Leg 117

Oman Margin

Twelve sites (Site 720 to Site 731) were drilled during Leg 117 which focused upon an investigation of global climate and ocean-atmosphere interactions over the last 10 m.y. on the Oman Margin, a region that has been under the influence of high biological productivity and high accumulation rates in one of the most fertile sectors of the world's oceans, the area of monsoonal upwelling along Arabia.

The sediment sections recovered during Leg 117 suggest that monsoonal upwelling may have been active since the middle Miocene, and that the oxygen-minimum zone may have fluctuated in oxygen content since this time. The high-productivity zone of upwelling waters and the extent of and location of the oxygen-minimum zone have been the most important factors for the distribution of sediment facies on the Oman Margin. The latter is controlled by water mass circulation, oxygen consumption by bacterial activity, and the extent of biological production in the upwelling areas. High organic carbon content, coupled with intervals of primary varve-like laminations, are preserved in upper Pliocene sediments recovered at most sites. They are interpreted to be an expression of an intensified oxygen-minimum zone, which episodically and effectively inhibited bioturbation during this time. The laminated, organic-rich interval may record a signal of significantly different oceanographic, and possibly atmospheric, circulation in the northwest Indian Ocean from about 2.5 to 1.5 Ma.

A pronounced cyclicity of sediment properties is observed, consistent across all sites. Patterns of sediment color, physical properties, and magnetic susceptibility of sediments from the late Miocene to the present reveal distinct periodicities (of 400, 100, 41, 23, and 19 k.y.) that match those of the Milankovitch mechanism. These sedimentary cycles seem to reflect variations in primary productivity, preservation of carbon and carbonate, and the proportion of eolian detritus. The magnetic susceptibility varies together with, and probably originates in, the eolian detritus (as indicated by the abundance of quartz).

Climatic signals recognized in the cores provide independent evidence of the variability in monsoonal intensity. Magnetic susceptibility data vary here directly with terrigenous quartz, which is thought to be eolian. Thus, the magnetic material that causes susceptibility variations may be part of the windblown dust from the Arabian peninsula or from the Iran-Makran region. The susceptibility signal thus may be directly related to wind direction and intensity.