The subtropical nutrient spiral
The subtropical nutrient spiral
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DOI：
10.1029/2003gb002085
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发表时间：
2003-12-04
影响因子：
5.2
通讯作者：
Doney, SC
中科院分区：
文献类型：
--
作者：
Jenkins, WJ;Doney, SC
我们利用He-3通量计技术对百慕大附近马尾藻海的新产量进行了一系列扩展的观测和更全面的分析。估计的年平均硝酸盐通量为0.84+/-0.26 mol m(-2) yr(-1)，这仅构成了物理输送到光带的硝酸盐，而不是来自生物来源（例如固氮或浮游动物迁移）的氮。结果表明，通量估计值与其他观测值在数量上是一致的，包括主温跃层中H-3+He-3储量的十年时间尺度演变和出口产量估计值。然而，我们认为，从长期来看，通量不能由局部的等平过程或等平过程提供。这些考虑使我们提出了一个三维途径，即在主温跃层内再矿化的营养物质返回到亚热带环流内季节性可达的层。我们将这种机制称为“营养螺旋”，将其描述为一系列步骤，其中(1)营养丰富的温跃层水被带入墨西哥湾流，(2)增强的深环流混合将营养物质向上移动到密度较低的地方，(3)去除和增强的等环流混合将这些水注入到环流再循环区域的季节性可达层中，(4)通过涡流起伏和冬季对流将营养物质提供给生物群。当营养物质被利用、输出，然后在温跃层内再矿化时，螺旋关闭。我们提出了有关螺旋特征的证据，并讨论了其在副热带海洋生物地球化学循环中运作的一些含义。
We present an extended series of observations and more comprehensive analysis of a tracer-based measure of new production in the Sargasso Sea near Bermuda using the He-3 flux gauge technique. The estimated annually averaged nitrate flux of 0.84+/-0.26 mol m(-2) yr(-1) constitutes only that nitrate physically transported to the euphotic zone, not nitrogen from biological sources (e.g., nitrogen fixation or zooplankton migration). We show that the flux estimate is quantitatively consistent with other observations, including decade timescale evolution of the H-3+He-3 inventory in the main thermocline and export production estimates. However, we argue that the flux cannot be supplied in the long term by local diapycnal or isopycnal processes. These considerations lead us to propose a three-dimensional pathway whereby nutrients remineralized within the main thermocline are returned to the seasonally accessible layers within the subtropical gyre. We describe this mechanism, which we call "the nutrient spiral,'' as a sequence of steps where (1) nutrient-rich thermocline waters are entrained into the Gulf Stream, (2) enhanced diapycnal mixing moves nutrients upward onto lighter densities, (3) detrainment and enhanced isopycnal mixing injects these waters into the seasonally accessible layer of the gyre recirculation region, and (4) the nutrients become available to biota via eddy heaving and wintertime convection. The spiral is closed when nutrients are utilized, exported, and then remineralized within the thermocline. We present evidence regarding the characteristics of the spiral and discuss some implications of its operation within the biogeochemical cycle of the subtropical ocean.