V22B-01 INVITED
Large Igneous Province Development and Environmental Impacts: some Recent Progress
The debate on causal connection between continental flood basalts and mass extinctions (ME) entered its modern phase in the mid-1980s. This was when attribution of the KT mass extinction to the Chicxulub impact was gaining widespread acceptance, and proponents of the volcanic theory were at first dismissed. Our group entered the debate in 1986 when we published evidence that the Deccan trap eruption in India had occurred at the time of the KT extinction, within uncertainty, and had lasted no more than a million years, based on magnetic stratigraphy. Many other correlations have been established over the following decades (with oceanic plateau (OP) eruptions and massive oceanic anoxia events (OAE) added to the picture). Courtillot and Renne reviewed the problem in 2003, and concluded that in most of 16 cases, most of the lava was erupted in about 1 Ma and sometimes less, with fluxes possibly exceeding over short times the entire crustal production of mid-ocean ridges. The correlation between dates of LIP eruption and ME or OAE has improved steadily with more data becoming available, to the point that the list of trap ages coincides with many major divisions in the geological time scale. The four largest mass extinctions in the last 260 Ma coincide with four traps, making a causal connection between the two hard to dismiss. Our recent work has focused on better measuring the number, volumes and durations of major volcanic pulses within the full lava pile and beginning to assess in a quantitative way their climatological consequences. We have completed a restudy of the Deccan [Chenet et al., 2008a and 2008b, in revision] and are currently analyzing the Karoo traps (M. Moulin et al, this session). We use in combination volcanology, K-Ar absolute dating and detailed analysis of paleomagnetic directions. We find that 3500m of the Deccan traps was erupted in some 30 major eruptive pulses, with volumes ranging up to 20,000km3. Some pulses with thicknesses attaining 200m were emplaced over distances in excess of 100 km. The total time of emission of all pulses could have been less than 10kyr, with most of the time recorded in a very small number of intervening alteration levels marking periods of volcanic quiescence. Based on geochronologic results of Chenet et al [2007] and paleontological results of Keller et al. [2008], we conclude that volcanism occurred in three rather short, discrete mega- pulses, with the largest around within C29r just before the KT boundary, the second largest shortly afterwards spanning the C29r/C29n reversal. Sulfur dioxide (likely the major agent of environmental stress) amounts and fluxes released by pulses would have ranged from 10 to 150 Gton and 1 to 3 Gton/yr respectively, over durations possibly as short as 100 years for each pulse. The input of the Chicxulub impact would have been on the same order as that of a very large single pulse. The impact therefore appears as important but incremental, neither the sole nor main cause of the Cretaceous-Tertiary mass extinctions. And volcanism alone is now the preferred suspect for most other ME and even OAE.
V22B-02 INVITED
The Chicxulub Impact Crater and the Cretaceous/Tertiary Boundary: Results from the Drilling Projects
Chicxulub crater has attracted considerable attention as one of the three largest terrestrial impact structures and its association with the Cretaceous/Tertiary boundary. Chicxulub is a 200 km-diameter multi-ring structure formed 65 Ma ago in the Yucatan carbonate platform in the southern Gulf of Mexico and which has since been buried by Tertiary carbonates. The impact lithologies and carbonate sequence have been cored as part of several drilling projects. Here we analyze the stratigraphy of Chicxulub from the borehole data and core analyses, with particular reference to studies on CSDP Yaxcopoil-1 and UNAM Santa Elena boreholes. Analyses of core samples have examined the stratigraphy of the cover carbonate sequence, impact breccia contact and implications for impact age, K/T global correlations and paleoenvironmental conditions following impact. The K/T age for Chicxulub has been supported from different studies, including Ar/Ar dating, magnetic polarity stratigraphy, geochemistry and biostratigraphy. A Late Maastrichtian age has also been proposed for Chicxulub from studies in Yaxcopoil-1 basal Paleocene carbonates, with impact occurring 300 kyr earlier predating the K/T boundary. This proposal calls attention to the temporal resolution of stratigraphic and chronological methods, and the need for further detailed analyses of the basal carbonate sections in existing boreholes and new drilling/coring projects. Stratigraphy of impact ejecta and basal sediments in Yaxcopoil-1 and UNAM boreholes indicates a hiatus in the basal sequence. Modeling of post- impact processes suggest erosion effects due to seawater back surge, block slumping and partial rim collapse of post-impact crater modification. Analyses of stable isotopes and magnetostratigraphic data for the Paleocene carbonate sequences in Yaxcopoil-1 and Santa Elena boreholes permit to investigate the post- impact processes, depositional conditions and age of basal sediments. Correlation of stable isotopes with the global pattern for marine carbonate sediments provides a stratigraphic framework for the basal Paleocene carbonates.
V22B-03 INVITED
On the Relationship Between Continental Flood Basalts and Environmental Crises
An empirical relationship between continental flood basalts (CFB's) and paleoenvironmental crises (PEC's), including mass extinctions, has been increasingly well established by geochronology. This is mostly due to an increase of stratigraphic and high quality isotopic age data over the past few years and a better understanding of the isotopic systems (e.g. decay constant bias; statistics). It now provides us with the opportunity to correlate CFBs to the stratigraphic timescale with an unprecedented accuracy and precision. Consequently, a new picture emerges: whereas some CFBs seem to be emplaced in a brief time frame (~ 1 Ma or less for the main volume) and coincide with major mass extinctions (e.g. Siberian Traps and the Permo-Triassic boundary) others such as the Karoo-Ferrar province have proven to comprise multiple brief pulses of magmatism collectively spanning 5 Ma or more and to be associated with only minor extinctions and moderate climate change. Perhaps the most intriguing case of all is given by the huge (ca. 1- 2×106) km3) Parana-Etendeka province (PEP) mainly erupted at 133-131 Ma, which did not coincide with a major well-defined extinction event, although a minor drop in sea-level and positive carbon isotope anomalies appear to have occurred. Temporal coincidence, and thus inferred causal relationships, are best established for three of the most significant extinctions in the Phanerozoic record which occurred at the Permo-Triassic, Triassic/Jurassic, and Cretaceous/Paleogene boundaries. These events coincided to within geochronological uncertainties with the Siberian, Central Atlantic and Deccan CFB's (respectively), which are among the largest CFB provinces known with >1.5×106) km3 of erupted magma in each case. The specific causality mechanism between CFB's and PEC's is uncertain, but most likely involves the atmospheric effects of volcanogenic volatiles, particularly CO2 and sulfate aerosols, whose competing effects have different timescales and might be expected to produce global cooling followed by global warming. A pyrogenic component, e.g. from coal measures in the CFB's wallrocks, is becoming recognized as a likely contributor to the volatile flux associated with CFB's. Attendant and intricately related phenomena such as eustasy, clathrate destabilization, acid rain and various other effects are likely contributors. Why did some CFBs have such drastic effects on the biosphere whereas some (i.e. PEP and Karoo CFBs) were so benign? We speculate that the dominant factors to explain weak effects from these large CFB's include unusually CO2- and S-poor, lithospherically dominated magmas in both cases coupled with (in the Karoo case) a lower eruption rate, the low-latitude position of southern Africa at this time and the paucity of explosive volcanism. Additional mitigating circumstances in the PEP case include a relative absence of appropriate wallrocks high in the magma ascent path contributing a pyrogenic component of CO2 and S, and significant independent increases in atmospheric CO2 that were already underway beginning at ca. 150 Ma, overwhelming the effects of the PEP.
V22B-04 INVITED
Volatile contents of magmas from the Deccan and Columbia River provinces: implications for atmospheric gas release from flood basalt eruptions
Sulphur (S) and chlorine (Cl) contents of magmas from the Mesozoic Deccan basalt province have been measured directly on rare, preserved glass inclusions within crystals and on glassy selvages in these ancient lava flows (Self et al., 2008). Lava flows of the Deccan Traps, India, were emplaced around 66-65 Ma ago. S and Cl concentrations range from high values of ~ 1400 ppm S and 500 ppm Cl in inclusions down to a few hundred ppm in lava selvages. The data indicate that the basaltic magmas of certain (and by implication, many) Deccan eruptions would have emitted up to 0.15 wt % SO2 and up to 0.03 wt % HCl, using an approach that accounts for the variable degree of melt evolution. Such values imply atmospheric releases of ~ 4 Tg of SO2 (and 0.8 Tg HCl) per cubic kilometer (km) of basaltic lava erupted, with most of this being released above the vents. Although eruptive volumes of individual Deccan flood basalt lava fields are not known, the SO2 masses released are indicated to be around 4000 Tg for a 1000 cubic km eruption. Similar, to slightly higher, values for S and Cl have been recently obtained by the same method on two other lava flow fields besides the already-studied Roza lava (Thordarson and Self, 1996) from the 15 Ma Columbia River flood basalt province (CRB) in the Pacific NW of the USA. Volumes of individual eruptive units are known for the CRB (those studied are from 1300-2600 cubic km) and it can be shown that the studied eruptions released SO2 masses in the range 8,000 to 12000 Tg, depending upon flow-field volume. In some cases, the vent areas for these eruptions can be explored. Understanding the eruptive style indicated by proximal deposits will help in future modeling of the atmospheric behavior of the eruption columns, and in heights attained. These results provide a solid basis for interpretation and modeling of the environmental impact of gas releases from past flood basalt activity, which has long been assumed to have been severe. The significance of flood basalt volcanism is that the erupted volumes, and hence the potential environmental pollution caused by the gases released, were immense on a scale compared to smaller-scale historic and Quaternary basaltic eruptive activity.