Lithium isotopes as indicators of meteorite parent body alteration

Hydrothermal processing on planetesimals in the early solar system produced new mineral phases, including those generated by the transformation of anhydrous silicates into their hydrated counterparts. Carbonaceous chondrites represent tangible remnants of such alteration products. Lithium isotopes are known to be responsive to aqueous alteration, yet previously recognized variability within whole rock samples from the same meteorite appears to complicate the use of these isotopes as indicators of processing by water. We demonstrate a new way to use lithium isotopes that reflects aqueous alteration in carbonaceous chondrites. Temperature appears to exert a control on the production of acetic acid‐soluble phases, such as carbonates and poorly crystalline Fe‐oxyhydroxides. Temperature and degree of water‐rock interaction determines the amount of lithium isotope fractionation expressed as the difference between whole rock and acetic acid‐leachable fractions. Using these features, the type 1 chondrite Orgueil (δ⁷Li_{(whole rock)} = 4.3‰; Δ⁷Li_{(acetic‐whole)} = 1.2‰) can be distinguished from the type 2 chondrites Murchison (δ⁷Li_{(whole rock)} = 3.8; Δ⁷Li_{(acetic‐whole)} = 8.8‰) and carbonate‐poor Tagish Lake (δ⁷Li_{(whole rock)} = 4.3; Δ⁷Li_{(acetic‐whole)} = 9.4‰). This initial study suggests that lithium isotopes have the potential to reveal the role of liquid water in the early solar system.     

Laboratory simulation of the salt weathering of schist: 1. Weathering of schist blocks in a seasonally wet tropical environment

Data describing sediment generation focusing on the temporal evolution of size gradation are required for the prediction of long‐term landform evolution. This paper presents such data for the salt weathering of a quartz‐chlorite schist obtained from the Ranger Uranium Mine in northern Australia. Rock fragment samples are subjected to three different climate regimes: (1) a dry season climate; (2) a wet season climate (both based on observations at the Ranger site); and (3) an oven‐drying sequence designed to test the sensitivity of the weathering process by exposing the rocks to more extreme temperatures. Two MgSO₄ salt solutions are applied, one being typical of wet season runoff and the other a more concentrated solution. Salt solution is applied daily in the wet season experiments and once only at the beginning of the dry season experiments. Results of the experiments reveal four stages of weathering. The kinetics of each stage are described and related to the formation of sediment of different sizes. Wet season climate conditions are shown to produce greater moisture variability and lead to faster weathering rates. However, salt concentrations in the wet season are typically lower and so when climate is combined with observed salt concentrations, the dry and wet season experiments weather at approximately equal rates. Finally, small variations in rock properties were shown to have a large impact on weathering rates, leading to the conclusion that rock weathering experiments need to be carefully designed if results are to be used to help predict weathering behaviour at the landscape scale. Copyright © 2006 John Wiley & Sons, Ltd.     

The international at-sea intercomparison of fCO2 systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The ‘International Intercomparison Exercise of fCO₂ Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO₂) systems, one discrete fCO₂ system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO₂ measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO₂, temperature measurements) and a common procedure for calculation of final fCO₂ were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO₂ in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO₂ measurements can be made in good agreement (±3 μatm) with underway fCO₂ data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO₂ system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO₂ results. Calculation of fCO₂ from high-quality total dissolved inorganic carbon (C_T) and total alkalinity (A_T) measurements does not yield results comparable in accuracy and precision to fCO₂ measurements.     

The Uptake of Colloidal Melanin from Seawater by Marine Bivalves

Abstract. A number of coastal organisms ( e.g ., bivalves) are known as important consumers of particulate organic carbon (POC) in the sea. In this paper we present new evidence that they are also important consumers of dissolved organic carbon (DOC) in the colloidal size range down to about 0.2 μm in diameter. Those colloids play an important role in the global flux of carbon in the seas.
We compare the uptake of colloidal DOC by marine bivalves of the North Sea and of the western Indian ocean.
To measure this uptake we made use of colloidal melanin as an alternative way to measure the uptake of colloidal DOC.     

Residual depth and residual geoid anomalies of the South Atlantic ocean

Abstract

We have obtained the residual depth and residual geoid anomalies of the South Atlantic Ocean and interpreted them in terms of upper mantle processes. Starting from the 5’ X 5’ SYNBAPS data, we computed the 1° X 1° mean water depth. We digitized a recent sediment thickness map of the South Atlantic and corrected for the isostatic sediment loading effects. A plot of the corrected basement depth against crustal ages shows a good match to the depth‐age curve of the plate model. We then subtracted the predicted plate model depths from the corrected basement depths and obtained the 1° X 1° residual depth. The residual depth anomalies have positive values over the topographic highs and relative negative values over the ocean basins. A prominent asymmetry is observed between the residual depth over the Argentine Basin and that over the Cape Basin.

We have obtained the 1° X 1° residual geoid of the South Atlantic by subtracting both the long wavelength features and the geoid variations due to the plate cooling from the 1° X 1° Seasat altimeter derived geoid. The long wavelength features are represented by the degree and order 10 geoid of GEM1OC, and the geoid variations due to the plate cooling effects are predicted by the plate model geoid‐age relationship. The residual geoid anomalies also show an asymmetry although weaker than the case of the residual depth, between the Argentine and Cape basins.

By taking the 5° X 5° averages, we removed possible plate bending effects on the depth and geoid anomalies. We then compared those two data sets with respect to the reported hot spot tracks in the South Atlantic. The residual depth generally shows positive values over the hot spot tracks, whereas the residual geoid does not show any sign of them. A prominent asymmetric feature of depth and geoid anomalies is observed over the Argentine and Cape basins. This asymmetry is probably caused by hotter and less dense materials beneath the Cape Basin. Hot spots or other mantle upwellings may be the heat sources.     

Matching carbon pools and fluxes for the Southern Ocean Iron Release Experiment (SOIREE)

The Lagrangian Southern Ocean Iron Release Experiment (SOIREE) allowed study of a gradually evolving iron-mediated phytoplankton bloom in water labelled with the inert tracer sulfur hexafluoride, SF₆. This article describes a pelagic carbon budget for the mixed layer in SOIREE and assesses the extent to which closure of the budget is achieved. Net community production (NCP) converted 837 mmol m⁻² of inorganic carbon to organic carbon in 12.0 d after the first iron addition. A large fraction (41%) of NCP remained as particulate organic carbon in the mixed layer of the iron-enriched patch, while 23% was lost by horizontal dispersion and 0–29% was exported. The closure of the carbon budget is hampered by the lack of measurements of dissolved organic carbon (DOC), by a major uncertainty in carbon export, and by use of empirical conversion factors in estimates of carbon biomass and metabolic rates. Lagrangian carbon-budget studies may be improved by direct measurement of all major carbon parameters and conversion factors. Carbon cycling in the SOIREE bloom resembled that in ‘natural’ algal blooms in the open Southern Ocean in some respects, but not in all. Daily NCP in the SOIREE bloom (70 mmol m⁻² d⁻¹) was higher than in natural blooms, partly because other studies did not account for horizontal dispersion, were for longer periods or included less productive areas. The build-up of POC stock and carbon export as a fraction of NCP in SOIREE were in the lower range of observations elsewhere.     

Commercial Possibilities for Stranded Conventional Gas from Alaska’s North Slope

Stranded gas resources are defined for this study as gas resources in discrete accumulations that are not currently commercially producible, or producible at full potential, for either physical or economic reasons. Approximately 35 trillion cubic feet (TCF) of stranded gas was identified on Alaska’s North Slope. The commercialization of this resource requires facilities to transport gas to markets where sales revenue will be sufficient to offset the cost of constructing and operating a gas delivery system. With the advent of the shale gas revolution, plans for a gas pipeline to the conterminous US have been shelved (at least temporarily) and the State and resource owners are considering a liquefied natural gas (LNG) export project that targets Asian markets. This paper focuses on competitive conditions for Asian gas import markets by estimating delivered costs of competing supplies from central Asia, Russia, Indonesia, Malaysia, and Australia in the context of a range of import gas demand projections for the period from 2020 to 2040. These suppliers’ costs are based on the cost of developing, producing, and delivering to markets tranches of the nearly 600 TCF of recoverable gas from their own conventional stranded gas fields. The results of these analyses imply that Alaska’s gas exports to Asia will likely encounter substantial competitive challenges. The sustainability of Asia’s oil-indexed LNG pricing is also discussed in light of a potentially intense level of competition.     

GRMHD/RMHD simulations & stability of magnetized spine-sheath relativistic jets

A new general relativistic magnetohydrodynamics (GRMHD) code “RAISHIN” used to simulate jet generation by rotating and non-rotating black holes with a geometrically thin Keplarian accretion disk finds that the jet develops a spine-sheath structure in the rotating black hole case. Spine-sheath structure and strong magnetic fields significantly modify the Kelvin-Helmholtz (KH) velocity shear driven instability. The RAISHIN code has been used in its relativistic magnetohydrodynamic (RMHD) configuration to study the effects of strong magnetic fields and weakly relativistic sheath motion, c/2, on the KH instability associated with a relativistic, γ=2.5, jet spine-sheath interaction. In the simulations sound speeds up to and Alfvén wave speeds up to ∼0.56c are considered. Numerical simulation results are compared to theoretical predictions from a new normal mode analysis of the RMHD equations. Increased stability of a weakly magnetized system resulting from c/2 sheath speeds and stabilization of a strongly magnetized system resulting from c/2 sheath speeds is found.