Flood Basalt Hosted Palaeosols: Potential Palaeoclimatic Indicators of Global Climate Change

Geoscience Frontiers xxx (2013) 1e9

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China University of Geosciences (Beijing) Geoscience Frontiers

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Review Flood basalt hosted palaeosols: Potential palaeoclimatic indicators of global climate change

M.R.G. Sayyed*

Department of Geology, Poona College (University of Pune), Camp, Pune 411 001, Maharashtra, India article info abstract

Article history: Since continental sediments (in addition to the marine geological record) offer important means of Received 29 December 2012 deciphering environmental changes, the sediments hosted by the successive flows of the continental Received in revised form flood basalt provinces of the world should be treasure houses in gathering the palaeoclimatic data. 27 July 2013 Palaeosols developed on top of basalt flows are potentially ideal for palaeoenvironmental reconstructions Accepted 16 August 2013 because it is easy to determine their protolith geochemistry and also they define a definite time interval. Available online xxx The present paper summarizes the nature of the basalt-hosted palaeosols formed on the flood basalts provinces from different parts of the globe having different ages. Keywords: Ó Continental flood basalts 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Palaeosols Elsevier B.V. All rights reserved. Weathering Palaeoclimates Global change

1. Introduction across multiple time and space scales. Such palaeoclimatic records further reveal the changes in the atmospheric chemistry and the Globally distributed climate change events affect oceanic, at- response of natural systems to the climate change as these events mospheric and terrestrial environments and Earth’s history reveals are affecting the global oceanic, atmospheric and terrestrial envi- that there were periods when the climate was significantly cooler ronments. As revealed by the Earth’s history there were periods and warmer than the present and these periods provide unique when climate was significantly either cooler or warmer than the opportunities for understanding global climatic change. These pe- present, and those extremes of the past provide unique set of case riods also offer data in modelling and predicting the global climate studies for global change. Although different potential causes have response to enhanced atmospheric greenhouse gases. This is been proposed for the global climatic change events, with a more because the variations in the atmospheric carbon dioxide concen- understanding of the relative phasing of these events, both trations are now linked to extreme past climate change record temporally and spatially, the nature of the cause-effect mechanism (global cooling to global warming) on almost every time-scale. can be deciphered. For much of the last 150 Ma Earth has experi- Climates during Earth’s history have remained sensitive to a large enced significant warmth, although long-term cooling has domi- number of forcing agents, and the Earth has certainly been nated the Earth for last 60 Ma. The periods of extreme warmth responding to such variety of perturbations. Although limited, one should offer data in modelling and predicting the global climate can understand the interactions of physical and biospheric systems response to increases in the atmospheric greenhouse gases. Thus the past warm climates should become the central theme in the future climate change modelling.

* Tel.: þ91 20 26454240; fax: þ91 20 26453707. Palaeoclimatic research indicates that climate change is pro- E-mail address: [email protected]. duced either by single forcing (e.g. volcanism), or by a combination of multiple forcing events that can bring about climate changes. Peer-review under responsibility of China University of Geosciences (Beijing) While climate change during the late Cenozoic has been attributed to the major episode of mountain building (Raymo and Ruddiman, 1992; Raymo, 1994) through enhanced chemical weathering and atmospheric p(CO2) drawdown, the global cooling of the Production and hosting by Elsevier EoceneeOligocene transition does not appear to be related to the

1674-9871/$ e see front matter Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2013.08.005

Please cite this article in press as: Sayyed, M.R.G., Flood basalt hosted palaeosols: Potential palaeoclimatic indicators of global climate change, Geoscience Frontiers (2013), http://dx.doi.org/10.1016/j.gsf.2013.08.005 2 M.R.G. Sayyed / Geoscience Frontiers xxx (2013) 1e9 increased mountain building (Rea, 1993). The weathering of con- Newark and Siberian flood basalts) a correlation with major tinental flood basalts also has been considered as a major sink of extinction events is feasible (Courtillot et al., 1996; Oslen, 1999), the atmospheric CO2 (Louvat and Allegre, 1998; Dessert et al., however, extinctions are also linked with the extraterrestrial im- 2001). The rapid extrusion of Deccan Traps (w1e4 Ma) and pacts (Alvarez et al., 1980; Retallack et al., 1998). concomitant subaerial weathering of such basalts together with an The work by Alvarez et al. (1980) trulybeganthedebatebetween increased flux of the weathered materials has affected contempo- the volcanically caused extinctions and bolide impact extinctions. raneous ocean chemistry as recorded in the marine Sr and Os Ever since the discovery of the Ir anomaly at Cretaceous-Tertiary geochemistry (Javoy and Courtillot, 1989; Vonhof and Smit, 1997; boundary (Alvarez et al.,1980) there were many announcements that Ravizza and Peucker-Ehrenbrink, 2003). However the contribu- most other mass extinctions have been correlated to impacts as well. tion of basaltic rocks to continental weathering flux is presently At one time or another, the Eocene-Oligocene, Triassic-Jurassic, poorly constrained since it is difficult to determine the relative Permian-Triassic, and late Devonian extinctions were all attributed contributions of the processes releasing the elements from the to impacts (Prothero, 2004) although the evidence for these has basaltic weathering (Kisakurek et al., 2004). Nonetheless, the na- diminished as it was more critically examined. Recently, however, the ture of weathering processes can be understood by stable isotope impact-volcanism debate has reopened in a new formulation studies of the light elements like Ca, Si, etc. (De La Rocha et al., (Courtillot,1999; Wignall, 2001) as the link between mass extinctions 2000; Schmitt et al., 2003). The fundamental question is whether and the flood volcanism has been strengthened by the discovery of flood basalts (Self et al., 2005) with major out gassing could lead to the temporal relationship between them (Wignall, 2001 and refer- significant environmental perturbations? For this more accurate ences therein). In this view, foreshadowed by an early correlation of modelling of dense atmospheric aerosol clouds and their effects on flood basalts and mass extinctions (Rampino and Stothers, 1988), atmospheric dynamics and chemistry is needed and in this sense flood basalts provide a general causal explanation for mass extinc- the continental flood basalt provinces of the world should provide tions. An impact did coincide with the Cretaceous-Tertiary boundary fundamental palaeoclimatic data from the associated contempo- (KTB) causing the extinction of one-third to one-half of the species. At raneous sediments hosted by the successive basaltic flows. KTB a large impact also occurred during a time of flood basalt eruption, producing a sudden spike of extinctions within a fauna already 2. Continental flood basalts and climate stressed and undergoing more gradual extinction due to the effects of Deccan volcanism (Alvarez, 2002). Keller et al. (2002) argued the Large Igneous Provinces (Table 1) have been formed at various extinction event was already under way as a result of Deccan volca- times in the geologic past and covered millions of square kilo- nism and so the projected environmental and climatic effects of these metres of the Earth’s surface (Coffin and Eldholm, 1994). Some mega-eruptions may be severe, and flood basalt events may have erupted in the sub-marine environment while other were erupted played a larger role in the mass extinctions than is commonly on land (Jerram and Widdowson, 2005). Those erupted on the land believed (Haggerty,1996). If the evidence for a flood basalt-extinction are referred to as continental flood basalts (e.g. Siberian Traps, link is compelling, we should accept that conclusion (Alvarez, 2002) Deccan Traps, Parana, Karro, etc.). Whether these eruptions after carefully examining the evidence. Although it is established that changed the climate and could this then have lead to the mass both impacts and volcanism occurred and correlate with some mass extinctions is yet to be fully understood. These flood basalts are extinctions, many aspects remain to be tested (Courtillot, 2002). supposed to be formed in a very short period of geologic time. In There are inherent asymmetries in comparing the evidence for most instances the greatest number of individual eruptions and the impact and volcanism, as recognized by Alvarez (2002),ascompeting largest volumes of lava probably occurred within a million year or explanations for mass extinctions. less (Self et al., 1997). Mafic volcaniclastic deposits exist in many Flood basalts erupt over intervals of one to a few million years, flood basalt provinces, and the eruptions that formed such deposits as against instantaneous impacts, and if an extinction occurs in that are meaningful in terms of potential atmospheric impacts and links interval, the flood basalt will be considered a candidate explanation with mass extinctions (Ross et al., 2005). for the extinction. Further absence of ejecta at mass extinction However it is necessary to correlate flood basalt episode with horizons also weakens the impact-extinction link claims. the host of related global geological factors like change in sea floor While not ruling out the contributions of impacts in some mass spreading rates, rifting events, increased tectonism and volcanism, extinctions White and Saunders (2005) do not consider the impacts sea level variations (Rampino and Self, 1999) and the reorganiza- as the primary cause but the outcome is strongly dependent upon tion of the tectonic plates (Cohen and Coe, 2002) that may be the conditions of the systems at the time of impact. These condi- associated with unusual climatic and environmental fluctuations tions include whether the biosphere was already under stressed leading to significant faunal changes. In at least three cases (Deccan, state due to the effects of flood basalts, or other terrestrial processes

Table 1 Correlations between ﬂood basalts and mass extinctions (after Haggerty, 1996).

Continental Flood Basalt Province K/Ar (Mys) 40Ar/39Ar (Mys) Extinction boundaries (Mys) Columbia River 17 1 16.2 1 Lower/Mid Miocene 14 3 Ethiopian 35 2 36.9 0.9 Eocene/OligoceneIr, mt/t, q 36 1 North Atlantic 62 3 60.5 Late Paleocene 59 1 Deccan 66 2 65.5 2.5 Cretaceous/TertiaryIr, mt/t, q 65 1 Madagascar 94 1 87.6 0.6 Cenomanian/TuronianIr 91 1 Rajmahal 110 5 116 1 Aptian/Albian 113 3 Serra Geral 130 5 132 1 Jurassic/Cretaceous 137 7 Antarctic 170 5 176 1 Bajocian/Bathonian 173 3 Karoo 190 5 190 3 Pliensbachian 193 3 Newark 200 5 201 1 End-Triassicq, Ir 211 8 Siberian 250 10 250 1 Permian/TriassicIr? 250 1

Several boundaries show stratigraphic evidence of large impact: shocked quartz (q), Microtektites/tektites (mt/t), and/or iridium (Ir).

Please cite this article in press as: Sayyed, M.R.G., Flood basalt hosted palaeosols: Potential palaeoclimatic indicators of global climate change, Geoscience Frontiers (2013), http://dx.doi.org/10.1016/j.gsf.2013.08.005 M.R.G. Sayyed / Geoscience Frontiers xxx (2013) 1e9 3 such as sea level changes or climate changes. Prothero (2004) of geologic time, despite their immense size (105e106 km3) with a referred to the intrinsic biological factors such as community dy- lifetime between 5 and 10 Ma (Jerram and Widdowson, 2005). namics and evolutionary trends within lineages as the possible Based on the published 40Ar/39Ar and U/Pb age determinations causes in assuming some apparent extrinsic causes (such as impact, Haggerty (1996) suggested that in most instances a peak tholeiitic volcanic eruptions or climate change) controlling the evolution of life. basalt output is achieved in less than 1e3 Ma. Stratigraphic details Although biotic effects of the basaltic flood volcanism cannot be also reveal the inter-eruptive intervals (periods of quiescence) in easily distinguished from those of large meteoric impacts the lava succession as indicated by palaeoweathering profiles, (Courtillot et al., 1996), the coincidence of both together may have interflow sediments and palaeoerosional surfaces with spatial and caused the most severe biotic changes (Keller, 2003; White and temporal distributions within the CFBPs (Widdowson et al., 1997). Saunders, 2005). It has been argued that volcanogenic CO2 pro- There is now a requirement for more elegant explanatory models duction could drastically enhance the atmospheric CO2 level lead- that can address the different phases and rates of volcanic activity, ing to global greenhouse effect, this is reflected in accelerated which will ultimately aid in understanding the coeval development chemical weathering (Louvat and Allegre, 1998; Dessert et al., of different geological environments and complex ecosystem 2001). Flood basalt volcanism thus provided both a source (erup- (Jerram and Widdowson, 2005). tion) and a sink (via weathering) of CO2 (Schwarz, 1997). The Rampino and Stothers (1988) found a good correlation between environmental impact of such enormous volumes of basalt out- the estimated initiation times of continental flood basalts and pourings includes cooling and/or greenhouse effects from the po- estimated times of mass extinctions. Table 1 shows a clear evidence 12 tential liberation of CO2 and SO2 aerosols in the order of 10 metric of correlation, on a quantitative level, between the initiations dates tons and proportionately large amounts of water vapour, halides of the continental flood basalts (Haggerty, 1996) and the estimated and ash, all produced in a brief volcanic event. The nature and ages of the mass extinction events (Rampino and Stothers, 1988). severity of the environmental effects of the eruptions and their Courtillot et al. (1996) have also plotted the ages of 12 CFBPs against potential impact on life is a major concern. In the case of flood the ages of 10 mass extinction events and found that seven of those basalt eruptions the indirect environmental effects in an oceanic events are contemporaneous with flood basalt and at least nine environment include changes in ocean chemistry, circulation and flood basalts are contemporaneous with a mass extinction event. oxygenation. In the terrestrial environment the effects include According to them there is no doubt that several large impacts have warming from CO2 and SO2 and acid rain resulting from the sul- occurred on Earth fairly recently, but apart from Chicxulub, they do phuric acid aerosols injected in to the stratosphere. However not coincide with extinctions. signature and amount of the minor volatile elements in these old lavas is difficult to obtain due to their age and altered state (Sharma 4. Basalt hosted fossil soils (intrabasaltic palaeosols) et al., 2004). Self et al. (2005) examined the manner and effect of volcanic gas release during the flood basalt eruptions on the Despite the fact that geological records of the ancient continents regional and global environment. According to these authors the are scarce, the continental palaeolandscapes and palaeoweathering important considerations are how often these gases were released records are vital in order to gain an understanding of global during the main eruptive pulse and the manner in which they were palaeoclimates (Widdowson, 1997a,b; Thiry et al., 1999) because introduced into the atmosphere by flood eruptions. However it is palaeosurfaces preserve the landscape with strong mineralogical possible to show whether the eruptions degassed vast amount of and geochemical fingerprints. On upland areas weathering pro- CO2 and SO2, if these species can be measured. For this more ac- cesses can operate continuously for long periods and they become curate modelling of the effects of these dense atmospheric aerosol eroded and rarely get preserved. However, in case of flood basalt clouds on atmospheric dynamics and chemistry further informa- landscapes the weathering profiles developed can get preserved tion is needed before the likely climatic impact of events such as more easily between successive lava episodes. These hiatuses flood basalts can be properly estimated. (giving rise to palaeoweathering profiles, interflow sediments and palaeoerosional surfaces) typically occur over a periods of 10e104 years (Jerram and Widdowson, 2005). However these have gener- 3. Duration of continental flood basalts and mass extinctions ally been overlooked since such periods have been considered as geologically insignificant with respect to the long term evolution of Continental flood basalt (CFB) volcanism has been considered to CFBPs. Jerram and Widdowson (2005) stressed to consider the be a prime candidate for causing most extinction events (McLean, temporal hierarchy of CFBPs, as the volcanically induced climate 1985; Courtillot, 1990; Courtillot et al., 1996). Its potential envi- and associated forcing agents that result in the profound oceano- ronmental impact is a function of size, rate of emplacement, loca- graphic and environmental changes, occur over 10e102 year pe- tion, more or less detrimental chemical composition, and riods. Clayey or earthy intrabasaltic layers (interflow sediments) submarine or subaerial emplacement (Courtillot et al., 1996). Other have recently captured the attention of Earth scientists globally internal processes, such as sea level regression and continental (Jolley et al., 2002; Ross et al., 2005) in order to understand the breakup, can be associated with volcanism without having to resort impact of continental flood basalts on the climates. to multiple causes (Courtillot et al., 1996). The apparent causal re- It is important to study the rate determining processes of lationships between flood basalts volcanism and mass extinctions weathering and soil formation in volcanic terrains since they fix the (Table 1) have highlighted the need for accurate age constrains on significant parts of CO2 due to chemical weathering (Dessert et al., continental flood basalts (Haggerty, 1996). The most compelling 2003; Sigfusson et al., 2008). In the formal lithostratigraphy of India evidence for a link between volcanism and extinctions comes from the intrabasaltic layers are known as bole beds (signifying hiatuses the comparison of the ages of flood basalt provinces and mass in basalt flow emplacement) which occur at the flow contacts extinction events (Wignall, 2001). Although there has been having colour variations (Fig. 1) from red to chocolate brown, considerable debate over the duration (e.g. Courtillot et al., 1986; earthy brown, green, purple grey and composing of an admixture of White et al., 1987; Richards et al., 1989; White and McKenzie, clay, silt and sand sized grains (Inamdar and Kumar, 1994). These 1989; Bakshi and Farrar, 1990; Renne and Basu, 1991; Campbell bole beds provide an opportunity to study products of chemical et al., 1992), the high-volume eruptive episodes of the Continen- weathering that can be a record of ancient weathering regimes in tal Flood Basalt Provinces (CFBP) occur over a relatively short period the Deccan basalts (Wilkins et al., 1994). The presence of red boles

Figure 1. Intrabasaltic bole beds occurring in the Deccan Traps near Pune. from the Deccan Traps of India (Chenet et al., 2008) is suggestive of The palaeosols developed on top of basalt flows are potentially significant time between the flood basalt flow units and the for- ideal for palaeoenvironmental reconstructions because protolith mation of such intertrappean boles is linked to either hydrothermal geochemistry may be determined directly rather than inferred deuteric alteration overprinting a weak weathering stage or pools from surrounding geology as in case of most palaeosols (Sheldon, of water retention. Earlier, Beckmann et al. (1974) recognized the 2003; Kisakurek et al., 2004). In addition intrabasaltic palaeosols friable red soils from the basalts from Queensland (Australia) as represent a defined time interval because they truncate the flow euchrozems and proposed that they have developed by long they formed upon and are in turn truncated by the successive flow. continued weathering under stable well drained conditions Thus the age of intrabasaltic palaeosols can be determined rela- without much erosion as against the development of black earths tively precisely as large numbers of flows are erupted in relatively which formed in sites prone to erosion and in situations with less short duration. The palaeoweathering patterns and palaeoclimatic water available for weathering. In order to refine models of the conditions can be deduced from the geochemistry and isotopic impacts of the Large Igneous Provinces (LIPs) on climate change signatures of these layers (Ghosh et al., 1995; Tandon et al., 1995; events we must first provide better constraints on the mafic vol- Tandon, 2002, etc.) developed during the inter-eruption intervals caniclastic deposits components which until now have been of the flood basalts. Further, they provide unique information about neglected. local variations of ancient climate and ecosystem (Solleiro- Studies have been carried out in France, Scotland, Ireland, West Rebolledo et al., 2003) because eruptions covered extensive areas Germany, Austria, Spain and North Atlantic (Singer and Ben-Dor, giving rise to palaeosols on different geomorphic positions, which 1987) and in Australia (Schafer and Mcgarity, 1980; MacNish are quite susceptible to spatial palaeoenvironmental diversity. To et al., 1987). Schwarz (1997) used deep ferralitic weathering pro- understand the environmentally driven changes marine records are files on the Vogelsberg basaltic massif in interpreting a global cli- well-resolved (Li and Keller, 1998; Abramovich et al., 1998, 2002), matic optimum during middle Miocene whereas Borger and while terrestrial climates in the same time frame are poorly known Widdowson (2001) reported on the comparative studies of the (Wilf et al., 2003). For the purpose of unifying global climate change Indian laterites and lateritic residues of southern Germany. model, it is important to know about the terrestrial events as Reddened flow tops and thin interflow sediments have also been terrestrial sediments (in addition to the marine geological record), identified during the Ocean Drilling Program (ODP) from the which offer important means of deciphering environmental southeast Greenland volcanic margin (Planke and Cambray, 1998) changes during mass extinctions. suggesting longer time interval between lava emplacements. The Monaro Volcanic Province (MVP) of New South Wales (Australia) 5. Selected examples of intrabasaltic palaeosols from the also contains ‘bole’ clays containing microscopic magnetite and continental flood basalts corundum (Taylor and Roach, 2005) intercalated with the lava pile. These boles are considered as original fine-grained clay-rich fer- Although Precambrian basalt derived palaeosols also have been ruginized regoliths, which were contact metamorphosed (Roach extensively studied (Button, 1979; Gay and Grandstaff, 1980; and Hill, 2005). Most researchers now consider that such intra- Retallack, 1986; Nesbitt and Young, 1989 and Nesbitt, 1992) the basaltic layers represent subaerial weathering products of basalt Phanerozoic intrabasaltic palaeosol layers have attracted attention saprolites or palaeosols, although their origin has remained only recently. In Phanerozoic rocks they range from Holocene to somewhat unresolved. In most reported cases “fritting” or “baking’ Tertiary (Singer et al., 1994) but much older layers of such materials has produced insignificant physical changes in them, with indi- have been identified extending up to Triassic (Tabor et al., 2004). vidual minerals being left mostly unaffected (Singer et al., 1994). Selected and interesting examples are presented below: Further, a number of them show evidence of paedogenesis having occurred during the inter-eruption intervals of flow basalts. 5.1. Basalt hosted palaeosols from late Triassic Ischigualasto A fundamental question is whether such palaeosols can be formation NW Argentina reliably useful for palaeoenvironmental reconstructions or whether those have been altered by baking or metasomatism (Sheldon, Tabor et al. (2004) used Triassic basalt hosted palaeosols to 2003). Extrusive igneous rocks (chiefly basalts) are important reconstruct the palaeoclimatic conditions. These soils were shown lithologic components of many depositional basins and paedogenic to be paedogenic vertisol that formed on well-drained flat terrain minerals formed from igneous lithologies in sedimentary basins with strongly contrasted wet and dry seasonal climate. The sig- have the potential to yield significant palaeoclimatic signatures nificant loss of bases indicates at least a moderate weathering while (Tabor et al., 2004). However the paragenetic sequence from hy- loss of carbonate and incipient development of kaolinite in the soil drothermal to supergene weathering in basalt lava flow is complex. profile suggest precipitation in excess of 760 mm/yr. However

5.5. Red interﬂow palaeosol intervals from the middle Miocene Picture Gorge Sub-Group of Columbia River basalt, Oregon

The Picture Gorge Sub-Group of Columbia River flood basalt province preserves a number of palaeosols that developed during the middle Miocene climatic optimum (Sheldon, 2003). Based upon geochemistry the palaeosols were identified as less weathered alfisols (Retallack, 2001; Sheldon et al., 2002). The middle Miocene climatic optimum was a time of global greenhouse condition within the generally cooling and drying climatic trends of the Cenozoic. All of the palaeosols of the Picture Gorge Sub-Group lack free carbonates indicating precipitation >500 mm/yr (Jenny, 1941). How- ever this relationship does not necessarily hold up for areas experiencing strongly monsoonal climates, where there may be large seasonal difference in evapo-transpiration and high mean annual precipitation. The mean annual rainfall is calculated to be 570e930 mm/yr while maximum annual temperature to be e Figure 2. Geographical extent of the Deccan Volcanic Province of the Indian subcon- 8 16 C(Sheldon, 2003) and there is no evidence to suggest that tinent (after Sayyed and Hundekari, 2006). Oregon was subjected to strongly seasonal climate in the Miocene.

These observations are consistent with the transient warm, wet sulphur (Thordarson et al., 1996), it has been suggested that envi- period (Miocene climatic optimum). ronmental degradation and extinction are the exacerbation of SO2. For instance at the Triassic/Jurassic boundary extinctions were not 5.6. Late Quaternary paedogenesis and volcanic sedimentation of caused by volcanic out gassing (Tanner et al., 2001) but by other Glacis de Buenavista, Central Mexico volcanic effects such as release of atmospheric aerosols or tecton- ically driven sea level changes. In case of the transient warming Solleiro-Rebolledo et al. (2003) studied the late Quaternary event due to Deccan volcanism, the combined duration in the sedimentary sequence of the Glacis de Buenavista, central Mexico marine and terrestrial realm is larger than predicted simply by CO2 and interpreted the sequence as a record of climate change and models (Ravizza et al., 2003). Indirect environmental effects interaction of paedogenesis and volcanic sedimentation. Here the include changes in ocean chemistry, circulation, and oxygenation, non-buried profile resulted due to paedogenesis under present especially from basaltic volcanism associated with large submarine humid conditions, on the recently deposited ash, giving rise to oceanic plateaus that may represent flood basalt eruptions in an andosol type horizon exhibiting moderate weathering. Another oceanic environment. Critical to this hypothesis is the rate at which palaeosol profile is the buried Pleistocene luvisol in loess-palaeosol the lava was erupted and the gas released. Some evidence suggests sequence characterized by advanced weathering. These palaeosols the eruptions happened over a long period, perhaps allowing the were never related to the past soil forming processes and envi- atmosphere to recover between eruptions. ronmental conditions (Solleiro-Rebolledo et al., 2003) and it is Oxidation and reduction are the key processes in the oxygen concluded that the sedimentary sequence of the Glacis de Buena- cycle and on long-term scale oxidation must balance oxygen vista is the result of paedogenesis interacted with the impulsive production to leave a steady amount of oxygen in the atmosphere. volcanic sedimentation. Volcanism releases reduced gases (like H2, CO, and H2S) into the atmosphere, which react with the oxygen. This would change the 6. Discussion redox conditions of the atmosphere and the soil-atmosphere interface, because large amount of oxygen would be consumed Mass extinction events are considered important in the history in oxidizing the reduced gases released from the volcanism. of the Earth, and some of these mass extinction events appear to be Therefore more pronounced greenhouse warming and nearly related to catastrophic causes such as asteroid impacts and flood anoxic conditions can be expected due to volcanism. The oceanic basalts. The possible volcanic effects of the continental flood ba- plateau basalts eruptions are also correlated with the global salts, in terms of serious climatic perturbations and mass extinc- oceanic anoxia (Kerr, 2005 and references therein) in which one of tions, have been advocated by many researchers (e.g. Hallam and the probable mechanisms for the depletion of dissolved O2 given þ Wignall, 1997; Rampino and Self, 1999; Wignall, 2001). However (Sinton and Duncan, 1997) is the complete oxidation of Fe2 , 2þ if there is a causal link between flood basalt events and mass ex- Mn ,H2S and CH4 from the hydrothermal solutions. Volcanism tinctions, then the environmental impact of the gases released from may also trigger the release of CH4 from clathrates buried at flood basalts must be considered (Rampino and Self, 1999; Self shallow depths beneath sea floor, which is rapidly oxidized to CO2 et al., 2005) as basalt eruptions are not particularly explosive but in the atmosphere (Wignall, 2001). During the early Triassic the when released suddenly, such a great volume of gas could produce atmospheric O2 levels apparently decreased while CO2 levels dire environmental consequences. Widdowson et al. (1997) and increased (Woods, 2005). A decrease in the atmospheric O2 is Cripps et al. (2005) argued for significant explosive volcanism in supported by the presence of berthierine in earliest Triassic the Deccan Traps. palaeosols (Sheldon and Retallack, 2002). Krull and Retallack Flood basalts can release huge amounts of CO2,SO2 and halo- (2000) indicated low soil oxygen in the earliest Triassic soils due gens and several kinds of environmental effects have been sug- to the conversion of CH4 to CO2. Consequently the release of re- gested, including climatic cooling from sulphuric acid aerosols, ductants from the solid Earth, either by continental uplift and greenhouse warming from CO2 and SO2 gases, and acid rain weathering or by geothermal degassing would control oxygen (Rampino et al., 1988; Kiehl and Shield, 2005; Saunders, 2005; Self consumption. Thus the examinations of oxidized and/or reduced et al., 2005, etc.). The initial release of SO2 can trigger a global mineral species in the palaeosols associated with the flood basalts volcanic winter, while later long term accumulation of CO2 may could be helpful in the palaeoclimatic reconstructions. lead to subsequent global warmingea volcanic summer (Self et al., One of the pivotal aspects to the idea of whether CFBPs funda- 2005). SO2, however, may also act as a greenhouse gas (Self et al., mentally altered climate is to determine the time scale interval 2005) if present in sufficient concentrations at low altitudes. between the groups of eruptions and individual major flood lava Wignall (2001) demonstrated that the regional warming by SO2 eruptions (Jerram and Widdowson, 2005) by analyzing the sedi- lasts only for few months as it rapidly reacts with water in the at- ments and fossil soils preserved between successive eruptive units. mosphere to produce sulphate aerosols that backscatter and absorb Features within continental flood basalt lava flows (Self et al., 1997) the suns radiation (Devine et al., 1984; Sigurdsson, 1990a,b) there and the presence of soil horizons at the Deccan Traps argue for a by producing cooling effects up to tens of years. gradual emplacement of the lava flows. In a flood basalt succession By contrast CO2 unlike other surficial gases, cannot be removed it would be desirable to obtain the age of individual lavas to rapidly because of its long residence period in the surficial systems determine how often these huge eruptions occurred, but the dating (102e105 years), the cumulative effects of successive lava eruptions methods used do not have sufficient accuracy to pinpoint individ- could be climatically significant (Wignall, 2001), leading to global ual flows in time-virtually a whole flood basalt field fits within the warming. Basaltic magmas are often rich in dissolved sulphur and errors on some of the dates. Migon and Lidmar-Berstrom (2002) sulphuric acid aerosols, formed from sulphur volatiles (largely SO2) argued that if the fossilized relict weathering mantles (saprolites) which are injected into the stratosphere by convective plumes are accurately dated and interpreted, may serve as an important rising above volcanic vents and fissures (Rampino and Self, 1999). complementary source of information about the long-term evolu- The most likely manner in which such flood basalt eruptions can tion history. According to them a fuller understanding of the role of influence the global environment is by degassing of sulphur (Jerram weathering in geological evolution will be achieved through dating and Widdowson, 2005) as against the release of CO2 (Caldeira and of saprolites, among which stable isotope analysis and K-Ar and Rampini, 1990). Since CFB eruptions release huge amounts of 40Ar/39Ar dating seem most promising.

Since flood basalt-hosted soils (interflow layers) were formed at have revealed the vital climatic information. Their frequent occur- the Earth’s surface they potentially represent high fidelity recorders rences in the flood basalt stratigraphy can help in evaluating the of the climatic and environmental conditions in which they were stepwise climate change record influenced by volcanism. formed. Mineralogical, geochemical and isotopic data can be compared with modern analogues to constrain the palae- Acknowledgements otemperature and palaeoprecipitation (Sheldon et al., 2002; Sheldon, 2002, 2003) to reveal the causal link between the flood I gratefully acknowledge Dr. Mike Widdowson for his help basalts, climates and extinction. This would help resolving the during preparing the manuscript. Thanks are also due to Prof. M. contentious issues related to the flood basalt volcanisms such as Santosh and Prof. Franco Pirajno for their critical and constructive wet and dry climate and whether climate change was the ultimate reviews that have helped much in revising and improving the cause of the extinction event rather than an extrinsic mechanism. manuscript. Principal of Poona College is also thanked for his More accurate modelling of dense atmospheric aerosol clouds, and encouragement and support. their effects on atmospheric dynamics and chemistry, is needed before the likely climatic impact of events such as flood basalts can be properly estimated. Appendix A. Supplementary data As seen from the flood basalt-hosted palaeosols from different temporal and spatial horizons it seems that they commonly indi- Supplementary data related to this article can be found at http:// cate quite high rainfall and good drainage with strong seasonality. dx.doi.org/10.1016/j.gsf.2013.08.005. Such conditions normally prevail in the humid tropical or sub- tropical climates (Bardossy and Aleva, 1990; Price et al., 1997) References producing lateritic soils. Lateritization is also be possible in cool climates (Taylor et al., 1992) but then they may take much longer Abramovich, S., Almongi-Labin, A., Benjamini, C., 1998. Decline of Maas- time to form than the laterites formed under humid tropical cli- trichtianpelagic ecosystem based on planktic foraminiferal assemblage change: mates (Hill et al., 2000). In case of Monaro Volcanic Province (MVP) implications for the terminal faunal crisis. Geology 26, 63e66. of New South Wales (Australia), which were formed under cold Abramovich, S., Keller, G., Adatte, T., Stinnesbeck, W., Hottinger, L., Stueben, D., Berner, Z., Ramanivosoa, B., Randriamanamantenasoa, 2002. 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