ESH - Post-Glacial Reef Accretion History and Drowned Shore- lines as Constraints on Abrupt Sea-Level Movements in the Northern Main Hawaiian Islands

夏威夷北部主要岛屿考艾岛和瓦胡岛的水下斜坡上有一系列的梯田，这些梯田是碳酸盐岩礁的化石，可以追溯到晚第四纪的海平面。每个珊瑚礁阶地面向大海的正面是一个近乎垂直的墙，展示了淹没的海岸线特征，如潮间带缺口、滩岩和化石珊瑚的生长（Fletcher和Sherman， 1995）。特别值得注意的是淹没的潮间带缺口，在全岛范围内可预测的深度发现，有保存完好的悬垂遮阳板。这些和相关的古海岸线特征表明，相对海平面出现了突然的跳跃，在缺乏同震位移的局部机制的情况下，最好的解释是冰川融化-水脉冲的产物。这些海岸线的出现让人想起在巴巴多斯（Fairbanks, 1989）和其他地方（Edwards等人，1993；Bard等人，1996;Montaggioni，提交）的珊瑚化石岩心中发现的融水脉冲。其他被淹没的海岸线也被描述过，比如在南佛罗里达（Locker et al., 1996）和新西兰（Carter et al., 1986）发现的海岸线。在夏威夷发现了四条被淹没的海岸线。最深的(大约。-110 ~ -120 m)可能与海因里希事件H-2（?）同时发生的海平面冰川低潮有关。在-90至-95米和-58至-60米的深度范围内，淹没的海岸线与巴巴多斯的融化水脉冲IA和IB的深度范围大致相同。然而，在某些情况下存在几米的差异。我们推测，-58至-60米的海岸线被切割成一个礁，可以追溯到最后一个主要的间期（阶段3）。最浅的海岸线为-24米，这与北大西洋一个显著的融水脉冲（Keigwin and Jones, 1995）和澳大利亚大堡礁陆架一条被淹没的海岸线（Larcombe et al., 1995）的记录有关。这个最浅的海岸线被大约190至240年前的化石碳所雕刻（第7阶段）。此外，三条最浅的海岸线在深度范围上与Blanchon和Shaw（1995）在分析巴巴多斯珊瑚序列时提出的有争议的灾难性上升事件（CRE’s）相关联。因此，有理由将夏威夷淹没海岸线系列解释为冰期后和全新世早期海平面突变事件的档案。然而，与其他全球记录的相关性（如果有的话）的可能性尚未得到检验，需要对各种类型的岩心样本进行详细检查，以确定溺水的年龄。这些特征与巴巴多斯和巴布亚新几内亚的构造活跃边缘、塔希提岛的稳定陆架或北大西洋的海洋地层学记录的海平面运动之间（如果有的话）的真正关系（如果有的话）存在很大的不确定性（Bond and Lotti, 1995）。此外，由于夏威夷的记录提供了通过分析被淹没的缺口（msl和mllw都与缺口形态的各个方面相关）来更好地约束古潮间带位置的可能性，它将为后冰期和全新世间隔两个特定时期的海平面行为提供更好的约束。在确定北大西洋的气候记录是区域性的还是全球性的，如果是全球性的，它对海平面上升的影响，夏威夷是一个重要的站点。海平面在最后一次冰消期融水事件中的作用在规模、年代或对海岸/陆架系统的影响方面还没有得到很好的限制。MacAyeal（1993）计算得出，Laurentide冰盖海因里希事件振荡的清洗阶段能够在250年内使全球海平面位置移动3.5米。然而，Clark等人（1996）提出南极冰盖是融水IA的来源。因此，海平面突变事件在过程和响应链中的位置仍然非常不明确。Blanchon和Shaw（1995）的模型虽然来自地质记录的解释，但仍然需要额外的测试和推广到太平洋盆地。塔希提岛的新记录提出了关于融水事件和气候事件发生的重要问题（Bard et al., 1996）。关于海因里希事件、融水脉冲和海平面运动的相对时间（Clark et al., 1996）的问题也仍然存在，在充分了解因果关系和来源之前，这些问题需要额外的年代测定。在深海漂流事件和淡水排放记录中，一个重要的缺失部分是海平面的作用。不受这些记录约束的组件。确定侵蚀缺口的时间并不容易。研究将集中在两个最浅的淹没海岸线，-24米和-58至-60米。我们将使用可测定的碳酸盐记录来确定海平面的年代和位置。冰期后和全新世的岩心珊瑚礁层序的年龄限制了沉积的退步历史。保存在被淹没的凹痕表面的潮间带动物化石组合，也许还有同时期暴露的海滩岩石，都将由潜水员用水力取样来获取样本，作为过去海平面的代用物。珊瑚生长在切断淹没海岸线的海底，后来又生长在海面上。这种珊瑚的暴露在古海岸线的底部和面罩上很普遍。从这些暴露的岩心中提取的文石样本（用XRD测定）的年龄为全新世早期和更新世晚期，将在SOEST-TIMS设施（由UH教员K.Ruben管理）和亚利桑那大学加速器放射性碳设施（G.S. Burr已同意作为地质年代学家参与）进行年代测定。来自古缺口弧线内的样品也将进行岩石学分析，以确定指示水汽带成岩作用的胶结历史（L. Montaggioni已同意参与）。我们也将取样稳定的光同位素含量（B. Popp， UH教员管理UH稳定的光同位素质谱仪设施，并已同意参与）。同样，在淹没的海岸线向陆地方向的珊瑚序列的岩心，标志着海平面在其突然运动之后的最终位置，将进行年代学、岩石学和地球化学的分析和描述。大多数岩心的目标是穿透每个钻孔点的整个全新世和后冰期序列。因此，我们将采用“基底日期”的概念，其中冰后/冰前接触以成岩蚀变为特征的地面为标志。标志着海平面首次上升（和稳定）的基本日期是一个重要的、公认的海平面指数点，它为美国被动边缘海岸的潮汐沼泽地层学群落在全新世中晚期相对海平面历史的研究提供了服务。在我们的案例中，我们将确定基底珊瑚日期的分布，因为它们与造成淹没凹槽的海平面运动有关。由于我们钻探了全新世剖面，我们还将获得形成瓦胡岛阶梯水深测量的化石礁带的样本。这些夏威夷珊瑚礁中原始文石的日期（TIMS-HAS）将提供对末次冰期结束和当前间冰期早期海平面运动的更好理解。在EAR-9317328项目期间，我们已经开发了在这些深度取心的能力，使用富氧呼吸气体进行减压，并采用冗余呼吸仔细控制安全程序

9710005 Fletcher The submerged slopes of the northern main Hawaiian islands of Kauai and Oahu are marked with a flight of terraces that are fossil carbonate reef tracts dating from sea-level stands during the late Quaternary. The seaward-facing front of each reef terrace is a near vertical wall that displays drowned shoreline features such as intertidal notches, beachrock, and growths of fossil coral (Fletcher and Sherman, 1995). Of special note are the submerged intertidal notches that are found island-wide at predictable depths with well-preserved overhanging visors. These, and related paleoshoreline features, indicate the occurrence of abrupt jumps in relative sea-level, which, in the absence of a local mechanism for coseismic displacement, are best explained as the product of glacial melt-water pulses. The occurrence of these shorelines is reminiscent of melt-water pulses identified in cores of fossil coral from Barbados (Fairbanks, 1989) and elsewhere (Edwards et al., 1993; Bard et al., 1996; Montaggioni, submitted). Other drowned shorelines have also been described, such as those found in south Florida (Locker et al., 1996) and New Zealand (Carter et al., 1986). Four drowned shorelines are identified in Hawaii. The deepest (approx. -110 to -120 m) probably correlates to the glacial lowstand of sea level which occurred around the same time as Heinrich Event H-2 (?). Drowned shorelines at depth ranges of -90 to -95 m and -58 to -60 m are approximately in the same depth range as Barbados melt-water pulses IA and IB, resp., however there are discrepancies of several meters in cases. We speculate that the shoreline at -58 to -60 m is cut into a reef dating from the last major interstadial (Stage 3). The shallowest shoreline, -24 m, correlates to records of a prominent melt-water pulse in the North Atlantic (Keigwin and Jones, 1995) and a drowned shoreline on the Great Barrier Reef Shelf of Australia (Larcombe et al., 1995). This shallowest shoreline is carved into fossil carbon ate of ca. 190 to 240 kyrs age (Stage 7). Additionally, the three shallowest shorelines correlate in their depth ranges with the controversial catastrophic rise events (CRE's) postulated by Blanchon and Shaw (1995) in their analysis of the Barbados coral sequence. Hence, there is a rationale for interpreting the Hawaiian submerged shoreline series as an archive of abrupt sea-level events during the post-glacial and early Holocene interval. However, the possibility of a correlation (if any) to other global records is untested and requires a detailed examination of various types of cored, datable samples to determine the age of drowning. Major uncertainty exists regarding the true nature of the relationship (if any) between these features and the stratigraphic record of sea-level movements from the tectonically active margins of Barbados and Papua New Guinea, the stable shelf of Tahiti, or the marine stratigraphy from the North Atlantic that identifies episodes of ice-rafted debris (Bond and Lotti, 1995). Further, because the Hawaiian record offers the possibility of better constraining the exact position of paleointertidal positions through analysis of the drowned notches (both msl and mllw correlate to aspects of notch morphology), it will provide an improved constraint on the behavior of sea level during two specific times of the post-glacial and Holocene interval. Hawaii is an important station in establishing whether the climatic record of the North Atlantic is of regional importance or carries a global signature, and if global, its influence on eustatic sea level. The role of sea level in melt-water events of the last deglaciation has not been well-constrained with regard to magnitude, chronology, or impact on the coastal/shelf system. MacAyeal (1993) calculates that a purge phase of Heinrich event oscillations of the Laurentide ice sheet is capable of producing a 3.5 m shift in the position of global sea level within 250 yrs. However, Clark et al(1996) propose the Antarctic ice s heet as the source for melt-water IA. Hence, the position of abrupt sea-level events in the chain of process and response is still very much ill-defined. The model of Blanchon and Shaw (1995), although derived from an interpretation of geological records, still requires additional testing and extension to the Pacific Basin. Important questions have been raised by the new records from Tahiti regarding the occurrence of meltwater events and climatic episodes (Bard et al., 1996). Questions also remain regarding the relative timing of Heinrich Events, melt-water pulses, and sea-level movements (Clark et al., 1996) that require additional dating before cause and effect relationships, and sources, are fully understood. An important missing component of deep-sea records of ice-rafting events and fresh-water discharge is the role of sea level...a component that cannot be constrained by those records. Dating an erosional notch is not easy. Research will focus on the two shallowest drowned shorelines, -24 m and -58 to -60 m. We will use datable carbonate records to define sea-level chronology and position. The age of cored coral reef sequences that are post-glacial and Holocene in age will constrain the back-stepping history of accretion. Fossil intertidal faunal assemblages preserved on the face of drowned notches, and perhaps contemporaneous beachrock exposures, will all be hydraulically cored by divers to obtain samples for age-dating as proxies for past sea level. Coral grew beneath the seas that cut the drowned shorelines, and later it grew above. Exposures of that coral are prevalent at the base and upon the visors of the paleoshorelines. Aragonitic samples (determined with XRD) from cores of these exposures, early Holocene and late Pleistocene in age, will be dated at the SOEST-TIMS facility (run by K.Ruben, UH faculty member) and at the University of Arizona accelerator radiocarbon facility (G.S. Burr has agreed to participate as geochronologist). Samples from within the arc of the paleo-notches will also be petrographically analyzed for cementation histories indicating vadose-zone diagenesis (L. Montaggioni has agreed to participate). We will also sample for stable light isotopic content (B. Popp, UH faculty member runs the UH stable light isotope mass spec. facility and has agreed to participate). Likewise, cores of coral successions landward of the drowned shoreline, marking the final position of sea level following its abrupt movement, will be chronologically, petrographically, and geochemically analyzed and described. The goal for most cores will be to penetrate the entire Holocene and post-glacial sequence present at each drill site. Hence, we will employ the concept of "basal dates" where the post-glacial/pre-glacial contact is marked by a subaerial surface characterized by diagenetic alteration. Basal dates marking the first incidence of newly risen (and stabilized) sea level are an important and recognized sea level index point that has served the tidal marsh stratigraphy community of the passive margin coasts of the U.S. in their investigations of relative sea-level history in the middle and late Holocene. In our case, we will determine the distribution of basal coral dates as they relate to sea-level movements responsible for the drowned notches. As a consequence of our drilling the Holocene section, we will also obtain samples of the fossil reef tracts that form the stair-step bathymetry of Oahu. Dates (TIMS-HAS) of pristine aragonite in these Hawaiian reefs will provide improved understanding of sea-level movements at the end of the last glacial and during the early phases of the present interglacial. We have developed the capability of coring at these depths during the course of our project EAR-9317328 using oxygen-rich breathing gases for decompression and carefully controlled safety procedures employing redundant breathing