Characterization of the Cretaceous Normal Superchron

数值模拟表明，稳定的(不可逆的)地球发电机在产生更强的偶极子场方面可能更有效，而且这种情况可能是由于核-地幔边界上横向变化的热通量的有利模式造成的。因此，弄清白垩纪正常超新纪(CNS)期间的平均场强是否真的异常高，对于理解地球发电机的工作原理和推断过去的下地幔条件都是重要的。在CNS期间，地球发电机停止反转的时间跨度约为121至83 Ma。研究人员提出了一项主要工作，以表征CNS期间的古地磁场强度和方向弥散。他们计划在马达加斯加、韩国、哥斯达黎加、格陵兰岛北部和斯瓦尔巴特群岛的野外工作中收集熔岩样本，并在CNS期间、之前和之后进行。他们还将从哥伦比亚河高原收集中中新世(反转率很高的时期)的玄武岩。这项研究试图回答以下几个尚未解决的重要问题：(I)CNS期间的平均强度是异常高、低还是正常，(Ii)CNS内部是否存在长期的强度趋势，以及(Iii)104年量级的时间尺度上的强度波动是否与最近的场相似或明显不同。对CNS发生的另一种猜测是，地磁场可以简单地凭借在核心运行的高度非线性的地球发电机过程，突然切换到和离开不可逆转的状态。研究人员将通过观察在CNS开始或结束之前，古强度或分散度是否存在可识别的时间趋势来检验这一假说。这个国际合作项目包括将资深科学家、博士后和学生聚集在一起，分享他们的知识和想法。科学成果将在国家科学基金会的支持下目前正在开发的综合古地磁数据库中提供，并将在期刊文章中传播。白垩纪正常超时时期的地磁场特征也具有社会意义，并引起了广大科学界的兴趣，他们试图了解地幔行为，并确定地柱、大型火成岩省形成和气候变化之间的潜在因果关系。

Numerical simulations have suggested that a stable (non-reversing) geodynamo may be more efficient at generating stronger dipole fields, and that this condition can arise from a favorable pattern of laterally varying heat flux across the core-mantle boundary. Thus it is important both for understanding how the geodynamo works and for inferring past lower-mantle conditions to find out whether or not the average field strength really was unusually high during the Cretaceous Normal Superchron (CNS), the span of time from around 121 to 83 Ma during which the geodynamo stopped reversing. The investigators propose a major effort to characterize the paleomagnetic field intensity and directional dispersion during CNS. They plan field work to collect lava samples in Madagascar, Korea, Costa Rica, N. Greenland, and Svalbard erupted during, just before, and just after the CNS. They will also collect basalt of mid-Miocene age (a time of high reversal rate) from the Columbia River Plateau. This research seeks answers to the following important questions that are still unresolved: (i) whether the average intensity during the CNS was unusually high, low or unexceptional, (ii) whether there were long-term intensity trends within the CNS, and (iii) whether the fluctuation of intensity on time scales of order 104 yr resembled that of the recent field or was distinctly different. An alternative conjecture for the occurrence of the CNS is that the geomagnetic field can switch abruptly into and out of a non-reversing state, simply by virtue of the highly nonlinear geodynamo processes operating in the core. The investigators will test this hypothesis by seeing whether there is an identifiable temporal trend in paleointensity or dispersion leading up to the beginning or end of the CNS. This international collaborative project includes bringing senior scientists, postdocs and students to work together and share their knowledge and ideas. The scientific results will be made available in the comprehensive paleomagnetic database currently under development with NSF support and will be disseminated in journal articles. The character of the geomagnetic field during the Cretaceous Normal Superchron also has societal relevance and interests a broad scientific community trying to understand mantle behavior and determine potential causal relationships between plumes, large igneous provinces formation and climatic changes.