Ventilation History of the Sulu Sea: Record of Abrupt Changes in the Tropical Western Pacific Thermocline

在未来几个世纪，高纬度冰盖迅速融化对海平面和海洋环流的潜在影响日益引起人们的关注。有证据表明，末次盛冰期的大冰原并没有以统一的速度融化，而是以一到两个被称为融水脉冲1A （MWP-1A； ~ 14600 - 13500年BP）和1B （MWP-1B； ~ 11450 - 11000年BP）的突然融化步骤崩解得最为迅速。MWP-1A在几个地方都有记录，但关于MWP-1B是否存在，以及在这段时间内海平面上升了多少，仍然存在争议。来自苏禄海盆地的初步数据表明，在MWP-1B的几个世纪里，地下海洋迅速变暖和变新鲜。这些结果表明，西太平洋发生了广泛的海洋学变化，但尚不清楚这种变化是否与海冰融化导致的海平面上升有关，还是与海洋气候系统中的其他变化有关。这项研究将利用苏禄海的沉积物档案，进一步了解与MWP-1B事件相关的海洋变化的起源，并记录在过去39.5万年（跨越三个冰期-间冰期旋回）中是否发生过类似的事件。该产品将是苏禄海温跃层条件和海平面相关通气史的一个新的独特的重建。更好地了解西热带太平洋温跃层的通气历史以及与高纬度冰盖融化事件的关系，将有助于更准确地预测未来的变化。该项目的更广泛影响包括支持博士生，将结果纳入教学，以及继续开展公共宣传活动。本项目将利用深海浮游生物和底栖有孔虫的地球化学测量，重建热带西太平洋苏禄海的温跃层和深海盆通风事件的时间。早期对苏禄海浅层有孔虫的地球化学研究提供了过去39.5万年中千年尺度的海洋表面变化的详细记录。苏禄海沉积物岩心MWP-1B段温跃层有孔虫Globorotalia tumida的初步氧同位素数据表明，三角洲- 18o的减少幅度大于先前测量的表层物种。这一意想不到的结果提出了以下问题：(1)这是冰川融水涌入热带太平洋温跃层的证据，还是与MWP-1B同时发生的亚洲季风相关信号？(2)如果这是一个冰盖融水信号，为什么在温跃层的响应比在地表的响应大？(3) MWP-1B期间温跃层性质的变化是否与海平面快速上升的脉冲有关，还是与西太平洋表层和温跃层水的西边界流平流变化有关？为了解决这些问题和其他问题，本项目将在苏禄海沉积岩心MD97-2141的全新世至海洋同位素阶段6 （~130,000 BP）的样品中，以高分辨率分析G. tumida（钙化~160-200 m）上的三角洲- 18o，并跨越冰期-间冰期过渡至~395,000 BP。这些数据将与选定间隔的Mg/Ca测量数据配对，以分离水温对有孔虫delta-18O测量变化的贡献。该项目还将利用在700米左右钙化的浮游有孔虫gloorotalia crasformis和底栖有孔虫Oridorsalis umbonatus的分析来评估苏禄海深层通风事件的时间。

The potential impacts of rapid high-latitude ice sheet melting on sea level and oceanographic circulation in the coming several centuries are of increasing concern. There is evidence that the great ice sheets of the last glacial maximum did not melt at a uniform rate, but instead disintegrated most rapidly in one or two abrupt melting steps known as Meltwater Pulses 1A (MWP-1A; ~14,600-13,500 years BP) and 1B (MWP-1B; ~11,450 to 11,000 years BP). MWP-1A has been documented in several locations, but there remains controversy over whether deglacial MWP-1B existed and how much sea level rose across that time interval. Preliminary data from the Sulu Sea basin indicate that the subsurface ocean rapidly warmed and freshened during the several centuries of MWP-1B. These results suggest wide-spread oceanographic changes in the western Pacific but it is not known whether this change is related to sea level rise driven by melting ice or some other change in the ocean-climate system. This research will utilize the sediment archive in the Sulu Sea to further understand the origin of the ocean changes tied to the MWP-1B event and document whether similar events have occurred over the last 395,000 years, a period spanning three glacial-interglacial cycles. The product will be a new and unique reconstruction of thermocline conditions in Sulu Sea and sea level related ventilation history. Developing a better understanding of the ventilation history of the western tropical Pacific thermocline and the relationship to high-latitude ice sheet melting events will facilitate more accurate predictions about future changes. Broader impacts of the project include support for a Ph.D. student, incorporation of results into teaching, and continued practice of public outreach activities.This project will reconstruct the timing of thermocline and deep basin ventilation events in the Sulu Sea in the western tropical Pacific using geochemical measurements of deep dwelling planktonic and benthic foraminifera. Earlier geochemical work on shallow dwelling foraminifera in the Sulu Sea has provided detailed records of millennial-scale surface ocean change over the last 395,000 years. Preliminary oxygen isotope data from the thermocline dwelling foraminifer Globorotalia tumida across the MWP-1B interval in a Sulu Sea sediment core indicates a larger decrease in delta-18O than in the surface dwelling species previously measured. This unexpected result raises the following questions: (1) is this evidence of glacial meltwater influx into the tropical Pacific thermocline or an Asian monsoon related signal at the same time as MWP-1B? (2) if this is an ice sheet meltwater signal, why is the response larger in the thermocline than at the surface? (3) is this change in thermocline properties at the time of MWP-1B related to pulses of rapid sea level rise or to changes in the western boundary current advection of surface and thermocline waters into the western Pacific? To address these and other questions this project will analyze delta-18O on G. tumida (calcifies ~160-200 m) at high resolution in samples from Sulu Sea sediment core MD97-2141 spanning the Holocene to Marine Isotope Stage 6 (~130,000 years BP), and across glacial-interglacial transitions back to ~395,000 years BP. These data will be paired with Mg/Ca measurements on selected intervals to isolate the water temperature contribution to measured changes in foraminifera delta-18O. The project will also utilize analyses of the planktonic foraminifera Globorotalia crassaformis that calcifies at ~700m and the benthic foraminifera Oridorsalis umbonatus to assess the timing of deeper ventilation events in the Sulu Sea.