late holocene continental shelf sediments off cuddalore east coast bay of bengal india geochemical implications for source area weathering and provenance in the present study a 60 cm sediment gravity core collected off the cuddalore shore 11 42 419 n 79 57 993 e from a water depth of 150 m was radiocarbon dated and analysed for major elements si al na k ca mg and p trace elements fe cr mn co ni cu zn and pb calcium carbonate and organic matter content to understand their behavior and probable sources and also to understand the monsoon variability the sediment core has been radiocarbon dated from around 4100 to 3100 bp high sand around 4000 3700 1500 bp with low caco3 content suggests high sediment flux coupled with higher precipitation corresponding to increased erosion during these periods and more fresh water runoff from rivers in the late holocene thus reflecting intensification of the sw and ne monsoon the results of the major elemental analyses show the fine grained detrital nature of sediments with high values of si and lower values of al and fe the present study clearly demarcates that the distribution patterns of cr cu pb and zn that are controlled by the distribution of the fine grained fractions the increased mn content towards the lower end of the sediment core denotes the allogenic supply of mn from oxic to sub oxic recycling within the sediment column in fine grain sediments the mean cia values 56 47 reflect immature moderately weathered sediments that are mostly derived from sandstone and clay

carbonate and carbon fluctuations in the eastern arabian sea over 140 ka implications on productivity changes biological productivity in the western arabian sea was higher during interglacial than glacial times in the eastern arabian sea productivity was higher during the glacials compared to interglacials which is in sharp contrast to the southwest monsoon intensity variations to examine temporal changes in productivity in the eastern arabian sea over the last 140 ka oxygen isotopes calcium carbonate and organic carbon on three cores sl 1 4 and sk 129 cr05 were analyzed oxygen isotope records display distinct glacial and interglacial transitions in the northeastern core sl 1 and eastern arabian sea core sl 4 both calcium carbonate and organic carbon variations show no significant systematic relationship with glacial and interglacials periods in the southeastern arabian sea core sk 129 cr05 calcium carbonate shows high and low values during interglacial and glacials respectively and temporal changes in organic carbon concentration are significant only during mis 5 differential variation of calcium carbonate and organic carbon concentration at the northeastern and southeastern arabian sea and between glacials and interglacials are attributed to regional differences in sedimentation rates dilution and preservation which modify the signal of carbonate and carbon production

characterization of seafloor pockmark seepage of hydrocarbons employing fractal a case study from the western continental margin of india here we apply quantitative technique to describe the seafloor seepages based on the multibeam backscatter and bathymetric investigations to characterize the pockmark morphology the variable seafloor backscatter strength for coarser seafloor sediments are related to the diagenesis derived from biodegraded seepages in this regard box counting method is used to estimate fractal dimension for backscatter imagery data of 398 blocks these blocks are further sub grouped into six classes depending on the spread of pockmark related seepages the study area lies 102 km west off marmagao along the central west coast of india which contains pre dominantly 70 gas charged sediments comparison between the estimated self similar fractals reveals that there is approximately 97 correlation between the box dbox and information dinfo dimensions box dimension derived fractal dimension values suggest that the seepages are more along the fault trace in deeper waters in comparison to sparsely distributed shallow water seepages besides this poor seepage is confined within the smooth to moderately rough seafloor it is established that the high backscatter strength along the upper slope of the pockmark region having higher fractal dimensions reflects multifractal behavior of seepage distribution entire area indicates patchy seepage patterns as supported by estimated fractal values showing intermittent fluctuations which emphasizes non linear behavior estimated self organizing criticality soc parameters for six representative blocks reveal that the nature of pockmark fault trace sediment nature coupled with slumping of pockmark s wall sediment movement due to bottom currents are controlling the dynamic balance in the area seepage system further our study emphasizing the multifractal behavior of seepage blocks clearly depicts the drift in the seepage pattern

formation and evolution of lateritic profiles in the middle amazon basin insights from radiation induced defects in kaolinite abstract the content of radiation induced defects rids in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle amazon basin brazil is investigated using electron paramagnetic resonance the paleodose registered by kaolinites ranges from 80 to 900 kgy present day dose rates of radiation determined from the whole rock u and th content range between 4000 and 40 000 mgy ka in most samples u and th concentrations are correlated suggesting that u has not been remobilized by lateritization this observation is consistent with the fact that 80 of the total u content is incorporated in resistant minerals such as zircon and ti oxides the heterogeneous distribution of u observed by induced fission tracks mapping makes it possible to neglect the radiation contribution of the u decay chains in the dose rate calculation the interpretation of the measured content of rids in kaolinite is then performed using the calculated present day dose rate and assuming equilibrium in the radioactive decay chains for the sedimentary samples the amount of rids is broadly correlated to the dose rate and provides apparent absolute ages older than 20 ma the rid contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 ma the high rid content of these lateritic kaolinites shows that their chemical isotopic and crystallographic properties are not representative of present day weathering conditions models assuming the dynamical equilibrium of kaolinites with local physical chemical conditions prevailing in lateritic soils are thus questionable alternatively our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo climates

lack of enhanced preservation of organic matter in sediments under the oxygen minimum on the oman margin the impingement of oxygen minima on continental margins is widely thought to promote the accumulation of sedimentary facies enriched in well preserved organic matter it is shown here however that such a relationship does not clearly apply to the productive oman margin in the arabian sea which hosts one of the most severe oxygen minima in the oceans measurements made on the 0 1 cm depth interval from fourteen box cores collected from the outer shelf upper continental slope area off oman show that 1 1 deposited organic matter is overwhelmingly of marine origin 2 2 there is no significant correlation between the abundance of sedimentary organic carbon corg and the bottom water o2 concentration 3 3 there is no relation between the sedimentary corg n ratio and bottom water o2 4 4 there is no correlation between the hydrogen index hi of the organic matter and bottom water oxygen there are however significant correlations between the corg n ratio and the i corg cr al and zr al ratios as well as between the corg n ratio and the hydrogen index these relationships are interpreted as being the result of winnowing and attendant reworking which produce a progressive increase in the corg n ratio and a decrease in the hi and the i corg ratio as organic matter becomes progressively degraded commensurate rises in the cr al and zr al ratios result from the increased proportion of heavy minerals associated with winnowed lag deposits overall these data suggest that the bottom water oxygen concentration has little effect in governing either the distribution or the degree of preservation of organic matter on this margin thus the generally high but spatially variable corg content of the sediments on the oman margin may not reflect the occurrence of an oxygen minimum but instead be the result of a high settling flux of organic matter supported by monsoon driven upwelling and post depositional redistribution of the organic material by hydrodynamic influences

lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the gulf of california a test of the hypothesis that hydrogen rich organic matter accumulates preferentially in sediments under oxygen minima has been carried out by examining the composition of laminated and homogeneous diatomaceous sediments of the guaymas basin in the gulf of california the laminated sediments occur where an intense oxygen minimum intersects the slope around the margins of the basin above and below the depth of the oxygen minimum the sediments are homogenized by a burrowing infauna the two sediment types have very similar bulk chemical and mineral compositions and the organic matter is overwhelmingly marine the total organic carbon contents and rock eval parameters hydrogen and oxygen indices of the two sediment types are not significantly different whereas a road organic carbon maximum in the homogeneous sediments is centered at roughly 1200 m depth on the slope of the basin this maximum lies deeper than the core of the oxygen minimum thus both the concentration and the nature of the organic matter in the sediments of this highly productive marginal marine basin are not related to the concentration of dissolved oxygen in the bottom waters

lack of organic matter accumulation on the upwelling influenced somalia margin in a glacial interglacial transition abstract monsoon induced upwelling has been operating off somalia for at least the last 160 000 years making the nw indian ocean one of the most productive oceanic areas in the world we have studied the sediments recovered during the indusom cruise of the r v marion dufresne from three different environments off this margin a core from the highly productive surface waters core md85 674 a core where the oxygen minimum intersects the slope core md85 664 and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production core md85 668 though the first two environments are a priori favourable to organic matter om accumulation no marine om enrichment can be observed in the cores to understand this anomaly emphasis is placed upon the transition from isotopic stage 6 to stage 5 because the end of the glacial stage 6 is characterised by intensified upwelling and primary productivity in this region whereas stage 5 experienced a marked decrease in productivity various palaeoenvironmental markers such as om v ni cu zn ba and p have been used to identify periods of high productivity and or bottom water oxygen depletion the results suggest that 1 the lack of marine om in two settings favourable to organic carbon storage may be due to the nature of primary productivity coccolithophorid production would not be prone to om accumulation in contrast to the contribution of om by diatoms or particularly naked phytoplankton dinoflagellates or other peridinians 2 ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of ba as a proxy indicator 3 the period of intensified upwelling in stage 6 as well as the stage 6 5 transition left no geochemical imprint on the sediments this means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments in other words the somalia upwelling does not participate to the so called biological pump and has no positive effect upon long term carbon storage

late quaternary protoperidinium cysts as indicators of paleoproductivity in the northern arabian sea the reliability of organic walled cysts of the heterotrophic dinoflagellate protoperidinium as paleoproductivity indicators and the influence of bottom water oxygenation on cyst preservation is assessed by using arabian sea records of the past 125 kyr as a natural laboratory multidisciplinary geochemical micropaleontological and palynological datasets are integrated to analyze the relationship between protoperidinium cyst concentrations and other paleoproductivity proxies differential preservation potential is quantified in order to establish threshold oxidative degradation values for a possible application of quantitative protoperidinium cyst records in paleoenvironmental reconstructions results indicate that variations in protoperidinium cyst concentration closely correspond to other marine productivity and or upwelling proxies although oxygenation will lead to significant cyst degradation and thus decreased concentrations down core patterns in protoperidinium cyst concentration still primarily reflect changes in sea surface productivity in view of differential preservation among dinoflagellate cysts down core variations in relative abundance of protoperidinium should be treated with caution

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

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

morphology of pockmarks along the western continental margin of india employing multibeam bathymetry and backscatter data this study addresses the morphology of pockmarks along the western continental margin of india using multibeam bathymetry and backscatter data here for the first time we have utilized the application of arcgis geographical information system for understanding the morphology of pockmarks from the western continental margin of india the pockmarks observed in water depths of 145 330 m are circular elliptical or elongated in plan view with an average length and width of 157 72 m and 83 19 m respectively the average pockmark relief and perimeter are 1 9 0 9 m and 412 181 m respectively the pockmarks have average areas and volumes of 10 759 m and 15 315 m respectively spatial separation that coincides with 210 m isobath divides the pockmarks into two groups with differing distributions and morphologies these pockmarks originated from seepages of biogenic or thermogenic gas or from pore fluids from deeper sources migrated vertically along the faults besides a possible structural control the pockmark morphologies are also affected by bottom currents and or by submarine slumping the average acoustic backscatter strength from pockmark centre is higher 35 db than the average backscatter of the total area 40 db which suggests their possible linkage to the precipitation of diagenitic minerals from biodegradation of seepage material

occurrence and exploration of gas hydrate in the marginal seas and continental margin of the asia and oceania region supplies of conventional natural gas and oil are declining fast worldwide and therefore new unconventional forms of energy resources are needed to meet the ever increasing demand amongst the many different unconventional natural resources are gas hydrates a solid ice like crystalline compound of methane and water formed under specific low temperature and high pressure conditions gas hydrates are believed to exist in large quantities worldwide in oceanic regions of continental margins as well as associated with permafrost regions in the arctic some studies to estimate the global abundance of gas hydrate suggest that the total volume of natural gas locked up in form of gas hydrates may exceed all known conventional natural gas reserves although large uncertainties exist in these assessments gas hydrates have been intensively studied in the last two decades also due to connections between climate forcing natural and or anthropogenic and the potential large volumes of methane trapped in gas hydrate accumulations the presence of gas hydrate within unconsolidated sediments of the upper few hundred meters below seafloor may also pose a geo hazard to conventional oil and gas production additionally climate variability and associated changes in pressure temperature regimes and thus shifts in the gas hydrate stability zone may cause the occurrence of submarine slope failures several large scale national gas hydrate programs exist especially in countries such as japan korea taiwan china india and new zealand where large demands of energy cannot be met by domestic supplies from natural resources the past five years have seen several dedicated deep drilling expeditions and other scientific studies conducted throughout asia and oceania to understand gas hydrates off india china and korea this thematic set of publications is dedicated to summarize the most recent findings and results of geo scientific studies of gas hydrates in the marginal seas and continental margin of the asia and oceania region

esr dating in quaternary geology abstract electron spin resonance esr a relatively new dating technique was first suggested by zeller et al 1967 zeller 1968 and levy 1968 it has made substantial progress in frequent applications in the last few years esr dating is based on the direct measurement of the amount of radiation induced paramagnetic electrons trapped in crystal defects these free electrons were generated by alpha beta and gamma radiation of natural radioelements e g u th k and have accumulated in the minerals over geologic time the esr age is obtained by dividing the total amount of accumulated radiation dose archaeological dose ad by the dose per year annual dose this review comprises four major parts the principles of the method the determination of the archaeological dose ad the determination of the annual dose and a compilation of datable materials several questions remain the evaluation of the ad the determination of the annual dose and particularly with respect to the nature and stability of the electron traps used for dating

radiation induced defects in kaolinites indirect assessment of radionuclide migration in the geosphere abstract defect centres are demonstrated in natural kaolinites by means of electron paramagnetic resonance the irradiation origin of these paramagnetic defect centres pdc is demonstrated by artificial irradiation four types have been used x rays rays he and pb2 ion beam implantation of kaolinites representing different formation conditions hydrothermal weathering synthesis several types of pdc designated a a and b centres are clearly identified and can be separated with respect to their relative stability one a centre is stable over geological periods moreover it is shown that defect centres in kaolinites reflect effectively the cumulative radiation dose the potential use of tracing radionuclide migration through pdc in kaolinite has been tested on three low temperature alteration systems some associated with u concentrations 1 a hydrothermal alteration system nopal i chihuahua mexico and 2 two weathering systems in tropical areas latentes from brazil and cameroon the kaolinites investigated show a significant concentration of pdc which corresponds with background natural irradiation the different types of pdc detected may form a record of successive irradiations of kaolinite linked directly to geochemical conditions prevailing during and after kaolinite growth because of their different thermal stability these different centres can be considered as being formed at different periods this makes a unique tool for tracing the dynamics of the transfer of radionuclides and allows the use of kaolinite as a very sensitive in situ dosimeter because some of the geological settings chosen for nuclear wastes are located in environments containing kaolinites pdc dosimetry could be directly useful to a safety assessment of these wastes

the ratio in the cenozoic bengal abyssal fan sediments and its use as a paleostream energy indicator abstract the tio 2 al 2 o 3 ratio in the cenozoic bengal abyssal fan sediments displays decisive potential as a paleostream velocity indicator the reason for this is that even in the finest fan suspendates hydrodynamic sorting determines the amounts of heavy tio2 rich minerals relative to lighter al2o3 rich clay minerals in five deep sea drilling project cores 213 217 which have been recovered in and south of the bay of bengal tio 2 al 2 o 3 ratios increase linearly with time from the late miocene to recent the increase reflects fan progradation and intensified bottom current activity on the southern bengal fan these processes are related to the himalayan elevation by factors such as the maturation of northern india river systems evolution of orographic monsoon rains and elevation attributed increases in denudation rates

mid late pleistocene osl chronology in western amazonia and implications for the transcontinental amazon pathway abstract the origin of the transcontinental amazon drainage system remains unrevealed sedimentary deposits formed from the neogene in the amazonas and solimoes basins constitute natural archives for reconstructing this event in space and time however paleoenvironmental and chronological analyses focusing on these deposits or even their basic mapping are still scarce to allow such investigation in this context primary interests are fluvial strata related to the lithostratigraphic ica formation mapped over a widespread area in western amazonian lowlands although long regarded as plio pleistocene in age this unit has not yet been dated and its overall depositional setting remains largely undescribed the main goal of the present work is to contribute for improving facies analysis and chronology of these deposits approaching an area in southwestern amazonia and another in northern amazonia which are located more than 1000 km apart despite this great distance the sedimentological and chronological characteristics of deposits from these two areas are analogous hence facies analysis revealed paleoenvironments including active channel abandoned channel point bar crevasse splay and floodplain which are altogether compatible with meandering fluvial systems similarly optically stimulated luminescence osl dating revealed thirty three ages ranging from 65 4 16 9 to 219 6 25 1 ky in addition to three outliners of 54 0 7 6 337 3 36 9 and 346 6 48 6 ky and nine 97 1 9 9 to 254 8 23 8 ky for the areas in southwestern and northern amazonia respectively these data lead to establish that deposits mapped as ica formation over a vast area of western brazilian amazonia have a mid late pleistocene age rather than the previously inferred plio pleistocene age it follows that if plio pleistocene deposits exist in this region they remain to be dated and must be restricted to a narrow belt in western amazonia as well as isolated occurrences underlying the mid late pleistocene strata characterized herein the combination of data from this work with previously published provenance studies supports main andean sediment sources only in the mid late pleistocene it is proposed that before this time the amazon river was restricted to eastern amazonia being separated from western amazonian drainage basins due to the presence of the purus arch erosion and or subsidence of this geological feature would have promoted the connection of these drainage systems ultimately with the expressive record of the transcontinental amazon pathway into the atlantic ocean in the mid late pleistocene

late cretaceous upwelling system along the southern tethys margin israel interrelationship between productivity bottom water environments and organic matter preservation organic rich upper cretaceous sequences in israel were deposited in an extensive highly productive upwelling linked system which prevailed along the southern tethys margin and lasted for 19 m y santonian to late maastrichtian an understanding of the spatial and temporal characteristics of this system was gained through detailed paleontological and geochemical analyses of subsurface sequences in two basins in israel representing an outer shefela and an inner zin facies belt the nature of the upwelling system and its effect on the sedimentary record is related to two basic environmental parameters namely paleoproductivity intensity and oxygen levels at the bottom the assessment of these parameters and their interrelationship has been performed through the development of paleontological foraminiferal criteria which are independent of the organic matter content following the establishment of these criteria it is concluded that the productivity reached its maximum intensity during the late campanian which was also the time of most notable differentiation between the center of the upwelling system in the inner belt and the less intense conditions in the outer basin this distribution is expressed in varied lithology organic rich carbonates phosphorites and siliceous rocks at the core of upwelling and a uniform lithology organic rich carbonates at the margin of this system the uniform lithology of the maastrichtian in both basins composed of organic rich carbonates is ascribed to a gradual weakening of productivity the bottom conditions in the inner belt during the late campanian the time of maximum surface productivity were near anoxic changing to more aerated dysaerobic conditions during the early maastrichtian in the outer belt a more aerated bottom dysaerobic prevailed throughout the late campanian to late maastrichtian the elevated organic matter content in both basins reflects the overall environment of high productivity its actual variations however seem to be unrelated to changes in surface productivity and bottom oxygen levels

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

the importance of the oxygen minimum zone and sediment geochemistry in the distribution of recent benthic foraminifera in the northwest indian ocean abstract the relationship between the distribution of benthic foraminifera and sediment type and depositional environment in the arabian sea is discussed the benthic foraminiferal fauna were sampled in nineteen recent surface sediment samples and geochemical variables of the sediment of the same samples were measured the water depths for the box core samples varies from 440 to 4040 m a total of 103 species and six species complexes were identified the geochemical properties were found to correspond well to the sediment type and depositional environment and six different sediment depositional environment types could be distinguished analysis of the benthic foraminiferal fauna reveals specific faunal assemblages that are closely related to these sediment depositional environment types

combined dating of goethites and kaolinites from ferruginous duricrusts deciphering the late neogene erosion history of central amazonia abstract this study focuses on the dating of millimetric pisoliths from ferruginous duricrusts located in central amazonia brazil by coupling detailed mineralogy and two relevant dating methods i u th he analysis of goethites by mass spectrometry ii analysis of radiation induced defects of kaolinites embedded in duricrusts by electron paramagnetic resonance spectroscopy epr three samples collected at different elevations in the landscape were selected the goethite corrected ages range from 1 1 15 2 ma and consistently increase with elevation epr was performed on kaolinites after several cycles of a deferration procedure considering extreme geochemical scenarii the ages of kaolinites are 1 7 7 1 ma for a closed system and 4 0 16 7 ma for an open system with 100 rn loss ages of goethites and kaolinites are close in the limit of the uncertainty of the methods they show important periods of duricrust formation at middle miocene and at late miocene pliocene data allow the estimation of lowering rates between 3 and 8 m ma in agreement with independent data available in the literature for the central amazonia region or even for erosion of other cratons in the world the two dating methods can be compared on common duricrust samples by revealing single or multi step weathering erosion episodes related to relictual paleosurfaces they also bring promising contributions to an advanced knowledge of ancient and complex lateritic geosystems

reactivation of precambrian faults on the southwestern continental margin of india evidence from gravity anomalies abstract gravimetric and bathymetric studies on the southwestern continental margin of india confirm the extension of onshore nw se nnw sse n s ne sw ene wsw and e w trending lineaments of precambrian age over a considerable distance into the offshore region the bight in the bathymetry off coondapoor the offsets of bessas de pedro bank and the cora divh island of the laccadive group the prathap ridge and the inferred mid shelf basement ridge suggest block movements on the southwestern continental margin the physiographic expression on the prathap ridge around 14 20 n and 72 50 e is unaffected by some of the ene wsw lineaments which probably indicates that these lineaments predate the evolution of the topographic expression as seafloor spreading advanced with respect to the carlsberg ridge some of the ene wsw and ne sw lineaments on the western continental margin appear to have been reactivated and block movements took place the presence of a basement ridge in the mid shelf and the shelf margin basement high prathap ridge west of the slope resembles the structural style of a passive continental margin

dating kaolinite from the neogene ica formation and overlying laterites central amazonia brazil constraints for a stratigraphic correlation abstract the neogene ica formation is a major sedimentary deposit in central amazonia brazil that is critical to the understanding and modeling of the evolution of the amazon basin however its chronostratigraphy is poorly defined mostly because it is not fossiliferous and it does not contain volcanic ash levels this study is the first attempt at the absolute dating of the sediments and overlying soils of the ica formation by electron paramagnetic resonance epr of radiation induced defects in kaolinite rids four profiles of unconsolidated sediments overlaid by lateritic soils from the barcelos region amazonia brazil were studied the concentration of rids was assessed from epr spectra through a simplified procedure on a selected set of samples excluding those showing too strong of interference from the signal of trace vanadyl complexes it is shown that in such sedimentary material both the microscopic distribution of thorium together with uranium and the origin of kaolinite either detrital or authigenic are critical for kaolinite dating considering an argued authigenic origin and two scenarios of th distribution it appears that most of the present chronologic data support the stratigraphic correlation with the madre de dios sedimentary formation in peru previously proposed in the literature for the ica formation 9 5 2 5 ma this study also shows a rejuvenation process of kaolinite both in the uppermost soil and at depth of the profiles this rejuvenation occurred during paleoclimatic periods already revealed in other weathering covers from the amazon basin and possibly resulted from downward weathering in the topsoil and water table or lateral weathering at depth

distribution and sources of organic matter in surface sediments of the eastern continental margin of india the sources and distribution of organic matter om in surface sediments of the eastern continental margin of india including the region influenced by river discharge were investigated using content molar c n ratios and stable isotopes of carbon and nitrogen despite relatively high water column integrated chlorophyll a concentrations were found in the continental shelf than the slope however the lower sediment organic carbon soc was found in the former than the latter region suggesting that in situ production did not play significant role on preservation of soc in the coastal bay of bengal the broad range of 13 c of soc 23 2 to 16 7 suggests that om is a broad mixture of terrestrial and marine om relative contributions from terrestrial c3 and c4 plants and marine sources are quantified as 34 23 and 43 respectively indicating that dominant source of allochthonous om 57 in the coastal bay of bengal relatively higher contribution of om from c4 plants were found in the sediments at off river krishna indicating that this region received detritus of agricultural crops such as jowar bajra and sugar cane which are dominant in its drainage basin during sw monsoon this study revealed that relatively high om preserved in the slope than shelf region along the coastal bay of bengal and the composition of om is primarily controlled by the type of agricultural crops and vegetation in the drainage basin of the river

sedimentary pigments and nature of organic matter within the oxygen minimum zone omz of the eastern arabian sea indian margin abstract sedimentary pigments carbon and nitrogen content and their stable isotopes were studied in three short cores collected from the oxygen minimum zone omz of the eastern arabian sea eas nine pigments including chlorophyll a and their degradation products were quantified using high performance liquid chromatography hplc astaxanthin followed by canthaxanthin and zeaxanthin were the major carotenoids detected in these cores the total pigment concentration was high in the core collected from 500 m water depth 6 5 gg 1 followed by 800 m 1 7 gg 1 and 1100 m 1 1 gg 1 depths respectively the organic carbon did not have considerable control on sedimentary pigments preservation pigment degradation was comparatively high in the core collected from the 800 m site which depended not only the bottom dissolved oxygen levels but also on the faunal activity as reported earlier the bottom water dissolved oxygen and presence of fauna have good control on the organic carbon accumulation and preservation at indian margin omz sediments the c n ratios and 13 c values for all the cores conclude the marine origin of organic matter and 15 n profiles revealed signature of upwelling associated denitrification within the water column

depletion of 13c in cretaceous marine organic matter source diagenetic or environmental sigal geochemical studies of cretaceous strata rich in organic carbon oc from deep sea drilling project dsdp sites and several land sections reveal several consistent relationships among amount of oc hydrocarbon generating potential of kerogen measured by pyrolysis as the hydrogen index hi and the isotopic composition of the oc first there is a positive correlation between hi and oc in strata that contain more than about 1 oc second percent oc and hi often are negatively correlated with carbon isotopic composition 13c of kerogen the relationship between hi and oc indicates that as the amount of organic matter increases this organic matter tends to be more lipid rich reflecting the marine source of the organic matter cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter average 13c values for organic matter from most cretaceous sites are between 26 and 28 and values heavier than about 25 occur at very few sites most of the 13c values of miocene to holocene oc rich strata and modern marine plankton are between 16 to 23 values of 13c of modern terrestrial organic matter are mostly between 23 and 33 the depletion of terrestial oc in 13c relative to marine planktonic oc is the basis for numerous statements in the literature that isotopically light cretaceous organic matter is of terrestrial origin even though other organic geochemical and or optical indicators show that the organic matter is mainly of marine origin a difference of about 5 in 13c between modern and cretaceous oc rich marine strata suggests either that cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis or that oc derived from cretaceous marine plankton was isotopically lighter by about 5 relative to modern plankton oc diagenesis does not produce a significant shift in 13c in miocene to holocene sediments and therefore probably did not produce the isotopically light cretaceous oc this means that cretaceous marine plankton must have had 13c values that were about 5 lighter than modern marine plankton and at least several per mil lighter than cretaceous terrestrial vegetation the reason for these lighter values however is not obvious it has been proposed that concentrations of co2 were higher during the middle cretaceous and this more available co2 may be responsible for the lighter 13c values of cretaceous marine organic matter

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

breccia hosted manganese rich minerals of alpi apuane italy a marine redox generated deposit mn rich beds occur within the brecce di seravezza formation bsfm intercalated between norian metadolostone megalodontic marble and hettangian marble in the alpi apuane italy the bsfm is a rockfall talus deposit that accumulates at the base of the footwall scarp of normal faults bordering extensional marine coastal basins the type sequence of metabreccia ends with a bed of chloritoid schist interpreted as the metamorphic equivalent of lateritic soils the four main types of mn mineral assemblages are 1 braunite piemontite and rare hausmannite 2 piemontite braunite hollandite and minor rhodochrosite and kutnahorite 3 braunite hollandite and rare piemontite the other minerals are quartz calcite muscovite phlogopite baryte and minor hematite rutile and apatite during the alpine orogeny pressures of 4 6 kb and temperatures of 350 380 c were attained mn assemblages with hematite and phlogopite indicate o2 10 8 for t 350 c and o2 10 6 when braunite appears on the mn rich rocks ti fe nb sn ta hf th ga rb pr k na nd were identified as detrital in origin and are positively correlated with al u is positively correlated with co pb bi mo the rare earth element patterns normalized to seawater show a negative slope towards the heavy rare earth elements with positive ce anomalies in some samples positive ce and eu anomalies in other samples and negative ce and positive eu anomalies in a third group the eu concentrations are nearly constant and the eu anomalies reflect differences in the concentration of other rare earth elements for ce a positive correlation with al2o3 sio2 indicate that a detrital signature prevails on that produced by duration of seawater exposure but a significant good correlation of la tm lu with mg could indicate at least for these elements a seawater signature only in some samples ba eu sr show anomalously high concentrations likely of hydrothermal origin however generally in all the chemical variation diagrams the bsfm samples fall outside the field of hydrothermal and hydrogenous deposits as defined in the literature for other mn deposits in addition metabasites and metavolcaniclastic beds are absent from the bsfm sequence and bsfm coastal basins were for the most part separated from the open ocean and related streams that must be excluded as possible mn sources owing to the arid climate of late triassic the runoff was negligible and the groundwater oxidizing and alkaline were unable to mobilize mn the bsfm basins are then the most suitable depositional environment for mn the water column was stagnant and stratified only where bottom of the basin was deeper than the oxic anoxic boundary reducing marine bottom waters percolated within the fractured basement and leached mn from disseminated minerals of metamorphic rocks and then rising along fault and fractures supplied mn to the bottom waters afterwards when tectonic uplift and sealevel drop brought the bottom into the oxic field mn precipitated and formed the mn rich matrix of the bsfm deposits

the influence of the sw monsoon on the deep sea organic carbon cycle in the holocene abstract results from long term sediment trap experiments carried out since 1986 in the western central and eastern arabian sea are combined with satellite derived wind fields and paleoceanographic information to link the intensity of the sw monsoon to organic carbon fluxes and its preservation in sediments the sw monsoon is characterized by the low level jet findlater jet that crosses the arabian sea almost parallel to the arabian coast the intensity of the findlater jet mainly controls the velocities of upwelling that occurs to the northeast of the jet since up welling in turn mainly governs the organic carbon fluxes in the western arabian sea variation in the strength of the findlater jet is the dominant factor determining the organic carbon fluxes on seasonal time scales changes in the subsurface nutrient concentrations due to variations in the surface ocean current systems seem to be another factor influencing the organic carbon fluxes mostly on interannual time scales the translation of sedimentary organic carbon burial rates into organic carbon fluxes according to jahnke 1996 global biogeochemical cycles 10 71 88 allows us to extend our reflections also to a millennium time scale this indicates that changes in the sw monsoon intensity as observed during the last decade could almost account for the range of organic carbon fluxes deciphered from the holocene record

late miocene onset of the amazon river and the amazon deep sea fan evidence from the foz do amazonas basin comment figueiredo et al 2009 1 analyzed sediments from two cores taken northeast of the mouth of the amazon river and concluded that the amazon river formed through a three stage sequence beginning at 11 8 11 3 ma this hypothesis conflicts in part or wholly with virtually every other recent

depth dependent δ 13 c trends in platform and slope settings of the campbellrand malmani carbonate platform and possible implications for early earth oxygenation highlights carbon cycle of neoarchean carbonate platform and potential oxygen oasis carbon isotopes reveal a shift to aerobic biosphere and increasing oxidation state rare earth element patterns reveal decrease in open ocean water influx rimmed margin architecture was crucial for evolution of aerobic ecosystems abstract the evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in earth s atmosphere during the archean proterozoic transition period the delay from the first biogenic production of oxygen and the permanent oxidation of earth s atmosphere during the early paleoproteorozoic great oxidation event goe indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates 13ccarb from 1 6 to 0 2 relative to vpdb and organic matter 13corg from about 40 to 25 relative to vpdb from the 2 58 2 50 gy old shallow marine campbellrand malmani carbonate platform south africa this indicates an increase in the burial rate of organic matter caused by enhanced primary production as well as a change from an anaerobic to an aerobic ecosystem trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent both supporting primary production and an increasing oxidation state of the platform interior these restricted conditions allowed that the dissolved inorganic carbon dic pool in the platform interior developed differently than the open ocean this is supported by coeval carbonates from the marginal slope setting which had a higher interaction with open ocean water and do not record a comparable shift in 13ccarb throughout the sequence we propose that the emergence of stable shallow water carbonate platforms in the neoarchean provided ideal conditions for the evolution of early aerobic ecosystems which finally led to the full oxidation of earth s atmosphere during the goe

distribution of organic carbon in surface sediments along the eastern arabian sea a revisit abstract one hundred twelve surface sediment samples along the western continental margin of india were analyzed for organic carbon oc elevated oc concentrations are associated with sediments deposited in areas where productivity is moderate to high very high oc concentrations as reported in earlier studies even for regions where productivity is comparatively low could not be confirmed the re analysis of selected samples by coulometry shows that oc concentrations determined previously by the wet oxidation method may be too high by up to more than a factor of two a broad similarity in the distribution of oc and bioproductivity is observed which is in accordance with other productivity proxies such as the abundance of planktonic foraminiferal species g bulloides and hydrogen indices hi our results indicate the importance of bioproductivity for oc accumulation along the western continental margin of india even though low dissolved oxygen concentrations in the water column sedimentation rate sediment texture clay mineral content dilution by quartz and carbonate as well as topography are considered responsible for the general oc distribution in this area

metamictization and chemical durability of detrital zircon we have investigated the effect that metamictization has on the weathering of zircon in detrital continental sediments and tropical soils of the amazon basin brazil the degree of radiation damage in the near surface region of the zircon grains was determined by raman microprobe in each of the four series investigated i e sediment podzol topsoil and subsoil horizons of lateritic soil the degree of radiation damage ranges from less than 1014 to 3 5 1015 decay mg the maximum degree of radiation damage coincides with the first percolation threshold of the metamictization process at 3 5 1015 decay mg below this threshold amorphous volumes in the structure of damaged zircon are not connected to each other the ranges of u th and pb contents in ppm measured by proton induced x ray emission pixe microanalysis are 100 u 7000 100 th 18000 and 100 pb 1300 chemical ages assessed from u th and total pb range between 0 15 ga and 2 8 ga this range is roughly consistent with the ages reported for the precambrian shields of the amazon basin 0 45 3 5 ga corresponding radiation doses range between 2 1015 and 3 1016 decay mg comparison of calculated doses with the degree of structural damage indicates that most of the zircon grains have experienced significant annealing however the degree of annealing differs from one grain to another thus the acute maximum limit observed for the degree of radiation damage of the whole zircon series is better explained by low temperature alteration or weathering processes than by thermal resetting following this interpretation our results provide evidence for a dramatic decrease in the chemical durability of zircon in natural weathering environments when the radiation dose exceeds 3 5 1015 decay mg below the first percolation threshold the zircon population survives the soil formation intact but more damaged zircons are dissolved during weathering alteration processes

the influence of humic substances on the geochemistry of iodine in nearshore and hemipelagic marine sediments iodine is characteristically enriched at the surface of hemipelagic and nearshore sediments deposited under oxygenated conditions in such sediments bulk i corg ratios usually decrease with depth to values which are characteristic of anoxic sediments reflecting a preferential release of i during early diagenesis there is some debate as to whether sedimentary i is associated with the iron oxyhydroxide phase or with the organic fraction and whether the decrease in i corg with depth is due to the dissolution of the iron oxyhydroxides or the decomposition of labile organic matter it is shown that in a surficial hemipelagic sediment sample and in a nearshore sediment core i is mainly associated with the organic fraction and moreover that humic substances are involved in the surficial iodine enrichment laboratory experiments on the uptake and release of i by and from sedimentary humic substances also suggest a mechanism whereby humic materials reduce iodate at the sediment water interface to an electrophilic i species which further reacts with the organic matter to produce iodinated organic molecules during burial this excess i could be displaced from the organic matrix by nucleophiles such as sulphide ions or thiosulphate thus providing a possible explanation for the decrease in i corg ratio with depth observed in many nearshore and hemipelagic sediments

surficial sediments of the arabian sea abstract coarse grained low carbonate sediments with high abundances of quartz and feldspar in the northerly areas of the arabian sea close to land reflect significant terrigenous sediment in fluxes primarily from the indus river and to some extent from other sources carbonate content of sediments increases gradually from the northernmost areas to about 18 21 n and reflects the waning influence of terrigenous sediment supply southward in areas farther from significant terrigenous influences the carbonate distribution also clearly reflects bathymetric control thus whereas the caco3 content is high on ridges it becomes less than 10 at about 4800 m ccd in the arabian basin a low carbonate band extends from the areas opposite the indus river to the deep areas 2000 3000 m southward and is confined to the easternmost indus fan valley relatively high bottomwater turbidities in this valley suggest that the low carbonate band may result in part from dilutions due to the influxes of terrigenous sediments transported and deposited by deep turbid water flows although high amounts of organic matter occur in the sediments of westerly northerly and easterly areas close to land it is along the indian continental margin at 200 1500 m water depths that the highest organic carbon values 5 10 are observed however high nutrient abundances in the surface waters and high contents of opal and siliceous fossil remains in the sediments off the arabian peninsula gulf of aden clearly reflect higher biogenic productivities in this region than along the indian margin the high organic matter content in the sediments of the indian margin is primarily due to its preservation which results from the impingement of low oxygenated waters on the sea floor at 200 1500 m water depths and from the high sedimentation rates calculations suggest that the degree of preservation of organic matter along the indian margin is about 15 80 times higher than that in the region off the arabian peninsula

gravity anomalies and crustal structure of the western continental margin off goa and mulki india abstract bathymetric and gravity data totalling 2000 line kilometres on the continental margin off goa and mulki india have been studied and prominent nnw sse and ene wsw trending free air gravity anomalies varying between 60 and 25 mgal have been identified using constraints from seismic refraction and reflection and well log results the thicknesses of the layers are estimated by computing two dimensional crustal structure models for the free air gravity anomalies the study suggests that 1 there are three layers varying in thickness from 0 5 to 4 0 km 0 5 to 12 0 km and from 14 0 to 26 0 km with densities of 2 3 2 6 and 2 9 g cm 3 which are interpreted as sedimentary granitic and mafic basaltic layers respectively that 2 there is a maximum thickness of 4 0 km of sediment over the shelf margin basin and that 3 the moho occurs at a depth of 24 30 km

geohistory analysis of bombay high region abstract geohistory diagrams have been constructed for three representative sites in the bombay high region offshore west coast india the model curve for 60 lithospheric injection matches the intitial part of a thermo tectonic curve for one of the sites where a complete tertiary sequence is developed the study suggests that the four paleogene unconformities recognised in this region were caused by eustatic falls a distinct phase of active subsidence occurred during the late middle late miocene probably due to more active spreading at the mid indian ocean ridge

karstic conduits and watershed identification based on the morphological characteristics of depressions in the yunnan guizhou plateau of china in the investigation of karstic form and underground karst water determination of the location and geometric properties of karstic conduits and underground watersheds is one of the greatest problems not yet adequately resolved in the karst terrain of the yunnan guizhou plateau of china karst depressions are a product of the formation and evolution of the groundwater system and the distribution of these depressions is a direct result of crustal uplift and the development of karstic conduits this terrain is characterized by differences in morphological characteristics of depressions between the region of recharge and the region of outflow discharge the region of recharge is marked extensively by circular and random depressions and is characterized by depressions with higher bottom elevations referred to here as higher depressions conversely the region of outflow and discharge is marked extensively by depressions that form long strips or irregular alignments that are distributed along with the karstic conduits and is characterized by depressions with lower bottom elevations referred to here as lower depressions here a methodology has been put forward to determine the possible location and principal directions of karstic conduits and underground hydrogeological watersheds based on surface morphological features of depressions in the region of the yunnan guizhou plateau china the methodology assumes that the contour map that results from determining the bottom elevations of karstic depressions is sufficient to determine the positions of underground hydrogeological divisions and karstic channels taking the two main conduits systems in guizhou province as an example a digital elevation model dem was derived from 1 10 000 scale topographic maps with arcgis 10 2 and the bottom elevations of depressions were marked then a contour map of the bottom elevations of the depressions was produced the directions of conduits and underground watersheds in the research area were identified from the valleys and ridges of the contour lines the methodology introduced in this paper offers a new means of determining the location and geometric properties of karstic conduits and extracting the spatial distribution of water systems in the region of the yunnan guizhou plateau china this methodology will be appropriate for investigating karstic areas with continuous crustal uplift and significant variation in the bottom elevation of depressions between the region of recharge and region of discharge

do benthic foraminifer records represent a productivity index in oxygen minimum zone areas an evaluation from the oman margin arabian sea abstract benthic foraminifer accumulation rates bfar and benthic to planktic foraminifer ratios b p have been suggested to represent an index of paleoproductivity in the world oceans it is shown here however that bfar and b p do not record the surface water productivity signal in the intense upwelling and oxygen minimum zone omz along the oman margin greater bfar and b p values are noticed during the last glacial period when the monsoon and upwelling strength were weaker and the surface water productivity was reduced this is in contrast to the suggestion that bfar and b p values are an indication of enhanced productivity on the other hand bfar and b p values were lower during the holocene as a consequence of intensified monsoon and upwelling and resulting enhanced productivity in the arabian sea this appears to suggest that bfar and b p do not reflect the productivity variations in the omz of the arabian sea we speculate that dissolved oxygen concentrations might instead be controlling the benthic foraminifer abundance

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

effects of tectonic deformation and landslides in the erosion of a mountain plateau in the transitional zone between the central and patagonian andes

organic matter preservation on the pakistan continental margin as revealed by biomarker geochemistry in order to assess whether the oxygen minimum zone omz in the arabian sea has an eaect on the preservation and composition of organic matter in surface sediments we investigated samples from three diaerent transects on the pakistan continental margin across the omz in addition to determining the total amount of organic carbon toc we analyzed the extractable lipids by gas chromatography combined gas chromatography mass spectrometry and compound specific stable carbon isotope measurements the extractable lipids are dominated by marine organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk and molecular isotopic composition sediments from within the omz are enriched in organic carbon and in several extractable lipids i e phytol n alcohols total sterols n c35 alkane relative to stations above and below this zone other lipid concentrations such as those of total n fatty acids and total n alkanes fail to show any relation to the omz only a weak correlation of toc with mineral surface area was found in sediments deposited within the omz in contrast sediments from outside the omz do not show any relationship between toc and surface area among the extractable lipids only the n alkane concentration is highly correlated with surface area in sediments from the hab and makran transects in sediments from outside the omz the phytol and sterol concentrations are also weakly correlated with mineral surface area the depositional environment of the indus fan oaers the best conditions for an enhanced preservation of organic matter the omz together with the undisturbed sedimentation at moderate rates seems to be mainly responsible for the high toc values in this area overall the type of organic matter and its lability toward oxic degradation the mineral surface area the mineral composition and possibly the secondary productivity by sedimentary bacteria also appear to have an influence on organic matter accumulation and composition 2000 elsevier science ltd all rights reserved

gas hydrates in india potential and development abstract the shallow sediments along the indian continental margin are good hosts for gas hydrates and the methane within gas hydrates has been prognosticated as more than 1500 times of india s present natural gas reserve production of even 10 from this natural reserve is sufficient to meet country s vast energy requirement for about a century hence it was felt necessary to map the most prospective zones of gas hydrates and evaluate their energy potential along the indian margin first of all we have updated the gas hydrates stability thickness map along the indian shelf to provide the spatial and depth domains within which gas hydrates can be looked for we have identified the bottom simulating reflectors bsrs the main marker for gas hydrates in the krishna godavari kg mahanadi andaman kerala konkan and saurashtra regions respectively the total organic carbon content toc sediment thickness and rate of sedimentation indicate that the cauvery and kerala laccadive basins are also prospective for gas hydrates seismic attenuation q 1 reflection strength instantaneous frequency and blanking have been computed to characterize the sediments containing gas hydrates and free gas the faulting or gas chimneys have also been used for the identification of gas hydrates we have developed several approaches based on seismic traveltime tomography full waveform inversion amplitude versus offset avo modeling and avo attributes each coupled with rock physics modeling and utilized them for the quantification of gas hydrates a large volume of multi channel and ocean bottom seismic data have been acquired in 2010 between 500 to 2500 m water depths in kg and mahanadi basins the new data exhibit wide spread occurrences of bsrs reveal new prospective zones of gas hydrates and are being modeled for the delineation of sediments hosting gas hydrates and evaluation of their resource potential efforts are on to develop suitable technology for exploitation we anticipate that free gas lying below gas hydrate bearing sediments can be produced economically in near future however it may take longer time to retrieve gas from gas hydrates

the pan amazonian ucayali peneplain late neogene sedimentation in amazonia and the birth of the modern amazon river system abstract we review the neogene geologic history of lowland amazonia in an attempt to focus attention on areas of agreement as well as areas in dispute in this research arena we reinterpret pre existing hypotheses present new data and discuss new insights intended to support a unified synthesis of the amazon basin as a single sedimentary basin albeit on a vast scale during the late miocene to middle late pliocene we document the ucayali peneplain as an isochronous pan amazonian geologic feature that formed following the early to mid miocene quechua i orogenic phase of andean tectonism peneplanation began possibly as early as 15 ma and terminated abruptly near the beginning of the late miocene quechua ii orogenic event at 9 5 9 0 ma subsequently a thin cover of sediments comprising the madre de dios formation began blanketing most of lowland amazonia excepting only the eastern subandean fold and thrust belt and isolated highlands within the basin the buried peneplain is readily observed in river cutbanks throughout amazonia as the marked often angular ucayali unconformity that separates eroded older often folded faulted and weathered moderately to well consolidated tertiary formations from unconsolidated near horizontal upper neogene deposits the dates of formation of major unconformities and subsequent depositional events at widely separated areas within the andes of bolivia ecuador and peru are coincident with that of the ucayali unconformity and deposition of the madre de dios formation and suggest that the events are linked to a common cause which is interpreted to be the still on going collision between the south american and nazca tectonic plates the madre de dios formation has three members the oldest of which documents a short lived high energy depositional environment followed by a moderate energy depositional environment both occurring at a time when drainage from the basin was unobstructed the upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed disorganized and took place over long distances with extremely low gradients the sedimentology of the madre de dios formation particularly the thick massive beds of clay and the widespread presence of paleodeltas and associated geomorphic features on the amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake lago amazonas or more probably within a complex series of interconnected mega lakes that occasionally united to cover most or all of lowland amazonia to a shallow depth from the latest miocene until 2 5 ma the presence of the ucayali unconformity and the relatively uniform lithostratigraphy basin wide of the fluvial fluviolacustrine and lacustrine sediments of the upper neogene madre de dios formation are consistent with the hypothesis that the amazon basin acted as a single undivided sedimentary basin in the late neogene the biostratigraphic correlation across important modern drainage divides of both micro and macro sized late miocene fossil vertebrates recovered from basal conglomerates of the madre de dios formation and the absence therefrom of fossil vertebrates of any other age is also consistent with this hypothesis two 40ar 39ar dates on ash deposits within the madre de dios formation corroborate the upper miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper pliocene age for the youngest sediments of the formation the modern amazon river drainage system was established in the late pliocene at 2 5 ma by the breaching of the eastern rim of the sedimentary basin as a result of the basin being overfilled or by headward erosion of the lower amazon river or both data on cenozoic mass accumulation rates and the chemistry of terrigenous sediments in the atlantic ocean obtained by ocean drilling project leg 154 ceara rise support the postulated timing of the establishment of the modern amazon river drainage system at 2 5 ma rather than the long held view that this event occurred in the late miocene we discuss the important role of ocean currents and sea level fluctuations on terrigenous mass accumulation rates on the ceara rise the postulated time of formation of the modern amazon river is nearly coincident with the onset of the plio pleistocene glacial climatic regime and the lowest sea level stands since the latest middle miocene this analysis indicates that modern amazonia is a product of terrain development within an erosional regime since 2 5 ma

on the sedimentological origin of down core variations of bulk sedimentary nitrogen isotope ratios 1 the bulk sedimentary nitrogen isotopic composition of two cores from nearby sites on the northern slope of the south china sea site 17940 and ocean drilling program odp site 1144 differs by up to 2 during the last glacial period given their close proximity both core sites are located in the same biogeographic zone and nutrient regime and it is thus unlikely that this offset is due to a true gradient in surface ocean conditions in an attempt to resolve this offset we have investigated the possible effects of two sedimentological parameters that can affect bulk sedimentary 15n namely the variable contribution of inorganic n to bulk n in the sediment and the grain size dependence of bulk 15n we find that neither effect singly or in combination is sufficient to explain the significant 15n offset between the two down core records by elimination the most likely explanation for the observed discrepancy is a different origin of both the organic and inorganic nitrogen at each site this study adds to the growing body of evidence highlighting the complex nature and origin of the sedimentary components in sediment drifts such as odp site 1144

unraveling weathering episodes in tertiary regoliths by kaolinite dating western ghats india abstract secondary minerals in soils can record climatic changes affecting continental surfaces over geological times their dating should refine our present knowledge about their potential periods of formation as well as their relations with the ongoing change of climate and erosion weathering regimes in the present study twenty kaolinite samples from two lateritic profiles of the karnataka plateau an intensively studied area in the southern india have been dated using electron paramagnetic resonance epr spectroscopy kaolinite ages vary between 0 229 0 24 ma to 40 73 15 37 ma four different groups of age can be identified with ages clustered around 1 0 3 5 9 0 and 39 0 ma these groups of age indicate local preferential weathering periods that coincide with distinct indian climatic events described in independent studies such as monsoon strengthening thus regional or subcontinental factors likely prevailed over global forcing in the imprint of climatic events in the regolith profiles these results confirm that despite their simple mineralogy laterites can contain several relictual and coexisting generations of secondary minerals and that epr dating of kaolinite contributes to unraveling the complex history of continental surfaces over geological periods

provenance of pliocene and recent sedimentary deposits in western amazonia brazil consequences for the paleodrainage of the solimoes amazonas river abstract integrated data on paleocurrents the morphology of detrital minerals and zircon grains chemical compositions and u pb geochronology reveal that the flow of the modern solimoes amazonas river has changed from west to east since the plio pleistocene this finding is supported by several lines of evidence including paleocurrent directions and detrital mineral assemblages in the ica formation and in recent sediments the ica formation which was most likely deposited during the pliocene has ne and se paleocurrents a high proportion of stable detrital mineral assemblages and u pb zircon ages that we interpreted as being derived from the amazonian craton e g the rondonian san ignacio and sunsas grenvillian geochronologic provinces and neighboring provinces including the neoproterozoic to cambrian brazilian pampean mobile belts a small proportion is derived from the cambrian to silurian famatinian continental arch another source is the precambrian and paleozoic basement from the andes cordillera which includes several metamorphic inliers in colombia peru and bolivia the overlying recent deposits have different provenances and are characterized by a more variable detrital assemblage with zircon grains that are enriched in trace elements and depleted in si and have mesoproterozoic ages in our interpretation the erosion of the iquitos arch after deposition of the ica formation allowed the westward expansion of the solimoes amazonas system in the plio pleistocene

late miocene onset of the amazon river and the amazon deep sea fan evidence from the foz do amazonas basin new biostratigraphic isotopic and well log data from exploration wells on the outer continental shelf and uppermost amazon deep sea fan brazil reveal that the amazon river was initiated as a transcontinental river between 11 8 and 11 3 ma ago middle to late miocene and reached its present shape and size during the late pliocene prior to the late miocene the continental shelf was a carbonate platform that received moderate siliciclastic sediment supply from the proterozoic basement in eastern amazonia average sedimentation rates on the amazon fan show three stages of development 1 11 8 6 8 ma ago low sedimentation rates 0 05 m ka prevailed on the fan because the amazon river was not yet entrenched and some sediments were partially trapped in continental basins 2 6 8 2 4 ma ago sedimentation rates 0 3 m ka increased the river entrenched and deposition fully migrated onto the amazon fan 3 2 4 ma ago to the present very high sedimentation rates 1 22 m ka with peaks of 11 m ka prevailed on the fan and the modern amazon river developed all these paleogeographic and depositional events are closely related to andean tectonism late miocene pliocene and were exacerbated by global cooling and sea level fall during the late miocene

interpreting rock eval pyrolysis data using graphs of pyrolizable hydrocarbons vs total organic carbon kerogen the major organic component of sedimentary rocks is commonly analyzed by rock eval pyrolysis plotting the data on a graph of s 2 vs total organic carbon toc and determining the regression equation is the best method for determining the true average hydrogen index and measuring the adsorption of hydrocarbon by the rock matrix such a plot also indicates the type of kerogen present and avoids the problem of increasing hydrogen index with total organic carbon content with the s 2 vs toc diagram the organic component of different suites of samples may be compared and their petroleum generation potentials established as an example the diagrams are used to evaluate the sedimentary environments and petroleum potential of the paleogene evaporitic sediments of the mulhouse basin alsace france

impact of cretaceous sea level rise and anoxic events on the mesozoic carbonate platform of yugoslavia the adriatic dinaric carbonate platform of yugoslavia was influenced by rapid sea level rise and an oceanic anoxic event during the cenomanian turonian open marine biota such as planktonic foraminifera radiolarians and locally even ammonites associated with and bracketed by successions of typical shallow water carbonates indicate partial drowning of substantial areas of the platform during this time suggestive furthermore that the rate of increase of water depth was locally great enough to outpace carbonate production the presence of carbon rich and fish bearing platy limestones commonly cherty as an associated coeval facies indicates the development of anoxic or euxinic environments and the stromatolitic laminations in such rocks are attributed to the action of bacterial mats it is suggested that an extensive column of deoxygenated water developed in the neighboring marche umbrian adriatic deep water basin and was carried on to the carbonate platform during the cenomanian turonian transgression

new evidence for enhanced preservation of organic carbon in contact with oxygen minimum zone on the western continental slope of india paropkari a l prakash babu c and mascarenhas a 1993 new evidence for enhanced preservation of organic carbon in contact with oxygen minimum zone on the western continental slope of india mar geol i 11 7 13 in order to evaluate the reason for organic enrichment on the western continental slope of india we provide rock eval pyrolysis data of 25 surficial sediment samples from this area results show that the organic matter on the slope is hydrogenrich type ii kerogen which suggests that the organic carbon there is mostly marine the low productivity of overlying waters rules out surface productivity as a possible cause of organic enrichment the association of higher hydrogen index hi values average 306 and higher organic carbon contents av 3 28 wt with the sediments of the upper slope in contact with the oxygen minimum zone omz clearly indicates enhanced preservation under reducing conditions in the anoxic waters of the omz it is suggested that the sediments of the upper slope in contact with the omz have excellent hydrocarbon generation potential and that these slope beds deserve preferential attention for hydrocarbon exploration

manganese iron and sulphur cycling on the pakistan margin abstract benthic biogeochemical processes in the oceans oxygen minimum zones omzs are of global significance yet they remain poorly defined here sedimentary manganese iron and sulphur cycling are studied at sites above within and below the omz on the pakistan margin of the arabian sea before and immediately after the southwest monsoon mechanisms and rates of sedimentary mn fe and s cycling demonstrated down slope variation according to the position of the omz no significant seasonal change was evident except at one station on the upper margin where bottom waters became hypoxic during the late to post monsoon sampling due to the shoaling of the upper omz boundary within the omz mn 4 reduction within the water column limited supply of reactive mn to the underlying sediments the majority of reactive mn reaching the sedimentary environment is reduced abiotically via reaction with fe 2 sediments within the omz act as a source of mn 2 to the overlying water below the omz mn 2 is re oxidised within the water column and is deposited on the surface of the underlying sediments manganese is then reduced microbially and re oxidised within the oxygenated surface sediments uniform surface sediment fe al ratios across the margin suggest that fe 3 reduction is lacking within the water column omz in contrast fe is actively cycled within the sediments with fe 3 reduction occurring via dissimilatory pathways an absence of potential fe 2 oxidants within the omz results in an efflux of fe 2 from the sediments to the water column below the omz greater oxidant availability ensures that fe is retained within the sediments measured sulphate reduction rates at all sites across the margin were surprisingly low 0 0 45 mmol m 2 d 1 relative to rates observed on other omz margins the abundance of fe oxides in the sedimentary environment appears to suppress sulphate reduction

large barchanoid dunes in the amazon river and the rock record implications for interpreting large river systems abstract the interpretation of large river deposits from the rock record is hampered by the scarcity of direct observations of active large river systems that is particularly true for deep channel environments where tens of meters deep flows dominate these conditions are extremely different from what is found in smaller systems from which current facies models were derived mbes and shallow seismic surveys in a selected area of the upper amazonas river in northern brazil revealed the presence of large compound barchanoid dunes along the channel thalweg the dunes are characterized by v shaped concave downstream crest lines and convex up longitudinal profiles hundreds of meters wide up to 300 m in wavelength and several meters high based on the morphology of compound dunes expected preserved sedimentary structures are broad large scale low angle concave up and downstream cross strata passing laterally and downstream to inclined cosets examples of such structures from large river deposits in the rock record are described in the silurian serra grande group and the cretaceous sao sebastiao and marizal formations in northeastern brazil as well as in triassic hawkesburry sandstone in southeastern australia and the plio pleistocene ica formation in the western amazon all these sedimentary structures are found near channel base surfaces and are somewhat coarser than the overlying fluvial deposits favoring the interpretation of thalweg depositional settings the recognition of large barchanoid dunes as bedforms restricted to river thalwegs and probably to large river systems brings the possibility of establishing new criteria for the interpretation of fluvial system scale in the rock record sedimentary structures compatible with the morphological characteristics of these bedforms seem to be relatively common in large river deposits given their initial recognition in five different fluvial successions in brazil and australia potentially enabling substantial improvements in facies models for large rivers

w b bull 2008 tectonic geomorphology of mountains a new approach to paleoseismology x 316 pp oxford blackwell publishing price 42 50 hard covers isbn 9781 4051 5479 6 over the last decade or so tectonic geomorphology has grown in popularity to become an integral part of the earth science curriculum the widely adopted introductory textbook tectonic geomorphology by burbank anderson provides an excellent overview of the diverse range of topics that encompass the subject but little else has been published that adequately builds on this foundation tectonic geomorphology of mountains a new approach to paleoseismology is a welcome arrival providing substantive and in depth exploration of mountain geomorphology over short timescales holocene to present that relate to the earthquake cycle the latter aspect

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

influence of marginal highs on the accumulation of organic carbon along the continental slope off western india abstract twenty five surficial sediment samples were collected from the central western continental margin of india which consists of diverse physiographic entities continental slope shelf margin basin marginal high and the deep arabian basin at depths between 250 and 2050 m the sediments were studied to assess the relative importance of productivity oxygen minima grain size and bathymetry on organic carbon accumulation preservation the total organic carbon toc content in the study area varies from 0 70 to 5 86 highest values of organic carbon are recorded on the marginal highs 5 12 5 86 followed by shelf margin basin 3 53 4 22 and the continental slope 1 80 3 84 the organic carbon content is relatively low in the arabian basin 0 98 1 46 the sediments from marginal highs are characterized by high oc contents relative to upper slope at equivalent water depth and also the deeper shelf margin basin the elevated organic carbon concentrations on marginal high and upper slope are associated with coarse grained biogenic sediments while those in the shelf margin basin with fine grained silty clays the enrichment of organic carbon on the marginal highs is attributed to productivity related biogenic sediments and their faster burial it is surmised that diverse topographic features in the slope and the associated hydrodynamic processes also play an important role in the enrichment of organic carbon

new geological framework for western amazonia brazil and implications for biogeography and evolution abstract although many of the current hypotheses to explain the origin and distribution of the amazon biodiversity has been based directly or indirectly on geological data the reconstruction of the geological history of the amazon region is still inadequate to analyze its relationship with the biodiversity this work has the main goal to characterize the sedimentary successions formed in the brazilian amazon in the neogene quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap radar image interpretation sedimentological studies and radiocarbon dating allowed the mapping of plio pleistocene to holocene units along the solimoes amazonas river brazil this integrated work led to the characterization of five sedimentary successions overlying miocene deposits of the solimoes pebas formation which include the following ica formation plio pleistocene deposits q1 37 400 43 700 14c yr b p deposits q2 27 200 14c yr b p deposits q3 6730 2480 14c yr b p and deposits q4 280 130 14c yr b p these deposits occur mostly to the west of manaus forming nw se elongated belts that are progressively younger from sw to ne indicating a subsiding basin with a depocenter that migrated to the ne the reconstruction of the depositional history is consistent with significant changes in the landscapes hence the closure of a large lake system at the end of the miocene gave rise to the development of a plio pleistocene fluvial system this was yet very distinct from the modern drainage with shallow energetic highly migrating braided to anastomosed channels having an overall northeast outlet this fluvial system formed probably under climatic conditions relatively drier than today s during the early pleistocene there was pronounced erosion followed by a renewed depositional phase ca 40 000 14c yr b p with the development of prograding lobes and or crevasse splays associated with a lake system i e fan delta and or fluvial flood plain areas after a period of erosion a fluvial system with eastward draining channels started to develop at around 27 000 14c yr b p the fluvial channels were overflooded in mid holocene time this flooding is attributed to an increased period of humidity with a peak between 5000 and 2500 14c yr b p the data presented herein support that rather than being a monotonous area the amazonia was a place with frequent changes in landscape throughout the neogene quaternary probably as a result of climatic and tectonic factors we hypothesize that these changes in the physical environment stressed the biota resulting in speciation and thus had a great impact on modern biodiversity

tracing kaolinites through their defect centers kaolinite paragenesis in a laterite cameroon stable structural point defects a centers have been studied in kaolinite using electron paramagnetic resonance epr the variation in the concentration of defect centers s a has been systematically investigated along seven lateritic weathering profiles from cameroon petrological and geochemical data have been obtained on the same samples a centers result from an external irradiation of kaolinite there is no relation between defect center concentration and textural changes the fe content of the kaolinite lattice and the crystalline order of this mineral on the contrary there does exist a relationship between the integrated intensity of the epr signal on one hand and the fe and the u th content of the bulk samples on the other a possible radiation source has to be sought in the radioactive elements disseminated through the weathered materials and mainly accumulated in ferruginous materials however the present day u th content is too low to explain the measured concentration of defect centers by taking into account the efficient chemisorption of radioactive elements on poorly crystallized iron oxides the a center defects are attributed to an inherited irradiation of the kaolinites during the first stages of rock weathering and their subsequent crystallization the major implication of the proposed model is that iron oxide gels precipitate at the weathering front and adsorb the radionuclides released by primary minerals the evolution of iron oxides toward a more crystalline structure leads to a release of the radioactive elements and the a centers are thus the memory of their transit at the same time the concentration of defect centers gives an efficient fingerprint of the successive generations of kaolinites during weathering processes

kaolinite dating from acrisol and ferralsol a new key to understanding the landscape evolution in nw amazonia brazil abstract ferralsols and acrisols are major types of soils of the amazon basin observed on various landform units comprising plateau surfaces incised hills at their edges and slopes the present study focuses on an acrisol developed on plateaus surfaces from northwest amazonia and a ferralsol from the convex hills of the incised plateau edges local geomorphologic models of weathering covers suggest that ferralsols are younger than plateau acrisols but the absolute chronology of their formation is still lacking this type of information is however critical to understand the evolution of northwest amazon basin landscapes and to identify its link with major climatic and geomorphologic events in this article kaolinite rich samples from soils and saprolites belonging to a transect in the sao gabriel da cachoeira region amazon state brasil are investigated based on their crystal chemical characters several types of kaolinites are identified using a previously developed methodology based on electron paramagnetic resonance epr spectroscopy crystallization ages are proposed for these different kaolinites saprolite kaolinites are dated from 6 to 3 6 ma in the acrisol profile and display significantly more recent ages

control of the preservation of organic matter by productivity and sulphate reduction in kimmeridgian shales from dorset uk abstract the kimmeridgian shales of dorset uk have a very wide range of total organic carbon toc content and hydrogen index hi these cyclic changes of organic carbon accumulation are usually interpreted in terms of biomass changes induced by variations in productivity the influence of such variations on the intensity of sulphate reduction and consequently on the preservation of organic carbon were studied the pyrite content does not reflect the full extent of sulphate reduction for sediments with a toc content greater than 10 when easily reducible iron is no longer available h 2 s is preferentially fixed as organic compounds in algal bodies a sulphate reduction index sri initial c org residual c org was calculated for each sediment on the basis of the sulphate reduction reaction stoichiometry and a drastic change in sulphate reduction intensity was seen for toc contents of about 10 above this value the sri is low and decreases with increasing toc content the sediments are rich in organic carbon composed of well laminated stacked algal bodies and were probably deposited at an oxygen depleted water sediment interface in this instance the quality and the amount of organic matter buried depends directly on the organic fluxes towards the redox interface it is assumed that the high relative input of organic matter which can be metabolized exceeds the availability of sulphate in the pore waters so that part of it escapes sulphate reduction and is probably degraded by other processes methanogenesis for toc contents less than 10 the sri is higher and variable the sediments show traces of burrowing and the depositional conditions are thought to be dysaerobic it is proposed that under such conditions bioturbation brings both sulphate and reducible iron into the sediment and that sulphate reduction mainly expressed by the presence of pyrite consumes all the available organic matter which can be metabolized the strong variability in the intensity of sulphate reduction depends on various parameters such as the genetic type of planktonic organic matter the method of transportation to the sediment and possibly the earlier oxic degradation during transportation

distributions and origins of clay minerals and quartz in surface sediments of the arabian sea abstract mineral distributions in surface sediments of the arabian sea north of 5 n reflect several sedimentary origins and processes smectite and quartz rich sediments occurring along the indian margin have been derived from the western regions of peninsular india in the northern and western most areas of the arabian sea sediments probably have been derived from several sources indus river iran makran region arabian peninsula and somalia these different sediment sources appear to have characteristically high amounts of quartz illite chlorite or palygorskite in addition smectite becomes a significant component in the sediments off somalia horn of africa the sediments derived from the peninsular india and indus river may have been dispersed by prevailing surface oceanic irculation in the region sediments from other sources probably have been transported by wind the sediments of the distal indus fan appear to be a mixture of sediments derived from the indus river iran makran and arabia somalia regions palygorskite clays from the arid regions of arabia somalia appear to have been transported by winds eastward to as far as the indian continental margin in the southernmost region of study relatively high kaolinite derived from the tropical soils of africa madagascar and southern india characterize the sediments

effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments in order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter we developed a new incubation method in contrast to most previous experiments 1 all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and 2 organic matter of varying lability was studied the production of co2 during incubations of sediment in glass test tubes corrected for dissolution precipitation of calcium carbonates was used as a measure of degradation rates the laboratory experiments using surficial and buried continental shelf and slope sediment from the open skagerrak northeastern north sea demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material fresh material was degraded with little difference in rates in the presence or absence of oxygen whereas old material was decomposed significantly up to 3 6 times faster with oxygen than without oxygen an implication of these findings is that bioturbation by exposing old buried material to oxygen may enhance integrated organic carbon oxidation in marine sediments this constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation the anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured we found however that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions which indicated that the oxic anoxic oxic redox transitions deposition under oxic conditions burial under anoxia and reexposure to oxygen promoted degradation our results therefore also suggest that the extent of long term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms