Effects of CO<Subscript>2</Subscript> Enrichment on Marine Phytoplankton

Rising atmospheric CO₂ and deliberate CO₂ sequestration in the ocean change seawater carbonate chemistry in a similar way, lowering seawater pH, carbonate ion concentration and carbonate saturation state and increasing dissolved CO₂ concentration. These changes affect marine plankton in various ways. On the organismal level, a moderate increase in CO₂ facilitates photosynthetic carbon fixation of some phytoplankton groups. It also enhances the release of dissolved carbohydrates, most notably during the decline of nutrient-limited phytoplankton blooms. A decrease in the carbonate saturation state represses biogenic calcification of the predominant marine calcifying organisms, foraminifera and coccolithophorids. On the ecosystem level these responses influence phytoplankton species composition and succession, favouring algal species which predominantly rely on CO₂ utilization. Increased phytoplankton exudation promotes particle aggregation and marine snow formation, enhancing the vertical flux of biogenic material. A decrease in calcification may affect the competitive advantage of calcifying organisms, with possible impacts on their distribution and abundance. On the biogeochemical level, biological responses to CO₂ enrichment and the related changes in carbonate chemistry can strongly alter the cycling of carbon and other bio-active elements in the ocean. Both decreasing calcification and enhanced carbon overproduction due to release of extracellular carbohydrates have the potential to increase the CO₂ storage capacity of the ocean. Although the significance of such biological responses to CO₂ enrichment becomes increasingly evident, our ability to make reliable predictions of their future developments and to quantify their potential ecological and biogeochemical impacts is still in its infancy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Eldgjá eruption: timing,long-range impacts and influence on the Christianisation of Iceland

The Eldgjá lava flood is considered Iceland’s largest volcanic eruption of the Common Era. While it is well established that it occurred after the Settlement of Iceland (circa 874 CE), the date of this great event has remained uncertain. This has hampered investigation of the eruption’s impacts, if any, on climate and society. Here, we use high-temporal resolution glaciochemical records from Greenland to show that the eruption began in spring 939 CE and continued, at least episodically, until at least autumn 940 CE. Contemporary chronicles identify the spread of a remarkable haze in 939 CE, and tree ring-based reconstructions reveal pronounced northern hemisphere summer cooling in 940 CE, consistent with the eruption’s high yield of sulphur to the atmosphere. Consecutive severe winters and privations may also be associated with climatic effects of the volcanic aerosol veil. Iceland’s formal conversion to Christianity dates to 999/1000 CE, within two generations or so of the Eldgjá eruption. The end of the pagan pantheon is foretold in Iceland’s renowned medieval poem, V?luspá (‘the prophecy of the seeress’). Several lines of the poem describe dramatic eruptive activity and attendant meteorological effects in an allusion to the fiery terminus of the pagan gods. We suggest that they draw on first-hand experiences of the Eldgjá eruption and that this retrospection of harrowing volcanic events in the poem was intentional, with the purpose of stimulating Iceland’s Christianisation over the latter half of the tenth century.     

Making the EU Emissions Trading System: The European Commission as an entrepreneurial epistemic leader

The EU has developed the first and largest international emissions trading system in the world. This development is puzzling due to the EU's scepticism to international emissions trading in greenhouse gases (GHGs) in the run-up to the 1997 Kyoto Protocol. This article analyses how the EU ETS was initiated in the first place mainly from the perspectives of Liberal intergovernmentalism (LI) and multi-level governance (MLG). LI emphasises change in the positions of the EU member states as the key to understand what happened and why, whereas MLG opens up for change in the position of supranational entrepreneurial leaders as the key explanation. The main conclusion is that entrepreneurial epistemic leadership exercised by the European Commission was crucial for making the EU ETS. The principal means of leadership involved building up independent expertise on how an EU ETS could be designed, and mobilizing support from state and non-state actors at various levels of decision-making. This type of leadership may be needed more generally to deal with challenges characterized by high scientific uncertainty and social complexity in which learning is pertinent, such as climate change.     

The effect of a vertical temperature gradient in the lithosphere on seismic and tectonic waves

A linear analysis of the stability of the lithosphere considered as a viscoelastic layer with an equilibrium vertical gradient of temperature is carried out. The problem is solved with a complete system of linearized equations of a continuous medium represented in the dimensionless form and containing a set of dimensionless parameters that determine thermomechanical properties of the lithosphere. As a result of the stability analysis, decrements are found that give the time dependence of perturbations and correspond to high-frequency seismic waves and low-frequency tectonic waves. The frequency and velocity of seismic waves are determined by the elasticity and inertial properties of the lithosphere, and their attenuation, by viscous properties of the lithosphere. The temperature gradient existing in the lithosphere influences seismic waves very weakly. On the contrary, the pattern of tectonic waves is controlled by the temperature gradient and viscous properties, while the effect of elastic and inertial properties on these waves is negligibly small. The stability of a viscoelastic lithosphere is examined using such rheological models as the Maxwell, standard linear, and Andrade media (the frequency of tectonic waves is zero in the Maxwell medium).     

The Mesoproterozoic in the Nordic countries

During the Mesoproterozoic, central Fennoscandia and Laurentia （Greenland） were characterized by a weakly extensional stress regime, as evident from episodic rapakivi granites, dolerite dykes, continental rift intrusives, sandstone basins and continental flood basalts. Along the southwestern active margin of Fennoscandia, the 1.64-1.52 Ga Gothian and 1.52-1.48 Ga Telemarkian accretionary events resulted in oceanwards continental growth. The 1.47-1.42 Ga Hallandian- Danopolonian event included high-grade metamorphism and granite magmatism in southern Fennoscandia. The pre-Sveconorwegian 1.34-1.14 Ga period is characterized by bimodal magmatism associated with sedimentation, possibly reflecting transcurrent tectonics. The Sveconorwegian orogeny involved polyphase imbrication of terranes between 1.14 and 0.97 Ga, as a result of a collision between Baltica and another major plate, followed by relaxation and post-collisional magmatism between 0. 96 and 0. 90 Ga. Recent geologic data support classical models restoring the Sveconorwegian belt directly to the east of the Grenville belt of Laurentia at 1.0 Ga. Fragments of Paleo-to Mesoproterozoic crust showing late Grenvillian-Sveconorwegian （1.00-0.92 Ga） magmatism and/or metamorphism are exposed in several tectonic levels in the Caledonides of Scandinavia, Svalbard and East Greenland, on both sides of the inferred Iapetus suture. Linking these fragments into a coherent late-Grenvillian tectonic model, however, require additional study.     

European summer temperature response to annually dated volcanic eruptions over the past nine centuries

The drop in temperature following large volcanic eruptions has been identified as an important component of natural climate variability. However, due to the limited number of large eruptions that occurred during the period of instrumental observations, the precise amplitude of post-volcanic cooling is not well constrained. Here we present new evidence on summer temperature cooling over Europe in years following volcanic eruptions. We compile and analyze an updated network of tree-ring maximum latewood density chronologies, spanning the past nine centuries, and compare cooling signatures in this network with exceptionally long instrumental station records and state-of-the-art general circulation models. Results indicate post-volcanic June–August cooling is strongest in Northern Europe 2 years after an eruption (?0.52?±?0.05 °C), whereas in Central Europe the temperature response is smaller and occurs 1 year after an eruption (?0.18?±?0.07 °C). We validate these estimates by comparison with the shorter instrumental network and evaluate the statistical significance of post-volcanic summer temperature cooling in the context of natural climate variability over the past nine centuries. Finding no significant post-volcanic temperature cooling lasting longer than 2 years, our results question the ability of large eruptions to initiate long-term temperature changes through feedback mechanisms in the climate system. We discuss the implications of these findings with respect to the response seen in general circulation models and emphasize the importance of considering well-documented, annually dated eruptions when assessing the significance of volcanic forcing on continental-scale temperature variations.     

Multiple sources for mineralizing fluids in the Charmitan gold(-tungsten) mineralization (Uzbekistan)

Mineral assemblages present within the Charmitan gold(-tungsten) quartz-vein mineralization have been investigated for their cathodoluminescence behaviour, chemical composition and noble gas isotope systematics. This inventory of methods allows for the first time a systematic reconstruction of the paragenetic relationships of quartz, scheelite, sulphides and native gold within the gold mineralization at Charmitan and provides the basis to utilise noble gas data in the discussion of sources and evolution of ore-forming fluids. The vein quartz is classified into four generations based on microscopic and cathodoluminescence investigations. Quartz I shows intense brittle deformation as associated scheelite I. Undeformed scheelite II overgrows scheelite I and has lower light rare earth element and higher intermediate rare earth element contents as well as higher strontium concentrations. Scheelite II is associated with the economic gold mineralization and formed during re-crystallisation and re-precipitation of material which was partly re-mobilised from early scheelite I during infiltration of gold-bearing fluids. Early stage native gold inclusions are often associated with stage 2 sulphides, scheelite II and bismuth tellurides and contain Ag (3.6–24.4 wt.%), Hg (≤1.0 wt.%) and Bi (≤0.2 wt.%). Later stage electrum grains occur in association with stage 3 sulphides and sulphosalts and contain Hg (<0.8 wt.%) and elevated Sb concentrations (up to 3.0 wt.%). Noble gas isotope data (³He/⁴He: 0.2-0.4 Ra) for hydrothermal ore fluids trapped in the gold-related sulphides and sulphosalts (stage 2 pyrite and arsenopyrite; stage 3 pyrite, sphalerite, galena and lead sulphosalts) suggest that diverse fluid sources were involved in the formation of the Charmitan gold deposit. These data are indicative of a small, but significant input of fluids from external, deep-seated (mantle and possibly lower crust) sources. A decrease in the input of mantle helium and an increasing role of crustal helium from early to later stages of the mineralization is suggested by the measured ³He/⁴He and ⁴⁰Ar*/⁴He ratios. Sulphides from ore veins in meta-sedimentary rocks contain higher portions of meteoric fluids than those in intrusive rock types as indicated by their lower ³He/³⁶Ar ratios. The ³He/³⁶Ar ratios in the meta-sedimentary rocks agree well with ratios typical of gold mineralizations in the Tien Shan gold province completely hosted by meta-sedimentary sequences, indicating intense fluid-wall rock interaction.     

Towards a complete magmatic barcode for the Zimbabwe craton: Baddeleyite U–Pb dating of regional dolerite dyke swarms and sill complexes

We present baddeleyite U–Pb ages of Neoarchaean to Palaeoproterozoic dyke swarms and the Mashonaland sill province in Zimbabwe. The 2575.0 ± 1.5 Ma age of the Umvimeela dyke is indistinguishable from the 2575.4 ± 0.7 Ma result (Oberthür et al., 2002) for a pyroxenite layer of the Great Dyke and testifies to synchronous emplacement of the Great Dyke and its satellites. Three samples of WNW- to NNW-trending dykes of the Sebanga swarm yielded ages of 2512.3 ± 1.8 Ma, 2470.0 ± 1.2 Ma and 2408.3 ± 2.0 Ma, the latter of which dates the Sebanga Poort Dyke of this swarm. These results suggest that emplacement took place over a protracted period which involved at least three generations of dykes within the swarm and, more importantly, invalidate previous inferences of a genetic link between the Sebanga swarm and the Mashonaland sills. Crystallisation ages of 1877 ± 2.2 Ma, 1885.9 ± 2.4 Ma and 1875.6 ± 1.6 Ma for three dolerite samples of the extensive Mashonaland sills from different parts of the Zimbabwe craton were also obtained. This is the oldest common igneous event that is recorded in the Zimbabwe and Kaapvaal cratons. Collectively with previous published geochronological and petrological evidence in favour of a major 2.0 Ga event within the Limpopo Belt, these results suggest that the Zimbabwe and Kaapvaal cratons did not form a coherent unit (Kalahari) until ca. 2.0 Ga.     

Paleomagnetic study of NeoArchean–Paleoproterozoic dykes in the Kaapvaal Craton

431 oriented samples were collected from 27 dolerite dykes at 17 sites, belonging to 2.95, 2.65, and 1.90 Ga swarms, that trend SE, E and NE, respectively from the Bushveld Igneous Complex into the eastern Kaapvaal Craton (ages determined by Olsson et al., 2010; Olsson in Söderlund et al., 2010). Samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility (AMS). For the 2.95 Ga SE-trending dykes high temperature/coercivity ‘P’ component has unblocking temperatures up to 590 °C and coercivity 40–90 mT and demonstrate SSW declination and intermediate positive inclination. Based on positive contact and conglomerate tests we argue for a primary origin of this component. The paleopole (BAD), calculated from ‘P’ component, does not correspond to any of the previously obtained Archean–Paleoproterozoic paleopoles for the Kaapvaal Craton, and represents a new key pole for 2.95 Ga. The high-coercivity ‘H’ component for the 2.65 Ga-old E-trending dykes has a SSW declination and steep positive inclination. Paleomagnetic pole (RYK), recalculated from this component, is close to the paleopoles, obtained by Wingate (1998) and Strik et al. (2007) for 2.78 Ga Ventersdorp volcanics. The third group, NE-trending dykes of the 1.90 Ga Black Hill swarm demonstrate an ‘M’ component with dual polarity high-coercivity component with SSE-declination and negative intermediate inclination. The paleopole (BHD), calculated from this component is close to the 1.87 Ga pole of the Kaapvaal Craton obtained by Hanson et al. (2004). Overprint directions include a very well developed thermo-chemical overprint (Dec = 329° Inc = −36°), which is believed to be associated with a ∼0.18 Ga regional ‘Karoo’ thermal event.