Abstract This book is a collection of papers focusing on various aspects of authigenic and diagenetic marine minerals and related elemental cycling. In essence, it belongs to a line of specialized volumes marking major steps of continued international cooperation. Closely following the establishment of UNESCO International Geological Correlation Programme (IGCP) Project 156 (Phosphorites) in 1978, SEPM Special Publication 29 (Marine Phosphorites, edited by Y. K. Bentor) was published in 1980 as a collection of stimulating papers stemming from a symposium on marine phosphorites held at the Xth International Congress on Sedimentology in Jerusalem in July 1978. Nearly simultaneously, a thematic set of special papers on “Phosphatic and Glauconitic Sediments” was published by the Journal of the Geological Society (Volume 137, 1980) following a meeting of the Geological Society on the same subject earlier that year. Led by P. J. Cook and J. H. Shergold (1978-1984) and later by W. C. Burnett and S. R. Riggs (1984-1988), IGCP Project 156 conducted 29 international and regional field workshops and symposia. Over the next two decades, a flourishing of international research on phosphorites and their related facies ensued, with many workers attempting to solve the many “Unsolved Problems” outlined by Bentor (1980) in his seminal introduction to SEPM Special Publication 29. A great deal of this research was submitted for Special Publication 52 of the Journal of the Geological Society of London, published in 1990, and edited by A. Notholt and I. Jarvis, who organized the final International Symposium of UNESCO Project 156 in Oxford in 1988.

Abstract: In this paper, we consider how long-term tectonic conditions and their effect on the surface environment of the earth interacted with the global carbonate cycle during the hothouse (greenhouse)–icehouse transition of the past 100 million years. Using the recalculated output of the Berner, Lasaga, and Garrels (BLAG) geochemical model as a template ( Berner et al., 1983 ; Lasaga et al., 1985 ), we computed changes in seawater carbonate chemistry for the past 100 m.y. Experimental dolomite and calcite precipitation rate data as a function of environmental conditions were used to calculate the ratio of rates of dolomite and calcite precipitation rates during this period of time. We conclude from these model calculations that the observed decrease in the ratio of dolomite to calcite in sedimentary carbonates deposited since the Late Cretaceous transgression was a result of changes in the ocean saturation state with respect to carbonate minerals and global surface temperature. Thus, the solution to the classical “dolomite problem” may lie in relatively small but coupled changes in the composition and temperature of the atmosphere and seawater.

Abstract: Conventional hydrologically-driven models of dolomite formation, though popular, often lack empirical support, and encounter fundamental chemical problems related to kinetic impediments in saline solutions: these include the high hydration energy of the magnesium ion, the extremely low activity of the carbonate ion, and the presence of even very low concentrations of sulfate. Although an organogenic dolomite model exists, it has been mainly limited in application to modern, organic-rich, deep marine sediments. However, growing evidence from modern and ancient sediments points to a greatly enhanced and fundamental role for benthic microbial communities in dolomite formation, linked to anoxic organic diagenesis driven by sulfate reduction during which all kinetic inhibitors to dolomite formation are overcome. Magnesium concentrated in cyanobacterial sheaths may be liberated during degradation into sulfate-free solutions of high ionic strength and carbonate ion activity, to become available for dolomite formation. Another possible source of magnesium in modified saline solutions is from dissociation of complexed ions. These processes, which provide the basis for a new organogenic model for dolomite formation, have operated throughout the geologic record on a range of scales, from relatively minor modern occurrences to the thick, extensive platformal dolostones of the Proterozoic which developed in association with microbially-dominated shallow marine environments. Analyses of lake waters and sediments from ephemeral Coorong lakes of South Australia reveal seasonally high carbonate alkalinities and magnesium concentrations in association with intense bacterial sulfate reduction and cyanobacterial degradation, indicating a genetic link between microbial mediation of ambient waters and dolomite formation. The ecosystem of the Coorong distal ephemeral lakes during late stages of evaporation provides a small scale analogue for biogeochemical processes operating in microbially-dominated shallow marine environments typical of the Proterozoic, where sulfate reduction would have been a major, shelf-wide phenomenon operating just beneath the sediment surface, continually driving biochemical modification of interstitial waters and sustaining high carbonate alkalinity in an underlying sulfate-free medium. Thin sections of late Archaean platformal Conophyton stromatolites from South Africa reveal progressive disintegration of constituent cyanobacterial sheaths accompanied by the appearance and increasing crystal size of replacive, authigenic dolomite in a calcite matrix, indicating that magnesium was derived from in situ degradation of cyanobacterial sheaths. Total loss of sheath material resulted in a coarse dolospar fabric preserving no evidence of its microbial origin. Petrographic analysis of silicified microbial shelf sediments, including cyanobacterial mat, ooids, peloids and intraclasts, from the Cambrian Eilean Dubh Formation of northwestern Scotland reveals a common diagenetic trend in which sequential anoxic biofabric degradation resulted in the appearance and progressive increase in abundance of dolomite, culminating in a dolospar fabric. Structureless dolostones may thus evolve through a sequence of ephemeral fabrics by authigenic precipitation or replacement linked to microbial degradation, but evidence for their origin depends on the preservation of successive stages in their development from organic-rich sediments to massive dolomites. The predominance of stromatolites and other microbialitic sediments in thick dolostone platformal successions throughout much of the Precambrian and early Paleozoic, indicates that organic diagenesis has played a major role in dolomite formation from the microbial to the global scale.

Abstract: Phosphorus is a critical element in the biosphere, limiting biological productivity and thus modulating the global carbon cycle and climate. Fluxes of the global phosphorus cycle remain poorly constrained. The prehuman reactive phosphorus flux, to the ocean is estimated to range from 0.7—4.8 x 10 12 g/yr. Uncertainty in the reactive phosphorus flux hinges primarily on the uncertain fate of phosphate adsorbed to iron oxyhydroxide particles which are estimated to constitute 50% or more of the chemically weathered-phosphorus river flux. Most reactive phosphorus is initially removed from seawater by burial of organic matter and by scavenging onto iron-manganese oxide particles derived from mid-ocean ridge (MOR) hydrothermal activity. Calculation of the oceanic phosphorus burial flux is complicated by early diagenetic redistribution of both oceanic and terrestrial phosphorus. Increased phosphorus input during periods of warm, humid climate is offset to some degree by increased burial rate as productivity shifts to expanded shallow-water estuary and shelf areas where phosphorus is rapidly decoupled from organic matter to form phosphorite. Phosphorus scavenging is greater if high sea levels are associated with increased MOR hydrothermal activity such as during the Late Cretaceous. Less phosphorus is derived from weathering during cool, dry climatic periods but a more direct transportation of phosphorus to the deep ocean, and a shift of productive upwelling regions to deeper water areas allows more phosphorus to be recycled in the water column. Lowered sea level results in less effective trapping of phosphorus in constricted estuary and shelf areas and in an increase ill the phosphorus flux to the deep ocean from sediment resuspension. A decrease in MOR spreading rates and the resulting decrease in phosphorus scavenging by iron-manganese oxide particles would result in more phosphorus for the biosphere. Orogeny and glaciation may accelerate chemical weathering of phosphorus from the continents when the increased particle flux is exposed to warm and humid climate. Large, reworked phosphorite deposits may proxy for short-term organic carbon burial and correspond to periods of increased reactive phosphorus input that cannot be accommodated by long-term organic matter and iron-oxide particulate burial.

Abstract: Until very recently, the effect of tectonisrn 011 the exogenic phosphorus cycle has been neglected. Currently, the subduction of phosphorus associated with organic matter buried in ocean sediment to the mantle represents a significant flux relative to other fluxes within the exogenic phosphorus cycle. Phosphorus is generally considered to have been the limiting nutrient for oceanic primary production over much of geologic time. Therefore, changes in the total exogenic phosphorus mass control, to a certain extent, the amount of phosphorus available for oceanic production. Over geologic time, the exogenic phosphorus mass is likely governed by plate tectonics; specifically, by the balance between input of newly formed crystalline rock and the output via subduction of oceanic crust and the overlying sediment to the mantle.

Abstract: The marine geochemistry of phosphorus links the burial rate of organic carbon in marine sediments to the oxygen content of the atmosphere and may serve as a major component of the system that controls atmospheric oxygen. The return flux of phosphate from marine sediments to seawater is an important part of the marine phosphorus cycle. This paper examines the relationship between the return flux of phosphate and the oxidation state of marine sediments, a necessary preliminary step in defining the efficacy of the oxygen control mechanism. The diffusive return flux of phosphate from marine sediments to overlying bottom waters was calculated for 193 published pore water phosphate profiles that met a number of stringent criteria for sediment core and pore water recovery and processing. The phosphate return fluxes, scaled to carbon regeneration fluxes, are significantly greater from highly reduced sediments than from highly oxidized sediments. In highly reduced sediments the return phosphate flux from carbon regeneration is frequently augmented by a large phosphate flux released during the reductive dissolution of ferric (oxy)hydroxides. in highly oxidized sediments the return phosphate flux can be somewhat less than the flux to be expected from carbon regeneration. The missing phosphate is probably adsorbed on ferric (oxy)hydroxides in these sediments. The strong coupling between the oxidation state of marine sediments and the return phosphate flux to seawater suggests that the marine phosphate cycle is indeed an important part of the system that stabilizes atmospheric O 2 . The total preagricultural return flux of P from marine sediments was ca. 12 × 10 11 mol P/yr. This rate is more than an order of magnitude larger than the riverine flux of total dissolved phosphorus to the oceans, ca. 0.3 × 10 11 mol P/yr. We estimate that the total continental preagricultural flux of reactive P that ultimately appears dissolved in ocean water was ca. 3.5 × 10 11 mol P/yr. The large flux of continental P to seawater, via direct input of riverine dissolved inorganic and organic P and via the diagenetic return flux from reactive continental particulate P deposited in marine sediments, indicates that the marine residence time of phosphate with respect to terrigenous inputs is ca. 10,000 years. This figure depends quite heavily on the fraction of terrigenous, particulate-phase phosphate that is released to seawater during diagenesis. Variations in this fraction can significantly affect the marine residence time of phosphate and the relative proportion of detrital versus authigenic phosphate phases in marine sediments.

Abstract: Rivers of South Asia carry dissolved inorganic phosphorus (DIP) at levels far greater than many of the world’s rivers such as the Amazon River and European rivers on whose phosphorus data average rates of DIP exported to the oceans are largely based. Similarly, all the major river systems in South Asia have sedimentary phosphorus values that are considerably higher than the world average. Unlike the Amazon or other large river system for which extensive data are available, organic phosphorus contributes very little to the total sedimentary phosphorus pool in South Asian rivers. Trapping of enormous amounts of sediment in the vast alluvial belts in the northern part of South Asia indicate that very little of the sedimentary phosphorus transported by these rivers is likely to be exported to the adjacent marine environment.

Abstract: Early-diagenetic aluminophosphate minerals (mainly florencite) are ubiquitous in ancient (Archean to Cretaceous) marine-deposited sandstones. The crystals are >10 μm in diameter and are mostly associated with thin coatings or pockets of detrital clay particles lining quartz grain surfaces. Aluminophosphate crystals are also found in altered detrital aluminosilicate grains (e.g. feldspar). The aluminophosphate minerals precipitated in sands deposited in shallow marine environments. Diagenetic textures and the presence of structural sulfate in the aluminophosphate minerals suggest that the authigenic crystals formed shortly after deposition, probably in the zones of sulfate reduction and methanogenesis. The aluminophosphate minerals formed as the concentrations of phosphate in the pore water increased and phosphate combined with various cations (e.g. calcium, barium, rare earth elements) to precipitate at sites of aluminum availability (e.g. detrital clay particles and aluminosilicate grains). Although the aluminophosphate minerals are volumetrically minor constituents (<0.05 wt%), the authigenic crystals, which are present in >90% of samples studied, are spatially and temporally widespread in ancient Australian sandstones. Burial flux estimates show that phosphorus is removed at a rate of ~5.6 × 10 -7 g/cm 2 -yr. This value suggests that aluminophosphate precipitation was probably an important sink for oceanic phosphorus in the past, and possibly significant in present-day coastal environments. The discovery of the magnitude of the authigenic aluminophosphate sink has implications for models of the marine phosphorus cycle.

Abstract: Sedimentary phosphorite formation has occurred episodically over geologic time. Substantia] phosphorite deposits were formed during P-giant episodes of which the most widespread and long-lasting were the Precambrian-Cambrian and the Cretaceous-recent. The aim of this review is to reevaluate these two episodes in the light of sedimentary Sr, Nd, S, and C isotopic records. Some, although not all of these phases of phosphorite formation were accompanied by increases in seawater 87 Sr/ s6 Sr unrelated to changes in seafloor spreading, which supports a possible link between orogeny and global weathering rates, P-input, and P-giant formation. Assessing the relationship between δ 13 C and phases of widespread phosphogenesis is more complicated due to the counterbalancing effects on seawater δ 13 C of productivity/organic matter deposition and subsequent phosphogenesis/early diagenetic carbon oxidation. It is recommended that proposed links between phosphogenesis and global changes of δ 13 C or P- input continue to be reexamined on a case-to-case basis. The application of secular trends in Nd isotopic ratios to unravelling the often vital role of paleocurrents in phosphorite formation is also discussed. In addition, recent isotopic research is outlined that has led to significant improvements in stratigraphic resolution around the Precambrian-Cambrian boundary. A temporal and causal connection is put forward between metazoan evolution, that is the introduction of bioturbation, fecal pellets, biomineralization, and filter feeders, which would have helped to concentrate mineral phosphate in sediments, and widespread phosphogenesis at this time.

Abstract: Calcium carbonate is known to be one of the sedimentary materials occuring with phosphorite in many of the phosphorite deposits. This relationship has been described for various geological periods and at various scales: mineable phosphorite alternating with barren carbonate; calcific or more often dolomitic phosphorite alternating with phosphatic dolomite or limestone. In the present study, we attempt to explain why there is a link between phosphorite and carbonate. This relationship does not presuppose a genetic link between minerals, although this link does exist where apatite can be recognized as the result of apatitization of pre-existing calcite. Calcite is a true biomineral that is directly produced by living marine organisms as their skeletons. It is deposited as a solid material before various diagenetic recrystallizations add their imprints. Apatite is an authigenic mineral, precipitated during early diagenesis from an organic matter (OM)-rich ooze. The OM must be furnished predominantly by naked and or siliceous organisms and its degradation processes are controlled by dysoxic conditions. The formation of limestones depends on a predominantly carbonate-producing productivity that is common because of the abundance of carbon. Phosphorite formation requires high quantities of OM related to specific noncarbonate-producing productivity that is less common bccause the availability of phosphorus is minor compared to that of carbon. The “phosphorite factory,” which produced the phosphorite deposits, and the “carbonate factory” are independent but complementary rather than competing processes; they do not use the same primary material, nor do they occur at the same phase of the sedimentary cycle. The occurrence of alternations of beds of phosphorite and beds of calcium carbonate is thought to be the record in the sediment of changes in the dominant type of bioproductivity in the water column. Shallow-water shelves without significant detrital influx are the environments where such frequent changes in the type of bioproductivity are known to occur easily in response to slight physical or chemical changes.

Abstract: Chemical components of most ooidal ironstones and phosphorites were initially derived from deeply weathered uplands and deposited in shallow waters of epeiric seas and continental margins that received very little clastic sediments. They were then commonly transformed during diagenesis largely by upwelling seawater and redeposiled in winnowed layers. Ironstones and phosphorites differ, however. Ironstones were first laid down as ferric oxide and kaolinite ooids and peloids, whereas phosphorites were formed first as phosphatic carbonate granules, crusts, and hardground. Ironstones were then transformed to berthierine (or rarely nontronite) during diagenesis, whereas phosphorites were transformed to carbonate fluorapatite (CFA). In terms of their distribution in the geologic record ironstones and phosphorites are also dissimilar. A few ooidal ironstones were deposited in Early Proterozoic time. Most accumulated during greenhouse phases of the Phanerozoic (Ordovician–Devonian and Jurassic–early Cenozoic). With the exception of the late Cretaceous–early Cenozoic deposits, major phosphorites are notable for their absence in rocks of those ages. In fact, phosphorites were most abundant in the Late Proterozoic and early Cambrian times.

Abstract: Precambrian banded iron-formations (BIPs) are chemical sediments of hydrothermal origin and consist of Fe-rich minerals with alternating layers of chert. Because microorganisms potentially played a role in their precipitation, the study of bacterial-mineral interactions at modern hydrothermal environments may provide small-scale analogues to those conditions under which they accumulated. Interestingly, microbial populations currently growing at hot springs and deep-sea vents are commonly encrusted in iron and silicate minerals. Iron biomineralization occurs either passively through interaction between the reactive sites of the cell and dissolved cationic iron from the hydrothermal fluid, or actively through chemolithotrophic iron-oxidation by bacteria such as Gallionella genera. Amorphous silica precipitates on individual bacteria through hydrogen bonding between hydroxy groups in the extracellular polymers and hydroxyl groups in dissolved silica, with some colonies becoming completely cemented together within a siliceous matrix up to several micrometers thick. Iron-silicates form due to reactions between dissolved silica and cell-bound iron. In these predominantly nonspecific processes, bacterial cells simply catalyze reactions that are rendered possible by the supersaturated conditions created by the sudden physical and chemical changes induced through venting. Diagenetic reactions, some of which are also catalyzed by microorganisms growing in the sediment, can further alter the mineralogy of these primary precipitates, leading to the formation of secondary magnetite and siderite. In this way, all of the main mineralogical components of BIFs can be associated with microbial activity.

Abstract: Barite occurs throughout the geologic record as massive beds, laminations, rosettes, and nodules. The most important scientific and economic occurrences of barite are stratabound and stratiform massive beds from the Early Archean and the Early to Middle Paleozoic. Paleozoic bedded barites are by far the most volumetrically significant deposits in the geologic record. Additional occurrences have been documented in some Middle Prolerozoic, Late Proterozoic, and Mesozoic rocks and in several localities on the modern ocean floor. Bedded barite is believed to have formed as emanations from seafloov sediments, as diagenetic replacements of preexisting minerals, or as direct precipitants due to biological fixation of barium in the water column. Direct field evidence to differentiate between these theories is often lacking or contradictory. Geochemical studies. particularly those that have employed δ 34 S and 87 Sr/ 86 Sr analyses of the barite, have proven very useful in understanding bedded barite genesis. The low solubility of barite relative to other natural salts has helped barite survive as a pseuodomorph of stratiform evaporite minerals in some Archean sedimentary sequences. Other examples of Archean barite appear to have a shallow water delrital or authigenic origin. Very low δ 34 S values of Archean barite are interpreted as indicating a low-sulfate ocean. Large deposits of Paleozoic bedded barite are typically found in fine-grained, organic-rich siliciclastic sequences and are associated with massive and disseminated sulfides, cherts, phosphorites, and less frequently limestones and volcanic rocks. δ 34 S analyses indicate that almost all bedded barite had a seawater sulfate source. The genetic link between Paleozoic bedded barites and sedimentary submarine exhalative Pb-Zn sulfide deposits has been established by field and geochemical study of deposits in western Canada and western Europe. 87 Sr/ 86 Sr analyses suggest that these bedded barites have a continental barium source. Economically important bedded barites in China, Arkansas, and Nevada have 110 significant sulfides or other hydrothermal manifestations. The clear association of dissolved barium and barite with biological cycles in the modern ocean, associations with phosphorites and cherts, and 87 Sr/ 86 Sr analyses that are comparable to contemporaneous seawater suggest that the Chinese, Nevada, and Arkansas barite deposits formed as biological precipitates 011 the seafloor. Bedded barite formed by this mechanism holds promise as an indicator of high paleoproductivity and open ocean sulfate reduction during selected periods of the Paleozoic. The lack of world class examples of bedded barite in Mesozoic and Cenozoic black shale sequences indicates a lack of open ocean sulfate reduction during these periods of geologic time.

Abstract: Phosphorites on the Peru shelf occur mainly as dark and dense conglomerate (“Diphosphates) and occasionally as soft, friable (“F”-phosphatcs) protocrusts, which appear as laterally extensive, centimeter–thick plates of bored, pbosphatized sediment. Several undisturbed and well–documented samples of these protocrusts were collected by box coring and submersible operations during an expedition of the R/N Seaward Johnson in 1992 . Three of these crusts were analyzed in layer-by-layer fashion for U-series isotopes, while two were analyzed for AMS- 14 C. and stable carbon isotopes. Unlike the D-phosphates, which may show a progressive decrease in their δ 13 C values (from 0 to –5%c) with a co-linear increase in structural CO 2 contents (from 2–6 wt. %), the protocrusts show a more restricted range with most values from 0 to –2%,δ 13 C and 4.2-4.7 wt.% CO 2 , indicative of their precipitation directly on or slightly below the seafloor. U-series disequilibrium results ( 230 Th/ 234 U, 231 Pa/ 235 U) indicate that the protocrusts are very young. They are so young, in fact, that corrections for“common” 230 Th and 231 Pa (that is, those amounts not associated with decay of the incipient uranium) are very important, and different interpretations (of growth direction, for example) are possible depending upon which assumptions are applied. AMS 14 C results are very consistent with the U-series ages in the topmost, highest-phosphate portion of these crusts. A few millimeters into the less phosphatic portions of the crusts, uncorrected 230 Th and 231 Pa ages diverge from the l4 C ages, probably because of contamination of these daughter nuclides from associated sediment grains. An isochron approach confirms that there is a minimal age difference between the upper and lower layers of the protocrusts. All protocrusts display a distinct trend of decreasing phosphate content with depth into the crust. Based on these observations, we propose a growth model that suggests that phosphatic protocrusts grow upward at rates of 2–9 mm/ky in response to downward diffusion of phosphate from an interstitial phosphate pore water spike just below the sediment-seawater interface. Later exhumation of protocrusts resulted in erosion into the more common and more complicated phosphatic hardgrounds, conglomerates, and nodules found scattered throughout the sediments of the Peru shelf upwelling zone.

Abstract: Phosphatic grains and nodules related to recent diatom oozes on the inner Namibian shelf as well as Pleistocene grains and nodules from the phosphorite deposit on the adjacent outer shelf have been studied by means of scanning electron microscopy combined with several analytical methods. Recent grain and nodules are being formed simultaneously in the same diatom oozes. Both consist of poorly, moderately, and well-lithificd varieties that reflect the process of progressive phosphatization, compaction, and crystallization of phosphatic matter along with dissolution and expulsion of nonphosphatic components. Similar processes are going on during the phosphatization of macro- and microcoprolites. The common varieties of apatite mineral morphologies are colloform masses without distinct texture, globules and globular aggregates with more or less pronounced radial crystallization, and micrometer-sized elongated particles and their aggregates of various shape, which are interpreted as diagenetically formed crystallites. All of these textures are related to each other, with the globular one becoming predominant in most lithified accretions. Reworked Pleistocene grains reveal textural features quite similar to the recent grains. They contain similar organic remains, which proves their common origin in the same environment. The chemical evolution of recent accretions in the course of their 1 ithification consists of gradual increases of phosphatic components and decreases of nonapatite components. The most striking feature of their geochemistry is the extremely low concentration of rare Earth elements in recent grains in contrast to their high concentration in Pleistocene grains. This might mean that phosphate absorbs these elements during prolonged contact with the ambient seawater.

Abstract: Phosphorite nodules recovered from the upper west Florida slope provide an analog for phosphogenesis under conditions of marginal upwelling that are significantly different from the regional, continental shelf-upwelling models applied to the majority of the southeastern United States. We believe that ferruginous (AI and All) and nonferruginous (BI and BII) phosphorite nodules record the episodic precipitation of francolite since the mid-Miocene (12–15 Ma) in response to positioning of the Gulf of Mexico Loop Current. It is likely that during sea level highstands, deflection of the Loop Current landward increases the C flux, establishing conditions suitable to the concentration of dissolved inorganic phosphorous (DIP) within redox stratified sediments. Francolite containing approximately 6.2% CO 2 (a-values: 9.316-9.324 Å) apparently precipitates in response to the early diagenesis of organic matter and/or Fe-P shuttling under suboxic to anoxic conditions (δ 13 C:-6.55-0.47%o PDB, δ 18 O: -4.92-2.07%o PDB), forming nonferruginous nodules (<6% Fe 2 O 3 ) and hardgrounds. Anhedral to subhedral francolite (ovoids, globules, and botryoids) encountered in BI and BII nodules is mineralogically consistent with primary nucleation, and appears to record a microbial component. Subsequent recrystallization of francolite, in response to submarine diagenesis, obliterates such fabrics, producing euhedral, hexagonal crystallites (AI and All) possessing a decrease in carbonate substitution to 4.9-5.5% CO 2 (a-values: 9.322-9.328 Å). During sea level lowstands, oxic conditions prevail, favoring Fe-enrichment of nodules (up to 22% Fe 2 O 3 ) at the expense of carbonate minerals and glaucony. This is consistent with the evolution of primary, BII nodules into ferruginous lithotypes (AI and All). Aragonite dissolution and the dissolution and/or reprecipitation of both calcite and francolite (δ 13 C: -1.09-0.67%c PDB, δ 18 O: -0.61-1.38%® PDB) within ferruginous nodules occurs during such intervals in response to changes in pore water redox and pH. Both petrographic observations and major element trends illustrate Fe-enrichment processes associated with loss of fine micritic cements and biogenic carbonate as a result of dissolution and/or replacement by FeOOH precipitates, as well as the oxidation of glaucony.

Abstract: A sediment core collected from a bathymetric high off Goa on the western continental margin of India has yielded phosphatic nodules at various subsurface depths (at 110, 150, 305, 355, 435, 500, 505, and 525 cm). The nodules are hosted by sediments of Pleistocene age. They are <1-5 cm in size, have carbonate flourapatitc (CFA) as a single autliigenic mineral phase, are free of detrital inclusions, and have very high P 2 0 5 contents (<30%). In addition, soft and hard phosphatic nodules have also been recovered from the eastern continental margin of India. Eight nodules from various subsurface depths of the sediment core from the western continental margin and two nodules from the eastern continental margin were analyzed for uranium and rare-earth elements (REEs) by inductively coupled plasma-mass spectrometry (ICP-MS). Total REE contents are very low (8–21 ppm) in western continental margin nodules and only slightly higher in eastern continental margin nodules (maximum is 42 ppm). REE abundances relative to shale (ΣREE in sample/ΣREE in shale) are less than 0.22 and are comparable to phosphatic nodules from the Namibian continental margin, and are slightly lower than the nodules from the Peruvian continental margin. Light REEs are depleted, with La n /Yb n r(La sample /La shale )/(Yb sample /Yb shale )] values lower than unity, indicating a minimal contribution from terrestrial sources. The cerium anomaly (Cc/Ce*), which is a measure of Ce fractionation relative to the neighboring REEs, ranges between 0.64 and 1.38 with most close to 1, indicating very little Ce fractionation. Uranium concentrations are very high in nodules from both margins. Low REE contents and LREE depletion collectively indicate either a seawater or porewater source for these elements. Since Ce is mostly stable in its trivalent stale and U precipitates under reducing conditions, the absence of Ce fractionation in association with U enrichment indicates that the nodules may have formed by autliigenic precipitation in reducing porewaters. High productivity in association with upwelling might have driven the accumulation of organic matter, which ill turn would help enrich phosphate in porewaters, eventually leading to the formation of high-grade phosphorites. Ce anomaly values in nodules do not correlate with paleoredox conditions in the water column, thus the REEs for these nodules are interpreted to mainly reflect porewater conditions.

Abstract: Investigations 011 green grains from sediments of the western continental margin of India, between Ratnagiri and Cape Comorin, (water depth 37-330 111) indicate the presence of verdine and glaucony facies. Verdine facies occurs over an area of about 100,000 km 2 , representing the largest sedimentary basin in the world associated with low fluvial input. Green grains occur as irregular grains, fecal pellets, and infillings/internal molds of microfossils. They are abundant on the shelf off river mouths and their distribution varies with sediment type. Grains vary from dark green to pale green or brownish green. Green grains studied here are a mixture of predominant authigenic green clay and detrital clay minerals and are altered. Both phyllite C and suspected phyllite V- (verdine minerals) associated green grains occur on the continental shelf (between 37 m and 100 111), the former being associated with the transition zone between inner and outer shelf and the latter with relict sands and reefs on the outer shelf. On the continental slope, suspected phyllite V occurs at depths between 100 111 and 235 111, followed by phyllite C at 280 m depth and glauconitic smectite of the glaucony facies at 330 m. As these grains are composed of a mixture of predominant authigenic clay, detrital clay, and some altered products, their major element composition differs from those of the green grains (reported elsewhere) that contain pure authigenic clay. The low REE contents and flat shale-normalized REE patterns suggest that the REEs were inherited from the substrate. We suggest that the size of the verdine deposit is related to the influx of iron rather than the amount of fluvial discharge. The color and morphology of the grains do not reflect the authigenic mineral or its evolution. Green grains 011 this margin formed at different times when the sea level was at different depths during the Late Quaternary. The distribution of verdine and glaucony facies on the southwestern margin of India is different from those of the distribution along the east coast of India, Senegal, and French Guiana margins, suggesting different paleogeography and subsidence history of the western Indian margin during the Late Quaternary.

Abstract: Phosphorite, limestone, basalt, and breccia/conglomerate occur on the flanks of central Pacific Cretaceous seamounts. Phospbatization occurred during the Tertiary, at or immediately below the sediment-water interface, within thin bodies of porous carbonate sediment and within all rock types that occur on the seamounts. The phosphorites are composed of carbonate fluorapatite (CFA), which lines pores and replaces carbonate and silicate protoliths; protolith fabric is locally preserved in fine detail. Phosphatization is typically most pervasive adjacent to pores, fractures, and the outer surfaces of samples. In order of decreasing abundance, diagenetic minerals associated with seamount phosphorites (SMP) include: CFA, Fe-Mn oxyhydroxides, palagonite, smectite, phillipsite, and barite. This mineral assemblage indicates that CFA formed under conditions consistent with low-temperature, open-circulation seafloor alteration of basalt. The common association between CFA and precipitation of Fe-Mn oxyhydroxides indicates that SMP formed under oxic to suboxic conditions. Phosphatization is characterized by mineralization fronts; the angular superposition of those fronts (successive mineralizing fluids coming from different directions) locally leads to irregularly shaped domains that differ in pervasiveness of phosphatization, degree of fabric preservation, and abundance of secondary minerals. Phosphatization is, therefore, controlled by a variety of factors including protolith mineralogy and fabric, degree of pore-fluid saturation with respect to CFA, rate of pore-fluid/seawater circulation (rock-water ratio), and number of previous episodes of phosphatization.