V24A-01
Emplacement Timescales and Environmental Impacts of Large Igneous Provinces: Constraints From the Re-Os Isotope System
Although widespread evidence indicates that the emplacement of 'large' LIPs (>106 km3) was in some way associated with extensive environmental change, it has proved very difficult to determine the precise nature of the link between the two. The unusually large size and rapidity of LIP emplacement make it hard to quantify their environmental impact by reference to the relatively small impact of present-day volcanic activity. For this reason, much research has focussed on interpreting evidence from the marine sedimentary record, which is the major repository of the history of environmental change. Although this approach is the best available, it is far from straightforward. For example, it is very difficult to correlate sedimentary records accurately with the rocks that represent the remnants of LIPs. Whilst the absolute age of LIP emplacement is often well defined from radiometric dating, a consequence of the relative brevity of their emplacement is that precise emplacement rates and durations often remain imperfectly constrained. New geochemical and isotopic techniques now make it possible to overcome some of these difficulties and enable us to identify unambiguously the timing and some of the environmental consequences of LIP emplacement. The Re-Os isotope system is particularly useful in this regard because the Os-isotope composition of juvenile mantle-derived basalts is distinctively unradiogenic (i.e. low 187Os/188Os ratios); also, the seawater Os-isotope composition responds rapidly because the marine Os residence time is relatively short (c. 40 ka). Chemical weathering of large volumes of LIP basalts, soon after their emplacement, thus imparts a characteristic unradiogenic Os-isotope signature to seawater; weathering rates can be estimated from simple mass balance calculations. The Re-Os system also differentiates very effectively the effects of LIP emplacement from those of a bolide impact because large amounts of Re are also released during LIP weathering, whereas bolides with abundant Os contain relatively little Re. The potential of the Re-Os system to further our understanding of LIP emplacement and its consequences will be illustrated using published and new data. For example, Os-isotope data from marine sedimentary successions that span the Triassic-Jurassic and Cretaceous-Paleogene boundaries show unequivocally that the emplacement of the Central Atlantic Magmatic Province and the Deccan traps, respectively, were implicated with contemporaneous environmental change (Cohen & Coe, Geology 2002; Cohen & Coe, Palaeo-3 2007; Ravizza & Peucker-Ehrenbrink, Science 2003). Importantly, in both these cases the isotope data demonstrate that basalt weathering (and hence LIP emplacement) commenced a relatively short but significant time before these two boundaries. Re-Os data also clearly link LIP emplacement with the development of some of the Mesozoic Oceanic Anoxic Events (OAE), for example during the Toarcian (Cohen et al., Geology 2004) and Cretaceous OAE 2 (Turgeon & Creaser, Nature 2008). However, the interpretation of Re-Os data that pertain to OAEs needs to be made with caution because of the competing effects of high continental weathering rates due to global warming and an enhanced hydrological cycle that often occurred at those times.
V24A-02
A high-resolution marine osmium isotopic record for the late Maastrichtian: the chemical fingerprints of Deccan volcanism
Variations in the marine Os isotope record have been reconstructed based on more than 150 analyses of sediments from 4 locations: the South Atlantic (DSDP 525); the Southern Ocean (ODP 690C); the Equatorial Pacific (DSDP 577) and the Tethys (Bottaccione Section in Gubbio, Italy). All records show 187Os/188Os values close to 0.6 throughout most of magnetochron C30n, and a 2-stage decline leading up to the KPB. The initial decline begins close to the C30n/C29r reversal boundary, with 187Os/188Os decreasing from plateau values of approximately 0.6 to values of 0.42. 187Os/188Os values stabilize at approximately 0.4 for 50 to 130 kyr, before beginning a second, more rapid decrease to very low, nearly chondritic, 187Os/188Os within the KP boundary interval. We interpret the initial decline as the result of Deccan volcanism and attribute the second decline to the Chicxulub impact event. Complementary platinum and iridium concentration data indicate that the length scale of diagenetic remobilization of platinum group elements from the KP boundary horizon is less than 1 meter at these sites. The length scale of the "Deccan-induced" Os isotope decline is larger, spanning up to 3 meters of section. This requires that the changes in oceanic inputs responsible for this initial Os isotope shift could not have been an instantaneous pulse, but rather spanned roughly 200 kyr. Although several recent studies have argued that Deccan volcanism may have contributed to the KT mass extinction, the marine Os isotope record suggests that the environmental consequences of Deccan volcanism are more likely concentrated early in magnetochron C29r, coincident with the initial decline in seawater 187Os/188Os.
V24A-03
Was Global Warming at the Paleocene-Eocene Boundary Terminated by Flood Volcanism?
The Paleocene-Eocene thermal maximum (PETM) has recently been attributed to greenhouse gases released from sedimentary basins in the Northeast Atlantic due to interaction with continental flood basalt magmatism. In the marine section in Denmark the alkaline Ash-17 has been dated at 55.1 plus minus 0.1 Ma and the PETM at 55.6-55.4 Ma. A similar alkaline tephra deposit in the uppermost part of the East Greenland flood basalt succession has also been dated at 55.1 plus minus 0.1 Ma and provides a linkage to Ash-17. Our recent results on the pressure of the coeval Skaergaard intrusion indicate that the majority of flood basalts erupted in less than 300,000 years. It is therefore possible to correlate the main flood basalt event with the interval immediately postdating PETM (55.4-55.1 Ma). This is consistent with a report of a small dinoflagellate cyst assemblage with a high proportion of Apectodinium homomorphum in one productive sample from sediments within the lower volcanics underlying the main flood basalt succession. The Apectodinium genus is usually abundant in the PETM interval. A scarcity of ash layers within the PETM interval also supports a correlation of the main flood basalt event with the overlying marine section including more abundant ash layers. The high eruption rate of the main flood basalts is likely to have resulted in atmospheric cooling caused by sulfuric acid aerosols produced from volcanic sulfur dioxide. Available estimates for volume and composition of the Northeast Atlantic flood basalts indicate that at least 36 teratonnes of sulfur dioxide was pumped into the atmosphere. This average 120 megatonnes per year over 300,000 years. For comparison, the historic Laki eruption in Iceland is estimated to have released 120 megatonnes sulfur dioxide over 5 months. We suggest that flood volcanism of the Northeast Atlantic terminated the global warming event at the Paleocene-Eocene boundary.
V24A-04
Evolution and Production of Calcareous Nannoplankton During the Cretaceous as Proxies of LIP-induced Oceanic Fertilization, Acidification and Anoxia
Through the Phanerozoic, biota have been intimately linked to Earth's degassing inducing major changes in composition and structure of the ocean-atmosphere system. Emplacement of large igneous provinces (LIPs) has been the primary natural source of atmCO2 with dramatic consequences on climate and ecosystems. During the mid-Cretaceous the Ontong Java-Manihiki and Caribbean Plateaus LIPs are recognized as responsible of pCO2 as high as 2000 ppm. Coeval biocalcification crises occurred in pelagic and neritic settings, suggesting a causal link between high concentrations of carbon dioxide and drops in benthic and planktonic calcifiers' efficiency. Within the oceanic biosphere, calcareous nannoplankton play a key-role as: (1) is widespread and consists of cosmopolitan and endemic taxa; (2) has a 220 My-long evolutionary history; (3) is one the most effective calcite producers; (4) is relevant for the C cycle; (5) is extremely sensitive to environmental variations. Diversity pulses of Cretaceous calcareous nannoplankton are grossly coeval with LIP construction, climate and sea-level changes, variations in ocean structure and composition, suggesting that evolutionary patterns are closely linked to environmental modifications. We explored time-intervals of LIP formation marked by nannoplankton adaptation/evolution, quantifying evolutionary rates, species richness, abundance, calcite production and morphometry. High-resolution investigations of the initial phase of both early Aptian oceanic anoxic event (OAE) 1a and latest Cenomanian OAE 2 pointed out major evolutionary changes, decreases in heavily calcified nannoliths and occurrence of dwarf coccoliths. Nannoplankton calcification crises and dwarfism is here interpreted as forced by rapidly increasing pCO2 during formation of the Ontong Java-Maniniki and Caribbean Plateaus. Alternatively or concurrently, calcification crash and dwarfism might result from enhanced fertility associated to OAE1a and OAE2 regardless of ocean alkalinity. However, such global nutrification episodes must be linked as well to LIP construction via supply of biolimiting metals. Contrary to common reasoning, we stress the fact that emplacement of Cretaceous LIPs did not cause extinctions among calcareous nannoplankton.
V24A-05
The Environmental Impact of Siberian Traps Volcanism
New high-precision 40Ar/39Ar data confirm that the Siberian Traps extend as far west as the Ural Mountains, and from the Kuznetsk Basin in the south to the Taimyr Peninsula in the north; an area encompassing some 5 million km2. The bulk of this volcanism occurred at about 250 Ma (Ar-Ar time). These data, plus new and published Ar/Ar data from the P-Tr section at Meishan, China, confirm that volcanism and the mass extinction were synchronous. Here, we explore the causal link between volcanism and extinction. The volcanism is associated with global super-greenhouse conditions and widespread shallow oceanic anoxia – perhaps the sine qua non of the marine mass extinctions. Injection of isotopically 'light' carbon is required to explain the characteristic and dramatic negative carbon isotope excursion preserved in ocean water proxies, but because the CIE occurs after the mass extinction, this suggests that the carbon pulse (from breakdown of methane hydrates, or magmatic burning of coal or other hydrocarbons) was not the fundamental cause of the extinction. Rather, we suggest that magmatic CO2 released during the eruptions (complemented by pyrogenetic CO2 and methane) led to progressive CO2 accumulation in the atmosphere-ocean system (rates of long-term removal of carbon by geological processes are significantly lower than volcanic injection). Atmospheric accumulation may have been amplified by short-term sulphate-induced volcanic winters that caused collapse of photosynthetic cycles by atmospheric temperature fluctuations and sunlight attenuation, thus inhibiting carbon draw-down. Subsequent warming of the deep ocean may have triggered the methane pulse, leading to the main CIE. What lessons can we take away for present climate change? Unlike in the Cenozoic, when atmospheric CO2 progressively decreased to low pre-industrial levels, throughout the Permian atmospheric CO2 levels fluctuated strongly, and may have been as much as 10x present-day by the time that Siberian volcanism began. Arguably, the earth system was much more vulnerable to the additional carbon loading from volcanism. The environmental impact of flood basalt eruptions may thus be strongly influenced by the prevailing global climate conditions and atmosphere-ocean composition.
V24A-06
The Impact on Environment of Large Igneous Provinces is Controlled by the Types of Sediment They Intrude.
Some but not all large igneous provinces (LIP) coincide with global warming episodes and major mass extinctions. The Cretaceous-Tertiary mass extinction coincides with flood volcanism in the Deccan province of India; the Permian-Triassic extinction with the emplacement of the Siberian Traps; and the End-Guadalupian mass extinction with the Emeishan LIP. In contrast, the enormous oceanic plateaus and the Karoo flood basalts had lesser effect on the biosphere. It has been suggested that the cooling effect of volcanic ash and sulphate aerosols injected into the stratosphere during Plinian eruptions may be the main cause of mass extinctions but the volume of erupted basalt of each large igneous province does not correlate well with the extent of mass extinction. An alternative explanation is that the global climatic changes associated with large igneous provinces are related to the volatiles released during the low-temperature contact metamorphism of the intruded rocks. Svensen et al. (2004, Nature 429, 542; 2007, EPSL 257, 554) have suggested that low- temperature destabilization of organic matter releases large amounts of CH4 and CO2. Here we propose that high-temperature contact metamorphism of carbonates and evaporites release immense quantities of greenhouse and toxic gases (CO2, SO2). The Panzhihua mafic-ultramafic sill, part of the Emeishan LIP, intruding the dolostones of the Sichuan Basin 261 Ma ago. The formation of periclase during high-temperature contact metamorphism of carbonates released at least ~62500 Gt CO2 and additional CO2 was released by lower temperature reactions such as the formation of calc-silicates or the degradation of organic matter. In contrast, the amount of magmatic CO2 released from the Emeishan lavas and intrusions, estimated using the approach of Self et al. (2006, EPSL 248, 518), was less than a third of sediment-derived CO2. The gases probably vented to the atmosphere on a short timescale, utilizing the permeability structure created during the emplacement event. The vented CO2 would have led to greenhouse conditions, and contributions from thermogenic gases could have created the end-Guadalupian negative carbon isotope excursion.
V24A-07
Did Deccan Volcanism or the Chicxulub Impact Cause the K-T Mass Extinction?
It is generally believed that the Chicxulub impact caused the Cretaceous-Tertiary (K-T) mass extinction. However, strong evidence from Mexico and Texas shows that this impact predates the K-T boundary and caused no species extinctions or any other significant environmental effects (Keller et al., 2003, 2007). The Chicxulub impact and K-T mass extinction are thus two separate and unrelated events and the biotic effects of this impact have been vastly overestimated. The real cause for the K-T mass extinction may now have been discovered in the Deccan volcanic eruptions of India. Recent discoveries reveal Deccan volcanism as the most likely cause for the K-T mass extinction for several reasons detailed in Chenet et al. (2007), Keller et al. (2008) and Self et al. (2008): (1) The main phase of Deccan Trap eruptions may have occurred over as little as 10,000 to 100,000 years. (2) The K-T mass extinction coincides with the end of this main phase of volcanism. (3) The longest lava flows (megaflows), spanning 1000 km across India and out to the Gulf of Bengal, mark this phase of Deccan volcanism and the mass extinction. (4) SO2 emissions associated with just one of these major volcanic pulses, or megaflows, are on the order of SO2 emissions estimated from the Chicxulub impact. (5) The total SO2 emissions during the main phase of Deccan volcanism are estimated at 30 to 100 times that of the Chicxulub impact. Thus, the short duration of volcanism and the repeated massive SO2 injections may have caused a deadly runaway effect that lead to the K-T mass extinction. Critical new data on the K-T mass extinction comes from investigations of Deccan Traps outcrops at Jhilmili, Madhya Pradesh, central India, quarry outcrops in Rajahmundry and subsurface cores drilled in the Krishna-Godavari Basin, eastern India, by the Oil and Natural Gas Corporation of India (ONGC). In eight subsurface cores examined, a total of 9 volcanic megaflows have been identified as occurring in very rapid succession. The biotic effects of these megaflows can be evaluated based on planktic foraminifera, which suffered the most severe mass extinction of all organisms. After the first megaflow up to 50% of the species disappeared and with each new megaflow more species died out culminating in near total mass extinction coincident with the last megaflow at the K-T boundary. After the mass extinction, no megaflows reached the Krishna-Godavari Basin for about 250-280 ky during which a low diversity early Danian assemblage of small new species evolved. The last major Deccan volcanic pulses began at the C29R/C29N boundary and appear to have been the cause for the long delay in the full biotic recovery. Deccan volcanism can thus explain both the K-T mass extinction and the long delayed biotic recovery that has been an enigma for so long. Chenet, A-L. et al. (2007) EPSL 263, 1-15; Keller, G. et al. (2003) ESR 62, 327-363; Keller, G., et al. (2007) EPSL 255, 339- 356; Keller, G. et al. (2008) EPSL 268, 293-311. Self, S. et al. (2008) Science, 319, 54-57.
V24A-08
Major Marine Seaway Across India During the K-T Transition: Evidence From Deccan Traps
Intertrappean beds in the main part of the Deccan Traps volcanic province of peninsular India are generally considered to be terrestrial deposits of late Maastrichtian age, lthough the precise position of the Cretaceous-Tertiary (K-T) boundary event has remained speculative. Recent investigations of the outlying Deccan Traps exposures around Rajahmundry near the southeastern coast, however, revealed the K-T event in intertrappean beds overlying the end of the main Deccan volcanic phase with the last phase of volcanic eruptions at the C29R/C29N transition (Keller et al., 2008). Further investigations in central India confirm these results and indicate that a major marine seaway existed across India during the K-T transition. The new evidence is from Deccan Traps at Jhilmili, Chhindwara District of central India, located about 800 km from the nearest ocean. Intertrappean sediments in this area have been considered as terrestrial deposition. Our multi-disciplinary investigations, including biostratigraphic, sedimentologic, mineralogic, chemo- and magnetostratigraphic analyses of the Deccan Traps and intertrappean sediments revealed: i) predominantly terrestrial to fresh water (lacustrine, palustrine) deposition with short marine incursions transporting planktic foraminifera and forming brackish-marine environments; ii) planktic foraminiferal assemblages that indicate an early Danian zone P1a age for these marine incursions; iii) the K-T boundary is above the last reversely magnetized (C29R) basalt flow, and iv) the upper basalt flow occurs near the C29R/C29N transition. These biostratigraphic and magnetostratigraphic ages corroborate the previous results from Rajahmundry and place the K-T boundary at the end of the main phase of Deccan Traps volcanism. Deposition at Jhilmili during the K/T transition thus occurred in predominantly terrestrial semi-humid to arid environmental settings with short aquatic intervals of fresh water ponds and lakes, followed by shallow coastal marine conditions with brackish marine ostracods and early Danian planktic foraminifera. The planktic foraminifera point to the existence of a major seaway that extended at least 800 km across India. Keller, G., Adatte, T., Gardin, S., Bartolini, A., Bajpai, S., 2008. Main Deccan volcanism phase ends near the K-T boundary: Evidence from the Krishna-Godavari Basin, SE India. Earth and Planetary Science Letters, 268, 293-311. Keywords: Deccan volcanism, K-T boundary, peninsular India, seaway