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

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.

The proposed research to be conducted on calcium carbonate samples acquired during IODP Expedition 329, a previously unexplored region in terms of ocean drilling, will greatly benefit my immediate research partners and a broad range of researchers across several disciplines worldwide. Specifically this research will provide detailed information regarding the timing of late stage hydrothermal alteration, the composition and origins of the hydrothermal fluid from which carbonate precipitated from, and the thermal conditions of late stage hydrothermal alteration within South Pacific Gyre basement.

Benefits to: IODP

The proposed research will help to address two fundamental objectives outlined in the initial science plan for IODP - 'defining the physical and chemical limits of subseafloor microbial communities' and 'The aging of the ocean crust'. This research will help address a key Expedition 329 objective: 'To determine how the basement habitats, potential activities and, if measurable, communities vary with crust age and hydrogeologic regime'. Our research will be publicised in internationally recognised peer-reviewed journals which will help to assert IODP's position of being at the forefront of pioneering high impact science.

Benefits to: Academic Researchers

Through this grant, our work will have significant impact in a wide range of disciplines concerned with trying to understand the subseafloor processes and the interaction between seawater and basement. These include: Microbiologists interested in understanding basement habitability and diversity of biomass, petrologists who research basement alteration processes and ocean crust aging, geochemists investigating fluid rock interactions in sub-seafloor hydrothermal systems, and palaeoclimate scientists who are interested in understanding ancient climate through ancient seawater compositions.

Benefits to: Public

This research will help answer important questions relating to Earth's climate and the nature of life in one of Earth's most extreme environments. Both topics are of high societal relevance and are at the forefront of public debate. University open day events, IODP press releases, and publication of articles to the wider public will be used to ensure the public are informed and inspired.

How does the proposed research generate impact?

Our proposed research will test a new ancient seawater Mg/Ca and Sr/Ca record that is independent of the established record based on benthic foraminifera. This will benefit palaeoclimate researchers who routinely use ancient seawater compositions to understand past climate variations. The timing and temperature estimates of late stage calcium carbonates will allow microbiologists to assess basement habitability over the entire range of the SPG at differing crustal ages, sediment piles and lithologies, and may infer the extent of life across all ocean crust on Earth.

Successful age dating of late stage carbonates within the proposed post cruise research will, for the first time, yield dates for late stage hydrothermal events within oceanic crust. This will profoundly impact on our understanding of the timing and duration of alteration of ocean crust and the rate of carbon uptake, opening the door for future age dating of calcium carbonates from other, previously sampled oceanic crustal sites.

What will be done (pathway to impact)?

To ensure that academic researchers benefit from our research, and to meet our publication obligations to IODP, our proposed research will result in the production peer reviewed research articles within 5 years of sailing on IODP Expedition 329. Opportunities will be taken to present research at relevant conferences, and during public engagement events. As PDRA Dr Christopher Smith-Duque sailed on IODP Expedition 344, he missed the IODP Expedition 329 post cruise scientific meeting at Honolulu (Hawaii), and therefore submitted a poster presentation of his research to date.