Ocean carbon cycling since the middle Miocene: testing the metabolic hypothesis

呼吸-有机物（食物）分解以提供能量，释放二氧化碳的过程-强烈依赖于温度。温度每升高10度，它的速度就快2.5倍。我们是呼吸的生物，但我们没有注意到这一点，因为我们的体温是受调节的，但冷血生物是这样，就全球进程而言，细菌和其他微生物也是如此。这就是为什么我们把食物放在冰箱里，也是为什么热带沼泽比温带沼泽更有生物活性。最近，地球系统科学家们逐渐意识到，如果我们要了解气候变化所涉及的一些重大全球反馈，就必须考虑微生物代谢的这种显著的温度依赖性，因此我们应该将其纳入地球系统计算机模型。有助于调节大气中二氧化碳含量的一个重要过程发生在海洋中，被称为“生物泵”。藻类在表面的透光区进行光合作用，形成食物链的基础。大部分有机物在表层被吃掉和呼吸，二氧化碳返回大气，但很大一部分下沉到更深的水中。大部分最终也会被细菌分解，但这里释放的二氧化碳与表面隔离。一些有机物质可以到达海底，在那里它可以被合并到沉积物中，形成未来的烃源岩。下沉到深海的有机物和沉积物形成的雨，产生了一个从大气到海洋的二氧化碳补偿“泵”。现在想象一下，由于气候变化，我们提高了水柱的温度。这对细菌来说是个好消息，因为它们能更有效地利用下沉的有机物。从海洋表面移除的碳较少，因此二氧化碳在大气中积累，直到恢复新的平衡。因为二氧化碳是一种重要的温室气体，当它在大气中时会导致全球变暖，这个过程理论上可能会加剧变暖过程，或者在一个冷却的星球上反过来工作。重要的是，我们要理解这种反馈在真实的世界中有多么重要，以及它在地球系统的其他部分可能产生的连锁反应。我们设计了一种研究地球过去的方法，使用海底的化石沉积物。我们计划采取一系列跨越过去1500万年的沉积物样本，以调查生物泵的效率。在此期间，地球已经显着冷却，因此我们预测海洋生态系统和生物泵的功能将发生重大变化。我们将研究分布在水柱中的有孔虫（大量出现的微观原生生物）化石外壳的化学成分。通过使用地球化学技术的组合，我们可以建立温度剖面，pH剖面和生物泵的强度。为了探索数据，我们将使用一个特别修改的版本的最先进的地球系统模型，将考虑到温度依赖性的代谢过程。然后，我们将使用该模型来研究其对一系列全球重要因素的影响，如有机碳埋葬模式和大气二氧化碳，并研究这些因素对未来气候变化的重要性。我们预测，在过去的1500万年里，全球变冷已经改善了深层生态位的氧合和食物供应。（所谓的海洋“黄昏区”），这将刺激深度的进化创新。我们将通过研究浮游生物在时间和空间上的深度丰度模式来测试这一想法，并调查在这种环境中是否有增强的进化。

Respiration - the process by which organic matter (food) is broken down to provide energy, releasing carbon dioxide - is strongly temperature-dependent. For every ten degrees increase in temperature, it occurs about 2 and a half times faster. We are respiring organisms but we don't notice this because our body temperatures are regulated, but cold-blooded creatures do, and so too do the most important respirers of all in terms of global processes - the bacteria and other microbes. This is why we put food in the fridge, and why a tropical swamp is a much more biologically active place than a temperate bog. Recently there has been a dawning realization among Earth System scientists that this marked temperature-dependency of microbial metabolism must be taken into account if we are to understand some of the big global feedbacks involved in climate change, and hence we should incorporate it into Earth System computer models.One important process that helps regulate the amount of CO2 in the atmosphere occurs in the ocean, and is called the 'biological pump'. Algae photosynthesize in the photic zone at the surface, forming the base of the food chain. Most of this organic matter gets eaten up and respired in the surface layer and the CO2 is returned to the atmosphere, but a substantial proportion sinks to deeper water. Most of it does, eventually, also get broken down by bacteria, but here the CO2 released is isolated from the surface. Some of the organic matter can reach the sea floor where it can be incorporated into sediments, forming the hydrocarbon source rocks of the future. The rain of organic matter sinking to the deep sea and sediments produces a compensatory 'pump' of CO2 from the atmosphere to the ocean. Now imagine we turn up the temperature in the water column as a result of climate change. This is good news for the bacteria which use up the sinking organic matter more efficiently. Less carbon gets removed from the surface ocean hence CO2 accumulates in the atmosphere until a new balance is restored. Because CO2 is an important greenhouse gas, contributing to global warming when it is in the atmosphere, this process could theoretically accentuate the warming process, or work the other way round on a cooling planet. It is important that we understand how important this feedback is in the real world, and what knock-on effects it may have in other parts of the Earth System. We have devised a way of studying it in the Earth's past, using fossil sediments from the sea floor. We plan to take a series of sediment samples spanning the last 15 million years across the oceans to investigate the efficiency of the biological pump. The planet has cooled markedly over this period so we predict major changes to the functioning of ocean ecosystems and the biological pump. We will study the chemical composition of fossil shells of foraminifera (microscopic protists that occur in large numbers) that lived distributed through the water column. By using a combination of geochemical techniques we can establish the temperature profile, pH profile, and strength of the biological pump.To explore the data we will use a specially modified version of a state-of-the-art Earth System Model that will take into account temperature-dependency of metabolic processes. We will then use the model to investigate its impact on a range of globally important factors such as patterns of organic carbon burial and atmospheric carbon dioxide, and investigate how important these factors are for future climate change.We predict that global cooling over the last 15 million years has produced improved oxygenation and food supply in deep planktonic niches (the so-called 'twilight zone' of the ocean) and that this would have spurred evolutionary innovation at depth. We will test this idea by studying plankton abundance patterns at depth in time and space and investigating whether there has been enhanced evolution in this environment.