After three decades of nearly unchallenged wisdom that a large meteorite, the Chicxulub impact on Yucatan, caused the end-Cretaceous mass extinction, evidence from over 150 Cretaceous-Tertiary boundary (KTB) sequences show that this impact predates the KTB by about 300,000 years and could not have been the cause for the mass extinction, and that the catastrophic effects of the Chicxulub impact have been vastly overestimated as no species went extinct as a result of this impact (see CHICXULUB DEBATE website).
The other end-Cretaceous catastrophe, Deccan Volcanism, has emerged as the most likely cause for the KTB mass extinction. Deccan volcanism has been advocated as potential cause for the KT catastrophe for over thirty years (e.g., McLean, 1978; 1985; Courtillot et al., 1986, 1988; Duncan and Pyle, 1988). But this hypothesis was considered unlikely because volcanism was generally believed to have occurred over about one million, leaving sufficient time for recovery between eruptions. A number of recent multi-disciplinary studies have changed this perception and directly link Deccan volcanism to the KTB mass extinction:
á Improved dating of the age and tempo of Deccan eruptions.
á Massive eruptions create the longest lava flows on Earth.
á Direct links between the main Deccan eruptions and the KTB mass extinction.
Figure 1. The massive Deccan volcanic mountain ranges reach up to a height of 3500 m and consist entirely of layered lava flows. (photo courtesy of Steve Self).
1. Deccan Traps, India
The Deccan volcanic province is one of the largest volcanic eruptions in EarthÕs history and today covers an area of 500,000 km2, or about the size of France, or Texas. The original size prior to erosion is estimated to have been at least twice as large. The volume of lava extruded is estimated to have been about 1.2 million km3 and today can be seen in mountains as high as the Alps (~3500 m) (Fig. 1).
Figure 2. The Deccan volcanic province in India today covers an area the size of France or Texas. The original size is estimated twice this size, but was reduced by erosion. Arrows show the direction of the longest lava flows on Earth, which span 1500 km across India and out into the Gulf of Bengal.
The longest lava flows on Earth
Deccan volcanism produced the longest lava flows on Earth, spanning over 1500 km from the main Deccan province across India to Rajahmundry and out into the Gulf of Bengal. Four to five of these longest lava flows occurred just prior to the KTB mass extinction. Another four occurred during the last Deccan eruption phase in the early Danian (Keller et al., 2008; Self et al., 2008).
Three main phases of Deccan eruptions
Recent studies indicate that Deccan volcanic eruptions occurred in three main phases with the initial and smallest Phase-1 at ~67.4 Ma; the relatively short main Phase-2 with ~80% of total Deccan volume ends in C29r below the KT boundary; and the last Phase-3 at the base of C29n in the early Danian (Chenet et al., 2007, 2008, 2009), about 280 ky after the Cretaceous-Tertiary boundary (KTB) mass extinction (Fig. 2).
Figure 3. 39Ar/40Ar and K/Ar dating of the main Deccan province reveals that the main phase of eruptions began during magnetochron C29r and ended at the KTB mass extinction. (modified after Chenet et al., 2007).
New Database: 2008-2010 and beyond
The new database linking the main phase-2 Deccan volcanism directly to the KTB mass extinction comes from various localities and is based on sediments between the longest lava flows (called intertrappean sediments) and from intervals above and below the volcanic sediments:
á Rajahmundry quarries: five quarries analyzed all show KTB mass extinction directly at the end of phase-2 volcanism (Keller et al., 2008; Malarkodi et al., 2010).
á Jhilmili, Chhindwara, central India: KTB also in intertrappeans above phase-2 volcanism; presence of marine microfossils indicates major seaway existed (Keller et al., 2009a, 2009b).
á Krishna-Godavari Basin: Longest lava flows of Phase-2 and Phase-3 volcanism in deep wells drilled by IndiaÕs Oil and Natural Gas Corporation (ONGC). Mass extinction complete by end of Phase-2.
á Meghalaya, northeastern India: one of the worldÕs most complete KTB sections reveals the environmental effects of Deccan volcanism about 800-1000 km from the main volcanic province (Gertsch et al., in press).
The first direct link between Deccan volcanism and the KTB mass extinction was established in four Rajahmundry quarries (Keller et al., 2008). The longest lava flows of the Phase-2 mega-eruptions are best known from these basalt quarries (Figs. 5, 6). Intertrappean sediments above Phase-2 basalts span up to 8 m of sediments deposited in a shallow near-shore marine environment. These intertrappean sediments contain earliest Danian planktic foraminiferal assemblages of zone P1a, which mark the evolution in the aftermath of the mass extinction. Deccan Phase-3 volcanic layers overlie the intertrappeans and mark the last Deccan eruptions.
Figure 5. Gauriputnam Quarry of Rajahmundry shows the intertrappean sediments with earliest Danian planktic foraminifera that evolved after the KTB mass extinction.Phase-2 eruptions underlie these sediments and Phase-3 eruptions are at the top.
In Rajahmundry quarries, multi-disciplinary studies of intertrappean sediments based on biostratigraphy, sedimentology, mineralogy and geochemistry, reveal that sediments were deposited in a shallow marine environment that fluctuated between supratidal, estuarine, lagoonal and open marine conditions, interrupted by periods of subaerial deposition (paleosoils). Changing sea levels are largely related to uplift and subsidence associated with Deccan volcanism. Planktic foraminifera and rare nannofossils mark deposition as early Danian zone P1a, which spans the first 280,000 years after the KTB mass extinction (see Figs. 6 and 7; Keller et al., 2008, 2009c).
Figure 6. Close-up of intertrappean sediments between Phase-2 and Phase-3 volcanic eruptions at the Gauriputnam Quarry of Rajahmundry.
Figure 7. Data from intertrappean sediments in the Government Quarry of Rajahmundry. See text.
Krishna-Godavari Basin, Andra Pradesh – Pulsed Mega-Eruptions
The Krishna-Godavari Basin is located seaward from Rajahmundry (Fig. 8). In this basin the lava flows are buried under 2000 m to 4000 m. Drilling by IndiaÕs Oil and Natural Gas Corporation has recovered numerous cores from this area and ten wells have been studied to evaluate the KTB and its relationship to Deccan volcanism.
Figure 8. Locations of ONGC wells studied in the Krishna-Godavari Basin.
Figure 9. Correlation between Phase-2 and Phase-3 eruptions in Rajahmundry Quarries and Phase-2 and Phase-3 eruptions in deep wells of the Krishna-Godavari Basin. (See text).
Correlation between lava flows of the Rajahmundry quarries and the Krishna-Godavari (K-G) Basin shows that in Rajahmundry the lava flows of Phase-2 and Phase-3 are fused or separated only by thin paleosoil or red bole layers (Fig. 9). In contrast, in the K-G basin four lava flows are clearly separated by intertrappean sediments in each volcanic Phase-2 and Phase-3. This demonstrates that Deccan volcanism during Phase-2 and Phase-3 occurred in rapid pulses with mega-eruptions flowing 1500 km across India and out into the Gulf of Bengal.
Between the two major phases of mega-eruptions, no volcanic flows reached Rajahmundry for about 280,000 years during the early Danian, when sediments were deposited in the intertrappean interval and the evolution of the first Danian species occurred.
Mass Extinction linked to Deccan Volcanism: K-G Basin
Sediments between the lava mega-flows in the Krishna-Godavari Basin yield information on the nature of environmental stress and the KTB mass extinction. The pattern of four and sometimes five mega-eruptions repeats itself in most wells of the Krishna-Godavari Basin in Phase-2 and three to four mega-eruptions are present in Phase-3. These are the longest lava flows, known on Earth spanning 1500 km across India and out into the Gulf of Bengal (Fig. 10).
Figure 10. Correlation of lava flows in deep wells of the Krishna-Godavari Basin.
Figure 11. Planktic foraminifera in the Krishna-Godavari Basin wells link Deccan Trap mega-eruptions of Phase-2 to the KTB mass extinction.
In the ONGC deep wells, the lava flows are separated by sediments, which indicate the elapsed time between flows (Fig. 11). Foraminifera are present in these intervals. Below Phase-2, a diverse late Maastrichtian assemblage exists. After the first mega-eruption there is a 50% reduction in species diversity and surviving species are dwarfed and show signs of dissolution (acid rain). No recovery occurred between mega-eruptions and after the fourth and last mega-eruption of Phase-2 the mass extinction was complete. The fact that there is no recovery in between the mega-eruptions indicates that they followed each other in rapid succession and may have caused a run-away effect leading to the KTB mass extinction.
There is no similar mass extinction associated with Phase-3 mega-eruptions, which suggests that there was time for recovery between eruptions. Phase-3 volcanism precedes full marine biotic recovery in the marine ecosystem after the mass extinction, including larger size and increased diversity. These results strongly suggest that Deccan volcanism played critical roles in both the KT mass extinction and the delayed marine biotic recovery that has been an enigma for so long.
Cretaceous Seaway and Dinosaur Nurseries
In Central India (Jhilmili, Chhindwara, Madhya Pradesh), intertrappean sediments between Phase-2 and Phase-3 lava flows consist mainly of terrestrial sediments, but also an interval deposited in a shallow lacustrine-brackish environment (Fig. 12) (Keller et al., 2009a, b).
Figure 12. In Jhilmili, Chhindwara, central India, intertrappean sediments between Phase-2 and Phase-3 sediments reveal deposition in a lacustrine-brackish environment.
Figure 13. Analysis of the Jhilmili intertrappean sediments revealed the presence of early Danian planktic foraminifera of zone P1, which suggests a seaway existed across India at KTB time.
The discovery of early Danian zone P1a planktic foraminifera in intertrappean sediments deposited in a lacustrine-brackish environment reveals a Cretaceous seaway existed 800 km into India, probably through the Narmada valley (Fig. 13). Dinosaur nesting sites surround this projected seaway and suggest that this was a major dinosaur nursing area in India (Keller et al., 2009a, b, c).
Meghalaya, NE India – Deccan Catastrophe Strikes
In Meghalaya, one of the most complete KTB sequences worldwide was recovered along the Um Sohryngkew River, which at KTB time was located about 800-1000 km from the main Deccan volcanic province (Fig. 4). This section provides critically important information on the effects of Deccan volcanism at 1000 km distance from the volcanic province. The information gained from this locality shows that the Deccan catastrophe was severe and caused near total devastation long before the final phase at the KTB mass extinction.
The KT boundary in this section is identified by the global defining- and supporting-criteria: mass extinction of planktic foraminifera, first appearance of Danian species, d13C shift, Ir anomaly (12 ppb) and KTB red layer. This section is particularly important in that it provides critical information on the environmental and biological effects of Deccan volcanism in a marine environment that was within 1000 km of the volcanic activity.
Figure 14. The KTB transition at Meghalaya, 1000 km from the main Deccan province, reveals super-stress conditions, early species extinctions and blooms of the disaster opportunist Guembelitria at the time of the main Phase-2 Deccan eruptions.
Multi-disciplinary analyses of this section includes biostratigraphy, faunal and stable isotope analyses, sedimentology, clay and bulk rock mineralogy, and analyses of major and minor trace elements, and platinum group elements (PGEs) (Gertsch et al., in press).
Major results based on the Um Sohryngkew River section from Meghalaya:
á Most complete KTB section known in India and comparable to the best sections worldwide (e.g., Tunisia, Texas, Spain).
á KTB red layer with major PGE and trace element anomalies
á Major Ir anomaly (12 ppb) indicates extraterrestrial origin as well as a significant component resulting from condensed sedimentation (P-enrichment), and redox fluctuations under sulfidic conditions (As, Co, Ni, Pb, Zn enrichments).
á Major d13C shift across the KTB comparable to the KTB d13C shift worldwide.
á Mass extinction coincides with red layer and Ir anomaly.
á First appearance of Danian species within 10 cm above the red layer and Ir anomaly and mass extinction.
á Super-stress environmental conditions in the latest Maastrichtian Micula prinsii and CF1 zones correlative with Deccan Phase-2.
á Super-stress conditions marked by the disaster opportunist Guembelitria blooms (>95%) and only rare and sporadic presence of other species.
á High chemical weathering rates indicate periodic acid rains associated with pulsed Deccan eruptions.
á High continental runoff resulted in major influx of nutrients leading to mesotrophic to eutrophic waters
á High chemical weathering rates, acid rains and high nutrient influx resulted in super-stress marine conditions that led to the early demise of nearly all planktic foraminifera and the blooms (>95%) of the disaster opportunist Guembelitria. Such blooms are best known from the aftermath of the KTB mass extinction, but have now also been identified globally (Pardo and Keller, 2008, Abramovich and Keller, 2009).
The Meghalaya results reveal devastating marine conditions surrounding the Deccan volcanic province during the main Phase-2 eruptions in C29 below the KTB and that these conditions led to regionally early extinctions followed by the global extinctions at the KTB.
Deccan and Gas Emissions – The Kill Effect
Environmental consequences of the massive Deccan eruptions were likely devastating mainly because of gas emissions, particularly SO2 and CO2. Sulfur dioxide gas released by volcanism and injected into the stratosphere forms sulfate aerosol particulates, which act to reflect incoming solar radiation and causes global cooling. Since sulfate aerosol has a short lifespan in the atmosphere, the cooling would be short-term (years to decades), unless repeated injections from volcanic eruptions replenished atmospheric sulfate aerosols and led to a runaway effect.
From Chenet et al. (2007, 2008, 2009) we know that Deccan volcanism occurred in a series of rapid, pulsed eruptions, with each of the 30 largest pulses estimated to inject up to 150 GT of SO2 gas, or the equivalent of the Chicxulub impact (e.g. 50-500 GT), over a very short time (possibly decades). By this estimate the total Deccan eruptions injected 30 to 100 times the amount of SO2 released by the Chicxulub impact.
It is not just the sheer volume of SO2 injection, but also the rapid succession of volcanic eruptions with repeated SO2 injections that would have compounded the adverse effects of SO2 leading to severe environmental consequences (e.g., cooling, acid rain, high weathering rates, high continental runoff and nutrient input into the oceans resulting in mesotrophic to eutrophic waters), preventing recovery and likely causing a run-away effect that led to extinctions.
15. Model showing interactions between Deccan volcanism, gas emissions, ocean
acidification, weathering, nutrient supply, climate warming and cooling and
Chenet A-L., Quidelleur X., Fluteau F., Courtillot V., 2007. 40K/40Ar dating of the
Main Deccan Large Igneous Province: Further Evidence of KTB Age and
short duration. EPSL 263: 1-15.
Chenet, A.-L., Fluteau, F., Courtillot, V., Gerard, M., and Subbarao, K.V., 2008.
Determination of rapid Deccan eruptions across the Cretaceous-Tertiary
boundary using paleomagnetic secular variation: Results from a 1200-m-thick
section in the Mahabaleshwar. Journal of Geophysical Research, 113, DOI:
Chenet, A.-L., Courtillot, V., Fluteau, F., Gerard, M., Quidelleur, X., Khadri, S.F.R.,
Subbarao, K.V., and Thordarson, T., 2009. Determination of rapid Deccan
eruptions across the Cretaceous-Tertiary boundary using paleomagnetic
secular variation: 2. Constraints from analysis of eight new sections and
synthesis for a 3500-m-thick composite section:Journal of Geophysical
Research, 114, B06103, doi:10,1029/2008JB005644, 2009.
Keller, G. and Abramovich, S., 2009. Lilliput Effect in late Maastrichtian
planktic Foraminifera: Response to Environmental Stress. Paleogeogr.,
Paleoclimatol., Paleoecol., 271, 52-68. doi:10.1016/j.palaeo.2008.09.007
Keller, G., Adatte T., Gardin, S., Bartolini, A., Bajpai, S., 2008a. Main Deccan
volcanism phase ends near the K-T boundary: Evidence from the Krishna-
Godavari Basin,SE India. Earth and Planetary Science Letters, 268: 293-313.
Keller, G., Adatte, T., Bajpai, S., Khosla, A., Sharma, R., Widdowson, M.,
Khosla, S.C., Mohabey, D.M., Gertsch, B., Sahni, A., 2009a. Early Danian
Shallow marine Deccan intertrappean at Jhilmili, Chhindwara, NE India:
Implications for Paleogeography. EPSL 282, 10-23. doi:10.1016/j.epsl.2009.02.016
Keller, G., Sharma, R., Khosla, A., Khosla, S.C., Bajpai, S., Adatte, T., 2009b. Early
Danian Planktic foraminifera from Intertrappean beds at Jhilmili, Chhindwara
District, Madhya Pradesh, India. J. Foram. Res., 39(1): 40-55.
Self, S., Jay, A.E., Widdowson, M., Keszthelyi, L.P., 2008a. Correlation of the
Deccan and Rajahmundry Trap lavas: Are these the longest and largest lava
flows on Earth? Journal of Vocanology and Geothermal Research, 172: 3-19.
Beyond India, multi-proxy studies also place the main Deccan phase in the uppermost Maastrichtian C29r below the KTB, as indicated by a rapid shift in 187Os/188Os ratios in deep-sea sections from the Atlantic, Pacific and Indian Oceans, coincident with rapid climate warming, coeval increase in weathering, a significant decrease in bulk carbonate indicative of acidification due to volcanic SO2, and major biotic stress conditions expressed in species dwarfing and decreased abundance in calcareous microfossils (planktic foraminifera and nannofossils, Fig. 2). These observations indicate that Deccan volcanism played a key role in increasing atmospheric CO2 and SO2 levels that resulted in global warming and acidified oceans, respectively, increasing biotic stress that predisposed faunas to eventual extinction at the KTB.
Figure 1. Over 150 KTB sections have been analyzed worldwide over the past 25 years by Keller and her students and collaborators. This research has resulted in a global database that permits the evaluation of the environmental and biological effects of both the Chicxulub impact and Deccan volcanism.
Planktic Foraminifera as Environmental Proxies
Planktic Foraminifera are excellent proxies for high-stress conditions associated with greenhouse warming, mesotrophic to eutrophic waters, marginal settings, and volcanically active regions during the Late Maastrichtian and early Danian. Sedimentary sequences analyzed from Israel, Egypt, Tunisia, Texas, Argentina, India and Indian Ocean (Fig. 1) reveal that the biotic response varies from optimum to catastrophic with the degree of biotic stress related to variations in oxygen, salinity, temperature, pH and nutrients (Pardo and Keller, 2008; Keller and Abramovich, 2009).
Figure 2. Size and shapes of tests of Maastrichtian planktic foraminifera species showing a continuum from small r-strategists to large complex k-strategists. R-strategy and opportunistic life strategy of Guembelitria are inferred by their minute test size, simple chamber arrangement and isotopically light d13C values. K-strategy species are inferred from large and complex test morphology, small populations and heavier d13C values. From Keller and Abramovich, 2009.
Planktic foraminifera vary in test sizes from very small to very large, from unornamented to highly decorated tests, from simple morphologies to the very complex (Fig. 2). These characteristics reflect the degree of environmental stress that species can tolerate and the particular ecological niches they tend to inhabit.
á K-strategists are large, morphologically complex, highly ornamented and very diverse species group. They utilize particular food sources and specialize in particular ecological niches; they have longer life spans and tend to produce only small numbers of offspring.
á R-strategists are the small to medium sized species with unornamented tests and simple morphologies. They have low species diversity, utilize a variety of food sources, live under variable environmental conditions, live short life spans and have a large number of offspring.
R-strategists thus optimize chances for survival, whereas k-strategists optimize the good life while it lasts.
From Optimum to High-Stress Conditions
Early stages of biotic stress result in diversity reduction and the elimination of large specialized species, followed by size reduction (dwarfism) of survivors and dominance of low O2 tolerant species (heterohelicids). At the extreme end of the biotic response are volcanically influenced environments, which cause the same detrimental effects as observed in the aftermath of the KT mass extinction, including the disappearance of most species and blooms of the disaster opportunist Guembelitria (Fig. 3).
Figure 3. The effects of increasing environmental stress upon planktic foraminiferal assemblages from optimum to catastrophe conditions shows the successive elimination of large, specialized k-strategy species, the survival of small r-strategy species, the overall dwarfing of these species and their great abundance. From Keller and Abramovich, 2009.
Late Maastrichtian environments span a continuum from optimum conditions to the catastrophic (mass extinctions) with a predictable set of biotic responses relative to the degree of stress induced by oxygen, salinity, temperature and nutrient variations as a result of climate and sea level changes and volcanism.
á Early stages of biotic stress result in diversity reduction and the elimination of large specialized species (k-strategists) leading to morphologic size reduction via selective extinctions and disappearances and intraspecies dwarfing of survivors.
á Later stages of biotic stress result in the complete disappearance of k-strategists, intraspecies dwarfing of r-strategists and dominance by low oxygen tolerant small heterohelicids.
á At the extreme end of the biotic response are volcanically influenced environments, which cause the same detrimental biotic effects as observed in the aftermath of the K-T mass extinction, including the disappearance of most species and blooms of the disaster opportunist Guembelitria.
Specialists under Stress (K-strategists)
Dwarfing (also called the Lilliput effect) of large specialized species has been observed in association with the latest Maastrichtian climate warming, which has been attributed to Deccan volcanism (Kucera and Malmgren, 1998; Olsson et al., 2001; Abramovich and Keller, 2003).
At Site 525A on Walvis Ridge, South Atlantic the latest Maastrichtian warm event is documented in a high resolution stable isotope analysis by Li and Keller (l998b, c) and tied to high-stress conditions in planktic foraminiferal assemblages by Abramovich and Keller (2003). The size reduction in these assemblages is over 50%. High-stress conditions also resulted in decreased abundance of Heterohelix species, and increased abundance of dwarfed specimens.
Fig. 4. Species dwarfing (50% reduced size) during the latest Maastrichtian C29r warm event at DSDP Site 525 on Walvis Ridge, South Atlantic (From Abramovich and Keller, 2003; stable isotopes from Li and Keller, 1998).
K-strategists meet Disaster
When a major environmental perturbation dramatically alters the ecosystem, the result may be mass mortality. The KTB mass extinction decimated planktic foraminiferal assemblages, eliminating all tropical and subtropical k-strategy taxa, which accounted for about 2/3 of the species assemblage. The k-strategistsÕ tenuous hold on survival even before KTB time is evident by the fact that their combined relative abundance was already less than 5% of the total foraminiferal population during the latest Maastrichtian zone CF1, which spans the last 300,000 years of the Maastrichtian.
Clearly, specialized species suffered mass mortality well before their extinction at the KTB. In the past, this abundance decline in large specialized species has been attributed to climate and sea-level changes, for lack of a better explanation. But this explanation was never very satisfactory because climate and sea-level changes occur continuously in Earth history without causing mass mortality and mass extinctions. It now appears likely that Deccan volcanism was the major cause for the early demise of these species as well as their later extinction at the KTB (see Deccan volcanism website).
Opportunists Inherit the World
Only r-strategists survived in the immediate aftermath of the mass extinction (e.g., heterohelicids, hedbergellids, guembelitrids, Fig. 8). Among this group, the subsurface dwellers Hedbergella and Heterohelix (H. globulosa, H. navarroensis) survived well into the Danian but with very reduced populations (Keller and Abramovich, 2009).
Only the disaster opportunist Guembelitria species thrived and dominated the assemblages (80-100%) after the mass extinction, then decreased as competition grew with the newly evolved Danian species.
Guembelitria, the smallest Cretaceous planktic foraminiferal species, are also the oldest survivors in foraminiferal populations and their morphotype is still around today. Stable isotope ranking indicates that they thrived in nutrient-rich surface waters where few or no other species survived (Pardo and Keller, 2008).
Opportunists and Volcanism
Biotic effects attributable to volcanism are still poorly understood. Studies of volcanic or pollution effects in Recent sediments, reveal decreased diversity, dwarfing and growth abnormalities in foraminifera (Yanko et al., 1994; Hess and Kuhnt, 1996). Similar biotic effects have also been observed in foraminifera associated with Deccan volcanism (e.g., Meghalaya). Perhaps the clearest example of the biotic effects of volcanism is found in the late Maastrichtian of DSDP Site 216 on Ninetyeast Ridge (Keller, 2003, 2005a). Another example is the NeuquŽn Basin of Argentina (Keller et al., 2007b).
Ninetyeast Ridge DSDP Site 216, Indian Ocean:
This locality tracks the passage of the oceanic plate over a superheated mantle plume during the late Maastrichtian (zone CF3). During this passage, lithospheric uplift led to the formation of islands built to sea level, and volcanic activity continued for more than 1 million years leading to catastrophic environmental conditions for marine life (Keller, 2003, 2005a).
The biotic effects were severe and immediate, eliminating all species in the vicinity of the volcanic eruptions. As Site 216 moved past the immediate reach of mantle plume volcanism, sediments changed from basalt to phosphatic volcanic clay and black vesicular glass, and environmental conditions improved sufficiently for the small disaster opportunists Guembelitria to return and dominate (85-100%). Only a minor component of other r-strategists returned at this time with all species dwarfed (<100 µm, e.g., Heterohelix, Hedbergella and Globigerinelloides), species richness only between 4 and 10 species and d13C values well below normal marine productivity (Fig. 5).
With varying intensity of volcanic influx over time, the disaster opportunists Guembelitria and low oxygen tolerant Heterohelix species alternately dominated, whereas the abundance of surface dwellers remained low. Only after significantly reduced volcanic influx, a change to glauconite-rich chalk and an abrupt increase in d13C values do Guembelitria disappear, species richness increase to 15 and species size increase, returning to near normal for r-strategists and signaling improved environmental conditions (Fig. 5).
Figure 5. Foraminiferal response to Ninetyeast Ridge volcanism during the late Maastrichtian.
Andean Volcanism – Effects in Argentina
During the late Maastrichtian (zones CF4-CF2) to early Danian, the NeuquŽn Basin of Argentina was adjacent to an active volcanic arc. Marine conditions were maintained through an open seaway to the South Atlantic. At the Bajada de JagŸel section sediment deposition occurred in a shallow inner-neritic to middle-neritic environment (50-100 m) with fluctuating sea level and dysaerobic conditions (Keller et al., 2007).
Volcanic influx into this environment occurred as ash fallout during eruptions and from continental runoff via erosion. Within this environment, planktic foraminifera mimic the post-KTB high-stress environment with alternating blooms of the disaster opportunist Guembelitria (G. cretacea and G. dammula) and low oxygen tolerant but dwarfed Heterohelix species (e.g.. H. globulosa, H. dentata, Zeauvigerina waiparaensis). Other small r-strategy species are rare (e.g., Hedbergella, Globigerinelloides aspera) (Fig. 6).
These high-stress assemblages suggest nutrient-rich surface waters and an oxygen depleted water column as a direct result of weathering and high continental influx, as indicated by clay and bulk rock minerals (Keller et al., 2007). Species richness is very low ranging from 2 to 7, except for a brief incursion of dwarfed k-strategists (e.g., Rugoglobigerina rugosa, R. macrocephala, Globotruncana arca, G. aegyptiaca, Gansserina gansseri) during climatic warming and a rise in sea-level (Keller et al., 2003, 2005).
Effects of Deccan Volcanism in the Tethys
Most studies have concentrated on Guembelitria blooms in the aftermath of the KTB mass extinction as evidence for the most severe environmental conditions (see review in Pardo and Keller, 2008). Less studied are the Guembelitria blooms of other high-stress periods, particularly during the late Maastrichtian. Indeed, it is those high-stress environments that provide insights to what may have happened at the end of the Cretaceous.
Unusual Guembelitria blooms were observed in the Qreya section of central Egypt during the late Maastrichtian. The high abundance of these disaster opportunists mimics blooms known globally in the aftermath of the KTB mass extinction (Keller, 2002). Similar Guembelitria blooms have been observed in Israel (Abramovich et al., 1998; Keller et al., 2004). Sediment deposition occurred in a middle-neritic environment in central Egypt and in a deeper outer-neritic environment in Israel.
Figure 7. Foraminiferal response to high-stress conditions in Egypt; alternating Guembelitria and Heterohelix blooms mark variations in intensity of environmental stress. Stress conditions may be related to Deccan volcanism.
At the Qreiya section, the KTB is marked by a thin clay layer and Ir anomaly above a bioturbated marly shale with an erosional surface. Hiatuses also reduced early Danian zones Pla, Plb and Plc (Fig. 7). Guembelitria blooms are present in the early Danian zones Pla and Plc, similar to other Tethys sections.
What sets Qreiya apart from other KTB sections are the Guembelitria blooms (50-70%) in the upper Maastrichtian zones CF4-CF3 and CF1. At times of low Guembelitria abundances, the small Heterohelix navarroensis dominates. Species richness is also very low (25-30 species) compared with similar paleodepths at Elles (40-45 species).
These Guembelitria blooms indicate that the late Maastrichtian of the eastern Tethys experienced similar high stress conditions as the lower Danian in the aftermath of the mass extinction. d13C data indicate only a minor (0.7 ä) negative excursion at the KT boundary, suggesting that primary productivity was already reduced during the upper Maastrichtian, as also indicated by the low species richness, Guembelitria blooms and small Heterohelix species. Low primary productivity is also indicated by the upper Maastrichtian reversal in the surface-to-deep d13C gradient, which is usually associated with the KTB productivity crash. It is possible that these Guembelitria blooms are due to high stress conditions related to the three phases of Deccan volcanism. Further work is needed to explore this possibility.
Tethys and the Effects of Deccan Volcanism
Deccan Phase-2 Volcanism:
In India outcrops and cores reveal Guembelitria cretacea blooms directly associated with the main Deccan phase-2 volcanism in chron 29r and correlative with zones CF1-CF2, which span the last 160ky and 120ky, respectively. These Guembelitria blooms are similar to those documented worldwide in previous studies done at a time when a connection to Deccan volcanism was highly speculative and no direct link to the KTB mass extinction had been established (Keller and Pardo, 2004; Pardo and Keller, 2008; Keller and Abramovich, 2009).
Figure 8. Correlation of Guembelitria bloom events in the late Maastrichtian of the eastern Tethys (Israel, Egypt) and Western Interior Seaway (Brazos, Texas) correlated with the climate record of South Atlantic DSDP Site 525A and Deccan volcanism phase-1, phase-2 and phase-3.
Today a link to global high-stress conditions (Guembelitria blooms) can be demonstrated for Deccan phase-2 leading up to the KTB mass extinction (Fig. 8) in India, the eastern Tethys (Israel, Egypt) and Texas (Keller and Benjamini, 1991, Abramovich et al., 1998; Keller et al., 2004, 2009). These high-stress conditions coincide with the global warm event in CF1-CF2 (see also Fig. 4) and directly correlate with the super-stress conditions documented in Meghalaya (see website on Deccan volcanism).
Deccan Phase-1 Volcanism:
Even the comparatively minor phase-1 of Deccan eruptions left its global mark. Analysis of ONGC wells from the Cauvery Basin reveal the first link to the onset of Deccan volcanism in zone CF4 (~67.5 m.y.) based on ash fall and blooms of the disaster opportunist Guembelitria cretacea. Faunal analysis of the same interval in the eastern Tethys (Israel, Egypt, Tunisia) and Texas reveal correlative Guembelitria blooms that indicate strong adverse global effects associated with phase-1 Deccan volcanism (Fig. 8).
Deccan Phase-3 – Volcanism
Deccan volcanic Phase-3 was the last eruption phase in the early Danian beginning at the base of C29n, about 280,000 ky after the KTB mass extinction. Although this volcanic phase was much smaller than Phase-2, four of the longest lava flows occurred at this time (see Deccan volcanism website).
The onset of Deccan Phase-3 coincided with the extinction of the early Danian index species Parvularugoglobigerina eugubina and P. longiapertura. In the eastern Tethys, this volcanic event can be linked to a major negative d13C shift, similar to the KTB event (Magaritz et al., 1992), and Guembelitria blooms similar to the KTB event. In fact, when this event was first recognized in 1991 (Keller and Bejamini, 1991) it was thought to be the KTB event. The major high-stress conditions indicated by the Guembelitria blooms and d13C shift can no be correlated with Deccan Phase-3 volcanism (Fig. 9). The long delayed (500 ky) recovery of the marine ecosystem after the mass extinction may now be explained by the adverse environmental conditions as a result of Deccan volcanism.
Figure 9. Guembelitria blooms and d13C shift marks Deccan Phase-3 in the eastern Tethys.
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