Implicit dates

Bones and teeth, however, are primary archaeological materials and are common to many archaeological sites. Bones bearing cut marks from stone tools are a clear proxy for human occupation of a site, and in the study of human evolution, hominid remains provide the primary archive material. Hence, many attempts have been made to directly date bones and teeth using the U-series method. Unlike calcite, however, bones and teeth are open systems. Living bone, for example, contains a few parts per billion (ppb) of Uranium, but archaeological bone may contain 1-100 parts per million (ppm) of Uranium, taken up from the burial environment. Implicit in the calculation of a date from °Th/U or Pa/ U is a model for this Uranium uptake, and the reliability of a U-series date is dependent on the validity of this uptake model. 

When a century is imputed by software, it is usually done based on something called a pivot point. The pivot point in SAS for determining the 100-year window is based on the YEARCUTOFF option, which is set by default to 1920. Let s look at an example to see what this means. The following SAS code uses both implicit and explicit century dates right at the YEARCUTOFF pivot point of 1920. 

Program 4.11 Using Implicit or Explicit Centuries with Dates... 

DATES DEFINED WITH IMPLICIT CENTURY data null ... 

Note that the implicit century DATA step produces dates in two different centuries, but that when you explicitly state the century there is no subsequent century confusion. You can define YEARCUTOFF as needed for your specific SAS applications, but it is wise to have a reasonable system-wide YEARCUTOFF default set. Also, if you use explicit centuries whenever possible, you minimize the implicit century risk. 

As an aside, Microsoft Excel s pivot-point year is 1930 by default. So, if you enter an implicit century date in Excel such as 01/01/29, it understands that as 01/01/2029, but 01/01/30 is understood as 01/01/1930. This is useful to know when implicit century data pass through Excel into SAS in some fashion. 

The simple definition of biological availability as the fraction of the total trace metal available for uptake by the biota implicitly assumes that all relevant organisms will have similar uptake characteristics. In addition the use of biological availability as a chemical parameter involves the assumption that it can be identified with particular chemical species or groups of species. Perhaps surprisingly many of the studies carried out to date have lent support to these assumptions, though the detailed mechanisms of the uptake processes remain unclear. Some of the inorganic chemical species identified as... 

As noted above, the SQT originally focused on sediment chemistry, toxicity, and community structure. It has been expanded to include bioaccumulation, sediment stability, CBR and TIE analyses. Schmidt et al. (2002) have provided a means for inclusion of data on habitat. Habitat is a key factor controlling the distributions of biota, which has been implicitly but not explicitly considered to date in the SQT (e.g., via measurements of sediment grain size and TOC). Habitat should be explicitly considered as part of the SQT in future. 

A wide variety of dynamical approximations have been applied to cluster dynamics and kinetics. Most calculations to date are based on simplified potentials and classical mechanics or statistical methods. In the near future, we can expect to see more work with detailed potential energy surfaces (both analytic and implicitly defined by electronic structure calculations) and progress in sorting out quantum effects and treating them more accurately. 

In fact, almost all polymer EL experiments to date have been interpreted in terms of the recombination of electrons injected from the cathode with holes injected from the anode, that is, a situation of double injection in an insulator, implicitly assuming that the carriers are not heated by the applied... 

The nearness of a major concrescence to our own time seems a self-evident fact, implicit in the excesses of all sorts that characterize the history of the twentieth century. Two methods for computing possible dates of a future concrescence have been found suggestive. The first is simple and involves simply propagating 67-plus—year cycles and their subsets forward in time, with various shifts in the alignment of the scales. The software... 

The chemical details of the initiation reaction can vary from one stem to another. Thus the carbenium ion can add on to the monomer double bond in a classical electrophilic reaction or it can abstract a hydride ion or an electron from the monomer. The first process seems to be the most common according to the experimental evidence gathered to date. Whatever the nature of this interaction, it remains to be established if the active ecies formed from them are also ionic and if so, if they are predcminantly free ions or ion pairs. This is undoubtedly the most delicate problem implicit in studies of this type. It has been often assumed that since the initiator was essentially composed of free ions in the low concentrations used it would follow that the same would apply to the drain carriers derived from it. However, it has become apparent that this asmmption is generally unjustified since the stability of the carbenium ions formed from the mono-... 

For the free energy of solvation calculation, however, it is difficult to discern the most accurate method. Recently, there have been numerous publications exploring the use of the cluster continuum method with anions. With regard to implicit solvation, there are no definite conclusions to the most accurate method, yet for the PB models the conductor-like models (COSMO CPCM) appear to be the most robust over the widest range of circumstances [23]. At this writing, the SMVLE method seems to be the most versatile, as it can be used by itself, or with the implicit-explicit model, and the error bars for bare and clustered ions are the smallest of any continuum solvation method. The ability to add explicit water molecules to anions and then use the implicit method (making it an implicit-explicit model) improves the results more often than the other implicit methods that have been used in the literature to date. 

---