Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

9A,12C/2004G302

Chemical States of Iron in the Deep Sea Sediments; in case of the Antarctic
Ocean
Katsumi SHOZUGAWA*1, Motoyuki MATSUO1, Akihito KUNO1, Hideki MIURA2.
Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo
153-8902, Japan
2
National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan

Introduction
Sea sediments provide a unique probe of theenvironment of the paleocean at the time of sedimentation.
A massive fluctuation is not seen in the content of thewhole sediment unless local event, like chimney, occurs
even if we determinate quantity of a heavy metal element
included in the sediment. It is difficult to identify the seaevent that happened on the ocean from the viewpoint of
the elemental determination. Provided that the element isin the sediment abundantly, the chemical states ofelement change depending on sea environment.
Specifically we paid attention to pelagic sea sediment of
the Antarctic Ocean where we estimated that theenvironment would change greatly after Pleistocene age.
Therefore, measuring the chemical states of iron withXANES spectra, we expect the reconstruction of theoxidation-reduction state of bottom water of sea at thetime of sedimentation.
Experiment
JOGMEC (Japan Oil, Gas and Metals NationalCorporation) collected sample cores at the deep sea offing
Antarctic Peninsula, located 60°11’08”S, 55°30’27”W in1997. The water depth of collecting station is 3531m. All
collected core samples were cut at every 1cm(~150mg)
immediately, and were kept at 4 degrees until themeasurement. Samples were also kept in N2 atmosphereto prevent from being oxidized with air. The XAFS
measurement was performed using synchrotron radiation
ring at BL-9A/12C, Photon Factory, KEK[1]. A Si (111)
double-crystal monochromator was used. The iron Kedge XANES spectra of the samples were measured in afluorescence mode using a Lytle-type detector. As
standard materials, Hematite, Olivine, Pyrite, Pyrrhotite
were also measured. Hematite and Olivine are for a classof Fe(III) and Fe(II).
Results and Discussion
Sample core has been no influence of human activities,
taking account of the sedimentation age and water depth.
Besides, we applied the depth of core to sedimentation
age with the dating value using 14C by JOGMEC. This
core has about 32000y.B.P. sedimentation age at thebottom. Figure1 shows the normalized iron XANES
spectra of the sediments by depth and standard materials.
The measurement performed 18 samples according to thesedimentation age. All spectra passed the middle ofHematite and Olivine near the K-edge. Contents of Fe(III)that is a characteristic of clay minerals tend to beabundant apparently.
Note that the K-edge. In the neighbourhood of K-edge,
with an expansion in Fig.1, it is evident that state ofoxidation-reduction of iron changed with the increasing
the sedimentation age. The spectrum of 22000y. B.P.
tends to be oxic relatively, in contrast, that of 10000y.
B.P. tends to be anoxic. The surface of core (5500y. B.P.)
passed middle of both. This trend was observed
continuously with all samples from pre-edge to K-edge.
Simultaneously we determined the quantity of iron withINAA (Instrumental Neutron Activation Analysis) overthe entire core; the content of iron was constant, around~1.8%, from the core-top to the bottom. Consequently, it
was noticeable that the chemical states of iron fluctuatedby the sedimentation age in spite of the constant ironcontent. This behaviour suggests the possibility that sea
sediment logged events of the oxidation-reduction state of
the AABW, Antarctic Bottom Water, which occurred
after Pleistocene age[2]. However, we cannot rule out apossibility that the sample includes the degeneration ofiron compounds after sedimentation. The further
discussion will be required about the effect of thesediment diagenesis.
Sedim ents

Nomalized Adsorption Coefficient1

PyritePyrrhotite
OlivineHematite

5500y.B.P.

10000y.B .P.

22000y.B.P.

7.106

7.114

7.122

7.130

7.138

Energy / keV

Figure1. Normalized iron K-edge XANES spectra of thesediments from various sedimentation age and standard
materials.
References[1] M.Nomura, A. Koyama, KEK Report. 95-15. (1996).
[2] Damuth et al., Nature. 290, 699-700. (1983).
*shozum@dolphin.c.u-tokyo.ac.jp

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