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

IODP探险队329号在66天的时间里从塔希提岛的帕皮提到新西兰的奥克兰，航行了超过11,000公里的海洋样带，从世界上最大的洋流系统南太平洋环流（SPG）中回收沉积物和玄武岩岩芯。也是地球上探索最少的地区之一。事实上，上一次在SPG内从海底回收样本的科学考察是在130多年前的挑战者号探险期间（1872-1876）。在IODP第329次探险中，我们在解决环流海底下的微生物生命（如果有的话）以及生命发生的条件（栖息地）方面取得了重大进展。我们还探索了海洋、海底沉积物和海底地壳在控制SPG横断面海底下的栖息地方面所起的作用。我们在三个地点采集了沉积物下的火山基底岩石样本。这些火山岩最初是由海底火山沿着洋中脊（洋中脊玄武岩）喷发形成的，经过数百万年的冷却，从洋中脊扩散到现在的位置。与通过裂缝和节理进入海洋地壳的海水（由喷发后冷却和构造运动引起）的化学相互作用导致次生矿物的形成，这些矿物填充裂缝空间，有时取代原始火山岩。这种“海底风化”过程导致海水和火山岩之间的化学物质转移，改变了进入海水和火山地壳的成分。我们基于低温（<100℃）次生矿物碳酸钙的研究将解决与这一过程相关的一些不确定性，进而将回答有关基底可居住性和全球地球化学循环的一些重要问题。存在于碳酸钙中的锶元素（Sr）非常有用，因为它在海水中的同位素组成随时间而变化，这一记录是众所周知的。我们将使用碳酸钙中的sr同位素组成来估计有多少海水与海中脊玄武岩混合，海中脊玄武岩具有非常不同但明确的同位素比例。此外，我们将使用锶同位素来部分限制碳酸钙形成的时间。由于铀(U)的同位素以已知的速度放射性衰变为铅（Pb）的稳定同位素“子”，因此，只要有适当的地质条件和适当的方法，就有可能确定矿物相形成的年代。结合到碳酸钙中的U和Pb将为海洋地壳海底风化过程的时间提供见解。我们将利用U和Pb同位素的测量来确定南太平洋环流中碳酸钙形成的年代。已知温度控制碳酸钙形成过程中氧(O)同位素吸收的偏好。因此，我们将使用碳酸盐中的o同位素来确定地层温度，以确定地层的热环境。对温度的了解将使我们能够对海底生物圈的范围加以限制。在方解石的晶体结构中，镁（Mg）和锶（Sr）的杂质可以变化地取代钙（Ca），从而推断出过去120 Myrs地质时期古代海水的Mg/Ca和Sr/Ca比值。这些比率代表了古代海水的化学成分，进而提供了对地壳、海洋和大气之间发生的地球化学过程的深入了解。我们的研究将为海洋地壳内微生物生命的潜在栖息地提供诱人的线索，为SPG内海底风化过程的时间提供见解。

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.