Distinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate
Distinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate
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DOI：
10.1029/2002gc000384
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发表时间：
2003-05
期刊：
影响因子：
3.7
通讯作者：
D. Sigman;Rebecca S. Robinson;Angela N. Knapp;A. Geen;D. McCorkle;Jay A. Brandes;R. Thunell
中科院分区：
文献类型：
--
作者：
D. Sigman;Rebecca S. Robinson;Angela N. Knapp;A. Geen;D. McCorkle;Jay A. Brandes;R. Thunell
在其岩床深度以下，圣巴巴拉盆地（SBB）通常处于低氧状态（[O2] ≤ 3 μM），只有短暂的通风期。与亚低氧的发展，硝酸盐的浓度随深度下降到盆地没有相关的减少磷酸盐，这表明供应到盆地的硝酸盐的相当大的一部分被去除反硝化。与SBB内的“氧化还原层”（好氧和亚氧沃茨之间的界面）中硝酸盐浓度的降低相一致，硝酸盐的15 N/14 N增加，正如从与反硝化相关的同位素分馏所预期的那样。然而，在15 N/14 N的硝酸盐的增加是远远小于发生在开放的热带北太平洋东部（ETNP）的硝酸盐损失量相当。盆地内的N物质浓度和硝酸盐18 O/16 O的测量结果都表明，低于预期的15 N富集在亚氧SBB中涉及反硝化作用，而不是由于一些未知的低15 N/14 N N到深层SBB的来源。硝酸盐供应和消费的一系列模型的计算表明，在盆地中的硝酸盐消费的程度是太小的水循环的差异来解释圣巴巴拉盆地和开放的ETNP之间的同位素差异。以往的研究表明，由于硝酸盐在沉积物孔隙沃茨中的扩散，沉积物反硝化作用的同位素效应可以忽略不计。因此，我们推断，小幅度的同位素富集的SBB水柱硝酸盐是由于在盆地内的沉积反硝化作用的重要性。假设水柱和沉积反硝化作用的同位素效应分别为每密耳25和1.5，我们的研究结果表明，沉积反硝化作用占亚氧SBB内硝酸盐损失的75%以上。
Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ∼ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the “redoxcline” (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15N/14N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15N/14N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18O/16O suggest that the lower‐than‐expected 15N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low‐15N/14N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.