Oxygen microprofiles and porewater chemistry in sediments of the Skagerrak

Sediment porewater oxygen profiles were measured with micro and needle electrodes in sediment cores of 27 stations in the Skagerrak (northeastern North Sea). Oxygen penetration depth ranged from 3 to 20 mm depth. Fluxes estimated from the oxygen gradients varied from 3 to 18 mmol m**-2 d**-1. Oxygen penetration and flux depend on water depth, but possibly more on the hydrological conditions, related to the import of fresh organic matter by primary production in the water column. Oxygen fluxes were not related to the total organic carbon (TOC) content of the sediments. Stations in the eastern part of the Skagerrak showed high burial rates of TOC. At 6 stations porewater chemistry of Fe, Mn and NO3- was strongly associated with the oxygen distribution. The average relative contribution of terminal electron acceptors to carbon mineralisation was estimated at 85% for O2, 0.5% for Mn, 4.5% for [NO3]3-, 1% for Fe and 9% for [SO4]2-. At one station the occurrence of exceptionally high solid manganese oxyhydroxides was probably related to an active internal manganese cycle.

(Table 1) Pigments isolated from DSDP Leg 56 Holes

A relatively well documented record of intermediate and late chlorophyll diagenesis in marine sediments now exists. Intermediate diagenetic stages include conversion of chlorins to DPEP-series porphyrins and subsequent chelation with nickel, vanadyl, and, in special cases, copper. Increasing thermal stress leads to etio-series generation and transalkylation (Baker, 1969; Baker and Smith, 1975; Baker et al., 1977; Palmer and Baker, in press). In contrast, the early transformations of clorophyll are still largely unknown. Very early diagenetic reactions must certainly include loss of magnesium, deesterification, decarboxylation, reduction of ring-conjugating groups, and finally, oxidative-aromatization of carbons 7 and 8 in ring IV to yield free-base porphyrins (Baker and Smith, 1973; Smith and Baker, 1974). Chlorins (7,8-dihydroporphyrins) are very difficult to isolate and identify, because of hydrocarbon impurities which absorb in the blue to violet region of the electromagnetic spectrum and which co-chromatograph with the pigments. Further complications possibly can arise from artifact formation during isolation. In the present study, twelve DSDP Leg 56 core samples, ranging in sub-bottom depth from 4 to 420 meters and in age from Pleistocene to middle Miocene, were analyzed for tetrapyrrole pigments. Chlorins, in concentrations ranging from about 4 to less than 0.002 µg/g sediment, wet weight, were the only tetrapyrroles found. A carotenoid (tetraterpene) was isolated from Section 434-1-3.

Geochemistry of tetrapyrrole, carotenoid, and perylene pigments in sediments at DSDP Holes 63-467 and 63-471

Twenty-one core samples from DSDP/IPOD Leg 63 were analyzed for products of chlorophyll diagenesis. In addition to the tetrapyrrole pigments, perylene and carotenoid pigments were isolated and identified. The 16 core samples from the San Miguel Gap site (467) and the five from the Baja California borderland location (471) afforded the unique opportunity of examining tetrapyrrole diagenesis in clay-rich marine sediments that are very high in total organic matter. The chelation reaction, whereby free-base porphyrins give rise to metalloporphyrins (viz., nickel), is well documented within the downhole sequence of sediments from the San Miguel Gap (Site 467). Recognition of unique arrays of highly dealkylated copper and nickel ETIO-porphyrins, exhibiting nearly identical carbonnumber homologies (viz., C-23 to C-30; mode = C-26), enabled subtraction of this component (thought to be derived from an allochthonous source) and thus permitted description of the actual in situ diagenesis of autochthonous chlorophyll derivatives.