One of the prime objectives of IODP Expedition 318, January to March 2010 (Wellington to Hobart) was to obtain an ultra high resolution record of the Holocene from drilling in the Ad´elie Basin. The Adelie Basin contains a 190 m thick sequence of diatomaceous ooze overlying a glacial diamict with an age of about 11 ka. Sediments obtained at Site U1357 reveal an ultra-high resolution record with the first annually resolved time series of oceanographic and climatic variability derived from the Southern ocean. Site U1357, was triple cored. Hole U1357A was split and processed during the expedition while Holes U1357B and C were sealed in nitrogen flushed bags and only split during the sampling party (June 27, 2010 - July 3, 2010). Data obtained from shipboard measurements of the archive half of U1357A documented an unrivaled record of secular variation (SV) of the geomagnetic field, which promises not only an unprecedented SV record but also will provide critical constraints for hole to hole correlation. During post-cruise sampling we bolstered our confidence in the reliability of this unique record, but also discovered that the magnetic mineralogy degrades extremely rapidly after exposure to air. While the discrete samples have been placed in nitrogen flushed bags for transport, immediate measurement is urgently required. Also, because the magnetic mineralogy is likely to be quite unusual (for example biogenic iron sulfides), it must also be characterized prior to as well as after oxidation. We therefore request funding via the RAPID funding mechanism. Support for the remaining Expedition 318 paleomagnetic efforts will be requested using the usual EOR mechanism.
The goal of this grant was to use an advanced form of electron microscopy called "cryogenic scanning electron microscopy", or cryo-SEM, to examine marine sediments collected as part of IODP Expedition 318 Site U1357. These sediments contain an unknown metastable mineral phase with magnetic properties. The unknown mineral appears to faithfully record the Earth's magnetic field, but sediment samples rapidly lose their magnetic recording after exposure to the atmosphere. Cryo-SEM has the ability to image oxygen sensitive sediments before they react. Low temperature magnetic measurements were also used to try and constrain the identification of this unknown mineral component. Our microscopy and magnetometry results allow us to constrain the most likely identification for this metastable phase as ferrihydrite, (Fe3+)2O3•0.5H2O. Ferrihydrite is known to be metabstable and commonly forms as a precursor to hematite and/or goethite. Oxidized sediments were observed to take on a yellow, tan color, consistent with the formation of goethite. Ferrihydrite is a hydrous ferric oxide mineral that is widespread in marine environments and is known to carry a metastable remanent magnetization. Ferrihydrite also has a high density of point defects within its atomic structure, which allows it to rapidly adsorb many environmentally important metal species, such as arsenic, lead, phosphate and organic molecules (e.g., humic and fulvic acids). It is known to be a precursor of more crystalline minerals like hematite and goethite by aggregation-based crystal-growth. However, its transformation in natural systems is frequently blocked by chemical impurities adsorbed at its surface, for example silica as most of natural ferrihydrites are siliceous (Carlson and Scwertmann, 1981). The high concentration of opaline silica in these sediment samples (see attached figure) suggests that if ferrihydrite is present, then its fate is closely entwined with that of the opaline silica. Other candidate minerals, such as greigite, pyyrhotite, and maghemite, were not observed during cryo-SEM examination.
