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

ESH - 冰期后珊瑚礁增生历史和淹没海岸线对夏威夷主岛北部海平面突变的限制

• 批准号: 9710005
• 负责人: Charles Fletcher
• 金额: $ 23.86万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1997
• 资助国家: 美国
• 起止时间: 1997-08-01 至 2000-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9710005&HistoricalAwards=false
• 关键词: ESH; Post; Glacial; Reef; Accretion

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

9710005 Fletcher 夏威夷北部主要岛屿考爱岛和欧胡岛的水下斜坡上有一系列阶地，这些阶地是第四纪晚期海平面上的化石碳酸盐礁区。 每个珊瑚礁阶地面向大海的前部是一个近乎垂直的墙，展示了淹没的海岸线特征，如潮间带凹口、海滩岩石和化石珊瑚的生长（Fletcher 和 Sherman，1995）。 特别值得注意的是全岛范围内可预测深度的水下潮间带凹口，其悬垂遮阳板保存完好。 这些以及相关的古海岸线特征表明相对海平面发生突然跳跃，在缺乏同震位移的局部机制的情况下，最好将其解释为冰川融水脉冲的产物。 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，1993；Bard 等，1996；Montaggioni，提交）化石珊瑚核心中发现的融水脉冲。 其他淹没的海岸线也有描述，例如在佛罗里达州南部（Locker 等人，1996 年）和新西兰（Carter 等人，1986 年）发现的海岸线。 夏威夷发现了四条被淹没的海岸线。 最深（约-110至-120米）可能与海因里希事件H-2（？）大约同时发生的冰川低位有关。 深度范围为 -90 至 -95 m 和 -58 至 -60 m 的淹没海岸线分别与巴巴多斯融水脉冲 IA 和 IB 的深度范围大致相同，但有时存在数米的差异。 我们推测 -58 至 -60 m 的海岸线被切割成可追溯到最后一个主要间质礁（第 3 阶段）的珊瑚礁。 最浅的海岸线为 -24 m，与北大西洋显着的融水脉冲记录（Keigwin 和 Jones，1995）和澳大利亚大堡礁陆架淹没的海岸线（Larcombe 等，1995）相关。 这条最浅的海岸线被雕刻成约 2000 年的碳酸盐化石。 190 至 240 kyrs 年龄（第 7 阶段）。 此外，三个最浅的海岸线的深度范围与 Blanchon 和 Shaw（1995）在巴巴多斯珊瑚序列分析中假设的有争议的灾难性上升事件（CRE）相关。 因此，将夏威夷水下海岸线系列解释为冰川后和全新世早期突然海平面事件的档案是有道理的。 然而，与其他全球记录的相关性（如果有的话）的可能性尚未经过测试，需要对各种类型的岩心、可数据样本进行详细检查，以确定溺水的年龄。 关于这些特征与巴巴多斯和巴布亚新几内亚构造活动边缘、塔希提岛稳定陆架的海平面运动地层记录或识别冰筏碎片事件的北大西洋海洋地层记录之间关系的真实性质（如果有的话）存在重大不确定性（Bond和Lotti，1995）。 此外，由于夏威夷记录提供了通过分析淹没凹口（MSL和MLLW都与凹口形态方面相关）更好地限制古潮间带位置的确切位置的可能性，它将在后冰期和全新世间隔的两个特定时期对海平面的行为提供改进的约束。 夏威夷是确定北大西洋气候记录是否具有区域重要性或具有全球特征，以及如果是全球性的，则确定其对海平面升降的影响的重要站。 海平面在上次冰消融的融水事件中的作用在规模、时间或对沿海/陆架系统的影响方面尚未得到很好的限制。 MacAyeal（1993）计算出，劳伦泰德冰盖海因里希事件振荡的清除阶段能够在 250 年内使全球海平面位置发生 3.5 m 的偏移。 然而，Clark 等人（1996）提出南极冰盖作为融水 IA 的来源。 因此，海平面突发事件在过程和响应链中的位置仍然非常不明确。 Blanchon 和 Shaw（1995）的模型虽然源自对地质记录的解释，但仍然需要额外的测试和扩展到太平洋盆地。 塔希提岛关于融水事件和气候事件发生的新记录提出了重要问题（Bard 等，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 等，1996）的问题仍然存在，需要在因果关系和来源得到充分理解之前进行额外的年代测定。 冰漂流事件和淡水排放的深海记录中缺少的一个重要组成部分是海平面的作用……这一组成部分不受这些记录的限制。 确定侵蚀切迹的年代并不容易。 研究将集中在两条最浅的淹没海岸线：-24 m 和-58 至-60 m。 我们将使用可数据化的碳酸盐记录来定义海平面年代和位置。 冰期后和全新世的核心珊瑚礁序列的年龄将限制增生的倒退历史。 淹没凹口表面保存的潮间带动物群化石，以及可能同时暴露的海滩岩石，都将由潜水员进行水力取芯，以获得用于年龄测定的样本，作为过去海平面的代表。 珊瑚生长在切割被淹没的海岸线的海底，后来又生长在上面。 这种珊瑚的暴露在古海岸线的底部和遮阳板上很普遍。 来自这些暴露岩心的文石样本（通过 XRD 确定），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 运营）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）进行测年。 还将对古凹口弧内的样本进行岩相学分析，以了解表明渗流带成岩作用的胶结历史（L. Montaggioni 已同意参与）。 我们还将对稳定轻同位素含量进行采样（夏威夷大学教员 B. Popp 负责运营夏威夷稳定轻同位素质谱设施，并已同意参与）。 同样，淹没海岸线向陆地的珊瑚演替核心，标志着海平面突然移动后的最终位置，将按时间顺序、岩石学和地球化学方式进行分析和描述。 大多数岩心的目标是穿透每个钻探地点的整个全新世和后冰期层序。 因此，我们将采用“基准日期”的概念，其中冰期后/冰期前的接触以以成岩蚀变为特征的地下表面为标志。 标志着海平面首次上升（和稳定）的基准日期是一个重要且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界服务于全新世中晚期相对海平面历史的调查。 在我们的例子中，我们将确定基础珊瑚日期的分布，因为它们与导致淹没凹口的海平面运动有关。 作为我们在全新世部分钻探的结果，我们还将获得形成欧胡岛阶梯式测深的化石礁带样本。 这些夏威夷珊瑚礁中原始文石的日期（TIMS-HAS）将有助于更好地了解末次冰期末期和当前间冰期早期阶段的海平面运动。 在 EAR-9317328 项目过程中，我们开发了在这些深度取芯的能力，使用富氧呼吸气体进行减压，并采用冗余呼吸来仔细控制安全程序