Detrital input and early diagenesis in sediments from Lake Baikal revealed by rock magnetism

A rock magnetic study was performed on sediment cores from six locations in Lake Baikal. For a comprehensive approach of the processes influencing the rock magnetic signal, additional data are presented such as total organic carbon (TOC), total sulphur (TS), opal, water content and relative variations in iron and titanium measured on selected intervals. In glacial sediments, the magnetic signal is dominated by magnetite, which is considered to be of detrital origin. This predominance of magnetite is interrupted by distinct horizons of authigenic greigite, probably confined to reductive microenvironments. In interglacial stages, besides dilution by biogenic silica and a decreasing detrital input, the weakness of the rock magnetic signal is also due to a reductive dissolution of magnetic particles. The magnetic assemblage is strongly linked to the redox history of interglacial sediment. In the oxidised bottom sediments of Lake Baikal, a biogenic magnetite is observed [Peck, J.A., King, J.W., 1996. Magnetofossils in the sediments of lake Baikal, Siberia. Earth Planet. Sci. Lett. 140 (1–4), 159–172]. After burial under the redox front, the magnetite is preferentially dissolved, and detrital hematite remains dominant when the sedimentation rate is low and when the residence time of the magnetite close to the redox boundary is long. During these low sedimentation rate conditions, the redox front is preserved [Granina, L., Müller, B. and Wehrli, B., 2004. Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal. Chem. Geol. 205 (1-2), 55-72]. At constant sedimentation rate and fast burial, the magnetite is preserved or transformed into greigite when sulphate-reducing conditions are reached in the sediment. In interglacial sediments, the magnetic assemblages depict changes in the sedimentation rate, which are traced using the ratio of magnetite over hematite (S-ratio). At the beginning of interglacials, the sedimentation rate is constant with an assemblage magnetite+greigite (high S-ratio), and at the end of some interglacials, the sedimentation rate decreases with a predominance of hematite (low S-ratio).     

Variation in Lake Baikal's phytoplankton distribution and fluvial input assessed by SeaWiFS satellite data

The current behaviour of selected climate proxies in Lake Baikal was assessed by remote sensing analyses of ‘Sea viewing Wide Field of view Sensor’ (SeaWiFS) satellite data. Suitable proxies include optically visible water constituents such as phytoplankton, suspended terrigenous matter and yellow substance. These limnological parameters reflect the present-day climate bioproductivity and the river discharge in the catchment area.A biological and geochemical ground truth data set for Lake Baikal was established with the help of members of the paleoclimate project ‘high-resolution CONTINENTal paleoclimate record in Lake Baikal’ (CONTINENT). For processing the SeaWiFS satellite data, the atmospheric correction was adapted to the case of Lake Baikal. Chlorophyll as a proxy for phytoplankton was quantified using global NASA ocean colour algorithms OC2 and OC4. In cases of no optical interferences by terrigenous input, the calculated chlorophyll concentrations in clear pelagic waters were within ±30% accuracy with the CONTINENT cruise data during the summers of 2001 and 2002. Within this range of accuracy, the SeaWiFS time series will be able to show the seasonal variations of chlorophyll of specified bio-optical provinces of Lake Baikal and of CONTINENT sites. In this study, the suspended matter as a proxy for the terrigenous input was calculated according to an empirical algorithm using ground truth data in the time frame of flooding events in summer 2001. The approach chosen correlates the suspended matter concentration with the remotely sensed parameter of ‘attenuation coefficient’ to account for the organic-rich terrigenous input that originates from the swampy watersheds.Seasonal and spatial information that is provided by the analyses of the SeaWiFS satellite data will assist paleoclimate researchers to interpret the autochthonous and allochthonous influences at the CONTINENT coring sites.     

A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy

Due to the major role played by diatoms in the biological pump of CO₂, and to the presence of silica-rich sediments in areas that play a major role in air–sea CO₂ exchange (e.g. the Southern Ocean and the Equatorial Pacific), opal has a strong potential as a proxy for paleoproductivity reconstructions. However, because of spatial variations in the biogenic silica preservation, and in the degree of coupling between the marine Si and C biogeochemical cycles, paleoreconstructions are not straitghtforward. A better calibration of this proxy in the modern ocean is required, which needs a good understanding of the mechanisms that control the Si cycle, in close relation to the carbon cycle.This review of the Si cycle in the modern ocean starts with the mechanisms that control the uptake of silicic acid (Si(OH)₄) by diatoms and the subsequent silicification processes, the regulatory mechanisms of which are uncoupled. This has strong implications for the direct measurement in the field of the kinetics of Si(OH)₄ uptake and diatom growth. It also strongly influences the Si:C ratio within diatoms, clearly linked to environmental conditions. Diatoms tend to dominate new production at marine ergoclines. At depth, they also succeed to form mats, which sedimentation is at the origin of laminated sediments and marine sapropels. The concentration of Si(OH)₄ with respect to other macronutrients exerts a major influence on diatom dominance and on the rain ratio between siliceous and calcareous material, which severely impacts surface waters pCO₂. A compilation of biogenic fluxes collected at about 40 sites by means of sediment traps also shows a remarkable pattern of increasing BSi:C_org ratio along the path of the “conveyor belt”, accompanying the relative enrichment of waters in Si compared to N and P. This observation suggests an extension of the Si pump model described by Dugdale and Wilkerson (Dugdale, R.C., Wilkerson, F.P., 1998. Understanding the eastern equatorial Pacific as a continuous new production system regulating on silicate. Nature 391, 270–273.), giving to Si(OH)₄ a major role in the control of the rain ratio, which is of major importance in the global carbon cycle.The fate of the BSi produced in surface waters is then described, in relation to C_org, in terms of both dissolution and preservation mechanisms. Difficulties in quantifying the dissolution of biogenic silica in the water column as well as the sinking rates and forms of BSi to the deep, provide evidence for a major gap in our understanding of the mechanisms controlling the competition between retention in and export from surface waters. The relative influences of environmental conditions, seasonality, food web structure or aggregation are however explored. Quantitatively, assuming steady state, the measurements of the opal rain rate by means of sediment traps matches reasonably well those obtained by adding the recycling and burial fluxes in the underlying abyssal sediments, for most of the sites where such a comparison is possible. The major exception is the Southern Ocean where sediment focusing precludes the closing of mass balances. Focusing in fact is also an important aspect of the downward revision of the importance of Southern Ocean sediments in the global biogenic silica accumulation. Qualitatively, little is known about the duration of the transfer through the deep and the quality of the material that reaches the seabed, which is suggested to represent a major gap in our understanding of the processes governing the early diagenesis of BSi in sediments. The sediment composition (special emphasis on Al availability), the sedimentation rate or bioturbation are shown to exert an important control on the competition between dissolution and preservation of BSi in sediments. It is suggested that a primary control on the kinetic and thermodynamic properties of BSi dissolution, both in coastal and abyssal sediments, is exerted by water column processes, either occuring in surface waters during the formation of the frustules, or linked to the transfer of the particles through the water column, which duration may influence the quality of the biogenic rain. This highlights the importance of studying the factors controlling the degree of coupling between pelagic and benthic processes in various regions of the world ocean, and its consequences, not only in terms of benthic biology but also for the constitution of the sediment archive.The last section, first calls for the end of the “NPZD” models, and for the introduction of processes linked to the Si cycle, into models describing the phytoplankton cycles in surface waters and the early diagenesis of BSi in sediments. It also calls for the creation of an integrated 1-D diagnostic model of the Si:C coupling, for a better understanding of the interactions between surface waters, deep waters and the upper sedimentary column. The importance of Si(OH)₄ in the control of the rain ratio and the improved parametrization of the Si cycle in the 1-D diagnostic models should lead to a reasonable incorporation of the Si cycle into 3-D regional circulation models and OGCMs, with important implications for climate change studies and paleoreconstructions at regional and global scale.     

Vivianite formation and distribution in Lake Baikal sediments

In an effort to better understand vivianite formation processes, four Lake Baikal sediment cores spanning two to four interglacial stages in the northern, central and southern basins and under various biogeochemical environments are scrutinized. The vivianite-rich layers were detected by anomalous P-enrichments in bulk geochemistry and visually by observations on X-radiographs. The millimetric concretions of vivianite were isolated by sieving and analysed by X-ray diffraction, scanning electron microscope (SEM), microprobe, infrared spectroscopy, inductively coupled plasma atomic emission spectrometry and mass spectrometry (ICP-AES, ICP-MS). All the vivianites display similar morphological, mineralogical and geochemical signature, suggesting a common diagenetic origin. Their geochemical signature is sensitive to secondary alteration where vivianite concretions are gradually transformed from the rim to the center into an amorphous santabarbaraite phase with a decreasing Mn content. We analysed the spatial and temporal distribution of the concretions in order to determine the primary parameters controlling the vivianite formation, e.g., lithology, sedimentation rates, and porewater chemistry. We conclude that vivianite formation in Lake Baikal is mainly controlled by porewater chemistry and sedimentation rates, and it is not a proxy for lacustrine paleoproductivity. Vivianite accumulation is not restricted to areas of slow sedimentation rates (e.g., Academician and Continent ridges). At the site of relatively fast sedimentation rate, i.e., the Posolsky Bank near the Selenga Delta, vivianite production may be more or less related to the Selenga River inputs. It could be also indirectly related to the past intensive methane escapes from the sediments. While reflecting an early diagenetic signal, the source of P and Fe porewater for vivianites genesis is still unclear.     

Late glacial and Holocene environmental change in the Lake Baikal region documented by oxygen isotopes from diatom silica

We investigate late glacial and Holocene climate change recorded in Lake Baikal using the oxygen isotope composition of diatom silica (δ¹⁸O_DIAT). Evaporation from the lake is minor, and the temperature fractionations of δ¹⁸O are unable to explain variations in the δ¹⁸O_DIAT record alone. Isotopically, low meltwater input from glaciers may have some influence on δ¹⁸O_DIAT, but the assumed periods of climatic warming and wastage do not coincide with large shifts in δ¹⁸O_DIAT. There is a gradual oxygen isotope lowering from 27.0‰ to 20.6‰ over the late glacial, while, during the Holocene, δ¹⁸O_DIAT values return to relatively high values. Previous studies of the modern oxygen and hydrogen isotope composition of Lake Baikal's inputs reveal that fluvial input to the lake's North Basin are isotopically lower than fluvial input from South Basin rivers. This north–south gradient of river δ¹⁸O and δD is mainly due to the greater input from isotopically low winter precipitation in the north and isotopically higher summer precipitation in the south. As a result, the δ¹⁸O_DIAT record from Lake Baikal can at least in part be explained by varying input from these sources related to seasonal changes in precipitation. Changes in atmospheric conditions may have a role in altering seasonality and the distribution of precipitation over Lake Baikal's catchment. A feedback mechanism is well known linking higher Eurasian spring snow cover extent (ESSC) to the development of anticyclonic conditions and low precipitation the following summer in the areas south of Lake Baikal. A simultaneous increase in the importance of depleted water (snowmelt) input from the north and decreased enriched summer precipitation in the south is needed to explain depletions in δ¹⁸O of lake water and subsequently δ¹⁸O_DIAT during colder periods. The opposite of this situation is required to enrich lake water during warmer periods. The analysis of δ¹⁸O from diatom silica is a useful proxy for environmental change, especially in lakes, like Lake Baikal, where carbonates are absent or diluted. However, analysis must be based on near pure diatom samples as even trace amounts of silt can have a dominating effect on δ¹⁸O_DIAT values.     

A paleoclimatic evaluation of marine oxygen isotope stage 11 in the high-northern Atlantic (Nordic seas)

A sediment core from the high latitude of the Northern Atlantic (Nordic seas) was intensively studied by means of biogeochemical, sedimentological, and micropaleontological methods. The proxy records of interglacial marine oxygen isotope stage (MIS) 11 are directly compared with records from the Holocene (MIS 1), revealing that many features of MIS 11 are rather atypical for an interglaciation at these latitudes.Full-interglacial conditions without deposition of ice-rafted debris existed in MIS 11 for about 10 kyr (∼398–408 ka). This time is marked by the lightest d18O values in benthic foraminifera, indicating a small global ice volume, and by the appearance of subpolar planktic foraminifera, indicating a northward advection of Atlantic surface water. A comparison with MIS 1, using the same proxies, implies that surface temperatures were lower and global ice volume was larger during MIS 11. A comparative study of the ratio between planktic and benthic foraminifera also reveals strong differences among the two intervals. These data imply that the coupling between surface and bottom bioproductivity, i.e., the vertical transportation of the amount of fresh organic matter, was different in MIS 11. This is corroborated by a benthic fauna in MIS 11, which contains no epifaunally-living species. Despite comparable values in carbonate content (%), reflectance analyses of the total sediment (greylevel) show much higher values for MIS 11 than for MIS 1. These high values are attributed to increased corrosion of foraminiferal tests, directly affecting the sediment greylevel. The reason for this enhanced carbonate corrosion in MIS 11 remains speculative, but may be linked to the global carbon cycle.     

Vegetation and climate variability during the Last Interglacial evidenced in the pollen record from Lake Baikal

A pollen record from the core sediments collected in the northern part of Lake Baikal represents the latest stage of the Taz (Saale) Glaciation, Kazantsevo (Eemian) Interglacial (namely the Last Interglacial), and the earliest stage of the Zyryanka (Weichselian) Glaciation. According to the palaeomagnetic-based age model applied to the core, the Last Interglacial in the Lake Baikal record lasted about 10.6 ky from 128 to 117.4 ky BP, being more or less synchronous with the Marine Isotope Stage 5e. The reconstructed changes in the south Siberian vegetation and climate are summarised as follows: a major spread of shrub alder (Alnus fruticosa) and shrub birches (Betula sect. Nanae/Fruticosae) in the study area was a characteristic feature during the late glacial phase of the Taz Glaciation. Boreal trees e.g. spruce (Picea obovata) and birch (Betula sect. Albae) started to play an important role in the regional vegetation with the onset of the interglacial conditions. Optimal conditions for Abies sibirica–P. obovata taiga development occurred ca. 126.3 ky BP. The maximum spread of birch forest-steppe communities took place at the low altitudes ca. 126.5–125.5 ky BP and Pinus sylvestris started to form forests in the northern Baikal area after ca. 124.4 ky BP. Re-expansion of the steppe communities, as well as shrubby alder and willow communities and the disappearance of forest vegetation occurred at about 117.4 ky BP, suggesting the end of the interglacial succession. The changes in the pollen assemblages recorded in the sediments from northern Baikal point to a certain instability of the interglacial climate. Three phases of climate deterioration have been distinguished: 126–125.5, 121.5–120, and 119.5–119 ky BP. The penultimate cooling signal may be correlated with the cool oscillation recorded in European pollen records. However, such far distant correlation requires more careful investigation.     

Climatic change in Central Asia during MIS 3/2: a case study using biological responses from Lake Baikal

A Marine Isotope Stage (MIS) 3/early MIS 2 section from a structural high along the east coast of the North Basin of Lake Baikal was analysed for diatoms, C/N ratios, and organic carbon isotope ratios. Diatoms were present throughout MIS 3 and early MIS 2, with high concentrations of the planktonic taxa Cyclotella sp. c.f. gracilis between 54 and 51.5 kyr BP indicating relatively warm, interstadial, conditions. Following a %TOC inferred climatic cooling between 43.2 and 39.1 kyr BP, evidence of a more muted δ¹³C_(organic) and %TOC inferred climatic warming from c. 39.1–34.7 kyr BP coincides with a period of very high diatom concentrations, indicating high aquatic productivity, at the Buguldeika Saddle in the South Basin of Lake Baikal. No evidence exists for a ‘Kuzmin’ catchment erosional event in the North Basin during MIS 3. This, however, may reflect the location of the coring site away from major riverine inputs. Abrupt climatic cooling at the culmination of both warm phases in the North Basin are associated, on the basis of the palaeomagnetic age-model and correlations to existing sites in Lake Baikal, with the initiation of Heinrich events 5 (c. 50 kyr BP) and 4 (c. 35 kyr BP), respectively, in the North Atlantic. The amount of organic material declines across the MIS 3/MIS 2 transition while constant C/N ratios suggest organic material to be predominantly derived from phytoplankton. An increase in δ¹³C_(organic) at the MIS 3/MIS 2 transition may therefore indicate changes in aquatic productivity, pCO₂ or the inorganic carbon pool.     

1000 years of climate variability in central Asia: assessing the evidence using Lake Baikal (Russia) diatom assemblages and the application of a diatom-inferred model of snow cover on the lake

The mainly endemic phytoplankton record of Lake Baikal has been used in this study to help interpret climate variability during the last 1000 years in central Asia. The diatom record was derived from a short core taken from the south basin and has been shown to be free from any sedimentary heterogeneities. We employ here a diatom-based inference model of snow accumulation on the frozen lake for the first time (r²_boot=0.709; RMSEP=0.120 log cm). However, palaeoenvironmental reconstructions have been improved by the use of correction factors, specifically developed for the dominant phytoplankton (Aulacoseira baicalensis, Aulacoseira skvortzowii, Cyclotella minuta, Stephanodiscus meyerii and Synedra acus) in the south basin of Lake Baikal. Cluster analysis identifies three significant zones in the core, zone 1 (c. 880 AD–c. 1180 AD), zone 2 (c. 1180–1840 AD) and zone 3 (c. 1840–1994 AD), coincident with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the period of recent warming, respectively. Our results indicate that S. acus dominated the diatom phytoplankton within zone 1 coincident with the MWP. S. acus is an opportunistic species that is able to increase its net growth when A. baicalensis does not. During this period, conditions are likely to have been unfavourable for the net increases in A. baicalensis growth due to the persistence of warm water in the lake, together with an increased length of summer stratification and delay in timing of the autumnal overturn. In zone 2, spring diatom crops blooming under the ice declined in abundances due in part to increased winter severity and snow cover on the lake. Accumulating snow on the lake is likely to have arisen from increased anticyclonic activity, resulting in prolonged winters expressed during the LIA. Thick, accumulating snow cover inhibits light penetration through the ice, thereby having negative effects on cell division rate and extent of turbulence underneath the ice. Consequently, only taxa whose net growth occurs during autumn overturn (C. minuta) predominate in the lake at this time. Diatom census data and reconstructions of snow accumulation suggest that warming in the Lake Baikal region started as early as c. 1750 AD, with a shift from taxa that bloom during autumn overturn to assemblages that begin to grow underneath the frozen lake in spring. Very recent increases and subsequent decline of S. acus in the surface sediments of the lake mirror monitoring records of this species over the last 50 years. Our study confirms that, over the last 1000 years, physical processes are important in determining planktonic diatom populations in the lake and highlights the value of integrated plankton, trap, and sediment studies for improving quantitative palaeoenvironmental reconstructions from fossil material.     

Helioseismology and the solar interior dynamics

The inversion of helioseismic modes leads to the sound velocity inside the Sun with a precision of about 0.1 per cent. Comparisoons of solar models with the “seismic sun” represent powerful tools to test the physics: depth of the convection zone, equation of state, opacities, element diffusion processes and mixing inside the radiative zone. We now have evidence that microscopic diffusion (element segregation) does occur below the convection zone, leading to a mild helium depletion in the solar outer layers. Meanwhile this process must be slowed down by some macroscopic effect, presumably rotation-induced mixing. The same mixing is also responsible for the observed lithium depletion. On the other hand, the observations of beryllium and helium 3 impose specific constraints on the depth of this mildly mixed zone. Helioseismology also gives information on the internal solar rotation: while differential rotation exists in the convection zone, solid rotation prevails in the radiative zone, and the transition layer (the so-called “tachocline”) is very small. These effects are discussed, together with the astrophysical constraints on the solar neutrino fluxes.     

Recent changes of water discharge and sediment load in the Zhujiang (Pearl River) Basin, China

The paper is concerned with identifying changes in the time series of water and sediment discharge of the Zhujiang (Pearl River), China. The gradual trend test (Mann–Kendall test), and abrupt change test (Pettitt test), have been employed on annual water discharge and sediment load series (from the 1950s–2004) at nine stations in the main channels and main tributaries of the Zhujiang. Both the Mann–Kendall and Pettitt tests indicate that water discharge at all stations in the Zhujiang Basin showed no significant trend or abrupt shift. Annual water discharges are mainly influenced by precipitation variability, while the construction of reservoirs/dams in the Zhujiang Basin had little influence on water discharge. Sediment load, however, showed significant decreasing trends at some stations in the main channel of the Xijiang and Dongjiang. More stations have seen significantly decreasing trends since the 1990s. The decreasing sediment load in the Zhujiang reflects the impacts of reservoir construction in the basin. In contrast, the Liujiang, the second largest tributary of the Xijiang, has experienced a significant upward shift of sediment load around 1991 likely caused by exacerbated rock desertification in the karst regions. The annual sediment load from the Zhujiang (excluding the delta region) to the estuary has declined from 80.4 × 10⁶ t averaged for the period 1957–1995 to 54.0 × 10⁶ t for the period 1996–2004. More specifically, the sediment load declined steadily since the early 1990s so that in 2004 it was about one-third of the mean level of pre-90s. Water discharge and sediment load of the Zhujiang would be more affected by human activities in the future with the further reservoir developments, especially the completion of the Datengxia hydroelectric project, and an intensification of the afforestation policy in the drainage basin.     

Shallow ocean overturning and the heat and carbon content of the Glacial Tropical Ocean

Mechanisms affecting the heat and carbon content of the Glacial Tropical Ocean (21,000 years BP) remain controversial. Exchange with the deep ocean via vertical mixing and the overturning circulation is one aspect that is clearly relevant to greenhouse gas concentrations and future climate change. We examined evidence of the possible role of the shallow overturning contribution on the Glacial Tropical Ocean temperature and carbonate ion concentration. Compared to the present, we find that the Glacial tropical upper ocean (0–1000 m) had enhanced vertical gradients in temperature and carbonate ion concentration, reduced turbulent diffusivity (vertical mixing rate) (by 20% or more), and weakened Ekman pumping in middle-latitudes, all consistent with reduced shallow overturning. The weakened property exchanges between the tropical upper ocean and the ocean below (cold with high total dissolved carbon-dioxide concentration) provide a unified explanation for both the unexpectedly-small sea surface cooling and the lower carbon-dioxide content in the Glacial tropical upper ocean and the atmosphere.     

Preliminary study of fluxes of major lipid biomarker classes in the water column and sediments of Lake Baikal, Russia

The application of biomarker climate proxies to lacustrine settings requires a thorough understanding of those processes that give rise to and mediate in the burial and preservation of organic matter in the sediments. This information is to date missing for Lake Baikal in Central Asia. The biogeochemistry of the lake cannot be considered analogous to that in other lacustrine environments given its enormous size, depth, remote location and unique biology. The aim of this paper is to report on the main compound classes in the water column and sediments. As part of an ongoing evaluation of the inputs and fluxes of organic matter in Lake Baikal, we have found that there are significant differences in the inputs and preservation of organic matter between the North and South Basins. Both basins have dominant algal input and a contribution from the terrestrial plants of the lake edges and surrounding environments. However, the concentration of organic material in the sediment traps and sediments of the South Basin is much higher than that of the North Basin. In addition, the South Basin contains much higher proportions of the more labile material than does the North Basin. This is likely due to the South Basin being free of surface ice for a much longer time than the North, and in consequence, primary producers having an overall longer productive season. There is some evidence of microbial activity in the sediment traps and sediments from both the North and South Basins, but it does not appear to be more predominant in one basin than the other. It is probable that the differences in the composition and concentration of biomarkers in sediments between basins is due to the length of and the intensity of the productive season, which is in turn influenced by the climate.     

Tolerance of allogromiid Foraminifera to severely elevated carbon dioxide concentrations: Implications to future ecosystem functioning and paleoceanographic interpretations

Increases in the partial pressure of carbon dioxide (pCO₂) in the atmosphere will significantly affect a wide variety of terrestrial fauna and flora. Because of tight atmospheric–oceanic coupling, shallow-water marine species are also expected to be affected by increases in atmospheric carbon dioxide concentrations. One proposed way to slow increases in atmospheric pCO₂ is to sequester CO₂ in the deep sea. Thus, over the next few centuries marine species will be exposed to changing seawater chemistry caused by ocean–atmospheric exchange and/or deep-ocean sequestration. This initial case study on one allogromiid foraminiferal species (Allogromia laticollaris) was conducted to begin to ascertain the effect of elevated pCO₂ on benthic Foraminifera, which are a major meiofaunal constituent of shallow- and deep-water marine communities. Cultures of this thecate foraminiferan protist were used for 10–14-day experiments. Experimental treatments were executed in an incubator that controlled CO₂ (15 000; 30 000; 60 000; 90 000; 200 000 ppm), temperature and humidity; atmospheric controls (i.e., ~ 375 ppm CO₂) were executed simultaneously. Although the experimental elevated pCO₂ values are far above foreseeable surface water pCO₂, they were selected to represent the spectrum of conditions expected for the benthos if deep-sea CO₂ sequestration becomes a reality. Survival was assessed in two independent ways: pseudopodial presence/absence and measurement of adenosine triphosphate (ATP), which is an indicator of cellular energy. Substantial proportions of A. laticollaris populations survived 200 000 ppm CO₂ although the mean of the median [ATP] of survivors was statistically lower for this treatment than for that of atmospheric control specimens. After individuals that had been incubated in 200 000 ppm CO₂ for 12 days were transferred to atmospheric conditions for ~ 24 h, the [ATP] of live specimens (survivors) approximated those of the comparable atmospheric control treatment. Incubation in 200 000 ppm CO₂ also resulted in reproduction by some individuals. Results suggest that certain Foraminifera are able to tolerate deep-sea CO₂ sequestration and perhaps thrive as a result of elevated pCO₂ that is predicted for the next few centuries, in a high-pCO₂ world. Thus, allogromiid foraminiferal “blooms” may result from climate change. Furthermore, because allogromiids consume a variety of prey, it is likely that they will be major players in ecosystem dynamics of future coastal sedimentary environments.     

The effect of gateways on ocean circulation patterns in the Cenozoic

Both geological data and climate model studies indicate that substantially different patterns of the global ocean circulation have existed throughout the Cenozoic. In a climate model study of the late Oligocene [von der Heydt, A., Dijkstra, H.A. (2006). Effect of ocean gateways on the global ocean circulation in the late Oligocene and early Miocene. Paleoceanography, 21, PA1011] a “northern sinking” type of circulation was found, with (shallow) deep water formation in both the North Pacific Ocean and the North Atlantic Ocean. This is in contrast to the present-day “conveyor” circulation, where there is deep water formation in the North Atlantic but not in the North Pacific. In order to explain these differences, we use an ocean general circulation model for idealized two-basin flows and study the effect of asymmetries in the continental geometry on the circulation patterns. Two types of asymmetry are considered: (i) the relative northward extent of the Pacific and the Atlantic basin, and (ii) the existence of a circum-global gateway at low latitudes. The more northward extent of the Pacific basin in the Oligocene makes the Conveyor solution less likely and facilitates deep water formation in the North Pacific compared to the North Atlantic. The low-latitude gateway on the other hand, allows salinity and heat exchange between the two main ocean basins and therefore leads to deep water formation in both the North Atlantic and the North Pacific.     

A new investigation of the secular drift of the Earth's pole

A new investigation of the secular drift of the Earth's pole was made based on nine long series of latitude observations. This led to the following conclusions: 1) During this century, the Earth's pole has been moving approximately along meridian 70 °W at a mean rate of 0.0016″/yr, much less than the 0.0035″/yr derived from the ILS sequence. 2) Relative to the North American Continent, the Ukiah Station located on the western shore of the U.S. shows a local drift of about 6 cm/yr toward north, in good agreement with the result from the new techniques. 3) Relative to the Europe-Asia plate, the whole North American Continent shows a northward drift of about 8 cm/yr. The Mediterranean shows a similar drift of about 6 cm/yr. 4) Three of five ILS stations, Ukiah, Gaithersberg and Carloforte, show significant drifts. Therefore, the Conventional International Origin (CIO), which is defined by the mean latitudes at 1903.0 of the five stations, is far from being fixed on the Earth's surface, and is not suitable as the origin of the Earth Reference System.     

Thinned simulations of extremely energetic showers in dense media for radio applications

This paper presents our results for full 3-D simulations of very-high to ultra-high energy electromagnetic cascades – and the associated coherent Cherenkov radiation – as might be produced by high-energy neutrino interactions in dense media. Using “thinning” techniques, we develop an algorithm based on the existing “ZHS” code, and demonstrate that the new “ZHS-thinned” code can produce fast and accurate results for showers up to . Using ZHS-thinned, we develop new parameterisations for the radiation from showers in ice, salt, and the lunar regolith, with a separate treatment of the megaregolith (deep regolith). Our parameterisations include for the first time a method to simulate fluctuations in shower length induced by the LPM effect. Our results, which avoid the pit-falls of scaling simulations from lower energies, allow improved calculations of the detection probability for experiments searching for high-energy neutrinos using the radio technique.     

Late Pleistocene deep-water circulation in the subantarctic eastern Atlantic

We present three new benthic foraminiferal δ¹³C, δ¹⁸O, and total organic carbon time series from the eastern Atlantic sector of the Southern Ocean between 41°S and 47°S. The measured glacial δ¹³C values belong to the lowest hitherto reported. We demonstrate a coincidence between depleted late Holocene (LH) δ¹³C values and positions of sites relative to ocean surface productivity. A correction of +0.3 to +0.4 [‰ VPDB] for a productivity-induced depletion of Last Glacial Maximum (LGM) benthic δ¹³C values of these cores is suggested. The new data are compiled with published data from 13 sediment cores from the eastern Atlantic Ocean between 19°S and 47°S, and the regional deep and bottom water circulation is reconstructed for LH (4–0 ka) and LGM (22–16 ka) times. This extends earlier eastern Atlantic-wide synoptic reconstructions which suffered from the lack of data south of 20°S. A conceptual model of LGM deep-water circulation is discussed that, after correction of southernmost cores below the Antarctic Circumpolar Current (ACC) for a productivity-induced artifact, suggests a reduced formation of both North Atlantic Deep Water in the northern Atlantic and bottom water in the southwestern Weddell Sea. This reduction was compensated for by the formation of deep water in the zone of extended winter sea-ice coverage at the northern rim of the Weddell Sea, where air–sea gas exchange was reduced. This shift from LGM deep-water formation in the region south of the ACC to Holocene bottom water formation in the southwestern Weddell Sea, can explain lower preformed δ¹³C_DIC values of glacial circumantarctic deep water of approximately 0.3‰ to 0.4‰. Our reconstruction brings Atlantic and Southern Ocean δ¹³C and Cd/Ca data into better agreement, but is in conflict, however, with a scenario of an essentially unchanged thermohaline deep circulation on a global scale. Benthic δ¹⁸O-derived LGM bottom water temperatures, by 1.9°C and 0.3°C lower than during the LH at deepest southern and shallowest northern sites, respectively, agree with the here proposed reconstruction of deep-water circulation in the eastern South Atlantic Ocean.     

Contrasting conditions preceding MIS3 and MIS2 Heinrich events

This paper presents an integrated multi-tracer study performed on piston cores recovered in the glacial ice-rafted detritus belt, stretching from Newfoundland to the Irish margin across the North Atlantic (40–55°N), in order to compare in detail the internal structure of each Heinrich event (HE). These tracers are IRD counts (quartz, dolomite, volcanic grains), their Nd isotopic composition and Ar–Ar datings of individual hornblende grains. A focus on the detailed structure of HE confirms that all intervals of massive sediment flux, specifically Heinrich layers HL1-to-5 (HLs), were dominated by North American, Laurentide ice-sheet surges from Hudson Strait, that are evident as far east as the Bay of Biscay (European margin). The sequences of events leading up to the HLs, however, present significant dissimilarities. One important difference is that HL2 and HL1 were preceded by “precursor events” (increases in the number of lithic grains per gram from non-Laurentide sediment sources). Sediment debris derived from near-simultaneous iceberg releases originating from the European ice-sheet are only detectable close to the European margin. In contrast there are no comparable precursor events before HL5 and HL4. This observation implies that precursor events are unlikely to be mechanistically linked to the triggering of HEs. The similarity of the HLs, against contrasting background conditions, is a significant observation that should add constraints to their origin.