Seawater Sr Isotopes at the Cretaceous/Tertiary Boundary

Martin, E.E. and Macdougall, J.D.
Earth and Planetary Science Letters (1991), v. 104, 166-180.

Seawater 87Sr/86Sr values 10-6 increase abruptly by 28 x 10-6 across the Cretaceous/Tertiary boundary (KTB). This small, but rapid shift is superimposed on the larger scale structure of the seawater Sr isotope curve. The time scale of radiogenic Sr addition appears to be too rapid to reconcile with sources associated with volcanism, and we show that the amount of Sr required to produce even this small increase is too large to be derived from: 1) a KT bolide of the size constrained by the Ir anomaly, 2) continental crust ejecta from the impact of such a bolide, 3) soot from global wildfires initiated by an impact, or 4) any combination of these sources. The probable source of the radiogenic Sr is enhanced continental weathering, but the high rate of increase appears to rule out processes such as sea level regression, glaciation or tectonism. A plausible mechanism for rapid addition of radiogenic Sr to the oceans is enhanced weathering associated with globally distributed acid rain (pH~1) which is a proposed by-product of a bolide impact.

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