Tracking Southern Ocean sea ice extent and frontal positions: Novel techniques based on oxygen isotope and Mg/Ca analyses of foraminifera

了解冰川-间冰期二氧化碳(CO2)循环的驱动因素是古气候学领域最重要的问题之一。人们早就知道，地球的冰盖在大气中二氧化碳含量较低时会膨胀，当二氧化碳含量上升时会融化，但其潜在的气候机制仍不清楚。南极洲周围南大洋海冰覆盖面的扩大可能限制了诸如末次冰川盛期(LGM；约20,000年前)等自然寒冷时期深海碳的释放。更大的海冰范围和海洋锋面的北移可能也导致了深海中的碳储存。因此，南大洋可靠的古气候记录对于理解长时间尺度上对大气二氧化碳的控制至关重要。这项拟议的研究将开发新的方法来制作这种记录，使用来自海洋沉积物岩心的微观化石的化学分析。这项拟议工作的教育影响包括支持康涅狄格大学的一名研究生和一名本科生。拟议的工作将侧重于根据底栖和浮游有孔虫的氧同位素比率(D18O)开发新的海冰范围替代指标。南大洋的海冰形成以接近冰冻的表面条件和负浮力强迫为特征，这在d18O形成了一个独特的垂直剖面。来自已发表文献的初步数据表明，现代浮游有孔虫和海底有孔虫之间的d18O差异得出了与现代观测相一致的海冰范围估计。我们还计划开发一个新的副热带锋位置的代理，基于有孔虫的镁/钙(镁/钙)和d18O分析。同时对海表面温度和海水d18O的测量捕捉到了副热带锋面的位置，这意味着基于镁/钙和d18O的重建可以用于追踪过去地质历史中的锋面位置。受这些初步结果的启发，我们建议使用南大洋大西洋段从~55°S到35°S的断面来：1)估计末次冰盖期间的海冰范围和表面浮力强迫；2)重建末次冰盖期间副热带锋的位置，以限制大西洋和印度洋之间的盆地间交换。这项拟议的工作可能会通过在古气候工具包中增加两种新方法来改变我们对南大洋的理解。D18O海冰代理将补充现有的基于硅藻的方法，它可以用于硅藻稀少的地区。同样，副热带锋面技术的发展将为研究冰川消融奠定基础，届时将援引锋面位置的变化来解释大气中二氧化碳的上升。鉴于这些技术与一系列时间尺度上的研究问题的相关性，我们预计我们的结果将引起古气候学界的广泛兴趣。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Understanding the drivers of glacial-interglacial carbon dioxide (CO2) cycles is one of the most important questions in the field of paleoclimatology. It has long been known that Earth’s ice sheets expand when atmospheric CO2 is low and melt as CO2 rises, but the underlying climate mechanisms remain unclear. Expanded sea ice coverage in the Southern Ocean around Antarctica may have limited the release of carbon from the deep ocean during natural cold periods such as the Last Glacial Maximum (LGM; ~20,000 years ago). Greater sea ice extent and northward movement of oceanic fronts may have also led to carbon storage in the deep ocean. Reliable paleoclimate records from the Southern Ocean are thus essential to understanding the controls on atmospheric CO2 over long timescales. The proposed study will develop new methods to produce such records, using chemical analyses of microscopic fossils from marine sediment cores. The educational impact of the proposed work includes support for a graduate student and an undergraduate at the University of Connecticut. The proposed work will focus on developing a new proxy for sea ice extent based on the oxygen isotope ratios (d18O) of benthic and planktonic foraminifera. Sea ice formation in the Southern Ocean is characterized by near-freezing surface conditions and negative buoyancy forcing, which creates a unique vertical profile in d18O. Preliminary data from the published literature shows that the d18O difference between recent planktonic and benthic foraminifera yields an estimate of sea ice extent consistent with modern observations. We also plan to develop a novel proxy for subtropical front position based on foraminiferal magnesium/calcium (Mg/Ca) and d18O analyses. Contemporary measurements of sea-surface temperature and the d18O of seawater capture the position of the subtropical front, implying that reconstructions based on Mg/Ca and d18O can be used to track frontal position in the geologic past. Motivated by these initial results, we propose to use a transect of cores from ~55ºS to 35ºS in the Atlantic sector of the Southern Ocean to: 1) estimate sea ice extent and surface buoyancy forcing during the LGM, and 2) reconstruct the position of the subtropical front during the LGM to constrain inter-basin exchange between the Atlantic and Indian Oceans. The proposed work could transform our understanding of the Southern Ocean by adding two new methods to the paleoclimate toolkit. The d18O sea ice proxy will complement existing diatom-based methods and it can be used in regions where diatoms are sparse. Similarly, development of the subtropical front technique will lay the groundwork for studies of the deglaciation when changes in frontal position have been invoked to explain rising atmospheric CO2. Given the relevance of the techniques to research questions across a range of timescales, we anticipate our results will be of broad interest to the paleoclimate community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.