wAbstract
@This study presents the isotopic compositions and concentrations
of dissolved and particulate iron from two seawater profiles of
the western and central equatorial Pacific Ocean, sampled during
the EUCFe cruise. Most of the Β56Fe values are positive
(relative to IRMM-14), from +0.01 to +0.58ρ in the dissolved fraction
(DFe) and from -0.02 to +0.46ρ in the particulate fraction (PFe).
The mean measurement uncertainty of }0.08ρ (2SD) allows the observation
of significant variations. Most of the isotope variations occur
in the vertical and not in the horizontal direction, implying
that each isotope signature is preserved over long distances within
a water mass.
@The thermocline waters of the Papua New Guinea (PNG) area, mostly
influenced by sedimentary inputs, display a mean Β56DFe
value of +0.37ρ (}0.15ρ, 2SD). This isotopic signature suggests
that the process releasing dissolved iron into the seawater in
this area is a non reductive dissolution of sediments (discharged
by local rivers and likely re-suspended by strong boundary currents),
rather than Dissimilatory Iron Reduction (DIR) within the sediment
(characterized by negative Β56DFe). These positive
Β56DFe values seem to be the result of a mean isotopic
fractionation of ’56FeDFe-PFe
= +0.20ρ (}0.11ρ, 2SD) produced by the non reductive dissolution.
At 0KN, 180KE, the Fe isotope signature of the Equatorial Undercurrent
(EUC) waters is identical to that of the PNG station within the
range of the uncertainty. This suggests that the dissolved iron
feeding the EUC, and ultimately the eastern Pacific high nutrient
low chlorophyll area, is of PNG origin, likely released by a non
reductive dissolution of terrigenous sediments.
@Significant Fe removals are observed within the thermocline and
the intermediate waters between the PNG and the open ocean stations.
The corresponding isotopic fractionations appear to be small,
with ’56Feremoved-SW Fe of -0.30}0.31ρ
to -0.18}0.12ρ (2SD) for DFe removal and of -0.10}0.04ρ to -0.05}0.31ρ
(2SD) for PFe removal. In the chlorophyll maximum of the open
ocean station, the isotopic fractionation associated with biological
uptake is estimated at ’56Fephyto-DFe
= -0.25}0.10ρ to -0.13}0.11ρ (2SD). Although these fractionations
are based on a limited dataset and need to be further constrained,
they appear to be small and to limit the transformations of the
iron source signatures within the ocean.
Keywords: Iron isotopes; Equatorial Undercurrent; Papua New Guinea;
equatorial Pacific; dissolved and particulate iron; biogeochemical
cyclex
1. Introduction
2. Sample location and water mass identification
3. Sampling and methods
4. Results of iron concentrations and Β56Fe
5. Discussion
@5.1. Papua New Guinea area (station 28)
@5.2. Thermocline waters in the EUC
@5.3. Antarctic Intermediate Water (AAIW)
@5.4. Biological uptake in the central equatorial Pacific (0KN,
180KE, station 14)
6. Conclusion
Acknowledgments
References