Iron and phosphorus speciations in DSDP Leg 92 sediments

We present a detailed study of the co-diagenesis of Fe and P in hydrothermal plume fallout sediments from ~19°S on the southern East Pacific Rise. Three distal sediment cores from 340-1130 km from the ridge crest, collected during DSDP Leg 92, were analysed for solid phase Fe and P associations using sequential chemical extraction techniques. The sediments at all sites are enriched in hydrothermal Fe (oxyhydr)oxides, but during diagenesis a large proportion of the primary ferrihydrite precipitates are transformed to the more stable mineral form of goethite and to a lesser extent to clay minerals, resulting in the release to solution of scavenged P. However, a significant proportion of this P is retained within the sediment, by incorporation into secondary goethite, by precipitation as authigenic apatite, and by readsorption to Fe (oxyhydr)oxides. Molar P/Fe ratios for these sediments are significantly lower than those measured in plume particles from more northern localities along the southern East Pacific Rise, and show a distinct downcore decrease to a depth of ~12 m. Molar P/Fe ratios are then relatively constant to a depth of ~35 m. The Fe and P speciation data indicate that diagenetic modification of the sediments is largely complete by a depth of 2.5 m, and thus depth trends in molar P/Fe ratios can not solely be explained by losses of P from the sediment by diffusion to the overlying water column during early diagenesis. Instead, these sediments are likely recording changes in dissolved P concentrations off the SEPR, possibly as a result of redistribution of nutrients in response to changes in oceanic circulation over the last 10 million years. Furthermore, the relatively low molar P/Fe ratios observed throughout these sediments are not necessarily solely due to losses of scavenged P by diffusion to the overlying water column during diagenesis, but may also reflect post-depositional oxidation of pyrite originating from the volatile-rich vents of the southern East Pacific Rise. This study suggests that the molar P/Fe ratio of oxic Fe-rich sediments may serve as a proxy of relative changes in paleoseawater phosphate concentrations, particularly if Fe sulfide minerals are not an important component during transport and deposition.

(Table 3) Contents of phosphorus, iron, and manganese in various zones of Fe-Mn nodules from the Pacific radiolarian belt

Phosphorus content in the surface layer of bottom sediments varies from 0.07% to 0.73%. Clayey radiolarian and radiolarian clayey oozes contain 0.12% P, miopelagic clays 0.21% P, and sediments with high iron and manganese concentrations 0.46% P (average contents). Phosphorus content of iron-manganese nodules varies from 0.14% to 0.39%, average 0.19%. Correlation between phosphorus contents in nodules and surrounding sediments is indicated indirectly by P/Fe ratio. Phosphorus is non-uniformly distributed in some nodules and sometimes correlates with iron. Accumulation of phosphorus in iron-manganese nodules is governed by a degree of manganese predominance in ore components.

XRF iron (Fe) counts of ODP Hole 171-1050A and 171-1051A

Upper Paleocene to lower Eocene sediments drilled at Ocean Drilling Program (ODP) Site 1051 (Blake Nose, off Florida) display well-defined orbital cycles, a detailed magnetic stratigraphy, and a suite of planktonic foraminiferal datums. We derived a cyclostratigraphy by using spectral analysis of high-resolution records of elemental concentrations obtained by an X-ray fluorescence (XRF) Core Scanner. XRF counts of iron serve as a proxy for the relative amount of terrestrial material. Sliding-window spectral analysis, bandpass filtering, and direct counting of precession and obliquity cycles yield minimum durations for magnetic polarity chrons C22 to C26 (~49 to ~61 Ma), calculations of sediment accumulation rates, as well as constraints on the timing of biostratigraphic and climatological events in the vicinity of the Initial Eocene Thermal Maximum (IETM). Durations of polarity chrons as represented in sediments drilled at Site 1051 were estimated using a conservative assignment of 41 k.y. for obliquity cycles and 21 k.y. for precession cycles. Combined polarity chrons C26r and C26n span 3.61 m.y., and chron C25r spans 1.07 m.y. Polarity chron C24r is estimated as 2.877 m.y. The interpretation of polarity chron C24n is ambiguous, but its duration is probably <1.23 m.y. Polarity chron C23r spans 0.53 m.y., chron C23n is 0.74 m.y., and chron C22r is 0.9 m.y. Spectral analysis through this interval indicates that spectral peaks shift through time and are related to changes in sedimentation rate in Site 1051. The sedimentation rates dramatically increased ~200 k.y. after the IETM and remained high for most of chron C24r.

(Table 1) Chemical compositions of iron-manganese phosphate crusts, phosphorites, volcanic products, and phosphate limestones from ocean seamounts

New data on phosphorites of Atlantic seamounts are presented and used in combination with published data to analyze sources of phosphorus in them.

Water chemistry and iron speciation in samples taken during SWEDARP 97/98 campaign with S.A. Agulhas to the Southern Ocean

The distribution and speciation of iron was determined along a transect in the eastern Atlantic sector (6°E) of the Southern Ocean during a collaborative Scandinavian/South African Antarctic cruise conducted in late austral summer (December 1997/January 1998). Elevated concentrations of dissolved iron (>0.4 nM) were found at 60°S in the vicinity of the Spring Ice Edge (SIE) in tandem with a phytoplankton bloom, chiefly dominated by Phaeocystis sp. This bloom had developed rapidly after the loss of the seasonal sea ice cover. The iron that fuelled this bloom was mostly likely derived from sea ice melt. In the Winter Ice Edge (WIE), around 55°S, dissolved iron concentrations were low (<0.2 nM) and corresponded to lower biological productivity, biomass. In the Antarctic Polar Front, at approximately 50°S, a vertical profile of dissolved iron showed low concentrations (<0.2 nM); however, a surface survey showed higher concentrations (1-3 nM), and considerable patchiness in this dynamic frontal region. The chemical speciation of iron was dominated by organic complexation throughout the study region. Organic iron-complexing ligands ([L]) ranged from 0.9 to 3.0 nM Fe equivalents, with complex stability log K'(FeL) = 21.4-23.5. Estimated concentrations of inorganic iron (Fe') ranged from 0.03 to 0.79 pM, with the highest values found in the Phaeocystis bloom in the SIE. A vertical profile of iron-complexing ligands in the WIE showed a maximum consistent with a biological source for ligand production and near surface minimum possibly consistent with loss via photodecomposition. This work further confirms the role iron that has in the Southern Ocean in limiting primary productivity.

X-ray fluorescence data (calcium and iron), bulk organic d13C, and dinoflagellate cysts of ODP Hole 189-1172D

The 'Paleocene/Eocene Thermal Maximum' or PETM (~55 Ma) was associated with dramatic warming of the oceans and atmosphere, pronounced changes in ocean circulation and chemistry, and upheaval of the global carbon cycle. Many relatively complete PETM sequences have by now been reported from around the world, but most are from ancient low- to midlatitude sites. ODP Leg 189 in the Tasman Sea recovered sediments from this critical phase in Earth history at Sites 1171 and 1172, potentially representing the southernmost PETM successions ever encountered (at ~70° to 65° S paleolatitude). Downhole and core logging data, in combination with dinoflagellate cyst biostratigraphy, magneto-stratigraphy, and stable isotope geochemistry indicate that the sequences at both sites were deposited in a high accumulation-rate, organic rich, marginal marine setting. Furthermore, Site 1172 indeed contains a fairly complete P-E transition, whereas at Site 1171, only the lowermost Eocene is recovered. However, at Site 1172, the typical PETM-indicative acme of the dinocyst Apectodinium was not recorded. We conclude that unfortunately, the critical latest Paleocene and PETM intervals are missing at Site 1172. We relate the missing section to a sea level driven hiatus and/or condensed section and recovery problems. Nevertheless, our integrated records provide a first-ever portrait of the trend toward, and aftermath of, the PETM in a marginal marine, southern high-latitude setting.

Iron isotope and trace metal record of Cenomanian-Turonian sediments

he global carbon cycle during the mid-Cretaceous (~125-88 million years ago, Ma) experienced numerous major perturbations linked to increased organic carbon burial under widespread, possibly basin-scale oxygen deficiency and episodes of euxinia (anoxic and H2S-containing). The largest of these episodes, the Cenomanian-Turonian boundary event (ca. 93.5 Ma), or oceanic anoxic event (OAE) 2, was marked by pervasive deposition of organic-rich, laminated black shales in deep waters and in some cases across continental shelves. This deposition is recorded in a pronounced positive carbon isotope excursion seen ubiquitously in carbonates and organic matter. Enrichments of redox-sensitive, often bioessential trace metals, including Fe and Mo, indicate major shifts in their biogeochemical cycles under reducing conditions that may be linked to changes in primary production. Iron enrichments and bulk Fe isotope compositions track the sources and sinks of Fe in the proto-North Atlantic at seven localities marked by diverse depositional conditions. Included are an ancestral mid-ocean ridge and euxinic, intermittently euxinic, and oxic settings across varying paleodepths throughout the basin. These data yield evidence for a reactive Fe shuttle that likely delivered Fe from the shallow shelf to the deep ocean basin, as well as (1) hydrothermal sources enhanced by accelerated seafloor spreading or emplacement of large igneous province(s) and (2) local-scale Fe remobilization within the sediment column. This study, the first to explore Fe cycling and enrichment patterns on an ocean scale using iron isotope data, demonstrates the complex processes operating on this scale that can mask simple source-sink relationships. The data imply that the proto-North Atlantic received elevated Fe inputs from several sources (e.g., hydrothermal, shuttle and detrital inputs) and that the redox state of the basin was not exclusively euxinic, suggesting previously unknown heterogeneity in depositional conditions and biogeochemical cycling within those settings during OAE-2.

Average fluorescence and dissolved iron and Fe-binding ligand characteristics during POLARSTERN expedition ANT-XXIV/3 in 2008

Organic complexation of dissolved iron (dFe) was investigated in the Atlantic sector of the Southern Ocean in order to understand the distribution of Fe over the whole water column. The total concentration of dissolved organic ligands ([Lt]) measured by voltammetry ranged between 0.54 and 1.84 nEq of M Fe whereas the conditional binding strength (K') ranged between 10**21.4 and 10**22.8. For the first time, trends in Fe-organic complexation were observed in an ocean basin by examining the ratio ([Lt]/[dFe]), defined as the organic ligand concentration divided by the dissolved Fe concentration. The [Lt]/[dFe] ratio indicates the saturation state of the natural ligands with Fe; a ratio near 1 means saturation of the ligands leading to precipitation of Fe. Reversely, high ratios mean Fe depletion and show a high potential for Fe solubilisation. In surface waters where phytoplankton is present low dissolved Fe and high variable ligand concentrations were found. Here the [Lt]/[dFe] ratio was on average 4.4. It was especially high (5.6-26.7) in the HNLC (High Nutrient, Low Chlorophyll) regions, where Fe was depleted. The [Lt]/[dFe] ratio decreased with depth due to increasing dissolved Fe concentrations and became constant below 450 m, indicating a steady state between ligand and Fe. Relatively low [Lt]/[dFe] ratios (between 1.1 and 2.7) existed in deep water north of the Southern Boundary, facilitating Fe precipitation. The [Lt]/[dFe] ratio increased southwards from the Southern Boundary on the Zero Meridian and from east to west in the Weddell Gyre due to changes both in ligand characteristics and in dissolved iron concentration. High [Lt]/[dFe] ratio expresses Fe depletion versus ligand production in the surface. The decrease with depth reflects the increase of [dFe] which favours scavenging and (co-) precipitation, whereas a horizontal increase in the deep waters results from an increasing distance from Fe sources. This increase in the [Lt]/[dFe] ratio at depth shows the very resistant nature of the dissolved organic ligands.

Iron analysis in bottle profiles off Mauritania

Iron solubility measurements in the Mauritanian upwelling and the adjacent Open Ocean of the Tropical Atlantic show for all stations lower values in the surface mixed layer than at depth below the pycnocline. We attribute this distribution to a combination of loss terms, chiefly photo-oxidation of organic ligands in the surface, and supply terms, predominantly from the release of ligands from the decomposition of organic matter. Significant correlations with pH, oxygen and phosphate for all samples below the surface mixed layer indicate that biogenic remineralisation of organic matter results in the release of iron binding ligands into the dissolved phase. The comparison of the cFeS/PO4**3- ratio with other published data from intermediate and deep waters in the Pacific suggests an enhanced release of iron chelators in the more productive Mauritanian upwelling zone.