Ocean circulation, nutrient cycling and atmospheric carbon dioxide

海洋的物理循环、海洋中营养物质的循环和深海中碳的储存之间的联系，对于理解温室气体大气二氧化碳(CO2)和地球气候过去和未来的变化至关重要。然而，目前尚不确定海洋营养循环和大气二氧化碳对全球海洋环流模式变化的敏感度有多高。营养物质通过海洋循环提供给阳光充足的表层海洋区域，然后生物利用这些营养物质生长，将碳储存在它们的软组织中。然后，这些生物中的一些会死亡并从表层海洋中掉出来，或者被其他生物吃掉，这些生物会排泄出它们的遗骸，这些遗骸也会从表面海洋中沉没。当这种物质下沉时，它被细菌消耗，细菌将其溶解，将其所含的碳和营养物质释放回深海。这种生物软组织从表层海洋到深层的运输，有助于将碳从大气中分离出来，使大气中的二氧化碳浓度保持在较低水平。尤其令人感兴趣的是表层海洋区域，在这些区域，供应的营养物质仍未被生物利用，这意味着深海中储存的碳较少。最大的这样的区域是南大洋，它在南极洲以北形成了一个环绕地球的环。如果全球海洋环流发生变化，使得更多的营养物质通过南大洋表层水域循环，那么总的来说，营养物质的生物利用效率将会降低，深海中储存的碳将减少，大气中的二氧化碳将会增加。相反，如果海洋环流发生变化，使得通过南大洋表层水域流通的营养物质减少，那么更多的营养物质将被利用，更多的碳将储存在深海中，大气中的二氧化碳将会下降。我们建议量化全球生物养分利用、深海碳储存和大气二氧化碳对海洋环流变化的敏感性(例如，对不同水团的形成速度)。我们将执行和分析多次运行的3D海洋数值模型，以调查和正式确定海洋循环、营养循环和通过下落的生物材料储存在深海中的碳量之间的联系。这些研究结果将被用来研究：(I)海洋环流的变化如何可能有助于冰河时代周期中大气二氧化碳的过去变化；(Ii)未来海洋环流的变化如何通过加速或减缓大气二氧化碳的增加来放大或抑制当前的人为气候压力；(Iii)通过试图提高部分表层海洋的生产力的“地球工程”提议，可以封存多少碳。

The link between the physical circulation of the ocean, the cycling of nutrients in the ocean, and the storage of carbon in the deep ocean, is fundamental to the understanding of past and future changes in the greenhouse gas atmospheric carbon dioxide (CO2) and the Earth's climate. However, it is currently uncertain how sensitive ocean nutrient cycling and atmospheric CO2 are to changes in global ocean circulation patterns. Nutrients are supplied to the sunlit regions of the surface ocean by ocean circulation, living organisms then use these nutrients to grow, storing carbon within their soft tissues. Some of these organisms then die and fall out of the surface ocean, or are eaten by other organisms, which excrete their remains that also sink out of the surface ocean. As this material sinks it is consumed by bacteria which 'dissolve' it, releasing the carbon and nutrients it contains back into the deep ocean. This transport of biological soft tissue, from the surface ocean to depth, helps to isolate carbon away from the atmosphere, keeping atmospheric carbon dioxide concentrations low. Of particular interest are areas of the surface ocean in which supplied nutrients remain unutilised by living organisms, meaning less carbon is stored in the deep ocean. The largest such area is the Southern Ocean, which forms a ring around the planet north of Antarctica. If global ocean circulation changes such that more nutrients are circulated through the surface waters of the Southern Ocean, then overall there will be less efficient biological utilisation of nutrients, less carbon will be stored in the deep ocean, and atmospheric CO2 will increase. Conversely, if ocean circulation changes such that fewer nutrients are circulated through the Southern Ocean surface waters, then more nutrients will be utilised, more carbon will be stored in the deep ocean and atmospheric CO2 will fall. We propose to quantify the sensitivity of global biological nutrient utilisation, deep ocean carbon storage and atmospheric CO2 to changes in ocean circulation (for example, to the rates of formation of different water masses). We shall perform and analyse multiple runs of a numerical 3D ocean model to investigate and formalise the link between ocean circulation, nutrient cycling and the amount of carbon stored in the deep ocean by falling biological material. These findings will then be used to examine: (i) How ocean circulation changes could have contributed to past changes in atmospheric CO2 during ice age cycles (ii) How ocean circulation changes in the future might amplify or dampen current anthropogenic climate forcing by accelerating or decelerating the increase in atmospheric CO2, (iii) How much carbon could be sequestered by 'geo-engineering' proposals to try and boost productivity in parts of the surface ocean.