ALTERATION OF VOLCANIC MATTER IN SEDIMENTS OF THE WOODLARK BASIN, (ODP LEG 180): EVIDENCE FROM TRACE ELEMENTS AND ISOTOPIC SIGNATURES IN INTERSTITIAL WATER

A change in geologic environment from continental rifting to seafloor spreading within a small region makes the western Woodlark Basin an attractive area to investigate lithospheric extension and its effects on the geochemistry of sediments and interstitial water (IW). Three holes drilled in the northern margin of the Woodlark Basin during OPD Leg 180 allowed characterization of the composition and in situ properties of a down-flexed prerift sedimentary basin and basement sequence unconformably onlapped by synrift sediments. Our data reflect how the evolution of the Woodlark basin affected sedimentation at the three sites, and how the input of volcanic matter influenced diagenesis. Studies of interstitial water and associated sediments indicate that similar reactions control the evolution of the IW composition throughout the Woodlark Rise sites. Paramount among these are: 1) bacterially mediated oxidation of organic matter, 2) diagenesis of biogenic carbonates, and 3) alteration of volcanic matter.

Reaction zones within the sediments are reflected in non-diffusive IW concentration profiles for selected elements. These zones are often constrained by changes in the composition and physical properties of the sediments. Although diagenesis of carbonates influences interstitial water profiles deep downhole, it is the occurrence of volcanic matter that imparts some of the more characteristic signatures to the IW.

Oxygen and Sr isotopic signatures of the IW help constrain the extent of alteration of volcanic matter and identify reaction zones. The range of d18O varies from +0.2 versus SMOW to nearly -3, with the largest gradients occurring across the Miocene unconformity. The shape of the profiles is characteristic of alteration of volcanic matter. Sr isotopes patterns are more complex reflecting contributions of different sources of Sr to an isotopic signature ranging from a contemporaneous seawater signature to a 87Sr/86Sr of 0.7072 observed at Site 1115 across the unconformity. Trace element compositions of clays isolated from sediments at Site 1109 also provide clues as to the provenance of the volcanic matter found throughout Woodlark Rise Sites.