161 Preliminary Report

Site 977

Site 977 is located south of Cabo de Gata in the eastern Alboran Basin, halfway between the Spanish and Algerian coasts, at a water depth of 1984 m. The site is situated south of the Al-Mansour Seamount in a 36 km-wide graben, which is bounded by the Yusuf Ridge to the south and the Maimonides Ridge to the north (Fig. 12). Seismic records at Site 977 show a lower sequence of tilted and hummocky reflectors with several internal unconformities. This sequence, which likely represents the earliest syn-rift sediments that filled the graben, is overlain by uniform, mostly horizontally layered, Pliocene-Pleistocene sediments. Our main objective at Site 977 was to penetrate ~650 m to sample the unconformity between these two seismic units; this unconformity was suspected to represent the seismic M-reflector.

Paleoceanographic objectives included documentation of Atlantic-Mediterranean water exchange from the Miocene to the Pleistocene and of productivity patterns in the eastern Alboran Sea. Productivity fronts develop today along the path of Atlantic water inflow and have possibly changed their position and intensity during the past as the pattern of water circulation changed.

Site 977 penetrated 598.5 m of Miocene(?)/Pliocene-to-Holocene sediments. The Pliocene/ Pleistocene boundary is at 266.95 mbsf. The lowermost section of Site 977 not only contains Pliocene planktonic foraminifer marker species, but also contains uppermost Miocene nannofossil marker species; thus, the biostratigraphic data are not conclusive as to whether the Miocene/ Pliocene boundary was reached at this site. A hiatus occurred in the lower Pliocene (Zones NN13, MPL3-MPL2) between 490.59 and 490.63 mbsf. Benthic foraminifers make up <1%-2% of the total foraminiferal assemblage. The presence of frequently abraded, shelf taxa suggests contamination through rare, downslope sediment transport. Average sedimentation rates at Site 977 are 148 m/m.y for the Pleistocene-Holocene, 98 m/m.y. for the upper to uppermost lower Pliocene, and 91 m/m.y. for the lower Pliocene.

The sedimentary sequence recovered at Site 977 was subdivided into two lithostratigraphic units, based on downhole changes in sedimentary structure and grain size.

Unit I (0-532.9 mbsf) contains Pliocene-Pleistocene sediments of an open-marine, hemipelagic facies. The sediments, which consist predominantly of nannofossil clay to nannofossil-rich, silty clay, are slightly to moderately bioturbated throughout the unit (Chondrites, Planolites, Zoophycos). Carbonate content ranges between 21% and 61%. The carbonate fraction consists of nannofossils (70%), micrite (19%), bioclasts (6%), and foraminifers (5%). Minor lithologies include diatom and nannofossil oozes and nannofossil-rich, diatomaceous, sandy, silty clay. Sand- and silt-rich layers are intercalated with the clay-rich sediments. Sharp basal contacts are common. Intervals of slumping are found throughout the unit.

Thirty-nine ORL's occur in Unit I, some of which resemble sapropels recovered at previous Sites 974 and 975. The ORL's are greenish in color and exhibit low magnetic susceptibilities. Organic carbon contents of the ORL's vary between 0.8% and 1.5%; carbon concentrations >2% were measured for some discrete ORL's (sapropels?). The appearance of ORL's at Site 977 is important, as it suggests that organic-rich sedimentation occurred basinwide in the western Mediterranean; organic carbon concentrations in some ORL's reach levels that are similar to those found in the eastern Mediterranean sapropels. Smear slide analysis shows that ORL's are enriched in opaque minerals and, locally, micrite. Fe-bearing, Mn-rich dolomite (kutnohorite, up to 30%) or siderite occurs at the top of many ORL's.

Unit I has been further subdivided into three subunits, based on changes in sedimentary structure.

Subunit Ia (0.0-417.4 mbsf). Visible structure in Subunit Ia sediments is minimal. Five minor slump intervals occur in this subunit with two associated intraclastic breccias. The breccias show inverse to normal grading with angular to subrounded clasts ranging to pebble size. Color banding and mottling are developed in sediments above, and below, the slumped intervals.

Subunit Ib (417.4-490.6 mbsf). This subunit contains parallel and cross-laminated to bedded clays with numerous slumps and intraclastic breccias. The subunit is Pliocene in age and represents an open-marine, hemipelagic facies. Zones of intense burrowing can be found throughout the unit, predominantly of Zoophycos and Planolites. Slumps are recognized from folding of fine laminations. The base of this subunit is just below a hiatus, spanning at least lower Pliocene Zone NN13.

Subunit Ic (490.6-532.9 mbsf). This subunit contains few sedimentary structures; laminations are rare, and soft sediment deformation has not been recognized. Bioturbation (Zoophycos, Chondrites, Planolites) ranges from slight to intense.

Unit II (532.9-598.5 mbsf) is composed of partly cemented, sandy gravel, which is early Pliocene to Miocene(?) in age. Recovery was less than 1% with only 2 cm of granule-rich sand and 40 cm of gravel being recovered from a cored interval of 57.8 m. The gravel consists predominantly of volcanic clasts (rhyodacite and rare shoshonitic to calc-alkaline basalt/andesite) and few sedimentary clasts (dolomitic mudstone and quartz sandstone, cemented by quartz and chlorite). These clasts are coated by a calcareous cement, suggesting that the gravel has been derived from the partly cemented sandy gravel.

Log data at Site 977 were acquired using the quad-combo and FMS tools. From 140 to 323 mbsf, the borehole was out of caliper (washed out). The FMS was run below this interval and obtained excellent images in the lower part of the hole, including the transition zone between clays and sandy conglomerates. After post-cruise processing, the logs will provide information about the lithology and structure of the gravel/sand interval below 531 mbsf in which core recovery was extremely low.

Sediments at Site 977 average 0.5% TOC and may reach 2.5% in ORL's. Organic C/N ratios of
28 ORL's, containing a minimum of 1% TOC, average 10.1, which is a value intermediate between unaltered algal organic matter and fresh land-plant material. These organic carbon-rich sediments likely contain partially degraded algal material with some admixed continental organic matter. Rock-Eval and elemental-source characterization suggests that marine organic matter in ORL's has been heavily oxidized. Rock-Eval TMax values indicate that organic matter in the upper section is thermally immature with respect to petroleum generation, whereas it is overmature in the deeper sediments; from this, we infer that heat flow at Site 977 was higher in the past.

Concentrations of headspace methane are high at Site 977. The source of the methane is probably in-situ microbial fermentation of marine organic matter. Concentrations of propane, iso-butane, and iso-pentane exceed those of ethane in sediments from about 200 to 450 mbsf. C3, C4, and C5 gases were likely produced by thermal degradation of sedimentary organic matter during some former period of elevated heat flow at this site.

Interstitial-water profiles at Site 977 are dominated by organic-matter degradation and carbonate diagenesis. Rapid sediment-accumulation rates at this site ensured that organic matter was buried more rapidly than dissolved oxygen could diffuse from the overlying seawater. Calcium and magnesium decrease in a zone of increased alkalinity, suggesting that dolomite or high-magnesium calcite has precipitated from solution. Silica increases markedly in the upper few meters, then gradually decreases with depth, except for a peak at 103.45 mbsf, which likely comes from dissolution of a diatomaceous ooze at 117.50 mbsf. Pore-water salinity remains below seawater concentrations until 362.30 mbsf, then increases to 46 g/kg at 506.50 mbsf. Lithium concentrations increase from surface concentrations to 202 ÁM at 477.70 mbsf; this may reflect a lateral migration of evaporitic brines from another section of the basin.

Downhole temperature and shipboard thermal conductivity measurements give a heat flow of
102 mW/m^2. This value is comparable to the range of 97-112 mW/m^2 in the region around Site 977.

Magnetic inclinations are mostly positive at Site 977. Some negative inclinations are recorded, but declinations remain constant in these sections compared to those with positive inclination, suggesting that a strong overprint obscures the magnetic signal at this site. Therefore, it was not possible to obtain a magnetostratigraphy at Site 977.

Changes in susceptibility in the upper 60 mbsf are possibly due to higher concentrations of terrigenous components. Thermal conductivity is between 1.0 and 1.5 W/m K. Bulk density, porosity, and void ratio increase rapidly with depth in the uppermost 50 mbsf and at a slower rate below 50 mbsf.

The post-Messinian stratigraphy established at Site 977 will provide time control on the seismic facies in the eastern Alboran Sea, yielding information on the timing of extensional structures identified in seismic data, and on the tectonic subsidence history of the basin. The seismic line acquired by JOIDES Resolution during the site approach shows that the Pleistocene deposits recovered at this site are faulted. The strong seismic reflector recognized in the area likely corresponds to the gravel-bearing interval that has been sampled to 598.5 mbsf. This seismic reflector correlates with the "M-reflector", the top of Messinian evaporites at our previous Sites 974 and 975. Drilling results at Site 977 suggest that, in the eastern Alboran Basin, the M-reflector corresponds to a strong erosional event, possibly from flooding during the early Pliocene.

The recovery of ORL's at Site 977 is an intriguing discovery that documents intervals with organic carbon concentrations, which in some cases reach levels found in sapropels from the eastern Mediterranean, across the western Mediterranean Sea. Establishing the timing of these events and coeval environmental conditions at Site 977 in relation to those at previous Sites 974 and 975 in the Tyrrhenian and Balearic seas will yield information on physical and biogeochemical boundary conditions in the western Mediterranean during these periods.

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