Constraints on oceanic N balance/imbalance from sedimentary 15N records
Constraints on oceanic N balance/imbalance from sedimentary 15N records
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DOI：
10.5194/bg-4-75-2007
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发表时间：
2007-01-01
期刊：
影响因子：
4.9
通讯作者：
Altabet, M. A.
中科院分区：
文献类型：
--
作者：
Altabet, M. A.
根据目前的最佳估计，现代海洋的氮循环严重不足。 N同位素预算在这方面提供了独立的地球化学约束，也是过去重建的唯一手段。总体而言，水柱反硝化消耗的 N-2 固定相对比例决定了稳态条件下海洋三角洲平均 N-15 的大小。几个因素（有机氮转化为氮、瑞利封闭和开放系统效应）可能会将水柱反硝化的有效分馏因子 (epsilon) 降低到 epsilon(den) 固有微生物值的一半左右。如果是这样，则平均海洋三角洲 N-15 接近 5%，这与水柱反硝化作用占 N2 固定源通量的 50% 的典型贡献是一致的。如果海洋氮源和汇的不平衡改变了这一比例，那么平均海洋三角洲 N-15 就会发生短暂的变化。使用一个简单的模型，将水柱反硝化作用改变+/- 30%或将N-2固定改变+/- 15%，在一个或更长时间的停留时间段内，平均海洋三角洲N-15会产生可检测的（> 1%）变化，同时海洋氮库存也会发生相应的变化。改变沉积反硝化不会导致 Delta N-15 发生变化，但会改变氮库存。来自被认为对海洋平均 Delta N-15 敏感的地点的沉积物 Delta N-15 记录均显示在过去 3 年前左右没有可检测到的变化，这意味着在最新的全新世期间海洋 N 预算保持平衡。时间尺度的不匹配是对与现代通量估计的明显冲突最有可能有意义的解释。净氮赤字和净盈余之间可能会出现十年到百年尺度的振荡，但在数千年的氮停留时间尺度上，总体上实现了净平衡。然而，文献中的沉积物三角洲 N-15 记录涵盖了自末次盛冰期以来的时期，显示出高达千分之几的偏移，这与冰消期期间持续的氮缺乏相一致，随后在全新世早期进行了重新调整和建立平衡。由于不平衡持续了一个氮停留时间或更长时间，海洋氮库存可能发生 10% 至 30% 的偏移。这一时期发生的气候和海洋变化显然一度克服了海洋生物地球化学维持氮平衡的能力。
According to current best estimates, the modern ocean's N cycle is in severe deficit. N isotope budgeting provides an independent geochemical constraint in this regard as well as the only means for past reconstruction. Overall, it is the relative proportion of N-2 fixation consumed by water column denitrification that sets average oceanic delta N-15 under steady-state conditions. Several factors ( conversion of organic N to N2, Rayleigh closed and open system effects) likely reduce the effective fractionation factor (epsilon) for water column denitrification to about half the inherent microbial value for epsilon(den). If so, the average oceanic delta N-15 of similar to 5% is consistent with a canonical contribution from water column denitrification of 50% of the source flux from N2 fixation. If an imbalance in oceanic N sources and sinks changes this proportion then a transient in average oceanic delta N-15 would occur. Using a simple model, changing water column denitrification by +/- 30% or N-2 fixation by +/- 15% produces detectable (> 1%) changes in average oceanic delta N-15 over one residence time period or more with corresponding changes in oceanic N inventory. Changing sedimentary denitrification produces no change in delta N-15 but does change N inventory.Sediment delta N-15 records from sites thought to be sensitive to oceanic average delta N-15 all show no detectible change over the last 3 kyr or so implying a balanced marine N budget over the latest Holocene. A mismatch in time scales is the most likely meaningful interpretation of the apparent conflict with modern flux estimates. Decadal to centennial scale oscillations between net N deficit and net surplus may occur but on the N residence timescale of several thousand years, net balance is achieved in sum. However, sediment delta N-15 records from the literature covering the period since the last glacial maximum show excursions of up to several parts per thousand that are consistent with sustained N deficit during the deglaciation followed by readjustment and establishment of balance in the early Holocene. Since imbalance was sustained for one N residence time period or longer, excursions in ocean N inventory of 10 to 30% likely occurred. The climatic and oceanographic changes that occurred over this period evidently overcame, for a time, the capacity of ocean biogeochemistry to maintain N balance.