Leg 172 Scientific Report

RESULTS (continued)

Intermediate Water Sites on the Blake Outer Ridge

Sites 1056-1059 are located at intermediate water depths (2177 to 2997 m) along the crest of the BBOR (Fig. 2), at locations ideal for monitoring Pleistocene to late Pliocene changes in production and flow of NADW. All four sites were cored within the advanced hydraulic piston corer (APC) at least three times to ensure complete recovery of the stratigraphic section, a prerequisite for high resolution paleoenvironmental studies. Site 1056 lies within the core of UNADW, Sites 1058 and 1059 are in the core of LNADW, and Site 1057 straddles the boundary between those two water masses (Fig. 4). It is expected that during glacial and stadial climate episodes paleoproxy data from Sites 1058 and 1059 will show increased nutrient content (reduced flow of LNADW), whereas shallower sites will show nutrient depletion (increased flow of UNADW).

The Blake Outer Ridge first becomes noticeable as a bulge in the continental slope at Site 1056; therefore, that site and those at greater depths should show progressively less influence of downslope sedimentary processes and greater influence of current-controlled sedimentation. Sites 1058 and 1059 provide a test location for the effects of current control. These two sites are only about 1 km apart, but seismic surveys indicate enhanced sediment accumulation in the upper part of Site 1059 relative to Site 1058. Because they underlie the same surface waters, any sedimentological and mass accumulation rate differences between them must be ascribed to the local effects of DWBC flow. Seismic data also show an approximately 1000-m-thick sediment sequence and a BSR, which is produced at the base of the gas hydrate stability zone.

The objectives of drilling at these sites were to (1) determine the history of circulation change in the western North Atlantic; (2) provide high-resolution sections for paleomagnetic study; (3) provide sections for studying geochemical processes related to gas hydrate formation and dissociation; and (4) monitor the extent of gas hydrate along the Blake-Bahama Outer Ridge.

Site 1056
The sedimentary succession recovered from the four holes at Site 1056 consists of a well dated ~155-m-thick interval of early Pleistocene to Holocene alternating clay and nannofossil ooze with variable amounts of foraminifers and diatoms (Fig. 3). Compared to the shallower water sites, Sites 1054 and 1055, there is much less evidence of downslope transport and current activity. The reworking of both planktonic foraminifers and calcareous nannofossils is surprisingly low. Stratigraphic correlation based on MST records from all holes suggest that we have recovered a complete stratigraphic sequence.

Two lithologic units were recognized (Fig. 3). Unit I (0 to 95 mbsf, Holocene to middle Pleistocene) is characterized by alternating clay and carbonate-rich nannofossil sediments. The unit is defined largely on the basis of relatively high carbonate content (up to 67%) and higher amplitude oscillations in color reflectance than the underlying unit. Distinct layers of red lutite, from 1 to 1.6 m thick, occur at 15, 64, and 90 mbsf in Hole 1056B. Unit II (95 to 155 mbsf; middle Pleistocene to early Pleistocene), is defined by predominant clayey lithologies with decreased carbonate content and color variability with respect to the overlying unit. Unit II has been subdivided into two subunits: Subunit IIA is a 5-m-thick convoluted clayey interval. Subunit IIB is fairly homogeneous, greenish to gray clayey lithologies (clay, silty clay, clayey silt, nannofossil clay, and, more rarely, nannofossil-clay mixed sediments). Nannofossil ooze layers, present in the overlying unit, are missing. Numerous thin silt layers occur at various levels suggesting winnowing by deep-water flow and/or deposition by turbidity currents. A 60-cm-thick biogenic sandy layer with a sharp lower contact occurs at 133 mbsf in Hole 1056B.

The succession contains abundant and well preserved calcareous nannofossils and common to few planktonic and benthic foraminifers with good to moderate preservation. Diatoms are few to common, and often fragmented. Seven calcareous nannofossil and planktonic foraminifer datum levels suggest sedimentation rates of 81-82 m/m.y. for the past 0.5 m.y. and 105-110 m/m.y. in the underlying interval. The oldest sediments recovered date to the early Pleistocene (between 1.25 and 1.58 Ma).

Magnetic susceptibility and magnetic intensities show cyclic variations that can be correlated with marine oxygen isotope stages for the past 600 k.y. (Fig. 7). Magnetic polarity assessment is difficult because of the effects of overprints, reduction diagenesis, and core disturbance due to gas expansion. The onset of the Brunhes Chron (0-0.78 Ma) is possibly located at 91 mbsf in Hole 1056D, with the top of the Jaramillo Subchron (0.99-1.07 Ma) at 111 mbsf.

Calcium carbonate contents fluctuate between 11% and 67%, gradually decreasing with increasing sediment depth. TOC contents vary between 0.1% and 1.14%, increasing downhole. C/N ratios of samples that contain more than 0.5% TOC average 5.5%, 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 1056 are typical of sediments in which sulfate reduction and methanogenesis occur. The sulfate reduction zone extends to only 16 mbsf, and carbonate precipitation probably occurs below that level. A chloride profile shows a trend of increasing concentration with depth. Anomalous excursions toward lower chloride values at discrete depths suggest the occurrence of gas hydrate, which could comprise as much as 5.5% of the sediment volume. The C1/C2 ratios and the absence of major contribution of higher molecular weight hydrocarbons indicate that the source for methane is most likely in situ bacterial methanogenesis resulting from decomposition of organic matter in the sediments.

Site 1056 provides an expanded succession ideally suited for high-resolution paleoceanographic studies on the climatic variability of the past 1.5 m.y. on both Milankovitch and millennial time scales. The alternations of nannofossil ooze and clay intervals are interpreted as switching of the climate system from interglacial to glacial times, respectively.The pattern of alternating lithologies in Unit I is likely to be a function of both changes in surface water productivity patterns and varying quality and quantity of terrigenous sediment input. On longer time scales, the transition from the 41-k.y. to the 100 k.y. climatic variability is obvious in the Site 1056 record, correlating with the transition from lithologic Unit II to Unit I.

Site 1057
The three holes at Site 1057 contain a well dated 136-m-thick interval of early Pleistocene to Holocene alternating nannofossil ooze and clay with variable amounts of foraminifers and diatoms. Compared to Site 1056, Site 1057 contains more silt layers in intervals inferred to be glacial epochs, and those intervals appear to be thicker. In contrast, the shallower site seems to have thicker interglacial episodes. Stratigraphic correlation based on MST records from all holes suggests that we have recovered a complete stratigraphic sequence.

Two lithologic units were recognized (Fig. 3). Unit I (0 to 80 mbsf; Holocene to middle Pleistocene) is characterized by alternating clay-dominated and carbonate-rich nannofossil sediments. The unit is defined largely on the basis of relatively high carbonate content (up to 64%) and higher amplitude oscillations in color reflectance than the underlying units. This unit correlates to Unit I at Site 1056. At Site 1057, however, the darker clay intervals are considerably thicker than the light nannofossil-rich layers, which is the opposite of what was observed at Site 1056, despite the short distance between the two sites. Unit II (78 to 138 mbsf; middle Pleistocene to early Pleistocene) was subdivided into two subunits: Subunit IIA (80 to 89 mbsf in Hole 1057A and from 78 to 85 mbsf in Hole 1057B) is contorted and deformed nannofossil clay and clay with nannofossils, without any lithologic contrast with the overlying and underlying units. Subunit IIB (85-90 to 137 mbsf) contains fairly homogeneous greenish to medium dark gray clayey lithologies (clay, nannofossil clay, clay with silt and nannofossils, and, more rarely, nannofossil-clay mixed sediments). The lack of nannofossil ooze layers, the lower carbonate content, and the low variability in color reflectance are the features that distinguish Unit IIB from Unit I. Silt layers occur at various levels suggesting winnowing by deep-water flow and/or deposition by turbidity currents.

Site 1057 contains abundant and well preserved calcareous nannofossils and common to few planktonic and benthic foraminifers with good to moderate preservation. Diatoms are few and often fragmented. Sedimentation rates were 70 m/m.y. in the past 0.5 m.y., and 125 m/m.y. between 0.5 and 1.25 Ma. Oldest sediments are early Pleistocene (between 0.96 and 1.25 Ma).

The magnetic polarity assessment is difficult because of the effects of overprints, reduction diagenesis, and core disturbance due to gas expansion. The onset of the Brunhes Chron (0-0.78 Ma) is tentatively located at 79 mbsf in Hole 1057A, with the Jaramillo Subchron (0.99-1.07 Ma) between 113 and 122 mbsf. These initial magnetic polarity data agree very well with biochronological results based on calcareous plankton. As at Sites 1055 and 1056, magnetic susceptibility and magnetic intensities show cyclic variations that correlate with marine oxygen isotopic stages (Fig. 6).

Calcium carbonate contents fluctuate between 11% and 64%, gradually decreasing with increasing sediment depth. TOC contents vary between 0.1% and 1.14%, increasing downhole. C/N ratios of samples containing more than 0.5% TOC average 9.0%, 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.

As at Site 1056, pore-water profiles from Site 1057 are typical of sediments in which sulfate reduction and methanogenesis occur. The top of the sulfate reduction zone is close to the seafloor. The onset of the methanogenic zone occurs at 15-20 mbsf, coincident with the level of zero pore water sulfate. The distributions of Ca and Mg concentrations suggest active Mg-enriched carbonate precipitation near the sulfate-methane interface and within the methanogenic zone. Chlorinity shows a trend toward lower values below 83 mbsf and suggests the occurrence of gas hydrate, which could occupy a maximum of 4.2% of the sediment volume. 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.

Site 1057 provides an expanded succession ideally suited for high-resolution paleoceanographic studies of the climatic variability during the past 1.0 m.y. on both Milankovitch and millennial time scales. On longer time scales, the transition from the 41-k.y. to the 100-k.y.climatic variability is obvious in the Site 1057 record, correlating with the transition from lithologic Unit II to Unit I. The alternation of nannofossil ooze and clay intervals in Unit I is interpreted as switching of the climate system from interglacial to glacial times, respectively. The pattern of alternating lithologies is likely to be a function of both changes in surface-water productivity patterns and varying quality and quantity of terrigenous sediment input. No clear evidence is available at this stage for the former process, although the latter process is supported by the changes in sedimentation rates and sediment type between Sites 1056 and 1057. The smaller scale differences between sites during the same climatic interval (i.e., carbonate-rich interglacials or clay-dominated glacials) can be explained by shifts of the Western Boundary Undercurrent Current (WBUC) between different depths in response to climate forcing. For instance, the fluctuating position and/or intensity of UNADW can be envisaged as a likely cause for the differences in sedimentation rates in glacial and interglacial times between Sites 1056 and 1057.

Sites 1058 and 1059
Sites 1058 and 1059 consist predominantly of rapidly accumulated nannofossil ooze, clay with nannofossils, and clays with minor amount of silt. The main component of lithologic variability occurs on decimeter to meter scales throughout the section in the form of cyclic changes in color that are mainly related to relative changes in the proportions of biogenic carbonates, detrital clay, and silt. Stratigraphic correlation based on MST records from all holes suggests that we have recovered a complete stratigraphic sequence at both sites.

Two lithologic units were recognized at the location of Sites 1058 and 1059 (Fig. 3). Unit I (0 to 112 mbsf at Site 1058, and 0 to 98 mbsf total depth [TD] at Site 1059; Holocene to late and middle Pleistocene at Sites 1058 and 1059, respectively) is characterized by cyclically alternating light gray nannofossil ooze and dark greenish gray clay containing reddish-brown layers. The latter are more common in the upper part of the unit. The unit is defined largely on the basis of relatively high carbonate content (up to 53%) and higher-amplitude oscillations in color reflectance than the underlying unit. Bioturbation, pyrite, and pyrite nodules are common in the olive-gray nannofossil clay. Silt-sized biogenic layers are present at the top of light colored nannofossil ooze intervals and have sharp upper contacts at Site 1058. Unit II (recovered only at Site 1058, between 112 and 164 mbsf TD; early Pleistocene) is dominantly clay, clay with silt, and clay with nannofossils. It is characterized by a lower and less variable carbonate content (9%-26%). The red lutite layers are missing and pyrite nodules occur throughout the unit.

These sites contain abundant to common calcareous nannofossils, which are generally well preserved. Reworking of mainly Cretaceous and Paleogene forms is present in the expanded section at Site 1059. Planktonic foraminifers are common to few with good to moderate preservation. Benthic foraminifers are rare to few and are generally moderately well preserved. Diatoms are relatively common and well preserved in the middle part of the succession.

Age control is provided by integrated calcareous planktonic biostratigraphy and magneto stratigraphy. At Site 1058, the oldest sediments extend to 1.24-1.58 Ma. Sedimentation rates average 95 m/m.y. in the past 0.5 m.y. and 110 m/m.y. in the underlying interval. At Site 1059, the oldest sediments date to 0.5 Ma and sedimentation rates average 120-140 m/m.y. over the past 0.5 m.y. Magnetic susceptibility and magnetic intensities show cyclic variations that evidently correlate with marine oxygen isotopic stages at least for the past 0.8 m.y. (Fig. 7). If correct, this correlation indicates that at Site 1059, late Pleistocene sedimentation rates may have been as high as 280 m/m.y.

Magnetic polarity assessment is difficult because of the effects of overprints, reduction diagenesis, and core disturbance due to gas expansion. The onset of the Brunhes Chron (0-0.78 Ma) is tentatively located between 89 and 99 mbsf in Hole 1058A, with the Jaramillo Subchron (0.99 1.07 Ma) between 116 and 126 mbsf. At Site 1059, only normal polarity directions were observed from the entire interval cored, which correlates to the Brunhes Chron. An excellent agreement is observed between the pass-through measurements of the archive halves and the best-fit directions obtained from progressive alternating field (AF) demagnetization of the discrete samples.

Calcium carbonate contents fluctuate between 6.6% and 53% at Site 1058 and between 9% and 49% at Site 1059. TOC contents vary between 0.1% and 1.14% at Site 1058 and between 0.2% and 1.14% at Site 1059. C/N ratios of samples containing more than 0.5% TOC average 6.5% and 7.0% at Site 1058 and 1059, respectively, 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 chemistry shows a very thin sulfate reduction zone at both Site 1058 (at 10 mbsf) and Site 1059 (at 15 mbsf). Carbonate precipitation probably occurs near the sulfate-methane interface and within the methanogenesis zone. At Site 1058, chlorinity gives equivocal indications concerning the presence of gas hydrate. Anomalous excursions towards lower chloride values at 94 and 139 mbsf indicate the possible occurrence of gas hydrate, which could comprise 1.5% of the sediment volume. At Site 1059, chlorinity decreases slightly downcore, indicating the presence of gas hydrate at depth. The cored interval (0-98.8 mbsf) does not show anomalous excursions towards fresher chloride values and should be free of gas hydrate. At both sites the C1/C2 ratios and the absence 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.

Sites 1058 and 1059 provide expanded sections ideally suited for high-resolution paleoceanographic studies on the climatic variability of the past 1.0 m.y. on both Milankovitch and millennial time scale. As at Sites 1056 and 1057, the transition from the 41-k.y. to the 100-k.y.climatic variability is obvious at Sites 1058/1059 and correlates with the transition from lithologic Unit II to Unit I. The alternations of nannofossil ooze and clay intervals are interpreted as switching of the climate system from interglacial to glacial times, respectively. It is noteworthy that at Site 1058, the tops of the carbonate beds are persistently sharp and overlain by coarser sediments, suggesting stronger currents at the end of interglacial periods. This contrasts with Site 1059, where the base of the carbonate unit seems characterized by an abrupt transition and the top is gradual, indicating stronger currents at the beginning of the interglacials.


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