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

跟踪南大洋海冰范围和锋面位置：基于氧同位素和有孔虫 Mg/Ca 分析的新技术

• 批准号: 2002425
• 负责人: David Lund
• 金额: $ 39.73万
• 依托单位: University of Connecticut
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-06-01 至 2024-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2002425&HistoricalAwards=false
• 关键词: Tracking; Southern; Ocean; sea; ice

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.

了解冰期-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，当大气中二氧化碳含量较低时，地球冰盖会膨胀，并随着二氧化碳含量的上升而融化，但潜在的原因却是。气候机制仍不清楚，南极洲周围南大洋海冰覆盖范围的扩大可能限制了末次盛冰期等自然寒冷时期深海碳的释放。约20,000年前）。因此，南大洋可靠的古气候记录对于了解长期尺度上对大气二氧化碳的控制至关重要。研究将开发新的方法来产生这样的记录，利用海洋沉积物岩心的微观化石的化学分析。拟议工作的教育影响包括支持康涅狄格大学的研究生和本科生。基于底栖和浮游有孔虫的氧同位素比（d18O）开发新的海冰范围替代方法。南大洋海冰形成的特点是接近冰冻的表面条件和负浮力强迫，从而形成了独特的垂直剖面。已发表文献的初步数据表明，近期浮游和底栖有孔虫之间的 d18O 差异可以估算出海冰范围。我们还计划根据有孔虫镁/钙 (Mg/Ca) 和 d18O 分析开发一种新的副热带锋位置代理，以捕获副热带位置。锋面，这意味着基于 Mg/Ca 和 d18O 的重建可用于跟踪地质过去的锋面位置。受这些初步结果的启发，我们建议使用样线。南大洋大西洋区~55°S 至 35°S 的核心，以：1) 估计末次盛冰期期间的海冰范围和表面浮力强迫，以及 2) 重建末次盛冰期期间副热带锋的位置，以限制之间的流域交换拟议的工作可以通过在古气候工具包中添加两种新方法来改变我们对南大洋的认识。d18O 海冰代理将补充现有的基于硅藻的方法。同样，考虑到副热带锋面技术与大气中二氧化碳浓度上升的相关性，副热带锋面技术的发展将为研究冰消作用奠定基础。跨越一系列时间尺度的研究问题，我们预计我们的结果将引起古气候界的广泛兴趣。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响进行评估，被认为值得支持审查标准。

项目成果

Tracking Southern Ocean Sea Ice Extent With Winter Water: A New Method Based on the Oxygen Isotopic Signature of Foraminifera

用冬季水追踪南大洋海冰范围：一种基于有孔虫氧同位素特征的新方法

• DOI: 10.1029/2020pa004095
• 发表时间: 2021
• 期刊: Paleoceanography and Paleoclimatology
• 影响因子: 3.5
• 作者: Lund, David C.;Chase, Zanna;Kohfeld, Karen E.;Wilson, Earle A.
• 通讯作者: Wilson, Earle A.