Timing and magnitude of C02 uptake by ocean crust through seawater interaction and carbonate veining

大洋中脊两侧的流体流动现在被认为是在长时间尺度上控制海洋和老化洋壳中许多元素收支的主要过程之一。这包括CO2，玄武岩地壳中的碳酸盐脉是一个重要的汇，但对各自的全球通量知之甚少。在三十多年的海洋钻探中，所有年龄的海洋地壳都被取样到允许评估最上层地壳中CO2吸收的深度，但通量估计只存在于少数几个地点。老地壳表明全球海洋吸收通量为2-4·1012摩尔CO2/年，而年轻地壳剖面表明吸收速率要小一个数量级。这些观测结果只有在地壳在其生命周期的大部分时间里持续吸收CO2的情况下才能得到调和，这与地震和热流证据不一致，这些证据表明，在20-50百万年之前，地壳的渗透性逐渐封闭，循环实际上已经关闭。我们建议，这一重大难题，在洋底研究可以解决全面和系统的分析碳酸盐脉从地壳部分，涵盖了一系列的年龄范围内的缓慢和快速扩展地壳。我们的方法有四个方面：（1）记录岩心档案以建立碳酸盐岩脉地壳清单，（2）通过激光烧蚀ICP-MS测定碳酸盐岩脉中的微量元素丰度，以确定碳酸盐岩沉淀的流体的成熟度，（3）通过激光烧蚀MC-ICP-MS分析适当的碳酸盐岩脉的Sr同位素定年，以及（4）测定脉晕中的地球化学变化。预期的结果将导致更好地估计海洋地壳和海洋之间的全球二氧化碳通量，并更全面地了解确定这一通量所涉及的时间和过程。这些数据将与在一项配套提案框架内获得的同位素数据（Ca、Sr、O、C）合并。

Fluid flow within the flanks of mid-ocean ridges is now recognized as one of the principal processes that control the budgets of many elements in the oceans and the aging ocean crust on long time scales. This includes CO2 for which carbonate veins in basalt crust impose an important sink, yet little is known about the respective global flux. In over thirty years of ocean drilling, ocean crust of all ages has been sampled to depths that allow an assessment of CO2 uptake in the uppermost crust, but flux estimates exisit only for a few sites. Old crust indicates global oceanic uptake fluxes of 2-4•1012 moles CO2/yr, whereas young crustal sections indicate uptake rates that are an order of magnitude smaller. These observations can only be reconciled if the crust takes up CO2 continuously throughout much of its life time, which is at odds with seismic and heat flow evidence suggesting progressive sealing of permeability and virtual shut down of circulation in crust older than 20-50 Myrs. We propose that this major conundrum in ocean floor research can be resolved by comprehensive and systematic analyses of carbonate veins from crustal sections that cover a range of ages of both slow and fast spread crust. Our approach is fourfold:(1) Logging of core archives to establish carbonate vein crustal inventories, (2) determination of trace element abundances in carbonate veins by laser ablation ICP-MS to determine the maturity of fluids from which the carbonate precipitated, (3) Sr isotope dating by laser ablation MC-ICP-MS analyses of suitable carbonate veins, and (4) determination of geochemical changes in vein halos. The expected results will lead to better estimates of the global CO2 flux between ocean crust and oceans and to a more comprehensive understanding of the timing and processes involved in setting this flux. The data will be merged with isotopic data (Ca, Sr, O, C) to be obtained within the framework of a companion proposal.