植被和气候对阿拉斯加和加拿大北部北极苔原地区多年冻土活动层厚度的影响

The active layer is the top layer of permafrost soils that thaws during the summer season due to increased ambient temperatures and solar radiation inputs. This layer is important because almost all biological activity takes place there luring the summer. The depth of active layer thaw is influenced by climatic conditions. Vegetation has also been found to have a strong impact on active layer thaw, because it can intercept incoming radiation, thereby insulating the soil from ambient conditions. In order to look at the role of vegetation and climate on active layer thaw, we measured thaw depth and the Normalized Difference Vegetation Index (NDVI; a proxy for aboveground plant biomass) along a latitudinal temperature gradient in arctic Alaska and Canada. At each site several measurements of thaw and NDVI were taken in areas with high amounts of vegetation and areas with little to no vegetation. Results show that the warmest regions, which had the greatest levels of NDVI, had relatively shallow thaw depths, and the coldest regions, which had the lowest levels of NDVI, also had relatively shallow thaw depths. The intermediate regions, which had moderate levels of NDVI and air temperature, had the greatest depth of thaw. These results indicate that temperature and vegetation interact to control the depth of the active layer across a range of arctic ecosystems. By developing a relationship to explain thaw depth through NDVI and temperature or latitude, the possibility exists to extrapolate thaw depth over large scales via remote sensing applications.     

Upper-atmosphere zonal winds: Variation with height and local time

The average rotation rate of the upper atmosphere can be found by analysis of the changes in the orbital inclinations of satellites, and results previously obtained have indicated that the atmospheric rotation rate appreciably exceeds the Earth's rotation rate at heights between 200 and 400 km.We have examined all such results previously published in the light of current standards of accuracy: some are accepted, some revised, and some rejected as inadequate in accuracy. We also analyse a number of fresh orbits and, adding these to the accepted and revised previous results, we derive the variation of zonal wind speed with height and local time. The rotation rate (rev/day) averaged over all local times increases from near 1.0 at 150 km height to 1.3 near 350 km (corresponding to an average west-to-east wind of 120 m/s), and then decreases to 1.0 at 400 km and probably to about 0.8 at greater heights. The maximum west-to-east winds occur in the evening hours, 18–24 h local time: these evening winds increase to a maximum of about 150 m/s at heights near 350 km and decline to near zero around 600 km. In the morning, 4–12 h local time, the winds are east to west, with speeds of 50–100 m/s above 200 km. We also tentatively conclude that, at heights above 350 km, the average rotation rate is greater in equatorial latitudes (0–25°) than at higher latitudes.     

Variations in air density from January 1972 to April 1975 at heights near 200 km

Variations in air density have been determined using the orbit of the satellite Cosmos 462, 1971-106A, which entered orbit on 3 December 1971 with an initial perigee near 230 km and inclination 65.75°, and decayed on 4 April 1975. Accurate orbits determined at 85 epochs give perigee height correct to about 200 m throughout the satellite's lifetime. Using these values of perigee height and orbital decay rates from NORAD elements, 604 values of air density at half a scale height above perigee have been evaluated. These densities have been compared with values from the COSPAR International Reference Atmosphere 1972, taking account of variations due to solar activity and geomagnetic disturbances, and day-to-night variations, to reveal the residual variations in density at a series of standard heights, 245, 240, 232 and 213 km.The main residual variation is semi-annual, with maxima usually in April and October, and minima usually in January and July; but it is irregular in phase and shape. The amplitude of the semi-annual variation is remarkably constant from year to year between 1972 and 1975, and considerably greater than that given by CIRA 1972: the April/July density ratio is 1.68, not 1.32 as in CIRA; the October–November maxima are all lower than the April maxima, whereas CIRA gives the opposite; the July minima are 18% lower than the January minima, as opposed to 10% in CIRA.A standardized semi-annual density variation for the early 1970s is presented, with January minimum of 0.94, April maximum of 1.28, July minimum of 0.77 and October–November maximum of 1.22. In addition, three other recurrent variations are recognizable: in each year the density has a subsidiary minimum in May and maximum in June; there are low values in mid November and high values in late December.     

Late Quaternary shedding of shallow-marine carbonate along a tropical mixed siliciclastic–carbonate shelf: Great Barrier Reef, Australia

Abstract The north-east Australian margin is the largest modern example of a tropical mixed siliciclastic/carbonate depositional system, with an outer shelf hosting the Great Barrier Reef (GBR) and an inner shelf dominated by fluvially sourced siliciclastic sediment wedges. The long-term interplay between these sediment components and sea level is recorded in the Queensland Trough, a 1–2 km deep N–S elongate basin situated between the GBR platform and the Queensland Plateau. In this paper, 154 samples from 45 surface grabs and six well-dated piston cores were analysed for total carbonate content, carbonate mineralogy and Sr concentration to establish spatial and temporal patterns of carbonate accumulation in the Queensland Trough over the last 300 kyr. Surface carbonate contents are lowest on the inner-shelf (<5%) and in the trough axis (<60%) because of siliciclastic dilution. Carbonate on the shelf is mostly Sr-rich aragonite and high-Mg calcite (HMC), whereas that in the basin is mostly low-Mg calcite. Once normalized to remove the effects of siliciclastic dilution, surface Sr-rich aragonite and HMC abundances decrease linearly to background levels ≈ 100 km seaward of the shelf edge. Core samples show that, over time, normalized aragonite and Sr abundances are greatest during periods of shelf flooding and lowest when sea level drops below the shelf edge. This is consistent with changes in the production of coral and calcareous algae, and the shedding of their debris from the shelf. Interestingly, normalized HMC concentrations on the slope peak during periods of major transgression, perhaps because of maximum off-shelf transport from inter-reef areas or intermediate water dissolution. After accounting for siliciclastic dilution, there are strong similarities in both spatial and temporal patterns of carbonate minerals between slopes and basins of the north-east Australian margin and those of pure carbonate margins such as the Bahamas. A limited set of basic processes, including the formation and breakdown of carbonate on the shelf, the transport of carbonate off the shelf and eustatic sea level, probably controls carbonate accumulation in slope and basin settings of tropical environments, irrespective of proximal siliciclastic sediment sources.     

Os isotopic constraints on Archean mantle dynamics

The Re-Os isotopic systematics of two ca. 2.7-Ga komatiite flows from Belingwe, Zimbabwe are examined. Rhenium and Os concentrations in these rocks are similar to concentrations in other Archean, Proterozoic, and Phanerozoic komatiites. Despite the excellent preservation of primary magmatic minerals, the Re-Os systematics of whole-rock samples of the komatiites show open-system behavior. Consistent model ages for several whole-rock samples suggest a disturbance to the system during the Proterozoic. Despite the open-system behavior in the whole rocks, Re-Os systematics for concentrates of primary magmatic olivine and spinel indicate generally closed-system behavior since the magmatic event that produced the rocks. Regression of the data for the mineral concentrates yields an age of 2721 ± 21 Ga, which is consistent with Pb-Pb and Sm-Nd ages that have been previously reported for the komatiites (Chauvel et al., 1993), and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.11140 ± 84 (γ_Os = +2.8 ± 0.8).The 2 to 3% enrichment in ¹⁸⁷Os/¹⁸⁸Os ratio of the mantle source of the komatiites, relative to the chondritic composition of the contemporaneous convecting upper mantle, most likely reflects either the incorporation of substantially older (≥ 4.2 Ga), Re-rich recycled mafic crust into the mantle source of the komatiites or the contribution of suprachondritic Os to the source from the putative ¹⁸⁷Os-enriched outer core. The former interpretation would indicate the Hadean formation and recycling of mafic crust. The latter interpretation would require early formation of a substantial inner core followed by upwelling of a mantle plume from the core-mantle boundary, at least as far back as the Late Archean. Either interpretation requires large-scale mantle convection during the first half of Earth history.     

Sensitivity analysis of a no-crossflow model for the transient flowmeter test

 Logarithmic sensitivities and plausible relative errors are studied in a simple no-crossflow model of a transient flowmeter test (TFMT). This model is identical to the model of a constant-rate pumping test conducted on a fully penetrating well with wellbore storage, surrounded by a thick skin zone, and situated in a homogeneous confined aquifer. The sensitivities of wellbore drawdown and wellface flowrate to aquifer and skin parameters are independent of the pumping rate. However, the plausible relative errors in the aquifer and skin parameters estimated from drawdown and wellface flowrate data can be proportionally decreased by increasing the pumping rate. The plausible relative errors vary by many orders of magnitude from the beginning of the TFMT. The practically important flowrate and drawdown measurements in this test, for which the plausible relative errors vary by less than one order of magnitude from the minimum plausible relative errors, can begin approximately when the dimensionless wellface flowrate exceeds q _D =q/Q≈0.4. During most of this stage of the test, the plausible relative errors in aquifer hydraulic conductivity (K _a ) are generally an order of magnitude smaller than those in aquifer specific storativity. The plausible relative errors in the skin hydraulic conductivity (K _s ) are generally larger than the plausible relative errors in the aquifer specific storativity when the thick skin is normal (K _s >K _a ) and smaller when the thick skin is damaged (K _s <K _a ). The specific storativity of the skin zone would be so biased that one should not even attempt to estimate it from the TFMT. We acknowledge Wiebe H. van der Molen for recommending the De Hoog algorithm and sharing his code. This research was partially supported by the US Geological Survey, USGS Agreement #1434-HQ-96-GR-02689 and North Carolina Water Resources Research Institute, WRRI Project #70165.     

Fieldwork and Tropicality in French Indochina: Reflections on Pierre Gourou's Les Paysans Du Delta Tonkinois, 1936

This paper examines the fieldwork undertaken by the distinguished French geographer Pierre Gourou (1900–99) in the Tonkin Delta (Red River Delta) of northern Vietnam in the 1920s and 1930s, and his wider configuration of “the tropical world” as a distinct space of knowledge and radical otherness. Gourou's fieldwork endeavours in French Indochina are interpreted in the light of recent work on “tropicality”: the idea that “the tropics” need to be understood as a western cultural construction and colonising discourse that essentialised the hot, wet regions of the world, and exalted the temperate world over its tropical counterpart. The paper focuses on Gourou's monumental 1936 study Les paysans du delta tonkinois, étude de géographie humaine. It is argued that in this study, and his later comparative work on the tropics, Gourou elaborated a distinct geographical variant of tropicality, but one that, ultimately, reinforced the essentialist logic and momentum of this discourse. Particular attention is paid to the geographical ideas, fieldwork techniques and discursive strategies that Gourou used in his 1936 study, and the French colonial context in which he worked. The article shows how Gourou appealed to western reason and science as tools of study, identified overpopulation as the key problem facing the Tonkin Delta, and suggested that colonial practices of modernisation had a limited place and ineffectual role in the rice plains of the region.     

Origin and significance of 13-hydroxychlorophyll derivatives in sediments

The authenticity of hydroxychlorophyll derivatives in sediments has been verified by subjecting pure chlorophyll preparations to the extraction method used for sediments. Model studies of chlorophyll autoxidation reveal hydroxychlorophyll as the major product formed in the presence of hydrogen peroxide. Its stability to further oxidation implies that hydroxychlorophyll derivatives are not precursors of aetioporphyrins and are more likely to produce cycloalkanoporphyrins via the processes of diagenesis. The occurrence of 13²-hydroxyphaeophytin a throughout a sediment core from Loch Ness confirms it to be a widespread chlorophyll oxidation product formed during early diagenesis. Profiles of hydroxychlorophyll derivatives in a sediment core from an Antarctic lake demonstrate their potential for use as markers of oxidation processes in palaeoenvironmental assessment.