State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact

TL;DR

High-precision ⁴⁰Ar/³⁹Ar dating of Deccan Traps plagioclase shows the magmatic system underwent a state shift within ~50,000 years of the Chicxulub impact and the Cretaceous-Paleogene boundary, after which ~70% of the Traps' total volume erupted in more massive, more episodic events. The authors argue the impact likely triggered the accelerated volcanism, and that the two mechanisms together drove the terminal-Cretaceous extinctions and suppressed postextinction marine recovery.

Key findings

• The Deccan system shifted from high-frequency, low-volume eruptions (Kalsubai Subgroup, compound pahoehoe) to low-frequency, high-volume sheet-lobe eruptions (Wai Subgroup), with the transition occurring across the Lonavala Subgroup; ~70% of the Traps' total volume was extruded after the shift.
• Kalsubai Subgroup: no less than ~70% erupted before the KPB over 173 ± 84 ky, representing 71 × 10³ km³ at a mean rate of ~0.4 ± 0.2 km³/year. The KPB (66.043 ± 0.010 Ma) fell less than 165 ± 68 ky after Kalsubai emplacement.
• Wai Subgroup rates were roughly double: ~300 × 10³ km³ extruded between the basal Bhimashankar and uppermost Ambenali Formations over 547 ± 241 ky (~0.6 km³/year), and 111 × 10³ km³ at 0.9 ± 0.3 km³/year from U/Pb zircon dates in the upper Ambenali and mid-Mahabaleshwar Formations.
• Interpolation of age-volume data places the transition within 50 ky of the Chicxulub impact; Wai Subgroup eruptions continued ~500 ky after the KPB, comparable to the lag before initial marine ecological recovery.

Methods

• New high-resolution ⁴⁰Ar/³⁹Ar plateau ages on igneous plagioclase (one to four aliquots per formation) from Kalsubai Subgroup formations and the Ambenali Formation, sampled across multiple Western Ghats sections; stepwise heating, dense bracketing with standards during irradiation, and characterization of interfering nuclear reactions.
• Ages were placed in a composite stratigraphic section and combined with previously reported U/Pb zircon dates and lava volume estimates (weighted by subgroup areal extent) to derive mean magma generation rates.
• Samples with ambiguous formation assignments were analyzed geochemically to confirm placement in the chemical stratigraphy.

Limitations

• Stated: available geochronologic data cannot resolve age variations below 100 ky except U/Pb zircon data, whose protracted age distributions require subjective interpretation; material suitable for U/Pb dating occurs only in rare magma segregation bodies and red boles, which are sparse in the Lonavala and Kalsubai Subgroups.
• Stated: formation thicknesses vary substantially across the province, so magma generation rates inferred from any single section may be misleading (hence the volume-weighted approach).
• Stated/evident: the close temporal coincidence of impact and accelerated volcanism makes it difficult to deconvolve their respective environmental effects; the authors also note an alternative explanation that increased red bole abundance in the Wai Subgroup could reflect post-KPB climate-driven weathering rather than decreased eruption frequency.