MABP Research Articles

Neogene palaeoceanography of Chatham Rise (Southwest Pacific) based on sediment geochemistry

Approximately one thousand sediment samples from ODP Site 1123 on the Chatham Rise, east of New Zealand, have been examined for inorganic elemental concentrations. ODP 1123 provides a record of sediment drift deposition under the Deep Western Boundary Current, the main inflow of deep water to the Pacific Ocean since the Early Oligocene, though a major hiatus spans the late Early Oligocene to the Early Miocene. Normalisation of the elemental concentrations by aluminium was used to allow for the effects of variable carbonate dilution. The elemental ratios were used as proxies for sediment composition and as palaeoceanographic indices. The samples were collected at a resolution designed to sample adequately any variation in elemental ratios at the scale of the Milankovitch orbital cycles. The sampled intervals span the Early Oligocene, Early Miocene, mid-Miocene and Late Pleistocene to Recent. Anomalous Si/Al, K/Al, Ti/Al values in the upper Pleistocene section, often associated with horizons of low carbonate, are attributed to tephras derived from North Island. Not all of the tephras detected geochemically had been detected visually in the cores. A total of 37 tephra events between 1.17 Ma BP and the present are recognised based on this and the shipboard investigations. The tephra events cluster at intervals of approximately 326 000 years (326 ka) perhaps due to variations in eruption frequency on North Island and/or to variations in the regional palaeowind intensity and direction. In the Late Pleistocene to Recent P/Al (inferred nutrient availability), percent calcium carbonate (%CaCO 3) and Ba/Al (inferred productivity) varied regularly at a period of 40 000 years with these factors lagging minimum global ice volumes (interglacials). During the mid-Miocene %CaCO 3, Ba/Al, P/Al and Si/Al all gradually increased with %CaCO 3 and P/Al showing regular 138 000-yr cyclicity and Ba/Al showing 44-ka cyclicity. Inferred productivity (Ba/Al) may have been rising in association with increasing nutrient availability (P/Al) at the same time as increased vigour of the Deep Western Boundary Current that was connected to a period of rapid ice-sheet growth in Antarctica. In the Early Miocene P/Al and Si/Al were much higher than subsequently and both %CaCO 3 and P/Al exhibited 131 000-yr cycles. By far the highest nutrient levels and inferred productivity at this site apparently occurred during the Early Oligocene as revealed by long-term changes in P/Al and Si/Al. A progressive rise in K/Al, but stable Ti/Al from the Early Oligocene to the Recent probably indicates increased proportions of illite in the clay mineral fraction of the drift sediments caused by increased flux of debris from the Southern Alps.

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO2 components—atmospheric CO2 and CO2 from in situ oxidation of organic matter. The δ13C values measured for the Fe(CO3)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most 13C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO2 components. δ13C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco2 value of about 1 × PAL, but the scatter in the measured δ13C values of organic matter permits a calculated maximum Pco2 of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO3)OH in MCGoeth and SAP correspond to soil CO2 concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO2 concentrations are similar to modern soils in warm, wet environments.The average δ13C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ13C value for all profiles of −5.4‰. Thus, the value of Δ13C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco2 and organic matter δ13C values were the same for both paleosol types. Furthermore, the atmospheric Pco2 calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco2 (permitted by one standard deviation of the organic matter δ13C value) of ∼5 × PAL.If, however, measured average δ13C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of 13C-depleted, labile organic compounds, calculated Permian atmospheric Pco2 using these 13C-enriched organic values would underestimate the actual atmospheric Pco2 using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO2 concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO2.The Fe(CO3)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in 13C relative to Fe(CO3)OH in goethites from soils in which there are mixtures of two isotopic CO2 components. Field-relationships and the δ13C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO2 components at the time of goethite crystallization. The three components were probably atmospheric CO2, CO2 from in situ oxidation of organic matter and CO2 from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO2 components, as recorded by Fe(CO3)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco2.

Late Miocene–Pliocene development of Asian aridification as recorded in the Red-Earth Formation in northern China

In northern China, the Late Miocene–Pliocene Hipparion Red-Earth Formation in the eastern Loess Plateau fills a gap of climate records between the well-known loess-soil sequences of the last 2.6 Ma and the Miocene loess-soil sequences from the western Loess Plateau. Earlier studies on type sections indicate that a major part of the Red-Earth Formation is also of wind-blown origin, covering the period from ∼7–8 to ∼2.6 Ma BP. Because the eolian dust deposited in the region originated from the Asian desert lands, the Red-Earth must contain a record of the aridification history of the Asian interior. In this study, the Xifeng type section in the eastern Loess Plateau is studied to assess the development of Asian aridification during Late Miocene–Pliocene time. Eolian dust deposition continued at the central Loess Plateau since ∼6.2 Ma BP, indicating that sizable desert lands in the interior of Asia and the Asian winter monsoon must have been constantly maintained during Late Miocene–Pliocene time, and were able to provide a significant amount of eolian dust. The aridity in the source areas was stronger from ∼6.2 to ∼5 Ma BP and weaker from ∼5 to ∼3.6 Ma BP. Two major aridification steps are observed at ∼3.6 and ∼2.6 Ma BP, respectively. The intensification of eolian deposition at ∼3.6 Ma BP is synchronous with a suggested uplift of portions of the Tibetan Plateau. The general aridification history is also highly consistent with the ongoing high-latitude cooling and the consequent expansion of Arctic sea-ice/ice sheets during this interval. Our results therefore suggest that both Tibetan uplift and ice-building processes in the northern hemisphere were two prominent driving forces behind the long-term desertification in the interior of Asia during Late Miocene and Pliocene time.

A river erosion estimate on the Loess Plateau: a case study from Luohe River, a second-order tributary of the Yellow River

Based on fieldwork and terrace ages, which were determined using 14C, TL and paleosol stratigraphy, a general model was established for the development of the Yellow River terrace system. The ages for the terraces and valley flats of the Yellow River system are T 6—1.67–0.85 Ma BP, T 5—0.85–0.47 Ma BP, T 4—0.47–0.10 Ma BP, T 3—0.10–0.007 Ma BP, T 2—7.0–0.7 ka BP, T 1—0.7–0.3 ka BP, the higher valley flat—0.3–0.15 ka BP and the lower valley flat 0.15–0 ka BP, respectively. Each terrace or valley flat and corresponding paleo-valley represents a river erosion/deposition cycle. Using this model and selected geomorphic parameters of terraces and paleo-valleys from 10 typical cross sections of Luohe River, a tributary of the Yellow River, an attempt is made here to estimate paleo-river erosion since the Pleistocene on the Loess Plateau.

Modelling Pliocene warmth: contribution of atmosphere, oceans and cryosphere

The relative role of the atmosphere, oceans and cryosphere in contributing towards middle Pliocene warmth (ca 3 Ma BP) is investigated using the HadCM3 coupled ocean–atmosphere general circulation model. The model was initialised with boundary conditions from the USGS PRISM2 data set and a Pliocene atmospheric CO 2 level of 400 ppmv and run for 300 simulated years. The simulation resulted in a global surface temperature warming of 3°C compared to present-day. In contrast to earlier modelling experiments for the Pliocene, surface temperatures warmed in most areas including the tropics (1–5°C). Compared with present-day, the model predicts a general pattern of ocean warming (1–5°C) in both hemispheres to a depth of 2000 m, below which no significant differences are noted. Sea ice coverage is massively reduced (up to 90%). The flow of the Gulf Stream/North Atlantic Drift is up to 100 mm s −1 greater in the Pliocene case. Analysis of the model-predicted meridional streamfunction suggests a global pattern of reduced outflow of Antarctic bottom water (AABW; up to 5 Sv), a shallower depth for North Atlantic deep water formation and weaker thermohaline circulation (3 Sv). The decrease in AABW occurs mainly in the Pacific rather than Atlantic Ocean. Model diagnostics for heat transports indicate that neither the oceans nor the atmosphere are transporting significantly more heat in the Pliocene scenario. Rather, these results indicate that the major contributing mechanism to global Pliocene warmth was the reduced extent of high-latitude terrestrial ice sheets (50% reduction on Greenland, 33% reduction on Antarctica) and sea ice cover resulting in a strong ice-albedo feedback. These results highlight the need for further studies designed to improve our knowledge regarding Pliocene terrestrial ice configurations.

Pleistocene fluvial systems of the Creuse river (Middle Loire Basin - Centre Region, France) [ Les systèmes fluviatiles pléistocènes de la Creuse (Bassin moyen de la Loire, Région Centre, France).

The Creuse River is a tributary of the Vienne River which joins the Loire Valley on its left bank downstream Tours (Centre Region, France). Recent field works has significantly increased the number of fossil fluvial formations identified in the Middle Creuse River Valley, has confirmed their penglacial character and shown the impact of recent tectonics and the magnitude of uplift which affect the geometry of the deposits on the northern edge of the Massif Central and the southern and western parts of the Pans Basin. Three sectors have been distinguished : - a "Massif Central" sector, with an endogenous bed-rock, where some five stepped higher alluvial formations are studied. - an "Intermediate" sector characterized by the opening of the valley when the river joins the Pans Basin. Eleven sandy formations, which constituted advancing fans in this actively tectonic zone, have been identified. - a "Pans Basin" sector in which the Creuse flows westwards along a major fault. Eight stepped terraces have been recognized here. Several geochronological data were obtained by electronic spin resonance (ESR) method applied to bleached alluvial quartz. The results show a good reproducibihty and generally a good coherence with the altimetry. They indicate, in close correlation with sedimentological and tectonic studies, that the response of the river is variable and depends of local geology and tectonic characteristics. From the results obtained on each sector, a first synthetic but schematic history of the fluvial system can be proposed for the Middle Creuse River valley between 1,7 Ma BP and today Some major tectonic movements are dated between 1,7 and 1,2 Ma, around 600 ka and between 450 and the present. On the other hand, the "Pont-de-Lavaud" Early Palaeolithic site with archaic pebble quartz industry was dated to the end of the Lower Pleistocene, around 1,1 Ma BP.

Petroleum migration, faults and overpressure. Part II. Case history: The Haltenbanken Petroleum Province, offshore Norway

Abstract Petroleum inclusion and geochemical data from core extracts were applied to deduce a model for oil migration, overpressure development and palaeo-leakage of oil from currently dry structures in the Haltenbanken Vest area. The existence of fluorescent oil type inclusions in quartz in the Smørbukk (Åsgard-2) field suggest that oil migrated into this structure 70–50 million years before present (Ma bp). This is also the case for the dry structures 6506/12-4, 6506/11-3 and 6506/11-1, west of the main Smørbukk Fault Zone. Black oil inclusions with medium gas/oil ratio (GOR) occur in these fields together with condensate-type petroleum inclusions. This suggests that the dry structures transformed from containing oil to condensate before leakage. Petroleum extracted from inclusions in these structures and in nearby fields have identical marine type II kerogen signatures. Source rocks at the Spekk Formation level in the current drainage area of Smørbukk and these dry structures, were immature 70–50 Ma bp and the Smørbukk Sør (Åsgard-3) field did not fill at this early time. Thus, oil must initially have entered into Smørbukk from areas to the W-SW, through the currently pressure sealing Smørbukk Fault Zone which today marks the westward limit of the Smørbukk field. Diagenesis in this fault zone caused the much later overpressure development and petroleum was lost from the 6506/12-4, 6506/11-3 and 6506/11-1 structures as overpressure built up regionally. Petroleum loss from these structures with their often thick seals must have occurred via self-propagating open-fracture-induced mechanisms. Lack of petroleum in the Cretaceous strata above these structures suggests that leakage occurred to even shallower strata. This could imply that the Cretaceous strata in Halten Vest were overpressured at the time of leakage. In contrast, the oil in the Cretaceous Lysing and Lange Formation (above the Jurassic reservoirs in Smørbukk and Smørbukk Sør) most likely originated (based on geochemistry and GORs) from the Jurassic reservoirs below and not from Cretaceous strata. This migration event would have been facilitated if it occurred before these sands became overpressured as they are today. Modelling suggests that the Spekk Formation became mature in the Smørbukk Sør region <10 Ma bp and microthermometry of oil inclusions from Smørbukk Sør supports filling during the past 10 Ma. This implies that caprock failure in the Halten Vest structures 6506/12-4, 6506/11-1 and 6506/11-3 most likely occurred after filling of the Smørbukk Sør and 6406/3-1 structures. Rapid regional burial during the past 10 Ma caused local migration of oil into Smørbukk Sør, Smørbukk and 6406/3-1 structures, and generation of high GOR oils in the deeper Halten Vest region. High GOR petroleum inclusions in the Halten Vest structures signify this event and suggests that caprock fracturing occurred after a gas-condensate had replaced oil in these traps. Rapid burial during the past 3 Ma is likely to have caused the current overpressure and associated leakage in Halten Vest. The fact that these traps did not later refill in this progressively subsiding and maturing basin must be related to trap pressures remaining too close to the actual fracture pressures.

Pliocene–Pleistocene stable isotope and paleoceanographic changes in the northern South China Sea

Based on the stable isotopic analysis of planktonic and benthic foraminifers from Ocean Drilling Program Core 1148 of the northern South China Sea (SCS), Pliocene–Pleistocene isotope stratigraphy and events have been reconstructed. The benthic foraminiferal δ 18O record shows that the Pacific intermediate water had a greater influence upon the SCS or the Pacific deep water above ∼2600 m was warmer before ∼3.2 Ma than at present. After that, the benthic δ 18O conspicuously increased during the ∼3.2–2.5 Ma period, in correspondence to the formation of the Northern Hemisphere ice sheet, whereas the planktonic δ 18O signal suggests a stepwise overall decrease of sea surface temperature during the ∼2.2–0.9 Ma period. Compared to the equatorial Pacific records, the decrease in planktonic ( Globigerinoides ruber) δ 13C during the ∼3.2–2.2 Ma period is particularly striking, suggesting that fertility of surface water increased noticeably. According to the modern δ 13C distribution of G. ruber in the northern SCS, it is inferred that the East Asian winter monsoon strengthened during this interval. Afterwards, there were several conspicuous decreases of G. ruber δ 13C at ∼1.7, 1.3, 0.9, 0.45 and 0.15 Ma BP, that is, about every 0.4 Ma, suggesting that the East Asian winter monsoon became episodically stronger. This is confirmed by changes in relative abundance of planktonic foraminifer species Neogloboquadrina dutertrei, a typical East Asian winter monsoon proxy. The deep-water δ 13C of the SCS is close to that of the Pacific, but lighter than that of the Atlantic, implying that the pattern of deep water originating mainly from the Atlantic and through the Pacific entering the SCS existed at least since the early Pliocene. After 1.4 Ma, the benthic δ 13C signal decreased conspicuously but with a periodicity of ∼100 ka, suggesting that the deep-water ventilation of the SCS was reduced, probably corresponding to a decrease of the North Atlantic Deep Water and/or further isolation of the SCS deep basin from the Pacific during glaciations.

Chemical weathering history of the southern Tajikistan loess and paleoclimate implications

Major chemical elements and Rb, Sr, Ba abundances were measured for loess samples from Chashmanigar loess-soil sequence in southern Tajikistan. And the degree of element loss and intensity of weathering of the samples were calculated. From these calculations we found that the paleosol horizons of the Chashmanigar section were chemically weathered to some extent and the weathering intensity of the soils is stronger than that of loess horizons, indicating that paleosols in southern Tajikistan were deposited in relatively warm and humid interglacial ages. In addition, the southern Tajikistan loess deposited before 0.9Ma BP was more intensely weathered than that deposited after 0.9Ma BP, which may imply the intensification of aridity in this area since 0.9Ma BP.

Variations of Sedimentary Environments during Late Cenozoic of Co Ngoin, Central Tibetan Plateau and Its Tectonic Uplift Implications

错鄂湖200m深井岩芯古地磁测年表明错鄂湖形成于约2.8Ma年前.沉积岩性组合粒度特征和磁化率变化揭示了约3次大的沉积环境变化过程反映了至少2次剧烈的青藏高原隆升过程同时孢粉组合也揭示了构造隆升导致的植被组合的变化.初步研究认为大的湖泊沉积环境变化主要是在青藏高原不断隆升的背景上进行的.2.8-2.5Ma和0.8Ma以来的沉积环境演化主要受构造运动的控制而2.5-0.8Ma环境演化过程更多的受到冰期-间冰期旋回气候变化的影响.;The paleomagnitism dating of the core Co Ngoin, Qinghai-Tibetan Plateau, a 200-m-depth core taken from Co Ngoin, indicates that the lake emerges at 2.8Ma BP. Lithological property, grain size and magnetic susceptibility (sus) reveal three major significant environmental variations and at least two tectonic uplifts during the last 2.8Ma. Accordingly, pollen associations open out the vegetation changes controlled by the tectonic uplift. Studies show that the major environmental changes were largely controlled by stepwise uplift of the Qinghai-Tibetan Plateau, and those numerous small changes overlapped on the large tectonic background are mainly controlled by the global climatic changes of glacial and interglacial cycles in Quaternary. The thickness of the strata above the B/M boundary is thinner, which might be resulted from the glacial cover to lead the shortage in stratigraphic thickness.

Linking growth episodes of zircon and metamorphic textures to zircon chemistry: an example from the ultrahigh-temperature granulites of Rogaland (SW Norway)

Abstract In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M 1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M 2 metamorphic episode. Zircon intergrown with or included within M 2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2 σ, n = 20). The youngest observed results are found in zircon outside M 2 minerals, some overgrown by M 3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2 σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

Trace metal signatures of Jurassic/Cretaceous black shales from the Norwegian Shelf and the Barents Sea

Black shale samples of Jurassic to Cretaceous age recovered during the ‘Norwegian Shelf Drilling Program’ between 1987 and 1991 from Sites 7430/10-U-01 (Barents Sea), 6814/04-U-02 (Norwegian Shelf near the Lofoten) and 6307/07-U-02 (Norwegian Shelf near Trondheim) were analyzed for major and trace elements. These laminated black shales are characterized by high total organic carbon (TOC) and total sulfur (TS) contents as well as by significant enrichments in several redox-sensitive and/or sulfide-forming trace metals (Ag, Bi, Cd, Co, Cr, Cu, Mo, Ni, Re, Sb, Tl, U, V, and Zn). Enrichment factors relative to ‘average shale’ are comparable to those found in Cenomanian–Turonian boundary event (CTBE) black shales and Mediterranean sapropels. The Re content is high in the studied black shales, with maximum values up to 1221 ng/g. Re/Mo ratios averaging 2.3×10 −3 are close to the seawater value. High trace metal enrichments and Re/Mo ratios close to the seawater value point to a dominantly anoxic and sulfidic water column during black shale formation. Interbeds with higher Re/Mo ratios, especially in high-resolution sampled core sections, point to brief periods of suboxic conditions. Additionally, enhanced Zn concentrations in the black shales from the Barents Sea support the assumption that hydrothermal activity was also high during black shale deposition. Trace metal signatures of black shales at different drill sites on a transect along the Norwegian Shelf are not only influenced by water depth but also by their location in the boreal realm. Metal enrichments are higher in the northern compared to the southern sites. Volgian (=Tithonian 151–144 Ma BP) black shales exhibit elevated trace metal contents in comparison to their Berriasian (144–137 Ma BP) counterparts. This probably reflects a change in the circulation pattern during periods of black shale formation. Therefore different paleoceanographic conditions, probably controlled by climatic change linked to the transgression of the paleo-sealevel and the North Atlantic opening, may have developed from the Volgian to the Berriasian.

Global middle Pliocene biome reconstruction: A data/model synthesis

The middle Pliocene warm interval (ca. 3 Ma BP) has been extensively studied. However, our knowledge concerning the global distribution of middle Pliocene biomes remains far from complete. This paper presents the results from a “first attempt” at simulating the distribution of different mid‐Pliocene biomes using an advanced numerical general circulation climate model (Hadley Centre Atmospheric Model Version 3) and the BIOME 4 vegetation model. The modeling indicates that during the middle Pliocene the geographical coverage of tundra type biomes may have been significantly reduced compared with the present day in the Northern Hemisphere. High‐latitude forests expand in the place of tundra forms of vegetation. Total area covered by forest increases for the Pliocene case compared with the present day. Arid deserts become less prevalent in the Pliocene scenario and are replaced by tropical xerophytic shrublands and savanna‐type vegetation. These results compare favorably with geological data in general and with the U.S. Geological Survey's PRISM2 middle Pliocene vegetation reconstruction, although data/model inconsistencies are apparent. Although some of these inconsistencies relate to the weaknesses of the climate and biome model employed, others identify deficiencies in the extant geological data set or the interpretation of this data. This modeled biome reconstruction will serve as a useful vehicle for aiding in future comparisons between geological data on middle Pliocene biomes and model predictions.

Magnitude of climate variability during middle Pliocene warmth: a palaeoclimate modelling study

The nature and magnitude of climatic variability during the period of middle Pliocene warmth (ca 3.29–2.97 Ma) is poorly understood. We present a suite of palaeoclimate modelling experiments incorporating an advanced atmospheric general circulation model (GCM), coupled to a Q-flux ocean model for 3.29, 3.12 and 2.97 Ma BP. Astronomical solutions for the periods in question were derived from the Berger and Loutre BL2 astronomical solution. Boundary conditions, excluding sea surface temperatures (SSTs) which were predicted by the slab-ocean model, were provided from the USGS PRISM2 2°×2° digital data set. The model results indicate that little annual variation (∼0.5°C) in SSTs, relative to a ‘control’ experiment, occurred during the middle Pliocene in response to the altered orbital configurations. Annual surface air temperatures also displayed little variation. Seasonally, surface air temperatures displayed a trend of cooler temperatures during December, January and February, and warmer temperatures during June, July and August. This pattern is consistent with altered seasonality resulting from the prescribed orbital configurations. Precipitation changes follow the seasonal trend observed for surface air temperature. Compared to present-day, surface wind strength and wind stress over the North Atlantic, North Pacific and Southern Ocean remained greater in each of the Pliocene experiments. This suggests that wind-driven gyral circulation may have been consistently greater during the middle Pliocene. The trend of climatic variability predicted by the GCM for the middle Pliocene accords with geological data. However, it is unclear if the model correctly simulates the magnitude of the variation. This uncertainty is derived from, (a) the relative insensitivity of the GCM to perturbation in the imposed boundary conditions, (b) a lack of detailed time series data concerning changes to terrestrial ice cover and greenhouse gas concentrations for the middle Pliocene and (c) difficulties in representing the effects of ‘climatic history’ in snap-shot GCM experiments.

Palaeomagnetism and magnetic properties of the Cappadocian ignimbrite succession, central Turkey and Neogene tectonics of the Anatolian collage

The Cappadocian ignimbrite succession of central-southern Anatolia comprises at least nine major and two minor calc-alkaline rhyolitic sheets emplaced at 1–2-Ma intervals between 11.2 and 1.1 Ma. It records the last phase of Neotethyan subduction during final emplacement of the Tauride orogen in southern Turkey. This study reports magnetostratigraphy and describes associated rock magnetic properties. Remanence resides in Ti-poor titanomagnetites. Haematisation is locally produced by post-emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation. Weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane. Directions of k max with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing. Older ignimbrites (Upper and Lower Göreme, Akdag-Zelve) from the Çardak Centre are all of normal polarity. Later ignimbrites, partly erupted from the Derinkuyu Centre, comprise the Sarımaden (R), Cemilköy (R), Tahar (R), Kızılkaya (R), Incesu (N) and Valibaba-Sofular (R) ignimbrites. The overall (reversed) group mean is D/ I=174/−51° ( N=10 units, R=9.84, α 95=6.6°, k=55) and all magnetisation directions from the Upper Göreme (9.0 Ma) onwards are rotated anticlockwise with respect to Eurasian and African palaeofields. This sense of rotation characterises most of central Anatolia and averages 9±5° in this sector. The rotation rate from 8 to 1 Ma BP was ∼1.25°/Ma but it appears to have accelerated during the latter part of the Quaternary to about an order higher than rates determined from GPS. Rotation has resulted from extrusion of fault blocks during tectonic escape of the Anatolian collage to the southwest and followed crustal thickening as the Afro-Arabia Plate has continued to impinge differentially into Eurasia. Cenozoic magnetic inclinations are systematically shallower across Anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates. Only part of this difference (∼400 km) can be accommodated by northward movement and crustal thickening in Central Anatolia since mid-Miocene times. Shallow inclinations observed in the Aegean region extend into Turkey; they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly. The pattern of neotectonic declinations across Anatolia shows rotations that are strongly anticlockwise rotated in the east near the Arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage. Rotations also seem to become generally younger towards the south. Crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the Tauride Arc and, by inference, the Cyprus Arc.

Lava–sediment interaction in desert settings; are all peperite-like textures the result of magma–water interaction?

This study reports on lava–sediment interaction in dry depositional settings focusing on the Early Cretaceous volcano-sedimentary sequence in the Huab Basin, NW Namibia, as a detailed example. Here an active aeolian sand sea (erg) system was progressively engulfed by lavas of the Etendeka Flood Basalt Province ∼133 Ma BP. This volcanic flooding resulted in: (1) the unprecedented preservation of large parts of the active dune system; (2) the development of a variety of sediment interlayers; and (3) a record of excellent examples of the dynamic interaction between lava and aeolian sand. The lava–sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava–sediment contacts at small scales. ‘Peperite’ like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands. Such interaction could involve lava cascading down the front of dune faces, or active ‘aa’ style flow fronts ‘bulldozing’ into and oversteepening dune faces causing rapid influx of sand into the active flow front. Resulting breccia horizons have been found up to 5 m thick. Locally, the aeolian sand is baked to a quartzite by the hot lava. In contrast, on other surfaces, the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction. In places, wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows. Peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with. However, examples from arid environments demonstrate that peperite-forming processes are diverse and may not always require water. Therefore, it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two. This leaves the presence, or lack, of water as an issue for interpreting the processes by which the peperite formed.

A relief-specific model of the ice age on the basis of uplift-controlled glacier areas in Tibet and the corresponding albedo increase as well as their positive climatological feedback by means of the global radiation geometry

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 20:1-7 (2002) - doi:10.3354/cr020001 A relief-specific model of the ice age on the basis of uplift-controlled glacier areas in Tibet and the corresponding albedo increase as well as their positive climatological feedback by means of the global radiation geometry Matthias Kuhle* Geographisches Institut der Universität Göttingen, Goldschmidtstraße 5, 37077 Göttingen, Germany *E-mail: mkuhle@gwdg.de ABSTRACT: The onset of the ice age era at ~2.75 Ma BP and its increasing intensity from ~1 Ma BP onwards cannot be explained by variations of the Earth¹s orbit. Evidence supporting a 2.4 million km2 ice sheet on the Tibetan plateau during the Last Glacial Maximum has led to the hypothesis that the resulting albedo-induced heat loss in the Earth¹s atmosphere may have triggered global ice ages. Recent data obtained from marine and terrestrial sediment records now confirm the climatic-ecological impact of a Tibetan glaciation; they also show that the development of Tibet¹s ice sheet was synchronous with the onset and intensification of global ice ages. KEY WORDS: Ice ages · Tibetan ice sheet · Quaternary climate Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 20, No. 1. Online publication date: February 12, 2002 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2002 Inter-Research.

Tectonic uplift and sedimentary evolution of the Jiuxi Basin in the northern margin of the Tibetan Plateau since 13 Ma BP

Sediments shed from the northern margin of the Tibetan Plateau, the Qilian Mountains, are widely deposited in the foreland basin, the Jiuxi Basin, archiving plenty of information about the mountain surface uplift and erosion history. The Laojunmiao section, 1960 m thick, representing the upper sequence of the Cenozoic basin sediments, is paleomagnetically dated to about 13—0 Ma BP. Detailed sedimentary study of this sequence has revealed five sedimentary facies associations which determine four stages of sedimentary environment evolution. They are: (I) the half-deep lake system before 12.18 Ma BP, (II) the shallow lake system between 12.18 and 8.26 Ma BP, (III) the fan delta dominated sedimentary system in dry climate between 8.26 and 6.57 Ma BP, and (IV) alluvial fan system since 6.57 Ma BP. The associated mountain erosion and uplift are suggested to have experienced three phases, that is, tectonic stable (13—8.26 Ma BP), gradual uplift (8.26— 3.66—0 Ma BP). The uplift at ~3.66 Ma BP is of great importance in tectonics and geomorphology. Since then, tectonic uplift and mountain building have been accelerated and become strong intermittent. At least three significant tectonic events took place with ages at

An outsider's review of the astronomical theory of the climate: is the eccentricity-driven insolation the main driver of the ice ages?

Although the astronomical theory of the climate (also known as the “Milankovitch” theory) explains a number of key features of the paleoclimate time series (especially frequency and phase distribution) in terms of orbital influence on the waxing and waning of ice sheets, it still faces several important unresolved challenges. In this review, we discuss five main challenges we believe the theory must resolve in order to survive: (1) The large amplitude of the ∼100 ky ice age cycles; (2) the switch from 41-ky- to 100-ky-long glaciations around 0.9 Ma BP; (3) the absence of significant spectral power at 413 ky over the past 1.2 Ma; (4) the variation in glacial cycle duration in the last 800 ky; and (5) the presence of non-orbital spectral peaks in the climate record. But in spite of these problems, we conclude that the Milankovitch theory may still be a viable one; at least in the original sense of Hays et al. [Science 194 (1976) 1121], that orbital changes somehow influenced the ice ages. Many investigators, ourselves included, appear to see the impact of orbital forcing in the data, clear evidence of astronomical forcing, as well as evidence which suggests that the climate system responds nonlinearly to all Milankovitch (orbital) frequencies.

Magnetic properties of the Tertiary red clay from Gansu

The study on magnetic properties of the red clay indicates that the red clay and loess-paleosol sequence have a common magnetic mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pedogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation in the red clay thus also provides an eolian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition is similar to the climatic shift from stage 5 (S1) to stage 2-4 (L1). This may suggest that during the Quaternary there is an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important environmental change from the underlying Cretaceous red sandstone. The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition.