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doi:10.2204/iodp.proc.329.201.2014

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 329 was conducted to investigate the subseafloor microbiology of the South Pacific Gyre. Its primary aim was to document the extent and character of life in sedimentary habitats with very low biomass and rates of activity (see the “Expedition 329 summary” chapter [Expedition 329 Scientists, 2011]). Although Expedition 329 focused on microbiology, the recovered cores also provide a unique opportunity to document a sedimentary system that has never been explored by scientific ocean drilling. The R/V JOIDES Resolution cored deep-sea sediment at seven sites (Fig. F1) located along two transects in the center of the South Pacific Gyre and spanning nearly the entire width of the Pacific plate in the Southern Hemisphere. The first transect progresses from the western edge of the gyre (Site U1365) to the gyre center (Site U1368) at ~26°S. The second transect at ~42°S goes from the central gyre (Site U1368) through the southern gyre edge (Site U1370) to the northern edge of the upwelling region south of the gyre (Site U1371). The more northern suite of sites has been continuously located off-shore and beneath the low-productivity gyre water for many tens of millions of years (see the “Expedition 329 summary” chapter [Expedition 329 Scientists, 2011]). Equally, the southerly sites of the second transect have been located in the southern portion of the present-day gyre (Sites U1369 and U1370) or south of the gyre (Site U1371) during this time. Particularly at Site U1371, chlorophyll-a concentrations and primary productivity are much higher than at all of the sites in the northern transect (see the “Expedition 329 summary” chapter [Expedition 329 Scientists, 2011]).

We recovered 208 sediment samples from the seven sites drilled (Sites U1365–U1371; Fig. F1) to analyze their particle size distribution. The primary objective of these analyses was to create a downcore profile of particle size at each site and to provide textural information that could be correlated with other types of data analyses made on adjacent samples. Samples were usually recovered at regular intervals, except for chert layers, which were avoided. Particle size distribution from this remote location can shed light on depositional conditions and diagenetic alterations and potentially provide information on eolian inputs and/or past deepwater circulation. Few studies have looked at particle size distribution in this area (see Rea, 1994, for a review).

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