Leg 80 Shipboard Organic Geochemistry

Abstract

The four sites drilled on the Irish continental margin (Goban Spur) yielded sediments ranging in age from Holocene to Barremian. Most of the sediments were deposited in well oxygenated waters, and the small amounts of organic matter they contain are highly oxidized. During a few time intervals from the Cenomanian to earliest Turonian, however, the oxygen content of the bottom waters reached very low levels, resulting in the deposition of homogeneous or laminated black sediments containing from 0.5 to 11% total organic carbon (TOC). The original organic matter was of mixed marine and terrestrial origin. The oxidizing-reducing cycles represented by interbedded black and light sediments are probably a result of changes in both circulation and productivity. The black sediments at Sites 550, 551, and 549 were probably deposited near the lower end, middle, and upper end, respectively, of an expanded oxygen-minimum layer. The oil and gas source potential of the laminated black sediments is very good to excellent. The organic-carbon-lean sediments deposited under oxidizing conditions have no oil or gas source potential. The thermal maturity of all sediments is low. TOC. The nannofossil oozes, which were deposited much more slowly, have lower TOC values. Unit 3 Unit 3 appears in Hole 548 from Cores 16 to 35 and in Hole 548A from Core 1 to Core 11, Section 3, 125 cm. The TOC values decrease and carbonate contents increase with depth. Sparse Rock-Eval data suggest that the organic matter is similar to that of the overlying units. Unit 4 Unit 4 appears in Hole 548A from Core 11, Section 3, 125 cm to Core 22, Section 6, 15 cm. It is made up of calcareous oozes that have very high carbonate contents and low TOC values. Because of the low TOC values, the few Rock-Eval analyses are not very reliable. Unit 5 Unit 5 appears in Hole 548A from Core 22, Section 6, 15 cm to Core 28, Section 6, 35 cm. The unit's carbonate contents are variable but in general much lower than in the Unit 4 sediments. The TOC values are in general slightly higher. The single Rock-Eval pyrolysis indicated no hydrocarbon source potential. Unit 6 Unit 6 appears in Hole 548A from Core 28, Section 6, 35 cm through Core 35. The sediments are nearly pure chalks and have very low TOC values. No Rock-Eval analyses were attempted. Site 549 Syn-and postrift sediments of Barremian to Pleistocene age were recovered from Site 549. As one moves uphole the sediments change from inner sublittoral to pelagic. In addition, a slightly metamorphosed sandstone, presumably of pre-rift Hercynian age, was recovered from the bottom of the hole. Site 549 is much more heterogeneous lithologically than Site 548, a difference that is clearly displayed in the geochemistry of the sediments. The monotonic decrease in organic carbon content with increasing depth of burial observed at Site 548 is not very evident at Site 549, although in the upper 30 m a slight decrease does occur (Table 2 and Fig. 3). The hydrogen and oxygen indices derived from Rock-Eval pyrolysis (Table 2) are plotted o...

Record of forearc devolatilization in low‐T, high‐P/T metasedimentary suites: Significance for models of convergent margin chemical cycling

“…Carbon‐reduced/nitrogen ratios are another useful indicator of the devolatilization of these elements, as both of these elements are provided to the sedimentary rocks largely by organic processes. Most sediments should have C/N ratios <20:1 [ Müller , 1977; Sweeney et al , 1978; Waples and Cunningham , 1985], which would be likely to be fractionated if either element was being lost due to devolatilization as organic detritus becomes purified (ultimately to graphite) and N is transferred into phyllosilicates. The vast majority of the high‐P/T metasedimentary rocks from this study fall into that range (Figure 10b).…”

Stable Isotopic Composition (d18OCO32–, d13C) of Early Eocene Fish-Apatite from Hole 913B: An Indicator of the Early Norwegian-Greenland Sea Paleosalinity

“…It is possible that the trend towards higher fish-apatite δ 13 C values from the dark gray sediment of Hole 913B, through the brown to greenish gray R0snaεs Clay Formation (Schmitz et al, 1996), to the pale, highly oxidized sediment of Hole 550 (Waples and Cunningham, 1985) indicates that early diagenetic isotopic reequilibration of the fish-apatite has taken place. This is also supported by the relationship between fish-apatite and foraminifer δ 13 C from the latter two sites.…”

Calcareous nannofossil biostratigraphy and assemblages of the Cenomanian‐Turonian boundary interval: Implications for the origin and timing of oceanic anoxia

“…In the South Ferriby Pit and Elsham Quarry sections of Humberside, England, the black shale has low organic carbon contents (commonly around 1-2%), and is bioturbated [Wood and Smith, 1978] and highly calcareous, containing abundant nannofossils and foraminifera. Black shale deposited at DSDP Site 551 on the Goban Spur is practically noncalcareous and quite siliceous with organic carbon contents ranging up to 10% [Waples and Cunningham, 1985]. The fine-scale sedimentary structure and lithostratigraphy of this interval are uncertain because drilling disturbance has mixed pieces of chalk and green volcanic claystone with black shale.…”