structural and anisotropy of magnetic susceptibility ams evidence for oblique impact on terrestrial basalt flows lonar crater india the anisotropy of magnetic susceptibility ams technique combined with the analyses of satellite images and the geological structure are examined on lonar crater a small 1 8 km diameter impact crater in india to evaluate the direction and obliquity of asteroid impact the advanced spaceborne thermal emission and reflection radiometer aster image 15 m resolution of the ca 52 ka lonar crater in the subhorizontal deccan basalt ca 65 ma shows that this simple bowl shaped impact crater has a near circular rim with a circularity of 0 95 most of the highly reflecting continuous ejecta blanket around the crater rim can be enveloped with an ellipse whose major e w axis is coincident with the diameter of the crater rim and minor n s axis is relatively displaced toward the west by 200 m the present ejecta distribution which appears to be close to its pristine shape extends to a distance of 700 m in all the directions from the crater rim except to the west where it extends to a distance of a little more than 1 km the circular shape of the crater rim the e w bilateral symmetry of the enveloping ellipse on the ejecta and the greater extension of the ejecta toward the west appear to be the result of an oblique impact from the east with an angle of incidence of 30 45 when compared with experiments the ams data suggest that the target basalts occurring at 2 km west southwest of the crater rim are highly shocked as indicated by the random orientation of their k 3 susceptibility axes in comparison to the unshocked basalts at 2 km east southeast of the crater the unshocked basalts show a bimodal distribution of susceptibility axes typical of lava flows moderate to strong westward shifts of the k 3 axes are seen for the majority of the shocked basalts on the crater rim and west southwest of the crater the shocked basalts also indicate an oblique impact from the east when compared with modeling and experiments a general lowering of degree of anisotropy of the lonar shocked basalts 1 01 compared to the surrounding unshocked basalts 1 03 is found to be a characteristic feature of impact crater target rocks variation in attitudes of the basalt flows on the lonar crater rim shows a bilaterally symmetrical distribution about an e w axial plane which includes quaquaversal dips of the flows all around the crater rim except to the west where overturned dips of the basalt flows are seen it appears that oblique impact and the symmetry in structural variations around the crater rim have a relationship for a small crater such as lonar

magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

monsoonal variability in the northeastern arabian sea during the past 5000years geochemical evidence from laminated sediments abstract a high resolution laminated sediment record from the oxygen minimum zone in the northeastern arabian sea off pakistan was studied to reconstruct climatic variations to infer the monsoon driven moisture history in the northeastern arabian sea during the past 5000 years we have used three types of independant proxies 1 varve thickness 2 stable isotope ratios of planktic foraminifera and 3 inorganic geochemical composition on the basis of these proxies we have subdivided the sedimentary record into five intervals the first time interval from 5000 to 3900 yr bp period i is dominated by summer monsoon activity deduced from ti al zr al k al and sr ca ratios and oxygen isotope data of planktonic foraminifera slightly enhanced winter monsoon conditions can be inferred for the period from 3900 to about 3000 yr bp period ii maximum precipitation during enhanced winter monsoon activity at about 3100 yr bp was followed by an onset of aridification at 3000 yr bp which continued until about 2000 2200 yr bp period iii the textural mineralogical and geochemical data suggest that this aridification was a gradual century scale process rather than an abrupt event comparison to late holocene records in nw and sw india the near east and tropical africa supports our interpretation that period iii from 3000 to 2000 yr bp is characterized by a gradual change in the atmospherical moisture budget over much of this area reduced monsoonal rainfall may be linked to variations of eurasian snow cover as deduced from varve thickness and element to al ratios period iv from 2000 to about 1500 yr bp is characterized by a summer monsoon dominated sequence with higher precipitation than during period iii the 18o record shows an abrupt shift at 2000 yr bp coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern arabian sea present day conditions in the makran area were established at 1500 yr bp period v based on inorganic geochemical data and meteorological records at several stations in the makran we conclude that period v is dominated by precipitation during the winter season here declined rainfall associated with the summer monsoon may be related to a less northward position of the intertropical convergence zone itcz during summer and a stronger influence of winter monsoon linked phenomenona

geochemical identification of impactor for lonar crater india the only well known terrestrial analogue of impact craters in basaltic crusts of the rocky planets is the lonar crater india for the first time evidence of the impactor that formed the crater has been identified within the impact spherules which are 0 3 to 1 mm in size and of different aerodynamic shapes including spheres teardrops cylinders dumbbells and spindles they were found in ejecta on the rim of the crater the spherules have high magnetic susceptibility from 0 31 to 0 02 si mass and natural remanent magnetization nrm intensity both nrm and saturation isothermal remanent magnetization sirm intensity are 2 am2 kg demagnetization response by the nrm suggests a complicated history of remanence acquisition the spherules show schlieren structure described by chains of tiny dendritic and octahedral shaped magnetite crystals indicating their quenching from liquid droplets microprobe analyses show that relative to the target basalt compositions the spherules have relatively high average fe2o3 by 1 5 wt mgo 1 wt mn 200 ppm cr 200 ppm co 50 ppm ni 1000 ppm and zn 70 ppm and low na2o 1 wt and p2o5 0 2 wt very high ni contents up to 14 times the average content of lonar basalt require the presence of a meteoritic component in these spherules we interpret the high ni cr and co abundances in these spherules to indicate that the impactor of the lonar crater was a chondrite which is present in abundances of 12 to 20 percent by weight in these impact spherules relatively high zn yet low na2o and p2o5 contents of these spherules indicate exchange of volatiles between the quenching spherule droplets and the impact plume

multiple monsoon controlled breakdown of oxygen minimum conditions during the past 30 000 years documented in laminated sediments off pakistan abstract late holocene laminated sediments from a core transect centred in the oxygen minimum zone omz impinging at the continental slope off pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years high sw monsoon controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon toc and redox sensitive elements ni v as well as by a low diversity high abundance benthic foraminiferal buliminacea association and high abundance of the planktonic species globigerina bulloides indicative of upwelling conditions surface water productivity was strongest during sw monsoon maxima stable omz conditions reflected by laminated sediments were found also during warm interstadial events preboreal bolling allerod and dansgaard oeschger events as well as during peak glacial times 17 22 5 ka all ages in calendar years sediment mass accumulation rates were at a maximum during the preboreal and younger dryas periods due to strong riverine input and mobilisation of fine grained sediment coinciding with rapid deglacial sea level rise whereas eolian input generally decreased from glacial to interglacial times in contrast the occurrence of bioturbated intervals from 7 to 10 5 ka early holocene in the younger dryas 11 7 13 ka from 15 to 17 ka heinrich event 1 and from 22 5 to 25 ka heinrich event 2 suggests completely different conditions of oxygen rich bottom waters extremely low mass and organic carbon accumulation rates a high diversity benthic fauna all indicating lowered surface water productivity during these intervals the omz was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods the abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the north atlantic heinrich events and dust peaks in the tibetan loess records the correlation of the monsoon driven omz variability in the arabian sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale

40ar 39ar dating of mineral separates and whole rocks from the western ghats lava pile further constraints on duration and age of the deccan traps abstract there has been an ongoing debate for almost two decades on the age and duration of deccan trap volcanism some of the best available ages have been determined using the 40 ar 39 ar method but differences in sample location laboratory procedure type of analyzed material and ages of monitors may have been responsible for much of the apparent spread in the data we have collected and analyzed samples from close to the bottom and top of the main lava pile in the western ghats near bombay we confirm previous findings that alteration and ar recoil strongly affect these rocks particularly measurements made on whole rocks when mineral separates mostly plagioclase are analyzed the spread is much reduced we find a mean age of 65 4 0 7 ma for five flows near the base of the section jahwar and igatpuri formations and 65 2 0 4 ma for a dyke cross cutting the poladpur formation below the topmost mahabaleshwar formation this implies that at least 1800 m out of the 2500 m composite trap section were erupted in a short interval less than 1 ma long and close to 65 5 ma i e close to the cretaceous tertiary boundary we find that older ages near 67 ma found by several authors are essentially based on whole rock samples which display disturbed spectra when this can be checked i e when the spectra are published

geochemical and isotopic signatures of surficial sediments from the western continental shelf of india inferring provenance weathering and the nature of organic matter abstract bottom sediments from thirteen transects sampled mainly at depth contours of 30 100 and 200 m along the western continental margin of india falling in two offshore sectors from dwaraka to goa and cape comorin to goa were analyzed for calcium carbonate major elements in silicate fraction trace and rare earth elements in the bulk fraction and carbon and nitrogen isotopes in organic matter to understand the provenance of sediments weathering patterns in the source areas and nature of organic matter major elements such as si ti mg and discrimination plots involving alkalies and silica have shown a distinct north south provinciality with the source signatures pointing at deccan basalt for sediments in the north from dwaraka to goa and gneisses and granulites of south india in the south shale normalized ree patterns and ree content suggest additional local sources mainly in the southwest coast of india degree of alteration determined using chemical index of alteration cia plagioclase index of alteration pia and a cn k plots have suggested that the sediments are moderately weathered despite the intense orographic rainfall in the source area variability in degree of chemical weathering is seen with the change in provenance in general the sediments north of goa with a provenance of deccan basalts show a higher chemical weathering index corg n molar ratios are mostly between 10 and 12 minimum and maximum being 6 9 and 14 respectively which are closer to values for marine organic matter this is corroborated by marine 13c values 18 2 to 21 6 while they broadly fall within the marine values heavier values are seen in northern transects off porbandar and dwaraka where the sediments have a provenance different from those in the south this could be due to a higher contribution from c4 type vegetation in the north consistent with heavier 13c values in terrestrial plants in source regions heavier 15n values are observed for the northern transects north of ratnagiri which is supported by the intense denitrification in the northern arabian sea

the first description and confirmation of the vista alegre impact structure in the parana flood basalts of southern brazil abstract the vista alegre structure centered at 25 57 s and 52 41 w has been recently proposed as a meteorite impact structure the 9 5 km diameter structure is located in the parana state of southern brazil within the parana basin which contains one of the largest and most extensive flood basalt provinces on earth the parana flood basalts belong to the serra geral formation and are temporally related to the opening of the south atlantic ocean having been dated at about 133 132 ma tholeiitic basalts dominate the western portion of parana state with some minor rhyodacites morphologically vista alegre has a prominent circular outline in the form of an incomplete ring of escarpments and an inner depression the presence of a central uplift is not obvious but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure these sandstones are possibly related to the triassic piramboia formation and or to the cretaceous botucatu formation these units are normally at stratigraphic depths of about 700 800 m below the present surface in this portion of the parana basin the structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick extensively used for agriculture as a result there are limited outcrops in the interior of the structure all of polymict breccias some of them melt bearing we report the extensive occurrence of shatter cones in the form of fine grained rock clasts within the polymict breccias the shatter cone bearing breccias occur at different locations within the structure separated by several kilometers the nested shatter cones range in size from about 0 5 to 20 cm for individual cones and up to half a meter for complete assemblages the shatter cones formed in fine grained parana flood basalt and might be the first examples of shatter cones in such a rock type in addition planar deformation features pdfs were found in quartz grains within sedimentary rock clasts of the polymict breccia these findings confirm the impact origin of the vista alegre structure

cosmic markers 40ar 39ar dating and paleomagnetism of the kt sections in the anjar area of the deccan large igneous province abstract bhandari et al bhandari et al geophys res lett 22 1995 433 436 bhandari et al geol soc am spec paper 307 1996 417 424 reported the discovery of iridium bearing sediments sandwiched between basalt flows in the anjar area kutch province india they concluded that the signature of the k t impact had been recorded and that onset of volcanism in the deccan traps preceded the k t boundary excluding the possibility of a causal connection this paper reports complementary analyses of anjar outcrops by a joint indo french team where we focused on cosmic markers iridium and spinels in the intertrappean sediments and 40ar 39ar dating and paleomagnetism of the lava flows anomalous ir concentrations up to 0 4 ng g are confirmed with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections despite careful search no ni rich spinels were found eight basalt samples provided 40ar 39ar results four on plagioclase bulk samples four on whole rocks spectra for whole rocks all indicate some amount of disturbance and ages based on plagioclase bulk samples seem to be consistently more reliable hofmann et al earth planet sci lett 180 2000 13 28 the three flows underlying the ir bearing sediments are dated at 66 5 ma and two overlying flows at 65 ma magnetic analyses both thermal and by alternating fields uncovered clear reversed primary components in the upper flows and more disturbed normal components in the lower flows with evidence for an additional reversed component there are reports bajpai geol soc india mem 37 1996 313 319 bajpai j geol soc london 157 2000 257 260 that the intertrappean sediments contain uppermost maastrichtian dinosaur and ostracod remains above the uppermost ir bearing level and may not be mechanically disturbed we propose the following scenario to interpret these multiple field and analytical observations deccan trap volcanism started within uppermost maastrichtian normal chron c30n at 66 5 67 ma in the anjar area volcanism then stopped at least locally and lacustrine sediments were deposited over a period that could be in the order of 1 2 ma the k t bolide impact was recorded as a deposit of ir and possibly though not necessarily spinels volcanism resumed shortly after the k t boundary within reversed chron c29r as witnessed by the three reversely magnetised overlying basalt flows dated 65 ma this was responsible for erosion and destruction of part of the uppermost sediments including spinels if there were any and heterogeneous and non uniform redeposition of ir at a number of underlying sedimentary levels this was also responsible for the partial remagnetisation of the underlying flows these findings generally confirm and complement those of bhandari et al bhandari et al geophys res lett 22 1995 433 436 bhandari et al geol soc am spec paper 307 1996 417 424 and are compatible with the occurrence of the k t impact at the paleontological k t boundary and of deccan trap volcanism straddling the boundary and starting before the impact anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors though still within normal chron c30n there is no indication contradicting the view that the bulk of deccan trap volcanism occurred over only three chrons c30n c29r c29n courtillot evolutionary catastrophes the science of mass extinctions cambridge university press 1999 courtillot et al earth planet sci lett 80 1986 361 374 vandamme et al rev geophys 29 1991 159 190

late holocene primary productivity and sea surface temperature variations in the northeastern arabian sea implications for winter monsoon variability variability in the oceanic environment of the arabian sea region is strongly 34 influenced by the seasonal monsoon cycle of alternating wind directions prominent and well studied is the summer monsoon but much less is known about late holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and highsurface ocean productivity in the northeastern arabian sea to establish the first high resolution record of winter monsoon variability for the late holocene we analyzed alkenone derived sea surface temperature sst variations and proxies of primary productivity organic carbon and 15n in a well laminated sediment core from the pakistan continental margin increased summer monsoon and weak winter monsoon intensities off pakistan are indicated from 400 b c to 700 a d by reduced productivity and relatively high sst at about 700 a d the intensity of the winter monsoon increased off pakistan as indicated by a trend to lower sst we infer that winter monsoon was still weak from 700 to 1400 a d because primary production did not increase despite decreasing sst declining sst and elevated biological production from 1400 to 1900 a d suggest invigorated convective winter mixing by strengthening winter monsoon circulation most likely a regional expression of colder climate conditions during the little ice age on the northern hemisphere the comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the asian monsoon system during the late holocene effected by shifts in the intertropical convergence zone

rapid and widespread dispersal of flood sediment on the northern california margin the dispersal of flood sediment from small river systems is a poorly studied yet potentially important aspect of active continental margin sedimentation in january 1995 during a flood with a 30 yr return period the eel river northern california delivered an estimated 25 3 106 t metric tons of fine grained 62 m sediment to the ocean the flood formed a distinct layer on the sea bed that was centered on the 70 m isobath extended for 30 km along shelf and 8 km across shelf and was as thick as 8 5 cm but contained only 6 106 t of sediment thus 75 of the flood derived sediment did not form a recognizable deposit but was instead rapidly and widely dispersed over the continental margin stratigraphic models of and compilations of sediment flux to active continental margins need to take the dispersive nature of small river systems into account

toward a robust normalized magnetic paleointensity method applied to meteorites abstract we propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs alternating field rem method it provides an estimate of the absolute paleointensity with an uncertainty of about a factor two contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization it is applicable to multicomponent magnetizations artificial or natural isothermal remanent magnetizations can be recognized and relaxation of natural remanent magnetization can be taken into account it is applicable for magnetizations carried by magnetite feni alloys and pyrrhotite the rem method is of particular interest to estimate paleointensities in meteorites as these materials are often characterized by complex multicomponent magnetization of various origins impact magnetization thermoremanence isothermal remanent magnetization and metastable magnetic minerals not suitable for thellier experiments however like other paleointensity methods the rem method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale for l ordinary chondrites an upper limit of 1 t is proposed for the paleofield tentative paleofield estimates in the 0 05 0 5 t range are proposed for ll ordinary chondrites which is much lower than previous results the first paleofield estimates for rumuruti chondrites indicate the presence of magnetic fields around 6 t during the last major impact on their parent body aubrites and hed achondrites paleofields of at least 10 t may suggest the existence of a dynamo field during the cooling of their parent body results from martian meteorites are scattered between 1 and 24 t and may represent the crustal magnetic field of the planet after dynamo shutdown carbonaceous chondrites provide contrasting results with possible evidence of strong field mt processes that clearly require additional studies

on the formation of laminated sediments on the continental margin off pakistan the effects of sediment provenance and sediment redistribution abstract the sedimentary processes and sediment sources contributing to the formation of laminated sediments along the upper slope off pakistan are unravelled using inorganic bulk sediment geochemistry of 43 surface cores from the pakistani continental margin and additional geochemical and pb and nd isotope data for different types of layers an important process everywhere along the margin is redeposition of fluvial derived detritus from the shelf onto the slope this process is of considerably higher intensity along the makran margin than on the indus margin trace element enrichment related to early diagenesis or surface productivity which is commonly detectable in bulk sediment composition is swamped by the high clastic supply in the makran region but may be observed in the indus region four types of layers are found in the laminated sediment cores from the upper slope they reflect different mechanisms of deposition and different sediment sources an alternating pattern of olive grey and black layers results from downslope redeposition of fluvial material over most of the year to which organic matter from sea surface production is added during the late summer monsoon season distinctive white to grey coloured layers along the makran slope originate from large scale expulsion of sediments from the makran accretionary wedge through mud volcanoes on the shelf subsequent erosion by waves and downslope redeposition these layers may dominate the sedimentary record within the makran accretionary wedge but are absent on the indus margin occasional red coloured turbidites which probably represent larger floods on the indus plain contribute to this mixture of varying sedimentary processes and sediment sources along the pakistani continental slope

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

lonar lake palaeomagnetic evidence of shock origin summary palaeomagnetic and rock magnetic investigations were carried out on oriented rock samples collected from the inner walls of lonar lake which is believed to be a meteorite crater these studies show systematic variation in some magnetic parameters q jn k and declination whereas at surrounding localities these parameters show random variation a soft secondary shock component acquired in the present field of d 9 i 47 k 47 6 1 6 4 has been successfully isolated by vector difference analysis whereas the stable primary vector is similar to that of lower tertiary period reverse magnetization nrm and irm of the upper and lower flows respond differently to af demagnetization as a function of their ha it is shown that the nrm carried by the lower flows is similar to that of irm probably reset by the meteorite impact because of their low h carriers it is suggested that such magnetic property variations are characteristic of shock origin and hence we propose an impact origin for lonar lake

active mud volcanoes on and offshore eastern makran pakistan to study the activity source and driving force of the venting of fluidized mud in the coastal makran we carried out reconnaissance surveys of two active onshore mud volcano fields chandragup and jebel u ghurab and of a newly born march 1999 offshore mud volcano malan island all studied on and offshore mud volcanoes line up along one southwest northeast trending structural lineament the axis of the dhak anticline isotopic data point to a bacterial origin of the gas mainly methane mixed benthic foraminiferal faunas and calcareous nannofloras suggest that the source level of the extruded mud is at a sub surface depth of 2 3 km observed mud discharge varied between 0 and 1 4 m3 h 1 and gas discharge rates between negligible amounts to 1 m3 s 1 mud temperatures in the crater lake of chandragup i are near ambient temperatures they rise slightly 1 5 c during episodes of modest mud outflow an area of 160 000 m3 of soft mud was vigorously extruded from the sea floor at a water depth of 10 m within days after 15 march 1999 forming malan island the island was destroyed within a few months after its birth by deep reaching wave action during the sw monsoon this was possibly aided by local subsidence of the mud volcano structure due to the volume loss following rapid degassing and mud extrusion

hydrothermal alteration at the lonar lake impact structure india implications for impact cratering on mars the 50 000 year old 1 8 km diameter lonar crater is one of only two known terrestrial craters to be emplaced in basaltic target rock the 65 million year old deccan traps the composition of the lonar basalts is similar to martian basaltic meteorites which establishes lonar as an excellent analogue for similarly sized craters on the surface of mars samples from cores drilled into the lonar crater floor show that there are basaltic impact breccias that have been altered by post impact hydrothermal processes to produce an assemblage of secondary alteration minerals microprobe data and x ray diffraction analyses show that the alteration mineral assemblage consists primarily of saponite with minor celadonite and carbonate thermodynamic modeling and terrestrial volcanic analogues were used to demonstrate that these clay minerals formed at temperatures between 130 c and 200 c by comparing the lonar alteration assemblage with alteration at other terrestrial craters we conclude that the lonar crater represents a lower size limit for impact induced hydrothermal activity based on these results we suggest that similarly sized craters on mars have the potential to form hydrothermal systems as long as liquid water was present on or near the martian surface furthermore the fe rich alteration minerals produced by post impact hydrothermal processes could contribute to the minor iron enrichment associated with the formation of the martian soil

the internal structure of lava flows insights from ams measurements ii hawaiian pahoehoe toothpaste lava and a ā abstract we studied the anisotropy of magnetic susceptibility ams of 22 basaltic flow units including s type pahoehoe p type pahoehoe toothpaste lava and a emplaced over different slopes in two hawaiian islands systematic differences occur in several aspects of ams mean susceptibility degree of anisotropy magnetic fabric and orientation of the principal susceptibilities among the morphological types that can be related to different modes of lava emplacement ams also detects systematic changes in the rate of shear with position in a unit allowing us to infer local flow direction and some other aspects of the velocity field of each unit a flows are subject to stronger deformation than pahoehoe and also their internal parts behave more like a unit according to ams the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes probably resulting from endogenous growth

aspects of the validation of magnetic remanence in meteorites meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution consequently the validation of these records is important in meteorite research the first step in the validation process should be the rem value the rem value is the ratio of natural remanence nrm to saturation remanent magnetization imparted by a 1 t magnetic field sirm the rem values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from allende c3v s1 bjurbole l4 s1 and chainpur ll3 s1 meteorites the rem values computed from published nrm and sirm data identify many orders of magnitude range in the rem values including rem values 100 10 3 these data suggest a dependence for the nrm intensity on the curatorial location from which the sample was obtained any earth rock acquiring thermoremanent magnetization trm in the geomagnetic field has a restricted range in rem mostly between 5 and 50 10 3 the exception being the mineral hematite in the multidomain size range the only terrestrial samples with rem much greater than 100 10 3 are those struck by lightning the rem value provides a physical basis for recognition between valid records and those that might be contaminated the isothermal remanence acquisition ra curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed rem rm sirm value in the case of the bjurbole and chainpur chondrules with rem values 100 10 3 the ra curve indicates that unrealistically large contamination magnetic fields would be required to give rem values greater than 100 10 3 this would suggest contamination other than by a hand magnet that is normally available to an experimenter this would require an explanation that would involve large magnetic fields during chondrule formation or some extraordinary remanence acquisition mechanism that remains to be described magnetic contamination experiments using 80 and 40 mt magnets demonstrate that the rem values and extent of modification of the magnetic vector record are mineralogy dependent and this is mostly related to the amount and characteristics of the mineral tetrataenite the complexity of the meteorite records suggest validation of the record as a first step the rem value is the first physical statement that can be made in this validation

variations in magnetic properties of target basalts with the direction of asteroid impact example from lonar crater india abstract the lonar crater in maharashtra state india has been completely excavated on the deccan traps basalt approximately 65 ma at approximately 570 47 ka by an oblique impact of a possible chondritic asteroid that struck the preimpact target from the east at an angle of approximately 30 45o to the horizon where the total duration of the shock event was approximately 1 s it is shown by our early work that the distribution of ejecta and deformation of target rocks around the crater rim are symmetrical to the east west plane of impact misra et al 2010 the present study shows that some of the rock magnetic properties of these shocked target basalts e g low field anisotropy of magnetic susceptibility ams natural remanent magnetization nrm bulk susceptibility and high coercivity and high temperature hc ht magnetization component are also almost symmetrically oriented with reference to the plane of impact studies on the relative displacements of k3 minimum ams axes of shocked basalts from around the crater rim and from the adjacent target rocks to the approximately 2 3 km west of the crater center suggest that the impact stress could have branched out into the major southwestward and northwestward components in the downrange direction immediately after the impact the biaxial distribution of ams axes in stereographic plots for the unshocked basalts transforms mostly into triaxial distribution for the shocked basalts although transitional type distribution also exists the degree of anisotropy p of ams ellipsoids of the shocked basalts decreases by approximately 2 when compared with those of the unshocked target approximately 1 03 the nrm am 1 values of the shocked basalts on the rim of the lonar crater do not show much change in the uprange or downrange direction on and close to the east west plane of impact and the values are only approximately 1 5 times higher on average over the unshocked basalts around the crater however the values become approximately 1 4 16 4 times higher for the shocked basalts on the crater rim which occur obliquely to the plane of impact the target basalts at approximately 2 3 km west of the crater center in the downrange also show a significant increase up to approximately 26 times higher in nrm the majority of the shocked basalt samples approximately 73 from around the crater rim in general show a lowering of rem except those from approximately 2 3 km west of the crater center in the downrange where nearly half of the sample population shows a higher rem of approximately 3 63 in average the shocked target basalts around the lonar crater also acquired an hc ht magnetization component due to impact these hc ht components are mostly oriented in the uprange direction and are symmetrically disposed about the east west plane of impact making an obtuse angle with the direction of impact the low coercivity and low temperature lc lt components of both the unshocked and shocked basalts are statistically identical to the present day field pdf direction this could be chemical and or viscous remanent magnetization acquired by the target basalts during the last 570 47 ka subsequent to the formation of the lonar crater the shocked lonar target basalts appear to have remagnetized under high impact shock pressure and at low temperature of approximately 200 300 c where ti rich titanomagnetite was the main magnetic remanence carrier

relationship between median destructive field and remanent coercive forces for dispersed natural magnetite titanomagnetite and hematite summary remanent acquisition curves remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well defined grain size fractions between 5 and 250 m of natural magnetite titanomagnetite and hematite from these curves the remanent acquisition coercive force h cr the remanent coercive force hcr and the median destructive field of irm h i were determined theoretically these parameters should be the same for an assembly of non interacting homogeneously distributed randomly oriented single domain grains for a given hematite specimen h cr hcr and h i have about the same value in spite of the strong grain size dependence of these parameters for each specimen of magnetite and titanomagnetite the value of h cr is larger than hcr which again is larger than h i however the ratios h cr hcr and h i hcr appear to have a different constant value an interesting relationship which appears to hold for dispersed magnetite titanomagnetite or hematite grains between 5 and 250 m independently of grain size quantity and packing density of the magnetic material is

40ar 39ar age of the lonar crater and consequence for the geochronology of planetary impacts asteroid impacts play an important role in the evolution of planetary surfaces in the inner solar system the large majority of impacts occur on bodies e g asteroids the moon mars covered by primitive igneous rocks however most of the impacts recorded on earth occur on different rock types and are poor proxies for planetary impacts the lonar crater is a 1 88 km diameter quaternary age crater fig 1 located on the ca 66 ma deccan basaltic traps in maharashtra india and is one of the very few craters on earth emplaced directly on basaltic lava flows we carried out 12 40 ar 39 ar step heating experiments on 4 melt rock samples in order to 1 obtain a precise age for the lonar crater 2 study the response of isotopic chronometers during impacts on mafic target rocks and 3 better understand the dating of extraterrestrial impact craters we obtained 10 plateau and 9 inverse isochron ages on various aliquots combination of selected data into a global inverse isochron yielded an age of 570 47 ka mswd 1 1 p 0 24 in comparison previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages ca 12 62 ka the measured 40 ar 36 ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40 ar degassing capacity of basaltic impact melt rocks the 40 ar 36 ar ratio of 296 5 1 7 is indistinguishable from the atmospheric composition and suggests that inherited 40 ar is absent from the melt rock this result substantiates diffusion models that predict a near complete degassing of low viscosity melt e g basalts during impact and demonstrates for the first time that inherited 40 ar is less problematic for 40 ar 39 ar dating of impact events in basaltic igneous rocks compared to si rich rocks these results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and as far as dating is concerned should be the preferred targets of sample recovery by future missions

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

structural effects of meteorite impact on basalt evidence from lonar crater india received 31 january 2005 revised 17 august 2005 accepted 26 august 2005 published 8 december 2005 1 lonar crater is a simple bowl shaped near circular impact crater in the 65 myr old deccan volcanic province in india as lonar crater is a rare terrestrial crater formed entirely in basalt it provides an excellent opportunity to study the impact deformation in target basalt which is common on the surfaces of other terrestrial planets and their satellites the present study aims at documenting the impact deformational structures in the massive basalt well exposed on the upper crater wall where the basalt shows upward turning of the flow sequence resulting in a circular deformation pattern three fracture systems radial concentric and conical fractures are exposed on the inner crater wall on the fracture planes plumose structures are common uplift and tilting of the basalt sequence and formation of the fractures inside the crater are clearly related to the impact event and are different from the preimpact structures such as cooling related columnar joints and fractures of possible tectonic origin which are observed outside the crater slumping is common throughout the inner wall and listric faulting displaces the flows in the northeastern inner wall the impact structures of lonar crater are broadly similar to those at other simple terrestrial craters in granites and clastic sedimentary rocks and even small scale experimental craters formed in gabbro targets as lonar crater is similar to the strength controlled laboratory craters impact parameters could be modeled for this crater provided maximum depth of fracture formation would be known citation kumar p s 2005 structural effects of meteorite impact on basalt evidence from lonar crater india j geophys res 110 b12402 doi 10 1029 2005jb003662

understanding oblique impacts from experiments observations and modeling natural impacts in which the projectile strikes the target vertically are virtually nonexistent nevertheless our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical recent theoretical observational and experimental work is improving this situation but even with the current wealth of studies on impact cratering the effect of impact angle on the final crater is not well understood although craters rims may appear circular down to low impact angles the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts experimental studies established that crater dimensions depend only on the vertical component of the impact velocity the shock wave generated by the impact weakens with decreasing impact angle as a result melting and vaporization depend on impact angle however these processes do not seem to depend on the vertical component of the velocity alone finally obliquity influences the fate of the projectile in particular the amount and velocity of ricochet are a strong function of impact angle

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

nrm irm s demagnetization plots an aid to the interpretation of natural remanent magnetization af demagnetization of thermoremanent magnetization trm and saturation isothermal remanent magnetization irm s are used to establish diagnostic criteria for primary trm and for secondary magnetization in igneous rocks in applying the approach to the interpretation of natural remanent magnetization nrm the demagnetization of irm s documents the nature of the magnetic phases in the rock while the demagnetization of the nrm gives information about the process of magnetization the ratio of nrm irm s measures the efficiency of the magnetization process

coccolithophore fluxes from sediment traps in the northeastern arabian sea off pakistan abstract we investigated coccolithophore fluxes from four sediment traps located in the northeastern arabian sea at the continental slope of pakistan two moorings wpt and ept western isastern pakomin trap consisting of one and two sediment traps were deployed at water depths of about 500 m and 1500 m they collected settling particles from october 1993 to february 1994 and from may 1995 to february 1996 at intervals of 22 and 24 days respectively a longer sampling was prevented probably due to high sedimentation events which occluded the trap cones both areas were characterised by a strong seasonality in coccolith fluxes high fluxes were recorded with up to 2000 10 6 coccoliths m 2 d 1 in the early sw summer monsoon and up to 5400 10 6 coccoliths m 2 d 1 in the early ne winter monsoon generally coccosphere fluxes exhibited a similar trend but were some orders of magnitude lower than coccolith fluxes twenty seven taxa were identified in this study all well to moderately well preserved and with a small range of diversity the species composition was dominated by gephyrocapsa oceanica florisphaera profunda and emiliania huxleyi making up about 70 of the assemblages during the sw summer monsoon high productivity in the surface waters of the eastern arabian sea may be linked to the upwelling system off oman apparently nutrient rich surface water derived from the southwestern upwelling areas off oman with high standing stocks were carried into this area by northeast flowing currents in contrast during the ne winter monsoon the locally observed high coccolithophore production is hypothesised to respond to local injections of nutrient rich deep water into the surface water due to sea surface cooling leading to convection consequently fluxes during the winter monsoon were in the same range as during the summer monsoon

palaeomagnetism of the vredefort meteorite crater and implications for craters on mars magnetic surveys of the martian surface have revealed significantly lower magnetic field intensities over the gigantic impact craters hellas and argyre than over surrounding regions the reduced fields are commonly attributed to pressure demagnetization caused by shock waves generated during meteorite impact in the absence of a significant ambient magnetic field lower than average magnetic field intensities are also observed above the vredefort meteorite crater in south africa yet here we show that the rocks in this crater possess much higher magnetic intensities than equivalent lithologies found elsewhere on earth we find that palaeomagnetic directions of these strongly magnetized rocks are randomly oriented with vector directions changing over centimetre length scales moreover the magnetite grains contributing to the magnetic remanence crystallized during impact which directly relates the randomization and intensification to the impact event the strong and randomly oriented magnetization vectors effectively cancel out when summed over the whole crater seen from high altitudes as for martian craters the magnetic field appears much lower than that of neighbouring terranes implying that magnetic anomalies of meteorite craters cannot be used as evidence for the absence of the planet s internally generated magnetic field at the time of impact

terrigenous plant wax inputs to the arabian sea implications for the reconstruction of winds associated with the indian monsoon author posting the authors 2005 this is the author s version of the work it is posted here by permission of elsevier b v for personal use not for redistribution the definitive version was published in geochimica et cosmochimica acta 69 2005 2547 2558 doi 10 1016 j gca 2005 01 001

detrital zircon and provenance analysis of late cretaceous miocene onshore iranian makran strata implications for the tectonic setting a multidisciplinary provenance study including sandstone framework heavy mineral analysis in situ u pb dating of detrital zircon and hf isotopic ratio analysis of dated zircons was undertaken on late cretaceous miocene deep marine turbiditic and deltaic sandstones of makran accretionary wedge se iran to determine their sedimentary provenance and tectonic setting sandstone framework modes reveal both magmatic arc rocks as a source of late cretaceous oligocene detritus and recycling of miocene sandstones heavy mineral assemblages cr spinel and blue amphibole indicate ophiolite and high pressure low temperature metamorphic rocks blueschists as a supplementary provenance in total 2931 laser ablation inductively coupled plasma mass spectrometry icp ms u pb detrital zircon ages on 21 sandstone samples yielded three major age peaks at ca 167 ma 88 7 ma and 48 9 ma also 241 in situ hf isotope analyses of dated zircons provide evidence for dominantly igneous source rocks two main detrital zircon ages are identified 1 abundant middle jurassic grains with hf isotopic compositions of continental crust suggesting a rifting related magmatic provenance and 2 late cretaceous eocene grains with hf isotopic compositions of continental crust and nondepleted mantle suggesting a continental magmatic arc provenance this change in provenance is attributed to the late cretaceous convergence between arabia and eurasia

target rocks impact glasses and melt rocks from the lonar impact crater india petrography and geochemistry the lonar crater india is the only well preserved simple crater on earth in continental flood basalts it is excavated in the deccan trap basalts of cretaceous tertiary age a representative set of target basalts including the basalt flows excavated by the crater and a variety of impact breccias and impact glasses were analyzed for their major and trace element compositions impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes comparable geochemical patterns of immobile elements e g rees for glass melt rock and basalt indicates minimal fractionation between the target rocks and the impactites we found only little indication of post impact hydrothermal alteration in terms of volatile trace element changes no clear indication of an extraterrestrial component was found in any of our breccias and impact glasses indicating either a low level of contamination or a non chondritic or otherwise iridium poor impactor

how accurately can nrm sirm determine the ancient planetary magnetic field intensity abstract the present study tested grain size dependence of thermoremanent magnetization trm saturation isothermal remanent magnetization sirm as well as its stability against demagnetization the trm sirm ratio is dependent on the grain size of magnetite suggesting that a strong constraint on grain size characterization is necessary to be used as a paleointensity proxy in planetary magnetism in addition the trm sirm ratio increases as the alternating field increases for fine grained magnetite accuracy of trm sirm was investigated using historic showa lava erupted in 1946 it was observed that the natural remanent magnetization nrm sirm ratio of 0 032 is comparable to the field intensity of 46 80 t however the uncertainty of nrm sirm was an order of magnitude larger than that of the companion thellier estimation therefore the nrm sirm ratio can only provide crude estimations on the absolute planetary magnetic field intensity in practice anisotropy correction is advisable to reduce the scatter of nrm sirm

sources of laminated sediments in the northeastern arabian sea off pakistan and implications for sediment transport mechanisms during the late holocene laminated sediments of the continental slope off the makran coast in the northern arabian sea are well known climate archives and record productivity as well as supply of material from land here

low field variation of magnetic susceptibility as a tool for magnetic mineralogy of rocks abstract low field variation of magnetic susceptibility was investigated on a collection of several hundreds specimens of various minerals and rocks using the kly 4s kappabridge the measurement is fully automated being executed in 21 distinct fields ranging from 2 to 450 a m in one frequency of 875 hz the measurement is rapid 7 min per specimen so that large collections of specimens can be investigated the results can be processed both graphically and mathematically for the latter processing parameters of two kinds were introduced one characterizes the susceptibility change with field the other one characterizes the field in which the susceptibility no longer obeys the rayleigh law and starts becoming more complex the results were evaluated statistically remarkable differences were revealed between individual minerals and between some rock types for example the field variation of susceptibility of pyrrhotite is in general an order of magnitude larger than that of titanomagnetite the susceptibility increase in pyrrhotite starts at the field an order of magnitude lower than that of titanomagnetite low field variation of susceptibility then appears as an interesting phenomeon that helps in the identification of magnetic minerals and in some cases also in assessing the compositional variation of them