Climate forcings and climate sensitivities diagnosed from atmospheric global circulation models

Understanding the historical and future response of the global climate system to anthropogenic emissions of radiatively active atmospheric constituents has become a timely and compelling concern. At present, however, there are uncertainties in: the total radiative forcing associated with changes in the chemical composition of the atmosphere; the effective forcing applied to the climate system resulting from a (temporary) reduction via ocean-heat uptake; and the strength of the climate feedbacks that subsequently modify this forcing. Here a set of analyses derived from atmospheric general circulation model simulations are used to estimate the effective and total radiative forcing of the observed climate system due to anthropogenic emissions over the last 50 years of the twentieth century. They are also used to estimate the sensitivity of the observed climate system to these emissions, as well as the expected change in global surface temperatures once the climate system returns to radiative equilibrium. Results indicate that estimates of the effective radiative forcing and total radiative forcing associated with historical anthropogenic emissions differ across models. In addition estimates of the historical sensitivity of the climate to these emissions differ across models. However, results suggest that the variations in climate sensitivity and total climate forcing are not independent, and that the two vary inversely with respect to one another. As such, expected equilibrium temperature changes, which are given by the product of the total radiative forcing and the climate sensitivity, are relatively constant between models, particularly in comparison to results in which the total radiative forcing is assumed constant. Implications of these results for projected future climate forcings and subsequent responses are also discussed.     

Transport and dispersal of organic debris (peat blocks) in upland fluvial systems

This paper assesses the mechanisms and pathways by which peat blocks are eroded and transported in upland fluvial systems. Observations and experiments from the north Pennines (UK) have been carried out on two contrasting river systems. Mapping of peat block distributions and appraisal of reach‐based sediment budgets clearly demonstrates that macro‐size peat is an important stream load component. In small streams block sizes can approximate the channel width and much of the peat is transported overbank. Local ‘peat jams’ and associated mineral deposition may provide an important component of channel storage. In larger systems peat blocks rapidly move down‐channel and undergo frequent exchanges between bed and bank storage. Results of peat block tracing using painted blocks indicate that once submerged, blocks of all sizes are easily transported and blocks break down rapidly by abrasion. Vegetation and bars play an important role in trapping mobile peat. In smaller streams large block transport is limited by channel jams. Smaller blocks are transported overbank but exhibit little evidence of downstream fining. In larger rivers peat blocks are more actively sorted and show downstream reduction in size from source. A simple model relating peat block diameter (D_p) to average flow depth (d) suggests three limiting transport conditions: flotation (D_p < d), rolling (d < D_p > d/2) and deposition (D_p > d/2). Experiments demonstrate that peat block transport occurs largely by rolling and floating and the transport mechanism is probably controlled by relative flow depth (d/D_p ratio). Transport velocity varies with transport mechanism (rolling is the slowest mode) and transport lengths increase as flow depth increases. Abrasion rates vary with the transport mechanism. Rolling produces greater abrasion rates and more rounded blocks. Abrasion rates vary from 0 to 10 g m^?1 for blocks ranging in mass from 10 to 6000 g. Copyright © 2001 John Wiley & Sons, Ltd.     

Liver lesions in winter flounder (Pseudopleuronectes americanus) from Jamaica Bay,New York: Indications of environmental degradation

Liver sections of winter flounder (Pseudopleuronectes americanus) collected from Jamaica Bay and Shinnecock Bay, New York, in 1989, were examined microscopically to determine the pervasiveness of liver lesions observed previously in Jamaica Bay winter flounder. Neoplastic lesions were not detected in fish from Jamaica Bay or the Shinnecock Bay reference site. Twenty-two percent of Jamaica Bay winter flounder examined (n=103) had unusual vacuolization of hepatocytes and biliary pre-ductal and ductal cells (referred to hereafter as the vacuolated cell lesion). The lesion, identical to that found in 25% of Jamaica Bay winter flounder examined in 1988, has previously been identified in fishes taken from highly polluted regions of the Atlantic coast (e.g., Boston Harbor, Massachusetts, and Black Rock Harbor, Connecticut). Prevalence of the vacuolated cell lesion in winter flounder from Jamaica Bay was significantly greater (p<0.0001) than in 102 specimens collected from Shinnecock Bay. Current scientific literature indicates vacuolated hepatocytes and cholangiocytes are chronically injured and that the extent of their deformity is consistent with the action of a hepatotoxicant. The high prevalence of vacuolated hepatocytes in Jamaica Bay winter flounder and absence of the lesion in flounder from reference sites strongly supports the hypothesis that this impairment is a manifestation of a toxic condition in at least some portions of Jamaica Bay.     

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules

The Bovedy L-group chondrite contains a light-colored poikilitic lithic fragment with olivine, low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na₂O, K₂O and P₂O₅ content indicate that the fragment represents a solidified, slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is heterogeneous, with a progressive increase in the bulk $\text{MgO}\text{FeO}$ ratio and in MgO content of olivines and low-Ca pyroxenes across its length. ${}^{39}\text{Ar}{}^{40}\text{Ar}$ analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of 0.5–0.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragment's and host's metallographic cooling rate of ~ 5 C/Myr (through 500°C) imprecise. However, the absence of martensite and the presence of kamacite. zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500°C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (κ) of this overburden was approximately comparable to that of the lunar regolith (10^?4cm²/sec), then the fragment was buried at a depth ≌ 6.5 km; if K = 10^?2 cm²/sec (similar to chondritic material), then the fragment was buried at a depth ?65 km.     

40Ar-39Ar age of the Shergotty achondrite and implications for its post-shock thermal history

Analyses of ⁴⁰Ar-³⁹Ar have been made on a whole rock sample and a maskelynite (feldspar) separate of the shocked Shergotty achondrite. The maskelynite gave a plateau age of 254 ± 10 Myr. The whole rock sample gave a complex release with apparent ages between 240 and 640 Myr. The slightly younger Rb-Sr isochron age of 165 Myr for Shergotty (Nyquistet al., 1978) suggests that the maskelynite as well as the whole rock was incompletely degassed. Reasonable Ar diffusion characteristics for Shergotty for shock heating temperatures of <400°C indicate D/a² of 10^?11?10^?13 sec^?1. The time required to lose 95% of the ⁴⁰Ar from the plagioclase would be ~10³–10⁴ yr. When this gas diffusion time is introduced into a thermal model of a cooling ejecta blanket of variable thickness, a post-shock cooling time of ? 10³ yr and a burial depth of ? 300 m are indicated for Shergotty. These conclusions are not seriously affected by uncertainties in the thermal model. Most likely the shock event occurred ~ 165 Myr ago, but no earlier than 250 Myr ago, when the Shergotty parent object experienced a collision in the asteroid belt. As a result of that collision, feldspar was converted to maskelynite, the K-Ar and Rb-Sr ages were completely or nearly completely reset, and the Shergotty meteorite was heated to <400°C and left to cool slowly inside the parent body.     

A lunar rock of deep crustal origin: sample 76535

Lunar sample 76535 is a coarse-grained troctolitic granulite exhibiting a texture indicative of long annealing times. It is composed of homogeneous crystals of plagioclase (58 per cent, An₉₆), olivine (37 per cent, Fo₈₈) and bronzite (4 per cent, En₈₆).Chromian spinel-bronzite-diopside (Wo₄₆En₅₀Fs₄) symplectic intergrowths commonly occur along olivine-plagioclase boundaries and as tiny inclusions within olivine grains. These symplectites apparently formed by a reaction of the type:

$\text{OI + An + Chromite → Opx + Cpx + Al-Mg-chromite}$

. The reaction is related to the experimentally determined reaction

$\text{OI + An = Opx + Cpx + Sp}$

of Kushiro and Yoder (1966). The enstatite content of the diopside coexisting with the bronzite indicates equilibration at about 1000°C. Thermodynamic calculations for 1000°C indicate that the symplectites formed at a minimum pressure of about 0.6 kb. Low alumina contents of the pyroxenes indicate equilibration near this minimum pressure.Clusters of the same assemblage found in the symplectic intergrowths, but containing accessory metal, troilite, Ca-phosphates, baddeleyite, plagioclase and/or K-feldspar occur sporadically throughout the rock. These apparent late stage products crystallized in the low temperature-high pressure region discussed above.Phase relations of co-existing metal phases indicate that the rock cooled at a few tens of degrees/my, corresponding to depths of 10–20 km below the lunar surface, in agreement with the above pressure estimate.We infer that 76535 represents an original cumulate deposited at a depth between about 10 and 30 km. The last liquid crystallized in the relatively high pressure-low temperature field opx + cpx + Al-Mg-chromite. Cooling was extremely slow and accompanied by extensive chemical and textural re-equilibration. Reaction to form the symplectites occurred during the late stages of re-equilibration.     

Analyse isotopique de l'air piégé dans la glace pour quantifier les variations de température

Isotopic measurements in polar ice core have shown a succession of rapid warming periods during the last glacial period over Greenland. However, this method underestimates the surface temperature variations. A new method based on gas thermal diffusion in the firn manages to quantify surface temperature variations through associated isotopic fractionations. We developed a method to extract air from the ice and to perform isotopic measurements to reduce analytical uncertainties to 0.006 and 0.020$\text{‰}$ for δ¹⁵N and δ⁴⁰Ar. It led to a 16±1.5 °C surface temperature variation during a rapid warming ($\text{?70}\text{000}$ yr). To cite this article: A. Landais et al., C. R. Geoscience 336 (2004).     

Population structure and reproductive characteristics of the gulf pipefish,Syngnathus scovelli, in Mobile Bay, Alabama

The gulf pipefish,Syngnathus scovelli, is the dominant syngnathid found in coastal regions of the Gulf of Mexico and is the only species in this region known to occur in both freshwater and saltwater habitats. Relatively little is known about the population and reproductive cycles ofS. scovelli, particularly for those found in low salinity environments. The focus of the current study was to collect population structure and reproductive data forS. scovelli from a low salinity habitat, Meaher Park, located at the mouth of Mobile Bay, Alabama. Sampling was conducted twice a month from January 2003 to January 2004. Environmental parameters, as well as population and reproductive parameters, were collected. Water temperature was the primary abiotic factor associated with both the appearance ofS. scovelli and their breeding cycle. Based on gonadosomatic (GSI) and brood pouch somatic indices (BPSI), females and males were reproductively active throughout the summer. Peaks in male GSI and BPSI were consistent with the subsequent appearance of a large number of juveniles in early fall. These event coincided with the higher temperatures seen during late summer and early fall. Sex ratios (male : female 1 : 1.64) and operational sex ratios (1 : 4.09) indicated that this was a female biased population. Although there was no difference in the average size of males and females in this population, the largest individuals collected were female. The female-biased sex ratio supports previous genetic analyses suggesting thatS. scovelli exhibits true sex-role reversal and that operational sex ratios are a reliable indicator of the intensity of mating competition and sex roles.     

Real-time measurement of volcanic SO2 emissions: validation of a new UV correlation spectrometer (FLYSPEC)

A miniaturized, lightweight and low-cost UV correlation spectrometer, the FLYSPEC, has been developed as an alternative for the COSPEC, which has long been the mainstay for monitoring volcanic sulfur dioxide fluxes. Field experiments have been conducted with the FLYSPEC at diverse volcanic systems, including Masaya (Nicaragua), Poás (Costa Rica), Stromboli, Etna and Vulcano (Italy), Villarica (Chile) and Kilauea (USA). We present here those validation measurements that were made simultaneously with COSPEC at Kilauea between March 2002 and February 2003. These experiments, with source emission rates that ranged from 95 to 1,560 t d⁻¹, showed statistically identical results from both instruments. SO₂ path-concentrations ranged from 0 to >1,000 ppm-m with average correlation coefficients greater than r ²=0.946. The small size and low cost create the opportunity for FLYSPEC to be used in novel deployment modes that have the potential to revolutionize the manner in which volcanic and industrial monitoring is performed.     

Rare and unusual mineral inclusions in diamonds from Mwadui, Tanzania

Syngenetic diamond inclusions from the Mwadui kimberlite reveal that an unusually fertile section of lithospheric mantle beneath the Central African Craton was sampled. This is shown by a very high ratio of lherzolitic to harzburgitic garnet inclusions (1:2) and low Mg/Fe-ratios in olivine and orthopyroxene. Geothermometry applied to the peridotitic inclusions indicates disequilibrium between non-touching inclusion pairs to be common. Disequilibrium between garnet-olivine and garnet-orthopyroxene pairs suggests successive iron enrichment during diamond formation, e.g. leading to the presence of harzburgitic garnet and lherzolitic olivine in the same diamond. Apart from the dominant peridotitic inclusion suite (88%), rare eclogitic inclusions occur (2%) and a number of uncertain paragenesis. Two diamonds, one with eclogitic garnets with moderate pyroxene solid solution and the other with a single ferro-periclase inclusion, suggest the contribution of a small sub-lithospheric component. The finding of the association Fe-FeO-Fe₃O₄ in one single diamond indicates diamond formation over a large range of f _O2 conditions, possibly along redox fronts. Steep compositional gradients may also be reflected by the joint occurrence of harzburgitic garnet and a SiO₂-phase in the same diamond. Alternatively the formation of the SiO₂-phase may be due to extreme carbonation of the peridotitic source. Further unusual findings include the exsolution of a silicate phase from magnetite inclusions, (i.e. primary solution of γ-olivine) and an ilmenite inclusion with an eskolaite (Cr₂O₃) component of 14.5 mol%, the latter together with harzburgitic paragenesis silicate inclusions. Received: 23 August 1997 / Accepted: 7 January 1998