Dating horizons

Date et al. (1983) found the existence of an Na20-depleted dacite mass with a lateral dimension of 1.5 x 3.0 km immediately below the ore horizon (Figs. 1.23 and 1.24) and the mass is useful indicator of exploration of Kuroko ore deposits. This Na20 depletion is considered to be due to the destruction of plagioclase attacked by potassium-... 

There are a few developments on the horizon that will increase our ability to date bones and teeth reliability. Both y- and a-spectrometric methods can measure Pa/ U and °Th/U and concordance between dates calculated using the two can provide a measure of reliability. However, the discordance between the two is not very sensitive to different uptake regimes, and it is difficult to resolve, for example, bones that have undergone EU from those that have undergone LU with the analytical errors commonly encountered in measurements by y- and a-spectrometry. On the other hand, it has been shown recently that TIMS can measure both isotopic ratios with a precision usually better than 1% (Edwards et al. 1997). TIMS measurements of Pa/ U and °Th/U have yet to be routinely applied to dating fossil remains, but in the future, concordance between the two decay series will provide further evidence of the validity of a particular uptake model to a particular sample. 

Needless to mention, the exact capturing of time presents further challenges in the analysis. Fundamentally, a decision has to be made on how the time horizon has to be represented. Early methods relied on even discretization of the time horizon (Kondili et al., 1993), although there are still methods published to date that still employ this concept. The first drawback of even time discretization is that it inherently results in a very large number of binary variables, particularly when the granularity of the problem is too small compared to the time horizon of interest. The second drawback is that accurate representation of time might necessitate even smaller time intervals with more binary variables. Even discretization of time is depicted in Fig. 1.8a. 

Glaciochemical horizons are intervals of core with substantially higher or lower than average concentrations of certain chemical constituents. If a historical event of known age can be correlated with the event horizon in the core, the assigned age of that interval can be used to confirm the depth-age relationship which has been determined from seasonal variations or other dating methods. In addition, in deep ice where annual layers are too thin to count seasons reliably and dating is only possible by model calculations [15,30], these horizons provide check points for calculated ages. 

The application of glaciochemical horizons to ice core dating is a recent and rapidly evolving development. A concerted search is under way to determine the impact of known events on the chemical composition of polar snow. So far, two types of universally applicable horizons have been detected in polar ice cores volcanic eruptions and Late Wisconsin dust. 

Other events, evidence of which has not yet been detected in polar ice, may also eventually serve as useful time markers in the future. Large meteors or meteor swarms that ablated in the upper atmosphere may have left a significant chemical impact on succeeding snowfalls. The same may be true for cometary collisions or brushes with a comet s tail. As with ancient volcanic eruptions, such events need not have been historically documented. If identified and accurately dated in one ice core, these events will serve as useful time horizons in other cores. 

Glaciochemical horizons not only provide checks on depth-age relationships but are of great intrinsic interest. Continuous chemical analysis of a well-dated core promises to greatly extend and refine the record of explosive volcanism [13]. Each well-dated eruption or other horizon then becomes of use as a time marker for cores where the dating is less precise. 

Demand patterns can also vary significantly ranging from cases where due dates must be obeyed to cases where production targets must be met over a time horizon (fixed or minimum). Changeovers are also a very important factor, which is particularly critical in cases of transitions that are sequence dependent on the products, as opposed to simple setups that are only unit dependent. 

We can then conclude that while the discrete time STN and RTN models are quite general and effective in monitoring the level of limited resources at the fixed times, their major weakness in terms of capability is the handling of relatively small processing and changeover times. Regarding the objective function, these models can easily handle profit maximization (cost minimization) for a fixed time horizon. Intermediate due dates can be easily modeled. Other objectives such as makespan minimization are more complex to implement since the time horizon and, in consequence, the number of time intervals, are unknown a priori (see [11]). 

We can conclude that the continuous-time STN and RTN models based on the definition of global time points are quite general. They are capable of easily accommodating a variety of objective functions such as profit maximization or makespan minimization. However, events taking place during the time horizon such as multiple due dates and raw material receptions are more complex to implement given that the exact position of the time points is unknown. 

Dated sediment layers (1939-1991) of a small German river were studied for their LAS concentrations and the gathered data were related to political, economic and technical developments in Germany [14]. As expected the sediments corresponding to the period 1939-1945 contained no LAS, since at that time soap was the most important surfactant used for laundry washing. A significantly different picture was obtained in sediments from 1949 to 1951 when a particular LAS was used along with soap. The values determined in the two core samples amounted to 1.4 and 1.3 mg kg-1, respectively. In the sediment horizons from 1954 to 1956 no LASs were detected, which is in accordance with the fact that the less expensive branched alkylbenzene sulfonate (ABS) had replaced the linear one. In the years from 1959 to... 

The footwall units below the ore horizon include the Watson Lake Rhyolite and the Bell River Complex intrusion. Both have U-Pb dates of 2724 Ma (Mortensen 1993), and the Bell River Complex is considered to the heat source which drove the hydrothermal circulation that created the ore deposits. 

Figure 2. left, reed-impressed, bitumen-encrusted, mudbrickfragment (AK) from Ali Kosh right, pottery fragment (AK1) recoveredfrom the Mohammad Jcffar horizon at Ali Kosh, dated to 7100 B.C. 

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