MAIN DECCAN PULSE OCCURED CLOSE TO THE KTB : INCREASING MULTIPROXY EVIDENCE

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Recent studies indicate that the bulk (80%) of the Deccan trap eruptions occurred over less than 0.8 m.y. in magnetic polarity C29r spanning the Cretaceous-Tertiary boundary (KTB) (Chenet et al, 2007, 2008). Detailed multiproxy studies from several sections from southeastern India (Rajhamundry, Andhra Pradesh) and central India (Jilmili, Madhya Pradesh) place the KTB event near the end of the main Deccan eruptive phase and indicate that Deccan volcanism could have been a major contributor to the mass extinction (Keller et al., 2008, 2009). Geochemical, mineralogical and micropaleontogical evidence from localities outside India suggest that this megapulse took place in the uppermost Maastrichtian C29r below the KTB (CF2-CF1 transition). For example, a rapid shift in ¹⁸⁷Os/¹⁸⁸Os ratios observed in three deep-sea sections (Atlantic, Pacific and Indian Oceans) are interpreted to mark the onset of the main Deccan pulse in C29r (Robinson et al., 2009). Foraminiferal oxygen isotope data from DSDP Site 525 (South Atlantic) show that the short rapid global warming in C29r (Li and Keller, 1998) coincides with the decline in ¹⁸⁷Os/¹⁸⁸Os ratios. A coeval increase in weathering is observed in Site 525 and Tunisia (Adatte et al., 2002) marked by dominant kaolinite clay assemblages. In the same interval a significant decrease in bulk carbonate content suggests acidification due to volcanic SO₂. Enhanced dissolution is also observed at DSDP Site 527 and Gubbio, Italy. Calcareous microfossils (planktic foraminifera and nannofossils) experienced major stress conditions expressed in species dwarfing and decreased abundance (Keller, 2005). These observations indicate that Deccan volcanism played a key role in increasing atmospheric CO₂ levels that resulted in global warming and enhanced greenhouse effect, which coupled with high SO₂ emissions, increased biotic stress and predisposed faunas to eventual extinction at the KTB.

Adatte et al., 2002, PPP, 2754, 1-32. Chenet et al, 2007 EPSL 263: 1-15; 2008 JGR 113: BO4101. Keller, 2005, AJES 52, 725-757; Keller et al., 2008 EPSL 268:293-311; 2009 EPSL 282: 10-23, Li and Keller, 1998, Geol. 26(11) :995-998, Robinson et al, 2009, EPSL, 281, 59–168.