OPP-PRF Investigating the influence of ocean temperature on Antarctic Ice Sheet evolution during the early to middle Pleistocene

南极冰盖的稳定性在预测未来海平面方面仍然存在很大的不确定性。目前，在相对温暖的水与冰川和冰架接触并从下面融化它们的地方，观察到最大的冰质量损失。这使得研究人员推测，海洋和冰之间的相互作用对决定冰盖的稳定性很重要，而暖水的增加将加速南极冰盖的损失，并导致下个世纪海平面的进一步上升。为了更好地预测未来冰盖的行为，了解南极洲周围过去冰与海洋的相互作用是至关重要的，尤其是在温暖时期和地球气候发生重大变化的时候。过去的南极冰块和海洋温度等环境条件可以利用沉积物重建，沉积物在海底积累时捕捉到环境记录。通过观察沉积物组成和分析沉积物中的地球化学特征，有可能拼凑出一份数亿年时间尺度上的气候变化记录。该项目将重建260万至70万年前的海洋上层温度和南极冰川后退/前进周期，其中包括中更新世过渡期，这是地球历史上标志着从4.1万年冰期周期向10万年冰期周期转变的时期。国际海洋探索计划第382次探险期间从斯科舍海收集的现有沉积物岩心将产生记录。中更新世过渡（MPT; ~1.25 ~ 0.7 Ma）标志着以倾角为主导的冰期-间冰期旋回（~41 kyr旋回）向以偏心率为主导的冰期-间冰期旋回（~100 kyr旋回）的转变。这种转变发生在地球轨道参数变化不大的情况下，表明内部气候反馈的作用。MPT伴随着大气pCO2的降低、深海碳储量的增加以及深水形成和分布的变化，这些都与南极边缘大气-冰-海洋相互作用有关。然而，由于南极冰盖（AIS）的反复推进，记录这种相互作用的更新世记录很少被保存在大陆架上；相反，它们被保存在南大洋盆地深处。本项目利用国际海洋发现计划（International Ocean Discovery Program）第382次探险期间从斯科舍海收集的连续更新世沉积物序列的良好保存和恢复，验证了以下假设：1)早更新世至中更新世（2.6-0.7 Ma）南大洋上层海洋温度在轨道时间尺度上的变化；2)南大洋温度与AIS进退周期共变。古温度将使用86碳的四醚指数（TEX86）重建，这是一种利用海洋古细菌生物标志物的代理。基于tex86的斯科舍海古温度记录将与AIS变动性记录进行比较，包括浮冰碎片。第382次考察队记录将与轨道定速气候时间序列和全球冰量和底部水温的底栖氧同位素记录进行比较，以确定上层海洋温度、AIS后退/前进和轨道气候强迫之间是否存在相关性。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。

Antarctic Ice Sheet stability remains a large uncertainty in predicting future sea level. Presently, the greatest ice mass loss is observed in locations where relatively warm water comes into contact with glaciers and ice shelves, melting them from below. This has led researchers to hypothesize that the interactions that occur between the ocean and the ice are important for determining ice sheet stability and that increased warm water presence will accelerate Antarctic ice mass loss and lead to greater sea level rise in the coming century. To better predict future ice sheet behavior, it is critical to understand past ice-ocean interactions around Antarctica, especially during warm periods and at times when Earth’s climate was undergoing major changes. Past Antarctic ice mass and environmental conditions like ocean temperature can be reconstructed using sediments, which capture an environmental record as they accumulate on the ocean floor. By looking at sediment composition and by analyzing geochemical signatures within the sediment, it is possible to piece together a record of climate change on hundred- to million-year timescales. This project will reconstruct upper ocean temperatures and Antarctic ice retreat/advance cycles from 2.6 to 0.7 million years ago, which encompasses the Mid-Pleistocene Transition, a time in Earth’s history that marks the shift from 41-thousand year glacial cycles to 100-thousand year glacial cycles. A record will be generated from existing sediment cores collected from the Scotia Sea during International Ocean Discovery Program Expedition 382. The Mid-Pleistocene Transition (MPT; ~1.25–0.7 Ma) marks the shift from glacial-interglacial cycles paced by obliquity (~41 kyr cycles) to those paced by eccentricity (~100-kyr cycles). This transition occurred despite little variation in Earth’s orbital parameters, suggesting a role for internal climate feedbacks. The MPT was accompanied by decreasing atmospheric pCO2, increasing deep ocean carbon storage, and changes in deep water formation and distribution, all of which are linked to Antarctic margin atmosphere-ice-ocean interactions. However, Pleistocene records that document such interactions are rarely preserved on the shelf due to repeated Antarctic Ice Sheet (AIS) advance; instead, they are preserved in deep Southern Ocean basins. This project takes advantage of the excellent preservation and recovery of continuous Pleistocene sediment sequences collected from the Scotia Sea during International Ocean Discovery Program Expedition 382 to test the following hypotheses: 1) Southern Ocean upper ocean temperatures vary on orbital timescales during the early to middle Pleistocene (2.6–0.7 Ma), and 2) Southern Ocean temperatures co-vary with AIS advance/retreat cycles. Paleotemperatures will be reconstructed using the TetraEther indeX of 86 carbons (TEX86), a proxy that utilizes marine archaeal biomarkers. The Scotia Sea TEX86-based paleotemperature record will be compared to records of AIS variability, including ice rafted debris. Expedition 382 records will be compared to orbitally paced climatic time series and the benthic oxygen isotope record of global ice volume and bottom water temperature to determine if a correlation exists between upper ocean temperature, AIS retreat/advance, and orbital climate forcing.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.