The timing and nature of inorganic calcium carbonate precipitation within exposed basalt from the South Pacific Gyre, IODP Expedition 329.

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IODP Expedition 329 sailed a transect of over 11,000 km of ocean over a period of 66 days from Papeete, Tahiti to Auckland, New Zealand to recover sediment and basalt drillcores from within the South Pacific Gyre (SPG), the world's largest ocean current system. and one of the most poorly explored regions of Earth. Indeed the last scientific expedition which recovered samples from the seafloor within the SPG was over 130 years ago during the HMS Challenger Expedition (1872-1876). During IODP Expedition 329 we made great strides in addressing what, if any, microbial life occurs beneath the sea floor within the gyre and in what conditions (habitats) life occurs. We also explored the role the oceans, seafloor sediment, and underlying ocean crust plays in controlling habitats beneath the seafloor across a transect of the SPG.At three sites we took samples of the volcanic basement rocks beneath the sediment. The volcanic rocks, initially formed from eruptions of sub-sea volcanoes along mid ocean ridges (mid ocean ridge basalt) have since cooled and spread from the mid ocean ridge over millions of years to their present location.Chemical interaction with seawater that enters the ocean crust through fractures and joints (caused by post eruption cooling and tectonic movement) results in the formation of secondary minerals that fill fracture space and sometimes replaces the original volcanic rock. This 'seafloor weathering' process results in chemical transfer between seawater and volcanic rocks, altering the composition of ingressing seawater and the volcanic crust. Our research, based on the low temperature (<100 deg C) secondary mineral calcium carbonate will address some of the uncertainties associated with this process and in turn will answer some important questions regarding basement habitability and global geochemical cycles.The element Strontium (Sr), which is present in calcium carbonate, is extremely useful since its isotopic composition in seawater varies through time and this record is well known. We will use the Sr-isotopic composition in calcium carbonate to estimate how much seawater has mixed with mid ocean ridge basalt, which has a very different but well defined isotopic ratio. In addition, we will use Sr isotopes to partially constrain the timing of calcium carbonate formation. Since isotopes of Uranium (U) decay radioactively into stable isotopic 'daughters' of Lead (Pb) at a known rate, it is possible, given the right geological conditions and appropriate methods, to date the formation of mineral phases. U and Pb incorporated into calcium carbonate will provide insights into the timing of seafloor weathering processes in oceanic crust. We will utilize measurements of U and Pb isotopes to date calcium carbonate formation within the South Pacific Gyre.Temperature is known to control the preference of Oxygen (O) isotopes uptake during calcium carbonate formation. We will therefore use O-isotopes in carbonates to determine the temperature of formation to determine the thermal environment of formation. Knowledge of temperature will allows us to place limits on the extent of the sub seafloor biosphere. Impurities of Magnesium (Mg) and Sr that variably replace (substitute) for calcium (Ca) in the crystal structure of calcite can be measured to infer past Mg/Ca and Sr/Ca ratios of ancient seawater for upto 120 Myrs of geological time. These ratios act as proxies for ancient seawater chemistry, which in turn provide insights into geochemical processes that take place between the Earth's crust, the oceans, and the atmosphere. Our research will offer tantalizing clues into the potential habitat for microbial life within oceanic crust, offer insights into the timing of seafloor weathering processes within the SPG.