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.

Miocene to Quaternary paleoenvironments and uplift history on the mid Norwegian shelf

Based on benthic and planktic foraminifera, Bolboforma, oxygen isotope measurements and seismic data, major changes in Miocene, Pliocene and Pleistocene paleoenvironments on the mid Norwegian shelf are discussed and a possible scenario of the late Cenozoic uplift history is given. The dating of the Neogene sequence has been done using foraminifera and Bolboforma. Four main assemblage zones have been identified with nine distinct subzones. Most of the Miocene sequence is preserved. The lower Miocene sediments contain only siliceous microfossils. A period of high fertility and upwelling in the study area prevailed. The early Miocene-early mid Miocene (15 Ma?) change from a siliceous to a calcareous rich microfauna, dominated by Nonion barleeanum, can be related to increased surface-water circulation due to overflow across the Iceland-Faeroe ridge. During the Miocene the temperature decreased in the study area. Evidence of increased amounts of coarser sediments may suggest that an uplift of the mainland areas occurred during the mid-late Miocene. Lower Pliocene sediments contain a foraminiferal fauna that seems to occur in slightly colder conditions than the late Miocene fauna suggesting a further cooling. Possibly, Arctic waters entered the study area in the early Pliocene. A very marked change in lithology (from compacted claystone to unconsolidated diamicton), fauna (from deep dwelling to shallow dwelling species) and seismic signature (from flat lying reflectors to prograding clinoforms) occurs during the mid?-late Pliocene. A two step cooling trend is indicated by the microfauna of these prograding wedges. (1) The first wedge buildups might have been associated with an uplift of the mainland during the early late Pliocene (mid Pliocene, ca. 4 Ma). However, the age determination is somewhat uncertain and may very well be of late Pliocene age. (2) The second step of wedge buildup is associated with a glacial phase where the dominating microfauna exists of arctic species. Large continental ice sheets might have occurred at this time reaching coastal areas and that possibly many of the geomorphological features such as the strandflat were made during this episode. The Pleistocene epoch is represented by an increased percentage of boreal foraminifera intermingled with high arctic species which indicates that interglacial-glacial cycles prevailed and the dynamics of the glacier system changed.

Foraminiferal Mg/Ca and Mn/Ca ratios across the Eocene-Oligocene transition

Constraining the magnitude of high-latitude temperature change across the Eocene-Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (d18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2-3 °C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal d18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical d18O gradients remained nearly constant at all sites between 35.0 and 32.5 Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2-3 °C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal d18O shift indicates that seawater d18O values increased by 0.6 ± 0.15 per mil across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60-130% modern East Antarctic ice sheet volume.

Geochemistry of sediment cores GeoB13820-1 and GeoB13863-1 from the western South Atlantic

Here, we present results from sediments collected in the Argentine Basin, a non-steady state depositional marine system characterized by abundant oxidized iron within methane-rich layers due to sediment reworking followed by rapid deposition. Our comprehensive inorganic data set shows that iron reduction in these sulfate and sulfide-depleted sediments is best explained by a microbially mediated process-implicating anaerobic oxidation of methane coupled to iron reduction (Fe-AOM) as the most likely major mechanism. Although important in many modern marine environments, iron-driven AOM may not consume similar amounts of methane compared with sulfate-dependent AOM. Nevertheless, it may have broad impact on the deep biosphere and dominate both iron and methane cycling in sulfate-lean marine settings. Fe-AOM might have been particularly relevant in the Archean ocean, >2.5 billion years ago, known for its production and accumulation of iron oxides (in iron formations) in a biosphere likely replete with methane but low in sulfate. Methane at that time was a critical greenhouse gas capable of sustaining a habitable climate under relatively low solar luminosity, and relationships to iron cycling may have impacted if not dominated methane loss from the biosphere.

Pore water and solid phase sulfur, carbon and iron data from samples collected in the Argentine Basin during expedition M78

Sulfur phases in the Argentine Basin.

Volcanic rocks from the submarine Vityaz Ridge: their age, chemical and isotopic compositions

Results of geological studies at the submarine Vityaz Ridge carried out during cruises 37 and 41 of R/V Akademik Lavrent'ev in 2005 and 2006 are reported. The studied area is located at an near-island trench of the slope in the central part of the Kuril Island arc. Morphologically it consists of two parts: an inner volcanic arc represented by the Great Kuril Range and an outer arc corresponding to the submarine Vityaz Ridge. Diverse rocks composing the basement and the sedimentary cover of the ridge were recovered by dredging. Based on K-Ar dating and geochemistry, volcanics were divided into Paleocene, Eocene, late Oligocene, and Pliocene-Pleistocene complexes. Each of the complexes reflects a tectonomagmatic stage in the ridge evolution. Geochemical and isotope data on the volcanics indicate contribution of ancient crustal material in the magma source and, correspondingly, formation of this structure on the continental basement. Two-stage model ages (TDM2) vary in a wide range from zero values in mafic rocks to 0.77 Ga in felsic varieties, pointing to presence of Precambrian protolith in the source of the felsic rocks of the Vityaz Ridge. The Pliocene-Pleistocene volcanics are classed with tholeiitic, calc-alkaline, and subalkaline series, which differ in alkali contents and REE fractionation. Values of (La/Sm)_n and (La/Yb)_n ratios vary from 0.74 and 0.84 in the tholeiitic varieties to 1.19 and 1.44 in the calc-alkaline and 2.32 and 3.73 in the subalkaline rocks. All three varieties occur within the same volcanic edifices and formed during differentiation of magmatic melts that were channeled along fault zones from the mantle source slightly enriched in crustal component.

Ba, Mn, and Mg record derived from Hoeglundina elegans of sediment core SLA-9

The evolution of productivity, redox conditions, temperature, and ventilation during the deposition of an Aegean sapropel (S1) is independently constrained using bulk sediment composition and high-resolution single specimen benthic foraminiferal trace metal and stable isotope data. The occurrence of benthic foraminifer, Hoeglundina elegans (H. elegans), through a shallow water (260 m) sapropel, permits for the first time a comparison between dissolved and particulate concentrations of Ba and Mn and the construction of a Mg/Ca-based temperature record through sapropel S1. The simultaneous increase in sedimentary Ba and incorporated Ba in foraminiferal test carbonate, (Ba/Ca)H. elegans, points to a close coupling between Ba cycling and export productivity. During sapropel deposition, sedimentary Mn content ((Mn/Al)sed) is reduced, corresponding to enhanced Mn2+ mobilization from sedimentary Mn oxides under suboxic conditions. The consequently elevated dissolved Mn2+ concentrations are reflected in enhanced (Mn/Ca)H. elegans levels. The magnitude and duration of the sapropel interruption and other short-term cooling events are constrained using Mg/Ca thermometry. Based on integrating productivity and ventilation records with the temperature record, we propose a two-mode hysteresis model for sapropel formation.