Mineralogy of ODP Hole 117-722B (Appendix)

Bulk mineralogy of the terrigenous fraction of 99 samples from ODP Site 722 on the Owen Ridge, western Arabian Sea, has been determined by x-ray diffraction, using an internal standard method. The sampling interval, approximately 4.3 k.y., provides a detailed mineralogic record for the past 500 k.y. Previous studies have identified important modern continental sediment sources and the mineral assemblages presently derived from each. These studies have also demonstrated that most of this material is supplied by southwest and northwest winds during the summer monsoon. A variety of marine and terrestrial records and general circulation model (GCM) simulations have indicated the importance of monsoonal circulation during the Pleistocene and Holocene and have demonstrated increased aridity during glacial times and increased humidity during inter glacials. The mineralogic data generated here were used to investigate variations in source area weathering conditions during these environmental changes. Terrigenous minerals present include smectite, illite, palygorskite, kaolinite, chlorite, quartz, plagioclase feldspar, and dolomite. This mineralogy is consistent with the compositions of source areas presently supplying sediment to the Arabian Sea. An R-mode factor analysis has identified four mineral assemblages present throughout the past 500 k.y.: quartz/chlorite/dolomite (Factor 1), kaolinite/plagioclase/illite (Factor 2), smectite (Factor 3), and palygorskite/dolomite (Factor 4). Chlorite, illite, and palygorskite are extremely susceptible to chemical weathering, and a spectral comparison of these factors with the eolian mass accumulation rate (MAR) record from Hole 722B (an index of dust source area aridity) indicates that Factors 1, 2, and 4 are directly related to changes in aridity. Because of these characteristics, Factors 1,2, and 4 are interpreted to originate from arid source regions. Factor 3 is interpreted to record more humid source conditions. Time-series of scores for the four factors are dominated by short-term (10-100 k.y.) variability, and do not correlate well to glacial/interglacial fluctuations in the time domain. These characteristics suggest that local climatic shifts were complex, and that equilibrium weathering assemblages did not develop immediately after climatic change. Spectral analysis of factor scores identifies peaks at or near the primary Milankovitch frequencies for all factors. Factor 1 (quartz/chlorite/dolomite), Factor 2 (kaolinite/plagioclase/illite), and Factor 4 (illite/palygorskite) are coherent and in phase with the MAR record over the 23, 41, and 100 k.y. bands, respectively. The reasons for coherency at single Milankovitch frequencies are not known, but may include differences in the susceptibilities of minerals to varying time scales of weathering and/or preferential development of suitable continental source environments by climatic changes at the various Milankovitch frequencies.

Planktonic foraminifera of ODP Site 117-722

Quantitative analysis of the late Cenozoic planktonic foraminiferal record from western Arabian Sea Site 722 revealed long term trends in the history of oceanography and climate. The modern western Arabian Sea surface waters are highly influenced by the monsoonal wind system. Summer upwelling, a result of southwestern winds, occurs along the coast of Somalia which produces distinct foraminiferal assemblages which are dominated by G. bulloides d'Orbigny. Consequently, variations in the distribution record of G. bulloides through time monitor the upwelling history associated with monsoonal activities. G. bulloides was a minor constituent of the foraminiferal fauna from the bottom of the hole (~14.5 Ma) until about 8.6 Ma ago. Then a rapid shift occurred toward higher values, up to 53% at 7.4 Ma. We interpret this rapid increase as a major step in the evolution of the monsoonal history. It is either the establishment of the system or at least a strong intensification of the monsoonal winds. At 5.5 Ma a significant drop of the G. bulloides relative abundance occurred which may indicate less upwelling, or alternatively other biota may have been favored in this period. From 5.0 Ma to Recent the G. bulloides record indicates that upwelling has occurred with minor fluctuations.

Stratigraphy of ODP Leg 117 sites

During the late early Miocene to early middle Miocene, the Owen Ridge was uplifted to a sufficient height as to be above the realm of turbidite deposition. Monsoonal-induced upwelling appears to have been initiated during the Miocene. On the Oman Margin, the effect of upwelling on the microplankton was established by the middle Miocene. However, the effects of upwelling on the Owen Ridge region were not realized until later, in the early late Miocene. A transition in the upwelling regime took place between the Pliocene and Pleistocene. While the Miocene and Pliocene sediments are dominated by the siliceous component, the Pleistocene sediments seem to be dominated by the calcareous component.

Magnetostratigraphy of ODP Leg 117 sites

Magnetic measurements were made on discrete samples from the Neogene pelagic and hemipelagic sediments recovered during ODP Leg 117. Polarity zones, usually identified for uppermost 200 m, were correlated to the geomagnetic polarity time scale referring to biostratigraphic datums. Quality and resolution of the magnetostratigraphy was partly limited by the weak and relatively soft magnetic character of almost all intervals, and core disturbance by gas expansion at some Oman Margin sites. Clear polarity records of the Brunhes and Matuyama chrons (C1 to C2r) were observed at Sites 724 and 727 on the Oman continental margin. Extended reversal records of the Pliocene were found at Site 722 on the Owen Ridge and Site 728 on the Oman Margin, and correlated to the Gauss to Gilbert chrons (C2A to C3).

Physical properties, grain size distribution and clay mineralogy of ODP Leg 117 sites

The fabric of sediments recovered at sites drilled on the Indus Fan, Owen Ridge, and Oman margin during Ocean Drilling Program Leg 117 was examined by scanning electron microscopy to document changes that accompany sediment burial. Two sediment types were studied: (1) biogenic sediments consisting of a variety of marly nannofossil and nannofossil oozes and chalks and (2) terrigenous sediments consisting of fine-grained turbidites deposited in association with the Indus Fan. Biogenic sediments were examined with samples from the seafloor to depths of 306 m below seafloor (mbsf) on the Owen Ridge (Site 722) and 368 mbsf on the Oman margin (Sites 723 and 728). Over these depth ranges the biogenic sediments are characterized by a random arrangement of microfossils and display little chemical diagenetic alteration. The microfossils are dispersed within a fine-grained matrix that is predominantly microcrystalline carbonate particles on the Owen Ridge and clay and organic matter on the Oman margin. Sediments with abundant siliceous microfossils display distinct, open fabrics with high porosity. Porosity reduction resulting from gravitational compaction appears to be the primary process affecting fabric change in the biogenic sediment sections. Fabric of illite-rich clayey silts and silty claystones from the Indus Fan (Site 720) and Owen Ridge (Sites 722 and 731) was examined for a composite section extending from 45 to 985 mbsf. In this section fabric of the fine-grained turbidites changes from one with small flocculated clay domains, random particle arrangement, and high porosity to a fabric with larger domains, strong preferred particle orientation roughly parallel to bedding, and lower porosity. These changes are accomplished by a growth in domain size, primarily through increasing face-to-face contacts, and by particle reorientation which is characterized by a sharp increase in alignment with bedding between 200 and 400 mbsf. Despite extensive particle reorientation, flocculated clay fabric persists in the deepest samples examined, particularly adjacent to silt grains, and the sediments lack fissility. Fabric changes over the 45-985 mbsf interval occur in response to gravitational compaction. Porosity reduction and development of preferred particle orientation in the Indus Fan and Owen Ridge sections occur at greater depths than outlined in previous fabric models for terrigenous sediments as a consequence of a greater abundance of silt and a greater abundance of illite and chlorite clays.

Lipid analyses of ODP Leg 117 samples

The solvent-extractable organic fractions of sediment samples from six Ocean Drilling Program Leg 117 sites were investigated by gas chromatography and gas chromatography-mass spectrometry. Sediments deposited in the Indus Fan (Site 720) as well as Miocene sediments from the Owen Ridge (Sites 722 and 731) contain almost exclusively organic matter of terrigenous origin. The organic matter in sediments from the Oman Margin (Sites 723, 725, and 728) and in the Pliocene/Pleistocene sections from the Owen Ridge is mainly of a marine origin with variable admixtures of terrigenous material. In these latter samples strong variations of the lipid composition and distribution are noted. However, the interpretation of the relation to potential biological sources is hampered by a lack of information on the possible lipid composition of appropriate organisms.