Determination of rapid Deccan eruptions across the Cretaceous‐Tertiary boundary using paleomagnetic secular variation: Results from a 1200‐m‐thick section in the Mahabaleshwar escarpment

Abstract

[1] Flow-by-flow reanalysis of paleomagnetic directions in two sections of the Mahabaleshwar escarpment, coupled with analysis of intertrappean alteration levels shows that volcanism spanned a much shorter time than previously realized. The sections comprise the upper part of magnetic chron C29r, transitional directions and the lowermost part of C29n. Lack of paleosecular variation allows identification of four directional groups, implying very large (40 to 180 m thick) single eruptive events (SEEs) having occurred in a few decades. Paleomagnetism allows temporal constraints upon the formation of 9 out of 23 thin red bole levels found in the sections to no more than a few decades; the two thickest altered layers could have formed in 1 to 50 ka. The typical volumes of SEEs (corresponding to magnetic directional groups) are estimated at 3000 to 20,000 km 3 , with flux rates $100 km 3 a À1, having lasted for decades. Flood basalt emission can be translated into SO 2 injection rates of several Gt a À1 , which could have been the main agent of environmental change. The total volume of SO 2 emitted by the larger SEEs could be on the order of that released by the Chicxulub impact. Moreover, each SEE may have injected 10 to 100 times more SO 2 in the atmosphere than the deleterious 1783 Laki eruption. The detailed time sequence of SEEs appears to be the key feature having controlled the extent of climate change. If several SEEs erupted in a short sequence (compared to the equilibration time of the ocean), they could have generated a runaway effect leading to mass extinction.

Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South Atlantic

“…Assuming a modal secular variation rate of ~2˚/100 years, lava flows of statistically homogeneous 297 direction can be placed into directional groups of flows separated by less than a century in eruption time, 298 or potentially one large eruptive event (Chenet et al, 2008). By implementing the test outlined in Chenet 299 et al, (2008), six groups are evident in the sequence: T03-T07, T10-T13, T15-T16, T19-T20, T25-T29, 300 and T36-T37 (Table 1).…”

Floral changes across the Triassic/Jurassic boundary linked to flood basalt volcanism

“…Two recent case studies, however, have significantly altered our perception of the devastating effects of subaerial flood basalt eruptions. Improved dating techniques show that the emplacement of the end-Cretaceous Deccan Traps proceeded in several extremely large and rapid pulses 35,36 38 .…”

The Siberian Traps and the End-Permian mass extinction: a critical review

“…However, using both U/Pb and Ar-Ar dates (corrected for the inter-technique differences), it is entirely possible that the bulk of the province was emplaced within less than 2 million years, but it may have been much shorter than this, possibly significantly less than 1 million years. Recent palaeomagnetic and radiometric studies of the Deccan Traps, India, suggest that the bulk of those lavas were erupted in less than 600000 years [27,28] . Timescales of 300000 years have been estimated [29] for the 4 to 6 km of lava erupted in East Greenland at 55 Ma.…”

“…A 'mean lifetime' of 30 to 35 ka for CO 2 in the atmosphere is much greater than the average eruptive frequency of flow fields (100 to 1000 years [27,28] ), so it is reasonable to consider average eruption fluxes. Assume that the bulk of the Traps were erupted in 600000 years, and that they have a total eruptive volume of 3×10 6 km 3 .…”

“…Furthermore, the Siberian eruptions may have been particularly sulphur rich, because some of the ascending magmas interacted with evaporate deposits [83] . Unlike the Pinatubo eruption, where an estimated 0.02 Gt of SO 2 equivalent were injected into the atmosphere [84] , a major flood basalt eruption of 1000 km 3 could release between 3 and 10 Gt [71,72,82] , and rapidly successive eruptive events could lead to massive atmospheric loading of sulphates [28,82] . The release would be sustained for the duration of the eruption, lasting several years or decades, and leading to a large reduction in the transmission of light through the atmosphere.…”