Abstract
Kau Bay (island of Halmahera, Eastern Indonesia) is a 470 m deep basin
separated from the Pacific Ocean by a sill that is at present only 40 m below
sea-level. The presence of this sill has two major implications. Firstly, during
Weichselian time, the sea-level dropped below the depth of the sill, Kau
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Bay
became isolated and fresh-water sediments were deposited. Secondly, the
exchange of water between Kau Bay and the adjacent Pacific Ocean is
limited. As a consequence, Kau Bay is at present a semi-euxinic marine basin
wherein low oxygen, but non-sulphidic, bottom waters alternate with anoxic
sulphidic bottom waters. These two interesting features are reflected in the
composition of the interstitial waters of the sediments.
The fact that fresh-water sediments are underlying marine sediments has
resulted in the downward diffusion of chloride and the other major
components of seawater. It also initiated ion-exchange reactions between the
fresh-water clays and the brackish-to-saline pore waters.
The distinct hydrography and chemistry of Kau Bay has resulted in the
accumulation of organic-carbon-rich sediments. The anoxic decomposition of
organic matter causes the release of ammonium, alkalinity, phosphate and
sulphide to the interstitial waters. This liberation of nutrients drives various
diagenetic reactions such as ion-exchange between dissolved ammonium and
exchangeable cations and the formation of authigenic minerals. It also causes
changes in the salinity and the speciation of the major components in pore
waters.
Sediments in Kau Bay can be divided into three distinct units: a Holocene
semi-euxinic marine facies (Unit 1), a transitional facies (Unit 2) and a
Weichselian fresh-water unit (Unit 3). These different units represent the
response of Kau Bay to the Holocene sea-level change. Within these
sediments a range of authigenic minerals has been identified and include
dolomite, calcite, pyrite and apatite. Dolomite appears to be forming in
specific intervals by direct precipitation from solution at a rate of a few
centimetres per 1 kyr. The formation of dolomite is believed to be initiated
in the zone of anoxic methane oxidation. The sediments contain about 1 wt%
pyrite-sulphur and 0.5 wt% acid-volatile sulphur. The reactive iron content
of the sediment limits the amount of iron that is sulphidized. Reactive iron
contents are shown to be correlated with the amount of terrigenous organic
matter. The marine sediments from unit 1 show no relation between organic
carbon and sulphur and have Degree of Sulfidization values that vary from
0.36 to 0.78
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