Abstract:
Radiometrically-dated basalts from Ontong-Java and Manihiki Plateaus in the western Pacific are evidence for the simultaneous formation of these huge features at about 124-122 Ma, although the uppermost basalts and deepest sediments from ODP 807C on Ontong-Java suggest a slightly subsequent, but probably distinctly separate event at about 121-120 Ma. Spreading rates increased between 121. 5 and 83. 5 Ma, although the temporal distribution of the spreading rate increase within this 38-m. y. period is unknown. Several responses to these large volcanic events probably resulted from accelerated hydrothermal processes delivering increased mantle effluents to the oceans. These responses occurred in the following stratigraphic order with the associated, approximate chronological ages, based in part on our redating of the base of the M0 magnetic reversed interval at 122 Ma. Strontium isotope ratios began to decline about 123 Ma. Nannoconid abundances began to decline, and radiolaria, planktonic foraminifera and calcareous nannofloras began to experience fluctuating abundances and speciation about 123-122. 5 Ma. Metal concentrations of Co, Mn, Pb, Yb, and Cu peaked about 122-121. 5 Ma (Duncan et al., 1997). All of these fluctuations preceded an abrupt change in sedimentation from predominately carbonate to a more reduced, organic-carbon regime whose basal member is the Selli black shale event (OAE-1a). Changes in planktonic communities and sedimentation culminated in a nannoconid "crisis" at about 121 Ma and in the Selli black shale at about 121-120 Ma. A large positive excursion of delta Carbon 13 followed the Selli event at about 120-119 Ma and strontium isotope ratios reached minimum values at about 116-113 Ma.These trends may have been accelerated towards their respective extrema by formation of the Nova-Canton Trough rift system at about 123-118 Ma, by smaller volcanic features in the northwestern Pacific that formed between 128-111 Ma, by others that have been subducted, or by still-existing features that have not yet been dated adequately.