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.