Carbon and nitrogen diagenesis in deep sea sediments

The sections penetrated on Leg 58 of the Deep Sea Drilling Project represent periods of geologic time during which depositional conditions apparently remained quite constant, thus offering an unusual opportunity to study the effects of diagenesis on organic material.Organic carbon and nitrogen contents decrease monotonically with increasing depth of burial before levelling off at minimum values of about 0.05-0.10 and 0.01%, respectively. The depths at which minima are reached vary from site to site, but the ages of the sediments at the minima are all about 2–5 Myr.These data indicate that diagenetic transformations are responsible for the gradual depletion of organic carbon and nitrogen. If diagenesis is at least partly the result of microbial activity, then the role of bacterial ecosystems in deep water sediments is much greater than has previously been thought.     

Analysis of causes for the beryllium-10 variations in deep sea sediments

A critical evaluation of the available ¹⁰Be data shows that its concentration of marine sediments appears to be affected by climate, cosmic ray (C.R.) intensity variations, magnetic reversals and changes in sedimentation rates in that order. Possible variations in the C.R. intensity would have averaged ≦ ±30% for selected periods of the order of 10⁵ yr during the last 2 m.y.     

Alteration of volcanic matter in deep sea sediments: evidence from the chemical composition of interstitial waters from deep sea drilling cores

Six Deep Sea Drilling Project (DSDP) Sites (252, 285, 315, 317, 336, 386) were examined for the chemical composition of the dissolved salts in interstitial waters, the oxygen isotopic composition of the interstitial waters, and the major ion composition of the bulk solid sediments. An examination of the concentration-depth profiles of dissolved calcium, magnesium, potassium, and H₂¹⁸O in conjunction with oxygen isotope mass balance calculations confirms the hypothesis that in DSDP pelagic drill sites concentration gradients in Ca. Mg. K, and H₂¹⁸O are largely due to alteration reactions occurring in the basalts of Layer 2 and to alteration reactions involving volcanic matter dispersed in the sediment column. Oxygen isotope mass balance calculations require substantial alteration of Layer 2 (up to 25% of the upper 1000 m). but only minor exchange of Ca, Mg, and K occurs with the overlying ocean. This implies that alteration reactions in Layer 2 are almost isochemical.     

The oxidation state of manganese in surface sediments of the deep sea

Iodometric titration of deep sea sediment from cores at MANOP sites M and H indicate a particulate Mn oxidation state between 3.3 and 3.9 in the top 5 cm. Results from site H show a minimum in the Mn oxidation state at 1.5 cm depth indicating Mn reduction is occurring above the zone of pore water Mn oxidation. Using a simple box model, the rate of reduction is calculated to be 19 mg/cm² kyr ? 77 mg/cm² kyr, at least 5 times the flux of Mn to nodules in this region. Although no Mn oxidation state minimum is observed in site M sediments, oxic reduction is inferred from a particulate mass balance, indicating that a large fraction of the Mn rain to this site is not preserved in the sediments and must be remobilized. We suggest that the process of reduction in oxic or suboxic environments near the sediment-water interface may be an important mechanism controlling the concentration of Mn in sediments and provides a mechanism for supplying diagenetic Mn to the nodules at site H.     

Model for the distribution of sulfate reduction and methanogenesis in freshwater sediments

A model, based on the in situ physiological characteristics of methanogens and sulfate reducers, was developed to describe the distribution of methanogenesis and sulfate reduction in freshwater sediments. The model predicted the relative importance of methane production and sulfate reduction in lakes of various trophic status and generated profiles of sulfate, acetate, methanogenesis, and sulfate reduction comparable to the profiles that are expected based on field studies. The model indicated that at sulfate concentrations greater than 30μM a sulfate-reducing zone develops because sulfate reducers maintain acetate concentrations too low for methanogens to grow. At lower sulfate concentrations a methanogenic zone develops because the dual limitations of low sulfate concentrations and acetate consumption by methanogens prevents sulfate reducers from growing. The model and a compilation of previously published field data indicate that, within the reported range of sulfate concentrations, the relative importance of methanogenesis and sulfate reduction in freshwater sediments is primarily dependent upon the rates of organic matter decomposition.     

The importance of atmospheric input of terrestrial organic material to deep sea sediments

The concentrations of lipids were determined in atmospheric particle, gas and rain samples collected from the tropical North Pacific to assess lipid sources, transport mechanisms and fluxes to the ocean surface. Four lipid compound classes (aliphatic hydrocarbons, fatty alcohols, fatty acid esters, and salts) all unequivocally show a terrestrial vascular plant source. These aerosol lipids originate from wind erosion of Asian and American soils and direct emission from vegetation. The major fluxes result from rain rather than dry deposition. These fluxes are large enough to have a major potential impact on the inventory of terrestrially derived lipid material found in deep-sea sediments. This has been showm for n-alkanes, fatty alcohols, fatty acids, total lipids and for organic carbon. By comparing atmospheric and sediment trap fluxes with sediment accumulation rates, it is suggested that some biogenic terrestrial material is more protected from degradation than marine-derived material.     

Extraction of Nd isotopes from bulk deep sea sediments for paleoceanographic studies on Cenozoic time scales

Nd isotopes preserved in fossil fish teeth and ferromanganese crusts have become a common tool for tracking variations in water mass composition and circulation through time. Studies of Nd isotopes extracted from Pleistocene to Holocene bulk sediments using hydroxylamine hydrochloride (HH) solution yield high resolution records of Nd isotopes that can be interpreted in terms of deep water circulation, but concerns about diagenesis and potential contamination of the seawater signal limit application of this technique to geologically young samples.In this study we demonstrate that Nd extracted from the > 63 μm, decarbonated fraction of older Ocean Drilling Program (ODP) sediments using a 0.02 M HH solution produces Nd isotopic ratios that are within error of values from cleaned fossil fish teeth collected from the same samples, indicating that the HH-extractions are robust recorders of deep sea Nd isotopes. This excellent correlation was achieved for 94 paired fish teeth and HH-extraction samples ranging in age from the Miocene to Cretaceous, distributed throughout the north, tropical and south Atlantic, and composed of a range of lithologies including carbonate-rich oozes/chalks and black shales. The strong Nd signal recovered from Cretaceous anoxic black shale sequences is unlikely to be associated with ferromanganese oxide coatings, but may be derived from abundant phosphatic fish teeth and debris or organic matter in these samples.In contrast to the deep water Nd isotopic signal, Sr isotopes from HH-extractions are often offset from seawater values, suggesting that evaluation of Sr isotopes is a conservative test for the integrity of Nd isotopes in the HH fraction. However, rare earth elements (REE) from the HH-extractions and fish teeth produce distinctive middle REE bulge patterns that may prove useful for evaluating whether the Nd isotopic signal represents uncontaminated seawater. Alternatively, a few paired HH-extraction and cleaned fish teeth samples from each site of interest can be used to verify the seawater composition of the HH-extractions. The similarity between isotopic values for the HH-extraction and fish teeth illustrates that the extensive cleaning protocol applied to fish teeth samples is not necessary in typical, carbonate-rich, deep sea sediments.     

Calcium carbonate dissolution in deep sea sediments: reconciling microelectrode, pore water and benthic flux chamber results

We report the benthic fluxes of O₂, titration alkalinity (TA), Ca²⁺, NO₃⁻, PO₄³⁻, and Si(OH)₄ from in situ benthic flux chamber incubations on the Ceara Rise and Cape Verde Plateau and compare them to previously published results. We find within analytical uncertainty that the TA flux is twice the calcium flux, suggesting that dissolution/precipitation of CaCO₃ is the principal mechanism controlling benthic TA and Ca²⁺ fluxes. At sites where the sediments contain significant (>35%) CaCO₃ and the overlying waters are supersaturated with respect to CaCO₃, the ratios of the total dissolution rate to the remineralization rate are significantly less than at all other study sites. We propose that these observations can be explained by precipitation of fresh CaCO₃ at the supersaturated sediment surface followed by redissolution deeper in the sediments because of metabolically-produced CO₂. A numerical simulation is presented to demonstrate the feasibility of this explanation. In addition, surface exchange reactions in high-CaCO₃ sediments coupled with high rates of particle mixing may also impact rates of metabolic dissolution and depress chamber-derived estimates of carbonate alkalinity and calcium benthic fluxes. These results suggest that at supersaturated, high CaCO₃ locations, previous models of sediment diagenesis may have overestimated the impact of metabolic dissolution on the preservation of CaCO₃ deposited on the sea floor.     

Iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids

Volcanogenic sediments are typically rich in Fe and Mn-bearing minerals that undergo substantial alteration during early marine diagenesis, however their impact on the global biogeochemical cycling of Fe and Mn has not been widely addressed. This study compares the near surface (0-20 cm below sea floor [cmbsf]) aqueous (<0.02 μm) and aqueous + colloidal here in after ‘dissolved’ (<0.2 μm) pore water Fe and Mn distributions, and ancillary O_2(aq), and solid-phase reactive Fe distributions, between two volcanogenic sediment settings: [1] a deep sea tephra-rich deposit neighbouring the volcanically active island of Montserrat and [2] mixed biosiliceous-volcanogenic sediments from abyssal depths near the volcanically inactive Crozet Islands archipelago. Shallow penetration of O_2(aq) into Montserrat sediments was observed (<1 cmbsf), and inferred to partially reflect oxidation of fine grained Fe(II) minerals, whereas penetration of O_2(aq) into abyssal Crozet sediments was >5 cmbsf and largely controlled by the oxidation of organic matter. Dissolved Fe and Mn distributions in Montserrat pore waters were lowest in the surface oxic-layer (0.3 μM Fe; 32 μM Mn), with maxima (20 μM Fe; 200 μM Mn) in the upper 1-15 cmbsf. Unlike Montserrat, Fe and Mn in Crozet pore waters were ubiquitously partitioned between 0.2 μm and 0.02 μm filtrations, indicating that the pore water distributions of Fe and Mn in the (traditionally termed) ‘dissolved’ size fraction are dominated by colloids, with respective mean abundances of 80% and 61%. Plausible mechanisms for the origin and composition of pore water colloids are discussed, and include prolonged exposure of Crozet surface sediments to early diagenesis compared to Montserrat, favouring nano-particulate goethite formation, and the elevated dissolved Si concentrations, which are shown to encourage fine-grained smectite formation. In addition, organic matter may stabilise authigenic Fe and Mn in the Crozet pore waters. We conclude that volcanogenic sediment diagenesis leads to a flux of colloidal material to the overlying bottom water, which may impact significantly on deep ocean biogeochemistry. Diffusive flux estimates from Montserrat suggest that diagenesis within tephra deposits of active island volcanism may also be an important source of dissolved Mn to the bottom waters, and therefore a source for the widespread hydrogenous MnO_x deposits found in the Caribbean region.     

Rare earth element geochemistry of South Atlantic deep sea sediments: Ce anomaly change at ~54 My

The geochemistry of the REE (rare earth elements) in oceanic sediments is discussed, based mainly on samples from DSDP Holes 530A and 530B, Leg 75, and Hole 525A, Leg 74. The proposed mechanisms for incorporation of the REE into the marine carbonate phases are adsorption, chiefly onto the carbonate minerals and on Sc, Hf, and Ta-rich Fe-Mn hydroxide flocs as carbonate coatings.The Ce anomaly of marine carbonate was used as an indicator of paleo-ocean water redox conditions: the bottom water of the Angola Basin was in a reducing condition in the Cretaceous. At ca. 54 My, the South Atlantic water condition became oxidizing, similar to the present seawater redox condition. This change was related to the improvement of circulation due to the widening of South Atlantic and the subsidence of water circulation barriers such as the Walvis Ridge and perhaps the Romanche Fracture Zone.The younger (Eocene-Recent) and older (Albian-Santonian) argillaceous sedimentary rocks from 530A (denoted as YSAB and OSAB respectively) show different degrees of Eu depletion with a transition period in between. The REE patterns of OSAB suggest a basaltic origin. The possible sources are Kaoko basalt in Southwest Africa or Namibia and the basaltic Walvis Ridge itself. The decrease in the area covered by Kaoko basalt due to erosion, the subsidence of the Walvis Ridge, and the improvement of water circulation led to changes in the Eu anomaly from Campanian to Paleocene, and resulted in the YSAB REE pattern. Changes in the Sm/Eu, La/Th, Th/Yb, Ti/Al₂O₃, FeO/Al₂O₃, and Hf/Al₂O₃ ratios suggest changes of average source rock composition from and esite to granodiorite.The REE abundances and patterns of younger sediments in the Angola Basin (YSAB) are very similar to those observed in NASC, PAAS, and ES sediments. The YSAB REE abundances and patterns may represent the average REE distribution of the exposed African continental crust. The strong resemblance of REE distributions of YSAB, NASC, PAAS and ES suggests thorough REE mixing from different sources and the uniformity of the average crustal compositions of different continents: Africa, North America, Australia, and Europe     

Biogenic and lithogenic magnetic minerals in Atlantic and Pacific deep sea sediments and their paleomagnetic significance

The carrier of the natural magnetization of deep sea sediments was characterized by mineralogical, electron microscopic, and rock magnetic investigations. Magnetic single domain (SD) and pseudo single domain (PSD) particles which are most important for the stable remanent magnetization were separated from the magnetic »coarse fraction« and concentrated as magnetic »fine fraction«. The magnetic coarse fraction consists of lithogenic magnetite and titanomagnetite, which often contains exsolution-lamellae of ilmenite. Both minerals are partially maghematized and occur isolated in the sediment or embedded in rock particles, in regionally different concentrations. The magnetic fine fraction consists of lithogenic magnetite and titanomagnetite and biogenic magnetite (magnetofossils = fossil bacterial magnetosomes), the latter generally maghematized.A graphical method is described which allows the classification and characterization of the magnetic fine fraction by demagnetization of the anhysteretic remanent magnetization (ARM) of whole sediment samples. Three groups with different magnetic properties can be distinguished, characterized by three ARM type-curves: Type A curves are associated with sediments from abyssal plaines. They show nearly identical ARM properties and are typical for magnetofossils.Type B curves are produced by sediments from the vicinity of volcanic regions. Their shapes are variable to a certain degree and indicate two lithogenic magnetic phases.Type C curves are found for sediments from submarine ridges and regions with input of terrigenous detritus. These curves have the largest deviations among each other indicating a magnetic multi-phase assemblage including magnetofossils.

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Zusammenfassung Die Träger der Magnetisierung von Tiefseesedimenten wurden mineralogisch, elektronenmikroskopisch und gesteinsmagnetisch untersucht. Magnetische Eindomänen (SD) und Pseudo-Eindomänen (PSD) Partikel, die für eine stabile remanente Magnetisierung wichtig sind, wurden als magnetische »Feinfraktion« von der magnetischen »Grobfraktion« abgetrennt. Die magnetische Grobfraktion besteht aus lithogenem Titanomagnetit und Magnetit die teilweise maghemitisiert sind und teilweise auch Ilmenit-Entmischungslamellen aufweisen. In regional unterschiedlichen Konzentrationen liegen sie frei im Sediment oder in silikatischer Matrix eingebettet vor. Die magnetische Feinfraktion besteht sowohl aus lithogenem Titanomagnetit und Magnetit, als auch aus biogenem Magnetit (Magnetofossilien = fossile bakterielle Magnetosomen); letzterer ist größtenteils maghemitisiert.Es wird eine grafische Darstellungsmethode beschrieben, die anhand von Untersuchungen der anhysteretischen remanenten Magnetisierung (ARM) von Sedimentproben eine Charakterisierung der magnetischen Feinfraktion erlaubt. Es lassen sich dadurch drei Gruppen mit unterschiedlichen magnetischen Eigenschaften unterscheiden, die durch drei Gruppen von ARM-Kurventypen charakterisiert sind.Kurventyp A wird bei Sedimenten aus Tiefsee-Ebenen beobachtet. Die ARM-Daten sind nahezu identisch und zeigen ein Verhalten, wie es für Magnetofossilien typisch ist. Kurventyp B tritt bei Sedimenten aus dem Einzugsbereich vulkanischer Gebiete auf. Er zeigt eine größere Variation und die Form der Kurven spricht für ein System aus zwei lithogenen magnetischen Komponenten.Kurventyp C gehört zu Sedimenten aus submarinen Rücken und dem Einzugsgebiet terrigener Schüttungen. Die Kurvenverläufe sind uneinheitlich und sprechen für ein magnetisches Mehrkomponenten-System mit Beteiligung von Magnetofossilien.

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Résumé Les minéraux porteurs du magnétisme dans les sédiments de mer profonde ont été explorés par les méthodes de la minéralogie, de la microscopie électronique et du magnétisme des roches. Les particules qui correspondent à un domaine magnétique unique (SD) et pseudo-unique (PSD), significatives pour un magnétisme rénanent stable, ont été concentrées comme «fraction magnétique fine» après séparation de la «fraction magnétique grossière». Cette dernière consiste en magnétite et titanomagnétite lithogéniques, qui renferment souvent des lamelles d'exsolution d'ilménite. Ces deux minéraux sont partiellement maghémitisés; ils se présentent isolés ou inclus dans des fragments de roches, avec des concentrations régionales diverses. La fraction magnétique fine consiste en magnétite et titanomagnétite lithogéniques, ainsi qu'en magnétite biogénique (magnétofossile = magnétosome fossile bactérien), cette dernière ordinairement maghemitisée.Les auteurs présentent une méthode graphique qui permet de caractériser la fraction magnétique fine à partir de l'examen du magnétisme rémanent anhystérique (ARM) de l'échantillon de sédiment. Cette méthode permet de distinguer trois groupes de propriétés magnétiques différentes, caractérisés par trois types de courbes ARM. Les courbes de type A caractérisent les sédiments de plaines abyssales; elles montrent des propriétés ARM presque identiques et sont typiques pour les magnétofossiles. Les courbes de types B sont fournies par les sédiments voisins des régions volcaniques; leurs formes varient dans une certaine mesure et indiquent un système à deux composants magnétiques lithogéniques. Les courbes de type C correspondent aux sédiments des crêtes sous-marines et des régions à apports terrigènes; ces courbes présentent entre elles des différences plus marquées, ce qui indique un système magnétique à composants multiples, comportant des magnétofossiles.

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A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic)

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement ( <6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1?N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24?h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe²⁺. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.     

137Cs in sediments of deep lakes

Sedimentation rate of suspensions (0.3–18.2 mm/yr) was determined on the basis of layer-by-layer distribution of ¹³⁷Cs (global and Chernobyl) in the cores of bottom sediments (BS) from 16 lakes located in the high and moderate latitudes of the Northern Hemisphere. Profiles of ¹³⁷Cs concentration in the BS cores were analyzed. Sedimentation rates in the limnic zones of deep-water lakes were calculated using ¹³⁷Cs accumulated in the finely-dispersed component of mineral suspension.     

Fossilization and degradation of intact polar lipids in deep subsurface sediments: A theoretical approach

Intact polar membrane lipids (IPLs) are frequently used as markers for living microbial cells in sedimentary environments. The assumption with these studies is that IPLs are rapidly degraded upon cell lysis and therefore IPLs present in sediments are derived from in situ microbial production. We used a theoretical approach to assess whether IPLs in surface sediments can potentially represent fossilized IPLs derived from the upper part of the water column and whether IPLs can be preserved during sediment burial. Previous studies which examined the degradation kinetics of IPLs show that phospholipids, i.e. ester-linked lipids with a phosphor-containing head group, degrade more rapidly than glycosidic ether lipids, i.e. ether-linked lipids with a glycosidically bound sugar moiety. Based on these studies, we calculate that only a minor fraction of phospholipids but a major fraction of glycosidic ether lipids biosynthesized in the upper part of the water column can potentially reach deep-sea surface sediments. Using a simple model and power law kinetic degradation parameters reported in the literature, we also evaluated the degradation of IPLs during sediment burial. Our model predicts a log-log relationship between IPL concentrations and depth, consistent with what has been observed in studies of IPLs in subsurface sediments. Although our results do not exclude production of IPLs in subsurface sediment, they do suggest that IPLs present in the deep biosphere may contain a substantial fossil component potentially masking in situ IPL production.     

Mineralogy,geochemistry, and radiocarbon ages of deep sea sediments from the Gulf of Mexico,Mexico

The mineralogy, geochemistry, and radiocarbon ages of two sediment cores (GMX1 and GMX2) collected from the deep sea area of the Southwestern Gulf of Mexico (∼876–1752 m water depth) were studied to infer the sedimentation rate, provenance, heavy metal contamination, and depositional environment. The sediments are dominated by silt and clay fractions. The mineralogy determined by X-Ray diffractometry for the sediment cores reveals that montmorillonite and muscovite are the dominant clay minerals. The sections between 100 and 210 cm of the sediment cores GMX1 and GMX2, respectively, are characterized by the G. menardii group and G. Inflata planktonic foraminiferal species, which represent the Holocene and Pleistocene, respectively. The radiocarbon-age measurements of mixed planktonic foraminifera varied from ∼268 to 45,738 cal. years B.P and ∼104 to 25,705 cal. years B.P, for the sediment cores GMX1 and GMX2, respectively. The variation in age between the two sediment cores is due to a change in sediment accumulation rate, which was lowest at the location GMX1 (0.006 cm/yr) and highest at the location GMX2 (0.017 cm/yr).The chemical index of alteration (CIA), chemical index of weathering (CIW), and index of chemical maturity (ICV) values indicated a moderate intensity of weathering in the source area. The total rare earth element concentrations (∑REE) in the cores GMX1 and GMX2 vary from ∼94 to 171 and ∼78 to 151, respectively. The North American Shale Composite (NASC) normalized REE patterns showed flat low REE (LREE), heavy REE (HREE) depletion with low negative to positive Eu anomalies, which suggested that the sediments were likely derived from intermediate source rocks.The enrichment factor of heavy metals indicated that the Cd and Zn concentrations in the sediment cores were impacted by an anthropogenic source. The redox-proxy trace element ratios such as V/Cr, Ni/Co, Cu/Zn, (Cu + Mo)/Zn, and Ce/Ce* indicated that the sediments were deposited under an oxic depositional environment. The similarity in major element concentrations, REE content, and the NASC normalised REE patterns between the cores GMX1 and GMX2 revealed that the provenance of sediments remained relatively uniform or constant during deposition for ∼4.5 Ma. The major and trace element based multidimensional discrimination diagrams showed a rift setting for the core sediments, which is consistent with the geology of the Gulf of Mexico.     

Conditions and mechanism for the formation of iron-rich Montmorillonite in deep sea sediments (Costa Rica margin): Coupling high resolution mineralogical characterization and geochemical modeling

Iron-rich smectite is commonly described in the diagenetic fraction of deep-sea sediment, as millimeter to centimeter aggregates dispersed in the sediment, or as a coating on sedimentary particles or nodules. This study examines several factors to elucidate formation mechanisms of a particular iron-rich smectite and its potential transformation to glauconite. The study combines a detailed mineralogical investigation on natural samples and a chemical modeling approach to assess mineralogical reactions and pathways.Transmission electron microscopy (TEM) observations and analytical electron microscopy (TEM-AEM) analyses were conducted on microtomed samples of millimeter- to centimeter-long green grains. These grains are widespread in pelagic calcareous sediment from the Costa Rica margin. They are composed of pyrites that are partially dissolved and are surrounded by amorphous or very poorly crystallized iron-rich particles. Iron-rich montmorillonite grows from an amorphous precursor and its formation requires the input of Si, O, Mg, K, Na and Ca; our results suggest that these inputs are supported by the dissolution of sedimentary phases such as volcanic glasses, siliceous fossils and silicates.Thermodynamic modeling of fluid-sediment interactions was conducted with the geochemical computer code PhreeqC, using mineralogical and pore fluid compositions from sediment samples and calculated estimates for thermodynamic constants of smectites that are not maintained by the computer code. Simulations confirm the possibility that the green grains are the product of pyrite alteration by seawater under oxidizing conditions. The extent of smectite production is controlled by the kinetics of pyrite dissolution and fluid migration. The absence of aluminum in the Costa Rica margin system explains the formation of an iron-rich montmorillonite instead of glauconite, whereas the presence of calcite that buffers the system explains the formation of an iron-rich montmorillonite instead of iron oxides.     

Diffusion of ions in sea water and in deep-sea sediments

The tracer-diffusion coefficient of ions in water, D_j⁰, and in sea water, $\text{D}{}_{\mathrm{j}}{}^1$, differ by no more than zero to 8 per cent. When sea water diffuses into a dilute solution of water, in order to maintain the electro-neutrality, the average diffusion coefficients of major cations become greater but of major anions smaller than their respective $\text{D}{}_{\mathrm{j}}{}^1$ or D_j⁰ values. The tracer diffusion coefficients of ions in deep-sea sediments, D_j,sed., can be related to $\text{D}{}_{\mathrm{j}}{}^1$ by $\text{D}{}_{\mathrm{j,sed.}}\text{= D}{}_{\mathrm{j}}{}^1\text{·}\text{α}\text{θ}{}^2$, where θ is the tortuosity of the bulk sediment and a a constant close to one.     

Quantitative measurement of the sea ice diatom biomarker IP25 and sterols in Arctic sea ice and underlying sediments: Further considerations for palaeo sea ice reconstruction

The lipid content of sea ice samples collected in 2011 and 2012 from Resolute Passage in the Canadian Arctic Archipelago was measured and compared with related samples obtained from the Amundsen Gulf in 2008. The highly branched isoprenoid (HBI) sea ice biomarker, IP₂₅, was found in sea ice samples from each study, consistent with its formation by diatoms during the spring bloom. Our analysis also revealed the occurrence of a number of sterols in Arctic sea ice and these were rigorously identified and quantified for the first time. Concentrations of IP₂₅ and sterols exhibited some variability between sampling studies, with somewhat higher values in samples from Resolute in 2012 than for the other two datasets, consistent with a general increase in biomass; however, major differences in biomarker concentration between sampling studies were not observed. An estimate of the proportion of Arctic sea ice diatoms that produce IP₂₅ (ca. 1–5%) was obtained by comparison of the concentration of IP₂₅ in the samples with those in laboratory cultures of known HBI-producing diatoms and cell enumeration in selected sea ice samples. The estimate is similar to the proportion of Haslea spp. in the same samples, providing further support to the suggestion that at least some species of the Haslea genus may be responsible for the biosynthesis of IP₂₅ and related HBI diatom lipids in Arctic sea ice and that IP₂₅ is made by a relatively small proportion of sea ice diatoms. In contrast, median sterol/IP₂₅ values were all substantially higher than those in cultures of HBI-producing diatoms, suggesting that sterols are made by the majority of sea ice diatoms. The sterol/IP₂₅ ratio was quite variable between locations and samples, likely as a result of differences in diatom assemblage; however, a comparison of individual and median sterol/IP₂₅ values in sea ice with those from surface sediments from different Arctic regions demonstrated that sterols from sea ice diatoms may, in some cases, have a significant impact on the sedimentary budget. This should be considered carefully for quantitative estimates of palaeo sea ice reconstruction using methods such as the PIP₂₅ index, which are based on the relative concentrations of IP₂₅ and sterols in Arctic marine sediment cores.