NASA Technical Reports Server (NTRS) 19940023797: Science observed: The mass-extinction debates

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KT Event and Other Catastrophes
SCIENCE OBSERVED: THE MASS-EXTINCTION DE-
References: [I] Wolbach W. S. et al. (1985) Science, 230,
BATES. W. Glen, StanfordUniversity Press, Stanford CA 94305-
167-170. [2] Wolbach W. S. et al. (1988) Nature, 334,665-669.
2235, USE, and c/o Mail Stop 930, U.S. Geological Survey, 345
[3] Carlisle D. B. and Bramman D. R. (1991)Nature, 352,708-709.
Middlefield Road, Menlo Park CA 94025, USA.
[4] Gilmour 1.et al. (1992) Science, 258, 1624-1626. [5] Hough
i
R. M. et al. (1993) Meteoritics, 28, 364-365. [6] Venkatesan M. I.
and Dahl J. (1989) Nature, 338, 57-60. [7] Gilmour I.et al. (1990) The upheaval triggered in 1980 by the Alvarez-Berkeley group
GSA Spec. Paper247,383-390. [8] Meyers P.A. and Simoneit B. R. impact hypothesis transformed the literature of mass extinctions [!
T. (1990) Org. Geochem., 16, 641-648. [10]Perch-Nielsen K. et al. from an unfocused, sporadic collection of papers that virtually ig-
nored extraterrestrial causes and treated endogenous ones only spar- i
(1982) GSA Spec. Paper 190, 353-371. [i I] Wolbach W. S. and
Anders E. (1989) GCA, 53, 1637-1647. [12] Wolbach W. S. ingly better to an integrated, diverse body of literature. Research
etal. (1990) GCA, 54, !133-1146. [13]Wolbach W. S. etal. (1990) programs organized seemingly overnight spawned collaborativeteams
whose members (often from distant, isolated disciplines) redirected
GSA Spec. Paper 247, 391-400. [14] Melosh H. J. et al. (1990)
Nature, 343, 251-254. [15] Koeberl C. and Sigurdsson H. (1992) : their careers in order to address the captivating, high-stakes issues.
GCA, 56, 2113-2129. [16] Gardner A.F. et al., this volume. _ The initial, generally skeptical cool reception of the impact hy-
[17] Hildebrand A. R.and Wolbach W. S. (1989) LPS XX, 414-415. _ pothesis might have been predicted for any of anumber of reasons:
Such an instantaneous catastrophe contravened Earth science' sreign-
ing philosophy of uniformitarianism; it was formulated from aform
of evidence (siderophile element anomalies) alien to the community
RESULTS OF BLIND TESTS TO RESOLVE CONTRO- charged with its appraisal; it advanced acausal mechanism thatavas
VERSIES: IRIDIUM AT GUBBIO; EXTINCTIONS AT EL improbable in terms of canonical knowledge; and it was proffered
KEF'. Compiled byR. N. Ginsburg 1from analyses ofF. Asaro 2,M. mainly by specialists alien toEarth and biological science, especially
Attrep Jr), J. I. Canudo 4, J. H. Crocket 5, U. Kr_ihenbtihl 6, -paleobiology.
B. Masters 7,H. T.Millard Jr. _,R. K. Olsson _,C. J. Orth 3,X. Orue- Early on it became clear that irrespective of which causal hypoth-
etxebarria i_,L.R.Quintana 3,and R.Rocchia tl, 1University of Miami, esis was chosen, the chosen one would be the strongest predictor of
Miami FL 33149, USA, 2Lawrence Berkeley Laboratory, Berkeley how the chooser would select and apply standards in assessing
CA 94720, USA, 3Los Alamos National Laboratory, Los Alamos evidence bearing on allsuch hypotheses. Less strong correlation also
NM 87545, USA,4Universidad de Zaragoza, Zaragoza 50009, Spain, appeared between disciplinary specialty and choice of hypothesis,
5McMaster University, Hamilton, Ontario LSS 4K l, Canada, and between disciplinary specialty and the assessment of evidence.
6Universit_it Bern, Bern CH-3000, Switzerland,7Sapolpa OK74066, =Such correlations varied with the level of specialization; the most
USA, xU.S. Geological Survey, Denver CO 80225, USA, _'Rutgers robust correlations appeared in the most restricted subspecialties--
University, New Brunswick NJ 08903, USA, I°Universidad del Pals the weakest in the broadest areas of science practice. The gestalt
Vasco, Bilbao E-48060, Spain, _tCNRS Laboratoire, Gif-sur-Yvette (mindset) seemingly engendered by the embrace of an extinction
91198, France. hypothesis overrode, or was stronger than, the intellectual predispo-
Sitions attributable to disciplinary specialty.
Two of the many controversies about KT events are based on The great majority of paleontologists rejected outright or es-
differing results of analyses of samples from the same outcrops chewed impact theory at its advent, and most who later came to
between two or more workers. One concerns the distribution of lr believe in an impact event(s) atthe KT boundary still deny impact(s)
as the main extinction cause. Many paleontologists regarded the
across the boundary in the much-studied sections at Gubbio, Italy;
the other involves the patterns of disappearance of planktonic duration, severity, and other aspects of the KT mass extinction in
Formanifera in the boundary section atEl Kef, Tunisia. At the 1988 terms of the fate of their own fossil taxa atthe extinction boundary;
Snowbird Conference itwas proposed that proponents of the differ- specialists inseverely affected fossil groups were most often among
ing interpretations join in collecting comprehensive samples for a those who spoke of a connection between impact and the mass
blind test to resolve the differences. extinction. Certain subspeciahy communities within paleontology
For the sections near Gubbio, 16 samples were selected for differed markedly in their opinions on the cause of mass extinction:
analysis, including those that could settle the controversy and place- For example, vertebrate paleontologists were almost unanimously
bos from other localities. The samples were ground in aball mill and opposed to impact-as-extinction-cause; in contrast, micropaleon-
splits were distributed blind for analysis oflr tosix analysts: Asaro, tologists---especially those treating planktonic calcareous forms--
Crocket, Kr_ihenbfihl, Millard, Orth, and Rocchia. The results to be were most often oppositely inclined.
presented have settled the controversy. Most cosmo- and geochemists, planetary geologists, impacting
For the El Kef sections, closely spaced samples were collected specialists, and students of Earth-crossing comets and asteroids were
across the boundary under the supervision of the two proponents of immediately sympathetic to impact theory in its entirety. Study of
differing interpretations. The samples were split and distributed impacting and closely related topics appeared to foster sympathy for
blind to four micropaleontologists: Masters, Canudo, Olson, and the impact hypothesis; however, most volcanologists (volcanic spe-
Orue-etxebarria. The results tobe presented indicate the status ofthe cialists) did not accept the volcanist hypothesis--which was not
conceived by volcanologists---but no unusual proportion of volca-
controversy.
nologists favored the alternative impact hypothesis.
Published authors and published supporters of alternatives to the
impact hypothesis of mass extinction, examined inallcases save one,
were opposed to the impact hypothesis atits advent. Irrespective of
LP1Contribution No. 825 45
their discipline, or however poorly informed, scientists rarely failed who exercised magisterial authority. Such doyens were most fre-
to embrace one of the mass extinction hypotheses. quently sought for their opinions on debated issues by both their own
The use of obsolete data and/or the omission of contrary evidence communities and the media; that was reflected in both the publica-
almost always punctuated published and oral arguments, and oppos- tions of science and the public. The rapid pace of the mass extinction
ing views were never treated atequal length. The gestalts or cognitive debates, with which only few could keep abreast, seemed to add to
frames of members of the opposing theoretical camps seemingly general reliance on the magister. Such two-step communication,
precluded mutually congruent viewpoints on any of the important from the world to the magister and then from the magister to the
debated issues or the assessment of evidence. The widely held view world, has been documented in other studies of conflicted ideas.
that such adversaries suffer an "incommensurability of viewpoint" Closure has not been reached on any of the many issues reticu-
seemed understated. lated inthese debates, but most Earth scientists are now convinced of
Impactors (proponents ofimpact asextinction cause) and volcanists atleast one impact atthe KT boundary and many are inclined to think
(proponents of volcanism as extinction cause) commonly used dif- in terms of multiple impacts, either instantaneous or spread over I-
ferent standards of appraisal and weighted the same evidence differ- 3m.y.; however, far from all who subscribe toimpact(s)---especially
ently. Application of standards and weighting of evidence varied among paleontologists--view impact(s) as the chief cause of the
widely even among those in the same theoretical camp. The impac- mass extinction(s).
tors argued mainly from canonical standards and claimed that their Research programs organized in the past decade to address the
hypothesis--backed by empirical evidence--facilitated clear pre- many issues that have arisen in the course of this upheaval continue
dictions with implicit directions for testing. The prediction of im- to generate new publications at a surprising rate (more than 2500
pact-generated, global, ballistic transport of impact products raised have already appeared). Modeling of impacts and mantle plumes and
expectations for many that impact evidence in addition tolrwould be their effects on a number of internal, crustal, and biospheric pro-
found atKT boundary sites--this provided important impetus for the cesses grow increasingly sophisticated with different research ap-
rapid formation of diversely comprised research teams. Almost al- proaches showing promising convergence intheir conclusions. The
ways, the members of the same collaborative team subscribed to the search for impact sites, which includes the reexamination of many
same extinction hypothesis. long-enigmatic structures, and the remapping and dating of great
Impact's opponents focused on the great range of variation in the flood basalt bodies are being actively pursued. Mass extinction
character of the boundary interval around the world, emphasizing horizons throughout the Phanerozoic are being scrutinized
that the geographic variations indicated a cause that was neither with methods, techniques, and instruments that did not exist just a
global nor instantaneous. Itwas aprime mission of impact's oppo- decade ago, and the broad character ofthe debates has forced unprec-
nents to demonstrate a lack of the ubiquitous, globally uniform edented interpenetrations of long isolated subdisciplines. The de-
effects claimed for an impact. Unlike the orthodox evidence that was bates continue, sustaining the opportunity to study the workings of
advanced for the radical impact hypothesis, impact's active oppo- science during a time of conflict over several related theories that
nents-mainly the voicanists--sought to undermine awide range of span multiple disciplines. The intensity, fast pace, and disciplinary
suppositions and vagaries that lay long hidden in established prin- diversity of this upheaval has opened windows on the workings of
ciples and methods; the volcanists searched for the weaknesses inthe science where only peepholes had been expected. These conclusions
orthodox standards and thus prompted much research at unprec- and others, and ahistorical overview of the debates are detailed byme
edented levels of refinement. The call for higher resolution and and adozen other scientists and social studies of science scholars in
greater detail required the development of new, or the acquisition of [I].
theretofore unused, methods, techniques, and instruments inanum- References: [1] (1994) The Mass-Extinction Debates: How
ber of disciplines; such needs were often fulfilled through the forma- Science Works in a Crisis, Stanford Univ., inpress.
tion of appropriately composed collaborative teams.
In the mid 1980s the postulate of stepped or multiple extinctions
near the KT boundary evolved through a series of recognizable
stages. The evidence for extinction steps was initially viewed as an BIOSTRATIGRAPH1C EVIDENCE OF THE KT BOUND-
anomaly in terms of the single-impact hypothesis and, as such, was ARY IN THE EASTERN GULF COASTAL PLAIN, NORTH
more or less dismissed early on bythe impactors. But it then became OF THE CHICXULUB CRATER. D. Habib, Geology Depart-
an increasingly serious problem as supporting data accrued and ment, Queens College and Graduate School, The City University of
stepped extinctions approached the status of a normative assump- New York, Rushing NY 11367-1597, USA.
tion. Ultimately, with further affirmative evidence, the stepped-
extinction idea, and allitcame toimply interms of impugning single- The KT boundary inAlabama iscurrently being restudied inlight
impact theory, evolved into a standard of appraisal that drove the of the proximity of this area to the Chicxulub impact crater in the
reassessment of the nature of extinctions atthe KT boundary. Atthat Yucatan Peninsula of Mexico [1,2]. Previous studies have suggested
point the newly born standard of multiple extinction steps--which that there isan apparent discordance between the position of the KT
had begun life as an anomaly in terms of the single-impact hypoth- boundary and the stratigraphy based on the Ir anomaly pattern [3],
esis--evolved into the primary forcing function that forced the paleomagnetic reversals [4], and the major biotic extinction event
reformulation of the single-impact hypothesis into a hypothesis of [5]. The purpose of this paper is to present the biostratigraphic
multiple impacts spanning enough time toaccommodate the extinc- evidence from dinoflagellate cysts [6], calcareous nannofossils [7],
tion steps. and planktonic foraminifera [8] to show that the mass extinction
Leadership of the various factions engaged inthe debates was, in datum is situated precisely at the KT boundary in Alabama and that
all cases, clearly in the hands of only one or very few senior leaders the low values of lr peaks inthe sediments directly above the bound-