geochemical and mineralogical control of different genetic types of deep sea nodules from the pacific ocean deep sea nodules from the northeast pacific nodule belt and the southeast pacific sonne basin being formed in areas bordering the equatorial zone of high biological productivity accumulate by two basically different growth processes a early diagenetic growth by supply from pore water and b hydrogenetic growth by supply from near bottom sea water these growth processes lead to different genetic types of nodules early diagenetic type a hydrogenetic type b and mixed type ab a further type ac very rich in mn is being formed by increasing influence of early diagenesis these types can clearly be distinguished by their shapes surface textures mineral constituents of oxide fraction internal microstructures and geochemistry a genetical classification is being proposed on the basis of statistically computed interelement relationships todorokite very poor in fe is the main mn phase in the early diagenetic substance mno2 intimately intergrown with feooh xh2o is the main phase in the hydrogenetic substance consequently an important difference can be pointed out the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of me2 e g mn2 ni2 cu2 zn2 whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles mobilization of mn2 and fractionation from fe is controlled by the amount of decomposing organic matter in the peneliquid layer of the sediments the main factor controlling the intensity of early diagenesis is the biological productivity in surface waters the crucial point of reversal at a mn fe ratio of about 5 obtained by hyperbolical regression of the analyses of nodules from the southeast pacific represents best concentrations in ni and cu mn fe quotients greater than 5 cause a decrease of ni and cu content nodules from the northeast pacific nodule belt generally contain higher concentrations in cu than nodules from the southeast pacific this can be explained by an additional supply of cu transported below ccd by siliceous plankton

noble gases in individual l chondritic micrometeorites preserved in an ordovician limestone we have measured the he and ne concentrations and isotopic ratios of individual sediment dispersed extraterrestrial chromite grains 63 180 mu m in diameter from an ordovician limestone in southern sweden in the same sediment many fossil meteorites were found and have been attributed to the l chondrite parent body breakup event similar to 470 ma ago in this analysis of 37 individual extraterrestrial chromite grains of l chondritic major element composition at least 35 similar to 95 contain surface implanted helium and neon of fractionated solar wind composition implying that these grains are relict parts of fossil micrometeorites of asteroidal origin similar to what has been observed in recent micrometeorites collected in the polar regions significant amounts of cosmogenic ne 21 were found in several grains resulting in cosmic ray exposure cre ages of up to similar to 77 myr these ages exceed both dynamical lifetimes for asteroidal micrometeorites of this size as well as cre ages observed in chromites from fossil meteorites from the same sediment beds significant contributions from terrestrial sources like nucleogenic and cosmic ray muon induced ne 21 can be excluded in the extraterrestrial chromites since 11 terrestrial chrome spinel grains from the same sediment beds did not contain any measurable ne 21 excesses having found micrometeorites of undisputed asteroidal origin with cosmic ray exposure ages of several 10 7 years implies that high cosmic ray exposure ages alone are not a good indicator of cometary origin in opposition to what has been suggested for recent micrometeorites and idps we propose instead that these grains collected their cosmogenic ne 21 while residing in the regolith layer of their parent body c 2009 elsevier b v all rights reserved less

metal diagenesis in oxic marine sediments the metal nutrient relationships observed for nickel and cadmium in the deep ocean are continued at the interface between seawater and oxidizing pore water this continuum results in pore water concentrations of these metals which are only slightly greater than near bottom seawater levels manganese concentrations in these oxidizing pore waters are also extremely low less than three times bottom water in contrast release in the boundary layer produces a maximum of dissolved copper which is 10 40 times ambient seawater assuming these pore waters are at steady state flux estimates based on these measurements suggest that the manganese in todorokite rich nodules of the central equatorial pacific was not supplied by upward diffusion through pore waters below the interface most nodular nickel is precipitated with manganese while nodular copper is supplied by diffusion

laser probe 40ar 39ar studies of the peace river shocked l6 chondrite abstract samples of matrix and melt glass from the shocked l6 chondrite peace river have been subjected to 40ar 39ar analysis using both stepped heating and localised outgassing by a laser probe stepped heating studies of three distinct samples of matrix gave indistinguishable and unusually well defined low temperature 40ar 39ar plateaux corresponding to an age of 450 30 ma in an attempt to investigate whether or not this plateau age is meaningful i e the time of a major collision event involving the parent body or is an artefact resulting from partial degassing during the fragmentation of the parent body which commenced the exposure of peace river to cosmic rays 40 ma ago we have used a laser probe to analyse ar from single 80 m diameter laser pits which release gas from a few micrograms of sample on this scale no areas have ages younger than the plateau age supporting the view that it is not an artefact measurements on the melt glass gave variable and meaningless ages in some cases greater than the age of the solar system due to retention of 40ar and loss of k during melting followed by rapid quenching this observation indicates the danger of attempts to date impact melt glass directly the observation that argon has been retained by the glass and lost by the matrix can be understood in terms of a range of plausible annealing histories

the ordovician chondrite from brunflo central sweden iii geochemistry of terrestrial alteration the fossil h chondrite brunflo found in a slab of ordovician limestone from central sweden is pervasively altered to an assemblage dominated by calcite and barite the meteorite is surrounded by a 15 20 cm wide zone of lighter colors than the unaffected limestone due to dissolution of hematite here we present detailed geochemical analyses of two meteorite samples 14 limestone samples at distances from 0 to 29 cm along two profiles from the meteorite and a reference sample of brunflo limestone element concentrations in brunflo and surrounding bleached limestone have been strongly disturbed during two stages of alteration early oxygenated and deep burial in the meteorite the ni rco ratio has changed from an initial value of 20 to 0 8 and redox sensitive elements like v as mo re and u are strongly enriched the sulfur isotope 34 composition of barite from brunflo d s sq 35 indicates initial loss of meteoritic sulfide followed by later accumulation of sea water sulfate as barite during deep burial under more reducing conditions reduction processes supported by an externally derived reductant possibly derived from alum shale underlying the limestone were largely responsible for the observed redox phenomena in spite of massive redistribution of many elements concentrations of pt ir and au remain at chondritic levels the geochemistry and mineralogy of alteration determined for brunflo are similar to

the oxidation state of manganese in marine sediments and ferromanganese nodules marine sediments and ferromanganese nodules from the pacific ocean have been analyzed for the omn ratio of solid manganese we tested six chemical methods and concluded that the iodometric and oxalate methods were equivalent and were the best choice in terms of accuracy and precision on natural samples we choose the iodometric method for most of our analyses because the oxalate procedure is a method of differences the ferromanganese nodules that we analyzed were all from manop site h and had mnfe ratios that ranged from 5 6 to 70 these nodules were invariably highly oxidized with omn values ranging from 1 90 to 2 00 our most precise analyses suggest that less than 1 of the total manganese is present as mn ii we also analyzed red clay and hemipelagic sediments from the eastern tropical pacific baja borderland and manop site h and carbonate ooze samples from the equatorial pacific these sediments are also highly oxidized omn 1 90 to 2 00 except when mn ii appears in the interstitial water as dissolved mn ii increases the value of the omn ratio in the solid phase decreases the omn ratio decreases to values as low as 1 40 this decrease appears to be due to a decrease in oxidized manganese by reduction however an increase in reduced manganese in the solid sediments by adsorption or mnco3 formation can not be ruled out in all cases

geochemistry of meteorite rich marine limestone strata and fossil meteorites from the lower ordovician at kinnekulle sweden abstract thirteen fossil meteorites 1 5 9 cm have been found in lower ordovician orthoceratite limestone at kinnekulle sweden the meteorites were recovered from a 3 5 m thick quarried interval in the middle of a 30 m thick section of orthoceratite limestone the section formed in an epicontinental sea at very slow sedimentation rates 0 2 cm ka the limestones of the lower two thirds of the section are reddish brown except in the quarried interval where a conspicuous 1 m thick grey limestone bed occurs ten of the meteorites were found in a thin bed below the grey limestone one was found in the grey limestone and two in the red limestone above the meteorites are almost completely pseudomorphosed primarily by calcite and barite they were identified by their content of chromite grains and chondrule pseudomorphs the central part of one of the meteorites shows iridium 800 ppb gold 550 ppb and chromium 4400 ppm concentrations in typical chondritic abundances centimeter resolution profiles for iridium and eight other elements have been established through the quarried interval at a site at 4 km distant from the meteorite producing quarry in the reddish brown parts ir concentrations carbonate free basis lie typically in the range 300 400 ppt compared with 50 ppt ir in average shale in the grey limestone the ir concentrations are lower around 100 150 ppt iridium correlates with iron and appears to be related to sedimentation rate and or iron redox state rather than to any variations in the composition of the siliciclastic fraction with estimated sedimentation rates as above the excess ir flux to the sediment was 30 ng cm 2 ma which is a factor 2 3 higher than the present total ir flux and one order of magnitude higher than the extraterrestrial ir flux to the pacific sea floor our data cannot discriminate to what extent the ordovician ir enrichments are related to extraterrestrial matter or to precipitation of ir from seawater in recent deep sea sediments seawater derived ir enrichments are associated with mn oxides but no similar association occurs in the orthoceratite limestone the ten meteorites in the bed below the grey limestone reflect minimum meteorite accumulation rates of one meteorite per 150 m 2 and 175 ka probably the ten meteorites are related to a local strewn field rather than to background influx minimum accumulation rates calculated on three meteorite finds 20 g in the entire quarried interval give one meteorite per 570 m 2 and 1 17 ma which is one order of magnitude higher than present day large area averaged meteorite influx rates the probability is low that the small area 2 of the sea floor studied would happen to be one where an unusually high number of meteorites struck our results may reflect that meteorite and possibly also extraterrestrial ir influx rates were substantially higher than at the present during a period in the early ordovician enhanced influx rates could be related to an asteroid collision in space

a survey of clasts and large chondrules in ordinary chondrites we report the results of a survey of clasts and large 5 mm chondrules macrochondrules within the 833 ordinary chondrites of the natural history museum collection thirty six macrochondrules and 24 clasts were identified and studied macrochondrules have textures and mineral assemblages like normal chondrules and so share a common origin clasts show evidence for fracturing from larger bodies and can be classified as either 1 chemically fractionated if they have major and trace element compositions differing substantially from most chondrules and clasts 2 impact melt clasts if they have microporphyritic textures and signs that indicate they are derived from shock melted chondritic material 3 microporhyritic clasts if they are similar to the last category but lack evidence for derivation through shock melting or 4 indeterminate clasts forming a diverse class that includes all those clasts that do not fit into the other categories

extraterrestrial chromite in latest maastrichtian and paleocene pelagic limestone at gubbio italy the flux of unmelted ordinary chondrites the distribution of sediment dispersed extraterrestrial ordinary chondritic chromite ec grains 63 m has been studied across the latest maastrichtian and paleocene in the bottaccione gorge section at gubbio italy this section is ideal for determining the accumulation rate of ec because of its condensed nature and well constrained sedimentation rates in a total of 210 kg of limestone representing eight samples of 14 28 kg distributed across 24 m of the bottaccione section only 6 ec grains were found an average of 0 03 ec grains kg 1 in addition one probable pallasitic chromite grain was found no ec grains could be found in two samples at the cretaceous tertiary k t boundary which is consistent with the k t boundary impactor being a carbonaceous chondrite or comet low in chromite the average influx of ec to earth is calculated to 0 26 grain m 2 kyr 1 this corresponds to a total flux of 200 tons of extraterrestrial matter per year compared to 30 000 tons per year as estimated from os isotopes in deep sea sediments the difference is explained by the ec grains representing only unmelted ordinary chondritic matter predominantly in the size range from 0 1 mm to a few centimeters in diameter sedimentary ec grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere the average of 0 03 ec grain kg 1 in the gubbio limestone contrasts with the up to 3 ec grains kg 1 in mid ordovician limestone that formed after the disruption of the l chondrite parent body in the asteroid belt at 470 ma the two types of limestone were deposited at about the same rate and the difference in ec abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup

asteroid breakup linked to the great ordovician biodiversification event the great ordovician biodiversification event coincides 470 million years ago with the break up of a large asteroid and the resultant frequent bombardment of earth with asteroid fragments

a global rain of micrometeorites following breakup of the l chondrite parent body evidence from solar wind implanted ne in fossil extraterrestrial chromite grains from china previous studies of limestone beds of mid ordovician age from both sweden and china show that the earth saw an at least two orders of magnitude increase in the influx of extraterrestrial material approximately 470 ma following the disruption of an l chondrite parent body in the asteroid belt recovered extraterrestrial material consists of fossil meteorites and sediment dispersed extraterrestrial chromite sec grains both with l chondritic origin ne isotope analysis of sec grains from one of the swedish limestone sections revealed that the vast majority of the grains were delivered to earth as micrometeorites in this study we extend the previous work both in time and geographically by measuring concentrations and isotopic ratios of ne in individual sec grains 60120 mu m in diameter from three different beds from a contemporary middle ordovician limestone section in china all of the chinese sec grains 44 in total contain surface implanted ne of fractionated solar wind composition implying that these grains were as in the case of the swedish sec grains delivered to earth as micrometeorites this gives further compelling evidence that the two to three orders of magnitude increase in the influx of micrometeoritic material following the breakup of the l chondrite parent body was indeed a global event the rain of micrometeorites prevailed for at least 2 myr the estimated time of the deposition of the topmost chinese bed after the breakup event less

cenozoic evolution of sediment accumulation in deltaic and shore zone depositional systems northern gulf of mexico basin abstract paleogeographic and volumetric lithofacies mapping of 18 cenozoic genetic sequences within the northern gulf of mexico basin quantifies the proportional sequestering of sediment within wave dominated shore zone vs deltaic systems through time three long term depositional phases are revealed by plots based on paleogeographic and sediment isochore maps of total shore zone system area and volume to total delta system area and volume sz d 1 sz d area and volume ratios are highly variable in paleocene through eocene sequences however typical volume ratios for major genetic sequences upper middle and lower wilcox queen city qc and yegua range between 0 2 and 0 6 minor sequences sparta sp jackson js which record very low rates and volumes of sediment accumulation have the greatest variability in their ratios 2 oligocene and miocene sequences display consistently high proportions of shore zone sediment sz d area ratios range from 0 6 to 1 0 and volume ratios cluster between 0 4 and 0 8 3 a substantial late neogene decrease in sz d ratios is presaged in the late miocene sequence pliocene and pleistocene sequences are uniformly characterized by very low ratios of

a single asteroidal source for extraterrestrial ordovician chromite grains from sweden and china high precision oxygen three isotope sims analysis we examined oxygen three isotope ratios of 48 extraterrestrial chromite ec grains extracted from mid ordovician sediments from two different locations in sweden and one location in south central china the ages of the sediments similar to 470 ma coincide with the breakup event of the l chondrite parent asteroid elemental compositions of the chromite grains are generally consistent with their origin from l or ll chondrite parent bodies the average delta o 17 parts per thousand deviation from the terrestrial mass fractionation line measured in situ from 15 mu m spots by secondary ion mass spectrometry sims of ec grains extracted from fossil meteorites from thorsberg and brunflo are 1 17 0 09 parts per thousand 2 sigma and 1 25 0 16 parts per thousand respectively and those of fossil micrometeorites from thorsberg and puxi river are 1 10 0 09 parts per thousand and 1 11 0 12 parts per thousand respectively within uncertainty these values are all the same and consistent with the l chondrite group average delta o 17 1 07 0 18 parts per thousand but also with the ll chondrite group average delta o 17 1 26 0 24 parts per thousand clayton et al 1991 we conclude that the studied ec grains from correlated sediments from sweden and china are related and most likely originated in the same event the l chondrite parent body breakup we also analyzed chromites of modern h l and ll chondrites and show that their delta o 17 values coincide with averages of delta o 17 of bulk analyses of h l and ll chondrites this study demonstrates that in situ oxygen isotope data measured by sims are accurate and precise if carefully standardized and can be used to classify individual extraterrestrial chromite grains found in sediments c 2009 elsevier ltd all rights reserved less

rates of delta progradation during highstands consequences for timing of deposition in deep marine systems estimated times required for 24 modern river systems to form a shelf edge delta range from 8 5 ka to 116 5 ka depending on fluvial sediment supply delta width shelf volume and shelf transport rates these values indicate that transport of sand into deep marine systems is likely to be significant during third order highstands of relative sea level factors such as shelf transport dynamics may slow delta progradation while submarine canyons cutting the shelf may reduce the time before deep marine deposition occurs interpreting ancient sand rich deep marine strata as lowstand deposits without sufficient palaeogeographic information may not therefore always be appropriate

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

the ordovician chondrite from brunflo central sweden i general description and primary minerals abstract the brunflo chondrite was found in the rodbrottet quarry near brunflo central sweden lat 63 5 n long 14 53 e the meteorite ca 11 cm 9 5 cm occurs in a polished slab of middle ordovician limestone which based on its content of conodonts belongs to the aserian segerstad limestone the meteorite fell in the iapetus ocean close to its eastern coast line the original minerals except chromite are replaced by secondary minerals mainly calcite remarkably even minute details of textures and structures are preserved in some areas the various types of chondrules and their frequencies could therefore be studied they are similar to present falls of ordinary chondrites of petrologic types 3 and 4 chromite occurs as up to ca 0 1 mm large grains microprobe analyses of 59 grains show that except for zn the composition is fairly constant and similar to h4 h5 chromites the zn content varies from 0 2 to 6 8 wt zno mean ca 0 6 wt two analyses show that also chrome spinel is present tentatively brunflo is classified as h4 h5

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

the origin of the brunflo fossil meteorite and extraterrestrial chromite in mid ordovician limestone from the garde quarry jamtland central sweden the brunflo fossil meteorite was found in the 1950s in mid ordovician marine limestone in the garde quarry in jamtland it originates from strata that are about 5 million years younger than similar limestone that more recently has yielded 50 fossil meteorites in the thorsberg quarry at kinnekulle 600 km to the south based primarily on the low tio2 content about 1 8 wt of its relict chromite the brunflo meteorite had been tentatively classified as an h chondrite the meteorite hence appears to be an anomaly in relation to the kinnekulle meteorites in which chromite composition chondrule mean diameter and oxygen isotopic composition all indicate an l chondritic origin reflecting an enhanced flux of meteorites to earth following the disruption of the l chondrite parent body 470 ma new chondrule size measurements for the brunflo meteorite indicate that it too is an l chondrite related to the same parent body breakup chromite maximum diameters and well defined chondrule structures further show that brunflo belongs to the l4 or l5 type chromites in recently fallen l4 chondrites commonly have low tio2 contents similar to the brunflo chromites adding support for brunflo being an l4 chondrite the limestone in the circle quarry is relatively rich about 0 45 grain kg 1 in sediment dispersed extraterrestrial chromite grains 63 mu m with chemical composition similar to those in l chondrites and the limestone 1 3 grains kg 1 at kinnekulle suggesting that the enhanced flux of l chondrites prevailed although somewhat diminished at the time when the brunflo meteorite fell less

ferromanganese nodules from manop sites h s and r control of mineralogical and chemical composition by multiple accretionary processes abstract the chemical composition of ferromanganese nodules from the three nodule bearing manop sites in the pacific can be accounted for in a qualitative way by variable contributions of distinct accretionary processes these accretionary modes are 1 1 hydrogenous i e direct precipitation or accumulation of colloidal metal oxides in seawater 2 2 oxic diagenesis which refers to a variety of ferromanganese accretion processes occurring in oxic sediments and 3 3 suboxic diagenesis which results from reduction of mn 4 by oxidation of organic matter in the sediments geochemical evidence suggests processes 1 and 2 occur at all three manop nodule bearing sites and process 3 occurs only at the hemipelagic site h which underlies the relatively productive waters of the eastern tropical pacific a normative model quantitatively accounts for the variability observed in nearly all elements zn and na however are not well explained by the three end member model and we suggest that an additional accretionary process results in greater variability in the abundances of these elements variable contributions from the three accretionary processes result in distinct top bottom compositional differences at the three sites nodule tops from h are enriched in ni cu and zn instead of the more typical enrichments of these elements in nodule bottoms in addition elemental correlations typical of most pelagic nodules are reversed at site h the three accretionary processes result in distinct mineralogies hydrogenous precipitation produces mno2 oxic diagenesis however produces cu ni rich todorokite and suboxic diagenesis results in an unstable todorokite which transforms to a 7 a phase birnessite upon dehydration the presence of cu and ni as charge balancing cations influence the stability of the todorokite structure in the bottoms of h nodules which accrete dominantly by suboxic diagenesis na and possibly mn 2 provide much of the charge balance for the todorokite structure limited growth rate data for h nodules suggest suboxic accretion is the fastest of the three processes with rates at least 200 mm 106 yr oxic accretion is probably 10 times slower and hydrogenous 100 times slower since these rates predict more suboxic component in bulk nodules than is calculated by the normative analysis we propose that suboxic accretion is a non steady state process variations in surface water productivity cause pulses of particulate flux to the sea floor which result in transient mn reduction in the surface sediments and reprecipitation on nodule surfaces

on the occurrence of ferromanganese micronodules from the sediments of the bengal fan a high terrigenous sediment input region studies of 13 samples 10 surface sediments and 3 cores from across the middle fan region of the bay of bengal reveal the presence of micronodules area 0 22 million km2 over the entire bay these micronodules have two dominant morphological forms botryoidal in the eastern bay and spheroidal in the central and the western regions of the bay the abundance of the micronodules was found to be higher in the eastern region of the bay in the cores the micronodules were generally found to decrease in abundance down core the core from the eastern region had micronodules down to deeper levels than the western side todorokite is the manganese mineral present in the micronodules associated mineral phases are quartz feldspar and phillipsite these micronodules have high mn 19 20 32 13 rather low fe 1 20 1 83 ni 0 10 1 14 zn 0 10 0 30 and co 0 01 0 04 and their mnfe ratio ranges from 10 55 to 25 42 the contents of mn ni cu and zn and the mnfe ratio decrease down core based upon the variations in the abundance morphology mnfe ratio of the micronodules and the elemental distributions and the omn ratio in the sediments remobilization of mn from deeper levels and its precipitation at the upper surfaces of the sediment column in an oxic environment is suggested the results of the present study also suggest that a high rate of sedimentation is not a limiting factor for the formation of micronodules although it might affect their growth

from the mid ordovician into the late silurian changes in the micrometeorite flux after the l chondrite parent breakup we present the first reconstruction of the micrometeorite flux to earth in the silurian period we searched 321 kg of condensed marine limestone from the late silurian cellon section southern austria for refractory chrome spinel grains from micrometeorites that fell on the ancient sea floor a total of 155 extraterrestrial spinel grains 10 grains 63 m and 145 in the 32 63 m fraction were recovered for comparison we searched 102 kg of similar limestone from the mid ordovician komstad formation in southern sweden this limestone formed within 1 ma after the breakup of the l chondrite parent body lcpb in the asteroid belt in the sample we found 444 extraterrestrial spinel grains in the 63 m fraction and estimate a content of at least 7000 such grains in the 32 63 m fraction our results show that in the late silurian 40 ma after the lcpb the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude almost down to background levels among the ordinary chondrites the dominance of l chondritic micrometeorites has waned off significantly from 99 in the post lcpb mid ordovician to 60 in the late silurian with 30 h and 10 ll chondritic grains in the late silurian primitive achondrite abundances are similar to today s value contrasting to the much higher abundances observed in pre lcpb mid ordovician sediments

morphology paleogeographic setting and origin of the middle wilcox yoakum canyon texas coastal plain the yoakum canyon is the largest of the gulf coast eocene erosional gorges and is interpreted as a buried submarine channel the canyon can be traced for 67 mi 108 km from the wilcox fault zone which defines the position of the early eocene shelf edge nearly to present outcrop in this paper we expand on previously published descriptions of the canyon and use a more extensive subsurface database decompaction of the canyon shale fill reveals that original depths of the canyon exceeded 3500 ft 1067 m apparent canyon wall slump scarps and a peripheral chaotic zone interpreted as an incipient slump feature are comparable to similar features of the late quaternary mississippi submarine canyon the yoakum canyon formed within the garwood subembayment to the west of and adjacent to the middle wilcox rockdale delta system quantitative mapping of facies adjacent to the yoakum shale indicate the following sequence of events l muddy distal deltaic and shelf facies of the lower middle wilcox subgroup were deposited during a retrogradation 2 a resurgence of progradation deposited upper middle wilcox deltaic facies atop the unconsolidated lower middle wilcox continental margin muds loading the shelf edge and initiating slump failure of the continental margin 3 headward erosion of the canyon across the shelf occurred contemporaneously with a subsidence induced transgression that coincided with a disruption in sediment supply the yoakum canyon was excavated by a combinat on of slumping and current scour 4 the canyon was filled by hemipelagic and prodelta muds of the upper wilcox subgroup

large spinel grains in a cm chondrite acfer 331 implications for reconstructions of ancient meteorite fluxes by dissolving 30400kg of marine limestone in hcl and hf acid our group has previously recovered common relict chromite grains approximately 63250m from ordinary chondritic micrometeorites that fell on ancient sea floors up to 500myr old here we evaluate if cm group carbonaceous chondritic material which makes up an important fraction of the micrometeorite flux today contains analogous grains that can be searched for in acid residues we dissolved 8g of cm2 meteorite acfer 331 in hf which yielded a characteristic assemblage of both transparent mg al and opaque cr spinels 28m we find on average 4 6 and 130 mg al spinel grains per gram in the 63250 and 2863m size fractions respectively these grains are mostly pink or colorless and often characterized by heterogeneous cr content black opaque cr spinel grains are absent from the 63m fraction but in the 2863m fraction we find approximately 65 such grains per gram meteorite the individual grains have a characteristic composition with heterogeneous major element compositions e g 44 461 7wt cr2o3 but narrow ranges for maximum tio2 0 61 6wt and v2o3 0 51 0wt concentrations the content of spinel grains in the 2863m fraction of cm meteorites appears comparable at the order of magnitude level with the content of 63m sized chromite grains in fossil l chondrites from ordovician limestone our approach of recovering meteoritic spinel from sediment may thus be extended to include cm meteorites but the smaller size fraction of the acid residues should be searched less

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

compositional variations in manganese micronodules a possible indicator of sedimentary environments manganese micronodules from various environments were analyzed with the electron microprobe chemical characteristics of marine manganese nodules were examined in relation with their depositional environments and compared with those of manganese macronodules the compositional variation has a connection with accumulation rates and chemical features of the associated sediments mg and ti tend to be enriched in micronodules relative to the host sediments these elements may produce authigenic mineral phases in micronodules these observations show that manganese micronodules are chemically formed under approximately the same condition as that for macronodules but coexisting pairs of micronodules and macronodules collected from the same sediments suggest that oxic diagenesis plays a more important role in the formation of micronodules than that of macronodules the chemical features of manganese micronodules collected from several marine environments such as a marginal sea hemipelagic regions and the cent

noble gases in fossil micrometeorites and meteorites from 470 myr old sediments from southern sweden and new evidence for the l chondrite parent body breakup event we present noble gas analyses of sediment dispersed extraterrestrial chromite grains recovered from similar to 470 myr old sediments from two quarries hallekis and thorsberg and of relict chromites in a coeval fossil meteorite from the gullhogen quarry all located ill southern sweden both the sediment dispersed grains and the meteorite gullhogen 001 were generated in the l chondrite parent body breakup about 470 myr ago which was also the event responsible for the abundant fossil meteorites previously found in the thorsberg quarry trapped solar noble gases in the sediment dispersed chromite grains have partly been retained during similar to 470 myr of terrestrial residence and despite harsh chemical treatment in the laboratory this shows that chromite is highly retentive for solar noble gases the solar noble gases imply that a sizeable fraction of the sediment dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites the grains in the oldest sediment beds were rapidly delivered to earth likely by direct injection into all orbital resonance in the inner asteroid belt whereas grains in younger sediments arrived by orbital decay due to poynting robertson p r drag the fossil meteorite gullhogen 001 has a low cosmic ray exposure age of similar to 0 9 myr based on new he and ne production rates in chromite determined experimentally this age is comparable to the ages of the fossil meteorites from thorsberg providing additional evidence for very rapid transfer times of material after the l chondrite parent body breakup

atypical ferromanganese nodules from pelagic areas of the central indian basin equatorial indian ocean abstract two box core samples one each from red clay and siliceous ooze sediment provinces from the central indian basin indian ocean were studied both cores contained ferromanganese nodules at their tops the red clay sediment also hosts buried nodules at a depth of 82 cm chemical characterization of the sediments indicates that early diagenetic remobilization of transitional metals is pronounced in the siliceous ooze sediment while in the red clay no such effect is discernible this is further corroborated by the abundance and dissolution patterns of microorganisms and ferromanganese micronodules occurring in the two sediment columns bulk chemical analyses indicate that all the nodules are of the hydrogenetic b type vernadite and todorokite are the major minerals both of which constitute laminated nondirectional colloform and cuspate microstructures petrographic studies yield evidence of recrystallization of todorokite and partial transformation of vernadite to todorokite the former process being more advanced in buried nodules microprobe analyses revealed that primary todorokite has a lower mn fe ratio and higher ni content than recrystallized todorokite in the same sample the mineralogical reconstitution in the nodules is attributed to postdepositional intranodule diagenesis the present study does not reveal the correlation between mineralogy and microstructure in the nodules which has been suggested for the nodules from the pacific neither can any mineral or microstructure be considered specific for any particular genetic model the studied nodules are therefore atypical the mineralogical and chemical reordering in these ferromanganese nodules raises the question of the viability of radiometric methods in determining growth rates of such nodules

petrographic classification of middle ordovician fossil meteorites from sweden the maximum diameter of chromite fecr2o4 grains within l chondrites reflects the petrographic type of the sample on the basis of our measurements of nine recent l chondrites l3 chromite d mac 34 50 mu m l4 87 150 mu m l5 76 158 mu m and l6 253 638 mu m this variation reflects the crystallization of the chromite grains during parent body thermal metamorphism we use this calibration to classify six fossil meteorites from the middle ordovician in sweden as type 3 or 4 to 6 the high flux of l chondrites at 470 ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent l chondrite falls the fossil meteorites have in places preserved recognizable chondrule textures including porphyritic olivine barred olivine and radiating pyroxene a large relict clast and fusion crust have also been tentatively identified in one fossil meteorite apart from chromite all of the original meteorite minerals have been replaced by carbonate and sheet silicate and sulfate during diagenesis within the limestone host the preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules the range 0 4 0 6 mm is consistent with measurements made in the same way on recent l chondrites less

a rain of ordinary chondritic meteorites in the early ordovician forty fossil meteorites with a total original mass of v7 7 kg have been recovered in the first systematic search for fossil meteorites pursued in an active quarry in lower ordovician 480 ma marine limestone in southern sweden the meteorites represent at least 12 different falls over a seafloor area of v6000 m 2 during 91 75 myr making the quarry one of the most meteorite dense areas known in the world geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably l chondrites mechanisms for meteorite delivery from the asteroid belt to earth were the same 480 ma as today however the flux was one to two orders of magnitude higher most likely reflecting the disruption of the l chondrite parent body at about that time this is a major event in late solar system history which may also have led to an enhanced flux of asteroids to earth during v30 myr fl 2001 elsevier science b v all rights reserved

40ar 39ar age of northwest africa 091 more evidence for a link between l chondrites and fossil meteorites abstract most 40ar 39ar ages of l chondrites record an event at approximately 500 ma indicating a large collisional impact at that time however there is a spread in ages from 400 to 600 ma in these meteorites that is greater than the analytical uncertainty identification of and correction for trapped ar in a few l chondrites has given an age of 470 6 ma this age coincides with ordivician fossil meteorites that fell to earth at 467 2 ma as these fossil meteorites were originally l chondrites the apparent conclusion is that a large impact sent a flood of l chondrite material to earth while material that remained on the l chondrite parent body was strongly heated and reset we have reduced 40ar 39ar data for northwest africa 091 using various techniques that appear in the literature including identification and subtraction of trapped ar these techniques give a range of ages from 455 to 520 ma and show the importance of making accurate corrections by using the most straightforward technique to identify and remove a trapped ar component which is neither terrestrial nor primordial an 40ar 39ar age of 475 6 ma is found for northwest africa 091 showing a temporal link to fossil meteorites in addition high temperature releases of northwest africa 091 contain evidence for a second trapped component and subtraction of this component indicates a possible second collisional impact at approximately 800 ma this earlier age coincides with 40ar 39ar ages of some h and l chondrites and lunar samples

depositional cycles and evolution of the paleogene wilcox strata gulf of mexico basin texas the wilcox group is one of the major thick clastic successions of the tertiary gulf coast the wilcox is conventionally subdivided into lower middle and upper parts detailed correlation of more than 700 well logs has identified two depositional cycles of the wilcox strata each of which is represented by four parasequence sets or high frequency genetic stratigraphic sequences two depositional cycles bounded by maximum flooding events were characterized by depocenter shifting shelf margin progradation and depositional system evolution and lasted several million years the sedimentary record of two depositional cycles documents two aspects of these cycles one aspect is that the shelf margin alternated between periods of active outbuilding and periods of relative stability or minor retrogradation another aspect of the cycle is that depositional system evolution displays a progression from progradation to aggradation to retrogressive transgression the time spans of these two cycles are 3 6 m y and 7 5 m y respectively based on the haq time scale each of the two depositional cycles generally corresponds to a pulse of laramide tectonism deposition of two cycles may also reflect a eustatic sea level overprint

upper wilcox rosita delta system of south texas growth faulted shelf edge deltas the rosita delta system a heretofore unrecognized major depositional system preserved in the deep upper wilcox of south texas was delineated by detailed correlation of approximately 500 well logs the rosita delta system comprises at least three delta complexes each of which can be traced up to tens of miles along strike and up to approximately 15 mi 24 km downdip basinward across the growth fault zone each delta complex thickens from about 600 ft 180 m to more than 3 000 ft 900 m the growth faults were activated by progradation of deltas over unstable prodelta slope muds at the contemporary shelf margin the three upper wilcox delta complexes studied in detail are from oldest to youngest the duval zapata and live oak deltas named for the counties in which they are centered each complex consists of several lobes some of which can be traced across the deep zones where the thickness increases by as much as tenfold owing to progradation over active growth faults characteristic coarsening upward progradational units are interpreted from electric log patterns to include prodelta shales delta front sandstones distributary channel and channel mouth bar sandstones and interdistributary shales and sandstones appreciable variability in sandstone distribution in the deltas may reflect changing importance of fluvial versus marine currents in distributing sediment along the delta front however all of the deltas prograded abruptly toward the shelf margin the gulfward extent of these delta complexes is unknown and it is concluded that appreciable quantities of sandstone remain to be explored in the deep subsurface of the trend

geology geography and humans battle for dominance over the delivery of fluvial sediment to the coastal ocean abstract sediment flux to the coastal zone is conditioned by geomorphic and tectonic influences basin area and relief geography temperature runoff geology lithology ice cover and human activities reservoir trapping soil erosion a new model termed bqart in recognition of those factors accounts for these varied influences when applied to a database of 488 rivers the bqart model showed no ensemble over or underprediction had a bias of just 3 across six orders of magnitude in observational values and accounted for 96 of the between river variation in the long term 30 years sediment load or yield of these rivers the geographical range of the 488 rivers covers 63 of the global land surface and is highly representative of global geology climate and socioeconomic conditions based strictly on geological parameters basin area relief lithology ice erosion 65 of the between river sediment load is explained climatic factors precipitation and temperature account for an additi

geochemistry of ferromanganese micronodules and associated mn and trace metals diagenesis at high terrigenous depositional site of middle fan region bay of bengal abstract the influence of sedimentation on the morphology growth and geochemistry of the micronodules and on mn enrichment in the sediments of the bay of bengal has been evaluated mn accumulation is 307 219 mg cm2 kyr 1 in cores having 20 17 cm kyr 1 sedimentation rate but drops to 34 17 7 mg cm2 kyr 1 in cores with 3 4 cm kyr 1 sedimentation rates however mn enrichment zone becomes thinner with increased sedimentation the abundance of the micronodules increases and they have been observed down to the deeper levels with higher contents of mn ni cu and zn from the areas of the lower sedimentation the results of the present study reveal that though the higher sedimentation may be conducive for higher mn accumulation in the upper surfaces of the sediments the growth abundance and enrichment of mn ni cu and zn in the micronodules are mostly regulated by their residence time within mn oxidation zone the influence of residence time on the morphology and the internal structures of the micronodules are also evaluated

towards a quantitative method for estimating paleohydrology from clast size and comparison with modern rivers abstract an outcrop based methodology is presented to reconstruct bankfull depth of fluvial channels from clast size based on well established geomorphological procedures results obtained from a lower old red sandstone ludlovian to emsian example from the midland valley basin scotland are calibrated against existing methodologies for reconstruction of bankfull channel depth in fluvial sandstones which utilize preserved channel thickness and preserved dune foreset height the proposed methodology can be applied to pebbly sandstones and conglomerates of fluvial origin it does include significant uncertainties but these are of a range similar to the existing approaches for reconstructing bankfull depth in sandy fluvial systems estimates of bankfull channel depth can be used to reconstruct paleo drainage area using paleo discharge data derived from geomorphological regional curves these regional hydraulic geometry curves are constructed for modern rivers with specific hydro physiographic catchment properties and constrain the relationship between bankfull depth and discharge and bankfull depth and drainage area by comparing the results of outcrop datasets with the regional curves from similar inferred climate regimes an estimate of paleo discharge and paleo drainage area can be derived utilizing these paleohydrological approaches from outcrops of fluvial strata allow quantitative constraints to be placed on paleogeographic reconstructions give a better understanding of paleoclimate and help to constrain provenance studies in addition comparison of the drainage areas and bankfull discharges obtained from this analysis for lower old red sandstone fluvial systems with global data for modern rivers reveals good agreement this indicates that using region specific hydraulic geometry curves to reconstruct ancient fluvial systems can be applied to large rivers interpreted to have a dryland setting possibly sourced from another climatic region as is commonly the case in modern dryland river systems it is recommended that by integrating several techniques more accurate estimates of paleohydrology can be obtained to better reconstruct ancient fluvial environments

relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material abstract chromite is the only common meteoritic mineral surviving long term exposure on earth however the present study of relict chromite from numerous ordovician 470 ma fossil meteorites and micrometeorites from sweden reveals that when encapsulated in chromite other minerals can survive for hundreds of millions of years maintaining their primary composition the most common minerals identified in the form of small analyses of recent meteorites holding both inclusions in chromite and corresponding matrix minerals show that for olivine and pyroxene inclusions sub solidus re equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former in the case of olivine the re equilibration has also affected the fayalite fa content lowering it with an average of 14 in inclusions for ca poor pyroxene the ferrosilite fs content is more or less identical in inclusions and matrix by these studies an analogue to the commonly applied classification system for ordinary chondritic matrix based on fa in olivine and fs in ca poor pyroxene can be established also for inclusions in chromite all olivine and ca poor pyroxene inclusions 1 5 m in chromite from the ordovician fossil chondritic material plot within the l chondrite field which is in accordance with previous classifications the concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates such as fossil meteorites and sediment dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts

the oxidation state of manganese in ferromanganese nodules and deep sea sediments from the central indian ocean abstract ferromanganese nodules from different sediment facies including siliceous ooze pelagic clay sediments from transition zone of siliceous and pelagic clay and seamount crusts from the central indian ocean were analysed for chemical composition mineralogy and bulk oxidation state of mn by the iodometric method despite considerable variation in chemical composition mn fe ratio times from 1 35 to 7 42 and mineralogy the o mn ratio or mno x solid hydrous manganese oxide does not vary much from 1 73 to 1 81 suggesting most of the mn in the manganese nodules in mn iv the oxidised upper zone in the sediment cores decreases in thickness from 15 cm in the north 5 s to 11 cm in the southern part 14 s of the central indian ocean iodometric titration of deep sea sediments yields o mn ratios of 1 29 1 95 sediments from the manganese nodule field have maximum solid mn concentrations and lower o mn ratios in the core top 0 3 cm with minimum mn and o mn ratios at 11 cm depth and thus may reflect the mechanism controlling the supply of diagenetic mn to the nodules in contrast sediments from the non manganese nodule field have comparatively low solid phase mn concentrations in the core top 0 3 cm and almost uniform o mn ratios with depth indicating that mn is not preserved in the sediments but may be remobilised and deposited somewhere else

the oxidation state of manganese in surface sediments of the deep sea iodometric titration of deep sea sediment from cores at manop sites m and h indicate a particulate mn oxidation state between 3 3 and 3 9 in the top 5 cm results from site h show a minimum in the mn oxidation state at 1 5 cm depth indicating mn reduction is occurring above the zone of pore water mn oxidation using a simple box model the rate of reduction is calculated to be 19 mg cm2 kyr 77 mg cm2 kyr at least 5 times the flux of mn to nodules in this region although no mn oxidation state minimum is observed in site m sediments oxic reduction is inferred from a particulate mass balance indicating that a large fraction of the mn rain to this site is not preserved in the sediments and must be remobilized we suggest that the process of reduction in oxic or suboxic environments near the sediment water interface may be an important mechanism controlling the concentration of mn in sediments and provides a mechanism for supplying diagenetic mn to the nodules at site h

determining the impactor of the ordovician lockne crater oxygen and neon isotopes in chromite versus sedimentary pge signatures abundant chromite grains with l chondritic composition in the resurge deposits of the lockne impact crater 458 myr old dia similar to 10 km in sweden have been inferred to represent relict fragments of an impactor from the break up of the l chondrite parent body at 470 ma this view has been challenged based on it cr and platinum group element pge patterns of the same resurge deposits and a reinterpretation of the origin of the chromite grains an impactor of the non magmatic iron meteorite type was proposed instead here we show that single grain oxygen and noble gas isotope analyses of the chromite grains from the resurge deposits further support an origin from an l chondritic asteroid we also present pge analyses and ir cr ratios for fossil l chondritic meteorites found in mid ordovician marine limestone in sweden the l chondritic origin has been confirmed by several independent methods including major element and oxygen isotopic analyses of chromite although the meteorites show the same order of magnitude pge and cr concentrations as recent l chondrites the elements have been redistributed to the extent that it is problematic to establish the original meteorite type from these proxies different pge data processing approaches can lead to highly variable results as also shown here for the lockne resurge deposits we conclude that the lockne crater was formed by an l chondritic impactor and that considerable care must be taken when inferring projectile type from pges in sedimentary ejecta deposits c 2011 elsevier b v all rights reserved less

high resolution depositional framework of the paleocene middle wilcox strata texas coastal plain the middle wilcox subgroup in the texas coastal plain of the gulf of mexico basin is consistently defined by maximum flooding surfaces associated with the big shale at the bottom and the yoakum shale at the top dated at 56 5 ma and 54 3 55 0 ma respectively two high resolution genetic stratigraphic sequences of the middle wilcox were delineated within the interval time span of 1 5 2 2 m y based on detailed correlation of approximately 700 well logs the sequence boundaries are identified as regionally continuous low resistivity markers within thin marine shale beds and are located stratigraphically between upward fining and upward coarsening log responses in the platform environment across the growth faulted shelf margin the sequence boundaries lie within thick mar ne shale wedges of the expanded downthrown sections each middle wilcox sequence has an average duration of about 0 75 1 1 m y and provides a high resolution stratigraphic framework for reconstructing depositional systems two high resolution genetic stratigraphic sequences a and b in ascending order are useful map units for facies mapping because they show gradational changes without pronounced relative base level falls displaying aggradational dominated stacking styles the interval isopach and net sandstone distributions of sequences a and b show no major sediment supply shift along strike during middle wilcox deposition downward shifting of deposition beyond the shelf margins occurred during slumping or relative sea level lowstand periods or when differential subsidence occurred within growth fault zones the middle wilcox sequ nce a contains an updip fluvial belt the calvert delta system the fayette strand plain system the san marcos barrier lagoon system the la salle delta system and downdip muddy shelf slope systems the middle wilcox sequence b resembles sequence a in that it includes an updip fluvial belt the calvert delta system and the la salle delta system however sequence b depocenters have prograded seaward from sequence a locations in the san marcos and rio grande areas the la salle delta system in sequence b is more extensive than that in sequence a and a third deltaic system named the wilson delta system localized in the san marcos area has replaced the interdeltaic barrier lagoon system of sequence a

noble gases in enstatite chondrites i exposure ages pairing and weathering effects cosmic ray exposure ages calculated from cosmogenic noble gas nuclides are reported for 57 enstatite e chondrites 43 of them were measured for the first time with a total of 62 individual e chondrites literature and this data corrected for pairing the observed spectrum of ages ranges between 0 07 and 66 ma three clusters seem to develop at about 3 5 8 and 25 ma respectively since the uncertainty of ages is estimated to be 20 in contrast to 10 to 15 for ordinary chondrites and the number of examined samples is still comparatively small these peaks have to be confirmed by more measurements regarding the two subgroups eh and el chondrites no systematic trend is apparent in the distribution of cosmic ray exposure ages several e chondrites yield significantly lower 38ar ages compared to those calculated from cosmogenic 3he and 21ne for these e chondrites we suggest a reduction of cosmogenic 38ar as a result of weathering in order to prove the possible influence of terrestrial alteration on the cosmogenic noble gas record of e chondritic material we simulated terrestrial weathering in an experiment of 12 weeks duration the treatment showed that a significant amount of cosmogenic 38ar is lost on earth by the influence of water

petrologic geochemical and experimental constraints on models of chondrule formation abstract the petrologic and geochemical properties of chondrules as well as results of experimental studies provide strong constraints on chondrule formation models nebular formation is indicated by the non mass fractionated oxygen isotopic compositions of bulk chondrules chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedral phenocrysts and glassy mesostases incomplete melting is indicated by the abundance of porphyritic chondrules which experiments demonstrate require relict nuclei and coarse relict grains the length of time that chondrules were hot is constrained by their retention of relict grains and moderately volatile elements rapid cooling of chondrules after formation is supported by the presence of zoned phenocrysts isotopic anomalies and dynamic crystallization experiments it is clear from the presence of relict grains enveloping compound chondrules and igneous rims that many chondrules were heated again after cooling the heating mechanism responsible for chondrule formation seems to have operated at varying intensities over large regions of the inner solar nebula for at least the time it took ambient nebular temperatures to cool from above 900 to below 600 k the chondrule formation mechanism provided a repeatable source of energy capable of highly localized melting characteristic of flash heating the occurrence of ferroan microchondrules with low melting temperatures within some chondrule rims indicates that chondrule formation did not occur exclusively in high temperature regions near the sun as required in bipolar outflow models mechanisms for forming chondrules that are consistent with the constraints include various flash heating models nebular lightning magnetic reconnection flares gas dynamic shock waves and radiative heating

a russian record of a middle ordovician meteorite shower extraterrestrial chromite at lynna river st petersburg region numerous fossil meteorites and high concentrations of sediment dispersed extraterrestrial chromite ec grains with ordinary chondritic composition have previously been documented from 467 1 6 ma middle ordovician darriwilian strata these finds probably reflect a temporarily enhanced influx of l chondritic matter following the disruption of the l chondrite parent body in the asteroid belt 470 6 ma in this study a volkhovian kundan limestone marl succession at lynna river northwestern russia has been searched for ec grains 63 mu m eight samples forming two separate sample sets were collected five samples from strata around the asaphus expansusa raniceps trilobite zone boundary in the lower middle kundan yielded a total of 496 ec grains in 65 5 kg of rock average 7 6 ec grains kg 1 but up to 10 2 grains kg 1 these are extremely high concentrations three orders of magnitude higher than background levels in similar condensed sediment from other periods ec grains are typically about 50 times more abundant than terrestrial chrome spinel in the samples and about as common as terrestrial ilmenite three stratigraphically lower lying samples close to the a lepidurusa expansus trilobite zone boundary at the volkhov kunda boundary yielded only two ec grains in 38 2 kg of rock 0 05 grains kg 1 the lack of commonly occurring ec grains in the lower interval probably reflects that these strata formed before the disruption of the l chondrite parent body the great similarity in ec chemical composition between this and other comparable studies indicates that all or most ec grains in these russian mid ordovician strata share a common sourcethe l chondrite parent body less

a critical evaluation of depositional parameters controlling the variability of organic carbon in arabian sea sediments abstract organic carbon distribution in the surficial sediments of the arabian sea to some extent mirrors the surface productivity being both high in the peripheral portions and low in the central region suggesting that the productivity is a primary source of organic matter however a critical examination reveals that the organic enrichment on the slope of the indian margin is two to four higher max 16 71 than on the slope of the arabian peninsula max 7 54 while the productivity in the former region is three to four times lower than the latter this observation suggests that the influence of productivity on organic enrichment is of secondary importance high concentrations 4 of organic carbon exactly coincide with the oxygen minimum zone 150 1500 m water depth evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon incidently the organic rich band along the western indian slope is wide long and highly concentrated as compared to that of the slope of the arabian peninsula and no organic enrichment is found on other continental slopes of the arabian sea although an equally intense oxygen minima impinge on the floor of the slope such a variability of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments rates of fine terrigenous sediment deposition shelf width slope gradient bottom currents and adsorption capacity of individual clay minerals all these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunction with various depositional parameters which determine the degree of preservation

determining chondrule size distributions from thin section measurements a method is described for correcting thin section derived chondrule sizes for three common sources of bias 1 the non equatorial sectioning of chondrules 2 the non zero thickness of thin sections and 3 the unequal probability of sectioning different size chondrules application of the correction procedure to chondrule data from co chondrites results in a reduction of mean and median chondrule diameters an increase in minimum diameters and transforms nearly log normal distributions to distributions that conform to a weibull probability function the weibull function form of co chondrule size distributions is similar to the form of chondrule distributions in ordinary chondrites obtained by disaggregation analyses

shelf margin architecture variability and its role in sediment budget partitioning into deep water areas abstract a review of 127 shelf margin accretion segments from 24 margins worldwide suggests that sediment flux can be low or high that shelf accommodation can be low medium or high and that climatic conditions can be icehouse or greenhouse the interplay of these three variables yields four main greenhouse margins and six main icehouse margins and thus has a fundamental control on shelf margin architectural styles providing new tools for predicting internal architecture of deep water systems in greenhouse settings there is commonly a strong supply drive because of higher temperatures more variable precipitation and higher sediment discharge but only modest shelf accommodation because of low frequency and low amplitude eustatic sea level changes low shelf accommodation is able to deliver significant volumes of sand sourced from either low or high sediment supply areas to deep water environments and residence time for deltas at greenhouse margins is likely to be relatively long because of the low amplitude of greenhouse sea level changes medium accommodation in contrast may overwhelm any low sediment supply but could be partly overwhelmed by high sediment supply which in turn can inhibit fans or encourage highstand fans in icehouse settings there is a strong cross shelf accommodation drive created by high frequency and high amplitude eustatic sea level changes thus causing significant and frequently repeated transits of river mouths and deltas across the shelf sandy basin floor fans created by low accommodation are predicted with either high or low sediment flux both medium and high accommodation can be volumetrically overwhelmed by high sediment supply which in turn cause greater volumes of sediment storage on the shelf resulting in volumetrically less significant highstand or transgressive submarine fans however a low supply may barely allow deltas to reach the shelf edge causing the deep water areas to be dominantly muddy

the distribution of rare earth elements in manganese micronodules and sediments from the equatorial and southwest pacific abstract thirty three manganese micronodules and 38 associated sediments from selected areas in the equatorial and southwest pacific have been analyzed for ree significant variations in composition of both micronodules and sediments between the areas are observed in general the avarage ree abundance in manganese modules in a given area is higher than that of the associated micronodules but the ce la ratio of the micronodules is in some cases higher than that of the nodules the avarage ce la ratio of the micronodules is always higher than that of the associated sediment but the ree abundance is lower in several cases the ce anomaly is dominantly positive for micronodules and negative for sediments negative ce anomalies are however found in micronodules from the crest of the east pacific rise epr where metallogenesis is taking place positive ce anomalies occur in sediments from the southwestern pacific basin between the epr and new zealand the highest ce la ratios of both micronodules and sediments are found in this region reflecting the well oxidized conditions prevailing there

paleocene wilcox cross shelf channel belt history and shelf margin growth key to gulf of mexico sediment delivery abstract shelf margins prograde and aggrade by the incremental addition of deltaic sediments supplied from river channel belts and by stored shoreline sediment this paper documents the shelf edge trajectory and coeval channel belts for a segment of paleocene lower wilcox group in the northern gulf of mexico based on 400 wireline logs and 300 m of whole cores by quantitatively analyzing these data and comparing them with global databases we demonstrate how varying sediment supply impacted the wilcox shelf margin growth and deep water sediment dispersal under greenhouse eustatic conditions the coastal plain to marine topset and uppermost continental slope succession of the lower wilcox shelf margin sediment prism is divided into eighteen high frequency 300 ky duration stratigraphic sequences and further grouped into 5 sequence sets labeled as a e from bottom to top sequence set a is dominantly muddy slope deposits the shelf edge of sequence sets b and c prograded rapidly 10 km ma and aggraded modestly 80 m ma characterizes sequence sets d and e which is associated with smaller 9 10 m thick on average and isolated channel belts this stratigraphic trend is likely due to an upward decreasing sediment supply indicated by the shelf edge progradation rate and channel size as well as an upward increasing shelf accommodation indicated by the shelf edge aggradation rate the rapid shelf edge progradation and large rivers in sequence sets b and c confirm earlier suggestions that it was the early phase of lower wilcox dispersal that brought the largest deep water sediment volumes into the gulf of mexico key factors in this lower wilcox stratigraphic trend are likely to have been a very high initial sediment flux to the gulf because of the high initial release of sediment from laramide catchments to the north and northwest possibly aided by modest eustatic sea level fall on the texas shelf which is suggested by the early flat shelf edge trajectory high amalgamation of channel belts and the low overall aggradation rate of the sequence sets b and c

a 3 d study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation x ray tomographic microscopy method and applications abstract a method is described for imaging in 3 d the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation x ray tomographic microscopy in ordinary chondrites chromite is the only common mineral that survives long term weathering on earth information about the silicate matrix of the original meteorite however can be derived from mineral inclusions preserved in the protecting chromite the inclusions are crucial in the classification of fossil meteorites as well as sediment dispersed chromite grains from decomposed meteorites and larger impacts as these are used for characterizing the past influx of material to earth but have previously been difficult to locate the method is non destructive and time efficient for locating inclusions the method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions the study of 385 chromite grains from eight chondrites h4 6 l4 6 ll4 ll6 reveals that inclusions are abundant and equally common in all samples almost two thirds of all chromite grains contain inclusions regardless of group and type the study also shows that the size of the inclusions and the host chromite grains as well as the number of inclusions within the host chromite grains vary with petrographic type thus the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content the study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites the study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite meaning that this technique is essential in the studies of ancient meteorite flux to earth

extraterrestrial chromite in middle ordovician marine limestone at kinnekulle southern sweden traces of a major asteroid breakup event the distribution of sediment dispersed extraterrestrial chromite grains and other cr rich spinels 63 mu m has been studied in middle ordovician orthoceratite limestone from two quarries at kinnekulle southern sweden in the thorsberg quarry all similar to 3 2 m thick sequence of beds previously shown to be rich in fossil meteorites is also rich in sediment dispersed extraterrestrial chromite grains typically 1 3 grains are found per kilogram of limestone in the nearby hallekis quarry the same beds show similarly high concentrations of extraterrestrial chromite grains but in samples representing the 9 m downward continuation of the section exposed at this site only 5 such grains were found in a total of 379 kg of limestone the extraterrestrial equilibrated ordinary chondritic chromite grains call be readily distinguished by a homogeneous and characteristic major element chemistry including 2 0 3 5 wt tio2 and stable v2o3 concentrations close to 0 7 wt terrestrial cr rich spinels have a wide compositional range and co exist with extraterrestrial chromite in some beds these grains may be derived for example from mafic dykes exposed and weathered at the sea floor considering lithologic and stratigraphic aspects variations in sedimentation rate cannot explain the dramatic increase in extraterrestrial chromite seen in the upper part of the composite section studied instead the difference may be primarily related to all increase in the ancient flux or extraterrestrial matter to earth in connection with the disruption of the l chondrite parent body in the asteroid belt at about this time the coexistence in some beds of high concentrations of chondritic chromite and terrestrial cr rich spinels however indicates that redistribution of heavy minerals on the sea floor related to changes in sea level and sea floor erosion and currents must also be considered less

thick turbidite successions from supply dominated shelves during sea level highstand emphasis on the association between relative sea level lowstand and the formation of sandy deep water fans has tended to downplay the significance of high sediment supply and its potential to create deep water fans even during sea level highstands the lance fox hills lewis shelf margin in southern wyoming suggests that high supply was critical in causing the accretion of this moderately wide maastrichtian shelf margin at a minimum rate of 47 8 km m y and the generation of large sand rich fans during every shoreline regression across the shelf it is surprising that fans developed from shelf margin clinoforms that show systematically rising shelf edge trajectories proxy for rising relative sea level as well as from those that show flat trajectories stable to falling relative sea level however the latter producing more sediment bypass resulted in bigger and thicker fans whereas the former produced somewhat smaller and thinner fans we term the former highstand fans and suggest caution in using the lowstand model for high supply systems

extraterrestrial chromite distribution across the mid ordovician puxi river section central china evidence for a global major spike in flux of l chondritic matter previous studies of mid ordovician limestone in sweden have shown that over a stratigraphic interval representing a few million years there is a two orders of magnitude enrichment in fossil l chondritic meteorites empty set 1 21 cm and sediment dispersed extraterrestrial chromite ec grains 63 mu m this has been interpreted as a dramatic increase in the flux of l chondritic matter to earth following the breakup of the l chondrite parent body which based on ar ar gas retention ages 470 6 ma of recently fallen meteorites occurred at about this time here we show that the general trend in the distribution of sediment dispersed ec grains can be reproduced in the puxi river section in central china a total of 288 kg of limestone was searched for chrome spinels in samples spanning the lower 8 m of the section representing the paroistodus originalis and lenodus antivariabilis conodont zones a total of 110 kg of limestone yielded only one ec grain similarly to the swedish sections ec grains begin to be common in the overlying l variabilis zone and remain common throughout the upper 9 m of the section representing the l variabilis yangtzeplacognathus crassus and l pseudoplanus zones in this part of the section 178 kg of limestone yielded 290 ec grains with an average chemical composition very similar to chromite from recent l chondrites in most of the beds over this interval one finds 1 4 ec grains per kilogram rock a clear two orders of magnitude enrichment relative to the lower part of the section small bed by bed variations in the ec content over the upper interval most likely reflect small variations in sedimentation rates the puxi river section contains only very rare terrestrial chrome spinel grains which can be distinguished already by their rounded abraded appearance compared to the angular pristine extraterrestrial spinels in the mid ordovician based on paleoplate reconstructions the puxi river site was positioned at mid latitudes on the southern hemisphere a couple of thousand kilometers east of the swedish sites the prominent enrichment of ec grains over the same stratigraphic interval in china and sweden is supporting evidence for a dramatic increase in the flux of l chondritic matter to earth shortly after the disruption of the l chondrite parent body in the asteroid belt c 2010 elsevier inc all rights reserved less