Tab. 2: Average geochemical composition of dolerites from Neuschwabenland

In western Neuschwabenland basic dikes occur in the Jurassic lavas and Permian sediments of Vestfjella as weil as in the Precambrian sedimentary-volcanogenic rock sequence of the Ahlmannryggen and in the Precambrian crystalline complexes of Heimefrontfjella and Mannefallknausane. The concentration of the dikes in Vestfjella is conspicuous. Two main directions of strike perpendicular to each other are recognizable, from which the NE-SW striking one is predominant. The direction of the dikes coincides with the Mesozoic and younger fracture tectonics. Age relationships by structural, petrographical and geochemical observations are confirmed by palaeomagnetic and radiometrie age determinations from PETERS et al. (1986). Considerations on the geochemistry of further dolerite occurrences from Antarctica and other regions of the Gondwana continent are pointed out. Finally comparisons with the analogous South African dike system show the geotectonic significance of the dolerite dikes for the break-up of Gondwana.

Lead and strontium isotope composition, mass accumulation rates and rare earth elements of selected samples from DSDP Sites 92-597 to 92-601

Most of the Pb isotope data for the Leg 92 metalliferous sediments (carbonate-free fraction) form approximately linear arrays in the conventional isotopic plots, extending from the middle of the field for mid-ocean ridge basalts (MORB) toward the field for Mn nodules. These arrays are directed closely to the average values of Mn nodules, the composition of which reflects the Pb isotope composition of seawater (Reynolds and Dasch, 1971). Since the Leg 92 samples are almost devoid of continentally derived detritus, it can be inferred that the more radiogenic end-member is seawater. The less radiogenic end-member lies in the very middle of the MORB field, and hence can be considered to reflect the Pb isotope composition of typical ocean-ridge basalt. The array of data lying between these two end-members is most readily interpreted in terms of simple linear mixing of Pb from the two different end-member sources. According to this model, eight samples from Sites 599 to 601 contain 50 to 100% basaltic Pb. Five of these samples have compositions that are identical within the uncertainty of the analyses. We use the average of these five values to define our unradiogenic end-member in the linear mixing model. The ratios used for this average are 206Pb/204Pb = 18.425 ± 0.010; 207Pb/204Pb = 15.495 ± 0.018; 208Pb/204Pb = 37.879 ± 0.068. These values should approximate the average Pb isotope composition of discharging hydrothermal solutions, and therefore also that of the basaltic crust, over the period of time represented by these samples ( 4 m.y., from 4 to 8 Ma). Sr isotope ratios show a significant range of values, from 0.7082 to 0.7091. The lower ratios are well outside the value of 0.70910 ± 6 for modern-day seawater (Burke et al., 1982). However, most values correspond very closely to the curve of 87Sr/86Sr versus age for seawater, with older samples having progressively lower 87Sr/86Sr ratios. The simplest explanation for this progressive reduction is that recrystallization of the abundant biogenic carbonate in the sediments released older seawater Sr which was incorporated into ferromanganiferous phases during diagenesis. Leg 92 metalliferous sediments have total rare earth element (REE) contents that range on a carbonate-free basis from 131 to 301 ppm, with a clustering between 167 and 222 ppm. The patterns have strong negative Ce anomalies. Samples from Sites 599 to 601 display a slight but distinct enrichment in the heavy REE relative to the light REE, whereas those from Sites 597 to 598 show almost no heavy REE enrichment. The former patterns (those for Sites 599 to 601) are interpreted as indicating moderate diagenetic alteration of metalliferous sediments originating at the EPR axis; the latter reflect more complete diagenetic modification.

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.