Leg 172 Scientific Report

RESULTS (continued)

Deep Water Sites on the Blake-Bahama Outer Ridge

The three deepest sites on the Blake-Bahama Outer Ridge depth transect (Sites 1060-1062; Fig. 2) were occupied to recover a continuous sequence of sediments from near the interface (approximately 4000-m water depth) between present-day NADW and AABW (Fig. 4). Sites 1060 (3481 m) and 1061 (4037 m) are on the crest of the Blake Outer Ridge (Site 1060 is near the position of Deep Sea Drilling Project [DSDP] Site 102). Both sites were cored with the goal of obtaining a history of circulation changes near the boundary between AABW and NADW.

Site 1062 (4763 m) is located in a 37-m-high mud wave situated about 400 m downslope and west of the crest of the Bahama Outer Ridge. This location is near the position of Core KN31 GPC-9, which has provided an important climate and deep-circulation record of the late Pleistocene (Keigwin et al., 1994). Presently, Site 1062 is under the influence of the WBUC, which is composed of 15% AABW at this depth. Waters of the WBUC follow a convoluted path along the topographic contours of the eastern and western flanks of the Blake-Bahama Outer Ridge and flow northwards at Site 1062. This deep current activity has generated a mud wave field from 26°N to 30°N in the area surrounding the location of Site 1062. Previous studies and the site survey data indicate lateral and vertical stratigraphic variability in the architecture of the mud wave, resulting from a higher depositional rate in the east flank than in the west flank and from migration of the mud wave with time. This variability may be a response to changing climate and ocean circulation patterns on both orbital and millennial time scales. A transect of eight holes drilled across the mud wave at Site 1062 documents changes in paleocirculation.

Other objectives at these sites were to (1) provide high-resolution sections for paleomagnetic study; and (2) monitor the extent of gas hydrate along the Blake-Bahama Outer Ridge.

Sites 1060, 1061, and 1062 have similar lithologies, which we grouped into two units (Fig. 3). Unit I is defined by its cyclically alternating light gray nannofossil ooze and dark greenish gray clay containing reddish brown layers. Because of shallow penetration, this is the only unit recovered at Site 1060. Unit II is recognized by its reduced abundance of pelagic carbonates and dominance of clays and silts. At Site 1061, Unit II appears more homogeneous than Unit I, whereas on the mud wave (Site 1062) the lithology is more heterogeneous because of the presence of shallow-water carbonate turbidites. Because the mud-wave field is in the Bahama Basin, a depression adjacent to the Bahama Banks and Blake Escarpment, it is subject to more turbidite deposition than Sites 1060 and 1061. Unit III at these sites is composed of clayey mixed sediments.

All three sites contain excellent paleomagnetic records, especially for the Brunhes Chron. There are no other long, well-dated paleomagnetic records of directional field variability available anywhere in the world for the time interval from 0.2 to 0.8 m.y. Preliminary analysis of the paleomagnetic secular variation at each hole of Sites 1060, 1061, and 1062 indicates convincingly that eight excursions may be present in the Brunhes Chron. These sites provide the material necessary for detailed paleoclimatical and sedimentological studies.

Site 1060
This site contains various combinations of Holocene and Pleistocene nannofossil ooze, clay and silt, with clay and clay with nannofossils being dominant. Bedding generally ranges from 0.2 to 2 m in thickness. Most lithological changes are gradual, although pronounced transitions can take place within a decimeter. One characteristic of this site is the degree of variability in the lithology, which is evident in the records of color reflectance and carbonate content of the sediment. Sediment color usually ranges from dark greenish gray in the clay, to light greenish gray in the sediments with more biogenic carbonate. In some intervals the clay and clay with nannofossil lithologies have a brownish gray to pale brown color or a reddish tinge. Calcite dominates the carbonate mineralogy and appears primarily in the form of calcareous nannofossils which can make up a significant proportion of the clay-size fraction in carbonate-rich sediments. Carbonate values range from 7%-44%.

Three calcareous nannofossil and planktonic foraminifer events suggest sedimentation rates of 240 m/m.y. The oldest sediments recovered are middle to early Pleistocene (between 0.46 and 0.96 Ma) in age.

Magnetic susceptibility and magnetic intensities show cyclic variations that appear to correlate with carbonate fluctuations and marine oxygen isotope stages (Fig. 7). Paleomagnetic studies indicate that the site is almost all Brunhes normal polarity with the Brunhes/Matuyama reversal occurring near the base of Hole 1060A. Reproducible records of magnetic field secular variation during the Brunhes Chron are present in Site 1060 cores.

Pore-water chemistry shows a very thin (13 m) sulfate reduction zone. Both calcium and magnesium decrease sharply with depth in the zone of sulfate reduction. In the zone of methanogenesis, alkalinity and magnesium decrease downhole, whereas calcium and strontium increase, suggesting dolomitization of carbonates. Methane is biogenic, based on methane:ethane ratios. No gas hydrate was directly observed, but chloride profiles freshen slightly with depth indicating that gas hydrate may underlie Site 1060.

Site 1061
At this site, the interval from 0 to 183 mbsf was cored several times and appears to provided a complete composite sequence down to the latest early Pleistocene. XCB coring at Hole 1061A from 152 to 350 mbsf recovered a sedimentary interval that spans 0.8 to 3.2 Ma. In addition to high-resolution Pleistocene objectives, Site 1061 was intended to provide a high-resolution record of climate and ocean changes associated with the onset of Northern Hemisphere glaciation (2.8 Ma) for comparison to other Atlantic records from ODP Legs 154 and 162. Although we XCB cored through the interval, sediment biscuiting, incomplete core recovery, and low sedimentation rates in the Pliocene lowered the value of Site 1061 as a middle Pliocene reference section.

Sediments at this location are predominantly composed of carbonate-rich and carbonate-poor units that alternate on a scale ranging from ~20 cm to ~10 m. The sediments range from nannofossil ooze in the most calcareous units (carbonate content of 20%-55%) to clay in the least calcareous units (carbonate content of 2%-15%). A few thin laminations were noted, presumably deposited by currents. Colors range from dark greenish gray to shades of orange and reddish brown. Three sedimentary units were distinguished on the basis of the bed thicknesses, carbonate content, and color. Additional information that helped to distinguish sedimentary units came from downhole logs.

Unit I (0-170 mbsf; Holocene to middle Pleistocene) is defined by cyclically alternating carbonate rich (20%-55% CaCO3) and carbonate-poor intervals (4%-15% CaCO3). Sediments in carbonate-rich intervals range from nannofossil ooze to nanofossil clay. In carbonate-poor intervals, sediments are clay, clay with silt, or clay with nannofossils. Colors range from light greenish gray to dark greenish gray alternating with reddish brown and reddish green intervals. This unit is well resolved on the downhole logs, with clear cyclic variations in gamma ray, bulk density, velocity, and resistivity. Unit II (170 to 260 mbsf; early Pleistocene) consists of alternating carbonate-rich and carbonate-poor intervals and is defined, in part, by the diminished carbonate contents (range 5%-20%), which are distinctly lower than in Unit I. Unit II sediments are nannofossil clay and clay with silt. Colors show limited variations from light to dark greenish gray. On downhole logs the unit has more constant gamma-ray counts and lower bulk density, resistivity, and uranium contents with respect to Unit I. A 40-cm-thick, multicolored lithified interval occurs at the top of Core 172-1061A-23X (215 mbsf), and marks the top of a gradual downhole increase in carbonate from 10% to 20%. Unit III (260-360 mbsf; early Pleistocene to middle Pliocene) is defined by alternating intervals of higher and lower carbonate with a somewhat higher carbonate content (2%-35%) than Unit II. The sediments are predominantly composed of nannofossil clay, clay with nannofossil, clay with silt, and clay. Colors include greenish gray (light and dark) and reddish brown, and there are numerous purple and green (diagenetic?) layers. Cores 1061A35X and 36X have distinctly less pyrite and perhaps lower carbonate (2%-20%) than the above sediments, and may represent a distinct subunit. Logs of this unit show increasing gamma-ray counts with an additional increase below ~315 mbsf and higher resistivity that decreases abruptly below 325 mbsf.

Eighteen calcareous planktonic biohorizons suggest that sedimentation rates were 227-228 m/m.y. during the past 0.5 m.y., 144 m/m.y. between 0.5 and 1.6 Ma, 87 m/m.y. between 1.6 and 2.0 Ma, 52 m/m.y. between 2.0 and 2.5 Ma, and 31 m/m.y. between 2.5 Ma and the bottom of the succession at 3.2-3.3 Ma.

Magnetic susceptibility and magnetic intensities show cyclic variations that evidently correlate with carbonate percentage fluctuations and marine oxygen isotopic stages (Fig. 7). Site 1061 has mixed magnetic polarity, with Hole 1061A ending in the Gauss normal chron. The Brunhes/Matuyama reversal occurs at 149 mbsf in Holes 1061A, C, and D, and appears to be a promising candidate for a high-resolution record of a polarity transition. Several excursions within the Brunhes Chron are reproducible among the holes.

As in previous Blake Ridge sites, pore-water chemistry shows a very thin sulfate reduction zone of about 13 m. Calcium and magnesium distribution indicate carbonate precipitation near the sulfate methane interface and within the methanogenic zone. Methane is biogenic based on methane:ethane ratios. No gas hydrate was directly observed, but chloride profiles freshen with depth, indicating gas hydrate may underlie Site 1061.

Downhole measurements were made from 350 to 77 mbsf in Hole 1061A, using two tool strings: the Triple-combo (resistivity, density, porosity, and natural gamma ray) and Formation MicroScanner (FMS)-Sonic (resistivity imager and sonic velocity) tools. The logs are generally of good quality. The borehole diameter was mostly around 12 to 13 in, with numerous thin wash outs up to 16 in. Resistivity, total natural gamma, and bulk density all show marked cyclicity down to 160 mbsf, i.e., during the past 0.8 m.y. This cyclicity is not evident between 160 and 260 mbsf and variations are more modest. Below 260 mbsf (2.5 Ma), bulk density, resistivity, and natural gamma all step up to higher values. The FMS resistivity images show a curious decimeter-scale banding in this lower section.

Site 1062
At Site 1062, we drilled eight holes on either flank and in the crest of the mud wave (Fig. 6). Holes 1062A, B, C, and D are on the east side of the mud wave, Holes 1062E and F are on the west side, and Holes 1062G and H are near the crest. Hole 1062B was cored by XCB from 2.6 to 4.0 Ma, and Hole 1062E was cored by XCB from 2.7 to 3.0 Ma. The holes located on the far west and far east sides of the mud wave are 1050 m apart. A total of 1000 m of sediments was cored with recovery in excess of 100%.

Three lithologic units were recognized (Fig. 3). Unit I (0 to 79 mbsf on the east flank, and 0 to 61 mbsf on the west flank; Holocene to middle Pleistocene) is composed of cyclically alternating dark gray to reddish brown clay layers (typically 5- to 10-m thick) and light olive gray nannofossil-clay mixed sediment layers (typically only 1.5-m thick). The transitions from carbonate-rich layers to clay layers are abrupt, whereas, those from clay-rich to carbonate-rich sediment are gradual. An entire sedimentary cycle typically occupies 10 to 20 m, and more than five of these cycles are contained in Unit I. Four distinctly reddish layers are also present that can be correlated to the red lutite layers observed at other shallower Leg 172 sites. Unit II (77.0 to 167 mbsf on the east flank and 61 to 117 mbsf on the west flank; middle Pleistocene to late Pliocene) is predominantly composed of interbedded clay and clay with nannofossils. Contacts are gradual and moderately to heavily bioturbated. The average thicknesses of clay layers and nannofossil-rich layers are 2 m and 1 m, respectively. Unit II also contains well-sorted foraminiferal or carbonate silt and sand layers, generally 7- to 12-cm thick, which are interpreted as carbonate turbidites. The source area of the turbidites was most probably the Bahama Banks. Unit III (167 to 239 mbsf on the east flank and 117 to 209 mbsf on the west flank; middle to early Pliocene) is dominantly dark greenish gray clay with nannofossils interbedded with clay, clay with silt, and nannofossil clay. Thin carbonate-rich layers are present. There is an increase of silt-sized lithologies with respect to the overlying units.

Calcareous nannofossils are the dominant microfossils at Site 1062. However, their abundance and preservation vary with depth and between holes. Samples with abundant and well-preserved nannoflora assemblages alternate with samples with a few partially dissolved specimens and those barren of nannofossils. Reworking is often strong, which makes recognizing the horizons around the Pliocene/Pleistocene boundary difficult at Site 1062. At Hole 1062E, the interval between 100 and 120 mbsf is apparently disturbed, as all the biostratigraphic markers appear displaced uphole relative to the ages indicated by magnetostratigraphy. Even though planktonic foraminifer abundance and preservation is highly variable, it is better than at Site 1061 which is 725 m shallower. Benthic foraminifers are rare to very rare, and moderately to poorly preserved.

Magnetic susceptibility and magnetic intensities show cyclic variations that correlate with oxygen isotope Stages 1 to 22 in all holes at Site 1062 (Fig. 7). Magnetic polarity stratigraphy is well defined down to the base of the Jaramillo Subchron. In the underlying interval, the presence of the turbidite layers makes the paleomagnetic record more difficult to interpret. The base of the Brunhes Chron is at 92-95 mbsf at Holes 1062A, B, and C on the eastern flank, whereas at Hole 1062E on the western flank, it is at 71.5 mbsf. The Jaramillo Chron is between 108.4 and 112.4 mbsf at Hole 1062B (eastern flank of the mud wave), but it is between 79 and 86 mbsf in Hole 1062E (western flank). Paleomagnetic directional changes, which likely reflect paleomagnetic secular variation, are observed within the Brunhes Chron on a variety of length scales from 10 cm to 10 m in all holes. When checked carefully in selected intervals, these correlate between holes as well as with directional changes observed at Sites 1060 and 1061.

Age control provided by calcareous planktonic biostratigraphy and magnetostratigraphy indicates that the sedimentation rates at Site 1062 were generally very high and that the successions recovered are well suited for paleoceanographic reconstruction at the orbital and millennial time scales. In addition, it indicates, in accordance with the evidence of the seismic images of the investigated mud wave, varying sedimentation rates in the east and west flanks. Specifically, in the east flank (Holes 1062A and B) sedimentation rates were 110 m/m.y. during the past 1 m.y., 32 m/m.y. between 1.0 and 2.5 Ma, and 73 m/m.y. between 2.5 and 2.86 Ma. In the west flank (Hole 1062 E), sedimentation rates were 82 m/m.y. during the past 1 m.y., 27 m/m.y. between 1.0 and 2.5 Ma, and 109 m/m.y. between 2.5 and 2.86 Ma

The interface between the sulfate and methanogenic zone is at 70 mbsf, deeper by 50 m than at the Blake Outer Ridge sites. The sulfate profile displays a distinct concave down profile suggesting that sulfate is predominantly consumed by microbially-mediated reactions with sedimentary organic matter. This, however, contrasts with the high methane levels in the lower part of the sulfate reduction zone and with the presence of a BSR in the area. Although gas hydrate was not directly observed, the chlorinity decreases with depth, indicating its presence above the BSR.

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