Insights into hydrothermal iron flux variability at slow-spreading ridges from deep drilling data and reaction-transport modeling

化学海洋学的最新发现指出，热液铁是全球尺度生物地球化学海洋循环的重要调节剂，因为铁的可用性通常限制浮游植物的生长，从而限制生物碳泵。有趣的是，这些来自GEOTRACES项目的新结果再次证实了大洋中脊群落之前的认识，即热液活动以及由此引起的沿大西洋中脊（MAR）的热液输出通量比其缓慢扩散速率所显示的要高得多。然而，对热液铁出口的控制以及这些控制在基性和超基性地质环境之间的差异仍然知之甚少。在这里，我们将把反应输运模型与IODP数据结合起来，以确定导致TAG和Rainbow热液区最含盐和富铁流体喷发的关键参数组合。当海水在热液系统循环过程中受热时，在与两相边界相交时，海水可能分裂为致密的含盐盐水相和较轻的低盐气相，金属在盐水相中聚集。因此，跟踪整个循环系统的盐水相是改进评估的关键之一。另一种方法是量化上升带周围热液矿物中铁的再沉积。我们建议回归到独特的ODP 158数据，并将其与反应-输运模型相结合，为卤水形成和铁输运提供新的见解。这项工作将由一项关于超镁铁质彩虹热液场的研究来补充，为此，一项国际努力正在提交IODP钻井建议。通过这两个案例的研究，我们期望对慢扩张脊的卤水形成和热液铁输出的关键控制因素有新的认识，这对全球生物地球化学循环非常重要。

Recent discoveries in chemical oceanography point to hydrothermal iron as an important modulator of global scale biogeochemical ocean cycles, as iron availability often limits phytoplankton growth and thereby the biological carbon pump. Interestingly, these new results from the GEOTRACES program re-confirm the preceding realization within the mid-ocean ridge community that hydrothermal activity and thereby the hydrothermal export flux along the Mid-Atlantic Ridge (MAR) is disproportionally higher than its slow-spreading rate would suggest. Yet, the controls of hydrothermal iron export and how those vary between mafic and ultramafic geological settings remain poorly understood.Here, we will combine reaction-transport models with IODP data to identify the critical combination of parameters that results in the venting of the most saline and iron-rich fluids at the TAG and Rainbow hydrothermal fields. When seawater is heated while cycling through a hydrothermal system, it may split into a dense and saline brine phase and a lighter low salinity vapor phase upon intersecting with the two-phase boundary, with the metals accumulating in the brine phase. Tracking the brine phase throughout the circulation system that feeds the vent sites is therefore one key to an improved assessment. Quantifying the re-deposition of Fe in hydrothermal minerals around the upflow zone is another. We propose to return to the unique ODP 158 data on the basaltic-hosted fault-controlled TAG hydrothermal system and to integrate it with reaction-transport models to provide new insights into brine formation and iron transport. This work will be complemented by a study on the ultramafic-hosted Rainbow hydrothermal field, for which an international effort of submitting an IODP drilling proposal is on the way. From these two case studies, we anticipate fundamental new insights into the key controlling factors of brine formation and hydrothermal iron export at slow-spreading ridges, which is now realized to be so important for global biogeochemical cycles.