Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125
Comunicações Geológicas (2012) 99, 2, 27-34
ISSN: 0873-948X; e-ISSN: 1647-581X
Iridium anomaly and extraterrestrial component in the clays
at the Cretaceous-Paleogene boundary in Denmark, Spain
and New Zealand
Anomalia de irídio e componente extraterrestre das argilas do limite
Cretácico – Paleogénico na Dinamarca, Espanha e Nova Zelândia
P. I. Premović1*, B. S. Ilić2
Artigo original
Original article
Recebido em 28/09/2011 / Aceite em 11/12/2011
Disponível online em Janeiro de 2012 / Publicado em Dezembro de 2012
© 2012 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The Cretaceous-Paleogene boundary clays at Højerup
(Denmark), Caravaca (Spain) and Woodside Creek (New Zealand) show
anomalous enrichments of iridium compared with the marine sedimentary
rocks. For the average iridium content of 465 ppb of CI carbonaceous
chondrite the estimate of CI chondrite proportions in the decarbonated
iridium-rich boundary layers, based on the integrated iridium fluencies, is
about 25 % at Højerup, 15 % at Caravaca and > 30 % at Woodside Creek.
These proportions are most likely too high due to a significant Ir influx
from the nearby marine or continental site to these sections.
Keywords: Fish Clay,
Carbonaceous chondrite.
Caravaca,
Woodside
Creek,
Iridium,
Resumo: As argilas do limite Cretácico – Paleogénico em Højerup
(Dinamarca), Caravaca (Espanha) e Woodside Creek (Nova Zelândia)
mostram enriquecimentos anómalos de irídio comparados com rochas
sedimentares marinhas. Para o conteúdo médio de irídio de 465 ppb do
condrito carbonáceo CI, a estimativa das proporções condríticas CI nas
camadas descarbonatadas ricas em irídio da zona de limite, baseado nas
fluências integradas de irídio, é cerca de 25% em Højerup, 15 % em
Caravaca e > 30% em Woodside Creek. Estas proporções demasiado
elevadas são provavelmente devidas a influxo significativo de Ir a partir
das regiões marinhas ou continentais próximas destas secções.
Palavras-chave: Argila, Caravaca, Woodside Creek, irídio, condrito
carbonáceo.
1
Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of
Niš, P.O. Box 224, 18000 Niš, Serbia.
2
Department of Pharmacy, Faculty of Medicine, University of Niš, 18000 Niš,
Serbia.
*Corresponding author / Autor correspondente: [email protected]
1. Introduction
In the original paper Alvarez et al. (1980) have first reported
anomalously high Ir concentrations in the Cretaceous-Paleogene
(KPB) boundary clays at Gubbio (central Italy, Fig. 1), Højerup
(the eastern Denmark, Fig. 1) and at Woodside Creek (the
northern part of the South Island of the New Zealand, Fig. 1).
They proposed an impact of extraterrestrial bolide to explain the
elevated Ir content at the KPB. Alvarez et al. also suggested that
approximately 60 times the mass of the impactor would have been
ejected into the atmosphere as impact dust. Almost simultaneously
with Alvarez et al., Smit & Hertogen (1980) reported an
anomalous Ir in the boundary clay at Caravaca (southeastern
Spain, Fig. 1). Since the Alvarez et al. discovery, the KPB has
been identified and studied at about 345 sites worldwide. Of these,
85 sites, representing all depositional environments, contain an Ir
anomaly. In general, Ir and other platinum-group elements (PGE:
collectively the elements Ru, Rh, Pd, Os, Ir and Pt) are invariably
enriched in the prominent boundary clays. Other trace elements (e.
g. heavy metals) are also relatively abundant in these clays.
Many researchers consider that the KPB impactor formed the
ca. 180 km crater at Chicxulub (Yucatan Peninsula, Mexico, Fig.
1). It has been suggested that the impactor was a carbonaceous
chondrite projectile - probably type CI (Kyte, 1998; Shukolyukov
& Lugmair, 1998; Frei & Frei, 2002; Quitté et al., 2003; Trinquier
et al., 2006), though it is still unclear whether it was a
carbonaceous chondrite or a comet. Indeed, comets are believed to
be primitive bodies with a composition like that of carbonaceous
chondrites (Gelinas et al., 2004). The anomalous Ir associated,
however, with the prominent boundary clay is consistent with the
high Ir content typical of most chondritic meteorites and
inconsistent with the general proposition of comets. Indeed, a
simple calculation shows that in the case of the ice-rich (>70 %)
comets the amount of Ir produced by an impact energy for a crater
of the Chicxulub size could be less than 0.001 % then that of an
asteroid.
In the past many researchers used the iridium concentration
and/or integrated amount of Ir to estimate the proportion of
chondritic component in the renowned boundary clays. This paper
aims to re-examine this method using comprehensive Ir data for
the boundary sections at Højerup, Caravaca and Woodside Creek
which are available and published by Schmitz (1988). These
stratigraphic sections are well preserved and distal (paleodistance:
>7000 km) to the proposed Chicxulub impact site. An Ir analysis
of the decarbonated KPB samples from Denmark, Spain and New
Zealand was carried out by Schmitz using instrumental neutron
activation analysis (INAA). Relative error in the precision of the
analyses ranges from 5 % to 10 %. Total uncertainties (including
accuracy errors) were up to 20 %.
For the sake of completeness, the Ir data related to the distal
KPB sections at Agost (Spain), Flaxbourne River (New Zealand),
Gubbio (Italy), Bidart (France), El Kef and Aïn Settara (Tunisia),
and in Ocean Drilling Program (ODP) Hole 738C (Kerguelen
Plateau, southern Indian Ocean) (Fig. 1) are also briefly discussed.
Although the sections studied systematically show an Ir
abundance anomaly, they differ from one another because local
sedimentation conditions under a strong continental influence.
Throughout this paper we make five reasonable postulates: (a) Ir
28
is wholly located in the non-carbonate fraction of the boundary
clays studied, i. e. the carbonate fraction of this section is
essentially an Ir diluents; (b) the Ir content of the carbonaceous
chondrites range from 406.0 ppb to 849.4 ppb (Table 1); (c) the
average content of Ir in CI chondrites is 465 ppb (Table 1); (d) all
Ir found in the boundary clays originated from the carbonaceous
chondrite (CC) impactor, especially of type CI; and (e) assumed
density of the boundary clays is about 2 g cm-3.
P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
carbonaceous chondrites typically contain 406.0 - 849.4 ppb Ir
(Table 1). So, a little addition of the CC component would be
necessary to critically increase the concentration of Ir in a
marine sedimentary rock. Thus, Ir is a very sensitive means of
examining the CC contribution to the marine KPB clays such as
the Fish Clay. Some authors suggested that Ir in the Fish Clay
was sourced by the Ir from seawater (Goldberg et al., 1986).
However, the average Ir concentration in the decarbonated BH
(ca. 100 ppb, see below) represents an enrichment factor of
about 1017 compared to seawater (ca. 2×10-15 ppb, Table 1).
Moreover, according to these authors, the residence time of Ir
in the oceans is about 1 million years. Thus, excess of Ir in the
BH could not, therefore, have been derived from the sea
reservoir.
Table 1. Concentrations of Ir [ppb] for carbonaceous chondrites, marine sediments
and seawater.
Tabela 1. Concentrações de Ir [ppb] para os condritos carbonáceos, sedimentos
marinhos e água do mar.
Fig.1. Geographic location of studied KPB clays.
Fig.1. Localização geográfica das argilas KPB estudadas.
2. Results and Discussion
Fish Clay. The lowermost Danian Fish clay Member of the
Rødvig Formation near the village of Højerup is a classic
marine KPB section. The lithology of the Fish Clay within this
boundary section characterizes three distinctive layers (from
bottom to top): a 3 cm thick (mainly black-to-dark) marl
(hereinafter BH) with 0.5 cm-thick basal red (goethite-rich)
sublayer, grey-to-brown marl and a light-grey marl (Fig. 2).
The red sublayer is underlain by (the latest) Maastrichtian
bryozoan-rich limestone (chalk) whereas the top marl is
overlain by (early) Danian Cerithium limestone (Schmitz, 1988;
Christensen et al., 1973; Schmitz, 1985; Elliott, 1993; Surlyk et
al., 2006). Geochemical studies show that the Ir profile (on a
whole rock basis) across the Fish Clay column is characterized
by a maximum just above the base of BH with an upward
gradual decrease (tailing-off) from its maximum (e. g. Schmitz,
1985). This gradual tailing off indicates that the Fish Clay
represent a relatively continuous and complete section. The BH
is considered to constitute the main part of the boundary
section, since it contains more than 95 % of the total Ir in the
Fish Clay (Premović, 2009). The mineralogy of BH is
comparatively simple, smectite and authigenic (mainly
biogenic) calcite being the principal components. Geochemical
evidence indicates that the BH was deposited under strong
anoxic conditions but the red sublayer under strong oxic
sedimentation conditions (Premović, 2009).
Premović et al. (2000) inferred that the BH was deposited
in a shallow marine <100 m water depth at Stevns Klint within
an interval of about 40 years. Wendler & Willems (2002)
considered that this layer represents the first decades or
centuries of deposition following the KPB impact event. Thus,
the average sedimentation rate in the BH is probably about 0.3
mm per year.
As pointed out above, the Ir enrichment of the KPB clays
is generally regarded as indicative of the presence of a CC
component. Table 1 shows that on average marine sedimentary
rocks usually contain the Ir bellow 1 ppb (Table 1) while
Fig.2. Expanded lithological log of the Fish Clay at Højerup based on Surlyk et al.
(2006).
Fig.2. Log litológico expandido das argilas em Højerup baseado em Surlyk et al.
(2006).
Kyte et al. (1985) estimated that the extraterrestrial component
in the basal 1 cm part of BH from the measured Ir concentration
(47.4 ppb) is about 10.5 %, corrected for about 20 % carbonate
content. According to Grieve (1997), chondrite-normalized relative
abundance of Ir indicates that the Fish Clay (on a whole-rock basis)
contains an admixture of about 10 % chondrite. Trinquier et al.
(2006) have shown that Cr isotopic signature of BH represents a
mixing of the CC matter with terrestrial material in a ratio up to 6.8
%. Recently, Osawa et al. (2009) estimated from the measured Ir
concentration (29.9 ppb) that the extraterrestrial material in the
basal part of BH was diluted to 1/19 (on a whole-rock basis),
assuming that Ir concentration of CC is 470 ppb. Of note, that an
impactor mass fraction globally dispersed after the Chicxulub
impact ranges between 22 % (Alvarez et al., 1980) to 50 %
(Vickery & Melosh, 1990).
Ir anomaly and meteoritic contribution
29
The INAA data (Schmitz, 1988) for Ir in the carbonate-free
fraction across the Fish Clay are presented in Fig. 3a. This profile
shows that the peak concentration of Ir in the decarbonated BH is
about 120 ppb. Using these data, we estimate that the
decarbonated BH is derived from about 6 - 30 % CC (Fig. 4a);
the CI chondrite proportion is about 22 % for an average Ir
concentration of about 100 ppb.
al., 2000). About 29 - 33 shocked quartz grains per gram are also
identified in this layer (Morgan et al., 2006). Of note, the total Ir
flux of the carbonate-free Fish Clay is about 615 ng cm-2 which
corresponds to about 22 % the type CI chondrite. For the average
sedimentation rate in the BH of 0.3 mm yr-1, the average
accumulation rate of Ir in the BH is ca. 3 ng cm-2 yr-1; the average
accumulation rate is about 6.5 µg cm-2 yr-1 CI chondrite.
Fig.3. Concentration profiles of Ir (on a carbonate-free basis) in the CretaceousPaleogene boundary clays at: (a) Højerup (Fish Clay), (b) Caravaca and (c)
Woodside Creek. The samples were analyzed with instrumental neutron activation
(INAA). Relative error in the precision of the analyses ranges from 5 % to 10 %.
Total uncertainties (including accuracy errors) were up to 20 % (Schmitz, 1988).
Fig.4. The CC contributions for the decarbonated boundary clays at (a) Højerup
(Fish Clay), (b) Caravaca and (c) at Woodside Creek. The lowest ( ) and highest
( ) CC contributions are based on the Ir concentrations of the carbonaceous
chondrites (see text).
Fig.3. Perfis de concentração de Ir (numa base sem carbonato) nas argilas do limite
Cretácico-Paleogénico em: (a) Højerup (Fish Clay), (b) Caravaca e (c) Woodside
Creek. As amostras foram analisadas com activação neutrónica instrumental
(INAA). O erro relativo na precisão das análises varia entre 5 % a 10 %. As
incertezas totais (incluindo erros de incerteza) chegaram até 20 % (Schmitz, 1988).
A better measure of an Ir anomaly in the boundary clay is
the integrated Ir fluence. For the BH, the fluence is estimated at
about 305 ng cm-2 after integration in the decarbonated segment
from 0.5 - 2.0 cm (Fig. 3a). (For comparison, an estimate of the Ir
fluence for the global impact deposit for the KPB is about 40 - 55
ng cm-2: Kyte, 2004). This value corresponds approximately to
about 0.35 - 0.75 g cm-2 CC. These estimates correspond to about
15 - 30 % CC; the CI contribution is about 25 %. Such a
significant proportion of CC is, however, not supported by any
mineralogical evidence. Indeed, if initially the precursor material
of BH contained a high percentage of the ejecta fallout, then
diagenetic alteration would have left a residue with high
concentrations of the impact markers. To our knowledge, the part
of BH above the red sublayer contains no altered meteoritic
fragments (Bauluz et al., 2000) and only about 0 - 6 shocked
quartz grains per gram (Morgan et al., 2006). On the other hand,
a simple calculation indicates that the underlying red sublayer
(with 15. 2 ppb of Ir, Fig. 3a) contains as low as about 3.3 % of
type CI chondrite material. In sharp contrast, this sublayer is
abundant (about 10 wt %) with the well-preserved impact-related
goethite-rich microspherules (Schmitz, 1985; Graup et al., 1992)
and with nano-size goethite grains (Bauluz et al., 2000; Wdowiak
et al., 2001) interpreted as altered meteorite fragments (Bauluz et
Fig.4. As contribuições CC para as argilas descarbonatadas do limite em (a) Højerup
(Fish Clay), (b) Caravaca e (c) em Woodside Creek. As contribuições CC mais
baixa ( ) e mais elevada ( ) baseiam-se nas concentrações de irídio dos condritos
carbonáceos (ver texto).
Our estimation (judging from the Ir concentrations and
integrated Ir fluence) may substantially underestimate the actual
CC contribution to the decarbonated BH. There are two reasons
for this. First, anomalous amounts of Ir in the decarbonated Fish
Clay display a gradual decrease from the BH upwards but this
metal is still relatively elevated (up to near 5 ppb) in the lightgray marl (Fig. 3a). These stratigraphically extended enrichments
of Ir may be explained only partly by the redox-controlled
remobilization (Colodner et al., 1992). Second, according to
Peucker-Ehrenbrink & Hannigan (2000), between about 40 %
and 80 % of the initial Ir budget could have been lost due to
surficial weathering of black shales (such as the BH) within
roughly 13, 000 years. Thus, because of the assumed
remobilization and/or weathering loss of the Ir, the original
proportion of CC in the carbonate-free part of BH actually could
be even considerably higher than presented above. The more
detailed discussion of this issue is beyond the scope of this paper.
Assuming that ejecta fallout (with an average density of
about 2 g cm-3) contained 50 % of the CC matter then the BH
portion in question would contain about 30 - 60 % of this (mostly
diagenetically altered) fallout. The CC matter (and associated Ir)
could be deposited in the Fish Clay by the direct airborne ejecta
fallout settling through the seawater column or they were
transported from the nearby marine or continental site to the Fish
30
Clay bed. Kyte et al. (1985) reasoned that Ir (and other
siderophiles) in the basal 1 cm thick part of BH is only
representative of primary Ir derived from the initial ejecta fallout
deposited during short time (a few days, months or years) after the
impact. According to these researchers, Ir above this layer is
probably secondary in origin and transported from the original
nearby site, which increased the primary Ir values. Wolbach et al.
(1988) proposed that secondary Ir is ejecta material eroded from
marine elevated sites to topographic lows already containing some
primary fallout. Premović (2009) has suggested that Ir in the Fish
Clay was probably fluvially transported from the soil on adjacent
land and redeposited in a shallow marine basin at Højerup. This
author argued that a predominant part of Ir in the BH ultimately
came from the CC material associated with ejecta fallout covering
nearby coastal soil. Indeed, the gradual decrease of Ir upsection
across the Fish Clay indicates the single input of Ir probably
associated with detrital sedimentation, consisting of smectite, silt
and humic materials (Premović, 2009, and references therein).
Consequently, the overestimated proportion of the CC matter in
the decarbonated BH is probably due to the high Ir (detritusassociated) input from the marine or continental site.
Anomalous Ir concentration (on a whole-rock basis) is not
limited to the Fish Clay but extends about 1 m above and below
the BH (Rocchia et al., 1987). They estimate that the duration of
the enhancement represents about 104 years. This may indicate an
extended contribution of Ir derived from an extraterrestrial source
other than the predominant and virtually instantaneous source of Ir
of the KP impactor. It is also possible that extension of Ir
enrichment originate from a relatively long terrestrial source, e. g.
Deccan Traps volcanism at the end of the Cretaceous period
which lasted millions of years. The third explanation is that this
tail represents postdepositional redistribution of Ir out of its
original peak position. This topic is, however, out of scope here.
The Caravaca boundary section. The sections at Agost, Caravaca
and El Kef are among the most continuous and complete marine
sections for the KPB transition. The Caravaca and Agost
boundary sections are located in the Betis Cordilleras
(southerneast Spain). The KPB section at Caravaca consists of a
ca. 1 cm-thick Ir-rich dark marl (BC) overlain by a grey-to-brown
marl, Fig. 3b. A basal 2-3 mm thick red sublayer of BC enriched
with Ir (ca. 47 - 57 ppb: Tredoux et al., 1989) is marking the
KPB. The clay association of these marls is dominated by
dioctahedral smectite (Ortega-Huertas et al., 1995). According to
these researchers, this smectite results from (a) the alteration of
marine volcanic rocks or (b) the erosion of continental soil.
Terminal Maastrichtian–basal Paleogene marls at Caravaca were
deposited in a middle bathyal environment (<500 m depth) (Smit,
1999). Abundant presence of goethite in the red sublayer
indicates that its deposition probably occurred under welloxygenated conditions.
The distribution of Ir in the carbonate-free fraction across
the boundary section at Caravaca is given in Fig. 3b. This profile
is based on the INAA measurements performed by Schmitz
(1988). The Ir values reach a profound peak concentration of
about 110 ppb in the decarbonated red sublayer. The main Ir peak
has tails both upsection and downsection, but the later is much
sharper than in the Fish Clay. As previously stated, the upward
tail feature infers a relatively continuous and complete section.
Using his concentration data, we estimate that the decarbonated
BC is derived from ca. 5 - 27 % CC (Fig. 4b); the CI chondrite
contribution is about 13 % for an average Ir concentration of
about 60 ppb.
The integrated Ir fluence of BC is estimated at about 100 ng
cm-2. This value corresponds to 0.20 - 0.40 g cm-2 CC. These in
P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
turn correspond to about 10 - 20 % CC; the CI chondrite
contribution is about 15 %.
Assuming that ejecta fallout (with an average density of
about 2 g cm-3) contained 50 % of the CC matter then the BW
portion in question would contain about 20 - 40 % of this (mostly
diagenetically altered) fallout. As previously stated for the BH, a
high percentage of the ejecta fallout would have left a residue
after diagenesis with high concentrations of the impact indicators
in the BC. The thin red sublayer contains numerous presumably
impact-derived goethitic microspherules (ca. 10 % of the total
weight: Schmitz, 1988), shocked zircons and Ni-rich spinels.
About 19 - 38 shocked quartz grains per gram were identified in
the BC (Morgan et al., 2006).
By analogy with the Fish Clay, we assume that Ir in the BC
was probably derived from carbonaceous chondritic component
of ejecta fallout accumulated on nearby adjacent land or marine
area. Indeed, the major carrier of this Ir was probably the
smectite which is most likely of detrital origin (Ortega-Huertas et
al., 1995). The decarbonated boundary section at Caravaca is
characterized with a similar stratigraphically extendeded
enrichments (<5 ppb) as the decarbonated Fish Clay, Fig. 3b.
Moreover, Rocchia et al. (1987) reported the Ir-rich interval at
Caravaca also contains above-background concentrations of Ir
(on a whole-rock basis) over 1.5 m of interval.
The Woodside Creek boundary section. The KPB section at
Woodside Creek is represented by a reddish (up to 1 cm thick)
clay-rich layer (BW) and overlying dark to grey-to-brown marls.
These layers were probably deposited in a shallow marine <500
m water depth (Morgan et al. 2006) probably under welloxygenated conditions. BW is underlain by the latest
Maastrichtian marl, Fig. 3c.
The INAA data for Ir (Schmitz, 1988) in the decarbonated
fraction of the boundary section at Woodside Creek are plotted in
Fig. 3c. The peak concentration of Ir of 460 ppb is located in the
carbonate-free BW which is one of the highest measured to date for
any KPB interval. Using Schmitz’s data, we estimate that in the
decarbonated BW is derived from about 60 % up to bizarre 115 %
CC (Fig. 4c); the type CI chondrite input averages about 100 %.
The Ir anomaly appears to be much sharper upward and
downward than at the Danian (Fig. 3a) and Spanish (Fig. 3b)
sites. Moreover, the decarbonated Ir-rich section at Woodside
Creek is characterized with high Ir enrichments (20-30 ppb) up to
about 5-6 cm above the BW, Fig. 3c.
Brooks et al. (1984) reported that a carbonate-free boundary
layer (0.8 cm thick) at another site of Woodside Creeks contains
about 153 ppb of Ir. This value corresponds to about 20 - 40 %
CC; the CI chondrite contribution is about 30 %.
Assuming that ejecta fallout (with an average density of
about 2 g cm-3) contained 50 % of the CC matter then the BH
portion in question would contain about >40 % of this (mostly
diagenetically altered) fallout. As in the case of BH, a high
percentage of the ejecta fallout would have left a residue after
diagenesis with high concentrations of the impact indicators in
the BW. To express precisely, the BW contains numerous
goethite-rich and organic microspherules (Schmitz, 1988); and,
about 2 - 8 shocked quartz grains per gram (Morgan et al., 2006).
Anomalous extraterrestrial Ir is also mostly associated with this
layer, Fig. 3c.
The Agost boundary section. The Agost boundary section is
similar to the neighbouring Caravaca section in lithology,
geochemistry and depositional history. As at Caravaca this
section is comprised of a dark (about 6-cm-thick) clay with a
basal 2-3 mm-thick red goethite-rich sublayer, Fig. 5. The dark
Ir anomaly and meteoritic contribution
31
clay layer is underlain with the latest Maastrichtian marl and
overlain by the grey-to-brown marl (Molina et al., 2005, and
references therein). This layer is deposited in the middle bathyal
environment, 600-1000 m deep (Coccioni & Galeotti, 1994).
boundary clay contains from about 0.25 - 5.0 % CC (Fig. 6b).
However, the decarbonated Ir-rich interval at Flaxbourn River is
characterized with another high Ir peak (ca. 20 ppb) about 1.5 cm
below the boundary clay, Fig. 6a.
Fig.5. (a) The Ir concentration profile (on a carbonate-free basis) across the
Cretaceous-Paleogene boundary section at Agost and (b) the CC contributions for
the decarbonated boundary section at Agost: lowest ( ) and highest ( ) CC
contributions (see above).
Fig.6. (a) The Ir concentration profile (on a carbonate-free basis) across the
Cretaceous-Paleogene boundary section at Flaxbourne River and (b) the CC
contributions for the decarbonated boundary section at Flaxbourne River: lowest ( )
and highest ( ) CC contributions (see above).
Fig.5. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do
limite Cretácico-Paleogénico em Agost e (b) as contribuições CC para a secção
descarbonatada do limite em Agost: contribuições CC mais baixa ( ) e mais
elevada ( ) (ver acima).
Fig.6. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do
limite Cretácico-Paleogénico em Flaxbourne River e (b) as contribuições CC para a
secção descarbonatada do limite em Flaxbourne River: contribuições CC mais baixa
( ) e mais elevada ( ) (ver acima).
Smit (1990) analyzed Ir (on a whole-rock basis) across the
dark clay. Using his Ir and carbonate content data, we computed Ir
concentrations in the non-carbonate (clay) fraction. These
calculated concentrations are plotted vs. stratigraphic height in
Fig. 5a. The results show that the highest Ir enrichment (ca. 30
ppb) is in the red sublayer with an upward gradual decrease from
this peak. We estimate that the decarbonated sublayer contains
about 4.0 - 7.0 % CC (Fig. 5b). The red sublayer contains
mineralogical evidence of extraterrestrial impact such as Ni-rich
spinels and diagenetically altered K-feldspar or Fe oxide
microspherules (Smit, 1990).
The Flaxbourne River boundary section. Beside at Woodside
Creek, the KPB has been identified in over 20 stratigraphic
sections in the New Zealand region and one of them is Flaxbourne
River (Hollis et al., 2003). Despite the lesser Ir enrichment in the
Flaxbourne section (21 ppb) compared to Woodside Creek (54100 ppb), the Flaxbourne section is considered to be more
complete (Strong, 2000; Hollis et al., 2003). The dark-to-black
boundary clay (average in thickness about 2 cm) in this section is
a calcareous limestone which is overlain by a dark limestone. This
clay in turn is underlain by the latest Maastrichtian marl, Fig. 6.
The INAA data for Ir (Strong et al., 1987) in the
decarbonated fraction of the boundary section at Flaxbourne River
are plotted in Fig. 6a. The peak concentration of Ir of about 21 ppb
is located in the carbonate-free boundary clay; more than about 85
% of the total Ir is distributed within the Ir-rich interval of -2 cm
to +6 cm. Using their data, we estimate that the decarbonated
The Gubbio boundary section. The classical Bottaccione
boundary shales near Gubbio originated at a deep marine bathyal
basin with a water depth over 1500 m (Kuhnt, 1990). A
stratigraphic profile for the basal part of these shales is shown in
Fig. 7. We roughly estimate that the CC matter in their
decarbonated fraction is <2 %. This estimation is based on the
peak Ir concentration (ca. 7 ppb: Crocket et al., 1988). These
researchers also found that the enrichment of Ir (on a whole-rock
basis) at Gubbio is not confined to the boundary shales but
extends approximately 2 m above and below the boundary level,
corresponding to 3.5 million years. According to Montanari
(1986), the boundary section at Gubbio bears evidence of
widespread contamination from the surrounding soil.
The Bidart boundary section. The boundary section at Bidart is
characterized by a sharp contrast between a marl of the latest
Maastrichtian and a 6-cm-thick layer of dark boundary clay with
a 2-mm-thick basal (iron-rich) red sublayer, Fig. 8a. This clay of
the lowermost Danian is overlain by brownish clay. The whole
interval is deposited in upper-middle bathyal depth (Alegret et
al., 2004). The red sublayer is marked by an Ir anomaly (Rocchia
et al., 1987) and the presence of Ni-rich spinels (Robin &
Rocchia, 1998). In Fig. 8a, we plot a Ir distribution(on a
carbonate-free basis) across the boundary clay using the Ir and
carbonate content data reported by Rocchia et al. (1987). We
roughly estimate that the carbonate-free fraction of the red
sublayer contains <2 % CC (Fig. 9a).
32
P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
clay is 55–65 cm thick, with the 2–3 mm (almost carbonate-free)
red (goethite-rich) sublayer (Keller et al., 1995); this sublayer
contains 5 - 10 % of the total Ir flux and most of Ni-rich spinels
(Robin et al., 1991). A stratigraphic column of basal part of this
clay is presented in Fig. 9b. Smectite is the main component of
the boundary clay at this site (Ortega-Huertas et al., 1998). This
clay passes upward into marl of the early Danian (Molina et al.,
2006). The Ir maximum of 4 ppb in the red sublayer (Robin et
al., 1991) corresponding roughly to <1.5 % (Fig. 9b).
The Aïn Settara boundary section. This section (about 50 km far
from the El Kef site) is similar to the El Kef section in lithology,
geochemistry and deposition history (Dupius et al., 2001); a
stratigraphic column of basal part of this section is shown in Fig.
8c. According to these researchers, (almost) carbonate-free
jarositic red sublayer is a few mm thick contains 3 ppb of Ir; this
corresponds roughly <1 % of the CI chondrite matter. We roughly
estimate that the red sublayer contains <1 % CC (Fig. 9c).
Fig.7. Expanded lithological log across the Cretaceous-Paleogene boundary section
at Gubbio (adopted from Premović, 2009).
Fig.7. Log litológico expandido ao longo do limite Cretácico-Paleogénico em
Gubbio (adaptado de Premović, 2009).
Fig.9. The CC contributions for the decarbonated boundary section at: (a) Bidart, (b)
El Kef and (c) Aïn Settara: lowest ( ) and highest ( ) CC contributions (see above).
Fig.9. Contribuições CC para a secção descarbonatada do limite em: (a) Bidart, (b)
El Kef e (c) Aïn Settara: contribuições CC mais baixa ( ) e mais elevada ( ) (ver
acima).
Fig.8. The Ir concentration profile (on a carbonate-free basis) across the CretaceousPaleogene boundary section at: (a) Bidart, (b) El Kef and (c) Aïn Settara.
Fig.8. O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite
Cretácico-Paleogénico em: (a) Bidart, (b) El Kef e (c) Aïn Settara.
El Kef boundary section. The base of the El Kef boundary section
in the central Tunisia has been officially designated as the
boundary global stratotype section and point (GSSP) for the KPB
(Cowie et al., 1989; Molina et al., 1996). At El Kef the boundary
ODP Hole 738. Schmitz et al. (1991) analyzed Ir (on a
whole-rock basis) across a 1m-thick-clay rich boundary section
in ODP Hole 738. They found that Ir concentrations are the
highest (18 ppb) in the dark boundary clay (lamina) a few
centimeters above the base of the clay-rich layer. Using their Ir
and carbonate content data, we computed Ir concentrations in the
non-carbonate (clay) fraction. These calculated concentrations in
Fig. 10a are plotted vs. stratigraphic height. The results show that
the highest Ir enrichment (ca. 60 ppb) is in the dark lamina. The
Ir concentrations decrease relatively sharply for about 1.5 cm
upsection and then remains nearly constant over a plateau of tens
of centimeters. We roughly estimate that the decarbonated
fraction of the dark lamina contains between 5 % and 15 % CC
(Fig. 10b). No shocked quartz or impact-derived microparticles
were reported to be found in the boundary section. Schmitz et al.
(1991) concluded that the clay fraction of this section studied is
predominantly derived from locally derived material.
Ir anomaly and meteoritic contribution
33
and endless patience for our effort to make this report as complete as
possible.
References
Fig.10. (a) The Ir concentration profile (on a carbonate-free basis) across the
Cretaceous-Paleogene boundary section at ODP Hole 738 and (b) the CC
contributions for the decarbonated boundary section at ODP Hole 738: lowest ( )
and highest ( ) CC contributions (see above).
Fig.10. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do
limite Cretácico-Paleogénico no buraco 738 do ODP e (b) as contribuições CC para
a secção descarbonatada do limite no buraco 738 do ODP: contribuições CC mais
baixa ( ) e mais elevada ( ) (ver acima).
In summary, the Ir content of marine boundary clays may be
influenced by various sedimentary and geochemical factors (e.g.,
erosion, redeposition, geochemical remobilization, and
weathering). The extent to which these factors operate is
dependent on the particular sedimentary site and the immediate
seafloor environment. Consequently, Ir may have been
concentrated or diluted in the boundary clay during
sedimentation and diagenesis, and afterwards. This makes
ambiguous any attempt to assess the contribution of the impactor
Ir to the Ir enrichment of the boundary clay or to use global
fluence of Ir to estimate the size of the impactor. The same
ambiguity goes for other PGE. In addition, in order to identify
chondritic elements in the boundary clay the researchers usually
present their trace element data normalizing first to Ir and then to
CI chondrites. It is clear from the above consideration that this
method is also rather ambiguous.
3. Conclusions
Estimates based on the integrated Ir amount show that the
decarbonated Ir-rich KPB layers at Højerup, Caravaca and
Woodside Creek appear to contain respectfully as much as 25 %,
15 % and >30 % of the CI matter for the average Ir content of
465 ppb CI chondrite. These percentages are most probably an
overestimation due to the high Ir input from the nearby marine or
continental site, which presumably increases primary Ir values in
these clays.
Acknowledgements
We are grateful to Dr. Adam Dangić for supplying constructive
comments. We also thank to Dr. Telmo Santos for his understanding
Alegret, L., Kaminski, M. A., Molina, E., 2004. Paleoenvironmental
recovery after the Cretaceous/Paleogene boundary crisis: Evidence
from the marine Bidart Section (SW France). Palaios, 19, 574-586.
Alvarez, L. W., Alvarez, W., Asaro, F. & Michel, H. V., 1980.
Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science,
208, 1095-1108.
Anbar, A. D., Zahnle, K. J., Arnold, G. L. & Mojzsis, S. J., 2001.
Extraterrestrial iridium, sediment accumulation and the habitability of
the early Earth’s surface. J. Geophys. Res., 106, 3219-3236.
Bauluz, B., Peacor, D. R., Elliott, W. C., 2000. Coexisting altered glass
and Fe–Ni oxides at the Cretaceous–Tertiary boundary, Stevns Klint,
Denmark: Direct evidence of meteorite impact. Sci. Lett., 182, 127-136.
Brooks, R. R., Reeves R. D., Xing-Hua, Y., Ryan, D. E., Holzbecker, J.,
Collen, J. D., Neall, V. E., Lee, J., 1984. Elemental anomalies at the
Cretaceous–Tertiary boundary, Woodside Creek, New Zealand.
Science, 226, 539-542.
Christensen, L., Fregerslev, S., Simonsen, A., Thiede, J., 1973.
Sedimentology and depositional environment of Lower Danian Fish Clay
from Stevns Klint, Denmark. Bull. Geol. Soc. Denmark, 22, 193-212.
Coccioni, R., Galeotti, S., 1994. K-T boundary extinction: geologically
instantaneous or gradual event? Evidence from deep-sea benthic
foraminifera. Geology, 22, 779-782.
Colodner, D. C., Boyle, E. A., Edmond, J. M., Thomson, J., 1992. Postdepositional mobility of platinum, iridium and rhenium in marine
sediments. Nature, 358, 402-404.
Cowie, J. W., Ziegler, W., Remane, J., 1989. Stratigraphic commission
accelerates progress, 1984–1989. Episodes, 112, 79-83.
Crockett J. H., Officer C. B., Wezel F. C., Johnson G. D., 1988.
Distribution of noble metals across the Cretaceous/Tertiary boundary
at Gubbio, Italy: iridium variation as a constraint on the duration and
nature of Cretaceous/Tertiary boundary events. Geology, 16, 77-80.
Dupuis, C., Steurbaut, E., Molina, E., Rauscher, R., Tribovillard, N.,
Arenillas, I., Arz, J. A., Robaszynski, F., Caron, M., Robin, E.,
Rocchia, R., Lefevre, I., 2001. The Cretaceous-Palaeogene (K/P)
boundary in the Aïn Settara section (Kalaat senan, Central Tunisia):
Lithological, micropalaeontological and geochemical evidence. Bull.
Inst. R. Sc. N. B-S, 71, 169-190.
Elliott, W. C.,1993. Origin of the Mg–smectite at the Cretaceous/Tertiary
(K/T) boundary at Stevns Klint, Denmark. Clay Miner., 41, 442-452.
Frei, R., Frei, K. M., 2002. A multi-isotopic and trace element
investigation of the Cretaceous–Tertiary boundary layer at Stevns
Klint, Denmark - inferences for the origin and nature of siderophile
and lithophile element geochemical anomalies. Earth Planet. Sci.
Lett., 203, 691-708.
Gelinas, A., Kring, D. A., Zurcher, L., Urrutia-Fucugauchi, J., Morton,
O., Walker, R. J., 2004. Osmium isotope constraints on the proportion
of bolide component in Chicxulub impact melt rocks. Meteorit.
Planet. Sci., 39, 1003-1008.
Goldberg, E. D., Hodge, V., Kay, P., Stallard, M., Koide, M., 1986. Some
comparative marine chemistries of platinum and iridium. Appl.
Geochem., 1, 227-232.
Graup, G., Palme, H., Spettle, B., 1992. Trace element stratification in the
Stevns Klint Cretaceous/Tertiary boundary layers. In: Stansbery, E. &
Reid, A. (Eds.) Proceedings of the 23th Lunar and Planetary Science
Conference. Lunar and Planetary Institute, Houston, 445-446.
Grieve, R. A. F., 1997. Extraterrestrial impact events: the record in the
rocks and the stratigraphic column. Palaeogeogr. Palaeoclimatol.
Palaeoecol., 132, 5-23.
Hollis, C. J., Strong, C. P., Rodgers, K. A., Rogers, K. M., 2003.
Paleoenvironmental changes across the Cretaceous/Tertiary boundary
at Flaxbourne River and Woodside Creek, eastern Marlborough, New
Zealand. New Zeal. J. Geol. Geophys., 46, 177-197.
Keller, G., Li, L., Macleod, N., 1995. The Cretaceous/Tertiary boundary
stratotype sections at El Kef, Tunisia: How catastrophic was the mass
extinction? Palaeogeogr. Palaeocl., 119, 221-254.
Kuhnt, W., 1990. Agglutinated foraminifera of western Mediterranean
Upper Cretaceous pelagic limestones (Umbrian Apennines, Italy, and
Betic Cordillera, southern Spain). Micropaleontology, 36, 297-330.
34
Kyte, F.T., 1998. A meteorite from the Cretaceous/Tertiary boundary.
Nature, 396, 237-239.
Kyte, F. T., 2004. Primary mineralogical and chemical characteristics of
the major K/T and Late Eocene impact deposits. In: Proceedings of
American Geophysical Union, Fall Meeting. Washington, USA,
#B33C-0272.
Kyte, F. T., Smit, J., Wasson, J. T., 1985. Siderophile interelement
variations in the Cretaceous-Tertiary boundary sediments from
Caravaca, Spain. Earth Planet. Sci. Lett., 73, 183-195.
Lodders, K., Fegley, B. J., 1998. The Planetary Scientist's Companion.
Oxford University Press, New York, 371.
Molina, E., Alegret, L., Arenillas, I., Arz, J. A., 2005. The
Cretaceous/Paleogene boundary at the Agost section revisited:
paleoenvironmental reconstruction and mass extinction pattern. J.
Iber. Geol., 31, 135-148.
Molina, E., Alegret, L., Arenillas, I., Arz, J. A., Gallala, N., Hardenbol, J.,
Von Salis, K., Steurbaut, E., Vandenberghe, N., Zaghbib-Turki, D., 2006.
The Global Boundary Stratotype Section and Point for the base of the
Danian Stage (Paleocene, Paleogene, "Tertiary", Cenozoic) at El Kef,
Tunisia - Original definition and revision. Episodes, 29, 263-273.
Molina, E., Arenillas, I., Arz, J.A., 1996. The Cretaceous/Tertiary
boundary mass extinction in planktic Foraminifera at Agost, Spain.
Rev. Micropaléont., 39, 225-243.
Montanari, A., 1986. Spherules from the Cretaceous/Tertiary boundary
clay at Gubbio, Italy: The problem of outcrop contamination. Geology,
14, 1024-1026.
Morgan, J., Lana, C., Kearsley, A., Coles, B., Belcher, C., Montanari, S.,
Diaz-Martinez, E., Barbosa, A., Neumann, V., 2006. Analyses of
shocked quartz at the global K-P boundary indicate an origin from a
single, high-angle, oblique impact at Chicxulub. Earth Planet. Sci.
Lett., 251, 264-279.
Ortega-Huertas, M., Martínez-Ruíz, F., Palomo, I., Chamley, H., 1995.
Comparative mineralogical and geochemical clay sedimentation in the
Betic Cordilleras and Basque-Cantabrian Basin areas at the
Cretaceous-Tertiary boundary. Sediment. Geol., 94, 209-227.
Ortega-Huertas, M., Palomo-Delgado, I., Martinez-Ruiz, F., 1998.
Geological factors controlling clay mineral patterns across the
Cretaceous-Tertiary boundary in Mediterranean and Antlantic
sections. Clay Miner., 33, 483-500.
Osawa, T., Hatsukawa, Y., Nagao, K., Koizumi, M., Oshima, M., Toh,
Y., Kimura, A., Furutaka, K., 2009. Iridium concentration and noble
gas composition of Cretaceous–Tertiary boundary clay from Stevns
Klint, Denmark. Geochem. J., 43, 415-422.
Peucker-Ehrenbrink, B., Hannigan, R. E., 2000. Effects of black shale
weathering on the mobility of rhenium and platinum group elements.
Geology, 28, 475-478.
Premović, P. I., 2009. Experimental evidence for the global acidification of
surface ocean at the Cretaceous-Palaeogene boundary: The biogenic
calcite-poor spherule layers. Int. J. Astrobiol., 8, 193-206.
Premović, P. I., 2009. The conspicuous red ’impact“ layer of the Fish
Clay at Hǿjerup (Stevns Klint, Denmark). Geochem. Int., 47, 513-521.
Premović, P. I., Nikolić, N. D., Tonsa, I. R., Pavlović, M. S., Premović,
M. P., Dulanović, D. T., 2000. Copper and copper(II) porphyrins of
the Cretaceous-Tertiary boundary at Stevns Klint (Denmark). Earth
Planet. Sc. Lett., 177, 105-118.
Quitté, G., Robin, E., Capmas, F., Levasseur, S., Rocchia, R., Birck, J. L.,
Allègre, C.J., 2003. Carbonaceous or ordinary chondrite as the
impactor at the K/T boundary? Clues from Os, W and Cr isotopes. In:
Stansbery, E. & Reid, A. (Eds.) Proceedings of the 34th Lunar and
Planetary Science Conference. Lunar and Planetary Institute, Houston,
USA, 1615-1616.
Robin, E., Rocchia, R., 1998. Ni-rich spinel at the Cretaceous–Tertiary
boundary of El Kef, Tunisia. Bull. Soc. Géol., 169, 365-372.
P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
Robin, E., Boclet, D., Bonte, P., Froget, L., Jehanno, C., Rocchia, R., 1991.
The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary
boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C
(Leg 122). Earth Planet. Sci. Lett., 107, 715-721.
Rocchia, R., Boclet, D., Bonté, P., Devineau, J., Jéhanno, C., Renard, M.,
1987. Comparaison des distributions de l`iridium observées à la limite
Crétacé-Tertiaire dans divers sites européens. Mem. Soc. Geol. Fr.,
150, 95-103.
Schmitz, B., 1985. Metal precipitation in the Cretaceous–Tertiary
boundary clay at Stevns Klint, Denmark. Geochim. Cosmochim. Acta,
49, 2361-2370.
Schmitz, B., 1988. Origin of microlayering in worldwide distributed Ir-rich
marine Cretaceous/Tertiary boundary clays. Geology, 16, 1068-1072.
Schmitz, B., Asaro, F., Michel, H. V., Thierstein, H. R., Huber, B. T.,
1991. Element stratigraphy across the Cretaceous-Tertiary boundary in
ODP Hole 738C. Proc. Ocean Drill. Program Sci. Results, 119, 719730.
Shukolyukov, A., Lugmair, G. W., 1998. Isotopic evidence for the
Cretaceous-Tertiary impactor and its type. Science, 282, 927-930.
Smit, J., Hertogen, J., 1980. An extraterrestrial event at the CretaceousTertiary boundary. Nature, 285, 198-200.
Smit, J., 1990. Meteorite impact, extinctions and the Cretaceous-Tertiary
Boundary. Geol. Mijnbouw, 69, 187-204.
Smit, J., 1999. The global stratigraphy of the Cretaceous-Tertiary
boundary impact ejecta. Annu. Rev. Earth Pl. Sc., 27, 75-113.
Strong, C. P., 1977. Cretaceous-Tertiary boundary at Woodside Creek, northeastern Marlborough. New Zeal. J. Geol. Geophys., 20, 687-696.
Strong, C. P., 2000. Cretaceous-Tertiary foraminiferal succession at
Flaxbourne River, Marlborough, New Zealand. New Zeal. J. Geol.
Geophys., 43, 1-20.
Strong, C. P., Brooks, R. R., Wilson, S. M., Reeves, R. D., Orth, C. J.,
Mao, X.-Y., Quintana, L. R., Anders, E. A., 1987. A new CretaceousTertiary boundary site at Flaxbourne River, New Zealand:
Biostratigraphy and geochemistry. Geochim Cosmochim Acta, 51,
2769-2777.
Surlyk, F., Damholt, T., Bjerager, M., 2006. Stevns Klint, Denmark:
Uppermost Maastrichtian chalk, Cretaceous–Tertiary boundary, and
lower Danian bryozoan mound complex. Bull. Geol. Soc. Denmark,
54, 1-48.
Tredoux, M., De Wit, M. J., Hart, R. J., Lindsay, N. M., Verhagen, B.,
Sellschop, J. P. F., 1989. Chemostratigraphy across the CretaceousTertiary boundary and a critical assessment of the iridium anomaly. J.
Geol., 97, 585-605.
Trinquier, A., Birck, J. L., Alegret, C. J., 2006. The nature of the KT
impactor. A 54Cr reappraisal. Earth Planet. Sci. Lett., 241, 780-788.
Vickery, A. M., Melosh, H. J., 1990. Atmospheric erosion and impactor
retention in large impacts: Application to mass extinctions. In:
Sharpton, B. & Ward, P. (Eds.) Global Catastrophes in Earth History.
Geological Society of America, Boulder, 289-300.
Walker, R. J., 2009. Highly siderophile elements in the Earth, Moon and
Mars: Update and implications for planetary accretion and
differentiation. Chem. Erde-Geochem., 69, 101-125.
Wdowiak, T. J., Armendarez, L. P., Agresti, D. G., Wade, M. L.,
Wdowiak, Y. S., Claeys, P., Izett, G. A., 2001. Presence of an iron rich
nanophase material in the upper layer of the Cretaceous–Tertiary
boundary clay. Meteorit. Planet. Sci., 36, 123-134.
Wendler, J., Willems, H., 2002. Distribution pattern of calcareous
dinoflagellate cysts across the Cretaceous/Tertiary boundary (Fish
Clay, Stevns Klint, Denmark): Implications for our understanding of
species-selective extinction. Geol. S. Am. S., 356, 265-275.
Wolbach, W. S., Gilmour, I., Anders, E., Orth, C. J., Brooks, R. R., 1988.
Global fire at the Cretaceous-Tertiary boundary. Nature, 334, 665-669.