The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and
reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico
volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are
available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen
independent cooling units in the Michoacán-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated
by 40Ar/39Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava
flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian
statistical parameters are k = 28, α95 = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically
undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study
of the scatter of the virtual geomagnetic poles giving SF = 15.9 with SU =21.0 and SL = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The
oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units
belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35
Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in
the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1.
Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field
within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine
sediments at about 590 ka. 相似文献
Four distinct approaches, that vary markedly in the spatial and temporal resolution of their measurement and process-level outputs, are used to investigate the daily and seasonal water vapour exchange in a 70-year-old Belgian Scots pine forest. Transpiration, canopy interception, soil evaporation and evapotranspiration are simulated, using a stand-level process model (SECRETS) and a soil water balance model (WAVE). Simulated transpiration was compared with up-scaled sap flow measurements and simulated evapotranspiration to eddy covariance measurements.
Reasonable agreement in the temporal trends and in the annual water balance between the two models was observed, however daily and weekly predictions often diverged. Most notably, WAVE estimated very low, to no transpiration during late autumn, winter and early spring when incident radiation fell below 50 W m−2 while SECRETS simulated low (0.1–0.4 mm day−1) fluxes during the same period. Both models exhibited similar daily trends in simulated transpiration when compared with sap flow estimates, although simulations from SECRETS were more closely aligned. In contrast, WAVE over-estimated transpiration during periods of no rainfall and under-estimated transpiration during rainfall. Yearly, total evapotranspiration simulated by the models were similar, i.e. 658 mm (1997) and 632 mm (1998) for WAVE and 567 mm (1997) and 619 mm (1998) for SECRETS.
Maximum weekly-average evapotranspiration for WAVE exceeded 5 mm day−1, while SECRETS never exceeded 4 mm day−1. Both models, in general, simulated higher evapotranspiration than that measured with the eddy covariance technique. An impact of the soil water content in the direct relationship between the models and the eddy covariance measurements was found.
The results suggest that: (1) different model formulations can reproduce similar results depending on the scale at which outputs are resolved, (2) SECRETS estimates of transpiration were well correlated with the empirical measurements, and (3) neither model fitted favourably to the eddy covariance technique. 相似文献
Natural Hazards - Human exposure to floods continues to increase, driven by changes in hydrology and land use. Adverse impacts amplify for socially vulnerable populations, who disproportionately... 相似文献
Natural Hazards - U. S. Military bases are especially vulnerable to tropical cyclones (TCs) because they concentrate extremely expensive equipment in a small area. A TC hit at a base can result in... 相似文献
We analyzed the distribution, diversity, and composition of western Mediterranean macroplankton (excluding gelatinous taxa) in the water column over depths of ca. 550–850 m, with special attention to near-bottom (0–1.5 and ca. 5–77 m above the bottom, mab) levels, and including data from three areas (off the coasts of Catalonia, and to the NW, and SE of Mallorca, Balearic Islands) in the period 1991–2008. Spatio-temporal changes in macroplankton abundance were evaluated as follows: (i) by seasonal sampling in 2007 off the Catalonian coast, (ii) by comparing Catalonian and Balearic Island slopes, and (iii) by comparing a fixed station on the Catalonian slope (at 550–800 m depth) at decadal (1991/1992–2007/2008) time scales. Diversity (in terms of species richness, S) was greater (i) at ca. 5–77 mab than at 0–1.5 mab, (ii) over the insular slopes of the Balearic Island (around Mallorca) than over the mainland Catalonian slopes, and (iii) in the period 1991/1992 than in 2007, likely related to higher values of the North Atlantic Oscillation (NAO) index in 1991/1992. In most analyses species composition was strongly influenced by the degree of stratification and homogenization of the water column in summer–autumn and winter–spring respectively and by location (longitude). Changes consisted mainly of higher density of macroplankton (e.g. abundance of the dominant euphausiids Nematoscelis megalops, Meganyctiphanes norvegica and Euphausia krohni and of the fish Cyclothone braueri) between June and October, parallel to an increase in the T and S close to the bottom. This coincided with changes in the flow of Levantine intermediate water (LIW) in the area. Aggregation of adult forms of the dominant species close to the bottom in summer–autumn could be favored because summer is the period of highest density of food – copepods, mainly Calanus helgolandicus – near the bottom off the Catalan slope. The formation of a thermocline in the water column and the reinforcement of the permanent thermohaline front at the shelf-slope break during summer at ca. 400 m in the Balearic Basin may also enhance this tendency toward greater aggregation of deep macroplankton under stratified water column conditions. 相似文献