Glacial Conditions Research Articles

Coccolithophore productivity at the western Iberian Margin during the Middle Pleistocene (310–455 ka) – evidence from coccolith Sr∕Ca data

Abstract. Coccolithophores contribute significantly to marine primary productivity and play a unique role in ocean biogeochemistry by using carbon for photosynthesis (soft-tissue pump) and for calcification (carbonate counter pump). Despite the importance of including coccolithophores in Earth system models to allow better predictions of the climate system's responses to planetary change, the reconstruction of coccolithophore productivity mostly relied on proxies dependent on accumulation and sedimentation rates and preservation conditions. In this study we used an independent proxy, based on the coccolith fraction (CF) Sr∕Ca ratio, to reconstruct coccolithophore productivity. We studied the marine sediment core MD03-2699 from the western Iberian margin (IbM), concentrating on glacial–interglacial cycles of Marine Isotopic Stage (MIS) 12 to MIS 9. We found that IbM coccolithophore productivity was controlled by changes in the oceanographic conditions, such as in sea surface temperature (SST) and nutrient availability, and by competition with other phytoplankton groups. Long-term coccolithophore productivity was primarily affected by variations in the dominant surface water mass. Polar and subpolar surface waters during glacial substages were associated with decreased coccolithophore productivity, with the strongest productivity minima concomitant with Heinrich-type events (HtEs). Subtropical, nutrient-poorer waters, increased terrigenous input, and moderate to strong upwelling during the deglaciation and early MIS11 are hypothesized to have attributed a competitive advantage to diatoms to the detriment of coccolithophores, resulting in intermediate coccolithophore productivity levels. During the progression towards full glacial conditions an increasing presence of nutrient-richer waters, related to the growing influence of transitional surface waters and/or intensified upwelling, probably stimulated coccolithophore productivity to maxima following the rapid depletion of silica by diatoms. We present conceptual models of the carbon and carbonate cycle components for the IbM in different time slices that might serve as a basis for further investigation and modelling experiments.

Glacial geomorphology of the High Gredos Massif: Gredos and Pinar valleys (Iberian Central System, Spain)

ABSTRACT We present a detailed geomorphological map of the landform assemblages originated by the two major paleoglaciers of the Sierra de Gredos mountain range in the Spanish Iberian Central System. Based on previous works, our map focused on the features formed by Gredos and Pinar paleoglaciers during the last glaciation and subsequent glacial events. Based on a remote sensing analysis and exhaustive field surveys, we identified with great accuracy the local distribution of glacial, periglacial, mass movement, structural, fluvial, and lacustrine features. We recognized three main glacial geomorphological formations representing: (i) the maximum glacial extension reached (peripheral deposits); (ii) the culmination of glacial conditions (principal moraines) and (iii) the local glacial withdrawal (internal deposits). Our map offers a renewed spatial framework on which to conduct higher-resolution glacial chronologies, especially of Late Glacial and Holocene glacial activity, providing key information for performing future paleoclimatic reconstructions of the northern hemisphere mid-latitudes.

A new bias-correction method for precipitation over complex terrain suitable for different climate states: a case study using WRF (version 3.8.1)

Abstract. This work presents a new bias-correction method for precipitation over complex terrain that explicitly considers orographic characteristics. This consideration offers a good alternative to the standard empirical quantile mapping (EQM) method during colder climate states in which the orography strongly deviates from the present-day state, e.g. during glacial conditions such as the Last Glacial Maximum (LGM). Such a method is needed in the event that absolute precipitation fields are used, e.g. as input for glacier modelling or to assess potential human occupation and according migration routes in past climate states. The new bias correction and its performance are presented for Switzerland using regional climate model simulations at 2 km resolution driven by global climate model outputs obtained under perpetual 1990 and LGM conditions. Comparing the present-day regional climate model simulation with observations, we find a strong seasonality and, especially during colder months, a height dependence of the bias in precipitation. Thus, we suggest a three-step correction method consisting of (i) a separation into different orographic characteristics, (ii) correction of very low intensity precipitation, and (iii) the application of an EQM, which is applied to each month separately. We find that separating the orography into 400 m height intervals provides the overall most reasonable correction of the biases in precipitation. The new method is able to fully correct the seasonal precipitation bias induced by the global climate model. At the same time, some regional biases remain, in particular positive biases over high elevated areas in winter and negative biases in deep valleys and Ticino in winter and summer. A rigorous temporal and spatial cross-validation with independent data exhibits robust results. The new bias-correction method certainly leaves some drawbacks under present-day conditions. However, the application to the LGM demonstrates that it is a more appropriate correction compared to the standard EQM under highly different climate conditions as the latter imprints present-day orographic features into the LGM climate.

Life histories and niche dynamics in late Quaternary proboscideans from midwestern North America

AbstractStable isotopes of mammoths and mastodons have the potential to illuminate ecological changes in late Pleistocene landscapes and megafaunal populations as these species approached extinction. The ecological factors at play in this extinction remain unresolved, but isotopes of bone collagen (δ13C, δ15N) and tooth enamel (δ13C, δ18O, 87Sr/86Sr) from midwestern North America are leveraged to examine ecological and behavioral changes that occurred during the last interglacial-glacial cycle. Both species had significant C3 contributions to their diets and experienced increasing levels of niche overlap as they approached extinction. A subset of mastodons after the last glacial maximum exhibit low δ15N values that may represent expansion into a novel ecological niche, perhaps densely occupied by other herbivores. Stable isotopes from serial and microsampled enamel show increasing seasonality and decreasing temperatures as mammoths transitioned from Marine Isotope Stage (MIS) 5e to glacial conditions (MIS 4, MIS 3, MIS 2). Isotopic variability in enamel suggests mobility patterns and life histories have potentially large impacts on the interpretation of their stable isotope ecology. This study further refines the ecology of midwestern mammoths and mastodons demonstrating increasing seasonality and niche overlap as they responded to landscape changes in the final millennia before extinction.

Scouring by rafted ice and cryogenic patterned ground preserved in a Palaeoproterozoic equatorial proglacial lagoon succession, eastern India, Nuna supercontinent

The Chaibasa Formation contains the oldest known record of ploughmarks formed by rafted ice preserved on Earth. The Palaeoproterozoic (Orosirian-Rhyacian, 1.86–2.1 Ga) Chaibasa Formation in the Dhalbhumgarh area, eastern India, represents sedimentation in an ice-contact proglacial lagoon affected by iceberg (bits and growlers) calving and lake-ice rafting. Ice scouring, both on muddy and sandy substrates, as well as ice “rosettes” developed as consequence of the ploughing action of floating icebergs that were moved by wind, meltwater floods and tidal currents on soft sediments. Muddy flats surrounding the proglacial lagoon underwent seasonal deep freezing conditions developing cryogenic patterned ground containing reticulate ice, ice veins and sand wedges, suggesting the occurrence of proglacial permafrost areas coeval with Proterozoic glaciers. The central part of the proglacial lagoon was characterized by deposition of glacial varves punctuated by pebble and cobble size angular dropstones accumulated in the basin as ice-rafted debris from floating ice. Evidences provided in this paper reinforced the idea that 1.86–2.1 Ga ago glacial conditions prevailed after the supposed ending of the Huronian glaciation. This paper provides detailed examples that can be used to recognize possible floating/drifted ice scours in other Precambrian successions, as these elements have largely passed undetected, being probably widely reported in the literature as generic soft-sediment deformation structures. Similar soft-sediment deformations from Precambrian clastic depositional systems should be re-evaluated as there is a significant possibility that most of the Precambrian iceberg-related structures had escape notice, as originally suggested Eyles et al. in 1997.

Late Pleistocene–Holocene environmental and climatic history of a freshwater paramo ecosystem in the northern Andes

ABSTRACTThis study presents the first multi‐proxy palaeoenvironmental and palaeoclimatic history for northern South America based on the palaeolimnological reconstruction of a pond located in a dry paramo at 3570 masl. During the Last Glacial Maximum (LGM), the study area was under glacial conditions, then during global events Heinrich Stadial 1 (HS1), Bølling–Allerød (BA), and the Younger Dryas (YD), the pond expanded, accumulation rates and proxies for erosion reached the highest values, indicative of humid conditions, with maxima in humidity during the BA and YD. Dry conditions and pond desiccation occurred in the Greenlandian–Northgrippian and by 6010 cal a bp the area was transformed into the mire of today. Comparisons with records from other sites in South America indicate that changes in humidity are most likely controlled by the Intertropical Convergence Zone, mainly during the glacial and postglacial, and by changes in the Pacific Ocean, more pronounced after the YD.

Paleoclimatic reconstruction in the Tatra Mountains of the western Carpathians during MIS 9–7 inferred from a multiproxy speleothem record

AbstractThe SC-3 speleothem from Szczelina Chochołowska Cave, located in the Tatra Mountains, was studied in detail. U-series dating and age–depth modeling allowed us to constrain the period of speleothem growth to between approximately 330 and 200 ka, that is, during Marine Isotope Stages (MIS) 9–7. The complementary use of stable isotope analyses, petrographic studies, and trace element analyses allowed the identification of warm and wet climatic conditions that were favorable for speleothem growth during MIS 9e and MIS 9c. Unfavorable climatic periods included the cold glacial conditions of MIS 8 and the MIS 9/MIS 8 transition. The breaks in the growth of the SC-3 stalagmite were most likely connected with a reduction in precipitation in MIS 9a and extreme hydrologic events during MIS 8. Comparisons with other European records suggest that the climatic variability recorded in the speleothem from the Tatra Mountains is not only a record of local environmental conditions but can also be linked to European climatic patterns during both interglacial and glacial intervals. This makes our study the northernmost paleoclimatic record for the whole Carpathian range and one of the very few records from those periods worldwide.

Pleistocene persistence and expansion in tarantulas on the Colorado Plateau and the effects of missing data on phylogeographical inferences from RADseq.

Montane species endemic to the "sky islands" of the North American southwest were significantly impacted by changing climates during the Pleistocene. We combined mitochondrial and genomic data with species distribution modelling to determine whether Aphonopelma marxi, a large tarantula from the nearby Colorado Plateau, was similarly impacted by glacial climates. Genetic analyses revealed that the species comprises three main clades that diverged in the Pleistocene. A clade distributed along the Mogollon Rim appears to have persisted in place during glacial conditions, whereas the other two clades probably colonized central and northeastern portions of the species' range from refugia in canyons. Climate models support this hypothesis for the Mogollon Rim, but late glacial climate data appear too coarse to detect suitable areas in canyons. Locations of canyon refugia could not be inferred from genomic analyses due to missing data, encouraging us to explore the effect of missing loci in phylogeographical inferences using RADseq. Results from analyses with varying amounts of missing data suggest that samples with large amounts of missing data can still improve inferences, and the specific loci that are missing matters more than the number of missing loci. This study highlights the profound impact of Pleistocene climates on tarantulas endemic to the Colorado Plateau, as well as the mixed nature of the region's fauna. Some animals recently colonized from nearby deserts as glacial climates receded, whereas others, like tarantulas, appear to have persisted on the Mogollon Rim and in refugia associated with the region's famous river-cut canyons.

A GIS‐based multi‐proxy analysis of the evolution of subglacial dynamics of the Quebec–Labrador ice dome, northeastern Quebec, Canada

AbstractThe glacial records of the inner‐core regions of the Laurentide Ice Sheet (LIS) document complex yet coherent patterns reflecting ice‐sheet change (e.g. ice‐divide migration), providing unique insights into past glacial conditions. This study develops a conceptual model of subglacial dynamics evolution within a major ice‐dispersal centre of the LIS in northeastern Quebec, Canada using a GIS‐based analysis of the surficial geologic record. Multiple proxies of subglacial conditions (subglacial streamlined landforms, lake density and lake area over thin drift/bedrock) were analysed through grid‐overlay techniques and then classified based on different proxy variables ranging from highly mobile warm‐based to immobile cold‐based conditions. An additional proxy (till blanket) was used to identify areas of thick till deposition, but with few proxies (few lake or landform metrics). Based on local ice‐flow reconstructions, the most ‘relict’ glacial terrain zone (GTZ1) has warm‐based conditions over 66% of its area and is remarkably well preserved, suggesting laterally extensive warm‐based conditions during the oldest identified ice‐flow phase. This relict glacial terrain is partially overprinted by two subsequent ice‐flow phases in spatially restricted zones in the northeast (73% warm‐based), east‐central (41% warm‐based), and northwest (33% warm‐based) of the study area. A zone of more sluggish conditions (only 3% warm‐based) was identified in the highlands at the centre of the study area, characterized by thin till cover, few landforms, yet with large patches of relatively abundant small lakes, indicative of areal scouring. No clear evidence of sustained cold‐based conditions (i.e. high chemical index of alteration values or high 10Be abundances) was found in the study area. These results suggest that warm‐based conditions (active erosion and/or deposition) were uniformly widespread during the earliest ice‐flow phase, later becoming more spatially restricted with broader sluggish ice conditions. These spatially restricted regions of warm‐based subglacial regimes were likely controlled by surrounding and down‐flow ice streaming. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Glacier and ocean variability in Ata Sund, west Greenland, since 1400 CE

To improve knowledge of marine-terminating glaciers in western Greenland, marine sediment cores from the Ata Sund fjord system, hosting two outlet glaciers, Eqip Sermia and Kangilerngata Sermia, were investigated. The main objective was to reconstruct glacial activity and paleoceanographic conditions during the past 600 years. Ice-rafted debris (IRD) was quantified by wet-sieving sediment samples and by using a computed tomography scan. Variability in relative bottom water temperatures in the fjord was reconstructed using foraminiferal analysis. On the basis of this, three periods of distinct glacial regimes were identified: Period 1 (1380–1810 CE), which covers the culmination of the Little Ice Age (LIA) and is interpreted as having advanced glaciers with high IRD content. Period 2 (1810–1920 CE), the end of the LIA, which was characterised by a lowering of the glaciers’ calving flux in response to climate cooling. During Period 3 (1920–2014 CE), both glaciers retreated substantially to their present-day extent. The bottom water temperature started to decrease just before Period 2 and remained relatively low until just before the end of Period 3. This is interpreted as a local response to increased glacial meltwater input. Our study was compared with a study in Disko Bay, nearby Jakobshavn Glacier and the result shows that both of these Greenlandic marine-terminating glaciers are responding to large-scale climate change. However, the specific imprint on the glaciers and the different fjord waters in front of them result in contrasting glacial responses and sediment archives in their respective fjords.

Emergence of regional cultural traditions during the Lower Palaeolithic: the case of Frosinone-Ceprano basin (Central Italy) at the MIS 11–10 transition

Decades of fieldwork in the Frosinone-Ceprano basin (Latin Valley, Latium, central Italy) have shed light on numerous open-air Lower Palaeolithic localities, delivering a human fossil calvarium, thousands of scattered faunal remains and a large collection of lithic industries, including core-and-flake type lithic series (mode 1) and Acheulean assemblages (mode 2). The continuously growing number of available geochronological data (obtained by 40Ar/39Ar on volcanic minerals, ESR/U-series on large mammal teeth and ESR on bleached fluvial quartz) allow today the construction of a reliable and precise chronological framework for the Lower Palaeolithic sites of this area of the Latin Valley. The archaeological horizons with bifaces all appear to belong to a relatively short Middle Pleistocene time range, between about 410 and 350 ka, coeval to the end of the interglacial MIS 11 and to the beginning of the following glacial MIS 10. The Acheulean tools are often associated with cores and flakes. Bifaces are mainly made on limestone, secondary flint and quartz. The archaeological corpus also yielded tools on fragments of large herbivore bones. Comparisons between technological strategies and palaeo-anthropological data at the global scale are now meaningful and enable us to decipher hominin behaviour at a more regional scale. Such careful work is becoming essential in the frame of the recent discoveries showing that the MIS 11–10 period was a pivotal period characterized by the appearance of several new archaeological features later associated with the Neanderthal lineage in Western Europe. We present here the first in-depth technological study of the Acheulean lithic corpus from the major archaeological sites from the Frosinone-Ceprano basin including the Campogrande localities (CG9 and CG10, intermediate and upper levels), Colle Avarone, Selvotta, Isoletta (level 4), Lademagne (upper and lower levels) and Masseria Castellone. For this work, we focus on biface shaping strategies and demonstrate technological features suggesting the existence of networks connecting the different human occupation sites. Technological data are compared with other penecontemporaneous Italian sites to discuss the hypotheses and characteristics of such early evidence of regionalization in Europe in this specific area of Central Italy. It seems to indicate that glacial conditions characterized by millennial rapid climatic oscillations could have been favourable to the development of specific vegetation propitious to human settlement in South-western Europe. European vegetation, as it drives the biomass availability for large herbivores, seems hence to have played a crucial role in the mobility and settlement of human groups.

Strong continentality and effective moisture drove unforeseen vegetation dynamics since the last interglacial at inland Mediterranean areas: The Villarquemado sequence in NE Iberia

Few continental palaeoenvironmental sedimentary sequences from Southern Europe are long enough to span the last interglacial period (Marine Isotopic Stage-MIS 5), the last glacial cycle (MIS 4 to 2) and the Holocene. El Cañizar de Villarquemado (North-Eastern Iberian Peninsula) is an exceptional sedimentary lacustrine sequence spanning the last ca. 135,000 years of environmental change in an area of inland Iberia characterized by Mediterranean climate with strong continentality. We present a multiproxy study which combines palynological, sedimentological and geochemical analyses framed by an independent, robust chronology. Hydrological and climate evolutions were reconstructed by sedimentological and geochemical proxies. Development of wetlands and shallow carbonate lakes support relatively humid conditions during MIS 6, till the onset of MIS 4, and during the Holocene. Palaeohydrological conditions were drier during MIS 5 (dominance of peat environments) than during the Holocene (more frequent carbonate-producing lakes). Sedimentological evidence indicates extremely arid conditions during MIS 3 with greater activity of alluvial fans prograding into the basin. Sedimentary facies variability highlights a large environmental and hydrological variability during MIS 2 and a rapid humidity response to the onset of the Holocene.Compared to classic Mediterranean sites, we found novel pollen assemblages for the end of MIS 6 and MIS 5 indicating that the vegetation cover was essentially represented by sustained high proportions of continentality-adapted taxa dominated by Juniperus during the relatively humid conditions since MIS 6 till the onset of MIS 4. Higher evapotranspiration in inland Iberia would have increased during periods of higher seasonal insolation maxima, impeding soil development and the usual mesophyte expansion during interglacials observed in other Mediterranean areas. Four main periods of forest development occurred in Villarquemado during MIS 5e, MIS 5c, MIS 5a and the Holocene; secondary peaks occurred also during MIS 3. During colder but still relatively humid MIS 4, junipers and Mediterranean taxa disappear but some mesophytes and cold-tolerant species persisted and Pinus became the dominant tree up to modern times. Pollen assemblages and geochemical data variability suggest a dominant control of seasonality and the impact of North Atlantic dynamics both during MIS 5 (cold events C18-C24) and full glacial conditions (HE and D-O interstadials). At millennial scales, steppe herbaceous assemblages dominated during the extremely arid conditions of MIS 3 and pines and steppe taxa during glacial period MIS 2. Villarquemado sequence demonstrates that the resilient behaviour of conifers in continental areas of inland Southern European regions is key to understand the glacial–interglacial vegetation evolution and to evaluate scenarios for potential impacts of global change.

Stability of the Atlantic overturning circulation under intermediate (MIS3) and full glacial (LGM) conditions and its relationship with Dansgaard-Oeschger climate variability

Paleoclimatic records reveal that millennial-scale climate variability during the Pleistocene was most pronounced during intermediate glacial conditions, like Marine Isotope Stage 3 (MIS3), rather than during interglacial and fully glaciated climates, like the Last Glacial Maximum (LGM). The rapid transitions between cold stadials and warm interstadials recorded in Greenland ice cores during MIS3, referred to as Dansgaard–Oeschger (D-O) events, have been correlated with millennial-scale climate variations worldwide. Although the origin of D-O events is a matter of controversy, striking evidence shows that variations in the strength of the Atlantic meridional overturning circulation (AMOC) were involved. Therefore, understanding the stability properties of the ocean circulation under different background climate conditions is key to understanding D-O millennial-scale climate variability. In the present study, the stability of the AMOC to northern high-latitude freshwater perturbations under MIS3 and LGM boundary conditions is investigated by using the coupled climate model CCSM3. Stability diagrams constructed from a large set of equilibrium experiments reveal a nonlinear dependence of AMOC strength on freshwater forcing under both MIS3 and LGM conditions. The MIS3 baseline state is close to an AMOC stability threshold, which makes the MIS3 climate unstable with respect to minor perturbations. A similar threshold behavior in AMOC stability is observed under LGM conditions; however, larger freshwater perturbations are necessary to pass the threshold and weaken the AMOC. The threshold’s displacement relative to the MIS3 background climate is attributable to differences in the atmospheric hydrologic cycle and North Atlantic sea ice transport. Different atmospheric moisture transports are attributable to thermodynamic and dynamic processes related to differences in greenhouse gas forcing and ice-sheet height between MIS3 and the LGM. We conclude that the higher stability of the AMOC during the LGM is a physically plausible explanation for millennial-scale D-O-type climate variability being suppressed under full glacial conditions, whereas minor perturbations in freshwater fluxes could have triggered D-O climate shifts during MIS3.

Carbon Export Buffering and CO2 Drawdown by Flexible Phytoplankton C:N:P Under Glacial Conditions

AbstractModern observations indicate that variations in marine phytoplankton stoichiometry correlate with the boundaries of major surface waters. For example, phytoplankton in the oligotrophic subtropical gyres typically have much higher C:N:P ratios (i.e., higher C:P and higher N:P ratios) than those in eutrophic upwelling regions and polar regions. Such a spatial pattern points to nutrient availability as a key environmental driver of stochiometric flexibility. Environmental dependence of phytoplankton C:N:P opens unexplored possibilities for modifying the strength of the biological pump under different climate conditions. Here we present a power law formulation of C:N:P flexibility that is driven by nutrients, temperature, and light. We embed the formulation in a global ocean carbon cycle model with multiple phytoplankton types and explore biogeochemical implications under glacial conditions. We find three key controls on export C:N:P ratio: phytoplankton physiology and community structure as well as the balance in regional production at the global level. Glacial inputs of iron and sea ice expansion are important modifiers of these three controls. We also find that global export C:N:P increases substantially under glacial conditions, and this strongly buffers global carbon export against decrease and draws down approximately 20 μatm of atmospheric CO2. These results point to the importance of including phytoplankton C:N:P flexibility in a mix of mechanisms that drive atmospheric CO2 over glacial‐interglacial time scale. Finally, our simulations indicate decoupling of nutrients, which may provide a resolution to the longstanding disagreement regarding nutrient utilization in the glacial Southern Ocean derived from different nutrient proxies.

Altitudinal, latitudinal and longitudinal responses of cloud forest species to Quaternary glaciations in the northern Neotropics

AbstractThe glaciations of the Quaternary caused changes in the geographical distributions of species associated with tropical montane cloud forests. The most obvious effect of the glacial conditions was the downward displacement of cloud forest species, thus giving opportunities for population connectivity in the lowlands. Considerable attention has been paid to these altitudinal changes, but latitudinal and longitudinal movements remain poorly understood in the northern Neotropics. Here, I use ecological niche modelling to generate palaeodistributions of small-eared shrews (Mammalia: Soricidae) closely associated with cloud forests in the mountain systems of Mexico and then retrodict their range shifts during the Last Glacial Maximum (LGM), one of the coldest periods of the Quaternary. The results suggest that cloud forest species not only migrated downwards in response to global cooling and dryness but also migrated latitudinally and longitudinally onto those slopes that maintained moist conditions (other slopes remained unsuitable during the LGM), thus revealing a hitherto unknown route for postglacial colonization of cloud forest species. This scenario of past distributional change probably had genetic and demographic implications and has repercussions for the identification of areas of refugia and postglacial colonization routes of cloud forest species.

Onset of permanent Taklimakan Desert linked to the mid-Pleistocene transition

Abstract The initial occurrence of desert landscape or eolian sand dunes is thought to have occurred long before the Pleistocene, and desertification was subsequently enhanced under cold, dusty glacial conditions. However, when and how the desert landscape persisted during both glacial and interglacial periods, defined as “permanent” desert here, remain elusive. Here, we present carbonate carbon isotope and grain-size records from the Tarim Basin, western China, revealing a detailed desertification history for the Taklimakan Desert. Our records demonstrate that after desiccation of episodic lakes at ca. 4.9 Ma, alternations of eolian sand dunes and fluvial and playa-like conditions persisted for a long period until 0.7 Ma in the Tarim Basin. The onset of permanent desert landscape around 0.7–0.5 Ma occurred concurrently with the climatic reorganization across the mid-Pleistocene transition. The occurrence of mountain glaciers on the Tibetan Plateau and atmospheric circulation changes may have controlled the formation and extreme aridification of the permanent desert in inland Asia since the mid-Pleistocene transition.

The KPP Trigger of Rapid AMOC Intensification in the Nonlinear Dansgaard‐Oeschger Relaxation Oscillation

AbstractMillennium time scale Dansgaard‐Oeschger oscillations of glacial climate, clear evidence for the occurrence of which was first provided on the basis of oxygen isotopic data from a Greenland ice core 25 years ago, have recently been shown to arise naturally (without explicit freshwater forcing) in a fully coupled modern climate model description of the interactions between the overturning circulation of the oceans, the atmosphere, and sea ice under maximum glacial conditions. The fast transitions from cold stadial to warm interstadial conditions in a typical D‐O oscillation are characterized by the appearance of an extensive polynya in the stadial sea ice cover of the Irminger Sea south of Denmark Strait that opens due to the onset of intense vertical mixing below the sea ice lid. Through detailed stability analysis of the water column in the region where the polynya first forms, together with analysis of the action of the KPP (Kappa Profile Parameterization) of stratified turbulence employed to represent water column diapycnal diffusivity, the authors show that the opening of the polynya is controlled by this turbulence parameterization. The relative contributions of the different components of the parameterization to polynya opening are investigated in order to better understand the rapid climate change that ensues. The authors furthermore show that the characteristic period of the model predicted Dansgaard‐Oeschger oscillation is also controlled by the detailed nature of this parameterization, a characteristic of the oscillation not previously explained.

Paleoclimatic changes and modulation of East Asian summer monsoon by high-latitude forcing over the last 130,000 years as revealed by independently dated loess-paleosol sequences on the NE Tibetan Plateau

The variability and interaction of the East Asian summer monsoon (EASM) and mid-latitude Westerlies have dominated Eurasian hydroclimates over orbital timescales and currently influence over a billion lives in Asia. Despite this, the mechanisms influencing EASM-Westerlies variability are still a matter of considerable debate. This study presents three loess-paleosol sequences in the NE Tibetan Plateau (NETP) dated at high-resolution by 50 K-feldspar pIRIR and 37 quartz OSL ages to produce the most detailed chronology for any loess record within the plateau. Combined with this chronology are sedimentology, magnetic susceptibility and grain size analyses with TraCE-21ka climatic modelling to constrain atmospheric controls on climatic changes over the NETP during the past 130 kyr. Our results indicate that deposition initiated by ∼130 ka with especially rapid rates during glacial periods, with maximum rates (140–180 g/cm2/ka) in early MIS 2. Over interglacial-glacial cycles, NETP climatic changes are consistent with warm-moist interglacial and cold-dry glacial conditions recorded in the monsoonal Chinese Loess Plateau and Westerlies-dominated Central Asia but anti-phase to Central Asia during interglacial and the Holocene substages. These results highlight how variation in EASM intensity strongly controls NETP moisture changes but that Holocene and last interglacial changes recorded in the NETP consistently lag 30°N summer insolation by 2–6 kyr. This study suggests high-latitude forcing effectively modulates EASM intensity and that factors such as ice volume, greenhouse gases, and freshwater input to northern Atlantic and Southern oceans critically influence NETP paleoclimatic conditions.

Glacial conditioning and paraglacial sediment reworking in Glen Croe (the Rest and be Thankful), western Scotland

Glen Croe, located near the western edge of the Loch Lomond and Trossachs National Park, is a well-known landslide hazard site in Scotland. Debris flows have repeatedly closed the A83 Rest and be Thankful road that passes through the valley, and considerable investment has been directed towards hazard risk reduction. However, little research has focused on the former glaciation and paraglacial response that have played an important role in governing current landscape processes at the site. This paper addresses the knowledge gap by investigating the glacial processes that have shaped and conditioned Glen Croe, and pathways that have characterised subsequent paraglacial sediment transfer. The large-scale valley form results from watershed breaching and interaction between glacier erosion and paraglacial rock slope failures. The distribution of thick glacigenic sediment is conditioned by deposition at former lateral ice margins, which was influenced by topography. Sediment reworking has resulted in accumulation of debris cones and alluvial fans in the upper part of the catchment, and growth of a delta at the outlet. Spatial connectivity mapping supports an interpretation whereby upper Glen Croe is poorly connected to the valley outlet, influencing sediment storage. In contrast, slopes in the lower part of Glen Croe are well connected. Sediment distribution in Glen Croe fits within the context of glaciated valley and paraglacial landsystems, allowing an understanding of sources and transport pathways. In upland infrastructure corridors this type of information is potentially helpful for understanding how landscapes might be affected by renewed sediment reworking under altered threshold conditions.

Glacial erosion promotes high mountains on thin crust

Every mountain range on Earth is unique in its tectonic evolution, but Quaternary climate cooling resulted in alpine landscapes with globally similar landforms such as horn-shaped peaks, cirques and overdeepened valleys. On a global scale, the impact of glacial and periglacial processes on the geometry of mountain ranges and the relative efficiency of glacial and fluvial erosion are still discussed. To address these issues, we analyse a novel, worldwide dataset of more than 16,000 mountain peaks. We relate peak height and steepness to mean elevation and crustal thickness. We show that the geometry of mountain ranges systematically varies with latitude and thus with climate. For peaks of similar height, the steepness increases towards high latitudes, while mean elevation and thickness of the supporting crust decrease. Modelling flexural isostasy based on the topographic load of mountain ranges demonstrates that the crustal thickness required to support peaks of similar height dramatically declines with intensity of glacial erosion. In concert with latitudinal trends in peak steepness, mean elevation and crustal thickness, this shows that Quaternary climate cooling reduces the topographic load by transforming the geometry of mountain landscapes from fluvial to glacial conditions. We conclude that the prevalence of glacial erosion in cold climate reduces the topographic load of mountain ranges and enables the growth of high mountain peaks even on moderately over-thickened crust.