Organic petrology of Neogene sediment from ODP Leg 117

Neogene sediments from three areas of the Northern Indian Ocean (Indus Fan, Owen Ridge, Oman Margin, ODP Leg 117) were studied in order to determine the amount, type, and preservation of organic matter as functions of the environments encountered. The work consisted of geochemical analyses on whole sediment (Total Organic Content and Rock Eval pyrolysis) and of petrographic studies on isolated organic matter by optical and scanning electron microscopy. In Indus Fan sediments, organic matter is present in low amounts, mainly as lignaceous fragments. A contrasting situation exists in Oman Margin sediments which are generally rich in amorphous autochtonous organic matter. Owen Ridge, located between Indus fan and Oman Margin areas, shows two phases of organic sedimentation as a consequence of the uplift of the ridge. The older phase (Oligocene to early or middle Miocene) is strongly influenced by detrital supply from the Indus, while the younger phase (middle Miocene to Pleistocene) is characterized by relatively high amounts of autochtonous organic matter. From a general point of view it appears that high amounts of organic matter are mainly due to good preservation of marine amorphous organic matter, such as in Oman Margin sediments and in upper pelagic levels of Indus Fan and Owen Ridge deposits. Low total organic carbon contents are correlated with low proportions of amorphous material in the total organic matter due to oxidizing conditions. This leads to a relative enrichment in components derived from resistant materials (lignin, chitin, or other resistant biopolymers) such as lignaceous fragments (Indus Fan) and/or fragments from benthic organisms and alveolate microplankton (Oman Margin).

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.

Radiolarian assemblages of ODP Leg 117 sites

Significant numbers of radiolarians ranging in age from late middle Miocene to Recent were recovered from six sites drilled on the Oman margin and Owen Ridge. Sparse faunas were recovered from five additional sites on the Oman margin and one site on the Indus Fan. Detailed range charts and biozonations are presented for most sites. The radiolarian assemblages are peculiar in that numerous common tropical forms, some of which are biomarkers, are absent or very rare. In addition, some species not usually found in tropical assemblages are present. These forms, indicative of up welling conditions, fall into three categories: (1) endemic upwelling: species endemic to upwelling and not previously described from the Indian Ocean; (2) displaced temperate: temperate forms not usually found in tropical waters; and (3) enhanced tropical: tropical forms which are more abundant and/or robust in areas of upwelling. Comparison of the Oman margin/Owen Ridge fauna with that recovered from the Peru margin upwelling area (ODP Leg 112) suggests that the assemblage may be globally diagnostic of upwelling conditions. The onset of upwelling is marked by the appearance of siliceous biota at about 11.9 Ma, and there is some indication of a decrease in the strength of the upwelling signal at about 9.6 Ma. A strong pulse in, or strengthening of, the upwelling mechanism is indicated by a marked fauna change at 4.7 Ma. There is a weaker signal, implying a change in upwelling conditions, at about 1.5 Ma.

Sea surface temperture reconstruction and planktonic foraminifera of ODP Hole 117-723A

In the western Arabian Sea (WAS), the highest seasonal sea surface temperature (SST) difference presently occurs between May and August. In order to gain an understanding on how monsoonal upwelling modulates the SST difference between these two months, we have computed SST for the months of May and August based on census counts of planktonic foraminifers by using the artificial neural network (ANN) technique. The SST difference between May and August exhibits three distinct phases: i) a moderate SST difference in the late Holocene (0-3.5 ka) is attributable to intense upwelling during August, ii) a minimum SST difference from 4 to 12 ka is due to weak upwelling during the month of August, and iii) the highest SST difference during the last glacial interval (19 to 22 ka) with high Globigerina bulloides % could have been caused by the occurrence of a prolonged upwelling season (from May through July) and maximum difference in the incoming solar radiation between May and August. Overall, variations in the SST difference between May and August show that the timing of intense upwelling in the Western Arabian Sea over the last 22 kyr has been variable over the months of June, July and August.