Date occupied: 24 April 1993
Date departed: 24 April 1993
Time on hole: 9 hr, 45 min
Position: 40°41.160'N, 12°7.380'W
Bottom felt (drill-pipe measurement at rig floor, m): 5289.1
Distance between rig floor and sea level (m): 11.42
Water depth (drill-pipe measurement from sea level, m): 5277.7
Total depth (from rig floor, m): 5294.50
Penetration (m): 5.40
Number of cores (including cores having no recovery): 1
Total length of cored section (m): 5.40
Total core recovered (m): 5.42
Core recovery (%): 100
Oldest sediment cored:
Depth (mbsf): 5.4
Nature: silt
Age: Pleistocene
Measured velocity (km/s): 1.5
Principal results: Site 898 is situated in the Iberia Abyssal Plain over an elliptical basement ridge within the ocean/continent transition (OCT) zone.
Geophysical modeling had predicted that the ridge lay within a part of the OCT, intermediate between thin oceanic crust to the west and thinned continental crust to the east, in which magnetic anomalies cannot be modeled by seafloor spreading. The site was one of a transect of sites across the OCT designed to study the petrological changes in the basement rocks within the OCT to identify the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. APC and XCB cores were obtained from two holes that penetrated 342 m of Pleistocene to late Oligocene sediments. Coring and logging plans were terminated by the loss of about 3340 m of drill pipe. This meant that insufficient drill pipe remained on board the ship for us to core an adequate basement section at this site.
Two lithologic units have been identified at Site 898.
Subunit IIB (172.2-339.7 mbsf) is a middle Miocene to late Oligocene silty claystone to clayey siltstone and nannofossil claystone with claystone. The Subunit mainly consists of calcareous contourites and terrigenous turbidites.
The sedimentary section provides a discontinuous fossil record from the Pleistocene through the late Oligocene. Calcareous nannofossils are generally present. Planktonic and benthic foraminifers are abundant to common in the Pleistocene and late Pliocene deposits, but are less common below. An unconformity from middle Miocene to late Pliocene represents a 10-m.y. hiatus.
Acoustic formations 1A and 1B are widely recognized on multichannel seismic reflection profiles on the western Iberia margin and have been locally dated by previous drilling. At Site 898, acoustic formation boundary 1A/1B correlates with the middle Miocene to late Pliocene hiatus at 162 mbsf and with the top of Subunit IIA. Seismic profiles around the Leg 149 drill sites show a layer of westward/southwestward inclined reflectors within acoustic formation 1B. The shape and extent of this layer suggest that it is part of a contourite drift, the top of which may have been cored near the base of Hole 898A.
Many potential magnetic reversals were observed in the almost complete sequence of APC/XCB cores from 0 to 177 mbsf. At greater depths, the cores were too weakly magnetized. The Brunhes to Gauss sequence of chrons has been tentatively identified in the APC cores (0-133 mbsf). At the centimeterscale, peaks in magnetic susceptibility values are associated with the terrigenous sandy layers in the turbidites of Unit I.
In-situ temperature measurements indicate that the vertical temperature gradient between the seafloor and 176 mbsf is about 41 °C/km and that the vertical conductive heat flow is about 61 mW/m2. Physical property measurements on the sediments exhibit a small but steady increase in density, seismic velocity, formation factor, and thermal conductivity, and a concomitant decrease in porosity, with depth.
Interstitial-water samples were obtained from Units I and II (3-336 mbsf). The pore-water chemistry (sulfate, alkalinity, ammonia, iron, manganese, calcium, magnesium, strontium, potassium, silica, chloride, and sodium) reflects the rapid deposition of the Pliocene-Pleistocene turbidites and the relatively slower deposition of the earlier sediments. The principal result is the surprisingly high sulfate concentration (up to 16 mM) in Subunit IIB. This has been attributed to a low sedimentation rate in the Miocene and Oligocene, low permeability, and the absence of reactive carbon. Concentrations of sulfate decrease rapidly from 3 to 71 mbsf as sulfate is consumed by reaction with organic carbon in Unit I.
Profiles of carbonate content vs. depth reflect a history of generally low biological productivity and deposition of hemipelagic sediment below the carbonate compensation depth (CCD), combined with delivery by turbidites of carbonate-rich material initially deposited above the CCD. Elevated organic carbon is found in Unit I (mean value 0.5%), whereas Unit II has much lower organic carbon. Relatively high concentrations of biogenic methane were encountered in headspace gas analyses of lithostratigraphic Unit I and Subunit IIA, but methane was essentially absent from all deeper sections.