A transport-reaction model was designed to identify the combination and importance of biogeochemical processes operating in four sites drilled during ODP Leg 207 (Demerara Rise, Equatorial Atlantic). Almost 100 Ma after their deposition, deeply buried Cretaceous black shales still act as active bioreactors in great sediment depths and control the biogeochemical reaction network of the whole sediment column. According to a model calibrated at the four drill sites through inverse modeling techniques, methanogenesis could be identified as a key process that dominates not only organic matter degradation but also sulfate availability through the anaerobic oxidation of methane above the black shales. A complete depletion of sulfate within the black shale sequences promotes the remobilization of biogenic barium that reprecipitates as authigenic barite at the top of the sulfate depletion zone. Temporal dynamics of degradation processes caused continuous shifts of the barite precipitation zone during burial, thus inhibiting the formation of an authigenic barite front or causing the dissolution of earlier formed fronts. Major deviations of pore water sulfate profiles from a linear gradient coincide with depths of decelerated or accelerated transport caused by local porosity minima or maxima. Model-determined reaction rates are by far lower than those found in shallower sediments due to the low metabolic activities that are characteristic for the Deep Biosphere. But even after almost 100 Ma, changing organic matter quality still influences the degradation within the black shale sequences, as it is indicated by model results. 相似文献
Integer GNSS ambiguity resolution involves estimation and validation of the unknown integer carrier phase ambiguities. A problem
then is that the classical theory of linear estimation does not apply to the integer GPS model, and hence rigorous validation
is not possible when use is made of the classical results. As with the classical theory, a first step for being able to validate
the integer GPS model is to make use of the residuals and their probabilistic properties. The residuals quantify the inconsistency
between data and model, while their probabilistic properties can be used to measure the significance of the inconsistency.
Existing validation methods are often based on incorrect assumptions with respect to the probabilistic properties of the parameters
involved. In this contribution we will present and evaluate the joint probability density function (PDF) of the multivariate
integer GPS carrier phase ambiguity residuals. The residuals and their properties depend on the integer estimation principle
used. Since it is known that the integer least-squares estimator is the optimal choice from the class of admissible integer
estimators, we will only focus on the PDF of the ambiguity residuals for this estimator. Unfortunately the PDF cannot be evaluated
exactly. It will therefore be shown how to obtain a good approximation. The evaluation will be completed by some examples. 相似文献
Most monazite reference materials (RMs) for in situ U‐Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix‐matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low‐U and low‐Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U‐Pb ID‐TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA‐(Q, SF, MC)‐ICP‐MS and SIMS) are within uncertainty of the ID‐TIMS data. U‐Pb LA‐(Q, MC)‐ICP‐MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA‐MC‐ICP‐MS analyses yielded homogeneous Sm‐Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and εNd(495 Ma) of ?18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng. 相似文献
Aquatic Geochemistry - Mountains are among the most sensitive ecosystems to climate change, and one of the most evident signs of climate-related effect is the continuous net loss of ice from the... 相似文献
Surface ocean currents are often of interest in environmental monitoring. These vectorial data can be reasonably treated as a finite realization of a complex-valued random field, where the decomposition in modulus (current speed) and direction (current direction) of the current field is natural. Moreover, when observations are also available for different time points (other than at several locations), it is useful to evaluate the evolution of their complex correlation over time (rather than in space) and the corresponding modeling which is required for estimation purposes. This paper illustrates a first approach where the temporal profile of surface ocean currents is considered. After introducing the fundamental aspects of the complex formalism of a random field indexed in time, a new class of models suitable for including the temporal component is proposed and applied to describe the time-varying complex covariance function of current data. The analysis concerns ocean current observations, taken hourly on 30 April 2016 through high frequency radar systems at some stations located in the Northeastern Caribbean Sea. The selected complex covariance model indexed in time is used for estimation purposes and its reliability is confirmed by a numerical analysis.
Mathematical Geosciences - Recent years have seen a steady growth in the number of papers that apply machine learning methods to problems in the earth sciences. Although they have different... 相似文献
The spatial and temporal patterns within the surf zone epibenthic assemblages were studied in a coastal fringe of Argentina to determine whether assemblage compositions, abundance, species richness and diversity vary spatially and temporarily. Sampling was conducted seasonally in two sandy beaches over 2 years with a benthic sledge used to collect the fauna in the upper centimeters of soft bottom sediments and the epifauna on the sediment surface. Physical variables were measured in the same coastal sites where biological sampling was conducted. A total of 58 morphospecies were collected. Peracarid crustaceans were the most abundant group. The mysid Pseudobranchiomysis arenae (new genus–new species) (29.73 ± 17.79 ind. per sample) and the isopod Leptoserolis bonaerensis (51.54 ± 22.35 ind. per sample) were the most abundant and common species and were present regularly throughout the sampling period. Differences in the surf zone community composition were found between the beaches; these differences could be related to variation in physical parameters such as sand grain size and wave climate, indicating the possible influence of the morphodynamic state of the beaches on the epibenthic assemblages. A seasonal abundance trend was detected, reflecting the changes in abundance of the two dominant species; the richness pattern was not easily detectable due to the sporadic appearance of non‐resident species in the surf zone, probably due to different causes, including dispersion by entry of water from surrounding areas, littoral currents and storms. The surf zone studied presents a complex and dynamic epibenthic community that appears to be influenced by the morphodynamic state of the beach and the dynamic of non‐resident species. 相似文献
The spatial and temporal distributions of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC)
was studied in the East-Frisian Wadden Sea (Southern North Sea) during several cruises between 2002 and 2005. The spatial
distribution of CDOM in the German Bight shows a strong gradient towards the coast. Tidal and seasonal variations of dissolved
organic matter (DOM) identify freshwater discharge via flood-gates at the coastline and pore water efflux from tidal flat
sediments as the most important CDOM sources within the backbarrier area of the Island of Spiekeroog. However, the amount
and pattern of CDOM and DOC is strongly affected by various parameters, e.g. changes in the amount of terrestrial run-off,
precipitation, evaporation, biological activity and photooxidation. A decoupling of CDOM and DOC, especially during periods
of pronounced biological activity (algae blooms and microbial activity), is observed in spring and especially in summer. Mixing
of the endmembers freshwater, pore water, and open sea water results in the formation of a coastal transition zone. Whilst
an almost conservative behaviour during mixing is observed in winter, summer data point towards non-conservative mixing. 相似文献
Solar Physics - The Maunder Minimum (1645?–?1715) was unique in terms of solar-cycle amplitudes and sunspot-position distributions registered in the last four centuries; however,... 相似文献