mobility of rhenium platinum group elements and organic carbon during black shale weathering this study investigates the effects of black shale weathering on the re os isotope system platinum group element concentrations and the degradation of organic matter samples from a weathering profile in late devonian 365 myr ohio shale show a pronounced decrease 77 in organic carbon corg near the present soil surface relative to the interior portion of the outcrop a similar trend is observed for total n 67 loss conversely organic phosphorus porg concentrations increase by 59 near the soil surface the decrease in corg is accompanied by a pronounced decrease in re 99 and to a lesser extent os 39 palladium and pt do not appear to be significantly mobile the effects of re and os mobility on the re os isotope system are significant none of the samples plots on a 365 myr isochron rather the samples define a trend with a slope corresponding to an age of 18 myr with an initial 187os 188os of 6 1 this indicates recent disturbance of the re os system isotope mass balance calculations imply that the labile fraction of os is significantly more radiogenic 187os 188os of 7 8 than the average of the unweathered samples 187os 188os of 6 4 based on data from this study the molar ratio of labile re to corg in ohio shale is estimated at 7 10 8 we estimate the present day riverine black shale derived re flux to seawater using literature data on re burial in anoxic marine sediments and assuming steady state between re release during black shale weathering and re burial in anoxic marine sediments then the labile re corg observed in this study implies that 0 5 tmol of corg is released annually from weathering of black shales a trace lithology in the continental crust this flux corresponds to 12 of the estimated annual co2 flux from oxidative weathering of sedimentary rocks the labile molar re os of 270 indicates that black shale weathering releases 130 mol os per year which accounts for 7 of the riverine os input to seawater the data from this study support the notion that the crustal cycles of labile corg re and os are tightly coupled gray shales which are less corg re and os rich but much more abundant in the continental crust than black shales are likely to be even more important continental sources of re and os to seawater

geochemistry and provenance of the middle ordovician austin glen member normanskill formation and the taconian orogeny in new england the austin glen member of the upper middle ordovician normanskill formation is a sandstone shale ysch succession deposited in the foreland of the taconian orogen petrographic major and trace element and nd pb isotopic data provide substantial constraints on its provenance lack of k feldspar and paucity of plagioclase in addition to the dominance of sedimentary rock fragments indicate that the source was dominated by recycled sedimentary components major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same no evidence of an ophiolitic or volcanic component was observed interpretation of nd isotopic characteristics are complicated by a partial resetting of the nd isotope system at about the time of sedimentation but indicate that the provenance of the austin glen member had a long term history of light rare earth element lree enrichment average tdm 1a8 ga furthermore nd isotopic compositions are extremely homogeneous end 8a1 0a6 1 s d n 23 at 450 ma the approximate depositional age indicating either a single source or very well mixed sources 207pb 204pb ratios are variable but within the range of pb isotopic compositions typically described as grenvillian the range of 207pb 204pb is greater than expected for the range of 206pb 204pb and suggests an additional component of pb possibly introduced during diagenesis the immediate source of the austin glen member may have been the accretionary prism that developed as older sediments of the laurentian margin were scraped off the basin oor incorporated within the accretionary prism and shed into the basin no evidence indicating the arrival of an undifferentiated island arc or continental fragment during the taconian orogeny has been found the data acquired during this study can be explained almost exclusively by grenville province source components but with possible additional contributions from older laurentian terranes and late proterozoic rift volcanics that are not readily quanti ed but likely to have been minor accordingly we conclude that the taconian orogeny in new england involved either 1 a continental arc that involved exclusively laurentia 2 collision of a continental block with identical geochemical characteristics as laurentia or 3 essentially no detritus from any exotic colliding block island arc or continental fragment reached the foreland basin at the time of austin glen deposition

the geochemistry of rhenium and osmium in recent sediments from the black sea abstract the os isotopic composition of recent sediments from the black sea are reported with the concentrations of re os and major and minor elements osmium concentrations range from 0 23 to 0 69 ppb and re concentrations range from 21 to 85 ppb concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon 187 os 186 os ratios range from 5 7 to 7 1 all of these values are less radiogenic than the estimated 187 os 186 os ratio of seawater and average continental crust material variations in the os isotopic composition of these sediments are attributed to mixing of a hydrogenous component 187 os 186 os ratio between 7 and 9 and a less radiogenic detrital component 187 os 186 os ratio between 1 and 4 a mixing model is used to determine the fraction of os in the sediment associated with each component and the burial fluxes of re and os in the black sea are calculated in addition the data are used to estimate the magnitude of systematic error which such initial os isotopic heterogeneity may introduce into re os age determinations of ancient organic rich sediments

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

an evaluation of elemental mobility during metamorphism of a turbidite sequence greenland group new zealand the extent of elemental mobility during lower greenschist to amphibolite facies metamorphism of a uniform turbidite suite greenland group new zealand has been evaluated using data for major elements and 19 trace elements simple comparison of average compositions at 2 wt al 2 o 3 intervals in the data suites shows little contrast between lower greenschist protolith and upper greenschist and amphibolite facies equivalents except for enrichment of cao and sr and loss of ba and rb particularly in the sandier end members division into psammitic and pelitic suites using tio 2 al 2 o 3 and zr al 2 o 3 ratios allows delineation of individual residual enrichment models on ti reference element plots and both lithotypes can be used to assess potential elemental mobility these plots show that a large number of elements ti al fe mg ce cr ga la nb ni sc th v zn and zr constitute immobile reference species with abundances equal to their equivalent lithotype in the protolith and little mass loss or residual enrichment k and rb also largely conform to the residual model but in the amphibolite facies some exchange between pelite and psammite may occur a number of elements show enrichment mn cu pb u na p or depletion y in a small number of samples but the significance of these contrasts is questionable due to relatively large variation in the protolith in contrast ca and sr show progressive and marked enrichment with increasing grade and ba and as are clearly depleted in amphibolite facies psammites in the amphibolite facies some si may have been lost by psammites and gained by the pelites although there has been no mass change from the suite as a whole a large part of the ca and sr enrichment in the amphibolite facies can be accounted for by metasomatic homogenization of calcareous concretions which occur in the lower grade protolith the metamorphism of the greenland group is thus considered to be essentially isochemical

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

the osmium isotopic composition of organic rich marine sediments abstract the osmium os concentration and 187 os 186 os ratio of several recent marine organic rich sediment samples from three widely separated sites have been measured os concentrations range from 0 095 to 0 212 ppb and 187 os 186 os ratios range from 8 2 to 8 9 the calculated fraction of hydrogenous os exceeds 78 in all samples thus the 187 os 186 os ratio of these samples reflects os isotopic composition of seawater the small range in measured 187 os 186 os ratio indicates that the os isotopic composition at these sites is fairly homogeneous the large magnitude of the os burial flux at these sites indicates the os burial in association with organic rich sediments is an important sink in the marine cycle of os these data also suggest that ancient organic rich sediments may provide a record of past variations in the os isotopic composition of seawater

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

genesis of upper proterozoic cambrian phosphorite deposits of india isotopic inferences from carbonate fluorapatite carbonate and organic carbon carbon and oxygen isotope analyses performed on sedimentary carbonates organic carbon kerogen and structurally bound co2 3 groups of carbonate fluorapatite francolite from four widely separated indian phosphorite provinces have shown that each of the deposits is characterized by specific basically facies dependent distribution patterns of 13ccarb 13ocarb 13corg and 13capat comparison of the carbon isotope composition of apatite co2 with that of the coexisting sedimentary carbonate has served to further constrain the genetic type of the individual phosphorite deposit investigated augmenting relevant geological evidence by an additional geochemical discriminator as a whole the results confirm the importance notably of carbon isotope work for elucidating the processes involved in sedimentary phosphorite formation

reos isotopes and major and trace element geochemistry of carbonaceous shales aravalli supergroup india impact of post depositional processes abstract this study presents re os isotopes and major and trace elemental geochemistry of carbonaceous black shales and a weathering profile developed on them from the paleoproterozoic deposits of the jhamarkotra formation lower aravalli group india these measurements provide information on the impact of post depositional processes particularly hydrothermal fluid interactions on the 187re 187os geochronology and element mobility in shales in contrast to previously reported compositions for black shales the re concentrations of the jhamarkotra shales are in general lower than their corresponding os concentrations the 187os 188os ratios of the black shales vary widely from 3 5 to 10 5 without any discernible systematic trend with 187re 188os range 0 4 to 17 7 excluding an outlier these 187re 188os ratios are lower by a few orders of magnitude than those generally reported for fresh black shales these observations indicate that the jhamarkotra shales though they seem visually well preserved have undergone post depositional modification resulting in open system behavior of re and os with re more mobile than os higher sio2 tio2 and k2o na2o ratios of the jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal possibly si and k rich fluids the sio2 tio2 ratios an index for hydrothermal alteration show negative correlation with re corg and positive correlation with corg n ratios 42 154 indicating loss of re and nitrogen during hydrothermal fluid interaction earlier studies have shown the presence of re porphyrins n containing organic compounds in black shales thus the observed co variations of sio2 tio2 with re corg and corg n ratios suggest mobilization of re from the porphyrins in the shales further positive correlation between sio2 tio2 and 187os 188os ratios indicates that significant amounts of radiogenic os component were re distributed throughout the jhamarkotra shales during the post depositional processes in the weathering profile developed on black shales the al2o3 normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering on the contrary the al2o3 normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile suggesting limited mobility over the depth of the profile the depth profiles of re al2o3 and 188os al2o3 and 187os 188os ratios in the weathering profile by and large reflect compositions of their presumed parent shales which have gone through significant post depositional changes

provenance tectonic setting and age of the sediments of the upper disang formation in the phek district nagaland abstract integrated petrographic and geochemical studies of sandstones and geochemical studies of shales of turbidites from the upper disang formation phek district nagaland have been carried out to determine their provenance weathering conditions and tectonic setting paleomagnetic studies were carried out for magnetostratigraphic purposes studies indicate that most of these sediments were derived from felsic and mafic sources with minor contribution from low to medium grade metamorphic rocks most of the felsic components have been transported from distant sources as evidenced from extensive reworking of grains the most likely source rocks are the granite granite gneiss of the karbi anglong crystalline complex to the west of the study area however the bulk of the sediments have been contributed from nearby basic and ultrabasic sources this would correspond to the fast rising naga ophiolite which probably emerged above sea level during the mid eocene prevailing high temperatures and humid climate caused intense chemical weathering of the source rocks the sediments from the west were transported great distances by turbidity currents into an easterly deepening basin sediments from the nearby east were rapidly dumped on the seafloor causing rapid mixing leading to textural and chemical immaturity paleomagnetic studies endorse published paleontological evidence to indicate that most of the sediments of the upper disang formation were deposited during the late eocene deposition took place in a westward migrating accretionary prism complex in an active margin setting at the convergence of the indian and burma plates this was a rapidly closing basin where anoxic conditions prevailed towards the end of the eocene this basin closed completely with the destruction of the tethyan ocean

direct radiometric dating of the devonian mississippian time scale boundary using the re os black shale geochronometer many phanerozoic time scale boundaries are characterized by oceanic anoxia and mass extinction events with the deposition of black shale the re os isotope system in black shale can be used to provide depositional ages for these rocks thus yielding direct radiometric ages for time scale boundaries we demonstrate that the re os black shale geochronometer can yield precise ages useful for time scale research through study of the devonian mississippian boundary within the exshaw formation canada the re os date determined places the devonian mississippian boundary at 361 3 2 4 ma 2 including uncertainty model 1 mean square of weighted deviates 1 2 in accord with recent calibration interpolated from u pb zircon dates 360 7 0 7 ma for the hasselbachtal section germany

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

re os systematics in pallasite and mesosiderite metal re os data on pallasite metal yield a sufficient range in re os to permit the determination of a whole rock isochron if pallasites from different groups are considered together the results are indistinguishable from the data obtained on groups iab iiab iid iiiab iva and ivb iron meteorites pallasite data were obtained on samples of the main group which have been considered related to group iiiab irons and on samples of the distinctive pallasites eagle station finmarken and marjalahti it appears that iron meteorites and pallasites crystallized within a time interval of 20 ma this result is consistent with the evidence from the short lived chronometers 107 pd 107 ag and 53 mn 53 cr it is also consistent with the evidence from 182 hf 182 w that the time interval over which molten feni metal from different iron meteorites equilibrated with and became segregated from silicates was 10 ma the well defined 187 re 187 os whole rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated feni metal and fes and then all crystallized within a 20 ma time interval this conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic the short time scale requires that the irons and pallasites were formed in small parent planets of 10 km radius or near the surface of larger bodies in contrast to the pallasites we find that feni samples from mesosiderites have a narrow range in re os so that it is not possible to determine a whole rock isochron for these samples the mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi stage evolution based on sm nd data on mesosiderite silicate clasts the re os results on mesosiderite metal are compatible with the model of melted or partially molten feni cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 ma after early formation and the feni was splashed onto the surfaces of other small differentiated planetesimals

extreme enrichment of polymetallic ni mo pge au in lower cambrian black shales of south china an os isotope and pge geochemical investigation extreme metal enrichments occur in the lower cambrian niutitang formation a thick black shale sequence deposited on the yangtze platform of south china and form a regionally distributed conformable polymetallic ni mo pge au sulfide horizon the origin for the extreme enrichment of metals in these black shales is discussed based on a comprehensive geochemical investigation of re os isotopes and platinum group elements the black shales of the lower cambrian niutitang formation in guizhou province yielded an re os isochron age of 535 11 ma with an initial 187 os 188 os ratio of 0 80 0 04 which is similar to the initial 187 os 188 os ratio 0 78 0 19 of the ni mo sulfide ores but slightly lower than the initial 187 os 188 os ratio 1 18 0 02 of lower cambrian black shales from the lesser himalaya india a comparison of os isotope data among the chinese ni mo sulfide ores the seafloor massive sulfide deposits and contemporaneous seawater suggests a possible similar hydrothermal forming mechanism for these deposits the pge concentrations pt anomaly pt pt pt pd ir pd au pd ratios of the chinese ni mo sulfide ores are similar to those metal rich black shales of submarine volcanogenic hydrothermal origin from czech republic namibia and canada but are different from those data of the ocean floor fe mn crusts that have scavenged their pge and other metals directly from seawater it is therefore suggested that the ni mo sulfide ores from the niutitang black shale sequence in south china had a submarine hydrothermal origin the anoxic environments and abundant organic matter in the ocean basin also played a key role for the extreme metal enrichments the discharge of hydrothermal fluids into the cambrian ocean may have had a great effect on life during the cambrian explosion 2007 elsevier b v all rights reserved

paleosalinity evolution of the paleogene perennial qaidam lake on the tibetan plateau climatic vs tectonic control as the largest cenozoic terrestrial intermountain basin in the tibetan plateau the qaidam basin is an ideal basin to understand the coupling controls of tectonics and climate on plateau lake evolution more than 10 000 data of chloride content from the paleogene sediments of the endorheic qaidam basin have been collected to monitor changes in lake paleosalinity results show that there were two prominent salinity increase events in the paleogene paleolake one in the late eocene 40 ma and the other in the transition from the eocene to oligocene eot 35 5 ma the first salinity increase event was evidenced by the expansion of saline water the connection of two previous separated saline centers and the deposition of thick bedded halite in a local sag which correlated with the late eocene s frequently fluctuated climate the other salinity increase event at the eot was characterized by the expansion of mesosaline water range which corresponded well to the drier condition of the oligocene icehouse climate the synchronous migration of the saline centers and depocenters demonstrates that the qaidam basin was a tectonically active saline basin where the activity of main faults controlled the occurrence and disappearance of supersaline center under the overall brackish background the deposition of over 200 m thick bedded halite during the late eocene in a local sag was the result of coupled effects by active tectonics and fluctuated climate

osmium isotope evidence for the regulation of atmospheric co2 by continental weathering the long term stability of earth9s climate throughout the phanerozoic stands in marked contrast to the dramatic fluctuations that have taken place on time scales as short as a few years reflecting the high efficiency of longer term climate regulation through negative feedbacks a fundamental mechanism is thought to involve control of co 2 in the ocean atmosphere system through continental weathering although unambiguous high resolution data supporting this hypothesis have hitherto not been available organic rich mud rocks from yorkshire england which were deposited during the toarcian oceanic anoxic event ca 181 ma early jurassic contain evidence of an exceptionally large excursion in the 187 os 188 os ratio of contemporaneous seawater from 0 4 to 1 0 the most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of 400 800 the os isotope excursion coincided with a well documented global 13 c excursion of 6 that affected all the major biospheric reservoirs of the time higher mean global temperatures caused global chemical weathering rates to increase substantially while in turn chemical weathering was very effective in reducing the elevated levels of atmospheric co 2 and the high temperatures to preexcursion levels

burial of redox sensitive metals and organic matter in the equatorial indian ocean linked to precession authigenic metals uranium cadmium and molybdenum organic carbon oc and total c37 alkenone totc37 concentrations were measured for the last 350 kyr in core md900963 located in the eastern equatorial arabian sea authigenic metal concentrations on a carbonate free basis range between 1 and 17 ppm 0 5 and 6 ppm and 0 5 and 4 ppm for u cd and mo respectively the profiles are characterized by well defined 23 kyr cycles between oxic and mildly suboxic conditions the redox sensitive metal profiles also follow variations in the concentrations of oc 0 2 0 9 and alkenones 0 2 6 7 ppm the coupled variations in inorganic and organic constituents are attributed to a 23 kyr cycle in primary production above site md900963 as suggested by clear correlations with independent micropaleontologic proxies primary productivity indices based on foraminifera and coccoliths and fragmentation of foraminiferal shells the 23 kyr cycles do appear to be primarily driven by productivity rather than changes in bottom water oxygen comparison with other records indicates that if this interpretation is correct productivity variations across much of the indian ocean have been dominated by precessional forcing with high productivity in phase with low summer insolation in the northern hemisphere this interpretation contrasts with the traditional attribu tion of enhanced productivity in the indian ocean with periods of high summer insolation copyright 2002 elsevier science ltd

the ciw index a new chemical index of weathering abstract a new chemical index of weathering that can be applied to modern soils and precambrian paleosis is proposed ciw al2o3 the value of this index increases as the degree of weathering increases and the difference between ciw index values of the silicate parent rock and soil or sediment reflects the amount of weathering experienced by the weathered material

further evaluation of the re os geochronometer in organic rich sedimentary rocks a test of hydrocarbon maturation effects in the exshaw formation western canada sedimentary basin abstract re os isotopic analyses of a single organic rich sedimentary rock unit ors of known depositional age and at three levels of regional hydrocarbon maturity show that hydrocarbon maturation does not affect the ability the 187re 187os chronometer to yield a depositional age for such rocks we present re os isotope analyses from the late devonian exshaw formation in the subsurface of the western canada sedimentary basin alberta and obtain a re os isochron age of 358 10 ma 2 model 3 1 666 10 11 a 1 for samples ranging from hydrocarbon immature to overmature this age is within uncertainty of the established absolute age for the exshaw formation hydrocarbon immature and mature plus overmature samples show no significant age differences if regressed individually indicating that hydrocarbon maturation did not greatly disturb the re os isotope system in the exshaw formation as such we propose that the re os geochronometer may be used as a reliable tool for measuring the depositional ages of ors regardless of their level of hydrocarbon maturity we find that minimizing natural variation in the initial 187os 188os ratio is more important than avoiding hydrocarbon maturation in obtaining precise re os ages in particular the exshaw formation appears to contain a nonhydrogenous component of unradiogenic os in addition to the hydrogenous os load a subset of exshaw formation samples with 5 total organic carbon toc which should best reflect the hydrogenous os load alone yields a very well fitted isochron having a depositional age of 358 9 ma 2 1 666 10 11 a 1 with an initial 187os 188os ratio of 0 59 0 05 model 3 mean square of weighted deviates mswd 1 8 the initial 187os 188os ratio of this regression may provide an estimate of the os isotopic composition of local seawater at the time of deposition

yttrium and rare earth elements in fluids from various deep sea hydrothermal systems rare earth element ree and yttrium y concentrations were measured in fluids collected from deep sea hydrothermal systems including the mid atlantic ridge mar i e menez gwen lucky strike tag and snakepit the east pacific rise epr i e 13 n and 17 19 s and the lau vai lili and manus vienna woods pacmanus desmos back arc basins bab in the south west pacific in most fluids y is trivalent and behaves like ho chondrite normalized y ree y reen concentrations of fluids from mar epr and two bab sites i e vai lili and vienna woods showed common patterns with lree enrichment and positive eu anomalies ree analysis of plagioclase collected at lucky strike strengthens the idea that fluid ree contents are controlled by plagioclase phenocrysts other processes however such as ree complexation by ligands cl f so42 secondary phase precipitation and phase separation modify ree distributions in deep sea hydrothermal fluids ree speciation calculations suggest that aqueous ree are mainly complexed by cl ions in hot acidic fluids from deep sea hydrothermal systems ree concentrations in the fluid phases are therefore influenced by temperature ph and duration of rock fluid interaction unusual y reen patterns found in the pacmanus fluids are characterized by depleted lree and a positive eu anomaly the demos fluid sample shows a flat y reen pattern which increases regularly from lree to hree with no eu anomaly these manus basin fluids also have an unusual major element chemistry with relatively high mg so4 h2s and f contents which may be due to the incorporation of magmatic fluids into heated seawater during hydrothermal circulation ree distribution in pacmanus fluids may stem from a subseafloor barite precipitation and the ree in demos fluids are likely influenced by the presence of sulfate ions

marine redox conditions in the middle proterozoic ocean and isotopic constraints on authigenic carbonate formation insights from the chuanlinggou formation yanshan basin north china abstract to improve our understanding of ocean chemistry and biogeochemical cycling following the termination of large scale paleoproterozoic iron formation if deposition 1 85 billion years ago ga we conducted a fe s c mo geochemical study of the 1 65 ga chuanlinggou formation yanshan basin north china despite the cessation of if deposition our results suggest the presence of anoxic but non euxinic ferruginous conditions persisted below the surface mixed layer for the deepest portion of the continental rifting basin and that this pattern is apparently independent of the local organic carbon content however our paired s isotope data of carbonate associated sulfate and pyrite suggest presence of sulfate in pore fluids which is not consistent with insufficient sulfate for bacterial sulfate reduction in the water column despite evidence for deposition under anoxic conditions sedimentary molybdenum mo concentrations are mostly not enriched relative to average continental crust this relationship is consistent with the notion that sulfide dominated conditions in the water column and or the sediments are required for mo enrichment and validates past assertions that mo enrichment patterns in ancient shales track both the local presence and global distribution of euxinia specifically in addition we identified extensive diagenetic carbonate precipitation in the upper chuanlinggou formation with only moderately negative 13 c values 3 4 1 4 we propose with support from a numerical model that these diagenetic carbon isotope values were most likely derived from precipitation of carbonates dominantly in the methanic zone within the sediments diagenetic carbonate precipitation in the methanic zone is likely to have been more extensive in the proterozoic than the phanerozoic due to porewater oxidant limitation

orbitally induced limestone marlstone rhythms in the albian cenomanian cismon section venetian region northern italy sedimentology calcareous and siliceous plankton distribution elemental and isotope geochemistry abstract a multidisciplinary study of the upper albian cenomanian portion of the cismon section venetian region northern italy was undertaken in order to characterize the cyclic alternations of carbonate rich and carbonate poor layers and to investigate their possible origin and cyclic patterns limestone semicouplets are characterized by abundant radiolarians and micarbs micron sized calcitic fragments common planktonic foraminifera strong bioturbation good oxygenation as expressed by the mn and v v ni parameters high si al ratio low k al in the absence of pyrite and organic matter the marlstone semicouplets are on the contrary frequently laminated rich in pyrite and organic matter with common fish remains in the absence of radiolarians the mn and v v ni parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions which allowed the preservation of organic matter the low si al ratio and high k al ratio the latter related to abundant feldspars indicate that marlstones received a substantial contribution from a terrigenous source the abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters the enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating during marlstone deposition surface waters were less fertile than in the limestone semicouplets but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils whereas the bottom waters were oxygen depleted to accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18o depleted values in the meantime the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate the cyclic organization of limestone marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles the alternating conditions are 1 efficient mixing highly fertile surface and near surface waters and a well oxygenated seafloor to 2 enhanced water runoff or decreased evaporation or both and oxygen depleted bottom waters we suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital climatic cycles in tune with the milankovitch cycles within this scenario the latest cenomanian bonarelli level is an exceptional event marked by a large positive 13c spike it is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near surface waters the enhanced production of organic matter was in excess for the oxygen available at the bottom floor thus allowing the organic matter to accumulate in the sediments

stable carbon isotopes of amorphous kerogens from phanerozoic sedimentary rocks amorphous kerogens from phanerozoic sedimentary rocks may be divided into two groups on the basis of their 13c values h amorphous kerogens have isotopic values in the range of 24 to 20 per mil and l amorphous kerogens have isotopic values in the range of 35 to 26 per mil the occurrence of amorphous kerogens in the intermediate isotopic range from one rock unit in the data set suggests a weak continuum may exist between these two groups phytoplankton and to a lesser extent zooplankton and bacteria are considered to be the precursors for both kerogen types a plausible explanation for this dichotomy may be found in the source of co2 utilized by phytoplankton which is considered to be controlled by environmental factors h amorphous kerogens are expected to occur in silled basins that are overlain by expansive well circulated deep 500 m water where the prevailing source of carbon in the photic zone is atmospheric derived co2 conversely l amorphous kerogens are expected to occur in restricted basins that are overlain by stratified shallow 200 m water where the prevailing source of carbon in the photic zone may be organic derived co2

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

geochemistry and re os geochronology of the organic rich sedimentary rocks in the jingtieshan fe cu deposit north qilian mountains nw china abstract the jingtieshan group in the north qilian mountains nw china is dominantly composed of banded iron formations bifs copper deposits and organic rich sedimentary rocks ors carbonaceous phyllite x ray diffraction analysis of the ors shows the mineral assemblage to be quartz clay minerals the total organic carbon contents show a range of 0 44 1 72 here we present the results of the geochemistry and re os geochronology of the ors from the jingtieshan group the high values of chemical index of alteration cia chemical index of weathering ciw plagioclase index of alteration pia and th u ratio indicate intense weathering the al 2 o 3 tio 2 zr sc th sc la th ratios high rare earth elements abundances light rare earth elements enrichment normalized to chondrite and distinctly negative eu anomalies suggest that the jingtieshan group ors were derived mainly from felsic volcanic units the new re os isochron age of 1308 100 ma 2 n 6 mswd 23 broadly overlap with the previous published ages determined using sm nd and u pb isotope systems the new age represents the depositional age of the jingtieshan group as well that of bif in the jingtieshan area furthermore the initial 187 os 188 os ratios 0 44 0 07 indicate that the os in the seawater was dominantly derived from hydrothermal fluids 75 the ce anomaly ce ce 0 95 1 00 and v v ni ratios 0 71 0 86 as well as the lack of enrichment in redox sensitive trace elements such as u v zn pb cu ni cr co and mn together with the presence of overlying bif suggest that the jingtieshan area represents a ferruginous deep water succession this and intense submarine hydrothermal activities contributed to the deposition of the jingtieshan bif

geochemistry of upper birimian sediments major and trace elements and nd sr isotopes and implications for weathering and tectonic setting of the late paleoproterozoic crust abstract the upper birimian paleoproterozoic formations consist mainly of a detrital sedimentary pile intercalated with calc alkaline volcanites that were accreted during the eburnean orogeny 2 1 ga the sediments of the upper birimian group were analyzed for their major and trace elements concentrations and their nd sr isotopic compositions the shales are enriched in al 2 o 3 and k 2 o relative to paas and depleted in the other mobile major elements and lile whereas the fe shales show enrichments in cao and mno and depletions in al 2 o 3 and k 2 o relative to paas but have similar depletion in lile together with the subarkose samples the shales have undergone significant post depositional k metasomatism as indicated in the a cn k diagram the calculated pre metasomatism cia values 83 92 indicates a high degree of source weathering compatible with steady state weathering under tropical to subtropical conditions the fe shales have lower cia values 46 57 reflecting less intense weathering conditions low eu anomalies and th sc ratios combined with high cr th ratios for certain samples indicate a significant input of basic detritus for the analyzed sediments provenance modeling indicates that the shales are best modeled with a mixture having 75 basalt 75b and 25 granite 25g the fe shales have a similar provenance with their compositions being modeled with a 80b 20g mixture the nd values of selected shales calculated at 2 1 ga the age of the deposition vary from 2 8 to 8 6 which indicate that the sediment received 60 100 of its detritus from a juvenile birimian volcanic arc therefore the birimian sediments deposited within an active continental margin setting mostly supplied by the adjacent volcanic arc

the marine environments encompassing the neoproterozoic glaciations evidence from c sr and fe isotope ratios in the hecla hoek supergroup in svalbard abstract the neoproterozoic era records several important events in earth history the associations between bif deposition snowball earth events and the redox state of seawater are the key to explain the re appearance of banded iron formations bif during the neoproterozoic unraveling ancient iron cycles provides important information about the linkage between the precipitation of bif and the redox condition in the ocean but current iron isotopic data are limited to sediments deposited only during the sturtian glaciations and ediacaran we conducted a detailed geological survey of the tonian to ediacaran sedimentary succession in nordaustlandet island svalbard our chemostratigraphies of 13 c carb from pre sturtian to post marinoan sedimentary successions in the polarisbreen group are consistent with those in coeval sediments we first analyzed iron isotope ratios 56 54 fe of individual pyrite grains with diverse shapes in carbonates black shales quartz arenites and diamictites in order to constrain the redox condition in seawater we found large variations in iron isotope ratios from ca 2 to ca 4 positive 56 54 fe values over 1 0 are only preserved in euhedral and aggregated pyrite grains in black shales of the backlundtoppen formation and the macdonaldryggen member of the elbobreen formation a partial oxidation of ferrous iron in seawater is necessary to explain such high 56 54 fe values which means that deep seawater was ferruginous ferrous iron rich before and after the sturtian glaciation this stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in neoproterozoic seawater this study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the sturtian glaciation if the deep ocean was intrinsically iron rich it is likely that neoproterozoic bifs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation

sources of osmium to the modern oceans new evidence from the 190pt 186os system high precision os isotope analysis of young marine manganese nodules indicate that whereas the composition of modern seawater is radiogenic with respect to 187os 188os it has 186os 188os that is within uncertainty of the chondritic value marine mn nodule compositions thus indicate that the average continental source of os to modern seawater had long term high re os compared to pt os analyses of loess and freshwater mn nodules support existing evidence that average upper continental crust ucc has resolvably suprachondritic 186os 188os as well as radiogenic 187os 188os modeling the composition of seawater as a two component mixture of oceanic cosmic os with chondritic os compositions and continentally derived os demonstrates that insofar as estimates for the composition of average ucc are accurate congruently weathered average ucc cannot be the sole continental source of os to seawater our analysis of four cambrian black shales confirm that organic rich sediments can have 187os 188os ratios that are much higher than average ucc but 186os 188os compositions that are generally between those of chondrites and average ucc preferential weathering of black shales can result in dissolved os discharged to the ocean basins that has a much lower 186os 188os than does average upper crust modeling the available data demonstrates that augmentation of estimated average ucc compositions with less than 0 1 additional black shale and 1 4 additional ultramafic rock can produce a continental end member os isotopic composition that satisfies the requirements imposed by the marine mn nodule data the interplay of these two sources provides a mechanism by which the 187os 188os of seawater can change as sources and weathering conditions change yet seawater 186os 188os varies only minimally

behavior of re and os during low temperature alteration results from himalayan soils and altered black shales re os analyses were performed on five soil profiles developed on silicate lithologies in the narayani drainage basin of the himalayas as well as on several altered black shales and associated alteration phases the results indicate that all of these samples have lost large amounts of re this re loss approached 100 in the black shale derived soils which also most probably lost substantial quantities of os the near constancy of os and re concentrations with depth indicates that this loss occurred quite early in the alteration process rather than progressively during the course of soil development a slight decrease in 187os 188os was observed toward the surface of the two black shale soil profiles possibly indicating a minor preferential mobility of radiogenic 187os for this lithology in the black shale derived saprolite and soils the loss of re and os was related to the loss of organic carbon the importance of this phase is underscored by the fact that organic material separated from a black shale sample accounted for nearly all of the os contained in the whole rock analysis of alteration phases from the surface of a weathered black shale showed that os liberated during alteration was partially reprecipitated in fe oxides with os isotopic compositions very similar to that of the original rock whereas re was removed with the fluid phase these results demonstrate the disproportionately large role that black shale weathering may play in determining the re and os contents of continental runoff and thus ultimately of seawater but they also emphasize the possible complexity of this process the data suggest that a large fraction of the os contained in black shales is released during chemical alteration which is apparently not the case for most other silicate rocks therefore the potential contribution of black shales to the dissolved os budget of runoff and river water may be even greater than that expected from the high os concentrations of these rocks the extent to which this os will affect the seawater os composition depends on how efficiently it is recaptured by secondary phases such as fe mn oxides in situ or during transport these results also underscore the high mobility of re with respect to os at every stage in the erosional process thus explaining the order of magnitude increase in re os ratio from the black shale source rocks to seawater

application of the 187re 187os system to black shale geochronometry the decay of 187re to 187os provides a tool for determining depositional ages of black shales re and os concentrations and os isotopic compositions of whole rock samples of the bakken shale a mississippian devonian boundary black shale yield a whole rock isochron with an age of 354 49 ma this age is in agreement with the accepted age of the bakken shale 360 ma the initial 187os186os ratio of the isochron is 6 2 3 3 this value is indistinguishable from the 187os186os ratios observed in modern black sea sediments the concentration of common os in bakken shale samples is strongly correlated with total nitrogen indicating that a large fraction of the os in these samples is associated with a hydrogenous component which overwhelms any os supplied to the sediment in association with detrital material or cosmic dust the dominance of the hydrogenous component imparts a relatively homogeneous initial 187os186os ratio to the sediment at the time of deposition the degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity the pattern of scatter is consistent with postdepositional mobilization of re and or os on a small spatial scale we suggest that this mobilization may be a consequence of petroleum formation and migration in the bakken shale

trace element and multi isotope geochemistry of late archean black shales in the carajas iron ore district brazil abstract the 250 300 m thick carajas formation in the carajas mineral province northern brazil consists of banded iron formation including giant high grade iron ore deposits and minor black shale overlying a thick pile 2 3 km of about 2 75 ga old metabasalt carbonaceous shale with pyrite and locally pyrrhotite rich patches from drillcore of the serra sul exploration project has up to 29 ppm mo iron speciation analysis indicates essentially ferruginous and for some samples likely euxinic depositional conditions positive 34 s isotope ratios of tris are between 0 3 to 10 7 with heavy data restricted to pyrrhotite free samples the data suggest microbial sulfate reduction under at least partially sulfate limiting conditions with later overprint by migrating solutions the black shale is affected by pronounced low temperature potassium metasomatism k 2 o na 2 o 100 up to 10 wt k 2 o as adularia related to diagenetic processes at 98 95 mo for continental input t dm of 2 8 3 1 ga according to nd isotope data and 0 9 98 95 mo for a hydrogenous component black shale samples from the bottom of the bif sequence have heavy mo isotope composition of up to 1 8 98 95 mo the significant mo isotope fractionation is either the result of an early and transient whiff of oxygen at 2 7 ga or the result of hydrothermal fluid overprint

precise re os ages of organic rich mudrocks and the os isotope composition of jurassic seawater rhenium and osmium isotope and abundance data have been obtained on precisely located samples from three suites of immature organic rich mudrocks from jurassic coastal outcrops in england the data provide accurate whole rock ages of 207 12 ma 181 13 ma and 155 4 3 ma for suites of hettangian toarcian exaratum subzone and kimmeridgian sensu anglico wheatleyensis subzone samples these new re os ages are indistinguishable within the assigned analytical uncertainties from interpolated depositional ages estimated from published geological timescales and establish the importance of the re os dating technique for chronostratigraphic studies early diagenetic pyrite nodules possess levels of re and os which are 1 2 orders of magnitude lower than in the enclosing organic rich mudrocks indicating that these elements had already been removed from sediment pore waters at the time of nodule formation thus the re os isotope system in these organic rich mudrocks has been closed since or from very soon after the time of sediment deposition because most of the re 98 and os 95 99 8 in the mudrocks is shown to be hydrogenous the 187os 188os i of the samples is interpreted to be that of contemporaneous seawater the data thereby provide the first estimates of the os isotope composition of jurassic seawater during the earliest jurassic hettangian the seawater 187os 188os ratio was extremely unradiogenic 0 15 it had increased to 0 8 at the end of the early jurassic toarcian 20 ma later while in the late jurassic kimmeridgian the seawater 187os 188os ratio was 0 59 the most likely explanation for the unradiogenic os isotope composition of hettangian seawater is that the contribution of unradiogenic os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic os from the continents at that time this interpretation is in line with observations suggesting that global weathering rates were low in the hettangian and that increased hydrothermal and volcanic activity preceded the break up of pangea the re os ratios of hettangian mudrocks and by inference of contemporaneous seawater are similar to those of mudrocks deposited at later times during the jurassic and argues against the unradiogenic os in hettangian seawater being derived from extraterrestrial meteoritic sources

deformation of helderberg limestones above the blind thrust system of the central appalachians helderberg limestones have been subjected to late burial solution and layer parallel shortening by cleavage formation with twinning they also record deformation during passive macrofolding due to emplacement of the underlying blind thrust system including passive cleavage rotation during flexural flow perhaps by grain boundary sliding cleavage intensification in gentle fold limbs and cleavage dilation in steep limbs by cleavage normal untwinned calcite fibers cleavage dilation was the second dilation event as bedding solution structures were previously opened with bedding normal fiber growth during layer parallel shortening thus pre existing solution structures were commonly exploited as fluid conduits and precipitation sites

relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas u s a analyses of 21 samples collected from a core of the 52 8 cm thick stark shale member of the dennis limestone in wabaunsee county kansas demonstrate four cycles with two orders of magnitude variations in contents of cd mo p v and zn and order of magnitude variations in contents of organic carbon cr ni se and u the observed variability in amounts and or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water column stratification and the resulting absence presence of dissolved o2 or h2s high cd mo u v zn and s contents a high degree of pyritization dop 0 75 0 88 and high high v v ni 0 84 0 89 indicate the presence of h2s in a strongly stratified water column intermediate contents of metals and s intermediate dop 0 67 0 75 and intermediate v v ni 054 0 82 indicate a less strongly stratified anoxic water column whereas low metal contents and low v v ni 0 46 0 60 indicate a weakly stratified dysoxic water column high p contents at the top of the organic matter rich intervals within the stark shale member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified h2s bearing water column indicates a combined halocline thermocline with the fresher near surface water the transport mode for the terrestrial organic matter the predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline relatively low amounts of degraded hydrogen poor organic matter characterize intervals deposited in a weakly stratified dysoxic water column the inferred variability in chemistry of the depositional environments may be related to climate variations and or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member