Interglacial Times Research Articles

North Atlantic Influence on the Glacial Amplitude of East Asian Millennial-Scale Monsoon Variability

Abstract Millennial-scale climate change is thought to be synchronous throughout the Northern Hemisphere and has been demonstrated to be strongly modulated by longer-term glacial–interglacial and orbital-scale processes. However, processes that modulate the magnitude of millennial-scale variability (MMV) at the glacial–interglacial time scale remain unclear. We present multiproxy evidence showing out-of-phase relationships between the MMV of East Asian and North Atlantic climate proxies at the eccentricity band. During most late Pleistocene glacial intervals, the MMV in North Atlantic SST and East Asian monsoon (EAM) proxies shows a gradual weakening trend from glacial inceptions into glacial maxima, inversely proportional to that of the North Atlantic ice-rafted detritus record. The inverse glacial age trends apply to both summer and winter monsoon proxies across the loess, speleothem, and marine archives, indicating fundamental linkages between MMV records of the North Atlantic and East Asia. We infer that intensified glacial age iceberg discharge is accompanied by weakened Atlantic meridional overturning circulation via changes in freshwater input and water column stability, leading to a reduction in North Atlantic SST and wind anomalies, subsequently propagating dampened millennial-scale variability into the midlatitude East Asian monsoon region via the westerlies. Our results indicate that the impact of North Atlantic iceberg discharge and the associated variability in water column stability at the millennial scale is a primary influence on hydroclimate instability in East Asia.

Dinoflagellate cysts as proxies of environmental, ocean and climate changes in the Atlantic realm during the quaternary

Over the last four decades, organic-walled dinoflagellate cysts have shown high potential as tracers of past sea-surface conditions during the Quaternary. These microfossils relate to the pelagic productivity of both phototrophic and heterotrophic protist organisms and are recovered in high numbers in almost all marine environment settings from the nearshore and estuarine systems to the distal continental margin. In polar environments, where other conventional proxies are rare or absent, dinoflagellate cysts showed a relatively high diversity of species and a close relationship with sea-ice cover duration, winter and summer temperature, and salinity, enabling quantitative reconstructions of several oceanic variables over time. From the temperate to the tropical latitudes of the Atlantic Ocean, their modern distribution highlights a response to primary productivity and seasonal contrasts in surface temperature. They also have proven that they could be used as tracers of eutrophication in stratified systems and can also highlight human impact on their distribution. In this paper, we present an overview of dinoflagellate cysts as ecological tracers in recent and past sediments of the Atlantic Ocean, from the Arctic to the Antarctic. We provide examples of their use as proxies in paleoclimatic-palaeoceanographic studies at glacial to interglacial time scales, with emphasis on the last ice age to recent (last 25 kyr), the northern North Atlantic and western-eastern tropic North Atlantic. We also discuss their potential as tracers of anthropogenic stress in coastal environments.

Review of the Voluta musica Complex

The distinctive morphology, color pattern, and isolated geographic locations of the various proposed taxa of the Voluta musica complex are discussed and specimens of the various taxa are illustrated. From the Miocene to the present, during interglacial times and the associated higher sea levels during the Neogene, evidence shows that populations of ancestral V. musica had become genetically isolated on offshore islands resulting in a species swarm of closely-related allopatric species. The following taxa are recognized as full species, and these include: Voluta musica, V. carneolata, V. thiarella, V. damula, and V. typica.

The Coupled Ice Sheet–Earth System Model Bern3D v3.0

Abstract Understanding climate variability from millennial to glacial–interglacial time scales remains challenging due to the complex and nonlinear feedbacks between ice, ocean, sediments, biosphere, and atmosphere. Complex climate models generally struggle to dynamically and comprehensively simulate such long time periods as a result of the large computational costs. Here, we therefore coupled a dynamical ice sheet model to the Bern3D Earth system model of intermediate complexity, which allows for simulating multiple glacial–interglacial cycles. The performance of the model is first validated against modern observations and its response to abrupt perturbations, such as atmospheric CO2 changes and North Atlantic freshwater hosing, is investigated. To further test the fully coupled model, the climate evolution over the entire last glacial cycle is explored in a transient simulation forced by variations in the orbital configuration and greenhouse gases and aerosols. The model simulates global mean surface temperature in fair agreement with reconstructions, exhibiting a gradual cooling trend since the last interglacial that is interrupted by two more rapid cooling events during the early Marine Isotope Stage (MIS) 4 and Last Glacial Maximum (LGM). Simulated Northern Hemispheric ice sheets show pronounced variability on orbital time scales, and ice volume more than doubles from MIS3 to the LGM in good agreement with recent sea level reconstructions. At the LGM, the Atlantic overturning has a strength of about 14 Sv (1 Sv ≡ 106 m3 s−1), which is a reduction by about one-quarter compared to the preindustrial. We thus demonstrate that the new coupled model is able to simulate large-scale aspects of glacial–interglacial cycles.

Patterns of valley incision beneath the Greenland Ice Sheet revealed using automated mapping and classification

The Greenland Ice Sheet covers an area of 1.7 million km2, equivalent to ∼79 % of the surface of Kalaallit Nunaat (Greenland) and ∼1.2 % of the Earth's land surface. The macro-scale geomorphology beneath the ice can provide a valuable record of past ice sheet behaviour, particularly during warm periods that may serve as analogues for present and future climates. However, despite extensive mapping of the landscape by airborne radar surveying, Greenland's subglacial geomorphology remains comparatively understudied. Here we construct an automated workflow to identify, extract, and quantify the morphology of valley cross-sectional profiles across Greenland, as observed in NASA Operation IceBridge radar data. We identify 5335 cross-sectional profiles and apply a supervised machine learning method to classify valleys based on their morphological similarity to those formed by glacial or fluvial incision elsewhere in the Northern Hemisphere. Approximately two thirds of the valleys are classified as ‘glacial’, some of which reflect active incision at the modern ice sheet margin, whereas others are situated beneath cold-based, slow-moving ice, indicating that they were incised under a different ice configuration earlier in Greenland's glacial history. The presence of ‘fluvial’ valleys in the low-lying interior of northern Greenland and in mountainous southern Greenland suggests parts of the inherited landscape formed under ice-free conditions during pre- or inter-glacial times have been preserved due to negligible long-term subglacial erosion rates. Some low-lying catchments show hallmarks of a combination of fluvial, glacial, and glacio-fluvial incision, hinting at complex interplays between valley-forming processes over the history of the Greenland Ice Sheet.

Spatiotemporal Discharge Variability of the Doce River in SE Brazil During MIS 6 and 5

The modern precipitation balance in southeastern (SE) Brazil is regulated by the South American summer Monsoon and threatened by global climate change. On glacial-interglacial timescales, monsoon intensity was strongly controlled by precession-forced changes in insolation. To date, relatively little is known about the spatiotemporal distribution of tropical precipitation in SE Brazil and the resulting variability of fluvial discharge on glacial-interglacial timescales. Here, we present X-ray diffraction-derived mineralogical data for the 150–70 ka period (marine isotope stage (MIS) 6 to MIS 5) from the Doce River basin. This area was sensitive to changes in monsoonal precipitation intensity due to its proximity to the South Atlantic Convergence Zone. The data, obtained from a marine sediment core (M125-55–7) close to the Doce river mouth (20°S), show pronounced changes in the Doce River suspension load’s mineralogical composition on glacial-interglacial and precessional timescales. While the ratio of silicates to carbonates displays precession-paced changes, the mineralogical composition of the carbonate-free fraction discriminates between two assemblages which strongly vary between glacial and interglacial time scales, with precession-forced variability only visible in MIS 5. The first assemblage, dominated by high contents of kaolinite and gibbsite, indicates intensified lowland erosion of mature tropical soils. The second one, characterized by higher contents of the well-ordered illite, quartz and albite, points to intensified erosion of immature soils in the upper Doce Basin. High kaolinite contents in the silicate fraction prevailed in late MIS 6 and indicate pronounced lowland soil erosion along a steepened topographic gradient. The illite-rich mineral assemblage was more abundant in MIS 5, particularly during times of high austral summer insolation, indicating strong monsoonal rainfall and intense physical erosion in the upper catchment. When the summer monsoon weakened in times of lower insolation, the mineral assemblage was dominated by kaolinite again, indicative of lower precipitation and runoff in the upper catchment and dominant lowland erosion.

Nitrogen isotope evidence for oxygenated upper ocean during the Cryogenian interglacial period

The Cryogenian interglacial period have witnessed dramatic changes in climate, oceanic environment and biological evolution. The nitrogen isotopic composition, as an important biogeochemical proxy, has the potential to track both the nutrient cycling and redox conditions in the past. However, nitrogen isotopic data during this interglacial time is rather limited. Here, we present integrated data for nitrogen isotopes (δ15N), as well as organic carbon isotopes (δ13Corg), iron (Fe) speciation, pyrite morphology and trace elements from the Cryogenian interglacial Datangpo Formation derived from a drill core from South China to figure out the nitrogen cycling and coeval redox states. The results reveal a gradual oxygenation bottom waters along with deepening O2-H2S chemocline from euxinic black shale in the lower unit (LU), to oxic siltstone in the upper unit (UU), which is consistent with the redox variation inferred in previous studies. Sedimentary δ15N values constantly remain positive (mean value> + 4‰) throughout the Datangpo Formation and display an increase from the LU to the UU. We interpreted the positive δ15N values in the LU as a result of denitrification related to the development of euxinic conditions. The further increase towards the UU associated with limited nutrient supply may indicate the existence of a stable nitrate reservoir in the upper water column, which was consistent with an increased oxygenation during the interglacial time. The elevated O2 levels and bioavailable nitrogen in seawater may have further contributed to the appearance of the earliest metazoan. In addition, our data lay good foundation for further global δ15N correlations among other interglacial successions.

Relationship between glacial CO2 drawdown and mercury cycling in the western South Atlantic: An isotopic insight

Abstract Pronounced changes in the deep Atlantic circulation occurred during glacial stages, which affected the global carbon distribution and the biogeochemical cycles of other elements. Previous studies demonstrated that oceanic mercury is sensitive to the same kind of processes that affect the carbon cycle in glacial–interglacial time scales. We used Hg isotopes to elucidate Hg cycling during the last two glacial–interglacial transitions in the subtropical western South Atlantic. Mass-dependent fractionation (MDF, δ202Hg) and mass-independent fractionation (MIF, Δ199Hg) show significant variations and shift toward higher and lower values during the penultimate and Last Glacial Maximum periods, respectively. δ202Hg variability is in-phase with periods of lower atmospheric CO2 and benthic δ13C, suggesting that MDF is affected by the same mechanisms that led to atmospheric CO2 drawdown during glacial stages. In contrast, terrestrial Hg input to our site, boosted by lower sea level and exposure of the continental shelf, dominates the Δ199Hg variability.

Revisiting Late Pleistocene Loess–Paleosol Sequences in the Azov Sea Region of Russia: Chronostratigraphy and Paleoenvironmental Record

Loess-paleosol sequences are the most intensively studied terrestrial archives used for the reconstruction of Late Pleistocene environmental and climatic changes in the Azov Sea region, southwest Russia. Here we present a refined chronostratigraphy and a multiproxy record of Late Pleistocene environmental dynamics of the most complete and representative loess–paleosol sequences (Beglitsa and Chumbur-Kosa sections) from the Azov Sea region. We propose a new chronostratigraphy following the Chinese and Danubean loess stratigraphic models that refines the subdivision of the Last Interglacial paleosol (S1) complex in two Azov Sea sites, resolve the uncertainty of the stratigraphic position of the weakly developed paleosol (L1SSm) in Beglitsa section, and allow for direct correlation of the Azov Sea sections with those in the Danube Basin and the Chinese Loess Plateau. More importantly, it may serve as a basis for better constraining local and regional chronostratigraphic correlations, and facilitate the interpretation of climatic connections and possible forcing mechanisms responsible for the climatic trends in the region. In addition, a general succession of environmental dynamics is reconstructed from these two vital sections, which is broadly consistent with other loess records in the Dnieper Lowland and Lower Danube Basin, demonstrating similar climatic trends at Glacial–Interglacial time scales. Furthermore, our results have important implications for the chronostratigraphic representativeness of Beglitsa as a key regional loess section and for the reconstruction of the temporal and spatial evolution of Late Pleistocene climate in the Azov Sea region.

The Southern Ocean diatom Pseudo-nitzschia subcurvata flourished better under simulated glacial than interglacial ocean conditions: Combined effects of CO2 and iron.

The 'Iron Hypothesis' suggests a fertilization of the Southern Ocean by increased dust deposition in glacial times. This promoted high primary productivity and contributed to lower atmospheric pCO2. In this study, the diatom Pseudo-nitzschia subcurvata, known to form prominent blooms in the Southern Ocean, was grown under simulated glacial and interglacial climatic conditions to understand how iron (Fe) availability (no Fe or Fe addition) in conjunction with different pCO2 levels (190 and 290 μatm) influences growth, particulate organic carbon (POC) production and photophysiology. Under both glacial and interglacial conditions, the diatom grew with similar rates. In comparison, glacial conditions (190 μatm pCO2 and Fe input) favored POC production by P. subcurvata while under interglacial conditions (290 μatm pCO2 and Fe deficiency) POC production was reduced, indicating a negative effect caused by higher pCO2 and low Fe availability. Under interglacial conditions, the diatom had, however, thicker silica shells. Overall, our results show that the combination of higher Fe availability with low pCO2, present during the glacial ocean, was beneficial for the diatom P. subcurvata, thus contributing more to primary production during glacial compared to interglacial times. Under the interglacial ocean conditions, on the other hand, the diatom could have contributed to higher carbon export due to its higher degree of silicification.

Late Quaternary variations in paleoerosion rates in the northern Qilian Shan revealed by 10Be in fluvial terraces

Understanding the pace of erosion through space and time may provide insight into how tectonic and climatic processes conspire to drive landscape evolution. However, determining whether erosion rates in a given locality have varied with past climate change remains challenging, and the mechanisms invoked to explain variations in past erosion rates are not well understood. In this study, we evaluate past rates of erosion during the late Quaternary along the northern Qilian Shan, the range bounding the northeastern margin of the Tibetan Plateau. We measured the concentrations of cosmogenic 10Be from fluvial deposits atop 5 terraces along the Hongshuiba River and average 10Be concentrations at the time of terrace formation. We also determined terrace ages by combining 5 10Be depth profiles and 15 OSL samples of overlying loess. Our results confirm previous dating of this terrace sequence using cosmogenic exposure ages of boulders and depth profiles in terrace gravels and indicate that terrace treads span glacial-interglacial cycles over the past ~200 ka. We exploit this exceptionally dense sampling of 10Be to evaluate differences in cosmogenic 10Be concentrations at the time of terrace formation along the Hongshuiba River. Analysis of 48 individual determinations of “inherited” 10Be concentrations reveal that fluvial sediment in terraces abandoned during the last glacial and penultimate glacial periods are 3–4 times greater than concentrations in sediment atop terraces formed during interglacial times. Paleoerosion rates within the Hongshuiba watershed appear to have been ~200 m/Myr during glacial episodes but increased to ~500–700 m/Myr during interglacial times, similar to recent erosion rates inferred from 10Be in modern sediment. These results point to likely changes in the mean residence time of sediment with the watershed that fluctuate in association with 100-ka glacial-interglacial cycles. We suggest that reductions in erosion rates associated with glacial conditions may have been driven by changes in catchment hydrology during cold and dry periods.

Pleistocene climate fluctuations drove demographic history of African golden wolves (Canis lupaster).

Pleistocene climate change impacted entire ecosystems throughout the world. In the northern hemisphere, the distribution of Arctic species expanded during glacial periods, while more temperate and mesic species contracted into climatic refugia, where isolation drove genetic divergence. Cycles of local cooling and warming in the Sahara region of northern Africa caused repeated contractions and expansions of savannah-like environments which connected mesic species isolated in refugia during interglacial times, possibly driving population expansions and contractions; divergence and geneflow in the associated fauna. Here, we use whole genome sequences of African golden wolves (Canis lupaster), a generalist mesopredator with a wide distribution in northern Africa to estimate their demographic history and past episodes of geneflow. We detect a correlation between divergence times and cycles of increased aridity-associated Pleistocene glacial cycles. A complex demographic history with responses to local climate change in different lineages was found, including a relict lineage north of the High Atlas Mountains of Morocco that has been isolated for more than 18,000years, possibly a distinct ecotype.

Atmospheric dust flux in northeastern Gondwana during the peak of the late Paleozoic ice age

The silicate mineral fraction of shallow marine carbonates archives dust contributions to the Central Persian Terranes along the northeastern margin of Gondwana (∼30ºS paleolatitude), enabling reconstruction of atmospheric dust loading and circulation for intervals of the late Paleozoic ice age. The Central Persian Terranes hosted cyclic deposition of warm water carbonates from middle Pennsylvanian to earliest Permian time, and our data set includes two ∼28 m sections from the Moscovian and Asselian sampled at 20 cm intervals. Bounding surfaces between successive cycles (high-frequency sequences) are recognized by either abrupt basinward shifts in facies or subtle exposure features; these high-frequency sequences range from 1 m to 5 m thick and are interpreted to record glacioeustatic variations. Time series analysis of the dust fraction through the studied interval supports the hypothesis of orbital forcing for the dust signal. The stratigraphic pattern of the dust flux indicates minimal flux during interglacial highstands (0.19−0.27 g/cm2/kyr) and peak flux during glacial lowstands (3.77−4.57 g/cm2/kyr) after accounting for hiatal time at sequence boundaries. Grain size analysis of the dust for all samples (n = 230) reveals modal sizes (volume-based) of 1−15 µm through the Moscovian interval and 10−75 µm through the Asselian interval. Dust deposition increased during glacial times relative to interglacial times by a factor of 16 to 19. Additionally, the Asselian interval exhibits higher dust flux overall relative to the Moscovian interval, which is interpreted to reflect the more extreme icehouse conditions of the Asselian. Variation in the dust content through the studied sections provides an indicator of temporal changes in atmospheric loading that varied at both glacial−interglacial and higher-frequency (<104 yr) scales. Geochemical data reveal that the Arabian−Nubian Shield and southwestern Pangaea (South America) are the most likely sources of dust deposition in the Central Persian Terranes, with sources shifting during different phases. Increased dust flux during glacials likely reflects multiple factors, including enhanced aridity in the source region, exposure of shelf regions, and potential changes in winds. However, the discrepancy in model reconstructions of the amplitude of glacial−interglacial dust variations indicates that increased production of dust sourced by dynamic glaciation played a large role in enhancing dust flux during glacial phases.

Global vegetation patterns of the past 140,000 years

AbstractAimInsight into global biome responses to climatic and other environmental changes is essential to address key questions about past and future impacts of such changes. By simulating global biome patterns 140 ka to present, we aimed to address important questions about biome changes during this interval.LocationGlobal.TaxonSpermatophyta.MethodsUsing the LPJ‐GUESS dynamic global vegetation model, we made 89 simulations driven using ice‐core atmospheric CO2 concentrations, Earth's obliquity, and outputs from a pre‐industrial and 88 palaeoclimate experiments run using HadCM3. Experiments were run for 81 time slices between 1 and 140 ka, seven ‘hosing’ experiments also being run, using a 1‐Sv freshwater flux to the North Atlantic, for time slices corresponding to Heinrich Events H0–H6. Using a rule‐based approach, based on carbon mass and leaf area index of the LPJ‐GUESS plant functional types, the biome was inferred for each grid cell. Biomes were mapped, and the extent and total vegetation biomass of each biome, and total global vegetation biomass, estimated.ResultsSubstantial changes in biome extents and locations were found on all vegetated continents. Although the largest magnitude changes were in Eurasia, important changes were seen in tropical latitudes and the Southern Hemisphere. Total global extent of most biomes varied on multi‐millennial (orbital) time scales, although some (e.g. Tropical Raingreen Forest) responded principally to the c. 100‐kyr glacial–interglacial cycle and others (e.g. Temperate Broad‐leaved Evergreen Forest) mainly to the c. 20‐kyr precession cycle. Many also responded to millennial contrasts between stadial (‘hosing’) and interstadial climates, with some (e.g. Tropical Evergreen Forest) showing stronger responses than to the multi‐millennial changes.Main conclusionsNo two time slices had identical biome patterns. Even equivalent Holocene and last interglacial time slices, and the last and penultimate glacial maxima, showed important differences. Only a small proportion of global land area experienced no biome change since 140 ka; many places experienced multiple biome changes. These modelling experiments provided little evidence for long‐term biome stability.

Antarctic Circumpolar Current Dynamics at the Pacific Entrance to the Drake Passage Over the Past 1.3 Million Years

AbstractThe Antarctic Circumpolar Current (ACC) is the world's largest current system connecting all three major basins of the global ocean. Our knowledge of glacial‐interglacial changes in ACC dynamics in the southeast Pacific is not well constrained and presently only based on reconstructions covering the last glacial cycle. Here we use a combination of mean sortable silt grain size of the terrigenous sediment fraction (10–63 μm, ) and X‐ray fluorescence scanner‐derived Zr/Rb ratios as flow strength proxies to examine ACC variations at the Pacific entrance to the Drake Passage (DP) in the vicinity of the Subantarctic Front. Our results indicate that at the DP entrance, ACC strength varied by ~6–16% on glacial‐interglacial time scales, yielding higher current speeds during interglacial times and reduced current speeds during glacials. We provide evidence that previous observations of a reduction in DP throughflow during the last glacial period are part of a consistent pattern extending for at least the last 1.3 Ma. The orbital‐scale cyclicity follows well‐known global climate changes from prevailing ca. 41‐kyr cycles in the early part of the record (1.3 Ma to 850 ka; marine isotope stage 21) across the mid‐Pleistocene transition into the middle and late Pleistocene 100‐kyr world. A comparison to a bottom water flow record from the deep western boundary current off New Zealand (Ocean Drilling Program Site 1123) reveals anti‐phased changes between the two sites. The enhanced supply of deep water along the DP and into the Atlantic Ocean during interglacials corresponds to a weakened flow of the SW Pacific deep western boundary current.

A record of dust deposition in northern, mid-latitude Pangaea during peak icehouse conditions of the late Paleozoic ice age

ABSTRACT The Wordiekammen Formation, a carbonate ramp on Spitsbergen developed on the Northern Pangaean margin in Moscovian (Carboniferous) through Sakmarian (Permian) time at a paleolatitude of 30–35° N. The study site on the Nordfjorden High was isolated from any source of fluvio-deltaic input, such that detrital material that occurs in this system experienced eolian transport, thus forming a proxy for atmospheric dust loading. We analyzed two intervals, of Moscovian (10 m) and Asselian (27 m) age, at 20 cm resolution, and identified five mid-ramp subtidal facies organized in upwardly shallowing, high-frequency sequences 3–5 m thick. High-frequency sequence boundaries commonly exhibit signs of subaerial exposure (e.g., Microcodium) developed atop subtidal facies, recording glacioeustatic falls (glacial phases), although the Moscovian section has a severe karst overprint attributable to prolonged exposure on a paleohigh. Samples were processed to isolate the silicate-mineral fraction (SMF), which includes both detrital silicate material and authigenic silica mostly in the form of (fine-sand-size) doubly terminated quartz crystals. Detrital cores in these crystals, together with other evidence, indicate recrystallization from fine-grained (silt- and clay-size) dust. Analysis of the dust record demonstrates that the Asselian (peak icehouse) had a significantly higher atmospheric dust load than the Moscovian (moderate icehouse). In the Asselian interval, dust input varies commensurate with glacial–interglacial cyclicity. Highest dust contents correspond to transgressive facies immediately above sequence boundaries, indicating peak atmospheric dust loading at lowstand to incipient interglacial times. Provenance data from detrital-zircon and whole-rock geochemistry indicate two distinct source regions for the dust. Dust from the Moscovian and lower Asselian intervals reflects a continental island-arc signature consistent with sourcing from the basement of northeast Greenland. Dust from the upper Asselian interval is more consistent with recycling from Devonian and Carboniferous strata of the east Greenland Caledonides, likely deflated from fluvial systems draining this orogenic system, indicating an expansion of regions of eolian deflation.

African climate response to orbital and glacial forcing in 140,000-y simulation with implications for early modern human environments

A climate/vegetation model simulates episodic wetter and drier periods at the 21,000-y precession period in eastern North Africa, the Arabian Peninsula, and the Levant over the past 140,000 y. Large orbitally forced wet/dry extremes occur during interglacial time, ∼130 to 80 ka, and conditions between these two extremes prevail during glacial time, ∼70 to 15 ka. Orbital precession causes high seasonality in Northern Hemisphere (NH) insolation at ∼125, 105, and 83 ka, with stronger and northward extended summer monsoon rains in North Africa and the Arabian Peninsula and increased winter rains in the Mediterranean Basin. The combined effects of these two seasonally distinct rainfall regimes increase vegetation and narrow the width of the Saharan-Arabian desert and semidesert zones. During the opposite phase of the precession cycle (∼115, 95, and 73 ka), NH seasonality is low, and decreased summer insolation and increased winter insolation cause monsoon and storm track rains to decrease and the width of the desert zone to increase. During glacial time (∼70 to 15 ka), forcing from large ice sheets and lowered greenhouse gas concentrations combine to increase winter Mediterranean storm track precipitation; the southward retreat of the northern limit of summer monsoon rains is relatively small, thereby limiting the expansion of deserts. The lowered greenhouse gas concentrations cause the near-equatorial zone to cool and reduce convection, causing drier climate with reduced forest cover. At most locations and times, the simulations agree with environmental observations. These changing regional patterns of climate/vegetation could have influenced the dispersal of early humans through expansions and contractions of well-watered corridors.

A continuous stable isotope record of last interglacial age from the Bulgarian Cave Orlova Chuka

AbstractRecent research shows that the last interglacial climate was more unstable in comparison to Holocene. Lack of suitable dating techniques and precisely defined absolute age benchmarks is one from main problems for present LIG studies. Therefore many of LIG chronologies base on indirect dating techniques like record alignment strategies. In this context, speleothems are valuable paleoclimate archives because of their capability to be dated by U-series method. In Europe LIG speleothem records are known mostly from western and central part of the continent. In this paper we present a 1,650 mm long stalagmite (ocz-6) from Bulgarian Cave Orlova Chuka. The ocz-6 stalagmite records the period of timeca. 129–112 ka. Ocz-6 stalagmite was analyzed in terms of stable isotopic composition of calcite and trace elements content. All analyzed geochemical proxies point to dynamic changes in the environment during the Last Interglacial time. At the time of interglacial development (129–126.5 ka), ocz-6 records shows systematic change in proportion of moisture delivered from Atlantic source and other sources. The beginning of last interglacial optimum is connected with a rapid change to more humid and warm conditions. During interglacial demise local climate become more dependent from regional settings.

Microbially-induced fluvial tufa in Gunna hills, Farafra Oasis, Egypt: Facies analysis and stable isotopes

This work addresses the characteristic sedimentary facies and stable isotopic (δ13C and δ18O) signature of the tufa unit that occurs in the uppermost part of the stratigraphic succession at North and South Gunna hills in the Farafra Depression (Western Desert, Egypt). This has been done to reconstruct the depositional setting and genesis of the tufa unit. The tufa unit occurs as chaotic dark fallen blocks and pile-like carbonate mound covering the slope of a marine Paleocene-Eocene substrate. In outcrop and at close-up view, the tufa unit displays three main lithofacies: laminated, massive and granular tufa, which can be interpreted as deposited in a fluvial setting. The microfacies analysis reveals the dominance of a variety of micro-textures including: a) crystalline laminated texture showing different shapes (bacterial-shrubs, crystal shrubs, and recrystallized robust palisades of leaf-like crystals), b) clotted-peloidal texture and c) clotted-oncoidal texture, all of them reflecting an influence of biotic processes (likely cyanobacterial activity) during tufa deposition. The moderate covariance (r = 0.53) between δ18O and δ13C suggests some evaporation effect during the tufa precipitation. Low δ13C values (−11.27‰ to −7.64‰ V-PDB) values indicate that soil CO2 is principally derived from C3 plants, pointing to flowing waters with low residence time. The recorded low δ18O values (−10.82‰ to −8.50‰ V-PDB) are also consistent with carbonates precipitating from meteoric waters related with pluvial episodes impoverished in 18O by continental effects. The pluvial period from which the tufa was formed could be related to air masses that crossed from the Atlantic-Mediterranean Sea during interglacial time period, bringing rain to the Sahara at least as far back as 450 000 yr (Late Pleistocene) before present.

Characteristics of modern carbonate contourite drifts

AbstractCarbonate environments inhabit the realm of the surface, intermediate and deep currents of the ocean circulation where they produce and continuously deliver material which is potentially deposited into contourite drifts. In the tropical realm, fine‐grained particles produced in shallow water and transported off‐bank by tidal, wind‐driven, and cascading density currents are a major source for transport and deposition by currents. Sediment production is especially high during interglacial times when sea level is high and is greatly reduced during glacial times of sea‐level lowstands. Reduced sedimentation on carbonate contourite drifts leads to early marine cementation and hardened surfaces, which are often reworked when current strength increases. As a result, reworked lithoclasts are a common component in carbonate drifts. In areas of temperate and cool water carbonates, currents are able to flow across carbonate producing areas and incorporate sediment directly to the current. The entrained skeletal carbonate particles have variable bulk density and shapes that lower the prediction of transport rates in energy‐based transport models, as well as prediction of current velocity based on grain size. All types of contourite drifts known in clastic environments are found in carbonate environments, but three additional drift types occur in carbonates because of local sources and current flow diversion in the complicated topography inherent to carbonate systems. The periplatform drift is a carbonate‐specific plastered drift that is nearly exclusively made of periplatform ooze. Its geometry is built by the interaction of along‐slope currents and downslope currents, which deliver sediment from the adjacent shallow‐water carbonate realm to the contour current via a line source. Because the periplatform drift is plastered on the slopes of the platforms it is also subject to mass gravity flow and large slope failures. At platform edges, a special type of patch drift develops. These hemiconal platform‐edge drifts also contain exclusively periplatform ooze but their geometry is controlled by the current around the corner of the platform. At the north‐western end of Little and Great Bahama Bank are platform‐edge drifts that are over 100 km long and up to 600 m thick. A special type of channel‐related drift forms when passages between carbonate buildups or channels within a platform open into deeper water. A current flowing in these channels will entrain material shed from the sediment producing areas. At the channel mouth, the sediment‐charged current deposits its sediment load into the deeper basin. With continuous flow, a submarine delta drift is built that progrades into the deep water. The strongly focused current forming the delta drift, is able to rework coarse skeletal grains and clasts, making this type of carbonate drift the coarsest drift type.