Early diagenesis and authigenic mineral formation in anoxic sediments of Kau Bay, Indonesia

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 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