Paleoceanography in Pelagic Clay of the South Pacific Gyre

Abstract:

A spatially and temporally expansive record of early Cenozoic high-latitude ocean history resides in the pelagic clay of the South Pacific Gyre (SPG). At the beginning of the Cenozoic, four sites drilled during IODP Expedition 329 were located between 40-62°S, which may have been the center of an ancient polar gyre. As the Pacific Plate migrated northward, these sites were subjected to major paleoceanographic changes including the onset of the Antarctic Circumpolar Current, Australian desertification, and Southern Hemisphere volcanism.

The SPG sediment is homogenous brown, zeolitic, metalliferous pelagic clay. Such sediment can be challenging for paleooceanographic research due its ultrafine grain size, slow accumulation rate, post-depositional alteration, and lack of biogenic material. However, our geochemical techniques embrace the authigenic nature of SPG clay to develop a constant-Co age model and track variations in sediment origin and accumulation. By combining sedimentation patterns with backtracked site paths, we produce an unprecedented characterization of the Cenozoic paleoceanographic evolution of the SPG.

We analyzed 47 major, trace, REE concentrations in 206 bulk sediment samples from 7 sites across the SPG, deposited as long ago as 100 Ma. For each sample, traditional geochemical partitioning techniques, Q-mode factor analyses, and multiple linear regressions allowed us to quantify contributions of six end-members: post-Archean average Australian shale (PAAS), Fe-Mn-oxyhydroxides, apatite, biogenic Si, and two distinct types of altered volcanic ash.

Mass accumulation of the PAAS end-member increased 12-18% throughout the Cenozoic, with the most rapid increase occurring just after the mid-Miocene when Australia became more arid. The Paleogene/Neogene boundary also marks a change in sedimentation, likely caused by a change in eolian activity and/or a change in authigenic processes due to changing bottom water conditions. Contributions from one kind of altered ash decreased throughout the Cenozoic while the second kind of altered ash increased in importance. This volcanic shift reflects either a large-scale change in the source of volcanic ash and/or different alteration of ash from the same source, possibly from changes in bottom water conditions or sedimentation rate.