Náttúrufræðingurinn - 1993, Síða 67

árekstrar meiri háttar loftsteina eða
halastjama virðast fylgja (Sepkoski 1990).
Það er engu líkara en lífríki jarðar sé
stjórnað af geimklukku sem slær á 26
milljón ára fresti. Enn er hart deild um
gildi og reyndar raunveruleika þessarar
tíðni, en það er til dæmis athyglisvert að
hún er svipuð og tíðnin í sveiflu sól-
kerfisins um miðás Vetrarbrautarinnar
(Rampino og Caldeira 1992).
HEIMILDIR
Alvarez, L.W., W. Alvarez, F. Asaro & H.B.
Michel 1980. Extraterrestrial cause for the
Cretaceous/Tertiary extinction. Science 208.
1095-1108.
Carlisle, D.B. 1992. Diamonds at the K/T
boundary. Nature 357. 119-120.
Covey, C„ S.J. Ghan, J.J. Walton & P.R.
Weissman 1990. Global environmental ef-
fects of impact-generated aerosols; results
from a general circulation model. Geologi-
cal Society of America Special Paper 247.
263-270. "
Gillot, P-Y., C. Jehanno, R. Rocchia &
Haraldur Sigurdsson 1991. Datation par la
methode potassium-argon de la limite
cretace-Peleogene en milieu marin: age des
verres de Beloc (Haiti). Compte Rendu
Academie Scientifique Paris 313. 193-199.
Hall, C.M., D. York & Haraldur Sigurdsson
1991. Laser 40Ar/39Ar step-heating ages
from single Cretaceous-Tertiary boundary
glass spherules. EOS 72. 531.
Haraldur Sigurðsson 1990. Assessment of the
atmospheric impact of volcanic eruptions.
Geological Society of America Special Pa-
per 247. 99-110.'
Haraldur Sigurðsson 1991. Heimsendisgler-
kúlurnar frá Haiti og loftsteinninn mikli.
Morgunblaðið 13. janúar 1991. 16-18.
Haraldur Sigurdsson, S. D’Hondt, M.A.
Arthur, T. Bralower, J.C. Zachos, M.
Fossen & J. Channell 1991 a. Glass from
the Cretaceous-Tertiary Boundary in Haiti.
Nature 349. 482-487.
Haraldur Sigurdsson, Ph. Bonté, L. Turpin,
M. Chaussidon, N. Metrich, M. Steinberg,
Ph. Pradel & S.D’Hondt 1991 b. Geochemi-
cal constraints on source region of Creta-
ceous/Tertiary impact glasses. Nature 353.
839-842.
Haraldur Sigurdsson, S. D’Hondt & S. Carey
1992. The impact of the Cretaceous/Ter-
tiary bolide on evaporite terrane and gen-
eration of major sulfuric acid aerosol. Earth
and Planetary Science Letters 109. 543-560.
Hermann, Y. 1990. Selective extinction of
marine plankton in the Paratethys at the
end of the Mesozoic era; a multiple interac-
tion hypothesis. Geological Society of
America Special Paper 247. 531-540.
Hjörtur Halldórsson 1954. Þættir úr ævisögu
jarðar. Félagsprentsmiðjan H.F., Reykjavík.
104 bls.
Koeberl, C. & Haraldur Sigurðsson 1992.
Geochemistry of impact glasses from the
K/T boundary in Haiti: relation to smect-
ites, and a new type of glass. Geochimica
Cosmochimica Acta, (í prentun).
Laubenfels, M.W., de 1956. Dinosaur extinc-
tion; one more hypothesis. Journal ofPale-
ontology 30. 207-218.
Níels Óskarsson, Örn Helgason, Haraldur
Sigurðsson & C. Koeberl 1992. Oxidation
state and volatile content of tektite glasses
from the Cretaceous/Tertiary boundary,
Haiti. Earth and Planetaiy Science Letters
(sent).
Penfíeld, G.T. & Z.A. Camargo 1981. Defini-
tion of a major igneous zone in the central
Yucatan platform with aeromagnetics and
gravity. Annual Meeting, Society of Explo-
ration Geophysics, Ahstracts 51. 37.
Pollack, J.B., O.B. Toon, T.P. Ackerman, C.P.
McKay & R.P. Turco 1983. Environmental
effects of an impact-generated dust cloud:
Implications for the Cretaceous-Tertiary
extinctions. Science 219. 287-289.
Rampino, M.R., & K. Caldeira 1992. Epi-
sodes of terrestrial geologic activity during
the past 260 million years: a quantitative
approach. Celestial Mechanics and dynami-
cal. Astronomy 51, (í prentun).
Sepkoski, J.J. 1990. The taxonomic structure
of periodic extinctions. Geological Society
of America, Special Paper 247. 33-44.
Sheehan, P.M. & D.E. Fastovsky 1992. Major
extictrions of land-dwelling vertebrates and
the Cretaceous/Tertiaiy boundary, eastern
Montana, (í prentun).
Shoemaker, E.M., R.F. Wolfe & C.S. Shoe-
61