Comparing sediment accumulation below the unproductive South Pacific Gyre (IODP expedition 329) with the highly productive central equatorial Pacific

IODP Expedition 329 cored deep-sea sediments at seven sites along two transects in the center of the South Pacific Gyre (SPG). Before this expedition, the SPG contained the largest portion of the seafloor that was never explored with scientific ocean drilling. Therefore, this expedition and the onshore post-cruise studies will advance scientific understanding across a broad front. The primary purpose of Expedition 329 is to document the extent and nature of microbial life in the sediments beneath the low-productivity heart of the ocean.

The SPG is sometimes described as Earth's largest oceanic desert. Its center is farther from continents than the center of any other gyre. Surface chlorophyll concentrations and primary productivity are lower in this gyre than in other regions of the world ocean. Wind-blown dust is at a minimum at this latitude. Mean sedimentation rates are among the lowest that occur on Earth, 1-2 orders of magnitude lower than rates in the equatorial Pacific. The SPG is thus an ideal region for exploring the nature of sedimentation in the extreme low-activity centre of an open-ocean gyre. This research will compare and contrast sediment accumulation below the unproductive South Pacific Gyre with accumulation in the highly productive central equatorial Pacific. The different physical environment as well as the very different particle compositions and sizes associated with clay-dominated deposits below the SPG should lead to interesting differences. The majority of deep-sea sedimentation studies have focused on sites relatively close to shore and beneath major upwelling zones, where biological productivity and organic flux to the seafloor are generally high. Little is known about the effects of weak bottom currents and related deep-water processes on sedimentation in ocean basins compared with their effects on continental margins and slopes.

Documentation of sedimentation processes below the SPG will require a range of analyses. I will use the seismic studies and site survey data (Cruise KNOX-02RR, aboard the R/V Revelle) and I will perform SediGraph grain size analyses. Variations in particle size can shed light on many facets of deep-sea sediments including depositional conditions and can provide indicators of relative flow speed. Grain size data from this remote location can also provide crucial information on eolian inputs and past atmospheric circulation.

In addition, the grain size measurements will contribute valuable information to the fundamental objectives of Expedition 329. In particular, it will help determine whether subseafloor life in the most organic-poor sediment is nourished to a significant extent by H2 from in-situ radiolysis of water (the molecular dissociation of water, H2O, caused by energetic alpha-particles produced by radioactive decay). Most subseafloor life in deep-sea sediments is maintained by products of organic matter degradation so this would be an important finding. Radiolysis rates will be determined from radioactive element concentrations by the scientific shipboard party, who require grain size data because particle size influences the rate of water radiolysis. As clay-rich sediment contains much higher concentrations of radioactive elements than other deep-sea sediment and, in fine-grained sediment, most alpha and beta production occurs within striking range of pore water, these deep-sea clays are anticipated to yield much higher rates of water radiolysis than other sedimentary environments.

Due to the unique nature of these sediments, in particular their very low organic matter content, we will also conduct a pilot study to determine the potential for future paleoceanographic and paleoclimatic reconstructions using specific biomarkers (i.e. fossil molecules).

This research will deepen our understanding of how pelagic sediments accumulate in the deep ocean. Relatively few precise grain size analyses of pelagic sediments have been reported so far.

Due to the South Pacific's remoteness, this project may seem unlikely to have direct UK relevance outside academia, but the remarkable story unfolding is that deep life can potentially be maintained by the products of radioactivity, via H2 released by radiolysis of water. This startling possibility has the potential for excellent outreach, as well as relevance to debates concerning the origin of life on Earth and other planets. The beneficiaries are therefore, besides other academics, a wider public interested in the natural world. This may be most immediately useful to UK teachers of biology and environmental science, in helping them to engage students in science.

To initiate the outreach, Nathalie will talk about science related to her experience in the Pacific and discuss upcoming expeditions in classroom and museum visits. This work will be done in collaboration with the Expedition Education Officer and using outreach facilities available through our adjacent museum. In school visits, students will learn firsthand about subjects that scientists have investigated during recent operations, including deep life, climate change and ocean observatories, as well as life onboard. Students will be able to examine recovered sediments and experiment with settling jars.

To engage with academic users, the results of this research will be reported at the Spring EGU (2012) and at UK meetings as appropriate. Grain size and geophysical data analysis will be published in Deep Sea Research and/or Paleoceanography.

Academic users will primarily be those interested in deep life, as the leg and post-cruise studies will contribute a significant microbiological dataset on life fueled, potentially, by H2 released by radiolysis. The grain size data will contribute to that effort, but also, along with geophysical analysis, to the understanding of South Pacific deep-water motions. The paleoceanography of this region is poorly understood, due to a lack of drilling and because traditional calcareous sample measurements cannot be carried out here. Therefore, evidence from grain size and geophysics will provide an important contribution to efforts to characterize the global ocean water movement history.

Rea et al showed how sediment accumulates so slowly (<1 cm/Ma) that it is still thinner than seismic resolution in many places here. As this area lies adjacent to the belt of Australian wind-blown dust, constraining the marginal incidence of dust will interest researchers deciphering Earth's wind patterns.

Nathalie herself will also benefit from this research in terms of education, expanding her experience further into physical oceanography, sedimentology and geophysics, as well as geomicrobiology.