Bhandari, N.

Blanchard, D. P.

MacDougall, J. D.

Murali, A. V.

English

Publications of the IAS Fellows

LPSC !XXI 819 
RELATIONSHIP OF DECCAN VOLCANISM TO GLOBAL K-T BIOTIC EXTINCI'IONS: A C ~ E  
OF UNFOUNDED ASSUMPTIONS? A. V. Murali, Lunar and Planetary Institute, 3303 NASA Road 1, 
Houston, TX 77058, D.'P. Blanchard, NASA/ Johnson Space Center, Houston, TX 77058, N. Bhandari, 
Physical Research Laboratory, ,Ahmedabad 380009, India, J. D. Macdougall, Scripps Institution of 
Oceanography, La Jolla, CA 92093 
In the debate about external versus internal causes for the K-T scenario, a spate of recent publications 
(1-5) suggest that there is a strong case in favor of the latter. 40Ar/39~r ages of Deccan volcanics (53)  and 
the observation of high iridium concentrations in Kilauea aerosols (6,7) are considered as the strong points 
that link the K-T Ir anomaly and possibly extinctions to the voluminous Deccan eruptions (l,3). However, our 
studies of the Deccan basalt province show that these connections are tenuous at best. 
Iridium anomalies: It is well  own that the Deccan Traps represent one of the most extensive flood basalt 
provinces in the world (-lo6 krn3); they occur mainly as flat lying tholeitic lavas (Rig. 1). Continental K-T 
sections (infratrappean Lameta beds and basal intertrappean beds with abundant dinosaur fossils) occur near 
the periphery of the province (Fig. 2) and several marine K-T sections (in Meghalaya and Tamitnadu states) 
also occur in the Indian subcontinent. We sampled numerous Deccan basalt flow sequences (Fig. 1) for 
chemical and isotopic studies (8,9). Eight selected samples of the Deccan flows representing major chemical 
groups were analysed by RNAA for iridium. The data show that the iridium in the Deccan basalts is low and 
variable (0.02 + 0.002 to ( 0.002 ppb). This is comparable to Columbia River basalt values (10). 
Based on detailed vertebrate and miaofossil evidence, Sahni and Bajpai (1988) identified continental 
K-T sections at Jabalpur (iatrappean Lameta beds) and Nagpur (Takli intertrappean beds). Detailed study 
of the Jabalpur section showed (12) that the iridium of these samples is ~ 0 . 2  ppb (below the detection limit of 
our INAA technique) indicating that there is no iridium anomaly in these layers. We carried out systematic 
studies of the Takli intertrappean beds and the marine K-T boundary clay layer from Meghalaya (F* 3 ) 
Iridium in Takli beds also is low ( c 0 2  ppb) and comparable to the Jabalpur section. This means that the 
continental K-T boundary sections in India do not show any significant iridium anomalies. However, the 
Meghalaya marine K-T layer shows a significant iridium &omaly (4.1 0 3  ppb) similar to many marine K-T 
sections (13) but much lower than values at S t e m  Klint, Denmark (87 ppb), Caravaca, Spain (44 ppb), 
Woodside Creek, New Zealand (28 ppb) etc. Were Deccan eruptions an important Ir source, much higher 
values would be expected in these nearby K-T sedions in the Indian subcontinent. It should be noted that, 
although lo5 fold Ir enrichments were observed in Kilauea aerosols, iridium has not been detected in 
numerous measurements of volcanic r i g l s e w h e r e  (7). 
Age of Deccan Volcanism: Recent Ar/ Ar age determinations of Deccan volcanics generally cluster near 
65 Ma and it has been argued that the entire province was erupted at -65 Ma within a very short time interval 
(53). This interpretation is based on a limited number of ages for surface samples and may be oversimplified. 
Furthermore among the surface flows sampled for the 40Ar/39Ar age determinations only nine samples (5) 
seem to have any stratigraphic control and the samples selected do not cover the region north of Indore or 
south of Poona (Fig. 3) which excludes a large part of Deccan basalt province. 
Deep Seismic Sounding (DSS) profiles aaoss the Deccan province are suggestive of trap thicknesses 
upto - 3-45 km in some areas (14). Much of this material must be buried. Unless these stratigraphically 
lower flows are dated neither the time of initiation nor the duration of Deccan volcanism can be constrained. 
If Deccan eruptions were caused by simple progression of the 'hot spot' from north to south as the 
plume model would suggest (IS), an -7-Ma age difference would be expected between flows erupted in north 
and those in the south based on the Indian plate motion of 14 cm/yr (16) and 1000-km trap, 
From this study we conclude that the global iridium spike cannot be attniuted to Deccan volcanism. 
Although it is clear that Deccan volcanism occured near K-T time, the present age data are inadequate to 
determine the time of initiation and total span of the Deccan volcanic episode. Systematic study of deep drill 
core samples may resolve this issue. 
REFERENCES: [I] Courtillot, V and Cisowski, S (1987), a @, 193. [2] Duncan, FL A and Pyle, D. G. 
(1988), Nature 3 Z  841. [3] Courtillot, V. et a1.(1988), ~ a k r e .  a 843. [4] ~ourtillot, V. (1988), EOS. 69,301. 
[5] Jaeger, J-J. et al. (1989), Geolog, 17.316. [q Zoller, W. H. et al. (1983), Science. a 1118. [7] Olmez et 
al. (1986), J. Geo~hvs. Re$, 91. 653. [8] Mahoney, J. J. et al. (1982), EPSL. @, 47. [9] Murali, A. V. et al 
O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System 
820 LPSC XXI 
RELATIONSHIP OF DECCAN VOLCANISM TO GLOBAL K-T EXIWCI'IONS? 
Murali, A V. et al. 
(1987), LPSC XVIII, 676. [lo] Ganapath~~ R, et al. (1974), Anal. Chirn. Act& 2 1. [ll] Sahni, A and Bajpai, 
S, (1988) J. Geol. Soc. India, a 3 8 2  [12] Murali, A. V. et al. (1989), LPSC XX, 740. [I31 Alvarez, et al. (1982), 
Geol. Soc. Am. SDI. Daper, 305. [14] Kaila, K L. (1989), Int. GmL ~ongress'28th (Abstr), 2, 145, [q 
Morgan, W. J, (1972), Am, Assw. Pet. Geol. Bull. & 203. [16] Pierce, J. W. (1978), Geo~hvs. J. R. Astron. 
a. z 2.77. 
Fig. 1 Decan Traps, India- Mdor tlorr coltunns sampled for tbe stody (Numben 
iadiate the heights of basalt columns in meters above or below MSL). 
1.- 
L = U Y n A E D S  
I=- 
0 9 5 0  4DObn x=maSAunc- 
-
SCALE t .r=.- 
-
YU 
F i  2 Continental K-T Boundmy sites around Bccern province. 
Fig. 3 Deccan province (stippled area), India and K-T boundary 
sites of the present strrdy. 
O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System 


Relationship of Decan volcanism to global K-T biotic extinctions: a case of unfounded assumptions?

http://repository.ias.ac.in/89243/1/9-Pub.pdf

Relationship of Decan volcanism to global K-T biotic extinctions: a case of unfounded assumptions?

Abstract

Similar works

Full text

Available Versions

Publications of the IAS Fellows