulation, extent and total above‐ground biomass of each biome were calculated globally and for three longitudinal segments corresponding to the major continental regions. Mean and minimum past extents of forest biomes, notably Temperate Summergreen Forest, in the three major continental regions strongly parallel relative tree diversities, hence supporting the hypothesis that past biome extents played an important role in determining present diversity. Albeit that they reflect the climatic consequences only of the faster Earth system components, simulated potential future biome patterns are unlike any during the past 800 ky, and likely will continue to change markedly for millennia if projected CO2 concentrations are realised.