Authigenic Oxides Research Articles

New data on the Turonian nodular phosphate-bearing bed in the Dobrindol Formation south of Kaspichan, NE Bulgaria

The Turonian nodular phosphate-bearing bed outcropping near Kaspichan consists of early diagenetically mineralized (carbonate-fluorapatite – CFA, glauconite and goethite) limestone lithoclasts (nodules), mineralized macrofauna, and non-mineralized intraclasts, embedded in a glauconite-CFA cement. The characteristics of the nodular bed are consistent with a formation under extremely low net sedimentation rate with nodules demonstrating evidence for reworking. The identified ammonite association assigns late Turonian age of the bed. XRF and LA-ICPMS analyses reveal low P, U, and REE-Y concentrations, with correlations between REE and the other authigenic oxides indicating that glauconite and goethite could be main REE concentrators along with CFA. Fluctuations in the redox conditions are implied by the authigenic mineral association, mineralization stages, presence of diverse fauna and ichnofissils, and applied geochemical proxies.

Coupled changes in western South Atlantic carbon sequestration and particle reactive element cycling during millennial-scale Holocene climate variability

AbstractContinental shelves have the potential to remove atmospheric carbon dioxide via the biological pump, burying it in seafloor sediments. The efficiency of marine carbon sequestration changes rapidly due to variations in biological productivity, organic carbon oxidation, and burial rate. Here we present a high temporal resolution record of marine carbon sequestration changes from a western South Atlantic shelf site sensitive to Brazil Current-driven upwelling. The comparison of biological records to rare earth element (REE) patterns from authigenic oxides shows a strong relationship between higher biological productivity and stronger particle reactive element cycling (i.e. REE cycling) during rapid climate change events. This is the first evidence that authigenic oxides archive past changes in upper ocean REE cycling by the exported organic carbon. In addition, our data suggest that Brazil Current-driven upwelling varies on millennial-scales and in time with continental precipitation anomalies as registered in Brazilian speleothems during the Holocene. This indicates an ocean–atmosphere control on the biological pump, most probably related to South American monsoon system variability.

The Nickel isotope composition of the authigenic sink and the diagenetic flux in modern oceans

We investigated Ni isotopes in a stratigraphic sequence of pelagic clays collected during ODP core leg 185 on site 1149 in Western Pacific Ocean near the Izu-Bonin subduction trench in order to determine the Ni isotope composition of the authigenic Mn-oxides sink and evaluate the composition of the diagenetic Ni input flux. This predominant oxic sink likely controls the Ni isotope budget in modern oceans. The sequence presented here is a 170 m-thick sequence of pelagic sediments deposited on some of the oldest oceanic crust of the seafloor, and the base was dated at 104 Ma. Nickel isotope values (δ60/58Ni relative to NIST SRM 986) vary in the range of 0.04 ± 0.04 to 1.03 ± 0.03‰. There is a trend between depth in the stratigraphic sequence and increasing Mn/Fe, Mn/Al, higher Ni concentration and heavier Ni isotope values. This trend is accounted for by authigenic Mn-oxyhdroxides precipitation in the sediment and scavenging of dissolved metals in the sediment porewaters. This indicates that authigenic oxide minerals in deep-sea pelagic clays are a relevant sink for Ni in modern oceans. Results enable us to determine the authigenic oxic output flux of 3.58 × 108 mol/yr of Ni with a Ni isotope composition of 1.2‰, this oxic output is ~0.4‰ lighter than the oxic output flux in FeMn crusts and nodules. This study shows that Ni isotope variations along the stratigraphic sequence could be the result of mixing between a pure pelagic clays end-member at ~0.1‰ and a pure Mn-oxyhydroxides end-member at ~1.2‰. We suggest that to keep the system at steady-state conditions the oxic output flux is compensated for by a diagenetic input flux of 3.7 × 108 mol/yr which is characterized by a highly fractionated Ni isotope composition of ~2.9‰.

Unraveling sedimentary processes in fluvial sediments from two basalt dominated watersheds in northern Idaho, USA

Basaltic sediments generated from the breakdown of the Columbia River Basalts (CRB) in the Lawyer Creek and Lapwai Creek watersheds of northern Idaho demonstrate that sediment composition in these fluvial systems is impacted by a number of processes, including, chemical weathering, sediment mixing, fluvial sorting, and diagenesis. Chemical weathering is documented in the compositional transformations that occur from the parent material to fluvial detritus, specifically in the leaching of labile cations (i.e., Ca, Mg, Na, and K), an overall increase in weathering indices, and the generation of secondary weathering products (i.e., clay minerals and X-ray amorphous phases) in these sediments. While the two watersheds mainly downcut through the Columbia River Basalt, the incorporation of material derived from local, glacially-derived soil and/or second cycle detritus from interbedded sandstone units significantly modifies the chemical and mineralogical signatures that would normally be used to fingerprint a mafic provenance, highlighting a potential difficulty for provenance reconstruction in sedimentary rocks derived from mixed sources. However, the influence of this mixing is less pronounced in the clay size fraction, highlighting the potential of the <2 μm sediments to preserve the chemical weathering history of the provenance during sedimentation. In addition to weathering and mixing, fluvial sorting is also recorded in the sediment composition and results in the separation of weathering products as a function of grain size. This process is first recognized from the enrichment of secondary weathering phases as grain size decreases, thus resulting in the most chemically altered material sorted into the clay size fraction of sediment deposits downstream. Lastly, early diagenesis of fluvial sediments in the CRB watersheds results in a Ce-anomaly that also correlates with grain size, suggesting the preferential formation of oxide phases in the fine-grained fraction. Furthermore, the most positive Ce-anomaly is recorded in an ancient stream deposit, suggesting that authigenic oxide precipitation initiates during sediment accumulation and advances as the deposit is cut off from the active stream channel.

40Ar/39Ar and (U–Th)/He – 4He/3He geochronology of landscape evolution and channel iron deposit genesis at Lynn Peak, Western Australia

(U–Th)/He geochronology of authigenic goethite cements from the Lynn Peak channel iron deposit (CID), Hamersley Province, Western Australia, reveals a history of mineral precipitation ranging from ca. 33 to 14Ma. Massive goethites from nearby weathering profiles at Roy Hill North, a possible source of detrital material during the aggradation of the Lynn Peak channels, yield (U–Th)/He results as old as ca. 64Ma. The combination of (U–Th)/He geochronology with incremental outgassing 4He/3He studies on proton-irradiated samples reveals that Lynn Peak goethites host radiogenic 4He in low retentivity (LRD) and high retentivity (HRD) domains and that the HRDs account for most of the sample mass and have lost very little of their original 4He over geologic time. Such high retentivity is especially notable given the goethites were collected from the surface, where they were subject to significant heating by solar irradiation. Minor contamination by detrital fragments of potentially 4He-rich primary phases (e.g., rutile, ilmenite, zircon) occurs in some samples. Fortunately, the 4He/3He method permits characterization of this extraneous 4He component, which is small (<10wt.% of the total 4He in the goethite) and can be corrected out in estimating the goethite formation age. These results indicate that the Lynn Peak channel was already aggraded and undergoing goethite cementation by ca. 33Ma.The history of aggradation and channel cementation independently measured through 40Ar/39Ar geochronology is consistent with that obtained from the (U–Th)/He and 4He/3He record. Laser incremental-heating 40Ar/39Ar geochronology of detrital and authigenic Mn oxides, primarily cryptomelane (KMn8O16·xH2O), from the same locality in the Lynn Peak channel reveals that detrital oxides are older than ca. 44Ma (and as old as ca. 65Ma) and authigenic oxides are younger than ca. 35Ma and as young as ca. 16Ma. Authigenic cryptomelane precipitation and channel cementation occurred throughout the Miocene, with a particularly strong period at around 20Ma. The 40Ar/39Ar geochronological results suggest that regional weathering profiles, developed before 44Ma and possibly as early as ca. 65Ma, were incised and partially eroded in the 44–35Ma interval. Semi-arid conditions promoted the transport of large volumes of sediments, aggrading the regional drainage system. Metasomatic reactions in the aggraded channels caused the ferruginization and, in some places, manganese replacement of the CIDs; goethite and K–Mn oxide cementation continued throughout the Oligocene and Miocene. Post-Miocene aridification contributed to the preservation of the cemented channel sediments, forming some of the largest readily mineable iron ore deposits on earth.

Accumulation and Partitioning of Heavy Metals by Bacterial Cells and Associated Colloidal Minerals, with Alteration, Neoformation, and Selective Adsorption of Minerals by Bacteria, in Metal-Polluted Lake Sediment

Energy dispersive X-ray microanalysis and transmission electron microscopy were used to investigate heavy metal accumulation and partitioning by a statistically meaningful assortment of bacteria and colloidal minerals in metal-polluted lake sediment. Visually selected external and internal parts of bacterial cells were analysed individually. Variations in element distribution and relationships provided clues to small-scale biogeochemical processes and their implications for microbial ecology. Most bacteria had mineral coatings of varying composition and texture formed by adsorption of detrital clay and precipitation of authigenic oxides and clay attributable to Fe and Al complexing by cell walls and fibrils. Fe oxide precipitated abiotically and indiscriminately on bacteria and minerals, but Mn oxide was purely biogenic, precipitating only on certain bacteria. Cell walls also complexed Cu; formation of mineral coatings interfered with Cu binding, but the coatings themselves scavenged Cu. Cu and Zn were bound preferentially by oxides, but Ni was bound mainly by clay. Many bacteria accumulated metals preferentially on cell exteriors or in cytoplasmic inclusions, implying different mechanisms of bioaccumulation and detoxification, whose original purpose may have been to concentrate nutrient trace metals from dilute solution and regulate their proportions. Bacteria commonly employed both mechanisms, suggesting adaptation to different metal species. Preferential intake and exclusion of metals also occurred, and exterior-interior partition coefficients of metals in cells correlated with specific metal properties, revealing clues to the processes involved. In addition, bacteria were important agents of diagenesis. Besides precipitating oxides, they degraded adsorbed clay, synthesised intracellular and extracellular clay, and adsorbed clay minerals selectively.

A first approach to study the mobility and behavior of lead in hypersaline salt marsh sediments: Diffusive and advective fluxes, geochemical partitioning and Pb isotopes

A geochemical study of interstitial water and solid phase sediment using bulk concentration and geochemical partitioning was undertaken in vertical sediment profiles to trace diagenetic processes of lead (Pb) in hypersaline salt marsh sediments. In addition, we measured the stable isotopic composition of Pb in order to distinguish its input sources. Concentrations of Pb increased from low or background values in the bottommost layer (< 60 cm depth), followed by fluctuations in the middle layer (20–60 cm) and peak values in the subsurface layer (3–5 cm). Pb associated to reactive fractions (e.g. OM, Fe–Mn oxyhydroxides and carbonates) accounted for 60% of that initially deposited. Stable Pb isotope data ( 206 Pb/ 207 Pb and 207 Pb/ 208 Pb) suggested that most of the Pb in the upper sediments (1.204 ± 0.002 and 2.469 ± 0.007) is still derived from the leaded gasoline combustion (1.201 ± 0.006 and 2.475 ± 0.005). Profile of dissolved Pb was related to those for ammonium, phosphates and dissolve Fe and Mn, which reveals the influence of the diagenetic reactions on the Pb behavior. OM, Fe–Mn oxyhydroxides and the sulfide minerals play a significant role for mobilizing and trapping the Pb. Metal mobilization was calculated considering an advective–diffusive system. The advective process constitutes the dominant mechanism of Pb mobilization. A low diffusive outflux with respect to the Pb mobilization rate suggested that most of the released Pb is retained in the sediments. Authigenic oxides precipitated at the oxic–suboxic layers (0–4 cm depth) and authigenic sulfide minerals formed Pb in the anoxic layers (7–20 cm depth) constituting the main scavengers for Pb that is diagenetically released. This retention has significant environmental implications because it reduces the availability and toxicity of Pb to biota, including humans.

Ferromanganese oxide deposits from the Central Pacific Ocean, I. Encrustations from the Line Islands Archipelago

Chemical and mineralogical analyses of a well-controlled suite of ferromanganese encrustations from the Line Islands Archipelago (Central Pacific) suggest that they represent purely hydrogenous deposits— i.e. they have formed through the slow accumulation of trace metal-enriched oxides directly from the water-column. Mineralogically they consist predominantly of δMnO 2 and amorphous FeOOH xH 2O. Compositionally, they are similar to δMnO 2 nodules from adjoining basinal areas but are enriched in both Mn (mean = 20.4%, max = 29.3%) and Co (mean = 0.55%, max = 1.57%). δMnO 2 is the most important trace metal bearing phase; strong associations are noted between it and Co, Mo, Ni, Zn, and Cd, whilst only Be is associated specifically with FeOOH. V, Sr and Pb are partitioned between the authigenic oxide phases, whilst Ti most probably occurs as TiO 2 xH 2O. Cu is contained in both aluminosilicate contaminant phases and Fe oxide phases. These relations are considered to reflect the differing scavenging behaviour of Mn and Fe oxides in the water column. Crusts from ~1–2 km are enriched in Mn and the Mn-related elements and exhibit higher Mn Fe ratios than deeper crusts, which are compositionally constant. The higher Mn Fe ratios may result from a supply of Mn from continental borderland sediments at these depths, which is transported horizontally by advective-diffusive processes. Since manganophile elements are enriched relative to Mn in the 1–2 km crusts, it is considered that the supply of Mn is scavenged by existing oxides, is oxidised and effectively occludes them. A higher proportion of oxide particles thus exhibit Mn oxide scavenging properties in the 1–2 km depth zone. The increased vertical flux of Mn resulting from the supply at ~1–2 km is not reflected by higher Mn Fe ratios in deeper crusts, so that the vertical flux of oxides is not simply related to the standing crop. The Mn Fe ratios of the crusts thus reflect the composition of suspended oxides at similar depths.

Trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments

Interstitial water samples from the Guatemala Basin and the coast of Baja California have been analyzed for manganese, iron, copper, nickel and nitrate. The data provide a systematic look at changes in trace metal diagenesis proceeding from red clay to highly reducing nearshore sediments. In red clay sediments, the nitrate concentrations suggest that only aerobic respiration is occurring. Manganese and iron are below detection. Nickel concentrations remain the same as in bottom seawater but copper shows a pronounced maximum just at the sediment/water interface. Proceeding to hemipelagic sediments, denitrification becomes increasingly important and manganese and iron remobilization occur in the sediments. The linear manganese and nitrate profiles suggest regions of production or consumption separated by zones of diffusion. This differs from the conventional picture of a continuous series of reactions within the sediments. Manganese reduction always occurs before iron reduction. The pore water nickel correlates well with manganese in these sediments, suggesting that nickel is associated with MnO 2 in the solid phase. The pore water flux ratio of manganese and nickel agrees well with the ratio in solid phase authigenic oxides. Copper still displays a core top concentration maximum as well as a second maximum associated with the remobilized manganese. The calculated ratio of the Cu/C flux ratios support the argument for copper remobilization during organic carbon oxidation. Comparison of the upward and downward diffusive fluxes with the rate of copper buried by sedimentation shows that at least half of the copper buried must be of diagenetic origin and less than 25% of the copper reaching the sediments is buried.