Carbon and Nitrogen Profiles in Deep-Sea Sediments: New Evidence for Bacterial Diagenesis at Great Depths of Burial

Self Cite

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...

Section: Kinetics Of Organic Carbon Diagenesiscontrasting

confidence: 51%

Section: Microbial Diagenesismentioning

confidence: 99%

Leg 80 Shipboard Organic Geochemistry

Waples¹,

Cunningham²

1985

Self Cite

“…It has been tempting to utilize C/N data generated on the Glomar Challenger as though the nitrogen measured were entirely organic (e.g., Waples and Sloan, 1980). As Arrhenius (1950), Stevenson and Cheng (1972), and Tucholke, Vogt, et al (1979) have noted, however, C/N ratios in sediments with low contents of organic matter may be strongly influenced by the inorganic nitrogen present as ammonia or as ammonium ion fixed on clays.…”

Section: Introductionmentioning

confidence: 99%

Organic and Inorganic Nitrogen in Sediments from Leg 80, Deep Sea Drilling Project

Waples¹

1985

Self Cite

Concentrations of organic and inorganic nitrogen have been measured on Leg 80 sediments. The inorganic nitrogen content is relatively constant, 0.02-0.03 wt.%. Because most of the inorganic nitrogen occurs as NH 3 or (NH 4) + fixed on clays, clay-poor sediments have lower inorganic nitrogen contents. Organic nitrogen content depends upon both the type and the quantity of organic matter present. In Leg 80 sediments, woody kerogens contain much less organic nitrogen than do kerogens of algal origin. Furthermore, pelagic samples of low organic carbon content have less organic nitrogen than predicted, because of loss during diagenesis. DSDP shipboard analytical procedures do not distinguish between organic and inorganic nitrogen. Great caution must therefore be exercised in interpreting C/N ratios.

“…More probably, reaction within the sediment leads to a decrease in organic carbon content with advancing time or increasing depth of burial. The regular decrease of organic carbon content with depth in marine sediments has been studied in both shallow (0-5 m) cores (Heath et al, 1977;Mueller and Mangini, 1980) and in DSDP (0-500 m) cores (Waples and Sloan, 1980). In a variety of environments, each with relatively uniform sediment accumulation rates, the decrease in organic carbon with time is consistent with first-order decay at half-lives that range from 15,000 to 866,000 years.…”

Section: Discussionmentioning

confidence: 90%

Diagenesis of Organic Matter, Isotopic Composition of Calcite Veins in Basement Basalt and Pore Water in SedimentBarbados Ridge Complex, Deep Sea Drilling Project Leg 78A

Claypool¹

1984

Sediments of the Barbados Ridge complex, cored on DSDP Leg 78A, contain low concentrations of acid-insoluble carbon (0.05-0.25%) and nitrogen (C/N 1.5-5) and dispersed Cj-C 6 hydrocarbons (100-800 ppb). The concentrations of organic carbon and 13 C in organic carbon decrease with depth, whereas the concentration of dispersed hydrocarbons increases slightly with depth. These trends may reflect the slow oxidation of organic matter, with selective removal of 13 C and slow conversion of the residual organic matter to hydrocarbons. Very minor indications of nitrogen gas were observed at about 250 meters sub-bottom at two of the drilling sites. Basement basalts have calcite veins with δ 13 C values in the range of 2.0 to 3.2‰ and δ 18 O SMOW values ranging from 28.5 to + 30.6‰. Interstitial waters have δ 18 O SMOW of 0.2 to -3.5‰ and δD SM ç W of -2 to -15‰. The oxygen isotopic composition of the calcite veins in the basement basalts gives estimated equilibrium fractionation temperatures in the range of 11 to 24°C, assuming precipitation from water with δ 18 O SMOW in the range of +0.1 to -l.O‰. This suggests that basalt alteration and precipitation of vein calcite occurred in contact either with warmer Campanian seawater or, later, with pore water, after burial to depths of 200-300 meters. Pore waters from all three sites are depleted in deuterium and 18 O, and dissolved sulfate is enriched in 34 S at Sites 541 and 542, but not at Site 543.