Dust, volcanic ash, and the evolution of the South Pacific Gyre through the Cenozoic

Abstract

We examine the 0-100 Ma paleoceanographic record retained in pelagic clay from the South Pacific Gyre (SPG) by analyzing 47 major, trace, and rare earth elements in bulk sediment in 206 samples from seven sites drilled during Integrated Ocean Drilling Program Expedition 329. We use multivariate statistical analyses (Q-mode factor analysis and multiple linear regression) of the geochemical data to construct a model of bulk pelagic clay composition and mass accumulation rates (MAR) of six end-members, (post-Archean average Australian shale, rhyolite, basalt, Fe-Mn-oxyhydroxides, apatite, and excess Si). Integrating the results with Co-based age models at Sites U1365, U1366, U1369, and U1370, we link changes in MAR of these components to global oceanographic, terrestrial, and climatic transformations through the Cenozoic. Our results track the spatial extent (thousands of kilometers) of dust deposition in the SPG during the aridification of Australia. Dispersed ash is a significant component of the pelagic clay, often comprising >50% by mass, and records episodes of Southern Hemisphere volcanism. Because both are transported by wind, the correlation of dust and ash MAR depends on the site's latitude and suggests meridional shifts in the position of atmospheric circulation cells. The hydrothermal MARs provide evidence for rapid deposition from the Osbourn Trough spreading ridge before it went extinct. Excess Si MARs show that the abrupt increase in siliceous productivity observed at Site U1371 also extended at least as far north as Sites U1369 and U1370, suggesting large-scale reorganizations of oceanic Si distributions~10-8 Ma in the southern SPG.

Cobalt‐based age models of pelagic clay in the South Pacific Gyre

“…[] and is detailed in Dunlea et al . []. Three separate digestions of an in‐house SPG sediment standard were analyzed with each batch and determined precision [(average)/(standard deviation) × 100] was ∼2% or better of the measured value for each element.…”

“…The partitioning calculations used in this study are detailed in Dunlea et al . [], and determine the mass fraction of sediment contributed by eolian dust, volcanic ash, apatite, Fe‐Mn oxyhydroxides, biogenic silica, and altered basalt, on a sample‐by‐sample basis. From the compositional mass fractions, we quantify the amount of Co d collectively sourced from eolian dust and volcanic ash and compare that to the Co t analyzed in each sample.…”

The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

“…It was originally held that the REEs are input to the oceans predominantly via rivers, with potential additions via dust where such inputs are relatively high (Byrne and Sholkovitz, 1996;Greaves et al, 1999;Tachikawa et al, 1999b;Goswami et al, 2014;Dunlea et al, 2015;Stichel et al, 2015; also see Bayon et al, 2004 for the alternative view that dust is a sink term). More recently, the emphasis of REE input to the oceans has shifted to greater consideration of a benthic continental margin source for REEs, as we will discuss further later (Spivack and Wasserburg, 1988;Jeandel et al, 1998;Goldstein and Hemming, 2003;Tachikawa et al, 2003;Arsouze et al, 2009;Rickli et al, 2009Rickli et al, , 2010Rickli et al, , 2014Carter et al, 2012;Grasse et al, 2012;Singh et al, 2012;Grenier et al, 2013;Wilson et al, 2013;Garcia-Solsona et al, 2014).…”

Quantifying K, U, and Th contents of marine sediments using shipboard natural gamma radiation spectra measured on DVJOIDESResolution

“…Samples were prepared and analyzed at Boston University and details of the analytical approach can be found in Dunlea et al . []. In brief, we first freeze‐dried and hand‐powdered the samples with an agate mortar and pestle.…”