Silicic acid uptake by marine diatoms and the biological pump of carbon during glacial periods: A case study in the eastern tropical Pacific

大气中二氧化碳浓度的变化对地球气候有着深远的影响。然而，这种变化的原因仍然鲜为人知。特别是，海洋在过去调节大气碳库中的基本作用仍有待明确评估。这些知识对于预测未来大气二氧化碳浓度和全球气候至关重要。目前提出的解释冰期二氧化碳含量较低的最合理假设涉及到碳的生物泵增加，特别是沉降颗粒中有机碳与无机碳的相对比例较高，即所谓的“降雨率比”。根据这一理论，增加降雨率比从而降低二氧化碳分压的一种方法是，通过增加向光带提供硅酸，使海洋表面的硅分泌多于分泌caco3的浮游植物。这一假设是基于这样的假设，即海洋碳和硅分泌生物对硅酸盐的吸收是耦合的，两者都取决于硅酸的有效性。然而，最近的原位和实验室实验表明，在铁限制条件下，铁的额外供应降低了硅质浮游植物对硅氮（和碳）的吸收比；也就是说产生了更多的有机物质来保存硅酸。富铁条件下硅酸保存导致蛋白石出口减少，有机碳和雨率比出口不变或增加。考虑到大量的沙尘携带的铁进入冰川海洋，基于蛋白石积累率的冰期-间冰期生物CO2泵强度变化解释目前是值得怀疑的。沉积物中硅藻（d30Si）的硅同位素信号可以与蛋白石和其他地球化学指标一起用于重建目前受铁限制的海洋地区冰期硅酸的保存/利用程度。东热带太平洋就是这样一个关键地区，那里存在大规模的铁限制，广泛的上升流和二氧化碳的排出，以及大的蛋白石积累率。我们建议将这一区域作为实验平台，并提出了加利福尼亚湾的初步数据，首次显示了千年尺度的d30Si，并证实硅藻对硅酸的吸收是控制蛋白石积累的重要因素。本文提出的研究解决了沉积物中蛋白石的产生和积累及其与降雨率比和碳的生物泵的关系的两个重要问题。(1)通过重建上升流区d30Si和蛋白石的富集，评估硅藻生长过程中的相对Si利用是否是沉积记录中蛋白石富集的重要控制因素。(2)通过对赤道东太平洋硅酸相对利用的天气重建，我们将评估该地区是否对降低冰川二氧化碳分压做出了贡献

Variations in atmospheric CO2 concentrations have a profound impact on earth's climate. However, the causes of such changes are still poorly known. In particular, the fundamental role of the ocean in modulating the atmospheric carbon reservoir in the past still has to be clearly assessed. This knowledge is essential for predicting atmospheric CO2 concentrations and global climate in the future. The most plausible hypothesis currently put forward to explain lower CO2 during Glacial periods involves an increased biological pump of carbon and in particular higher relative proportion of organic carbon to inorganic carbon in settling particles, the so-called 'rain rate ratio'. According to this theory, one way to increase the rain rate ratio, and hence decrease pCO2, is to favour Si-secreting over CaCO3-secreting phytoplankton in the surface ocean by increasing the supply of silicic acid to the euphotic zone. This hypothesis is based on the assumption that marine carbon and silicate uptake by Si-secreting organisms are coupled and both depend on silicic acid availability. However, recent in situ and laboratory experiments have shown that under iron-limited conditions, additional supply of Fe decreases the Si to N (and C) uptake ratio by siliceous phytoplankton; that is more organic matter is produced conserving silicic acid. Silicic acid conservation under Fe replete conditions could lead to lower export of opal but unchanged or increased export of organic carbon and rain rate ratio. Given the much larger input of dust-borne Fe to glacial ocean, opal accumulation rate based interpretations of glacial-interglacial change in the intensity of the biological CO2 pump are currently suspect. The silicon isotopic signal of diatoms (d30Si), in sediments could be used along with opal and other geochemical proxies to reconstruct the degree to which silicic acid is conserved/utilised During glacial period in areas of the ocean currently limited by Fe. The Eastern Tropical Pacific is one such key area where there is large-scale limitation of Fe, widespread upwelling an out gassing of CO2 and large opal accumulation rates. We propose to use this area as a test bed and present preliminary data from the Gulf of California which shows for the first time millennial scale d30Si and confirms that silicic acid uptake by diatoms is an important factor controlling opal accumulation. The proposed study addresses two important questions surrounding opal production and accumulation in sediments and its relationship to rain rate ratio and the biological pump of carbon. (1) By reconstructing d30Si and opal accumulation in upwelling regions we will assess whether or not relative Si utilisation during diatom growth is an important control on opal accumulation in sedimentary records. (2) Through synoptic reconstruction Silicic acid relative utilisation in the Eastern Equatorial Pacific we will gauge if this regions contribution to lower glacial PCO2