Sr and ⁸⁷Sr/⁸⁶Sr in rivers draining the Deccan Traps (India): implications to weathering, Sr fluxes, and the marine ⁸⁷Sr/⁸⁶Sr record around K/T

Abstract

The concentration of dissolved Sr and its ⁸⁷Sr/⁸⁶Sr has been measured in the headwaters of the Krishna River System and west flowing Western Ghat rivers, all of which have their drainage almost entirely in the Deccan Traps. The Sr concentration follows that of Ca and Mg with Sr/Ca and Sr/Mg ratios similar to that of Deccan basalts, suggesting that all these alkaline earths are released to waters nearly congruently from the Deccan basalts during chemical weathering. The ⁸⁷Sr/⁸⁶Sr range from 0.70614 to 0.70986, within that reported for the Deccan basalts. The dissolved Sr flux from the Deccan calculated from the measured data is ~1.35 × 10⁸ moles yr^-1, ~0.4% of the global riverine flux, which is nearly the same as the proportion of area covered by Deccan basalts relative to the global drainage. The flux, however, is a factor of ~3 lower than that reported for the Narmada-Tapti-Wainganga (NTW) rivers draining the northern Deccan. This difference in Sr flux among rivers draining the various regions of Deccan could be natural spatial/temporal variations and/or due to supply of Sr to NTW rivers from nonbasaltic sources. Model calculations on the role of emplacement and weathering of Deccan on marine Sr isotope evolution around KTB and late Tertiary show that Deccan basalts can be an important contributor to the decline in ⁸⁷Sr/⁸⁶Sr during these periods. It is shown that to account for the pre-KTB dip in ⁸⁷Sr/⁸⁶Sr, the flux requirement from Deccan at that time would have to be several times the contemporary flux with ⁸⁷Sr/⁸⁶Sr of 0.705-0.708. Considering that the area of Deccan at KTB was about thrice the present area and that the weathering rate of Deccan basalts may have been much higher in early stages following eruption, such high fluxes at about KTB seem feasible. The disproportionately higher flux requirement for Sr is similar to that invoked to explain the pre-KTB decrease in ¹⁸⁷Os/¹⁸⁸Os. The calculations also show that the supply of unradiogenic Sr from basalts can have significant control on the long-term, ~66 to ~55 Ma, decline of marine ⁸⁷Sr/⁸⁶Sr.