N. Bhandari et al., CHEMICAL PROFILES IN K T BOUNDARY SECTION OF MEGHALAYA, INDIA - COMETARY, ASTEROIDAL OR VOLCANIC/, Chemical geology, 113(1-2), 1994, pp. 45-60
The Cretaceous-Tertiary boundary (KTB) exposed as a 1.5-cm-thick limon
itic layer in the Um Sohryngkew River basin in Meghalaya is marked by
enhanced concentrations of Ir, Co, Ni, Os, Fe, Zn, Sb (by factors of 4
to approximately 1200) and REE (by factors of 1.7 to approximately 5)
compared to the Cretaceous shales. Ir concentration is generally < 0.
01 ng g-1 in shales away from the boundary but gradually increases by
a factor of > 10 in approximately 2-m-thick ''broad band'' around the
KTB, abruptly reaching a peak concentration of 12.1 ng g-1 within the
limonitic layer. The Os/Ir ratio at the KTB is approximately 0.37 but
in the adjacent shales it is approximately 1.8, suggesting that the so
urces of these elements in the peak and the broad band may be differen
t. Cretaceous planktonic foraminifera and dynocysts start disappearing
before the Ir-rich limonitic layer was deposited but several diminuti
ve Cretaceous foraminifera survive after this layer. The observations
indicate that the extinctions are confined to a short span of time but
are probably not as abrupt as anticipated by the asteroidal impact hy
pothesis. The chemical data are discussed in terms of cometary, astero
idal and volcanic hypotheses. The profiles of Ir and Os/Ir can be unde
rstood in the frame work of a model of cometary impact in which cometa
ry debris falls onto the Earth preceding and succeeding the fall of th
e cometary nucleus.