161 Preliminary Report



Site 975


Site 975 is located on the South Balearic Margin between the Balearic Promontory (Menorca and Mallorca islands) and the South Balearic-Algerian Basin. The site was drilled at the edge of a small basin perched on the east-dipping slope of the Menorca Rise, at a water depth of 2415 m (Fig. 9). Upon approaching the site location, navigation, bathymetric, seismic reflection, and 3.5-kHz data were collected to verify the position of the drill site.

The primary objective at Site 975 was to continuously core the Pliocene-Pleistocene sedimentary sequence on the Menorca continental rise to obtain a complete stratigraphic section that would allow us to document the history of surface- and deep-water variations in the western Mediterranean. Site 975 is located in a key position to monitor the history of eastbound inflowing Atlantic waters, and of westbound outflowing Mediterranean waters. The site, which is a central tiepoint along the trans-Mediterranean paleoceanographic drilling transect, will allow us to correlate environmental conditions in the eastern Mediterranean and western Mediterranean.

Triple APC and double XCB coring was chosen to ensure continuous sediment recovery at Site 975. The two deepest holes recovered 317.1 m (Hole 975B) and 313.7 m (Hole 975C).

The stratigraphic sequence at Site 975 ranges from uppermost Pleistocene/Holocene (Subzone NN21b) to uppermost Miocene (Zone NN12). The sediments contain abundant and well-preserved Pleistocene to lower Pliocene foraminiferal assemblages. Upper Miocene samples (G. conomiozea Zone) contain poor to moderately well-preserved foraminifers. The biostratigraphic data indicate that sedimentation rates at Hole 975B were 70.5 m/m.y for the Pleistocene/Holocene, 48.9 m/m.y. for the upper Pliocene, and 53.8 m/m.y. for the lower Pliocene.

Sediments at Site 975 have been divided into three lithostratigraphic units, based on downhole changes in lithology and sedimentary facies.

Unit I (Hole 975A, 0.0-9.5 mbsf; Hole 975B, 0.0-305.2 mbsf; Hole 975C 0.0-306.3 mbsf; Hole 975D, 0.0-0.149.9 mbsf) consists of Pliocene-to-Pleistocene nannofossil or calcareous clay, nannofossil or calcareous silty clay, and nannofossil ooze. The carbonate content of these sediments varies between 30% and 70% (average 47%) and increases slightly with depth. Nannofossils are the major component of the carbonate fraction; locally, foraminifers and micrite may each constitute up to 30% of carbonate sediments. The terrigenous sediment fraction includes clay minerals, quartz, and minor amounts of feldspar and accessory minerals. Color banding and bioturbation are common throughout Unit I, but are especially prominent below 150 mbsf. Graded and/or laminated foraminiferal-rich sandy or silty layers were found throughout the unit.

In Unit I, we encountered 38 sapropels of Pleistocene-to-Pliocene age, containing up to 2.8% TOC. These sapropel layers are well correlated between all holes at Site 975. Organic C/N ratios of the sapropels average 12.7, implying that the sapropels likely contain algal material that has been partially degraded during sinking to the seafloor. Rock-Eval analyses of sapropels suggest that their organic matter consists of a mixture of partially oxidized Type II algal material and Type III land-plant material.

Unit II (Hole 975B, 305.2-307.0 mbsf; Hole 975C, 306.3-306.9 mbsf) consists of Pliocene/Miocene(?) light-colored, carbonate-rich sediments. The major lithologies are finely interlaminated to thinly interbedded gray micrite and greenish-gray, micritic, silty clay. Rare thin beds of graded, calcareous, silty sand contain abundant foraminifers and bioclasts. Mineral assemblages encountered in these silty sands include quartz, feldspar, micritic calcite, clay minerals, and minor amounts of dolomite, glauconite, chlorite, celestite, and gypsum. Sedimentary structures and composition strongly suggest an intertidal origin for sediments of Unit II.

Unit III (Hole 975B, 307.0-317.1 mbsf; Hole 975C, 310.7-313.7 mbsf) is composed of 4.4 m and 1.6 m of an upper Miocene evaporite sequence at Holes 975B and 975C, respectively. The major lithology is light, olive-gray to moderate olive-brown gypsum, which occurs as nodular, finely laminated, and coarse crystals in a micrite matrix. Thin (1- 2 cm) intervals of grayish-green clay to micrite-rich clay are present as a minor lithology. The evaporites of Unit III are composed of two broad cycles, which begin with a clay or micrite-rich clay interval, overlain by thinly bedded, gypsiferous chalk.

The sediments of Unit I are interpreted to have been deposited in an open-marine environment.
The gradual decrease in carbonate content toward the sediment surface may reflect a shift from dominantly hemipelagic to pelagic conditions from the Pliocene to the Pleistocene. The finely laminated beds in the lower intervals of Unit I may reflect periodic fluctuations of terrigenous input. The boundary between Units I and II likely marks the change from a shallow, intertidal environment during the latest Miocene(?) to open-marine conditions during the Pliocene. The wavy laminations of the micritic intervals in Unit II are indicative of algal or microbial mat layering. Calcareous, silty, sand intervals at the base of Unit II may have formed in a beach or channel environment. The evaporite sequence in Unit III is consistent with deposition in a supratidal environment. Unit III can be correlated with the top of the Messinian evaporite sequences (just below the M reflector) that are known elsewhere in the Mediterranean.

Meter-scale slump folds occur at 114.5 and 143 mbsf in Hole 975B. Slumping may have been related to the very gentle east-northeast dip of the entire Neogene and Pleistocene sequence, which is visible in seismic profiles at the site. Steeply dipping, conjugate fractures in laminated, micritic, silty clay and micrite at 305.6 mbsf in Hole 975B appear to have been affected by pressure solution, producing stylolites; vertical stylolites are visible at exactly the same level in Hole 975C. Brittle boudins occur in a sand layer within micrite at 306.4 mbsf. Neither of these features is likely to have any regional tectonic significance. The lack of tectonic deformation and the paucity of slump structures suggest that Site 975 was tectonically inactive during Pliocene- Pleistocene times.

A strong magnetic overprinting made it difficult to determine the primary magnetization at Site 975. The magnetic overprint is characterized by high intensities of natural remanent magnetizations, high coercivity, strong positive inclinations close to the present field (60°), and a PRR. Intervals of shallow and negative inclinations are tentatively correlated among the four holes. A very short interval of negative inclination is found at ~10 mbsf. Negative inclinations are recorded between 25 and 45 mbsf for Holes 975B, 975C, and 975D. A third interval of negative inclination occurs between 50 and 100 mbsf in Holes 975B and 975C, and between 50 to 80 mbsf in Hole 975D. The third interval of negative inclination may be the Brunhes/Matuyama boundary, which, according to the sedimentation rates as defined by biostratigraphic datums, should be at ~60 mbsf.

P-wave velocity increases with depth from 1.5 km/s at the surface to 4.8 km s-1 at 310 mbsf in the evaporitic sequence.

Individual cores were correlated between the four holes using magnetic susceptibility, GRAPE, and 550-nm spectrophotometer data. Excellent results were achieved from the seafloor to approximately 150 mbsf, the limit of APC coring. Above 150 mbsf, a continuous record is readily identifiable using intervals in Holes 975A, 975B, 975C, and 975D. Independently correlated sapropel layers on the depth-shifted core data yielded composite depths of the layers that were generally within 20 cm of each other.

Downhole temperature data measured at five depths in Hole 975C, combined with thermal conductivity data, indicate a heat flow of 81 mW/m^2.

Interstitial water profiles at Site 975 are strongly influenced by the presence of the evaporitic sequence at the base of Site 975. Interstitial calcium and sulfate profiles both exhibit a gentle decrease between the sediment/water interface and 46.55 mbsf, and then increase to near linear gradients toward the base of the hole. Dissolution of gypsum in the Messinian evaporites provides the deep source for the calcium and sulfate and explains why sulfate depletion does not occur at this site, even though bacterial sulfate reduction is the main diagenetic process. Calcium, and particularly strontium, may also originate from interactions of the Messinian brines with biogenic carbonates. The high, interstitial-lithium concentrations suggest the presence of late-stage brines in the evaporites, and the decrease in potassium with depth indicates an absence of potassium-bearing salts. The interstitial-water sulfate gradient suggests that organic-matter degradation below 5.55 mbsf occurs mainly by sulfate reduction. Between the sediment/water interface and 5.55 mbsf, organic-matter degradation is occurring by aerobic degradation and manganese reduction, as evidenced by the manganese-mobilization peak at 5.55 mbsf.

Samples containing at least 1% TOC were selected from Hole 975B for extraction and analysis of C37 alkenone biomarkers and for calculation of sea-surface paleotemperatures. Sea-surface paleotemperatures have fluctuated over a 7°C range in the Balearic Sea during the Pleistocene. A similar record of fluctuating paleotemperatures was established for the eastern Mediterranean during Leg 160. The paleotemperatures calculated for the Balearic Sea are consistently ~5°C cooler than those estimated at times of sapropel deposition in the eastern areas.

Hole 975C was logged between 42.0 and 309.0 mbsf, using the quad-combo, FMS, and GLT tool strings. The resistivity profile is homogeneous throughout the section, with values increasing from 0.7 m at the top of the hole to 1.0 m at the bottom of the hole. Caliper data from the quad-combo and the FMS show erratic variations in hole diameter from 150 mbsf downward, where drilling switched from APC to XCB coring.

Site 975 achieved three significant results. First, by multiple APC and XCB coring at offset holes, a continuous Pliocene-Pleistocene sedimentary sequence was retrieved, which will enable high-resolution documentation of paleoceanographic conditions in the western Mediterranean. Second, 38 sapropel layers were recovered at this site; Site 975 thus extends the geographic limit of documented sapropel occurrence farther west, from the Tyrrhenian Sea to the central western Mediterranean Basin. Discovery of sapropels at Site 975 warrants revision of the paleoceano-graphic concepts that relate the formation of sapropels to paleoceanographic changes in the eastern Mediterranean. Third, a well-preserved sequence of uppermost Messinian evaporites and early Pliocene hemipelagic-to-pelagic sediments was retrieved in two holes at Site 975. The high-quality XCB cores will enable a detailed study of the environmental transition from restricted Messinian conditions to open-marine conditions during the early Pliocene.



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