Biostratigraphy and Geochronology in the 21st Century
M. D. Simmons, W. A.Berggren, R. O. Koshkarly, B. J. O'Neill,
R. W. Scott, and W. Ziegler
This paleontological theme deals with two related but distinct subject
areas--biostratigraphy and geochronology. Since the focus of this meeting
is paleontology, with regard to geochronology we concentrate on its relationship
Biostratigraphy and geochronology underpin much of geoscience. No matter what aspect of geology one is working on, the most common question posed by geologists is "what age is it"? Biostratigraphy and geochronology provide the framework for answering that question.
Biostratigraphy is the study of the temporal and spatial distribution of fossil organisms. The limited stratigraphic range of many fossil taxa is used for correlation, typically by means of biozonation schemes (i.e. intervals characterized by a species or group of species). When interpolated with numerical age information derived from radiometric dating, biozones and the divisions of the geological time scale containing biozones (periods, epochs, stages, etc.) can be given chronological values, a process sometimes referred to as biochronology.
Alongside the interpretation of depositional environments, biostratigraphy is one of the products of paleontological studies with which non-paleontological geoscientists are most familiar. Indeed, in these days of job insecurity, many paleontologists now refer to themselves as biostratigraphers, in an effort to demonstrate the applied nature of their work. As noted above, biostratigraphy and geochronology are fundamental building blocks of many wider-ranging geoscience studies. They provide the common language of geology, which enables the dating and correlation of rocks, whether this is at a global scale, between basins, within a basin or within an oil field. They provide an understanding of the duration and periodicity of geological events (e.g. the rate of sediment accumulation) and the framework to study the process of evolution. Biostratigraphy and geochronology are fundamental aspects of all geological training programs in education, with all courses providing some tuition in the construction and interpretation of the geological time scale.
Despite the powerful and wide-ranging implications of biostratigraphic and geochronological studies, research into these areas has suffered from underfunding in recent years. There appears to be a miscomprehension that "biostratigraphy is all done" (Simmons & Lowe, 1996), meaning that all fossils are described, their idenity clearly established, their stratigraphic ranges understood and there being a stable and precisely dated geological time scale. Whilst biostratigraphers continue to strive to reach this elusive goal, there remain many significant gaps in our understanding. Some biozonation schemes, especially those utilizing planktonic or nektonic fossil groups (e.g. ammonites, planktonic foraminifera, nannofossils, conodonts) are approaching a degree of consensus and stability (e.g. Bolli et al., 1985; Bralower et al., 1995, Sandberg & Ziegler,1996). With reference to microfossils this in part relates to the immense dataset collected during the DSDP and ODP cruises. However, other schemes, especially those utilizing benthonic fossil groups, often contain a high degree of uncertainty related to stratigraphic ranges being revised by new records, improved dating and taxonomic revision. The resolution of biozonation schemes can vary markedly - zonation schemes based on pelagic fossils for the Cretaceous or Jurassic can have a resolution better than 0.3my (Cope, 1993), whilst zonations in the same intervals based on some of the common groups of benthonic fossils have resolutions in the order of several million years. Can we do much about this? The answer is probably yes, but only with further detailed taxonomic work. For example, detailed work on conodonts (Ziegler & Sandberg, 1994; Sandberg & Ziegler, 1996) has shown what is possible-zonal resolution of 0.1my or less is possible for Devonian conodont-based biozonation schemes.
If biostratigraphy and geochronology can be said to underpin geology,then taxonomy can be said to underpin biostratigraphy. An essential prerequisite of any biostratigraphic study is a good taxonomic understanding of the fossils involved (Cope, 1993; Loydell, 1993; Simmons & Lowe, 1996). However, taxonomic studies are in serious decline, perhaps because otheir benefits remain hideen or perhaps because there is a belief that they too, "are all done". There is no doubt that much work remains to be done to establish the idenity and stratigrpahic range, not only of more obscure fossil groups, but of those also most commonly used in biozonation and correlation such as the planktonic foraminifera. Biostratigraphy can only be as good as the taxonomy on which it is built.
Notwithstanding the remarks above, we can take pleasure in the fact that for many geological time periods, we are approaching a reasonable knowledge of the stratigraphic distribution of many fossil groups and have developed reasonably stable regional and inter-regional biozonation schemes using these groups. This appears to be particularly true for the planktonic and nektonic fossil groups. However, there often remains some uncertainty in the way in which zonation schemes for different fossil groups relate to each other and the geological time scale. Consider, for example the issues surrounding the Campanian - Maastrichtian boundary. It is now clear that the standard zonation scheme using planktonic foraminifera across the Campanian - Maastrichtian boundary needs to be recalibrated against the ammonite standard and other definitions (Simmons et al., 1996). Although published time scales (e.g. Haq et al., 1988; Harland et al., 1990) show the extinction of the planktonic foraminifera Globotruncanita calcarata to be a proxy for the Campanian - Maastrichtian boundary, various studies (Schonfeld & Burnett, 1991; Hancock et al., 1992; Obradovich, 1993) have shown that this extinction probably predates the boundary by between 1.5 - 3My. Clearly many sediments regarded as Early Maastrichtian on the basis of planktonic foraminiferal biostratigraphy will have to be reassessed as Late Campanian!
In some regions of the world (e.g. the Neogene Paratethyan realm), localized stratigraphic nomenclature has been developed because of the presence of endemic faunas. Distinctive facies and isolation from the world ocean. The correlation of these localized nomenclatures with the standard geological time scale is often still far from certain (see Jones & Simmons, 1996) for a review of Paratethyan stratigraphic issues). Calibration using a combination of magnetostratigraphy, radiometric dating, relative sea-level change and, where possible, plankton stratigraphy, is required. Inter-regional correlation is not only inhibited in regions where basins are extremely isolated. Provincialism in planktonic and nektonic organisms has been widespread during many periods of the Earth's history, thus precluding ease of correlation. Hancock et al. (1993) have succinctly illustrated this problem with reference to the Late Cretaceous ammonite biostratigraphy for the Western Interior of the USA and Northwest Europe, where the correlation between the separate zonation schemes for the two areas remains problematic. However, problems such as this should not be viewed as insurmountable and to the detriment of biostratigraphy. They can be overcome by taking an integrated approach - in the case of the Cretaceous example cited, by also taking into account other fossil groups, using Milankovitch cyclicity, relative sea-level change, and, where available, radiometric dates.
All major geological eras have had working groups of geoscientists investigating their subdivision in the last two decades. Given the inadequacy of many stage stratotypes in terms of their faunal paucity and stratigraphic completeness, particular emphasis has been placed on the definition of stage boundaries and the location of stage boundary stratotypes (GSSPs - Global Stratotype Sections and Points). Despite the enormous difficulties in agreeing on the definition of a stage boundary caused by uncertainty in the ranges of fossil groups, paleogeographic provincialism, political and personal preferences and pressures, many working groups are now approaching some sort of consensus about the definition of many stage boundaries. For example, the Devonian Subcommission has reached agreement on the recognition of all Devonian stage boundaries using conodont bioevents. The working groups on Cretaceous stage boundaries, after a series of meetings and exchange of ideas, are approaching consensus over the biostratigraphic definition of each stage boundary within the Cretaceous, and the denotation of each stage boundary within the Cretaqceous, and the location of a boundary stratotype. It is intended to publish the final decisions at the International Geological Congress to be held in Rio Di Janeiro in 2000. Refinements in radiometric dating allow for continuing improvement in the precision and accuracy with which these stage boundaries can be assigned a numerical chronostratigraphic value.
The advent of sequence stratigraphy has had a major impetus in furthering biostratigraphic studies (Simmons & Williams, 1992; Armentrout, 1996; Emery & Myers, 1996). This is for two reasons. Firstly, the framework of sequence stratigraphy is time. Essentially this means that sequence stratigraphic studies require a biostratigraphic framework in which to place the organization of sequence boundaries, maximum flooding surfaces and systems tracts. Attention has therefore focused on the development of biozonation schemes suitable for this purpose. Secondly, the global eustatic sea-level curves published by Haq et al. (1988) has led to the desire to locate synchronous relative sea-level changes in stratigraphic sections around the world, and, of importance to some workers, to relate these to the global eustatic curve. We might argue that such calibration is often beyond the precision of biostratigraphy (although see Johnson et al (1985); Owen (1996) for examples where calibration is possible), and in any case, local tectonically derived relative sea-level changes are likely to overprint any eustatic signal (for discussion see Aubry, 1991), but nonetheless, the desire for correlation and calibration of sea-level changes has led to a small renaissance in biostratigraphy.
In the early 1990's there was a marked downturn in the employment prospects for paleontologists, including biostratigraphers. The oil industry (the traditional means of employment for many micropaleontological biostratigraphers) recruited few new biostratigraphers, and major companies "outsourced" their biostratigraphic workforce, turning them into external contractors and effectively dispersing applied resaerch teams. At the same time paleontological positions in universities were being replaced with environmental geologists (but see Flessa & Smith, in this volume) and cutbacks in government meant that fewer biostratigraphers were being employed by geological surveys. However, as far as the oil industry is concerned it appears that the corner has been turned with regard to employment. The cyclical nature of the industry and an upswing in exploration generated by a stable oil price has resulted in the renewed need for biostratigraphic expertise. But more importantly, industrial biostratigraphers have refocused their efforts on assisting production and development rather than exploration, as oil companies concentrate on exploiting more of their existing reserves rather than discovering entirely new fields. This has required a pragmatic approach to biostratigraphy, developing biozonation schemes that, rather than using classical zonal indices, use local acme events and slight changes in assemblage characteristics to develop high resolution biozonation schemes, which are probably of very localized (i.e. field-wide) significance (Simmons & Lowe, 1996). Consequently, biostratigraphy has proved itself to be a major tool in developing an understanding of reservoir architecture and continuity, and in developing strategies to maximize production. At the same time, the high resolution biozonation schemes so developed, are particularly useful for wellsite work, especially if the wells are being drilled horizontally to maximize exposure of the well bore to the pay zone. In this case the well can be "biosteered" using pragmatic, high resolution biozonation schemes (Shipp & Marshall, 1995). The result of this (at least in the UK), has been an upturn in the demand for micropaleontologists/biostratigraphers, especially to carry out wellsite work.
Historically, the results of industrial biostratigraphy have not always been disseminated into the public domain, or between companies or contractors. This was perceived as a commercial advantage for the individual companies concerned. However, the real added-value of biostratigraphy is now seen to be in the interpretation of the data rather than from in-house taxonomy or biozonation schemes. Consequently, in the Gulf of Mexico, for example, biozonation schemes and "open" nomenclature are being standardized between companies and will be published in due course.
The demand from end-users of biostratigraphy (both within and without industry) for greater resolution and precision have resulted in an increasing trend for biostratigraphers to treat their data statistically and apply quantitative and semi-quantitative techniques. The recent SEPM special publication on graphic correlation (Mann & Lane, 1995) is a case in point. Quantitative techniques can be applied in a sequence stratigraphic sense to reinforce the power of this approach (e.g. Neal et al, 1994). Concomitant with the increased interest in quantitative biostratigraphy are advances in technology that enable biostratigraphic data to be manipulated more rapidly. This includes data management software such as StrataBugs and Ragware, and access to paleontological databases such as PalCat, PaleoVision and Compustrat. These technological innovations mean that biostratigraphers, often criticized for the speed of their data gathering and interpretation, can develop their interpretations faster.
The last few years have seen developments in non-paleontological methods of stratigraphic correlation (Dunay & Hailwood, 1995), particularly chemostratigraphic techniques such as the measurement of 86Sr/87Sr isotope ratios (e.g. Smalley et al., 1994), carbon and oxygen isotopes (e.g. Jenkyns et al., 1994; Mitchell et al., 1996; Vahrenkamp, 1996) and whole rock geochemistry (e.g. Racey et al., 1995). However, these techniques are still in their infancy, can have a degree of provenance control or be influenced by diagenetic alteration, and most importantly still require biostratigraphy to calibrate them. In any case, biostratigraphy remains a cost effective means of correlating and dating sediments, although we have to accept that as we move into the 21st Century the chemical signatures of sedimentary rocks will be increasingly used for the same purpose.
The 1990's have seen a continuing refinement and, at least for the Mesozoic and Cenozoic, a stabilization of the numerical scale of geological time beginning with Harland et al. (1990) and more recently, the publication of Berggren et al. (1995). The reasons for this partly lie in the gathering of more data with more precise techniques of radiometric measurement (e.g. Obradovich's (1993) high resolution 40Ar/39Ar laser fusion dates from bentonites of the Western Interior of the USA) and the introduction of techniques that can obtain precise radiometric dates from extremely small sample material such as SHRIMP (Sensitive High Resolution Ion Microprobe) (e.g. Roberts et al., 1995) and fission track (e.g. Hurford & Carter, 1991) analysis of zircon. The database of radiometric dates is continually being expanded, allowing older, spurious, data to be rejected and new statistical analysis of the spread of radiometric dates relevant to a given stratigraphic horizon or boundary to be undertaken to arrive at a consensus on the date to be ascribed. The increasing availability of glauconite based radiometric ages from sedimentary successions allows for easier interpolation with biostratigraphy (see Fischer & Gygi (1989) for an example), although the reliability of glauconite derived ages in sediments older than Late Cretaceous is questionable (Harland et al., 1990).
The continuing recognition of Milankovitch cyclicity (the periods of
orbital precession (~21ky), obliquity (~41ky) and eccentricity (~100ky
and ~400ky) in the geological record permits the development of an astrochronological
time scale giving a notable refinement to our knowledge of the duration
and dating of events within the geological time scale (for examples see
Gale, 1990; Einsele et al., 1991; Hilgen et al.,1993; Schwarzacher 1993;
Herbert et al., 1995; House & Gale, 1996; Sageman et al., 1997). For
example, in the Late Cretaceous in particular, recognition of Milankovitch
cycles, coupled with high resolution radiometric dates (Obradovich, 1993)
provides a very precise control for the chronostratigraphic calibration
of the biozonations for this time period (Gradstein et al., 1994, 1995).
A challenge for the future will be to expand the use of astrochronology
to other parts of the geological column. We only have to look to Pleistocene
studies (e.g. Weaver, 1993) to see what can be achieved by the integration
of biostratigraphy, Milankovitch cyclicity and stable isotope stratigraphy
- stratigraphic resolution in the order of 1-5ky is easily possible.
Needs And Aims For The Future
It is anticipated that the continuing demand from end-users of biostratigraphy and geochronology will be for continued refinement of biozonation schemes and time scales with improvements in accuracy, precision and the measurement of uncertainty (Kidd & Hailwood, 1993). With reference to biostratigraphy this will require (i) improvements in our taxonomic understanding of key fossil groups; and (ii), integration across fossil disciplines and with geochronology. It will be essential that we continue to develop our understanding of the identity and stratigraphic range of key fossil groups and integrate this knowledge to develop biostratigraphic schemes of the highest resolution. We need to continue to improve the geologic time scale by rigorous definition (GSSPs) of chronostratigraphic units calibrated by accurate radioisotope dating, and characterized by paleontological and geochemical (stable isotope) means. Milankovitch cyclicity can be used to provide supplemental information on the duration of stages and hence the dating of stage boundaries.
Both biostratigraphy and geochronology contain elements of uncertainty, yet this is not always appreciated by the end-user of our work, often leading to the accusation that we "have changed our minds" if at some point in the future we refine the information we have provided. Our geoscience colleagues need to be made acutely aware that numerical age values ascribed to the geological time scale will change; definitions of stage boundaries will change; and the calibration of biozonation schemes against the time scale will also change, as will the schemes themselves.
We cannot escape the fact that chemostratigraphic techniques will become increasing en vogue as a means of correlation and dating sedimentary rocks. What we must ensure therefore is that chemostratigraphic schemes are calibrated by, and integrated with, biostratigraphy and geochronology.
There is no doubt that the profile of biostratigraphy and geochronology can be raised if they can be applied to improving our understanding of changes in ecosystems and trends in environmental change and the effects of such changes. This is not just a matter of raising the profile of the subject - these are important issues for all of us. Studies on past climate change and sea-level change and sea-level change can be used to analyze future trends (Pirazzoli, 1996). For example, the Caspian Sea region is, following the break-up of the Soviet Union, becoming the focus for economic investment. Yet the level of the Caspian Sea is known to be rising rapidly, endangering buildings, infrastructure, homes and lives. Studies on past changes in the Caspian Sea level can help us to predict future trends and the magnitude of such changes. But studies of past climates and sea-levels require stratigraphic control - the role of biostratigraphy and geochronology.
If all the above aims are met, the value of biostratigraphy will be
appreciated by the wider geoscience community and recruitment and resourcing
of biostratigraphers and geochronologists will move in a positive direction.
This will be enhanced if biostratigraphers and geochronologists take a
proactive role in ensuring that their disciplines are being used to maximum
effect - this requires maintaining a broad knowledge of geology as well
as specialist skills, and a willingness to demonstrate the benefits of
the subject areas rather than waiting to be asked. Bearing this in mind,
we need to publicize the application our work has. Textbooks such as those
by Jenkins (1993) and Jones (1996) are therefore a step in the right direction.
Major Problems To Overcome
Worst Case Scenario
Biostratigraphy and geochronology continue to be underfunded and undervalued.
The existing knowledge base continues to be eroded. Biostratigraphers are
unable to refine and integrate their ideas because the fundamental taxonomic
base supporting the subject cannot progress. Geoscientists turn increasingly
to chemostratigraphy to answer their questions on correlation, despite
the value of such techniques being uncertain and uncalibrated by biostratigraphy.
The definition of stages and stage boundaries remains unresolved. No agreement
is reached between geochronologists on the accuracy and precision of time
scales. Those geoscientists remaining in biostratigraphy and geochronology
become increasingly isolationist and divorced from the mainstream of geoscience.
They are reactive; waiting to be asked the right questions.
Best Case Scenario
The value of biostratigraphy and geochronology is appreciated by the
general geoscience community, both within academia and industry. Resources
are put in place for refinements to be made in biostratigraphy by supporting
taxonomic research and allowing biostratigraphers of different fossil disciplines
to integrate their results and collaborate with geochronologists to accurately
date biozonations and the geological time scale. The clear utility of biostratigraphy
results in recruitment and training of specialists in the subject area,
allowing the knowledge base to be maintained and built upon. Chemostratigraphic
techniques are calibrated by, and integrated with, biostratigraphy and
geochronology. Biostratigraphers and geochronologists receive a broad training
and exposure to geology and can appreciate how their work impacts on that
of other geoscientists. They are proactive and attempt to ascertain how
their work can help others. Concern about present-day and future environmental
change means that by using "the past is the key to the present",
high impact studies into past climates and sea-level change proliferate
requiring stratigraphic control.
This manuscript benefitted from critical reviews by Mike Bridgood, Nigel
Trewin, Dave Owen, Marcelle DouDager-Fadel, Bob Jones and Steve Lowe.
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Ziegler, W. & Sandberg, C.A. 1994. Conodont phylogenetic-zone concept. Newsletters in Stratigraphy, 30, 105-123.
Dr. Mike Simmons--Topic Coordinator
Department of Geology
University of Aberdeen
Meston Building, King's College
Abderdeen,AB9 2UE, UNITED KINGDOM
Dr. W. A. Berggren
Woods Hole Oceanographic Institute
Woods Hole, MA 02543-1047 USA
Dr. Reyhan Koshkarly
APTI Geologic Services
10450 Stancliff Road
Houston, TX 77099
Dr. R. W. Scott
Precision Stratigraphy Associates
3734 So. Darlington
Tulsa OK 74135
Prof. W. Ziegler
Mr. Brian J. O'Neill
Shell Offshore Inc.
Rm: OSS-3208,P.O. Box 61933
New Orleans LA 70161 USA
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