Figure 1: Core locations, surface winds, excess P at 20-m depth and main surface currents.
可以被生物利用的或“被固定的”氮驱动着浮游植物的生产力及向深海的碳输出。但关于控制固氮的全球速度和空间分布的因素，仍有很多问题有待回答。现在，古生物地球化学数据显示，在过去160000年北大西洋所存在的一个23000年的固氮周期，也许可以通过过量磷的可获得性响应于区域海洋环流由轨道驱动的变化所发生的变化得到最好的解释。（生物谷 Bioon.com）
生物谷推荐的英文摘要
Nature doi:10.1038/nature12397
Changes in North Atlantic nitrogen fixation controlled by ocean circulation
Marietta Straub, Daniel M. Sigman, Haojia Ren, Alfredo Martínez-García, A. Nele Meckler, Mathis P. Hain & Gerald H. Haug
In the ocean, the chemical forms of nitrogen that are readily available for biological use (known collectively as ‘fixed’ nitrogen) fuel the global phytoplankton productivity that exports carbon to the deep ocean1, 2, 3. Accordingly, variation in the oceanic fixed nitrogen reservoir has been proposed as a cause of glacial–interglacial changes in atmospheric carbon dioxide concentration2, 3. Marine nitrogen fixation, which produces most of the ocean’s fixed nitrogen, is thought to be affected by multiple factors, including ocean temperature4 and the availability of iron2, 3, 5 and phosphorus6. Here we reconstruct changes in North Atlantic nitrogen fixation over the past 160,000 years from the shell-bound nitrogen isotope ratio (15N/14N) of planktonic foraminifera in Caribbean Sea sediments. The observed changes cannot be explained by reconstructed changes in temperature, the supply of (iron-bearing) dust or water column denitrification. We identify a strong, roughly 23,000-year cycle in nitrogen fixation and suggest that it is a response to orbitally driven changes in equatorial Atlantic upwelling7, which imports ‘excess’ phosphorus (phosphorus in stoichiometric excess of fixed nitrogen) into the tropical North Atlantic surface5, 6. In addition, we find that nitrogen fixation was reduced during glacial stages 6 and 4, when North Atlantic Deep Water had shoaled to become glacial North Atlantic intermediate water8, which isolated the Atlantic thermocline from excess phosphorus-rich mid-depth waters that today enter from the Southern Ocean. Although modern studies have yielded diverse views of the controls on nitrogen fixation1, 2, 4, 5, our palaeobiogeochemical data suggest that excess phosphorus is the master variable in the North Atlantic Ocean and indicate that the variations in its supply over the most recent glacial cycle were dominated by the response of regional ocean circulation to the orbital cycles.
