The Carolina Slope and Rise region is well known for its abundant gas hydrate (Paull, Matsumoto, Wallace, et al., 1996). Seismic data show at least 1000 m of sediment accumulation with a prominent BSR, which occurs at the base of the gas hydrate stability zone.
Presently, the environment overlying the Carolina Slope is influenced by the Gulf Stream and by the shallowest waters of the Deep Western Boundary Current (DWBC), which are thought to originate in the southern Labrador Sea (Pickart and Smethie, 1993). During glacial times, when production rate of the lower component of NADW decreased and the upper component increased (Boyle and Keigwin, 1987; Oppo and Fairbanks, 1987; Lehman and Keigwin, 1992), Sites 1054 and 1055 were probably influenced by glacial North Atlantic Intermediate Water. The presence of high-latitude clay mineralogies and red sediments of Nova Scotian origin on the Carolina Slope could be advective tracers for the flow of intermediate-depth waters (Heezen et al., 1966; Hathaway, 1972).
Drilling objectives at Sites 1054 and 1055 were to (1) monitor the shallowest reaches of UNADW in Pleistocene time; (2) provide high-resolution sections for paleomagnetic study; and (3) provide sections for studying geochemical processes related to gas hydrate formation and dissociation. In addition, the sedimentary record recovered at Site 1055, together with that at Sites 1054 and 1056, may prove useful for testing models of continental slope sedimentation during glacial lowering of sea level.
The sedimentary succession recovered from the three holes drilled at Site 1054 consists of a well dated 200-m-thick interval of latest middle Pliocene (Piacenzian) to Holocene nannofossil ooze and silty clay, with variable proportions of biogenic and siliciclastic components. Biogenic carbonate and opal are present throughout the succession. Three lithologic units were recognized (Fig. 3). Unit I (0-19 meters below seafloor [mbsf], Holocene to middle Pleistocene) consists of clayey and silty mixed sediments and is defined largely on the basis of relatively abundant carbonates (up to 67%) and higher amplitude oscillations in color reflectance than the underlying units. Unit II (19 to 119 mbsf; middle Pleistocene to early Pleistocene) consists of interbedded mixed sediment and silty foraminifer sand with decreased presence of carbonate and color variations. Unit III (119 to 200 mbsf; early Pleistocene to the latest middle Pliocene) consists of mixed sediment with increased clay content. There is evidence of downslope transport with as many as 20 thin carbonate turbidites and debris-flow deposits observed at Hole 1054A, but their thickness and occurrence among Site 1054 holes suggests spatial variability.
The succession contains abundant and well preserved calcareous nannofossils and planktonic and benthic foraminifers. Diatoms are common to abundant and exhibit moderate to good preservation. Pteropods are well represented and become abundant in several discrete layers. Siliceous flagellates (including silicoflagellates and ebridians) and radiolaria range from trace to common occurrences with good to moderate preservation. The abundance of diatoms suggests that conditions of high productivity might have characterized the area of Site 1054 in the past 2.5 m.y. Reworking and displacement of shallow-water forms is surprisingly low, in spite of the sedimentological evidence of downslope transport and current activity.
Paleomagnetic results indicate that the Brunhes Epoch (0-0.78 Ma) is possibly located at 26 mbsf in Hole 1054A, with the Jaramillo event (0.99-1.07 Ma) between 27 and 31.5 mbsf. Below 75 mbsf in Hole 1054A, the magnetization signature is weak and difficult to interpret owing to drilling overprints.
Sedimentation rates were 46 m/m.y. in the past 0.5 m.y., 15 m/m.y. between 0.5 and 1.0 Ma, 120 m/m.y. between 1.0 and 2.0 Ma, and 75 m/m.y. between 2.0 Ma and the bottom of the succession at 2.5 Ma. Low sedimentation rates between 0.5 and 1.0 Ma are associated with an interval where there is evidence of removal and redeposition of sediments by bottom currents.
Calcium carbonate contents fluctuate between 30% and 67% with an average value of 46.5%, gradually decreasing with increasing sediment depth. The carbonate cycles probably reflect glacial/interglacial fluctuations. Total organic carbon (TOC) content varies between 0.7% and 1.84% with an average value of 1.2%. Organic C/N ratios between 5 and 10 are indicative of predominantly marine organic material. Results of Rock-Eval analysis indicate that organic matter is thermally immature with respect to petroleum generation.
Pore-water profiles from Site 1054 are typical of sediments in which sulfate reduction and methanogenesis occur. Sulfate concentrations decrease from seawater values at the top of the hole to values less than 1 mM at 48 mbsf. The onset of the methanogenic zone occurs at 48 mbsf, coincident with the level of zero pore-water sulfate. The boundary between sulfate reduction and methanogenesis is very sharp, presumably because utilization of methane by sulfate-reducing bacteria prevents significant diffusive penetration of methane into the overlying sulfate reduction zone. The high C1/C2 ratios and the absence of major contribution of higher molecular weight hydrocarbons suggest that the source for methane is most likely in situ bacterial methanogenesis resulting from decomposition of organic matter in the sediments. Although Site 1054 is located in a well-characterized gas-hydrate area, gradients of chloride are absent, suggesting that gas hydrate was not recovered in cores from Site 1054.
The sedimentary succession recovered from the five holes at Site 1055 is a 130-m-thick interval of early Pleistocene to Holocene nannofossil ooze and silty clay with variable proportions of biogenic and siliciclastic components. Biogenic carbonate and opal are present throughout the succession. Two lithologic units were recovered(Fig. 3). Unit I (0 to 80 mbsf; Holocene to early Pleistocene) is composed of interbedded layers of nannofossil ooze, nannofossil ooze with silt, clayey nannofossil ooze, silty clay, silty clay with foraminifers, clay with silt, and clay. The occurrence of pteropods in several silty clay layers and the presence of dolomitized clay concretions in Section 1055D-6H-1 are noteworthy. Unit II (80 to 128 mbsf; early Pleistocene) is composed of predominantly massive, structureless, and homogeneous dark greenish gray clay with siliceous, calcite-cemented burrow fills. Although nannofossils are present throughout Unit II, the nannofossil-rich layers found above are completely absent.
Fauna and flora at Site 1055 are rich and diverse. Calcareous nannofossils and planktonic foraminifers are abundant and well preserved, whereas benthic foraminifers are rare to few but well preserved. Diatoms are less abundant than at Site 1054 and exhibit moderate to good preservation. Paleomagnetic results indicate that the upper 100 m of Hole 1055B lie within the Brunhes Chron, although below this level the magnetization is weak and difficult to interpret owing to drilling overprints.
Sedimentation rates, based on six calcareous nannofossil and planktonic foraminifer events, increased with decreasing age from 30 m/m.y. for the interval from 1.0 to 1.3 Ma, to 79 m/m.y. in the interval 1 Ma to 0.5 Ma, and to 120 m/m.y. for the past 0.5 m.y. Multisensor track (MST) investigations document that recovery was complete over the entire sequence (0-129 mbsf) with good overlap across core breaks.
Calcium carbonate content fluctuates between 34% and 67%, with an average value of 48%, and gradually decreases downhole. The carbonate cycles probably reflect glacial/interglacial fluctuations. TOC varies between 0.3% and 2.10%. Organic C/N ratios lie mainly between 5 and 35, with an average value of 6, and are indicative of predominantly marine organic material. Results of Rock-Eval analysis indicate that organic matter is thermally immature with respect to petroleum generation.
Pore-water profiles from Site 1055 are typical of sediments in which sulfate reduction and methanogenesis occur. Sulfate concentrations decrease from the seawater value at the top of the core to values less than 1 mM at 25 mbsf. The shallower and thinner sulfate reduction zone at Site 1055 compared to Site 1054 is probably due to the labile nature of the sedimentary organic matter at Site 1055. The onset of the methanogenic zone occurs at 25 mbsf, coincident with the sulfate consumption. High C1/C2 ratios and the absence of major contribution of higher molecular weight hydrocarbons suggest that the source for methane is most likely in situ bacterial methanogenesis resulting from decomposition of organic matter in the sediments. Although Site 1055 is located in a well-characterized gas-hydrate area, gradients of chloride are absent, suggesting that no gas hydrate was recovered.
To 172 Results: intermediate-water sites 1056-1059
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