Was the deep Atlantic dominated by southern source waters during the LGM? A conservative view based on the oxygen isotopic ratio of benthic foraminifera

冰期-间冰期大气CO2循环的成因一直是古气候学领域未解决的重要问题之一。人们早就知道，当大气中二氧化碳含量低时，地球的冰盖会膨胀，当二氧化碳含量上升时，冰盖会收缩。然而，调节二氧化碳水平的潜在气候机制仍不清楚。一种可能性是海洋环流的变化影响了海气交换。据推测，在最后一个冰河时期（~2万年前），南大洋深水的扩张限制了碳的释放。因此，重建过去的海洋环流对于理解是什么在长时间内调节大气中的二氧化碳至关重要。在这项研究中，我们将采用一种新的方法，利用海洋沉积物岩心中微观化石的氧同位素分析来重建海洋环流。各个层次的学生，从高中到本科再到研究生，都将参与这项研究。末次盛冰期（LGM ~ 20000年前）的地球气候特征是大气CO2浓度较低，地表温度较低，全球海平面较低。大西洋环流也可能以北大西洋发出的深水浅滩和南大洋的深海侵入为特征。我们对环流的认识主要基于微化石的碳同位素分析，表明北大西洋南部的水源取代了北部的水源。主宰大西洋深处的南部水源的图像已经成为LGM的一个定义特征，就像众所周知的二氧化碳分压、表面温度和海平面的变化一样熟悉。然而，最近钕同位素的结果表明北大西洋深处主要受到北部水源的影响，挑战了LGM环流的权威观点。这项工作的目标是利用氧同位素作为循环示踪剂，确定这两种情况中哪一种最可能是正确的。微化石的氧同位素组成是保守的，因为它主要随温度的变化而变化。一旦在海面上确定了氧同位素比率，它就只能通过海洋环流和混合来改变。另一方面，微化石的碳同位素组成是非保守的，因为它也受到生物过程的影响，损害了它作为循环示踪剂的效用。根据一种保守的水团示踪剂，提出的工作将通过创建LGM大西洋的第一个部分，使古气候界受益。氧同位素结果还将有助于验证其他环流代用物，并改进海洋环流及其在碳循环中的作用的模式模拟。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The cause of glacial-interglacial atmospheric CO2 cycles remains one of the most important unresolved questions in the field of paleoclimatology. It has long been known that Earth’s ice sheets expand when atmospheric CO2 is low and shrink when CO2 rises. The underlying climate mechanisms that regulate CO2 levels however, remain unclear. One possibility is that changes in ocean circulation influence air-sea gas exchange. It has been hypothesized that expansion of deep waters from the Southern Ocean limited the release of carbon during the last ice age (~20,000 years ago). Reconstructing ocean circulation in the past is therefore essential to understanding what regulates atmospheric CO2 over long time periods. In this study, we will use a new method to reconstruct the ocean circulation using oxygen isotope analyses of microscopic fossils from marine sediment cores. Students from all levels, high school to undergraduate to graduate, will participate in this research.Earth’s climate during the LGM (Last Glacial Maximum ~20,000 years ago) was characterized by lower atmospheric CO2 levels, cooler surface temperatures, and lower global sea level. The Atlantic Ocean circulation was also likely marked by shoaling of deep waters that emanate from the North Atlantic and incursion of abyssal waters from the Southern Ocean. Our understanding of the circulation is based mainly on carbon isotope analyses of microfossils that suggest southern source waters replaced northern source waters in the North Atlantic. The image of southern source waters dominating the deep Atlantic has become a defining feature of the LGM, as familiar as well-known changes in pCO2, surface temperature, and sea level. More recently, however, results from neodymium isotopes imply the deep North Atlantic was influenced primarily by northern source waters, challenging the canonical view of the LGM circulation. The goal of the proposed work is to determine which of these two scenarios is most likely correct using oxygen isotopes as a circulation tracer. The oxygen isotopic composition of microfossils is conservative because it varies mainly as a function of temperature. Once the oxygen isotope ratio is set at the sea surface, it changes only through ocean circulation and mixing. The carbon isotope composition of microfossils, on the other hand, is non-conservative because it is also influenced by biological processes that compromise its utility as a circulation tracer. The proposed work will benefit the paleoclimate community by creating the first section for the LGM Atlantic based on a conservative water mass tracer. The oxygen isotope results will also facilitate validation of other circulation proxies and lead to improved model simulations of the ocean circulation and its role in the carbon cycle.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.