BREVIA
Comet or Asteroid Shower in
the Late Eocene?
Roald Tagle1 and Philippe Claeys2*
In the late Eocene [⬃34 to 36.5 million years ago
metary origin. The PGE concentration of com(Ma)], the deep-water carbonates of the Massigets is unknown. Nonetheless, it is unlikely that
nano section (Italy) were enriched in extraterresa cometary bolide would display PGE elementrial 3He, indicating a higher delivery of interplantal ratios similar to those of L-chondrites. Cometary dust particles (IDPs) to Earth (1). This ⬃2.5ets are believed to be primitive bodies with a
million-year–long increase in the flux of IDPs is
composition like that of carbonaceous chonattributed to the arrival in the inner solar system of
drites, which differ in PGE concentrations from
long-period comets, triggered by a perturbation of
ordinary chondrites (Fig. 1). Because asteroids
the Oort cloud (1). The two largest terrestrial
may form ⬃1% of the inner Oort cloud obcraters in the Cenozoic, the 100-km Popigai (datjects (4), it cannot be excluded that a comet
ed 35.7 ⫾ 0.2 Ma) in northern Siberia and the
shower would include an L-chondrite body;
85-km Chesapeake Bay (dated 35.5 ⫾ 0.6 Ma)
however, this is a rather remote possibility.
just offshore Virginia, formed during the same
Popigai is most likely an asteroid impact. The
time interval (2); they are likely the result of
probability that it coincides with a comet
cometary impacts (1).
shower is also low. A 5-km object, required
Meteorites are enriched in
platinum group elements (PGEs)
compared to crustal rocks. The
PGE elemental ratios in the rocks
melted by an impact compared
with those of known meteorites
document the composition of the
crater-forming projectile. Popigai
impact-melt rock (32 samples)
and target lithologies (3 samples)
were analyzed for PGEs with a
NiS fire assay combined with inductively coupled plasma mass
spectrometry (ICP-MS) (3) (table
S1). Major and trace element
compositions, measured by
x-ray fluorescence, were homogeneous, but all impact-melt
samples showed a major enrichment (up to 15 times higher) in
PGEs compared to the target
lithologies. The meteoritic contamination of the Popigai melt
rock is ⬃0.2 weight percent; its
PGE pattern and Ni/Cr ratio are
chondritic. The Popigai PGE elemental ratios systematically overlap the field of ordinary chondrites
(Fig. 1A). Combining ratios of elements with low (Pd and Rh) and
high (Pt and Ru) condensation
temperatures refines the identification of the projectile to an L- Fig. 1. (A and B) PGE elemental ratios in the impact melt of the
chondrite (Fig. 1B), whose source Popigai crater compared to the elemental ratios for different
types of chondrites. The error bars represent one standard
is possibly the type S(IV) asteroids deviation. Chondrite types: L, low iron; LL, low iron and low
from the inner asteroid belt.
metal; H, high iron; CV, carbonaceous Vigarano; CM, carbonaAn L-chondrite bolide is dif- ceous Mighei; CI, carbonaceous Ivuna; EL, enstatite low iron;
ficult to reconcile with a co- EH, enstatite high iron.
492
to produce Popigai, strikes Earth only once
every ⬃26 million years (5).
The large impacts and the increase in IDPs
of the late Eocene could be the results of a
major collision in the asteroid belt. This possibility was discarded by Farley et al. (1) because
they relied on modeling that estimated a decay
of IDP production rate over much longer periods than the 2.5-million-year IDP enrichment
observed at Massignano. However, in the case
of asteroid collision, recent work supports time
scales of a few million years for the delivery of
asteroidal material toward Earth (6). A major
collision in the asteroid belt could produce dust
and inject a large number of fragments into
resonances capable of sending them into Earthcrossing orbits. An asteroidal collision would
explain both the high delivery of IDPs and the
occurrence of two large impacts, one of them
by an ordinary chondrite, over an interval of a
few million years. This event may be comparable to the disruption ⬃480 Ma of the Lchondrite parent body, which induced the higher accretion rate of micrometeorites and cosmic
dust found in the middle Ordovician marine
limestone of Sweden (7). This asteroidal collision hypothesis can be tested: The identification
of the Chesapeake Bay crater projectile may
confirm (or not) the asteroidal origin of the flux
of IDPs and larger objects of the late Eocene.
References and Notes
1. K. A. Farley, A. Montanari, E. M. Shoemaker, C. S.
Shoemaker, Science 280, 1250 (1998).
2. A. Montanari, C. Koeberl, Impact Stratigraphy (Lecture
Notes in Earth Sciences 93, Springer, Berlin, 2000).
3. Materials and methods are available as supporting
material on Science Online.
4. P. R. Weissman, H. F. Levison, Astrophys. J. 488, L133
(1997).
5. B. M. French, Traces of Catastrophe (Contribution No.
954, Lunar and Planetary Institute, Houston, 1998).
6. A. Morbidelli, B. Gladman, Meteorit. Planet. Sci. 33,
999 (1998).
7. B. Schmitz, T. Haggstrom, M. Tassinari, Science 300,
961 (2003).
8. We thank the three anonymous reviewers for substantially improving this manuscript and J. Erzinger
for his assistance with the ICP-MS and for providing
access to the analytical facilities at the GeoForschungsZentnum-Potsdam. Supported by Funds for
Scientific Research–Flanders (Belgium), a Vrije Universiteit Brussel–Onderzoeksraad grant, and the
Deutsche Forschungsgemeinschaft (grant nos. Cl147/
2-1 and Cl147/2-2).
Supporting Online Material
www.sciencemag.org/cgi/content/full/305/5683/492/
DC1
Materials and Methods
References and Notes
Table S1
30 March 2004; accepted 13 May 2004
1
Institut fuer Mineralogie, Museum fuer Naturkunde,
D-10099 Berlin, Germany. 2Department of Geology,
Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium.
*To whom correspondence should be addressed. Email: [email protected]
23 JULY 2004 VOL 305 SCIENCE www.sciencemag.org
Comet or Asteroid Shower in the Late Eocene?
Roald Tagle and Philippe Claeys
Supporting Online Material
Sample
The 32 unaltered samples of impact melt and the 3 target rock gneisses were collected at
several locations on the Western side of the Popigai crater in Siberia (GPS coordinates: 71º
45 530′ N & 110º 15 171′ E).
Methodology
The NiS fire assay combined with ICP-MS analytica method is described by (1). The
samples are finely powdered in corundum ball mill. PGE concentrations are determined at
the GeoForschungZentrum in Potsdam, dependably down to 0.06 ng/g Ru, 0.01 ng/g Rh, 0.14
ng/g Pd, 0.06 ng/g Ir, 0.1 ng/g Pt, and 0.13 ng/g Au. The weight of the samples varied
between 10 and 50 g to ensure representative and reproducible analyses. With less than 10 g,
the values obtained by repetitive analyses displayed significant variability probably because
of the tendency of PGE to form micro-nuggets. The accuracy of the PGE measurements was
tested by recurring analyses of three certified reference materials: TDB-1, WGB-1 both with
low and WMG-1 with high PGE concentrations. The PGE + gold results are presented in
Table 1. Between 20 and 150 g of the same powder were analyzed for major and trace
elements by x-ray fluorescence spectroscopy (XRF) on glass beads with a SIEMENS SRS
3000 instrument, at the Museum of Natural History in Berlin. The melt rock and the target
lithologies are homogenous in term of their major and trace element composition confirming
the data of (2).
References
1. H.-G. Plessen, J. Erzinger, Geostandards Newsletter 22, 187-194 (1998).
2. J. Whitehead, R. A. F. Grieve, J. G. Spray, Meteoritics & Planetary Science 37,
632-647 (2002).
Table 1. Concentrations in ng/g (of ppb) of the PGE (+gold) in the Popigai impact melt and
target rocks. The values measured on 3 international references standards (Canadian
Certified References Material Project, CCRMP) compared to their specified values are
also presented. Values with error are certified, concentrations without errors are values
proposed by CCRMP. Analytical error for each element is given in italics.
Sample
error
Impact melt sample no.
58
59
102
103
228
230
56
57
107
33
34
35
229
231
106
105
144
134
61
62
154
124
140
152
121
146
136
147
125
143
133
135
Average
Standard dev.
Ru [ng/g]
± 0.05
Rh [ng/g]
± 0.02
Pd [ng/g]
± 0.06
Ir [ng/g]
± 0.05
Pt [ng/g]
± 0.08
Au [ng/g]
± 0.07
1.38
1.65
2.72
2.96
1.78
1.33
1.30
1.26
2.09
1.04
0.96
0.43
1.42
1.23
2.15
3.61
1.52
1.64
1.72
2.18
1.85
1.77
1.80
2.41
2.41
2.16
1.88
1.21
1.06
1.85
1.84
2.35
1.77
0.58
0.31
0.39
0.63
0.65
0.44
0.32
0.29
0.30
0.52
0.21
0.20
0.19
0.38
0.37
0.52
0.83
0.33
0.40
0.35
0.48
0.46
0.44
0.46
0.52
0.54
0.51
0.48
0.26
0.29
0.49
0.43
0.52
0.42
0.14
1.21
1.51
2.52
2.78
2.02
1.60
1.16
1.11
2.04
0.51
0.57
0.61
1.55
1.52
1.71
3.31
1.35
1.69
1.63
2.06
1.92
1.71
1.75
2.04
2.22
2.02
2.15
1.01
1.08
1.89
1.78
1.58
1.75
0.48
0.80
1.01
1.71
1.73
0.97
0.57
0.75
0.82
1.23
0.48
0.55
0.43
0.96
0.65
1.41
2.31
0.68
0.92
0.85
1.19
0.87
0.94
0.99
1.27
1.34
1.13
1.09
0.60
0.72
1.00
0.98
1.16
0.96
0.38
2.14
3.20
4.00
3.89
5.63
1.97
2.07
2.20
2.67
1.97
1.52
1.48
2.49
1.68
3.17
5.54
1.84
2.55
2.25
3.05
2.78
2.63
2.75
3.60
3.46
3.03
3.09
1.52
1.96
2.85
2.57
3.08
2.58
0.84
1.35
1.49
1.27
2.27
0.85
0.78
0.71
1.17
1.32
1.00
0.67
1.72
0.09
0.44
0.65
1.71
0.60
0.43
0.91
1.22
1.00
0.79
0.39
0.92
1.06
1.60
1.18
0.69
0.61
1.17
0.32
0.31
0.87
0.43
Target rocks
gneiss
garnet gneiss
garnet gneiss
0.16
u.l.d.
0.24
0.05
0.03
0.07
0.45
0.21
0.38
0.16
u.l.d.
0.14
0.69
0.40
0.61
1.63
0.57
0.27
Standard
TDB-1 (CCRMP)
TDB-1 (n = 3 )
0.3
0.24 ± 0.07
0.7
0.42 ± 0.03
22.4
20 ± 1.0
0.15
0.12 ± 0.004
5.8 ± 1.1
4.8 ± 1.0
6.3 ± 1.0
4.0 ± 0.8
WGB-1 (CCRMP)
WGB-1 (n = 8)
0.3
0.16 ± 0.01
0.32
0.15 ± 0.04
13.9 ± 2.1
10 ± 2.9
0.33
0.18 ± 0.06
6.1 ± 1.6
3.7 ± 1.1
2.9 ±1.1
0.5 ± 0.2
46 ± 4
45 ± 2.4
731 ± 35
688 ± 33
110 ± 11
85 ± 13
WMG-1 (CCRMP).
35 ± 5
26 ± 2
382 ± 13
WMG-1 (n = 3)
27 ± 2.5
25 ± 1
338 ± 32
n=number of analyses, u.l.d = under limit of detection