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最重要的悬而未决的问题之一。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缩小。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，在上一次冰河时期(约20000年前)，南大洋深海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在很长一段时间内调节大气二氧化碳的因素是至关重要的。在这项研究中，我们将使用一种新的方法来重建海洋环流，方法是对海洋沉积物岩心的微观化石进行氧同位素分析。从高中到本科生到研究生，所有级别的学生都将参与这项研究。末次冰川盛期(约2万年前)地球气候的特点是大气二氧化碳水平较低，地表温度较低，全球海平面较低。大西洋环流的特点还可能是来自北大西洋的深水变浅，以及来自南大洋的深海侵入。我们对环流的理解主要是基于对微化石的碳同位素分析，这些分析表明，北大西洋的南方水源取代了北方的水源。南方水源主宰大西洋深处的图像已经成为末次冰盖的一个决定性特征，就像众所周知的二氧化碳、表面温度和海平面的变化一样熟悉。然而，最近，来自钕同位素的结果表明，北大西洋深处主要受到北方源水的影响，挑战了LGM环流的经典观点。拟议工作的目标是使用氧同位素作为循环示踪剂来确定这两种情况中哪一种最有可能是正确的。微体化石的氧同位素组成是保守的，因为它主要随着温度的变化而变化。一旦海洋表面的氧同位素比率被设定，它只会通过海洋环流和混合而改变。另一方面，微化石的碳同位素组成是非保守的，因为它还受到生物过程的影响，这些过程损害了其作为循环示踪剂的用途。这项拟议的工作将使古气候界受益，因为它将基于一种保守的水团示踪器为LGM大西洋创建第一个区段。氧同位素结果还将有助于其他环流指标的验证，并有助于改进海洋环流及其在碳循环中的作用的模型模拟。该奖项反映了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。

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.