| 19281 |
abstract |
In the marine environment, authigenic ferromanganese (Fe–Mn) oxides precipitate from
seawater, incorporating many dissolved trace elements (such as Nd, Pb) whose isotopic
composition is a direct proxy for ambient seawater. Thus, isotopic studies of the
Fe–Mn oxyhydroxide component leached from sedimentary records can provide information
on changes in erosional input or ocean circulation changes. Such studies may be complicated,
however, by the presence of detrital material containing a Fe–Mn oxide component,
which may mask the true ‘seawater’ signal. In addition, the formation of Fe–Mn oxyhydroxides
in the marine environment remains poorly understood. In particular, the phase (carbonates,
detrital particles) that controls the delivery of authigenic Fe–Mn oxides to the sediments
has not yet been determined. In this study, we have analysed the REE and Nd isotopic
compositions of Fe–Mn oxyhydroxide fractions dispersed in marine sediment cores from
the Cape and Angola basins, in the SE Atlantic (cruise IMAGES II). For the Angola
Basin deep-sea core, located at 1000 km south of the Congo River mouth, both the REE
and Nd isotopic compositions (εNd) measured in recent Fe–Mn oxyhydroxide fractions
are typical of Congo River-borne Fe–Mn oxides. This shows that ‘preformed’ Fe–Mn oxides
associated with detrital fractions can locally contaminate the ‘seawater’ signal recorded
by ‘authigenic’ oxyhydroxides. By contrast, examination of REE distributions, Nd isotope
data and mass accumulation rates in Cape Basin sediments shows paradoxically that
the flux of Fe–Mn oxides to sediments is controlled by aeolian particles from the
nearby Namib Desert, even though their REE and εNd compositions point clearly to an
‘authigenic’ origin. It is proposed that partial dissolution of aeolian dust occurs
in the water column, releasing into solution its easily leachable Fe–Mn component.
The dissolved Fe2+ and Mn2+ would then fuel the reprecipitation of authigenic Fe–Mn
oxyhydroxides, scavenging additional dissolved trace elements from the ocean, most
probably from deep water masses. In this case, calculations suggest that aeolian deposition
acts as a net sink for dissolved Nd in the Cape Basin, rather than a source, and allows
us to estimate the rate of removal of dissolved Nd associated with atmospheric deposition,
yielding a global oceanic residence time for Nd (τNd) of between 500 and 1400 years. |
| 19281 |
label |
Bayon, Germaine; German, Christopher R.; Burton, Kevin W. ; Nesbitt, Robert W.
and Rogers, Nick W. (2004). Sedimentary Fe–Mn oxyhydroxides as paleoceanographic
archives and the role of aeolian flux in regulating oceanic dissolved REE. Earth
and Planetary Science Letters, 224(3-4) pp. 477–492. |
| 19281 |
label |
Bayon, Germaine; German, Christopher R.; Burton, Kevin W. ; Nesbitt, Robert W. and
Rogers, Nick W. (2004). Sedimentary Fe–Mn oxyhydroxides as paleoceanographic archives
and the role of aeolian flux in regulating oceanic dissolved REE. Earth and Planetary
Science Letters, 224(3-4) pp. 477–492. |