Paleotemperature Changes Research Articles

Novel insight into arsenic enrichment in aquifer sediments under different paleotemperatures from a molecular-level characterization of sedimentary organic matter

The heterogeneous distribution of As in sediments is governed by the abundance and type of SOM, which is closely associated with the depositional environment. However, few studies have revealed the effect of depositional environment (e.g., paleotemperature) on As sequestration and transport in sediments from the perspective of the molecular characteristics of sedimentary organic matter (SOM). In this study, we characterized the optical and molecular characteristics of SOM coupled with organic geochemical signatures to illustrate in detail the mechanisms of sedimentary As burial under different paleotemperatures. We identified that alternating paleotemperature changes result in the fluctuation of H-rich and H-poor organic matter in sediments. Further, we found aliphatic and saturated compounds with higher nominal oxidation state of carbon (NOSC) values predominate under high-paleotemperature (HT) conditions, while polycyclic aromatics and polyphenols with lower NOSC values accumulate under low-paleotemperature (LT) conditions. Under LT conditions, thermodynamically favorable organic compounds (higher NOSC values) are preferentially degraded by microorganisms to provide sufficient energy to sustain sulfate reduction, favoring sedimentary As sequestration. Under HT conditions, the energy gained from the decomposition of low NOSC value organic compounds approaches the energy required to sustain dissimilatory Fe reduction, leading to sedimentary As release into groundwater. This study provides molecular-scale evidence of SOM that indicates LT depositional environments favor sedimentary As burial and accumulation.

Multiproxy Paleosol Evidence for Jurassic Paleoclimate Fluctuations in the Sichuan Basin, SW China

AbstractPaleoclimate reconstruction of continental environments has been hampered by the limited evidence. A thick sequence of Jurassic continental deposits in the Sichuan Basin, Southwest China yields abundant paleosols that may offer valuable insights regarding Jurassic climate scenarios. A succession of 169 paleosols belonging to Protosols, Calcisols, and Argillisols from 23 detailed stratigraphic sections was recognized and characterized macro‐ and micromorphologically and assessed for mineral and geochemical compositions. Quantitative paleoclimate reconstructions using bulk geochemical proxies, the depth to and the stable oxygen isotopic composition of paleosol carbonates indicated a predominant alternation of semiarid and arid cool/warm‐temperate climatic conditions punctuated by several episodes of subhumid and humid climates that generally prevailed in the Sichuan Basin during the Jurassic. The estimated paleoatmospheric CO2 concentrations (pCO2) from calcic paleosols yielded a low range of ∼104 ± 58 to ∼610 ± 152 ppmv during the Middle Jurassic. The terrestrial paleotemperature changes in the Sichuan Basin coincided with the pCO2 variations, which probably resulted from global geological events (e.g., volcanic activities, magmatic and oceanic events, and the ephemeral caps development) in the Middle Jurassic. Jurassic climatic fluctuations in the basin were likely attributed to true polar wander due to global plate motion, megamonsoon effect linked to global and regional paleogeography, and regional paleotopography.

Responses of silicate weathering intensity to the Pliocene-Quaternary cooling in East and Southeast Asia

Silicate weathering plays a key role in maintaining a habitable climate on Earth. Although the climate-weathering link has been verified by observations on river geochemistry and weathering profiles, it remains a big challenge to quantify this link in the geological past mainly because sedimentary records of weathering intensity are complicated by other factors such as tectonics, sediment provenance and sorting effects. To clarify the climate dependence of weathering on different spatial and temporal scales, 4 Myr weathering intensity records determined by Chemical Index of Alteration (CIA), are compiled from 12 sites along the East and Southeast Asian margins ranging from ca. 10°N to 40°N. All the CIA records show overall decreasing trends, corresponding to global cooling. The CIA stacks that indicate regional and continental-scale weathering intensity are produced by binning and averaging the CIA data from each site. To explore the weathering intensity-temperature relationship, literature data on sea surface temperature (SST) anomaly are presented for the paleo-temperature changes at comparable latitudes. The results show that the decrease in CIA stack since 4 Ma is larger in East Asia (71-82 in Pliocene down to ca. 62 in late Quaternary) than in Southeast Asia (73-75 in Pliocene to ca. 72 in late Quaternary), in part due to the amplified midlatitude cooling. The correlation analyses indicate the temperature sensitivity of stacked CIA record in Southeast Asia (ca. 0.8 CIA units/°C) is consistent with modern observations of surface soils and river sediments. In comparison, the response of weathering intensity to temperature change is more sensitive in East Asia, with a magnitude of ca. 2.4 CIA units/°C, which may relate to dynamic source-to-sink processes and weathering regime changes. This study provides the first observation on silicate weathering responses to temperature changes at various latitudes since the Pliocene, showing the similar responses, but with different magnitudes. Our findings provide new insights on continental weathering mechanism and late Cenozoic cooling.

The interplay of Malm carbonate permeability, gravity-driven groundwater flow, and paleoclimate\xa0\u2013 implications for the geothermal field and potential in the Molasse Basin (southern Germany), a foreland-basin play

The Molasse Basin in Southern Germany is part of the North Alpine Foreland Basin and hosts the largest accumulation of deep geothermal production fields in Central Europe. Despite the vast development of geothermal energy utilization projects especially in the Munich metropolitan region, the evolution of and control factors on the natural geothermal field, more specifically the time-varying recharge and discharge governing groundwater and heat flow, are still debated. Within the Upper Jurassic (Malm) carbonate aquifer as the main geothermal reservoir in the Molasse Basin, temperature anomalies such as the Wasserburg Trough anomaly to the east of Munich and their underlying fluid and heat transport processes are yet poorly understood. We delineate the two end members of thermal–hydraulic regimes in the Molasse Basin by calculating two contrasting permeability scenarios of the heterogeneously karstified Malm carbonate aquifer along a model section through the Wasserburg Trough anomaly by means of two-dimensional numerical thermal-hydraulic modelling. We test the sensitivity of the thermal-hydraulic regime with regard to paleoclimate by computing the two Malm permeability scenarios both with a constant surface temperature of 9 °C and with the impact of paleo-temperature changes during the last 130 ka including the Würm Glaciation. Accordingly, we consider the hydraulic and thermal effects of periglacial conditions like permafrost formation and the impact of the numerous glacial advances onto the Molasse Basin. Thermal-hydraulic modelling reveals the effect of recurrent glacial periods on the subsurface targets of geothermal interest, which is minor compared to the effect of permeability-related, continuous gravity-driven groundwater flow as a major heat transport mechanism.

Evaluation of the sources and seasonal production of brGDGTs in lake Sihailongwan (N.E. China) and application to reconstruct paleo-temperatures over the period 60–8 ka BP

Paleotemperature changes during the Late Quaternary are mainly derived from marine and ice core records. However, there are relatively few temperature reconstructions from the East Asian Monsoon region, which has hindered our mechanistic understanding so far. To address this issue, a new time series of paleo-temperature was developed, based on the analysis of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from the well-dated sedimentary record of Sihailongwan maar lake (Lake SHL). This was achieved in two steps. Firstly, the sources and timing of seasonal production of brGDGTs were evaluated in the modern settings of Lake SHL. Secondly, using this information, we attempted to correct the temperature bias from soil derived brGDGTs using a binary mixing model, and a quantitative reconstruction of the paleo-temperature was derived from the Lake SHL brGDGTs sequence for the period of ∼60-8 ka BP. The results show that brGDGTs in Lake SHL are produced predominantly in summer and autumn (June–November). The sediment record reveals several important points: 1) the summer-autumn temperatures over the period ∼60–8 ka BP varied between ∼7 °C and ∼12.5 °C and the coldest period occurred during ∼25-21 ka BP when the mean summer-autumn temperature was ∼8.2 °C, 2) the summer-autumn temperature in Northeast China began to increase at ∼21 ka BP, and gradually warmed until the early Holocene in agreement with what has been recorded in southern China, most likely forced by summer insolation at 65°N.

Sedimentary characteristics and evolution of Domanik facies from the Devonian–Carboniferous regression in the southern Volga-Ural Basin

The Domanik facies are organic matter-rich siliceous–clay–carbonate sediments of Upper Devonian to Lower Carboniferous age in the Volga-Ural and Timan-Pechora petroleum provinces.Different studies have been inferred for these sediments across the Eastern European (Russian) Platform, such as sedimentary environments, tectonic and hydrogeological regimes, paleotemperature changes and so on. However, less attention are paid to the petroleum formation in this area, especially that the link between low amplitude uplift and petroleum concentrated into fractured zones in the center of the sags is not clear. In this study, we particularly explained this relationship from analyzing the sedimentary evolution of the Domanik facies in the southern Volga-Ural Basin (southeastern Russian Platform). The results show that the sediments in this area were formed during a period of regression in uncompensated sags and depressions from the Middle Frasnian Semiluksk Chron (Late Devonian) until the Early Tournaisian (Early Carboniferous). The Semiluksk Formation in this area is dominated by typical Domanik carbonate sediments, which are dark, siliceous, clayey and bituminous, with complex faunal assemblages. From the Upper Frasnian – Lower Tournaisian, carbonates are developed and become less abundant as the axial parts of the Kama–Kinel sags narrow. We propose that during the formation of the Domanik facies when the basin reached anomalous isostatic gravity, the reefs that grew widely on the slope subsided due to the differential compaction of the basin, at the same time the center of the basin underwent uplift. This process formed tectonic fractures and petroleum could migrate and concentrate in the low-amplitude intrabasinal uplifts. Therefore, these uplifts could be characterized by maximal basin thicknesses, sediments with high petroleum concentrations and abnormally high pressure, and thus could be reasonable zones for the exploration and development of low porosity and low permeability Domanik reservoirs.

A high-resolution Middle to Late Permian paleotemperature curve reconstructed using oxygen isotopes of well-preserved brachiopod shells

Climatic warming has been widely cited as a driver for many mass extinction events in Earth history, including the middle-late Permian Guadalupian-Lopingian boundary (GLB) mass extinction and end-Permian mass extinction (EPME) investigated in this study. However, the role of warming in driving the EPME event is under debate because current paleotemperature studies indicate the rapid increase in sea-surface temperature postdate the onset of the EPME event, and a reliable paleotemperature record across the GLB event is lacking. Here, we present a high-resolution and successive δ18O record using low-Mg calcite shells of brachiopods (bLMCs) from a single stratigraphic section in central China to reconstruct paleotemperature changes from the middle to the latest Permian. After a rigorous screening for diagenesis, our data show a ∼1.0‰ decrease in δ18Ocalcite (and thus climatic warming) during the middle Capitanian, followed by a gradual ∼2.0‰ increase (climatic cooling) during the late Wuchiapingian. From the latest Wuchiapingian to the early Changhsingian, δ18O decreased gradually and reached the lowest values of -4.0‰ (and thus rapid climatic warming) immediately before the Permian-Triassic boundary. The two warming events are coeval with the onset of the Emeishan Large Igneous Province and the Siberian Traps Large Igneous Province, respectively, suggesting the two volcanic events are the plausible drivers of these climatic warming events. A close comparison of paleontological and geochemical records indicates that the warming events occurred predate the GLB and EPME events, this is in contrast with conodont δ18Oapatite data which document the rapid climatic warming lagged the onset of the extinction event. The comparison between δ18O of bLMCs and δ18O of conodonts from the same section shows that the long-term trends are similar, but, at higher temporal resolution, discrepancies are present, entailing further investigation in the near future. Our study confirms that δ18O of the brachiopod shells underwent important changes during the end of Paleozoic, highlighting that sea surface temperature is a key factor to understand the biosphere history, since it changes simultaneously with the biological crises, and that bLMCs are invaluable archive for tracking physical and chemical conditions of past oceans.

Oxygen and Carbon Isotopes of Cold-water Corals—Reconstructing Paleotemperature Changes and Calcification Mechanism

Oxygen and Carbon Isotopes of Cold-water Corals—Reconstructing Paleotemperature Changes and Calcification Mechanism

Hypostratotype of the Bashkirian Stage of the Carboniferous System (Askyn River, Bashkortostan): Lithology, Isotopes (δ13C, δ18O), and Carbonate Depositional Settings

The isotopic composition and lithology of the hypostratotype of the Bashkirian Stage (Pennsylvanian) in the Askyn River section (Bashkortostan, western slope of the South Urals) are studied. The isotopic study (δ13C, δ18O) was conducted taking the carbonate lithology into account. The succession studied is composed of Upper Serpukhovian, Bashkirian, and Lower Moscovian carbonates of various lithological types, changeable throughout the section. The Serpukhovian carbonates are characterized by a relatively light carbon isotopic composition (negative values of δ13C), whereas the Bashkirian and Moscovian sediments have a heavy composition (mainly positive values of δ13C). The carbon isotope anomaly of the first order, fixed at the Mississippian–Pennsylvanian boundary and having a global distribution, is recorded in the studied hypostratotype section slightly higher than the uniformly accepted boundary level. This may indicate that the section contains “transitional beds” at the base of the Bashkirian. In the studied carbonates, a dependence (of the second order) of the isotopic composition of carbon and oxygen on the lithological type of rocks is sometimes recorded. The variation in the values of δ13C and δ18O in carbonates reflects changes in the conditions in which they were formed (temporary and local changes in paleotemperature, bioproductivity, desalination) and epigenetic changes.

Controlling factors and the paleoenvironmental significance of chemical elements in Holocene calcareous root tubes in the Alashan Desert, Northwest China

AbstractIn the hinterland of the desert, valuable archives of paleoenvironmental evolution are scarce. Calcareous root tubes (CRTs) have a strong potential for reconstructing paleoenvironmental conditions. It is still unclear, however, whether chemical elements in the CRTs can provide insights into paleoenvironmental conditions. In this study, the major- and trace-element composition of 32 CRT samples from the Alashan Desert were analyzed by X-ray fluorescence spectrometry. Results showed that the elemental composition and content change were controlled by the parent material and climatic conditions at the time of CRT formation. Ca, Mg, and Sr were significantly affected by climate, whereas the enrichment of P is likely related to the growth of plants. Higher (lower) Mg/Ca and Sr/Ca ratios corresponded to higher (lower) effective moisture and a higher (lower) Mg/Sr ratio indicated a higher (lower) temperature during the middle Holocene (8–5 cal ka BP). The reconstruction results for effective moisture were consistent with those in the Asian monsoon margin of northwestern China, which were caused by higher monsoon precipitation and lower evaporation. Therefore, chemical elements in the CRTs can reflect changes in paleo-effective moisture and paleotemperature at a millennial resolution in this area.

Campanian-Maastrichtian paleotemperature and paleodepth changes along Tethyan transect, North Sinai, Egypt

Planktic foraminiferal analyses of the Campanian-Maastrichtian sediments from four sections a long a Tethyan transect in north Sinai, enabled to recognize twelve planktic foraminiferal zones and subzones ranges from CF11 to CF1, in addition to Globotruncanita elevata Zone. The planktic morphotypes were basically subdivided into three groups, Deep taxa (DT), Surface taxa (ST) and subsurface e Taxa (SsT) to provide insights into paleotemperature changes. Q-mode cluster analysis as well as the relative dominance of planktic groups revealed the recognition of long term warming in the Campanian with warmest interval in Globotruncana ventricosa. Whereas minor cold events intervened at 80, 78.4, 75.4, 73.6 and particularly at 72 Ma indicates the Campanian/Maastrichtian boundary. The Maastrichtian (CF7-CF1) were characterized by successive warm-cool intervals superimposed on long term cooling, the late Maastrichtian (CF3-CF1) in particular, is characterized by alternate warm cold interval, where the last event was cooling, prior to K/T boundary. The Campanian- Maastrichtian paleodepth in North Sinai indicate that the cooling event much consistent with regression phases and equivalent to global eustasy whereas, the warm interval is consistent with the transgression phases. It is concluded that the late Maastrichtian was much controlled with local tectonics since CF3 and prior the K/T boundary.

Mechanisms of salinization in a middle Eocene lake in the Tanggu area of the Huanghua Depression

The Sha-3-5 Submember of the Shahejie Formation in the Tanggu area of the Huanghua Depression contains analcime-bearing dolomite, indicating salinization of an ancient lake during deposition. Solar evaporation and hydrothermal salinization have both been proposed as ways to generate these saline conditions. Based on a comprehensive analysis of core data, thin sections, spore and pollen data, elemental geochemistry, pyrolysis results, and vitrinite reflectance, we assessed the mechanisms driving salinization in light of provenance evolution, geothermal evolution, paleoclimate, mineral responses to evaporation, and the relationship between fluid temperature and salinity. The vertical profiles of ΣREE, Eu/*Eu, and Tmax exhibit little variation, and the maximum paleotemperatures attained by organic matter are comparable to present-day drill hole temperature. No major changes in provenance or paleotemperature and no petrological fabrics that would indicate a hydrothermal input are observed in the studied section. However, evidence for a semi-humid to semi-arid climate, primary evaporitic textures and structures, and a positive relationship between fluid temperature and salinity all support evaporation as the primary mechanism driving rising salinity. Solar evaporation, rather than hydrothermal eruption, appears to have been the primary factor driving lake salinization during middle Eocene deposition of the dolomitic Sha-3-5 Submember.

What caused the five mass extinctions?

Although a majority of biologists are convinced that a mass extinction is underway on earth today, the human history with direct observatory data is too short to predict its future trends. At least five mass extinctions occurred during the Phanerozoic Eon, causing the rapid extinction of at least 75% of existing marine species; they also seriously affected species diversity on land once the terrestrial ecosystem developed. The causes and consequences of these mass extinctions have become the most useful analogs for understanding whether the current global ecosystem is experiencing an extinction event. Previous multidisciplinary studies of the extinction patterns of fossil groups and concurrent environmental changes of the five mass extinctions during the past 500 million years (occurring in the end-Ordovician, Late Devonian Frasnian-Famennian, end-Permian, end-Triassic, and end-Cretaceous) suggested that no catastrophic event wiped out all organisms on earth. However, all five mass extinctions were associated with serious environmental deterioration and major paleoclimatic changes. The end-Ordovician mass extinction, occurring 445.2–443.8 million years ago, consists of two phases separated by an interval dominated by the cold Hirnantia fauna. The wax and wane of glaciation and associated widespread anoxia were the major causes of the two phases of the end-Ordovician mass extinctions. The Late Devonian mass extinction consists of a few different events from the Givetian to the Devonian-Carboniferous boundary, of which the most important is the Kellwasser event around the Frasnian-Famennian boundary. This extinction most seriously affected the coral and stromatoporoid reefs, and caused the extinction of two brachiopod orders (Pentamerida and Atrypida). As of yet, there is no consensus on the cause of this extinction event. Global cooling related to the widespread development of the terrestrial vegetation ecosystem and marine anoxia are the two most plausible scenarios. Although impact events were reported from a few horizons in the Late Devonian, they cannot account for the multiple phases of the Late Devonian mass extinctions. The end-Permian mass extinction about 252 million years ago has been universally documented as the most serious, which caused the disappearances of about 95% of all marine and 75% of all terrestrial species. Based on the latest high-precision geochronology data from South China, this extinction happened within an interval of less than 61 thousand years. This extinction was associated with a sharp negative excursion of δ 13Ccarb. A rapid temperature rise of 6–8 °C also occurred within the extinction interval. The Siberian Traps eruption and volcanism in South China, triggered by the dispersal of the supercontinent Pangea, is the most plausible explanation for the end-Permian mass extinction. The end-Triassic mass extinction at 201.564±0.015 Ma also seriously affected both marine and terrestrial ecosystems. Amphibians and reptiles both suffered a great loss during this extinction, and they were subsequently replaced by highly diverse dinosaurs. Marine conodonts and Ceratitida became extinct, and the sponge Demospongea and the brachiopod order Spiriferinida were also greatly affected. Recent studies suggest that the extinction stage may have extended over 10–20 million years, and the volcanism of the Central Atlantic Magmatic Province may be the cause. The end-Cretaceous mass extinction has been the best-known event among the general public because it caused the extinction of various dinosaurs that prevailed during the Mesozoic Era. The cause of this extinction has been documented as the extraterrestrial Chicxulub impact event. However, detailed paleontological studies suggested that the extinction is more likely to have been caused by another major volcanism event, the massive eruption of the Deccan Traps. In summary, global changes in atmospheric CO2 and paleotemperature (both icehouse and greenhouse), oceanic acidification, sea-level changes, and anoxia triggered by massive volcanic eruptions are the most plausible causes of the past extinctions. Massive volcanism not only ejected a huge amount of CO2 and volcanic sulfates, but also caused a massive release of thermogenic CO2 and methane stored in the deposits of inland basins and continental shelves. Extraterrestrial impact, supernova explosion, and solar flares could instantaneously wipe out all organisms on earth, but they are not the main causes of the five mass extinctions experienced in the history of the earth.

Fidelity of plant-wax molecular and carbon isotope ratios in a Holocene paleosol sequence from the Chinese Loess Plateau

Loess-paleosol sequences from the Chinese Loess Plateau (CLP) provide important archives for paleoclimate reconstruction in the Quaternary. Lipid biomarkers from the CLP have been shown to be useful as molecular paleoclimate proxies. Here, we report a Holocene record of n-alkane ratios and compound specific carbon isotope compositions from the Weinan section on the southern margin of the CLP. Throughout the latest deglaciation (L1-1) and Holocene (S0) layers, the average chain length, carbon preference index (CPI), relative abundance of low molecular weight to high molecular weight n-alkanes, and the δ13C values of n-C29 and n-C31 alkanes varied closely with the changes in paleotemperature and aridity as recorded by tetraether lipids from the same soil samples. Our results are consistent with the expectation that the CLP C4/C3 plant ratio is sensitive to monsoon intensity. Furthermore, the carbon isotope offset between n-C29 and n-C31 alkanes (Δδ13C29–31) correlated closely with the paleoclimate record. Existing data for C4 and C3 herb plants do not support the idea that the close relation between Δδ13C29–31 and paleoclimate results from the isotopic difference between C4 and C3 plants. Instead, it is likely that it arises from climate effects on vegetation dynamics. Although more work is required to confirm this deduction, it appears that the Δδ13C29–31 has potential to be considered a novel paleoclimate proxy in loess-paleosol sequences and similar settings.

Historical colonization and dispersal limitation supplement climate and topography in shaping species richness of African lizards (Reptilia: Agaminae).

To what extent deep-time dispersal limitation shapes present-day biodiversity at broad spatial scales remains elusive. Here, we compiled a continental dataset on the distributions of African lizard species in the reptile subfamily Agaminae (a relatively young, Neogene radiation of agamid lizards which ancestors colonized Africa from the Arabian peninsula) and tested to what extent historical colonization and dispersal limitation (i.e. accessibility from areas of geographic origin) can explain present-day species richness relative to current climate, topography, and climate change since the late Miocene (~10 mya), the Pliocene (~3 mya), and the Last Glacial Maximum (LGM, 0.021 mya). Spatial and non-spatial multi-predictor regression models revealed that time-limited dispersal via arid corridors is a key predictor to explain macro-scale patterns of species richness. In addition, current precipitation seasonality, current temperature of the warmest month, paleo-temperature changes since the LGM and late Miocene, and topographic relief emerged as important drivers. These results suggest that deep-time dispersal constraints — in addition to climate and mountain building — strongly shape current species richness of Africa’s arid-adapted taxa. Such historical dispersal limitation might indicate that natural movement rates of species are too slow to respond to rates of ongoing and projected future climate and land use change.

A regional record of expanded Holocene wetlands and prehistoric human occupation from paleowetland deposits of the western Yarlung Tsangpo valley, southern Tibetan Plateau

AbstractThe Asian Monsoon, which brings ~80% of annual precipitation to much of the Tibetan Plateau, provides runoff to major rivers across the Asian continent. Paleoclimate records indicate summer insolation and North Atlantic paleotemperature changes forced variations in monsoon rainfall through the Holocene, resulting in hydrologic and ecologic changes in plateau watersheds. We present a record of Holocene hydrologic variability in the Yarlung Tsangpo (YT) valley of the southern Tibetan Plateau, based on sedimentology and 14C dating of organic-rich black mats’ in paleowetlands deposits, that shows changes in wetlands extent in response to changing monsoon intensity. Four sedimentary units indicate decreasing monsoon intensity since 10.4 ka BP. Wet conditions occurred at ~10.4 ka BP, ~9.6 ka BP and ~7.9–4.8 ka BP, with similar-to-modern conditions from ~4.6–2.0 ka BP, and drier-than-modern conditions from ~2.0 ka BP to present. Wetland changes correlate with monsoon intensity changes identified in nearby records, with weak monsoon intervals corresponding to desiccation and erosion of wetlands. Dating of in situ ceramic and microlithic artifacts within the wetlands indicates Epipaleolithic human occupation of the YT valley after 6.6 ka BP, supporting evidence for widespread colonization of the Tibetan Plateau in the early and mid-Holocene during warm, wet post-glacial conditions.

A regional record of expanded Holocene wetlands and prehistoric human occupation from paleowetland deposits of the western Yarlung Tsangpo valley, southern Tibetan Plateau

The Asian Monsoon, which brings ∼80% of annual precipitation to much of the Tibetan Plateau, provides runoff to major rivers across the Asian continent. Paleoclimate records indicate summer insolation and North Atlantic paleotemperature changes forced variations in monsoon rainfall through the Holocene, resulting in hydrologic and ecologic changes in plateau watersheds. We present a record of Holocene hydrologic variability in the Yarlung Tsangpo (YT) valley of the southern Tibetan Plateau, based on sedimentology and 14C dating of organic-rich ‘black mats’ in paleowetlands deposits, that shows changes in wetlands extent in response to changing monsoon intensity. Four sedimentary units indicate decreasing monsoon intensity since 10.4 ka BP. Wet conditions occurred at ∼10.4 ka BP, ∼9.6 ka BP and ∼7.9–4.8 ka BP, with similar-to-modern conditions from ∼4.6–2.0 ka BP, and drier-than-modern conditions from ∼2.0 ka BP to present. Wetland changes correlate with monsoon intensity changes identified in nearby records, with weak monsoon intervals corresponding to desiccation and erosion of wetlands. Dating of in situ ceramic and microlithic artifacts within the wetlands indicates Epipaleolithic human occupation of the YT valley after 6.6 ka BP, supporting evidence for widespread colonization of the Tibetan Plateau in the early and mid-Holocene during warm, wet post-glacial conditions.

Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

High-resolution SIMS oxygen isotope analysis on conodont apatite from South China and implications for the end-Permian mass extinction

Understanding the interplay of climatic and biological events in deep time requires resolving the precise timing and pattern of paleotemperature changes and their temporal relationship with carbon cycle variations and biodiversity fluctuations. In situ oxygen isotope analyses of conodont apatite from South China enables us to reconstruct high-resolution seawater temperature records across the Permian–Triassic boundary (PTB) intervals in the upper slope (Meishan), lower slope (Shangsi), and carbonate platform (Daijiagou and Liangfengya) settings. Constrained by the latest high-precision geochronological dates and high-resolution conodont biozones, we can establish the temporal and spatial patterns of seawater temperature changes and assess their potential connections with the carbon cycle disruption and biodiversity decline. We find a rapid warming of ~10°C during the latest Permian–earliest Triassic that postdated the onset of the negative shift in δ13Ccarb by ~81kyr (thousand years), the abrupt decline in δ13Ccarb by ~32kyr and the onset of mass extinction by ~23kyr, which contradicts previous claims that the extreme temperature rise started immediately before or coincided with the onset of mass extinction. Our new evidence indicates that climate warming was most likely not a direct cause for the main pulse of the end-Permian mass extinction (EPME), but rather a later participant or a catalyst that increased the pace of the biodiversity decline. In addition, a prominent cooling is recorded in the earliest Changhsingian, with the main phase (a drop of ~8°C in ~0.2Ma) confined to the lower part of the Clarkina wangi zone and synchronous with the positive limb of the carbon isotope excursion (CIE) around the Wuchiapingian–Changhsingian boundary (WCB) in Meishan and Shangsi. Further long-term and high-resolution studies from other sections are needed to confirm the full contexts and underlying dynamics of the WCB “cooling event”.

Late Quaternary paleoenvironmental changes revealed by multi-proxy records from the Chukchi Abyssal Plain, western Arctic Ocean

Late Quaternary paleoenvironmental changes in the western Arctic Ocean are revealed by multi-proxy records of core 03M03 from the Chukchi Abyssal Plain (CAP). Proxy parameters include lithology, grain size fractions, and mineralogy and petrology of ice-rafted detritus (IRD), element contents, biogenic components, δ18O, δ13C and Mg/Ca of planktonic foraminifera Neogloboquadrina pachyderma (sin.) (Nps). Seven IRD (>250μm) peaks are interpreted as marking detrital input by rafting sea ice or icebergs during MIS 3 interstadials and early MIS 1. High MnO, CaO and MgO contents and high Ca/Al and Mg/Al ratios during MIS 3 and MIS 1 correspond to increases in ice-rafted detrital carbonates and the synchronous declines in siliciclastic elements (e.g., Al2O3, Fe2O3). Therefore, these warmer periods were characterized by a high detrital carbonate input entrained in icebergs from the Canadian Arctic Archipelago coeval with an increased input of Mn through rivers and/or coastal erosion. Relatively stable contents of siliciclastic elements and their ratios in the grayish sediment units are interpreted from turbid surface water plumes or nepheloid flows delivered by meltwater and/or brine rejection from ice-sheet margins at the Arctic Ocean periphery. Relatively stable clay- and silt-sized fractions were attributed mainly to sea ice entrainment over glacial–interglacial cycles. High foraminiferal abundances in the brown units during MIS 3 and 1 are related to enhanced calcareous plankton productivity under more open water conditions and/or the incremental input of Atlantic water masses. Relatively high TOC and opal contents in the grayish units of MIS 3 appear to have accumulated by lateral transport of organic matter from the Chukchi shelf to the deep abyssal plain. Lower contents of biogenic material in the brown units probably result from increased dilution by rapid IRD deposition, and from early diagenetic degradation. Depletions in Nps-δ18O and -δ13C concurrent with high foraminiferal abundances and IRD peaks within the brown units of MIS 3 and 1 are indicative of meltwater pulses, as previously documented across the Arctic Ocean. However, several Nps-δ18O and -δ13C depletions between the brown units B3 and B2 in MIS 3 could have resulted from enhanced sea ice formation. The Nps-Mg/Ca has the potential to record paleotemperature changes in the Arctic region, but still awaits a better calibration of the Nps-Mg/Ca–temperature relationship with results from core-tops, sediment traps, and plankton tows.