Organic carbon, sulphur, and iron in recent semi-euxinic sediments of Kau Bay, Indonesia

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. During Weichselian time, the sea-level dropped below the depth of the sill and freshwater sediments were deposited. These sediments are rich in organic matter ( C org = 2–6 wt% ) with terrigenous isotope characteristics ( δ 13C org = −28 to −30%. ), and are also rich in sulphur (S ≈ 4 wt%). This sulphur enrichment is interpreted to be a late diagenetic feature related to diffusion of dissolved sulphate and sulphide from the marine sediments into the freshwater sediments; the sulphur in these sediments has a δ 34S value of + 15%.. The overlying Holocene sediments consist of marine muds deposited in alternating low-oxygen and H 2S-bearing bottom waters. The marine sediments are also rich in organic matter (C org = 2–6 wt%), but of mixed terrigenous and marine origin ( δ 13C = −25 to −19%. ). Moreover, the sulphur content of these Holocene sediments is lower (S = 1–2 wt%) and isotopically lighter ( δ 34S = −20%. ). In addition to pyrite, large amounts of acid-volatile sulphur (AVS) compounds accumulate in Kau Bay. The reactive iron content of the sediment limits the amount of iron that is sulphidized, while the supply of oxidants controls the limited conversion of AVS to pyrite. Total and reactive iron contents are strongly correlated with the amount of terrigenous organic matter as a consequence of their common provenance and depositional histories. The marine sediments in Kau Bay show no simple relation between organic carbon and sulphur.