geochemical variability in distal and proximal glass from the youngest toba tuff eruption the youngest toba tuff ytt erupted at ca 74 ka is a distinctive and widespread tephra marker across south and southeast asia the climatic human and environmental consequences of the ytt eruption are widely debated although a considerable body of geochemical data is available for this unit there has not been a systematic study of the variability of the ash geochemistry intrinsic magmatic and extrinsic post depositional chemical variations bring fundamental information regarding the petrogenesis of the magma the distribution of the tephra and the interaction between the ash and the receiving environment considering the importance of the geochemistry of the ytt for stratigraphic correlations and eruptive models it is central to the ytt debate to quantify and interpret such variations here we collate all published geochemical data on the ytt glass including analyses from 67 sites described in the literature and three new samples two principal sources of chemical variation are investigated i compositional zonation of the magma reservoir and ii post depositional alteration post depositional leaching is responsible for up to ca 11 differences in na2o k2o and ca 1 differences in sio2 al2o3 ratios in ytt glass from marine sites continental tephras are 2 higher in na2o k2o and 3 higher in sio2 al2o3 with respect to the marine tephra we interpret such post depositional glass alteration as related to seawater induced alkali migration in marine environments crystal fractionation and consequential magmatic differentiation which produced order of magnitude variations in trace element concentrations reported in the literature also produced major element differences in the ytt glass feo al2o3 ratios vary by about 50 which is analytically significant these variations represent magmatic fractionation involving fe bearing phases we also compared major element concentrations in ytt and oldest toba tuff ott ash samples to identify potential compositional differences that could constrain the stratigraphic identity of the morgaon ash western india no differences between the ott and ytt samples were observed

contribution of grain size and compositional data from the bengal fan sediment to the understanding of toba volcanic event abstract toba tephra layers in cores from the bengal fan have been studied to characterize their granulometric mineralogic and geochemical features the layers are silt sized sediments with no significant non volcanic contamination the samples are very homogeneous glass shards have moderate hydration about 6 water a calc alkaline character and a rhyolitic composition the major and trace element compositions indicate an orogenic affinity and coincide with literature data for the youngest toba tephra level ytt 75 ka x ray diffraction xrd data indicate that the glass is the dominant phase with minor feldspar biotite and quartz the grain size analyses of the beds show discontinuous vertical variation and no linear trends from south to north over about 1500 km the vertical granulometric gap of the layers probably represents a single pyroclastic emplacement unit subjected to pre depositional particle aggregation

climate volcanism feedback and the toba eruption of 74 000 years ago abstract a general feedback between volcanism and climate at times of transition in the quaternary climate record is suggested exemplified by events accompanying the toba eruption 74 000 yr ago the largest known late quaternary explosive volcanic eruption the toba paroxysm occurred during the 18o stage 5a 4 transition a period of rapid ice growth and falling global sea level which may have been a factor in creating stresses that triggered the volcanic event toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co ignimbrite ash clouds to heights of at least 32 5 km which may have led to dense stratospheric dust and sulfuric acid aerosol clouds these conditions could have created a brief dramatic cooling or volcanic winter followed by estimated annual northern hemisphere surface temperature decreases of 3 to 5 c caused by the longer lived aerosols summer temperature decreases of 10 c at high northern latitudes adjacent to regions already covered by snow and ice might have increased snow cover and sea ice extent accelerating the global cooling already in progress evidence for such climate volcanic feedback following milankovitch periodicities is found at several climatic transitions

potential atmospheric impact of the toba mega eruption 71 000 years ago an approx 6 year long period of volcanic sulfate recorded in the gisp2 ice core about 71 000 5000 years ago may provide detailed information on the atmospheric and climate impact in the toba mega eruption deposition of these aerosols occur beginning of an approx 1000 year long stadial event but not immediately before the longer glacial period beginning approx 67 500 years ago total stratospheric loading estimates over this approx 6 year period range from 2200 to 4400 mt of h sub 2 so sub 4 aerosols the range in values is given to compensate for uncertainties in aerosol transport magnitude and longevity of the atmospheric loading may have led directly to enhanced cooling during the initial two centuries of this approx 1000 year cooling event 25 refs 2 fig 1 tab

a melt inclusion study of the toba tuffs sumatra indonesia the toba caldera in northern sumatra indonesia is the site of earth s largest quaternary volcanic eruption this eruption dated at 74 ka produced the 2800 km3 youngest toba tuff ytt and ash fall quartz bearing silicic pumices ranging from 68 to 77 wt sio2 indicate that the ytt magma was zoned compositionally and mineralogically prior to the ytt eruption two other silicic quartz bearing tuffs known as the middle toba tuff mtt and the oldest toba tuff ott were erupted from toba at 0 501 ma and 0 840 ma respectively although the volatile contents of the toba magmas are poorly constrained aerosols generated by the ytt eruption are generally thought to have caused a global volcanic winter an evaluation of the pre eruptive dissolved gas contents of the ytt magma is fundamental to understanding the global effects of this eruption we used melt inclusions in quartz crystals to determine the dissolved h2o co2 s cl and f contents of the ytt mtt and ott magmas quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data melt inclusions from the ytt mtt and ott are rhyolitic in composition 73 77 wt sio2 and have overlapping indistinguishable major element trends inclusions from the large eruptions ytt and ott have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses in contrast the least evolved melt inclusions occur in the most silicic samples and either overlap with or are less evolved than their matrix glasses these patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt while those in the most silicic melt formed in situ several lines of evidence suggest that crystal settling of early formed quartz crystals in long lived magma bodies was an important process in the ytt and ott magmas trace element data from the ytt melt inclusions have wide ranges rb 250 830 ppm ba 10 485 ppm and mostly support these models however incompatible trace elements are enriched 2 3 over their matrix glass compositions in all samples in the smaller mtt melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in situ crystallization ftir analyses indicate that the ytt mtt and ott melts contained about 4 0 5 5 2 0 5 5 and 2 0 5 5 wt h2o respectively co2 contents in all units are generally 100 ppm and cl is 2000 ppm water gradients occur in all 3 tuffs cl gradients are evident only in the ytt and the least silicic samples from all units had no detectable co2 sulfur contents in melt inclusions are low in all samples 32 ppm and overlap with matrix glass s contents applying broad degassing constraints about 1014 g of h2so4 aerosols were likely loaded into the atmosphere during the ytt eruption 2 orders of magnitude less than previous petrologic estimates considerably more cl and f about 1015 g of each may have been part of the ytt aerosol cloud

newly discovered eastern dispersal of the youngest toba tuff volcanic glasses with minor mafic mineral fragments such as biotite and hornblende found in deep sea sediments of the south china sea basin scsb have been clearly identified as eruptive products of the youngest toba tuff ytt northern sumatra indonesia the tephra layer occurs between marine oxygen isotopic event 5 1 79 3 ka and event 4 22 64 1 ka with an interpolated age of 74 0 ka which is in good consistence with previous radiometric dating 73 75 ka and ice core dating 71 5 ka of the ytt the tephra consists predominantly of bubble wall shards with minor elongated vesicles of pumice fragments geochemical characteristics of the tephra such as high total alkali content high 87 sr 86 sr ratio and uniformity of their compositions all suggest that the recovered tephra is of the youngest toba tuff this finding supports an extended dispersal of coarse 63 mm glass shards over 1500 km northeast of the toba caldera a direction opposite to what previously conceived while providing a better documentation of the distribution extent of the toba ash this report points to the need to reestimating the eruptive volume of the ytt and re evaluating its environmental impact q 2000 elsevier science b v all rights reserved

the stratigraphy and paleogeography of the middle paleolithic open air site of ein qashish northern israel abstract the middle paleolithic site of ein qashish is located on the floodplain of the qishon stream northern israel which drains into the mediterranean sea it is located at the inlet to a narrow water gap between mt carmel and tiv on hills the excavation and 3 trenches up to 5 m deep provide the stratigraphy and allow a reconstruction of the paleogeography and depositional environments the archaeological layer osl dated to about 64 4 ka is composed of carbonate gravel and overlies 66 4 ka mis4 black heavy alluvial clays deposited by the qishon stream which represents marsh environment the site is covered by another clay unit representing a second marsh period between 50 3 and 41 3 ka mis3 this sedimentary complex is unconformably covered by coarse gravel and reddish clay matrix dated to between 15 1 0 7 and 10 5 0 5 ka which is derived from wadi qashish a steep mountainous stream of mt carmel the mineralogical analyses of the clays indicate high quartz concentrations and smectitic is which support the interpretation of qishon basin origin versus illitic is from mt carmel slopes the duration of the prehistoric human activity is estimated at sea level during the occupation period was between 100 and 60 m b s l and the coastline was located 10 7 km westward the proposed mechanism for the marsh development is a repeated episodes of blockage of the qishon stream outlet at the narrow water gap by aeolian sand transported by westerly winds inland over the exposed continental shelf the accumulation of sand and sandstone over the coastal plain during this period supports this idea b based on hydrological correlation with the lake lisan levels the flows in the qishon stream were low with large component of baseflows characterizing warmer and wetter periods that could not cut through the sand blockage and forced the water upstream into marshes the marshes drained during the colder and drier heinrich events h6 h5a and h4 which were characterized by higher frequency of large floods

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

fresh pumice from the central indian basin a krakatau 1883 signature the occurrence of pumice in the central indian basin cib has been a controversy some are of the opinion that these pumices are erupted from a volcano along the ninetyeast ridge while others suggest the indonesian arc volcanism as their probable source recently it is also suggested that the pumice are the products of in situ silicic volcanism in the cib this study was initiated in order to resolve this controversy about the origin of the fresh pumice from the cib the fresh pumice pieces of different sizes and shapes were collected from the surficial sediments of the cib from an average depth of about 5000 m 12 s 76 e glass is the main phase comprising about 95 volume and resembles the matrix glass in krakatau 1883 dacitic pumice which is also very poor 5 in the modal mineral composition the mineral and glass analyses were carried out with the help of the electron microprobe the orthopyroxenes clinopyroxenes plagioclases and spinels are the mineral phases present in these pumices the pyroxenes have similar wo en and fs components to those previously reported from the 1883 krakatau eruption products geochemically these fresh pumices have a high sio2 content 71 and high total alkali content 8 similar to 1883 eruption products of krakatau thus based on the similarities in mineralogy and glass chemical composition it is inferred that these fresh pumices are the products of the fierce 1883 eruption of the krakatau volcano in the sunda strait of the indonesian arc

deep sea ash layers reveal evidence for large late pleistocene and holocene explosive activity from sumatra indonesia abstract deep sea tephra layers sampled from sediment cores collected within and adjacent to the sunda trench of offshore sumatra reveal evidence for five previously undocumented and apparently large minimum volume 0 6 6 3 km3 volcanic explosivity index values of 4 5 explosive eruptions over the last 31 000 years with a presumptive source of mainland sumatra chemical analysis of glass shards and 14c age constraints are used to distinguish the five tephra layers as well as a sixth that likely correlates with the youngest toba tuff ytt the tephra layers are labeled v 1 through v 6 relative to their north to south positioning along the sunda trench the three tephra layers taken from cores west of central sumatra v 3 v 4 v 5 are well constrained by 14c age determinations whereas less reliable sedimentation rate estimates are available for the northern v 1 v 2 and southern v 6 tephra layers deposition of the northernmost tephra layer v 1 was likely accompanied by seismicity as two chemically indistinguishable tephras are separated by 12 cm of course grained turbidite layer v 2 shows a strong chemical resemblance to the ytt and age estimates do not rule out the correlation with the exception of a likely correlation with the ytt no other correlations were made between the tephras analyzed in this study with the marine or terrestrial record from the published literature the most frequent widespread and youngest marine tephra layers were found in the central region of the study area layers v 3 v 4 and v 5 were all deposited within the last 17 thousand years with minimum eruptive volumes of 0 6 to 5 2 km3 a complex depositional sequence of layer v 6 is estimated at 27 5 ka and may be associated with late pleistocene ignimbrite volcanism of southern sumatra the ages and suggested minimum volumes represented by the deep sea tephra layers are consistent with an active volcanic arc and demonstrate the need for further terrestrial studies

local and regional scale impacts of the 74 ka toba supervolcanic eruption on hominin populations and habitats in india abstract the toba supervolcanic eruption of 74 000 years ago is argued to have devastated homo sapiens populations causing a human population bottleneck through a combination of rapid global climatic deterioration and ecological disruption following the widespread ash fall this dramatic eruption the largest in at least the last 2 million years is hypothesized to have shaped the genetic structure and diversity of human populations today past assessments of toba s human impacts however have been predominantly theoretical using archaeological and geological evidence from sites in india this study addresses toba s local scale impacts on hominin technology behavior demography and habitats in two valleys in india the jurreru and middle son valleys using paleoclimatological and volcanological data regional scale impacts throughout india are inferred a refugia hypothesis is presented that argues for variation in the scale of toba s impact throughout india driven by regional differences in monsoonal dynamics geography and topography areas of north west india the indo gangetic plain and parts of the deccan to the east of the western ghats are argued to have been the most affected whereas areas of southern and eastern india may have preserved the largest refugia for hominin populations

himalayan sedimentary pulses recorded by silicate detritus within a ferromanganese crust from the central indian ocean a central indian ocean deep water seamount hydrogenous ferromanganese crust ss663 crust contains variable 7 23 amounts of detrital material silicate detritus taking into account the growth rate of the authigenic component the accumulation rate of the silicate detritus has been calculated which shows an overall decrease during the past 25 7 2 ma growth history of the specimen this silicate detritus displays ond 0 between 37 7 and 312 7 and 87 sr 86 sr between 0 7083 and 0 7215 the sr nd isotopic compositions record the variation in the mixture of two detrital end members throughout the growth history of the ss663 crust while one end member clearly appears to be himalayan derived material the other component is less well defined but could be related to the volcanogenic component from the indonesian back arc basin we have unscrambled the proportion of these two components in the silicate detritus of the ss663 crust to quantify the variation in the accumulation rate of the himalayan derived silicate detritus during the last 25 ma the result shows three periods of high accumulation rates modern around 10 ma and prior to 16 ma these sedimentary pulses have already been recognised and described in the turbidites from the distal bengal fan which is located more than 1000 km away from the ss663 crust location suggesting that these sedimentary pulses are of regional significance rather than of local importance the timings of elevated contents of the himalayan derived silicate detritus in the specimen during the neogene closely superimpose the periods of the himalayan uplift along the main central thrust the main boundary thrust and the main frontal thrust this further emphasises the tectonic control of the sediment production by the erosion of an active orogenic range and its distribution over a vast area additionally close superimposition of the early miocene detritus pulse in the ss663crust on the riverine sr isotopic positive excursion suggests that the physical and chemical weathering of the himalaya was strongly coupled during the early miocene while the spatial decoupling appears to have taken place only around 12 14 ma 2002 elsevier science b v all rights reserved

major element geochemistry of glass shards and minerals of the youngest toba tephra in the southwestern south china sea an ash layer newly discovered in core md01 2393 from the southwestern south china sea has been studied in order to characterize its major element features the layer 4 0 cm thick light grayish and silt size occurs right at the marine isotope stage 4 5 transition ca 74 kyr ago the morphology and geochemistry of glass shards combined with oxygen isotope and carbonate stratigraphy confirm the youngest toba eruption in northern sumatra as the origin of the ash layer major element data on mineral crystals i e biotite plagioclase and hornblende from the ash layer suggest that biotite is phenocrystic while hornblende and some plagioclase are xenocrystic implying that these xenocrysts were incorporated into the youngest toba magma before the eruption q 2005 elsevier ltd all rights reserved

multiple ash layers in late quaternary sediments from the central indian basin abstract we have investigated three sediment cores collected from water depths 5000 m along the transect 76 30 e in close proximity to a fracture zone in the central indian basin cib the cores yielded five volcanic horizons of which four have visual and dispersed shards rhyolitic glass shards of bubble wall platy angular and blocky types were retrieved from various stratigraphic horizons in the cores the abundance of glass shards composition of bulk sediments and 230thexcess ages of the host sediments were used to distinguish the volcanic horizons of the four volcanic horizons three are now newly reported and correspond to ages of 85 107 109 and 142 146 ka while the fourth horizon is of 70 75 ka by using trace element ratios and cr and nb based normative calculations cryptotephra has been identified for the first time from the cib sediment the cryptotephra forms the fifth ash horizon and is of 34 ka a comparison with the published data on volcanic tephra in and around the indian ocean indicate the shard rich horizon srh of 70 75 ka to resemble the younger toba tuffs ytt while the other volcanic horizons that were deposited during different time periods do not correlate with any known marine or terrestrial records these tephra layers have produced a tephrostratigraphic framework across the tectonically and volcanically complex regions of the cib due to the lack of terrestrial equivalents of these tephra it is hypothesized that the newly found volcanic horizons may have been derived from submarine volcanic eruptions multiple layers of submarine volcaniclastic deposits found at water depths as great as 5300 m reaffirm the growing belief that submarine phreatomagmatic eruptions are much more common in the intraplate region of the indian ocean than previously reported

abyssal sediment erosion in the central indian basin evidence from radiochemical and radiolarian studies abstract radiochemical analyses of three spade cores collected around 78 e between 10 s and 12 s from the abyssal depths in the central indian basin yield an average accumulation rate of 2 mm ka the ratios of 230th flux in the sediments to its production rate in the overlying water column fa fp are extremely low 0 2 this fact and the absence of collosphaera invaginata first appearance datum 150 200 ka an index radiolarian species of neogene radiolarian zone 1 indicate intense erosion and lateral transport of younger sediments from this region the radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent antarctic bottom water entering the central indian basin through the northern saddles of the ninety east ridge

late quaternary tephra in the new ireland basin papua new guinea sediment cores were recovered from the new ireland basin east of papua new guinea in order to investigate the late quaternary eruptive history of the tabar lihir tanga feni tltf volcanic chain foraminifera delta 18 o profiles were matched to the low latitude oxygen isotope record to date the cores which extend back to the early part of delta 18 o stage 9 333 ka sedimentation rates decrease from 10 cm 1000 yr in cores near new ireland to similar to2 cm 1000 yr further offshore the cores contain 36 discrete ash beds mostly 1 8 cm thick and interpreted as either fallout or distal turbidite deposits most beds have compositionally homogeneous glass shard populations indicating that they represent single volcanic events shards from all ash beds have the subduction related pattern of strong enrichment in the large ion lithophile elements relative to morb but three distinct compositional groups are apparent group a beds are shoshonitic and characterised by 1300 ppm sr high ce yb and high nb yb relative to morb group b beds form a high k series with morb like nb yb but high ce yb and well developed negative eu anomalies whereas group c beds are transitional between the low k and medium k series and characterised by flat chondrite normalised ree patterns with low nb yb relative to morb a comparison with published data from the tltf chain the new britain volcanic arc and backarc including rabaul and bagana on bougainville demonstrates that only group a beds share the distinctive phenocryst assemblage and shoshonitic geochemistry of the tltf lavas the crystal and lithic rich character of the group a beds point to a nearby source and their high sr ce yb and nb yb match those of tanga and feni lavas a youthful stratocone on the eastern side of babase island in the feni group is the most probable source group a beds younger than 20 ka are more fractionated than the older group a beds and record the progressive development of a shallow level magma chamber beneath the cone in contrast group b beds represent glass rich fallout from voluminous eruptions at rabaul whereas group c beds represent distal glass rich fallout from elsewhere along the volcanic front of the new britain arc c 2003 elsevier b v all rights reserved

moho map of south america from receiver functions and surface waves 1 we estimate crustal structure and thickness of south america north of roughly 40 s to this end we analyzed receiver functions from 20 relatively new temporary broadband seismic stations deployed across eastern brazil in the analysis we include teleseismic and some regional events particularly for stations that recorded few suitable earthquakes we first estimate crustal thickness and average poisson s ratio using two different stacking methods we then combine the new crustal constraints with results from previous receiver function studies to interpolate the crustal thickness between the station locations we jointly invert these moho point constraints rayleigh wave group velocities and regional s and rayleigh waveforms for a continuous map of moho depth the new tomographic moho map suggests that moho depth and moho relief vary slightly with age within the precambrian crust whether or not a positive correlation between crustal thickness and geologic age is derived from the pre interpolation point constraints depends strongly on the selected subset of receiver functions this implies that using only pre interpolation point constraints receiver functions inadequately samples the spatial variation in geologic age the new moho map also reveals an anomalously deep moho beneath the oldest core of the amazonian craton

the origin of ferro manganese oxide coated pumice from the central indian ocean basin pumice clasts partially and fully coated with ferro manganese oxide from the central indian ocean basin ciob were analysed for major trace and rare earth elements and glass and mineral grain chemistry to assess their possible source these pumice clasts have 95 glassy matrix with 5 of plagioclase ortho pyroxene amphibole and spinel mineral grains pumice is rhyolitic with high content of silica and total alkalis and show strong light rare earth element enrichment with la 100 times enriched compared to chondritic abundance and exhibits a pronounced negative eu anomaly both the partially and fully coated pumice clasts have a similar composition but this and their glass and mineral orthopyroxene chemistry differs from all tuffs known to have erupted from the toba caldera complex the partial to complete fe mn oxide coating suggests the pumice probably predates activity from the toba caldera their general composition is consistent with a volcanic arc origin and show similarities to rhyolitic eruptives from sumatra the distribution of partially and fully coated pumice in the ciob overlaps with that of uncoated pumice from the voluminous ytt eruption suggesting that other large rhyolitic calderas active in sumatra indonesia similarities in composition to ash layers from the ciob suggest a possible late miocene late pleistocene age for these pumices

a millennial scale u37k sea surface temperature record from the south china sea 8 n over the last 150 kyr monsoon and sea level influence we report 150 kyr records of u37 sea surface temperature sst and carbonate content with 150 200 year sampling resolution from the southern south china sea scs in the northern margin of the western pacific warm pool wpwp using images core md972151 the glacial interglacial sst change is about 5 c for termination ii 23 5 to 28 5 c and 4 c for termination i 24 to 28 c the simultaneous rise of sea level and u37 sst during termination i suggests that sea level change has played an important role in the scs sst variability by influencing the exchange of tropical ocean warm surface water with the scs through the sunda shelf region sill depth 30 50 m when sea level drop was less than 30 m relative to today s md972151 sst was constantly above 27 c a situation which occurred during mis 5e part of mis5c and 5a and over the last 11 kyr sea level change was also the main control on carbonate content in the scs this was lower during low sea level stands due to the dilution effect of enhanced terrigenous inputs within the limitations of our age model millennial scale sst oscillations 1 5 c over a few hundred years occurred frequently during the entire glacial period correlating reasonably well with the hulu monsoon record from eastern china and the dansgaard oeschger d o cycles recorded for high latitude regions in the northern hemisphere such correlations suggest a possible teleconnection between higher latitude atmospheric forcing and low latitude climate oscillations in the western pacific tropical region via the asian monsoon system carbonate content also reveals millennial scale oscillations during the glacial period with higher content correlated with higher sst millennial scale fluctuations of sst and carbonate contents at this low latitude scs provide additional evidence that high frequency climate changes are global phenomena

ash from the toba supereruption in lake malawi shows no volcanic winter in east africa at 75 ka the most explosive volcanic event of the quaternary was the eruption of mt toba sumatra 75 000 y ago which produced voluminous ash deposits found across much of the indian ocean indian peninsula and south china sea a major climatic downturn observed within the greenland ice cores has been attributed to the cooling effects of the ash and aerosols ejected during the eruption of the youngest toba tuff ytt these events coincided roughly with a hypothesized human genetic bottleneck when the number of our species in africa may have been reduced to near extinction some have speculated that the demise of early modern humans at that time was due in part to a dramatic climate shift triggered by the supereruption others have argued that environmental conditions would not have been so severe to have such an impact on our ancestors and furthermore that modern humans may have already expanded beyond africa by this time we report an observation of the ytt in africa recovered as a cryptotephra layer in lake malawi sediments 7 000 km west of the source volcano the ytt isochron provides an accurate and precise age estimate for the lake malawi paleoclimate record which revises the chronology of past climatic events in east africa the ytt in lake malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change implying that the eruption did not significantly impact the climate of east africa and was not the cause of a human genetic bottleneck at that time

major and trace element compositions of surface sediments from the lower bengal fan implications for provenance discrimination and sedimentary environment abstract the bengal fan the largest submarine fan in the world receives a large amount of terrestrial material from the himalayas and the indian peninsula making it an ideal area for studying land sea interactions in different time scales using the major and trace element compositions of 98 surface sediment samples from the lower bengal fan we identified the provenances of the sediments calculated the relative contributions of each provenance and further assessed their controlling factors the statistic results of the major and trace elements confirm that terrigenous materials comprise the majority on the sediment and that the study area can be divided into three depositional provinces the main sources of sediments are the himalayas and indian river systems which can be distinguished in the fe al k al scatter diagram based on calculation results of a two end member mixing model the relative contributions of the himalayas and the indian rivers are 55 and 45 respectively however the spatial distributions of sediments from these two sources in the three provinces exhibit a large variation the himalayan contributions decrease from 63 in province i southeast part of the study area to 53 in province ii central part of the study area and 45 in province iii northwest part of the study area correspondingly the contributions of indian rivers increase from province i through province iii during the high sea levels of modern time seasonal monsoonal currents play a key role in controlling the sediments transport from south asia continent to the lower fan

ultra distal tephra deposits from super eruptions examples from toba indonesia and taupo volcanic zone new zealand abstract voluminous rhyolitic eruptions from toba indonesia and taupo volcanic zone tvz new zealand have dispersed volcanic ash over vast areas in the late quaternary the 74 ka youngest toba tuff ytt eruption deposited ash over the bay of bengal and the indian subcontinent to the west the 340 ka whakamaru eruption tvz deposited the widespread rangitawa tephra dominantly to the southeast in addition to occurrences northwest of vent extending across the landmass of new zealand and the south pacific ocean and tasman sea with distal terrestrial exposures on the chatham islands these super eruptions involved 2500 km 3 and 1500 km 3 of magma dense rock equivalent dre respectively ultra distal terrestrial exposures of ytt at two localities in india middle son valley madhya pradesh and jurreru river valley andhra pradesh at distances of 2000 km from the source caldera show a basal primary ashfall unit 4 cm thick although deposits containing reworked ash are up to 3 m in total thickness exposures of rangitawa tephra on the chatham islands 900 km from the source caldera are 15 30 cm thick at more proximal localities 200 km from source rangitawa tephra is 55 70 cm thick and characterized by a crystal rich basal layer and normal grading both distal tephra deposits are characterized by very fine ash with high pm 10 fractions and are crystal poor glass chemistry stratigraphy and grain size data for these distal tephra deposits are presented with comparisons of their correlation dispersal and preservation using field observations ash transport and deposition were modeled for both eruptions using a semi analytical model hazmap with assumptions concerning average wind direction and strength during eruption column shape and vent size model outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated with tephra fallout modeling based on observed ytt distal tephra thicknesses indicate a relatively low

late jurassic intraplate faulting in eastern australia a link to subduction in eastern gondwana and plate tectonic reorganisation abstract eastern gondwana was subjected to subduction processes during the middle late jurassic but how these processes affected intraplate deformation in eastern australia is poorly understood here we present 40ar 39ar k ar and rb sr geochronological data from illitic clay bearing fault gouges associated with the northern part of the 200 km long n striking dextral strike slip demon fault in eastern australia we show a major range of geochronological ages at 162 99 0 74 152 1 4 8 ma indicating that the demon fault was active during the late jurassic this period partially coincides with the middle late jurassic deposition of widespread ash fall tuffs in the clarence moreton surat and eromanga basins we propose that middle late jurassic intraplate tectonism in eastern australia was influenced by subduction processes farther east which produced extensive calc alkaline magmatism in new zealand from 170 ma a global plate reorganisation event related to the development of early middle jurassic sea floor spreading of the pacific plate possibly acted as the driving mechanism responsible for the intensification of magmatism and intraplate faulting in eastern gondwana

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

new geochemical evidence for the youngest toba tuff in india abstract new geochemical data on tephra samples from a layer present at several archeological sites in india support correlation of this layer to the youngest toba tuff erupted from northern sumatra about 74 000 yr ago the data show that the indian tephra layer is not a correlative of older tephra erupted from toba as has been suggested on the basis of artifact assemblages previously published geochemical data on the indian tephra beds was based on bulk ash samples containing mineral and clay contaminants and the resulting variability in analyses did not allow identification or discrimination of individual eruptive events our new data were collected on individual glass shards and small purified glass separates which have greater resolving power in fingerprinting acheulian and paleolithic artifacts found at some of the indian tephra sites do not reflect the antiquity of the tephra bed as they occur in fluvial sediments and may be reworked

individual glass shard trace element analyses confirm that all known toba tephra reported from india is from the c 75 ka youngest toba eruption uncertainty over the identity and age of toba tephras across peninsular india persists with radiometric age dates contradicting earlier compositional data which have been used to identify this important stratigraphic marker as the youngest toba tuff ytt to address this issue new single glass shard analyses have been performed for samples from morgaon and bori north western india which have recently been dated at c 800 ka these and indeed all toba tephra samples thus far analysed from india show the presence of four populations of glass shards defined by their ba y ratio which uniquely identifies them as products of the c 75 ka youngest toba eruption confirmation that the ytt fingerprint is characteristic comes from new analyses of oldest toba tuff ott glass shards from five sites in the indian ocean these are compositionally identical to layer d from the odp site 758 sediment core c 800 ka and belong to a single low ba population clearly different from ytt these analyses show that there is essentially no reworked ott material in the ytt eruption and indicate unequivocally that all known toba tephra occurrences in india belong to the c 75 ka youngest toba eruption

ytt ash from quaternary sediments of kapileshwar area purna alluvial basin central india abstract a thick pocket of volcanic ash 2 5 m represented by various shades of gray powdery massive to laminated in nature hosted in yellowish brown thinly bedded laminated silty clay lithocolumn has been studied for its preservational and geochemical aspects the ash column shows preservations of both primary and reworked ashes which are distinguishable on the basis of their field criteria including colors bedding geometries and thicknesses petrological study of the same reveals high representation of glass shards with bubble wall blocky and pumice morphologies minor occurrences of light and heavy minerals and unidentifiable admixture of fine grained sediments geochemistry of both bulk ash and glass shards through icp aes and epma respectively suggest high percentages of sio2 al2o3 na2o and k2o ree data generated through icp ms show strong negative eu anomaly the effects of weathering or aqueous solution on the geochemistry are also noticeable as evident by the variations in the values of both mobile and immobile elements however the data obtained from both bulk ash and glass shards are within the limits of ytt composition calm to very low energy environment of deposition is suggested for their deposition paleoclimatic proxies represented by rhizoliths rhizospheres rhizolith balls ant traces and vertebrate remains preserved in the pre tephra succession have also been commented

excess aluminum in deep sea sediments of the central indian basin abstract the aluminum concentration in the marine sediments has long been considered an indicator of terrigenous abundance the al ti values 48 5 of the central indian basin subsurface siliceous sediment are up to three times that of post archean australian shale paas the estimation of structurally unsupported al al ex from six sediment cores suggests that an average of 40 of total al is from noncrustal sources higher al ti or al ex values are often but not exclusively associated with abundant volcanic unaltered glass shards electron microprobe analysis of 60 of these glass shards reveals that they are rhyolitic in composition and of youngest toba 74 ka origin from northern sumatra these glass shards have very high al ti ratio of 175 mean al ti values are also above the paas in sediment where glass shards are absent which could be due to scavenging of dissolved al by biogenic components

first toba supereruption revival little has been known about the earliest toba eruptive episodes that created the largest known caldera complex of quaternary age here we report evidence for the eastward dispersal of the oldest toba tuff in south china sea sediments to 2500 km away from the source the tephra deposits occur below the brunhes matuyama geomagnetic boundary 778 ka and slightly above the australasian microtektite layer 793 ka calibrated by astronomically tuned oxygen isotope stratigraphy the middle pleistocene toba eruption occurred during the deglaciation at 788 2 2 ka according to the tephra occurrence between marine isotope stages 20 and 19 this refined age is in good agreement with the 4 0 ar 3 9 ar date of 800 20 ka for the toba tephra layer d from ocean drilling program odp site 758 but significantly younger than the commonly cited ar ar age of 840 30 ka the eruption expelled at least 800 1000 km 3 dense rock equivalent of rhyolitic magma on the basis of the widespread tephra fall deposit in the basins of the indian ocean and the south china sea in spite of its exceptional magnitude the timing of this major eruption does not indicate a causal linkage between this event and a long term global climatic deterioration

new occurrence of youngest toba tuff in abyssal sediments of the central indian basin abstract volcanic glass and pumice found in siliceous abyssal sediments of the central indian basin south of the equator have previously been assigned various origins including intra basin volcanism indonesian arc and krakatau rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the youngest toba tuff erupted at 74 ka from northern sumatra the correlation extends the distribution of coarse 63 m glass shards some 1500 km south of the previously known fallout zone and into the southern hemisphere reaching latitudes of ca 14 s this provides evidence for bi hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption such dispersal could have facilitated the global impact of the eruption

youngest toba tuff in the son valley india a weak and discontinuous stratigraphic marker abstract investigation of the climatic and environmental impacts of the youngest toba tuff ytt 74 ka bp eruption of toba volcano sumatra is crucial for understanding the consequences of the eruption for contemporaneous human populations the middle son valley in india was the first locality on the indian subcontinent where the ytt was reported the ash bed forms a discontinuous layer stretching for over 30 km along the river here we report on the stratigraphic contexts of ytt ash layers in alluvial deposits of the middle son valley in order to reconstruct the taphonomy of the ash deposits and the dynamic of their deposition although the distal ash has been studied since the 1980s its stratigraphic integrity and the mechanisms and pathways involved in its transport and deposition have bit previously been assessed we find that the ytt occurrences in the middle son valley may not be reliable chronostratigraphical markers for millennial scale palaeoenvironmental reconstruction

monsoon related changes in sea surface productivity and water column denitrification in the eastern arabian sea during the last glacial cycle sea surface salinity reconstruction for the holocene and the last glacial maximum lgm suggests that the lgm eastern arabian sea was more saline by 1 5 psu than the holocene due to weakened freshwater flux to the region and intensified winter monsoons time series of sedimentary organic carbon corg p alkenone and y 13 corg for the last 100 kyr together indicate increased glacial productivity significantly reduced sedimentary y 15 n during the lgm indicates however diminished water column denitrification in spite of increased productivity the distinct decoupling of denitrification from productivity during the lgm can be explained by vigorous ventilation of the thermocline as a result of simultaneously intensified formation of high salinity water and deep winter mixing in northern arabian sea related to the extreme cold climate the closely comparable time series y 15 n records across the arabian sea suggest basin wide homogenisation of the isotopic signal this probably resulted from combined monsoon dependent surface water mixing due to changes in the relative strength of reversing surface circulations and intermediate water mixing due to changes in north south salinity gradient d 2005 elsevier b v all rights reserved

the 73 ka toba super eruption and its impact history of a debate abstract toba volcano in northern sumatra is located at the intersection of two major tectonic lineaments in one of the most seismically active regions in the world and its caldera is considered the largest quaternary caldera on earth the most recent toba explosive eruption at 73 ka was an order of magnitude larger than that of tambora in 1815 and has a volcanic explosivity index of 8 the exceptional magnitude of this super eruption and the widespread distribution of the younger toba tuff ytt in marine cores in the indian ocean the arabian sea and the south china sea have prompted sustained debate as to its global and regional impact on climate ecosystems and prehistoric human populations however despite climate modelling and appeals to a variety of proxy climatic data including geochemistry pollen analysis archaeology and stable isotopes the extent of the impact remains unclear in order to achieve better progress in the future greater care is needed to obtain higher resolution and more precise chronologies from the marine and terrestrial proxy records used to test whether or not the 73 ka toba super eruption had a significant environmental impact records from speleothems and ice cores have much to offer in this regard

rayleigh wave group velocity tomography of the borborema province ne brazil from ambient seismic noise ambient seismic noise has traditionally been regarded as an unwanted perturbation that contaminates earthquake data over the last decade however it has been shown that consistent information about subsurface structure can be extracted from ambient seismic noise by cross correlation of noise simultaneously recorded at two seismic stations the empirical green s function for the propagating medium between them can be reconstructed moreover for periods less than 30 s the seismic spectrum of ambient noise is dominated by microseismic energy and because microseismic energy travels mostly as surface waves the reconstruction of the empirical green s function is usually proportional to the surface wave portion of the seismic wavefield in this paper we present 333 empirical green s functions obtained from stacked cross correlations of one month of vertical component ambient seismic noise for different pairs of seismic stations in the borborema province of ne brazil the empirical green s functions show that the signal obtained is dominated by rayleigh waves and that dispersion velocities can be measured reliably for periods between 5 and 20 s the study includes permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region resulting in a combined network of 34 stations separated by distances between approximately 40 and 1 287 km fundamental mode group velocities were obtained for all station pairs and then tomographically inverted to produce maps of group velocity variation for short periods 5 10 s the tomographic maps correlate well with surface geology with slow velocities delineating the main rift basins potiguar tucano and reconcavo and fast velocities delineating the location of the precambrian sao francisco craton and the rio grande do norte domain for longer periods 15 20 s most of the velocity anomalies fade away and only those associated with the deep tucano basin and the sao francisco craton remain the fading of the rio grande do norte domain fast velocity anomaly suggests this is a supracrustal structure rather than a lithospheric terrain and places new constraints on the precambrian evolution of the borborema province

ash layers and pumice in the central indian basin relationship to the formation of manganese nodules abstract manganese nodules in areas of biosiliceous sedimentation in the central indian basin cib have three main types of nuclei 1 fragments of altered ash layer 2 felsic pumice and 3 indurated residual sediments red clay type morphological variations among nodules are due to the form of the nuclei the type of accretionary process diagenetic or hydrogenetic accretion and thickness of the crust piston cores in the same areas sampled up to four tephra layers interstratified in red clay all of which underlie condensed series of pliocene to recent siliceous oozes the in situ tephra layers and fragments of tephra in the core of the nodules are similar in composition they are completely altered to smectite and smectite zeolite tephra horizons are 10 35 mm thick and have sharp contacts with the red clay rare earth element ree patterns zr hf and nb content and ti al ratios strongly suggest that this altered volcaniclastic material originated as distal fallout from the indonesian volcanic arc iva and was deposited in late neogene time positive ce ce anomalies indicate that alteration of this tephra may have occurred at the sediment water interface prior to burial under hydrogenous conditions rhyolite pumicelapilli are water laid and are variably replaced by fe mn oxides inducing a higher metal content in the bulk nodules pumice constitutes one of the main types of nuclei of diagenetic nodules that are found in the deep sea troughs and plains where biosiliceous deposits are thick petrological characteristics of the pumice suggest that volcanic eruptions from toba in northern sumatra are the most likely source tectonic deformation of the indian plate may have affected the sedimentary sequence causing uplift and outcrop of ash layers on the topographic highs and slopes eroded fragments of these ash layers and waterlaid pumice fragments have contributed significantly to the large extent of mn nodules in the cib

surface wave tomography from microseisms in southern california received 5 april 2005 revised 23 may 2005 accepted 9 june 2005 published 26 july 2005 1 since it has already been demonstrated that point topoint seismic propagation green functions can be extracted from seismic noise it should be possible to image earth structure using the ambient noise field seismic noise data from 148 broadband seismic stations in southern california were used to extract the surface wave arrival times between all station pairs in the network the seismic data were then used in a simple but densely sampled tomographic procedure to estimate the surface wave velocity structure within the frequency range of 0 1 0 2 hz for a region in southern california the result compares favorably with previous estimates obtained using more conventional and elaborate inversion procedures this demonstrates that coherent noise field between station pairs can be used for seismic imaging purposes citation sabra k g p gerstoft p roux w a kuperman and m c fehler 2005 surface wave tomography from microseisms in southern california geophys res lett 32 l14311 doi 10 1029 2005gl023155

the toba volcanic event and interstadial stadial climates at the marine isotopic stage 5 to 4 transition in the northern indian ocean abstract the toba volcanic event one of the largest eruptions during the quaternary is documented in marine sediment cores from the northeastern arabian sea on the crest of the murray ridge and along the western indian continental margin we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5 4 boundary with the chemical composition of the youngest toba tuff time series of the uk 37 alkenone index planktic foraminiferal species magnetic susceptibility and sediment accumulation rates from this interval show that the toba event occurred between two warm periods lasting a few millennia using toba as an instantaneous stratigraphic marker for correlation between the marine and ice core chronostratigraphies these two arabian sea climatic events correspond to greenland interstadials 20 and 19 respectively our data sets thus depict substantial interstadial stadial fluctuations in sea surface temperature and surface water productivity we show that variable terrigenous eolian sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record within the provided stratigraphic resolution of several decades to centennials none of these proxies shows a particular impact of the toba eruption however our results are additional support that toba despite its exceptional magnitude had only a minor impact on the evolution of low latitude monsoonal climate on centennial to millennial time scales

reply to the comment on environmental impact of the 73 ka toba super eruption in south asia by m a j williams s h ambrose s van der kaars c ruehlemann u chattopadhyaya j pal p r chauhan palaeogeography palaeoclimatology palaeoecology 284 2009 295 314 abstract haslam and petraglia raise three broad questions concerning our paper environmental impact of the 73 ka toba super eruption in south asia by martin a j williams stanley h ambrose sander van der kaars carsten ruehlemann umesh chattopadhyaya jagannath pal parth r chauhan palaeogeography palaeoclimatology palaeoecology 284 2009 295 314 a what was the relationship between the 73 ka toba eruption and the cold stade between the dansgaard oeschger interstades 20 and 19 b what was the regional impact of the eruption on vegetation c what was the possible effect of the eruption upon humans and mammals in response to questions a and b we note that the 73 ka toba eruption was followed by several centuries of intense cooling and wind blown dust accession in the greenland gisp2 ice core by a change from forest to grassland or open woodland in central india carbon isotopic analysis and in the wider region pollen analysis of a marine sediment core in the bay of bengal in regard to c the genetic evidence is as yet too imprecisely dated to demonstrate causality as is the archaeological evidence cited by haslam and petraglia in favour of minimal impact future work supported by higher resolution chronologies than are presently available is needed to provide a less equivocal picture of the environmental impact of the 73 ka toba eruption

the toba caldera complex abstract the toba caldera in indonesia is one of the most remarkable volcanic features formed during quaternary geologic time its rich history of research for over a century has yielded important information on the physical volcanology of silicic calderas and super eruptions geochemical evolution of silicic magma bodies and geophysical imaging of active sub volcanic systems during the past 1 3 my the toba area has erupted intermediate composition lavas followed by intermediate pyroclastics three quartz bearing silicic tuffs and most recently intermediate to silicic lavas this pattern represents the incremental assembly and periodic eruption of a crustal magma body of batholithic proportions the apparent migration of activity to the west may have implications for the next toba super eruption

did the 73 ka toba super eruption have an enduring effect insights from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models abstract the potential impact of the 73 ka toba super eruption upon global and regional climate terrestrial ecosystems and prehistoric human populations remains unclear evidence from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models has provided some useful working hypotheses further progress requires a substantial improvement in the accuracy precision and resolution of the chronologies of each of the marine and terrestrial proxy records used to reconstruct the environmental impact of this extreme event

first toba supereruption revival comment and reply comment the article by lee et al 2004 1 suggests that at least one supereruption from the toba caldera sumatra was associated with a climatic warming rather than a cooling trend the background to this is that rampino and self 1992 2 1993 3 originally suggested that the supereruption

environmental impact of the 73 ka toba super eruption in south asia abstract the cooling effects of historic volcanic eruptions on world climate are well known but the impacts of even bigger prehistoric eruptions are still shrouded in mystery the eruption of toba volcano in northern sumatra some 73 000 years ago was the largest explosive eruption of the past two million years with a volcanic explosivity index of magnitude 8 but its impact on climate has been controversial in order to resolve this issue we have analysed pollen from a marine core in the bay of bengal with stratified toba ash and the carbon isotopic composition of soil carbonates directly above and below the ash in three sites on a 400 km transect across central india pollen evidence shows that the eruption was followed by initial cooling and prolonged desiccation reflected in a decline in tree cover in india and the adjacent region carbon isotopes show that c 3 forest was replaced by wooded to open c 4 grassland in central india our results demonstrate that the toba eruption caused climatic cooling and prolonged deforestation in south asia and challenge claims of minimal impact on tropical ecosystems and human populations

the indian ocean nodule field petrotectonic evolution and ferromanganese deposits abstract the indian ocean nodule field ionf is significant from several points of view roughly bordered by 10 s to 16 30 s and 72 e to 80 e and located within the central indian ocean basin ciob the field hosts the world s second largest and second high grade manganese nodule deposit after the equatorial nodule belt in the north pacific ocean moreover the crust underlying this field is characterised by unique morphotectonic signatures owing to its formation between 60 and 49 ma under three variable spreading conditions fast intermediate and slow from the indian ocean ridge system iors the nodule field has been surveyed both extensively more than 0 4 million km2 area and intensively comprising of a large geophysical data set and geological sample inventory during the last two decades several morphotectonic features such as seamounts hills ridge normal lineaments and ridge parallel lineations have disturbed the apparently smooth topographic gradient 1 7000 of the seafloor here variations in the rate of spreading and formation of new oceanic crust along the ridge crest during more than one episode of india eurasia collision are imprinted in the ionf based on the nature of the ridge parallel lineations which are related to the rate of crustal accretion the field is divided in to four sectors a b c and d from north to south sectors a and c were formed at a fast rate of spreading 90 95 mm year half rate and sectors b and d were formed at an intermediate 55 mm year and slow 26 mm year rates respectively the predominance of tensional stress in sectors a and c caused asymmetrical flexuring of the seafloor resulting in widely spaced faults and folds with low amplitude and large wavelength in contrast the seafloor flexuring in sector d are closely spaced long symmetrical and of high amplitude the timing and intensity of the collision of india with eurasia is constrained by the variable intensity of these flexures suggesting probably a soft touch at 58 ma and the hard collision at about 51 ma the nodule field hosts several seamounts both as isolated entities and in linear chains which are arranged parallel to the flow lines along the direction of absolute motion of the indian plate the distribution morphology and growth patterns of a majority of these seamounts are related to spreading rate suggesting their formation at the ridge crest however many of the seamounts show more than one stage of growth with local intraplate volcanism contributing to the enlargement of the larger ancient seamounts varieties of volcanics such as tholeiitic basalts spilites ferrobasalts and pumice occur within the ionf the alteration of some of these volcanics has resulted in palagonitisation of the glass and formation of zeolites subsequently during its journey away from the ridge crest to the abyssal areas the crust underlying the nodule field witnessed intraplate volcanism this is evident from the addition of younger rocks at the base of the ancient seamounts inconsistent growth of volcanoes eruption of ferrobasalt corresponding to the areas of high amplitude magnetic zones and presence of volcanogenic hydrothermal materials vhm of 10 ka age these findings collectively hint that the ionf is geodynamically unstable and may have been volcanically active in the recent past during the last 8 ma growth of authigenic ferromanganese deposits in the form of manganese nodules and crusts has occurred in the nodule field the deposits occur at an average water depth of 5000 m the basinal geomorphology intraplate tectonic deformations and volcanic eruptions considerably influence the formation development morphology mineralogy and composition of these deposits the data show that the large seamounts reverse faults and fracture zones fzs supply rock fragments as seeds for the nodule formation the hydrogenous precipitation from the overlying water column is the primary source of metal accumulation in the nodules the secondary relatively weak intraplate eruptions along the base of ancient seamounts or lineaments subsurface igneous activity and diagenetic remobilisation of metals have also played significant role in the growth and enrichment of the deposit based on a large data set we estimate the contribution of various physico chemical parameters and model the probable conditions of formation of the ferromanganese deposits in the ionf the model also hints at the location where the resource exploitation should be rewarding

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

toba ash layers in the south china sea evidence of contrasting wind directions during eruption ca 74 ka two cores from the southern south china sea contain discrete ash layers that mainly consist of rhyolithic glass shards on the basis of the specmap time scale the ash layers were dated to ca 74 ka the age of the youngest toba eruption in northern sumatra this link is supported by the chemical composition of the glass which is distinct from volcanic glass supplied from the philippines and the northern south china sea but is almost identical with the chemistry of the toba ash the youngest toba ash layers in the south china sea expand the previously known ash fall zone over more than 1800 km to the east the dispersal of ashes from sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that 1 the toba eruption probably happened during the southeast asian summer monsoon season and 2 the volume of erupted magma was larger than previously interpreted

early to middle pleistocene tephrochronology of north island new zealand implications for volcanism tectonism and paleoenvironments thick volcaniclastic sequences of early to middle pleistocene age in southern north island new zealand contain rhyolitic tephra beds that record the early history of the taupo volcanic zone tvz at least 54 different tephra beds are recorded and their chronology is defined by glass fission track ages and paleomagnetism the tephra beds span the interval ca 2 0 0 6 ma and provide an event frequency of 1 19 k y significantly higher than the frequency of sheet forming ignimbrites preserved in the tvz at this time 1 100 k y the distal tephra beds thus provide a record of volcanism not revealed in the proximal volcanic region and suggest a major period of explosive activity at ca 1 79 1 60 ma several important marker horizons are identified pakihikura tephra 1 63 ma mangatewaiiti tephra 1 24 ma potaka tephra 1 00 ma kidnappers b tephra 1 00 ma and kaukatea tephra 0 88 ma these tephra beds allow direct correlation between 1 marine and nonmarine facies and 2 the fore arc and back arc regions of new zealand the tephra beds provide a framework for a paleoenvironmental reconstruction of the southern north island volcaniclastic transport routes from the tvz to basins in the south and southeast and through the site of present mountain ranges supplied material to a terrestrial lowland fore arc area in the interval 1 64 0 7 ma uplift and deformation since 0 7 ma have disrupted paleodrainage routes diverting them to the north and southwest

palaeoenvironmental response to the 74 ka toba ash fall in the jurreru and middle son valleys in southern and north central india distal deposits of rhyolitic volcanic ash from the 74 ka supervolcanic eruption of toba in northern sumatra are preserved in numerous river valleys across peninsular india the toba eruption is hypothesized to have resulted in climate change and the devastation of ecosystems and hominin populations this study reports the results of the analysis of sediments and stratigraphical sequences from sites in the jurreru and middle son valleys in southern and north central india the aim of the study is to determine the extent of palaeoenvironmental change in both valleys as a result of the ash fall inferences based on evidence from the jurreru valley are more detailed where pre and post toba palaeoenvironmental changes are divided into seven phases the results indicate that ash fall deposits in both valleys underwent several phases of reworking that possibly lasted for several years indicating that ash was mobile in the landscape for a considerable period of time prior to burial this could have enhanced and lengthened the detrimental effects of the ash on vegetation and water sources as well as animal and hominin populations

nature source and composition of volcanic ash in sediments from a fracture zone trace of rodriguez triple junction in the central indian basin abstract volcanic glasses associated with pumice micronodules and palagonite like lithic fragments were recovered from a volcanic terrain in a fracture zone defined as rodriguez triple junction trace in the central indian basin morphologically the tephra consist predominantly of bubble wall shards with the co occurrence of platy and blocky types rarity of vesicles in the shards suggests that they are formed in depths deeper than 4000 m below the volatile fragmentation depth vfd energy dispersive spectrometer eds analyses have shown that the tephra have sio2 contents ranging from 74 to 77 suggesting that they are mainly rhyolitic and formed from melts of siliceous composition the depositional age of the tephra layers determined by radiolarian assemblages and excess 230th decay profile corresponds to an age older than 180 kyr the tephra are rare in sediments younger than 239 kyr the beginning of the volcanic eruption contributing this volcanic ash however cannot be estimated occurrence of large sized tephra suggests proximal source probably from a local volcanic source compositional and morphological contrasts age constraints occurrence of large sized tephra and tectonic setting of the cored site location suggest an intraplate volcanism triggered by reactivation of tectonic activity in the fracture zone trace of the indian ocean triple junction during the late pleistocene