Dissolved Si isotope composition measured in water samples during METEOR cruises M77/3 and M77/4@en

Silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon (Si). In this study the distribution of the Si isotope composition of dissolved silicic acid (d30Si(OH)4) was analyzed in the water column of the Eastern Equatorial Pacific (EEP) where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Samples were collected at 7 stations along two meridional transects from the equator to 14°S at 85°50'W and 82°00'W off the Ecuadorian and Peruvian coast. Surface waters show a large range in isotope compositions d30Si(OH)4 (+2.2 per mil to +4.4 per mil) with the highest values found at the southernmost station at 14°S. This station also revealed the most depleted silicic acid concentrations (0.2 µmol/kg), which is a function of the high degree of Si utilization by diatoms and admixture with waters from highly productive areas. Samples within the upper water column and the OMZ at oxygen concentrations below 10 µmol/kg are characterized by a large range in d30Si(OH)4, which mainly reflects advection and mixing of different water masses, even though the highly dynamic hydrographic system of the upwelling area off Peru does not allow the identification of clear Si isotope signals for distinct water masses. Therefore we cannot rule out that also dissolution processes have an influence on the d30Si(OH)4 signature in the subsurface water column. Deep water masses (>2000 m) in the study area show a mean d30Si(OH)4 of +1.2±0.2 per mil, which is in agreement with previous studies from the eastern and central Pacific. Comparison of the new deep water data of this study and previously published data from the central Pacific and Southern Ocean reveal substantially higher d30Si(OH)4 values than deep water signatures from the North Pacific. As there is no clear correlation between d30Si(OH)4 and silicic acid concentrations in the entire data set the distribution of d30Si(OH)4 signatures in deep waters of the Pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct Si isotope signatures including Lower Circumpolar Deep Water (LCDW) and North Pacific Deep Water (NPDW).硅同位素是研究溶解硅（dissolved silicon, Si）循环的有力工具。本研究针对全球最大低氧区（Oxygen Minimum Zones, OMZs）之一所在的东赤道太平洋（Eastern Equatorial Pacific, EEP）水柱，分析了溶解硅酸的硅同位素组成（d30Si(OH)4）的分布特征。采样沿两条经向断面的7个站位开展，断面位于厄瓜多尔与秘鲁沿岸的西经85°50′与西经82°00′海域，从赤道延伸至南纬14°。表层水体的d30Si(OH)4同位素组成范围跨度较大（+2.2‰至+4.4‰），最高值出现在最南端的南纬14°站位。该站位同时呈现最低的硅酸浓度（0.2 μmol/kg），这一现象源于硅藻对硅的高强度利用，以及与高生产力海域水体的混合作用。上层水柱及氧浓度低于10 μmol/kg的低氧区内的样品，其d30Si(OH)4分布范围广泛，这主要反映了不同水团的平流输送与混合过程——尽管秘鲁沿岸上升流区极强的水文学动态性，使得无法识别出对应不同水团的明确硅同位素信号。因此，我们无法排除溶解过程对次表层水柱中d30Si(OH)4同位素特征的影响。研究区深层水体（水深>2000 m）的d30Si(OH)4平均值为+1.2±0.2‰，这与此前东太平洋及中太平洋的相关研究结果一致。将本研究获得的深层水体数据与此前发表的中太平洋、南大洋深层水体数据对比后发现，其d30Si(OH)4值显著高于北太平洋深层水体的同位素特征。鉴于整个数据集内d30Si(OH)4与硅酸浓度之间无明确相关性，太平洋深层水体的d30Si(OH)4分布特征主要被认为是若干具有特定硅同位素特征的端元水团混合的结果，这些端元水团包括环极深层下水（Lower Circumpolar Deep Water, LCDW）与北太平洋深层水（North Pacific Deep Water, NPDW）。