Isotopic and Chemical Signatures of Paleophreatic Lenses at Enewetak Atoll: Constraints on Pleistocene Sea Level

本文对埃尼韦塔克环礁后中新世层序中的方解石胶结物进行了同位素和地球化学研究。Enewetak环礁KAR-1孔上新世序列中方解石胶结物的碳、氧同位素初步数据表明，这些胶结物侵位于大气降水潜水透镜中，而87Sr/86Sr数据表明这些成岩事件发生在更新世和上新世。与这些胶结物有关的古海相透镜的位置可以提供更新世海平面低水位的定量估计。KAR-1孔的方解石胶结物和OOR-17孔的相当于年龄的沉积物需要稳定的和放射性同位素数据的组合，以区分与连续的和时间上分离的古潜水透镜有关的胶结相，并评估各个潜水胶结带的地层分布。这种“化学胶结”地层学将能够识别多期潜水胶结作用，并记录它们的分布。因此，这项研究提供了一个机会：1)更好地了解海平面高频变化时期的大气成岩系统的动力学；2)通过监测古海平面透镜的位置，直接估计更新世海平面低水位的大小。最终，这一新的海平面记录可用于将层序地层学和有孔虫三角洲18O地层学的记录校准到海平面。

This is an isotopic and geochemical investigation of calcite cements in the post-Miocene sequence of Enewetak Atoll. Preliminary carbon and oxygen isotope data from microsamples of calcite cements within the Pliocene sequence of Core KAR-1 at Enewetak Atoll indicate that these cements were emplaced in meteoric phreatic lenses, while 87Sr/86Sr data places these diagenetic events in the Pleistocene and Pliocene. The position of paleophreatic lenses associated with these cements can provide quantitative estimates of Pleistocene sea-level lowstands. A combination of stable and radiogenic isotope data are needed form calcite cements from Core KAR-1 and from age-equivalent sediments of Core OOR-17 to discriminate cement phases related to successive and temporally separate paleophreatic lenses and to assess the stratigraphic distribution of individual phreatic cement zones. This "chemical cement" stratigraphy will allow multiple episodes of phreatic cementation to be identified and their distribution documented. Thus, this study offers an opportunity to: 1) better understand the dynamics of the meteoric diagenetic system during times of high frequency sea-level change, and 2) directly estimate the magnitude of Pleistocene sea-level lowstands by monitoring the position of paleophreatic lenses. Ultimately this new sea level record can be used to calibrate the records of sequence stratigraphy and foraminiferal delta 18O stratigraphy to sea level.