Refining the bounds of Marine Isotope Stage 5a and 5c peak sea level: Insights from 3D modeling of glacial isostatic adjustment

海洋阶地和古代海洋海岸线的其他地质标志可以限制过去温暖时期的海平面高度，那里的冰与今天地球上的冰相似或更少。然而，地球科学家在声明地质标志目前的海拔等于很久以前的海平面高度时，必须谨慎地纠正冰盖生长和融化引起的地壳局部构造位移和运动。在后者的例子中，大陆冰盖，就像那些在最后一次冰川盛期横跨加拿大和美国北部的冰盖一样，向下加载冰化的地壳，并产生一个隆起的隆起，分别向南延伸到北美东海岸和西海岸的佛罗里达/加勒比海和南加州/下加利福尼亚。这种物理原理类似于一个人坐在柔软的床垫上造成的变形。就像床垫的类比，当大陆冰盖融化(人站起来)时，加载的部分开始反弹，或反弹，而隆起的部分开始放松或下降。然而，对于地壳来说，从加载到卸载的调整需要许多数千年的时间才能完成。出于这个原因，地球科学家使用计算机模型研究地壳如何变形，以“修正”过去海岸线的地质标志的高度，从而确定过去的海平面高度；通过假设地球有一个简单的、分层的内部结构，在横截面上看起来像一个同心的令人瞠目结舌的糖果，地球科学家可以在台式计算机上模拟地球的变形。在这里，研究人员建议用更真实的地球成分横向和垂直层来更新这个计算机模型，这些层是由地球内部的地震波成像确定的(很像超声波)。PI建议使用更强大的计算机来完成这一建模工作。PI将把计算机模型的输出与已公布的全球分布的海洋梯田和地质海岸线标志的高度汇编进行比较，这两个时期发生在约8万年前和约10万年前的最后一个冰河时代。这项研究将改进对全球海平面和过去这些冰河时代变暖间隔时间的估计。这项拟议的研究将培训一名海平面研究方法的博士生；科学家们将与俄勒冈州科学与工业博物馆(OMSI)一起创建一个动手活动，展示地质学家如何利用古代海平面的地质档案和地球结构的全球地球物理模型来了解过去气候状态下冰盖的稳定性。全球冰川均衡调整模型(GIA)利用从海洋阶地和其他海岸线标志重建的过去海平面的地理模式来确定过去暖期的全球平均海平面(GMSL)和等效冰量。汇编了已发表的全球分布的海洋同位素5a和5c高位标记物的高程，分别为~80ka和~100ka，为研究轨道进动周期产生的GMSL峰值提供了机会。最近GIA对这一全球地质标志汇编的区域子集进行了分析，得出了在当前海平面以下约5至40米之间变化的MIS5a和5c峰值GMSL的不同估计。这些分析采用了地球粘弹性结构的独立一维模型--即深度变化但横向均匀--最适合于古海平面标志物的区域子集。然而，这些一维模型不能适应来自全球的全套地质约束，因此在GMSL重建中引入了偏差。在这里，研究人员建议采用GIA的三维有限元模型，该模型既能反映最近高分辨率地震横波层析成像所推断的地球粘弹性结构的横向和径向复杂性，又能改善GIA产生的海平面地理变化模式的数值预测与MIS5a和5c峰值海平面地质指标的全球汇编之间的拟合。这项工作将改进对MIS5a和5c峰值全球平均海平面的估计，并进一步限制冰冻圈在过去这些相对冰期变暖的间隔期间的稳定性。因此，拟议的研究是一个及时的机会--也是必要的下一步--进一步加深我们对地质记录所证明的冰冻圈变化的规模和速度的了解。最后，该项目的结果将通过改进区域构造抬升速率与正在进行的地震危险性研究(例如，整个美国太平洋海岸)直接相关。该奖项反映了NSF的法定使命，并已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。

Marine terraces and other geological markers of an ancient ocean's shoreline can constrain the height of sea level for past warm periods with similar or less ice than exists on Earth today. However, geoscientists must be careful to correct for local tectonic displacement and motion of the Earth's crust that arises from the growth and melt of ice sheets when stating that a geological marker's present elevation equals the height of sea level long ago. In the case of the latter, continental ice sheets, like those that existed across Canada and the northern U.S. at the last glacial maximum, loaded down the glaciated crust and created a raised bulge that extended south as far as Florida/Caribbean and southern California/Baja California on the east and west Coasts of North America, respectively. This physics is similar to the deformation caused by a person sitting on a soft mattress. Like the mattress analogy, when continental ice sheets melt (the person stands up), the loaded portion begins to bounce back, or rebound, and the raised portion begins to relax down, or subside. For the Earth's crust, however, the adjustment from being loaded to unloaded takes many, many thousands of years to complete. For this reason, geoscientists use computer models for how Earth's crust deforms to "correct" the elevations of geological markers for past shorelines and therefore determine past sea level heights; by assuming that the Earth has a simple, layered, internal structure, that looks like a concentric jaw-breaker candy in cross-section, geoscientists can model Earth's deformation on a desktop computer. Here the investigator proposes to update this computer model with more realistic lateral and vertical layers of Earth's composition as determined from seismic-wave imaging of Earth's interior (much like an ultrasound). The PI proposes to use more powerful computers to complete this modeling effort. The PI will compare the computer model outputs to a compilation of published elevations of globally distributed marine terraces and geological shoreline markers for two periods of warmth during the last ice age that occurred ~80 and ~100 thousand years ago. This research will refine estimates of global sea level and during these past intervals of ice-age warmth. The proposed research will train a PhD student in sea level research methods; together with the Oregon Museum of Science and Industry (OMSI), the scientists will create a hands-on activity of how geologists use the geological archive of ancient sea level and global geophysical models of Earth's structure to understand the stability of ice sheets during past climate states.Global geophysical models of glacial isostatic adjustment (GIA) utilize the geographic pattern of past sea level reconstructed from marine terraces and other shoreline markers to determine global mean sea level (GMSL), and equivalent ice volume, during past warm periods. A compilation of published elevations of globally distributed markers for Marine Isotope Stage (MIS) 5a and 5c high stands, ~80 and ~100 ka, respectively, offer the opportunity to examine the magnitude of peak GMSL arising from an orbital precession cycle. Recent GIA analyses of regional subsets of this global compilation of geological markers have produced discrepant estimates of MIS 5a and 5c peak GMSL that vary between ~5 to 40 m below present sea level. These analyses adopted independent one-dimensional models of Earth's viscoelastic structure- that is, depth varying but laterally homogenous- that best fit regional subsets of paleo-sea level markers. However, these 1D models could not fit the full suite of geological constraints from around the globe, thus introducing biases into GMSL reconstructions. Here, the investigator propose to adopt a three-dimensional finite element model of GIA that captures both the lateral and radial complexity in Earth's viscoelastic structure as inferred from recent studies of high-resolution seismic shear wave tomography in order to improve the fit between numerical predictions of the geographically variable pattern in sea level arising from GIA and a global compilation of geological indicators of MIS 5a and 5c peak sea level. This exercise will refine estimates of MIS 5a and 5c peak global mean sea level and further constrain cryosphere stability during these past intervals of relative ice-age warmth. As such, the proposed research represents a timely opportunity-and necessary next step- to further our understanding of the magnitude and rate of cryosphere change as evidenced by the geological record. Finally, the results of this project will be directly relevant to ongoing studies of seismic hazard (for example, across the U.S. Pacific coast) through refinements of regional rates of tectonic uplift.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.