An Investigation of the Relation Between Mio-Pliocene Sea Level Changes and Dolomitization Events Imprinted on a SouthPactific Carbonate Platform

我们将对南太平洋纽埃岛（一个凸起的珊瑚环礁）新近纪剖面的大块白云岩和主礁碳酸盐进行同位素、元素化学和岩石学研究。最近在巴哈马和巴巴多斯进行的研究表明，白云化可能是由海水和（或）海水衍生的流体介导的，可能是偶发的，可能与涨落有关。最近在纽埃钻取的岩心将那里已知的部分从上新世延伸到中新世下部，并揭示了地层和暂时分离的白云岩，这些白云岩与海平面下降和上升的多个不同的印记共存。纽埃的前更新世构造史是简单的下沉，地下水系统仅限于海洋和大气水，岩性系统仅限于清洁的碳酸盐，这为研究大规模白云岩的形成方式和时间提供了最小的构造和化学干扰。这项拟议调查的目的是以比以前更高的可信度确定：(1)与替代白云化有关的流体的确切性质；(2)白云化过程中水岩相互作用程度；(3)白云岩相对于化石地下水位的地层位置；(4)白云石化相对沉积的时间和持续时间；(5)白云石化事件与涨落之间是否存在关联。18O/16O、13C/12C、87Sr86Sr比值、Ca、Mg、Sr和微量元素浓度、XRD晶体化学分析、白云岩、寄主碳酸盐、共存方解石-白云岩对、同时期白云岩和海相方解石胶结物的岩石学和SEM研究将是本研究的主要技术手段。这项研究的结果将提供新近系白云岩和寄主碳酸盐岩的精确年代剖面，这将对同沉积白云化的起源以及海平面周期性变化与白云岩幕式的关系等更基本的问题产生重要影响。它还将评价同沉积白云岩相对于寄主碳酸盐的印记在多大程度上保留了参与白云化的流体的地球化学印记，从而有助于解决白云岩作为过去地表过程监测仪的效用问题。此外，在这一大洋中碳酸盐岩台地上记录的海平面印记将有助于详细重建中新世-上新世时期的海平面记录，从而为大陆边缘上/下覆盖印记所获得的Vail-Haq海平面记录提供额外的约束。

We will undertake isotopic, elemental chemistry, and petrographic studies of the massive dolomites and host reef carbonates in the Neogene section at Niue, a raised coral atoll in the South Pacific. Recent studies in the Bahamas and Barbados have suggested that dolomitization may be mediated by sea water and/or sea water derived fluids, may be episodic, and may be related to eustatic fluctuations. Cores recently drilled at Niue have extended the known section there from the Upper Pliocene to probably the Lower Miocene and have revealed stratigraphically and temporary discrete dolomites coexisting with multiple and distinct imprints of sea level falls and rises. Niue's pre-Pleistocene tectonic history of simple subsidence, a groundwater system limited to marine and meteoric waters, and a lithologic system limited to clean carbonates offers minimal tectonic and chemical interference to the study of how and when massive dolomites form. The objectives of this proposed investigation are to determine with higher confidence than previously achieved (1) the exact nature of the fluids involved in the replacive dolomitization; (2) the degree of water-rock interaction during the dolomitization; (3) the stratigraphic position of the dolomites with respect to the fossil water table; (4) the timing and duration of dolomitization vis-a- vis deposition, and (5) whether there is a relation between dolomitization episodes and eustatic fluctuations. 18O/16O, 13C/12C, and 87Sr86Sr ratios, Ca, Mg, Sr, and trace element concentrations, XRD crystal chemistry analysis, and petrographic and SEM studies of the dolomites, the host carbonates, co-existing calcite-dolomite pairs, and coeval dolomites and phreatic calcite cements will be the chief techniques used in this study. The results emerging from this investigation will provide a closely dated section of Neogene dolmites and host carbonates which have an important bearing on the more fundamental problem of the origin of synsedimentary dolomitization and implications concerning the relation between periodic sea level changes and dolomite episodicity. It will also evaluate to what degree synsedimentary dolomites preserve the geochemical imprint of the fluids involved in dolomitization vis-a-vis the imprint of the host carbonates and therefore contribute to the question of dolomites' utility as monitors of past surficial processes. In addition, the sea level imprints documented on this mid-oceanic carbonate platform will allow a detailed reconstruction of the sea level record during the Mio-Pliocene time, thus providing additional constraints to the Vail-Haq sea level record obtained rom onlap/offlap imprints on continental margins.