161 Preliminary Report
Site 974 is located in the central Tyrrhenian Sea on the lowermost eastern continental margin of Sardinia. The site lies in a north-south trending, small, deep basin, between the Tyrrhenian Central Fault and the De Marchi Seamount. The basin is underlain by thinned continental crust, which surrounds areas of Pliocene and Pleistocene oceanic crust (the Vavilov and Marsili basins) to the south and southeast. Site 974 lies about 300 m west-northwest of Site 652, which was drilled during ODP Leg 107 in 1986 (Fig. 8).
The objectives at Site 974 were to obtain a complete Pleistocene-through-Pliocene sedimentary sequence that would contain a continuous record of organic-rich depositional events (sapropel intervals) in the Tyrrhenian Sea, and to retrieve a comprehensive record of the coeval volcaniclastic deposits within this sequence. At Site 974, we recovered 198 m (Hole 974B) and 202.2 m (Hole 974C) of Pleistocene-Pliocene sediments. At the base of the sedimentary sequence in Hole 974B (203.7 mbsf) and Hole 974C (204.5 mbsf), sediments of latest Miocene age were reached. Recovery of a continuous sedimentary sequence was achieved at Site 974 by triple APC coring to 165 mbsf, and by double XCB coring to total depth. This drilling strategy proved successful in ensuring complete stratigraphic coverage.
Cores recovered at Site 974 contain abundant, mostly well-preserved Pleistocene to earliest Pliocene/latest Miocene calcareous nannofossil and foraminiferal assemblages. Abundance and preservation of nannofossils and foraminifers are poor in sediments at the base of Holes 974B and 974C; this may reflect a transition from nonmarine to marine environment during the latest Miocene (Messinian). Benthic foraminifers were rare to absent. The biostratigraphic data indicate that sedimentation rates at this site steadily increased with age, from 30.7 m/m.y. during the early Pliocene to over 34.1 m/m.y. during the late Pliocene to 45.4 m/m.y. during the Pleistocene/ Holocene.
Sediments at Site 974 are subdivided into four lithostratigraphic units, based primarily on nannofossil (carbonate) content. Unit boundaries were clearly correlated among Holes 974A, 974B, 974C, and 974D.
Unit I (Hole 974B, 0-88.9 mbsf; Hole 974C, 0-90.1 mbsf; Hole 974D, 0-89.8 mbsf) consists of Pliocene-to-Pleistocene nannofossil-rich clay to nannofossil-rich silty clay. Carbonate contents within this unit average 29% by weight. The sediments are locally bioturbated and exhibit thin to medium color banding.
A total of 36 sapropels, ranging in thickness from 2 to 20 cm, were identified within Unit I. Total organic carbon (TOC) contents of the sapropels reach up to 6.4%. Some of these layers are thin beds (1-2 cm) with less than 2% TOC. Organic matter atomic C/N ratios and Rock-Eval analyses indicate that the sapropels mostly consist of partially degraded (oxidized) Type II algal and microbial material. At the base of Unit I, some sapropels have been faulted and deformed; reverse faulting has repeated some of these intervals in Hole 974D, whereas normal faulting may have removed some others in Holes 974B and 974C.
Numerous crystal-rich and vitric ash layers are present in Unit I. Ash and volcaniclastic layers range from a few millimeters to about 12 cm in thickness. Several ash layers are normally graded and faintly laminated, showing medium to high bioturbation. Rounded pumice grains are present in a few of these layers. From shipboard observations, it is difficult to determine whether all volcaniclastic intervals are primary, or if they consist of reworked material. Ash beds are locally altered to zeolite (phillipsite and analcime?) and clay minerals. Cemented, ash-rich, zeolitic concretions were found at the top of several ash layers. The boundary between Units I and II is located in a zone where slumping intervals alternate with undisturbed sediments.
Unit II (Hole 974B, 88.9-199.32 mbsf; Hole 974C, 90.1-200.14 mbsf; Hole 974D, 89.8- 163 mbsf) consists of Pliocene nannofossil clay and nannofossil ooze, with minor amounts (1%-5%) of foraminifers. Ash beds are few and are locally altered to clay minerals and zeolites. Carbonate contents within the nannofossil clay and nannofossil ooze intervals average 50%. A few sapropels are present in the lower part of Unit II.
The sediments of Units I and II correspond to deposits that accumulated in an open-marine environment with periodic influx of pyroclastic material. The shift from more pelagic deposits in Unit II to more hemipelagic sediments in Unit I may reflect greater terrigenous input during the Pleistocene.
Unit III (Hole 974B, 199.32-200.47 mbsf; Hole 974C, 200.14-200.97 mbsf) is a very thin, variegated, brownish-gray to blackish-red, transitional unit that separates Pliocene marine sediments from the Messinian sequence. The unit is characterized by horizontally laminated to cross-laminated(?) silt to silty clay with local clay interbeds. The Unit II/Unit III boundary was placed at the Miocene/Pliocene age boundary, based on the last occurrence of in-situ planktonic foraminifers.
Unit IV (Hole 974B, 200.47-203.86 mbsf; Hole 974C, 200.97-202.48 mbsf) consists of a single graded(?), gray, siliciclastic interval, comprising a basal coarse-to-medium sand that grades upward into cross-laminated, very fine sand to cross- or parallel-laminated silt. The sandy and silty fraction is formed of carbonate and siliciclastic grains and possibly includes gypsum.
Structural features of primary significance include changes in bedding dips and the presence of microfaults and slumps. Dips of up to ~15° in coherent sections are interpreted to reflect tectonic tilting of the sedimentary sequence. Microfaults with normal-sense displacements of 1 cm, or less, are abundant from about 30 mbsf downward, with dips mainly in the range of 45°-60°. Between 90 and 110 mbsf, intervals of slump folding, on scales from 5 cm to 2 m, and disrupted and contorted bedding, including slump-related reverse-faults, have been identified in all holes. Slumping and tectonic deformation at this site suggest that the area was tectonically active during the Pliocene and Pleistocene.
Natural remanent magnetization (NRM) was strong in all cores (about 100 mA/m), i.e., far above the noise level of the cryogenic magnetometer. After 25 mT alternating field (AF) demagnetization, the values decreased to about 10 mA/m. Inclinations throughout the cores are close to present-day values at the site (+60°). The consistently positive inclinations appear to have been caused by drill string overprinting, which makes magnetostratigraphy difficult at this site. Declination was obtained by reorienting the cores, using Tensor tool measurements. The large scatter observed for declination is likely the result of spurious secondary magnetization. The apparent spurious declination is not removed after 25 mT demagnetization. Additional measurements on working and archive halves of some cores, as well as on discrete samples, confirmed that the bulk magnetization is perpendicular to the split section in the core. Similar observations during Leg 154 were used to infer "pervasive radial remagnetization" (PRR), possibly induced by the cutting shoe, but the processes which produce PRR remain unclear.
Physical property measurements (thermal conductivity, index properties, and natural gamma radiation) all showed good hole-to-hole correlation, with due allowance for variation on short vertical scales and variances in sampling locations. High-resolution (2-10-cm-scale) profiles of GRAPE density, magnetic susceptibility, and color reflectance were used to construct a composite depth section from Holes 974B, 974C, and 974D. Distinctive features in each of the cores were aligned, using a common depth scale. Shifted positions of the cores were used to splice together a composite section. This composite section provides the most complete and least disturbed stratigraphic record at this site.
Downhole temperature measurements were made with the ADARA temperature tool at five depths in Hole 974B. The temperature data were reduced to in-situ values and combined with the thermal conductivity measurements to determine heat flow. The best-fitting linear regression to the data has a slope of 157 mW/m^2 (n=5, R=0.998), which is virtually identical with the 160 mW/m^2 value obtained at Site 652 during ODP Leg 107, also at this location. These values fit well with a young age for the Tyrrhenian Basin.
Interstitial water profiles at Site 974 show steady increases with depth in salinity, chlorinity, calcium, strontium, ammonium, and lithium values. Evaporites, especially halite, anhydrite, and gypsum, known to occur below the sediments cored at Site 974, can provide the source for these variables. Potassium and, to a certain extent, pH and alkalinity steadily decrease with depth. The downhole increase in calcium and strontium is most likely the result of an interaction of Messinian brines with background carbonates. High lithium concentrations suggest the presence of late-stage evaporitic brines. Ammonium concentrations are generally low, reflecting limited decomposition of organic matter. The sulfate concentration indicates that organic matter degradation at this site occurs mainly by sulfate reduction.
Logging in Hole 974C included the quad-combo, formation microscanner (FMS), and geochemical tool (GLT). Two main log units were identified in close correspondence with lithostratigraphic Units I and II. High log variability in Log Unit I (36.0-90.0 mbsf; Pleistocene-Pliocene hemi-pelagic sediments) indicates relatively heterogeneous lithologies. High variations in thorium content in this unit appear to be related to ash layers. Uranium content is likely related to the ORL's. Log Unit II (90.0-186.0 mbsf; Pliocene pelagic to hemipelagic sediments) shows a homogeneous response of all three tools.
Site 974 successfully achieved its goals in that we obtained a continuous sequence of Pliocene-Pleistocene sediments that contain a striking record of sapropel deposition in the Tyrrhenian Sea. Within this sequence, substantial volcaniclastic deposits have been found. Further studies, which will combine biostratigraphy, tephrochronology, geochemistry, and isotope stratigraphy at Site 974, will focus on developing a latest Cenozoic-to-Holocene paleoceanographic model at this site. This will allow us to constrain the timing and origin of sapropels in the Tyrrhenian Sea, and to compare the pattern of sapropel occurrence in the western Mediterranean with that in the eastern Mediterranean.
To Site 975
161 Table of Contents