A new model evaluating Holocene sediment dynamics: Insights from a mixed carbonate-siliciclastic lagoon (Bora Bora, Society Islands, French Polynesia, South Pacific)
A new model evaluating Holocene sediment dynamics: Insights from a mixed carbonate-siliciclastic lagoon (Bora Bora, Society Islands, French Polynesia, South Pacific)
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DOI：
10.1016/j.sedgeo.2016.08.002
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发表时间：
2016-08-15
影响因子：
2.8
通讯作者：
Camoin, Gilbert F.
中科院分区：
文献类型：
--
作者：
Isaack, Anja;Gischler, Eberhard;Camoin, Gilbert F.
大堡礁的碳酸盐-硅质碎屑混合泻湖具有作为沉积档案的巨大潜力，重点研究古环境和古气候变化以及事件沉积。沉积物来源包括泻湖碳酸盐产物、边缘生物礁和火山腹地。碳酸盐岩-硅质碎屑陆相混合型滨海泻湖已被广泛研究，但其孤立的海洋泻湖却被广泛忽视。本文根据沉积学、古生物学、年代学和地球化学资料，建立了博拉博拉堡礁泻湖全新世沉积动力学模型。泻湖序列始于代表低水位体系域的更新世土壤。随着全新世海平面的上升淹没了碳酸盐台地，泥炭在当地堆积了大约10650-9400年BP。碳酸盐-硅质碎屑混合沉积开始于8700-5500年前，代表海侵体系域。在此期间，沉积物的特征是粒度较粗，含有大量来自火山腹地的陆源物质以及主要在泻湖内产生的碳酸盐沉积物。硅质碎屑含量在整个全新世都呈下降趋势。海平面上升到现代水平后，在礁顶和泻湖之间形成了砂坪，为礁源物质创造了运输通道，导致在高位体系域期间约6000-3009年前以碳酸盐为主的沉积。然而，泻湖主要是细粒物质的输运和堆积，而较粗的颗粒则留在了进积的沙坪上。从大约4500-500年BP开始，颗粒大小、指示初级生产力的总有机碳、反映碳酸盐有效性和泻湖盐度的定性指标Ca和Cl元素强度出现了显著变化。这种模式可能表明事件(再)沉积，并与在附近塔哈湖发现的同期事件沉积相关，这些沉积被认为是由气旋活动增加引起的。相应地，火山腹地侵蚀和径流的加剧以及较低的泻湖盐度将与反复登陆气旋期间的强降雨有关。来自边缘礁区的更多粗粒沉积物将被输送到泻湖中。然而，自中全新世以来，作为陆源沉积物输送指标的钛/钙和铁/钙比值一直在下降。此外，底栖有孔虫动物群并不证实沉积物从礁湖到泻湖的运输。或者，更高的水动力能量条件的明显开始可以用更持久的东南信风和高于现在的海平面来解释，这被认为是南太平洋全新世的中晚期。持续的大风会将更多的公海水冲进泻湖，提高初级生产力和泻湖内的远洋生物数量，同时降低泻湖的盐度。我们提出了全新世中晚期向粗粒沉积模式的转变，以反映沙坪/泻湖边缘的泥沙负荷剥落，这是由于在高水位体系域的较强贸易和高于现今海平面的时期，砂坪/泻湖边缘的斜坡变得过陡，导致即使在泻湖中心也有沉积物的再沉积。现代条件，包括海平面下降到现代水平，达到了大约1000年前，泻湖充填在很大程度上是由泻湖内产生的以碳酸盐为主的细粒沉积物决定的，来自海军礁。据估计，通过砂坪的泻湖可容空间的填充量比通过泻湖背景沉积的填充量高出6倍，并强调了砂坪进积的重要性。与通常假设的细粒泻湖序列中的粗粒沉积层代表风暴或风暴高活动期引起的冲刷事件相反，我们假设了一个包括海平面、气候变化和堡礁泻湖地貌变化的新的长期泻湖沉积动力学模型。(C)2016爱思唯尔B.V.保留所有权利。
Mixed carbonate-siliciclastic lagoons of barrier reefs provide great potential as sedimentary archives focusing on paleoenvironmental and paleoclimatic changes as well as on event deposition. Sediment sources include lagoonal carbonate production, the marginal reef and the volcanic hinterland. Mixed carbonate-siliciclastic continent attached coastal lagoons have been intensively studied, however, their isolated oceanic counterparts have been widely disregarded. Here, we present a new model of Holocene sediment dynamics in the barrier-reef lagoon of Bora Bora based on sedimentological, paleontological, geochronological and geochemical data. The lagoonal succession started with a Pleistocene soil representing the Lowstand Systems Tract. As the rising Holocene sea inundated the carbonate platform, peat accumulated locally similar to 10,650-9400 years BP. Mixed carbonate-siliciclastic sedimentation started ca. 8700-5500 years BP and represents the Transgressive Systems Tract. During that time, sediments were characterized by relatively coarse grain size and contained high amounts of terrestrial material from the volcanic hinterland as well as carbonate sediments mainly produced within the lagoon. Siliciclastic content decreases throughout the Holocene. After the rising sea had reached its modern level, sand aprons formed between reef crest and lagoon creating transport pathways for reef-derived material leading to carbonate-dominated sedimentation ca. 6000-3009 years BP during the Highstand Systems Tract. However, mainly fine material was transported and accumulated in the lagoon while coarser grains were retained on the prograding sand apron. From ca. 4500-500 years BP, significant variations in grain-size, total organic carbon as indicator for primary productivity, Ca and Cl element intensities as qualitative indicators for carbonate availability and lagoonal salinity are seen. Such patterns could indicate event (re-)deposition and correlate with contemporaneous event deposits found in the lagoon of nearby Tahaa, which are supposed to be induced by elevated cyclone activity. Correspondingly, enhanced erosion and run-off from the volcanic hinterland as well as lower lagoonal salinity would be associated with intense rainfall during repeated cyclone landfall. Increased amounts of coarse-grained sediment from marginal reef areas would be transported into the lagoon. However, Ti/Ca and Fe/Ca ratios as proxies for terrigenous sediment delivery have incessantly declined since the mid-Holocene. Also, benthic foraminiferal faunas do not validate reef-to-lagoon transport of sediment. Alternatively, the apparent onset of higher hydrodynamic energy conditions can be explained by more permanent southeast trade winds and higher-than-present sea level, which are supposed for the mid-late Holocene in the south Pacific. Sustained winds would have flushed higher amounts of open ocean water into the lagoon enhancing primary productivity and the amount of pelagic organisms within the lagoon while lowering lagoonal salinity. We propose the shift towards coarser-grained sedimentation patterns during the mid-late Holocene to reflect sediment-load shedding of sand aprons due to oversteepening of slopes at sand apron/lagoon edges during times of stronger trades and higher-than-present sea level of the Highstand Systems Tract, which led to redeposition of sediment even within the lagoon center. Modern conditions including a sea-level fall to modern level were reached ca.1000 years BP, and lagoonal infill has been determined to a large part by fine-grained carbonate-dominated sediments produced within the lagoon and derived from the marignal reef. Infill of lagoonal accommodation space via sand aprons is estimated to be up to six times higher than infill by lagoonal background sedimentation and emphasizes the importance of the progradation of sand aprons. Contrary to the commonly supposed assumption that coarse grained sediment layers within fine-grained lagoonal successions represent overwash events induced by storms or periods of higher storm activity, we postulate a new model of long-term lagoonal sediment dynamics including sea level, climatic change and geomorphological variation of the barrier reef lagoon. (C) 2016 Elsevier B.V. All rights reserved.