In Search of a Key Paper

by Moya K. Mason

Using the GeoRef database in Dialog Select on the Internet, I did a subject search for Chicxulub Crater, specifying articles in English, without a restriction on year. I got a total of forty seven hits. The last, and therefore, oldest one, was an article by Alan R. Hildebrand and associates, called Chicxulub Crater: A Possible Cretaceous/Tertiary Boundary Impact Crater on the Yucatan Peninsula, Mexico. The citation is as follows:

Hildebrand, Alan R.; Penfield, Glen T.; Kring, David A.; Pilkington, Mark; Camargo Z.; Jacobsen, Stein B.; Boyton, William V. 1991. Chicxulub Crater: A Possible Cretaceous/Tertiary Boundary Impact Crater on the Yucatan Peninsula, Mexico. Geology, Vol. 19, No. 9, pp. 867-871.

Since this paper was the oldest one found in the database, it was believed to be a key paper on the subject. This was later confirmed by consultation with someone who is quite knowledgeable in scientific matters. Key papers are important because they "follow the history or methodology of an idea from its first communication to the present, locating current research based on earlier published works or findings, create a list of experts within a field of research, and analyze the impact of published research" (Katz 1987, 9).

The Importance of the Hildebrand Impact Crater

Until 65 million years ago, dinosaurs lived on this planet; within a short period of time they were extinct. Not only did the dinosaurs suddenly disappear, an examination of a variety of fossils show that 70% of all living species, both plant and animal, disappeared along with them. A mass extinction took place, resulting in the death of most living creatures. The death of the dinosaurs has always been one of the greatest mysteries to face scientists. Many have speculated about why they disappeared.

A very controversial theory has suggested that the destruction of living species 65 million years ago was triggered by the collision of a huge asteroid with the Earth, which would have created a crater at least 150 km wide. An impact of this magnitude would have caused a tremendous amount of energy to be generated within a short period of time, along with acid rain, incineration, greenhouse warming, cold temperatures, and darkness - a deadly combination. Evidence for such an occurrence has been deposits of "iridium-rich clay found at the boundary of the Cretaceous and Tertiary deposits known as the K/T Boundary" (Hildebrand 1991,868).

During an aerial survey of the region in 1978, Pemex, Mexico's national oil company, located what it believed was a buried impact crater nearing 180 km. in diameter close to the town of Chicxulub. For unknown reasons, this information was not passed along to scientists actively looking for such a site, nor to the scientific world, until 1990. In 1981, the discovery was announced at a petroleum geology conference, but the information was not given to the K/T Boundary researchers. By 1991, Alan Hildebrand and his associates from the University of Arizona, Harvard University, and the Geological Survey of Canada had concluded that:

The Chicxulub Crater is the largest probable impact crater on Earth. Its position and target rock composition satisfy many of the characteristics required for the K/T crater, and it may have a K/T boundary age. This impact may have caused the K/T extinctions (Hildebrand 1991,871).

A Search of Science Citation Index

Since 1991, Hildebrand has continued to work on the mystery surrounding the Chicxulub Crater. Many other scientists have also used the Hildebrand research to conduct experiments of their own. To find out how often Chicxulub Crater: A Possible Cretaceous/Tertiary Boundary Impact Crater on the Yucatan Peninsula, Mexico has been cited by others doing research, I used SCI or SciSearch. The database includes all the cited references (footnotes and bibliographies) for each article it indexes. When I originally did the search using the online version of Dialog Select, only forty four articles were found that cited the 1991 paper, most of them from the field of microbiology. The results were surprising since only one of the papers was on topic. I began the process all over again, this time using Dialog at the local university. The numbers changed dramatically, providing 120 citations. Although the 1991 paper is a seminal piece of work, it has not been identified as a SCI Citation Classic, nor a SCI Research Front.

By retracing the search steps that led to the incorrect answer, I perused the help files and realized that the online version of Dialog Select provided incorrect instructions, telling users to insert the first three letters of an author's first name into a search, rather than the author's initials. Using the initials A.R., I tried again, and the numbers went from 44 to 120. Why those incorrect instructions were posted on the website is unknown, but they caused me to retrieve a batch of citations that were off topic, and obviously not in any way connected to Alan R. Hildebrand. With the new results in hand, the next step was to find out how many of the citing articles were subject-related.

By looking at the abstracts for all 120 citing articles, there was no question that each of them was subject-related and on topic. Under certain circumstances, a cited reference search might yield articles that are off topic or of little use. Authors might use one small aspect of a study in their own work - something that relates to the study at hand, or simply refer to it in passing. Either way, it must be cited. For example, a researcher may be studying specialized stratigraphy in the Southern Hemisphere and refer to the area around Chicxulub, Mexico, using some findings from the work done by Hildebrand and associates. Of course, the paper has nothing to do with asteroids hitting Earth millions of years ago, but would undoubtedly come up in a SciSearch. The citing of articles of a tertiary manner is unavoidable, and sometimes results in the retrieval of unwanted materials.

Additionally, there is the problem of padding bibliographies, which is endemic within certain fields of study. This means that certain authors will cite papers in their references, even though they have not actually read the articles or incorporated the concepts contained in them. Authors will sometimes do this to make an article look more academic or prestigious, by citing important authors or having a large number of citations.

The only other disadvantage of using SCI instead of a conventional subject index is the cost. When compared to other databases, SCI is extremely expensive and should only be used after considering the financial expenditure.

As previously stated, the articles citing Hildebrand's 1991 paper were on topic. What follows is a small sample of the titles to prove the point.

1. Size and Morphology of the Chicxulub Impact Crater.

2. Re Os Isotope Systematics as a Diagnostic Tool for the Study of Impact Craters and Distal Ejecta.

3. Extraterrestrial Impact Events: The Record in the Rocks and the Stratigraphic Column.

4. Energy, Volatile Production, and Climatic Effects of the Chicxulub Cretaceous/Tertiary Impact.

5. When the Sky Fell on Our Heads: Identification and Interpretation of Impact Products in the Sedimentary Record.

6. Magnesioferrite Spinel in Cretaceous/Tertiary Boundary Sediments of the Pacific Basin Remnants of Hot, Early Ejecta From the Chicxulub Impact.

7. Impact of Meteorites.

8. Models and Causes of Mass Extinction.

9. Impact a Natural Hazard in Planetary Evolution.

10. Impact Winter and the Cretaceous/Tertiary Extinctions Results of a Chicxulub Asteroid Model.

11. Yucatan Karst Features and the Size of Chicxulub Crater.

12. Cretaceous Tertiary (Chicxulub) Impact Angle and its Consequences.

13. Chicxulub, the Ideal Crater.

14. Target Earth: Evidence for Large Scale Impact Events.

15. Did the European Dinosaurs Disappear Before the K/T Event? Magnetostratigraphic Evidence.

Discussion of Results: Advantages of Citation Indexing in Comparison to Subject Indexes

Assessing the quality of scientific publications has long been a major problem - the age old struggle between quantity and quality. The academic community had no choice but to view prolific writers as creators of quality work. That was until the introduction of the Science Citation Index (SCI), a reliable tool that measures the significance of individual scientists' contributions by counting the number of times they are cited by other researchers. Citation indexing is a method of organizing documents in a way that overcomes many of the shortcomings of more traditional indexing methods. SCI lists all bibliographic references appearing in an increasingly large number of journals, allowing the number of citations an individual receives to be tabulated and used as an indicator of the relative scientific significance or quality of that individual's publications.

The primary advantage of citation indexing is that it identifies relationships between academic publications that are often overlooked in a subject index. A second important advantage is that the compilation of citation indexes is especially well suited to the use of machine indexing methods that do not require indexers who are subject specialists (Katz 1987,171). Furthermore, citations, which are bibliographic descriptions of documents, tend to bring together material that would never be collated by subject indexing, because they "are not vulnerable to scientific and technological obsolescence as are the terms used in subject indexes" (Katz 1987,172). A traditional topic search is dependent on your knowledge of the field's vocabulary and on how well you formulate your search statement. A cited reference search leads you to relevant data through the experts themselves. Since this type of search is not language dependent, it will reveal changes in terminology and phrases used in your area of research (Katz 1987,174).

Studies have shown that a citation search yields relevant results that could not be found by conducting a subject search. It overrides the need for indexers to provide exact terms of controlled vocabulary to connect papers, a practice that has its share of pitfalls. Instead, related articles are retrieved via cited reference searching. For instance, a search for citations to journal articles allows you to perform more comprehensive searches of the literature and retrieve all information relevant to a particular subject, in spite of how limited your initial knowledge might be on the topic. You may know the name of a researcher who published an important article on a subject several years ago, but are not sure how to find more current research on the same topic. By using the author's name, a cited reference search can lead you to recent papers and everything that has been published since that one known article. Citation indexing is a very powerful tool that allows you to move forward in time. SCI can also be helpful to the scholar interested in finding criticisms of their work. This can be done by discovering who is referencing the information and how it is being used to support current research. The citation index makes this possible.

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Katz, William A. 1987. Introduction to References Work. New York: McGraw Hill Book Company.

Citation Data is Subtle Stuff a Primer on Evaluating Scientist's Performance. April 6, 1987. The Scientist, Vol. 1, No. 10, p. 9.

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