Coupled radiocarbon and neodymium isotopes: Improved reconstructions of deep-ocean circulation change

温室气体（如二氧化碳、CO2）进入大气所导致的气候变化是21世纪及以后人类面临的最重大和最紧迫的挑战之一。多年来，科学家们一直在开发海洋-大气系统的模型，目的是预测二氧化碳排放增加将如何影响我们的气候。然而，在有信心地预测未来的气候行为之前，有必要不仅调查过去的气候行为，还要调查二氧化碳在各种自然储存库之间的移动情况。利用主要来自海洋沉积物的信息，气候科学家已经确定，在过去的几百万年里，地球的气候经历了巨大的变化，从类似于今天的温暖气候到北半球大部分地区被冰覆盖的“冰川”气候。这些冰期-间冰期旋回是由入射太阳辐射变化引起的外部强迫以及大气中温室气体CO2浓度变化引起的内部反馈引起的。虽然人们知道二氧化碳对过去的气候变化很重要，但我们仍然不知道二氧化碳水平是如何或为什么会发生这样的变化。气候科学家利用宇宙辐射在高层大气中产生的放射性碳（14C）来了解二氧化碳是如何以及何时在深海和大气之间转移的。深海是地球上千年时间尺度上最大的二氧化碳储存库。作为深海通风的代表，14C非常有用，但其解释具有挑战性。一种合适的“染料示踪剂”可以极大地增强这种有价值的代用物的效用，这种示踪剂可以揭示深水团块的来源和混合情况，这两者都会影响14C数据的解释。在这里，我们建议使用一种新开发的、可能非常强大的水团来源和循环示踪剂，钕（Nd）同位素。最近的一项开创性研究表明，底栖有孔虫是过去海水Nd同位素组成的有用档案，这代表了一个非常有前途的进一步发展途径。我们的目标是进行必要的研究，以充分证明这种示踪剂的有效性，特别是与14C结合使用时。初步调查之后将进行两个案例研究，将这种令人兴奋的新示踪剂与两个深海沉积物岩心的14C数据结合起来。我们的目标是利用这些数据来了解在过去的2万年里，14C（以及二氧化碳）是如何在大气和深海之间移动的，以及这是如何受到深海环流模式的影响的。

Climate change resulting from the introduction of greenhouse gases (e.g. carbon dioxide, CO2) into the atmosphere is one of the most significant and pressing challenges facing mankind in the 21st century and beyond. For years, scientists have been developing models of the ocean-atmosphere system, with the aim of predicting how increased CO2 emissions will affect our climate. Before future climatic behaviour can be predicted with any confidence, however, it is necessary to investigate not only how the climate has behaved in the past but also how CO2 has moved between the various natural reservoirs. Using information primarily derived from marine sediments, climate scientists have ascertained that for the last few million years the Earth's climate has undergone dramatic changes, from warm conditions similar to the present day to 'glacial' conditions, when much of the northern hemisphere was covered with ice. These glacial-interglacial cycles are brought about through external forcing by variations in incident solar radiation combined with internal feedbacks due to changes in the atmospheric concentration of the greenhouse gas CO2. Although CO2 is known to be important for past climate change, we still do not know exactly how or why CO2 levels have changed as they have. Climate scientists use radiocarbon (14C), which is produced in the upper atmosphere by cosmic radiation, to understand how and when CO2 has been transferred between the deep ocean, which is the Earth's largest reservoir of CO2 on millennial timescales, and the atmosphere. As a proxy for deep ocean ventilation 14C is incredibly useful but its interpretation is challenging. The utility of this valuable proxy can be greatly enhanced by a suitable 'dye tracer' that can reveal the sourcing and mixing of deep-water masses, both of which can impact the interpretation of 14C data. Here we propose to employ a newly-developed and potentially very powerful tracer of water mass sourcing and circulation, neodymium (Nd) isotopes. A recent pioneering study has demonstrated that benthic foraminifera are a useful archive of past seawater Nd isotope composition, and that this represents a very promising avenue for further development. We aim to conduct the necessary research to fully demonstrate just how effective this tracer can be, particularly when combined with 14C. An initial investigation will be followed by two case studies that will combine this exciting new tracer with 14C data from two deep-sea sediment cores. We aim to use these data to understand how 14C (and hence CO2) has moved between the atmosphere and the deep oceans over the last 20,000 years, and how this has been influenced by deep ocean circulatory patterns.