Dynamics of a Snowball Earth ocean
Dynamics of a Snowball Earth ocean
复制标题
DOI：
10.1038/nature11894
复制
发表时间：
2013-03-07
期刊：
影响因子：
64.8
通讯作者：
Tziperman, Eli
中科院分区：
文献类型：
--
作者：
Ashkenazy, Yosef;Gildor, Hezi;Tziperman, Eli
地质证据表明，在新元古代（大约7.5亿至6.35亿年前），海洋冰至少两次延伸到赤道（1，2），激发了雪球地球假说，即地球被全球冰覆盖（3，4）。在可能的雪球地球气候中，海洋环流和混合过程将设定决定冰厚度的融化和冻结速率（5，6），将影响光合生物的生存（4，5，7 -9），并可能为地球化学和沉积学观测的解释提供重要的限制（4，10）。在这里，我们表明，在一个雪球地球，海洋会很好地混合和。其特征是动力环流（11），具有强烈的赤道纬向翻转环流、纬向赤道急流、发达的涡流场、强烈的沿岸上升流和对流混合。这与雪球地球情景（3）中通常预期的缓慢海洋形成对比，因为厚冰层覆盖使海洋与大气强迫隔绝。由于强烈的对流混合，海洋的温度、盐度和密度在垂直方向上是均匀的，或者在少数地方是弱分层的。我们的研究结果是基于一个耦合冰流和海洋环流的模型，并由大约一公里厚的全球冰盖下的弱地热热通量驱动。与现代海洋相比，雪球地球海洋具有更大的垂直混合率，以及海洋涡旋的水平混合。强大的环流和海岸上升导致大陆附近的融化速率比先前估计的大十倍（6，7）。虽然我们不能解决全球冰盖存在的争论（10，12，13），我们讨论的光合作用活动的营养供应和带状铁形成的影响。我们对海洋动力学的见解和限制可能有助于解决雪球地球的争议时，结合未来的地球化学和地质观测。
Geological evidence suggests that marine ice extended to the Equator at least twice during the Neoproterozoic era (about 750 to 635 million years ago)(1,2), inspiring the Snowball Earth hypothesis that the Earth was globally ice-covered(3,4). In a possible Snowball Earth climate, ocean circulation and mixing processes would have set the melting and freezing rates that determine ice thickness(5,6), would have influenced the survival of photosynthetic life(4,5,7-9), and may provide important constraints for the interpretation of geochemical and sedimentological observations(4,10). Here we show that in a Snowball Earth, the ocean would have been well mixed and. characterized by a dynamic circulation(11), with vigorous equatorial meridional overturning circulation, zonal equatorial jets, a well developed eddy field, strong coastal upwelling and convective mixing. This is in contrast to the sluggish ocean often expected in a Snowball Earth scenario(3) owing to the insulation of the ocean from atmospheric forcing by the thick ice cover. As a result of vigorous convective mixing, the ocean temperature, salinity and density were either uniform in the vertical direction or weakly stratified in a few locations. Our results are based on a model that couples ice flow and ocean circulation, and is driven by a weak geothermal heat flux under a global ice cover about a kilometre thick. Compared with the modern ocean, the Snowball Earth ocean had far larger vertical mixing rates, and comparable horizontal mixing by ocean eddies. The strong circulation and coastal upwellimg resulted in melting rates near continents as much as ten times larger than previously estimated(6,7). Although we cannot resolve the debate over the existence of global ice cover(10,12,13), we discuss the implications for the nutrient supply of photosynthetic activity and for banded iron formations. Our insights and constraints on ocean dynamics may help resolve the Snowball Earth controversy when combined with future geochemical and geological observations.