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Journal Title
Marine Geology
ISSN
00253227
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Item type:Publication, Low-temperature alteration of dredged volcanics from the Southern Chile Ridge: additional information about early stages of seafloor weatheringA suite of submarine volcanic rocks from the Southern Chile Ridge has been examined in order to investigate the early stages of low temperature alteration. Alteration in these samples proceeded as follows: (1) Fe-staining on sample surface and along fractures, (2) filling of vesicles with secondary material, (3) breakdown of glassy matrix, (4) breakdown of microcrystalline matrix, and (5) breakdown and replacement of olivine. Plagioclase and pyroxene were sometimes found to be slightly altered along internal fissures. Secondary or alteration phases generally showed high K (3–5 wt.%), Fe (30–70 wt.%) and low Al (<2 wt.%) contents. The most common secondary minerals were Fe-oxyhydroxides, celadonite/Fe-oxyhydroxide (a mixture between celadonite and Fe-oxyhydroxide) and saponite. The formation of secondary minerals is controlled by the oxidation state which is a direct result of seawater to rock ratio. Fe-oxyhydroxides and celadonite/Fe-oxyhydroxide form during oxidative diagenesis, whereas saponite forms during non-oxidative diagenesis. The final alteration products, however, are controlled by external factors, e.g., sedimentation rate or formation of a manganese crust. Ca has been consistently removed, Mg has been added to some rocks and lost from others, and Si has either been removed or remained unaffected. Fe and H2O increases are accompanied by an increase in Fe3+/Fe2+. Wherever increases in K, Rb and Cs were documented, the enrichments are consistently in the order Cs>Rb>K. During initial stages of alteration the behavior of some trace elements such as rare-earth elements (REE), Ba, Zr, Hf, Ta, Nb, and Mo are solely controlled by the precipitation of Mn-rich Fe-oxyhydroxides. The preferred incorporation of Ce into Mn-rich Fe-oxyhydroxides may be a principal factor explaining the Ce depletion in seawater. We conclude that the earliest stages of submarine weathering are controlled by Eh and pH gradients between the rock and seawater. In the absence of a buffer, oxidation of ferrous iron causes a decrease in solution pH.journal articleBand:159Heft:1-467 45 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Contourite and mixed turbidite-contourite systems in the Mozambique Channel (SW Indian Ocean): Link between geometry, sediment characteristics and modelled bottom currentsOceanic currents can profoundly reshape the seafloor and even modify the characteristics of turbidite systems. Multiple erosional and depositional features directly formed by bottom currents (i.e. contourites), as well as by the interaction between bottom currents and turbidity currents or turbidite systems (i.e. mixed turbidite-contourite systems) have been identified in the Mozambique Channel (SW Indian Ocean) in multibeam bathymetry, seismic reflection data, sub-bottom profiler images and sediment cores. In this study, we characterise the morphology, stacking pattern and sedimentary characteristics of these sedimentary systems and analysed the properties of bottom currents at these systems using a hydrodynamic numerical model. Modelled bottom currents are the highest at abraded surfaces and moats, but they also display a relatively high variability, suggesting that the observed erosion is not the result of a constant or persistent current but rather of episodes of intense circulation. Modelled bottom currents at contourite terraces are not significantly different from currents at related plastered drifts, where accumulation is enhanced. The formation of contourite terraces can thus not solely be explained by the mean oceanic circulation and eddies, implying that other processes such as internal waves may play a relevant role in their formation. Three different types of mixed turbidite-contourite systems were observed: one characterised by asymmetric channel-levee systems formed by the synchronous interaction of bottom currents and turbidity currents, one characterised by a phased interaction that resulted in the erosion of the channel flanks by bottom currents, and another one in which both synchronous and phased interaction played a relevant role in the evolution of the system. Finally, we propose a simplified classification of contourites that can be applied to any contourite system worldwide, and that comprises erosional and depositional features, including muddy and sandy contourite deposits.journal article146 191