Experiments combining different cleaning and analytical techniques indicate that multiphase magnetizations may be quite common. However, these may not be recognized because of limited experimental work. Alternating field (AF) cleaning is often the only treatment applied to igneous and metamorphic rocks; thermal and/or AF cleanings are usually the only treatments applied to sedimentary rocks. In many instances, AF and thermal treatments are limited to 100 mT and 550°C respectively. Investigations based on such limited cleaning often fail to detect some of the phases of magnetization contained in the rock. Failure to detect one or more phases means that valuable data are not recovered and the whole magnetic history of the rock has not been unfolded. Most importantly, the undetected phase might be the initial so that a secondary magnetization can easily be mistaken for the initial with an erroneous interpretation as a result. It is therefore imperative to recognize all phases of magnetizations and it possible to separate them.Procedures that have been used to recognize and unravel multiphase magnetizations are described. These procedures make use of chemical, thermal and AF cleaning treatments, two-stage demagnetization, vector analysis, slicing of specimens and vector diagrams. The combination used depends on the rock studied. For example, it is found that AF followed by thermal treatment can be very useful for the study of igneous rocks; chemical leaching is by far the most effective cleaning technique for the study of red beds. A three-phase model describing the magnetizations process of certain red beds is proposed. The slicing of specimens is used to explain intermediate directions with respect to field reversals. It is shown that graphical representation by vector diagrams can greatly facilitate the interpretation of the results. The examples show that, although a statistically well defined magnetization may be obtained after limited cleaning, it cannot be assumed to be the initial. One must ascertain that another magnetization has not remained undetected. This necessitates detailed and extensive experimental work using and devising new analytical procedures in an attempt to unfold the whole magnetic history of the rock. It is noted that tentative polar paths constructed from results obtained after inadequate experimental work cannot be up-graded by adding more data points of doubtful or unproven quality. The evolutionary process of polar paths is dependent upon increasing the reliability of palaeomagnetic results. 相似文献
To better understand the mechanisms relating to hydrological regulations of chemical weathering processes and dissolved inorganic carbon (DIC) behaviours, high-frequency sampling campaigns and associated analyses were conducted in the Yu River, South China. Hydrological variability modifies the biogeochemical processes of dissolved solutes, so major ions display different behaviours in response to discharge change. Most ions become diluted with increasing discharge because of the shortened reactive time between rock and water under high-flow conditions. Carbonate weathering is the main source of major ions, which shows strong chemostatic behaviour in response to changes in discharge. Ions from silicate weathering exhibit a significant dilution effect relative to the carbonate-sourced ions. Under high temperatures, the increased soil CO2 influx from the mineralisation of organic material shifts the negative carbon isotope ratios of DIC (δ13CDIC) during the high-flow season. The δ13CDIC values show a higher sensitivity than DIC contents in response to various hydrological conditions. Results from a modified isotope-mixing model (IsoSource) demonstrate that biological carbon is a dominant source of DIC and plays an important role in temporal carbon dynamics. Furthermore, this study provides insights into chemical weathering processes and carbon dynamics, highlighting the significant influence of hydrological variability to aid understanding of the global carbon cycle. 相似文献
Remote sensing images of AD 1991–2011 and field observations help evaluate shoreline changes (erosion and accretion) in Puducherry and Tamil Nadu states of southeastern India. A minor harbor was constructed during AD 1986–1989 in the coast of Puducherry, and it initiated the gradual process of shoreline modification. In the subsequent years, beaches located toward the north of the harbor suffered erosion (?0.12–?4.19 m/year) and there was accretion (0.27–7.25 m/year) in the southern beaches. However, the man-made structures (seawall and groin) have reduced the shoreline changes after AD 2004. In the last two decades, the rate of erosion area-wise gradually decreased (0.24–0.013 km2/year) and accretion remained constant (0.019 km2/year). Our results suggest that accretion happened in the southern side of the breakwaters and erosion occurred in the northern part. Presence of groins structures in the region in the northern part has also provoked accretion in the south and erosion in the northern side close to the State of Tamil Nadu.
We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia
that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi
River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced
since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and
suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf
of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring
through late summer would ensure that the metric is representative of the system in any given year and useful in a public
discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters,
and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi
River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines
of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased
eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately,
bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses
on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern
Gulf of Mexico. 相似文献
Lake water levels change under the influences of natural and/or anthropogenic environmental conditions. Among these influences are the climate change, greenhouse effects and ozone layer depletions which are reflected in the hydrological cycle features over the lake drainage basins. Lake levels are among the most significant hydrological variables that are influenced by different atmospheric and environmental conditions. Consequently, lake level time series in many parts of the world include nonstationarity components such as shifts in the mean value, apparent or hidden periodicities. On the other hand, many lake level modeling techniques have a stationarity assumption. The main purpose of this work is to develop a cluster regression model for dealing with nonstationarity especially in the form of shifting means. The basis of this model is the combination of transition probability and classical regression technique. Both parts of the model are applied to monthly level fluctuations of Lake Van in eastern Turkey. It is observed that the cluster regression procedure does preserve the statistical properties and the transitional probabilities that are indistinguishable from the original data. 相似文献
Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO2). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87Sr/86Sr and 143Nd/144Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low 87Sr/86Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments. 相似文献