Facies variations in response to Holocene sea-level and climate change on Bora Bora, French Polynesia: Unravelling the role of synsedimentary siderite in a tropical marine, mixed carbonate-siliciclastic lagoon
Facies variations in response to Holocene sea-level and climate change on Bora Bora, French Polynesia: Unravelling the role of synsedimentary siderite in a tropical marine, mixed carbonate-siliciclastic lagoon
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DOI：
10.1016/j.margeo.2017.05.002
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发表时间：
2017-08
期刊：
影响因子：
2.9
通讯作者：
Anja Isaack;E. Gischler;J. Hudson;F. Anselmetti;S. Buhre;G. Camoin
中科院分区：
文献类型：
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作者：
Anja Isaack;E. Gischler;J. Hudson;F. Anselmetti;S. Buhre;G. Camoin
通过对采自两个振动岩心的70个高分辨率沉积物样品的微相和统计分析，在波拉波拉障壁-礁泻湖中识别出5种碳酸盐-硅质混合沉积相。从全新世海平面变化和气候变化的角度对沉积相和相序进行了解释。迎风泻湖核心的特点是sideritic泥灰岩杂砂岩和有孔虫sideritic杂砂岩，沉积约7700年BP（年前）在早中期全新世海侵。当时，由于气候潮湿，火山岛的含铁矿物受到广泛的风化和侵蚀，在泻湖沉积物中形成了同沉积菱铁矿。菱铁矿颗粒中δ 18 O（+0.32 ~+0.54 ‰）的富集表明其形成于海洋-大气混合环境。菱铁矿的形成是由于有机物质的微生物降解，δ 13 C值（− 13.61至− 14.48‰）的减少导致泻湖沉积物中的还原条件，并导致铁在溶解的碳酸氢盐存在下还原。菱铁矿的化学成分在核上发生变化，从核底部的相对高Fe（91-95摩尔%）和低Mn（5-6摩尔%）到核顶部的相对低Fe（83-88摩尔%）和高Mn（11-16摩尔%）。由Mn，Ca和Mg在颗粒边缘的Fe的替代说明了孔隙水化学的变化，对更多的含氧条件，并反映海平面上升和降雨量增加在早中期全新世。全新世中晚期气候干燥，铁的输入量减少，菱铁矿的比例下降，在岩芯上部接近零。在背风泻湖核心，菱铁矿是再次常见的下部，并减少了丰富的核心。全新世中期，软体有孔虫泥灰岩和软体杂砂岩在迎风岩芯中堆积了约5400-3500年BP。在这一时期的早期，轮虫和粟粒有孔虫占主导地位。它们能耐受环境压力，如水质、营养物或盐度的变化。从中全新世到现在，textularid有孔虫是常见的两个核心，并表示正常的海洋泻湖条件。自中晚全新世海平面上升和下降到现代水平，泥岩占主导地位的两个核心。在过去的1000年里，迎风泻湖中的珊瑚碎片数量增加，这可能是全新世中晚期边缘礁向泻湖进积的结果。晚全新世以来，波拉波拉岛迎风面的动力阻碍了沉积物的输移和沙裙向泻湖的扩展。在过去的1000年里，在迎风核心中越来越多的peloids，主要是硬化的粪便颗粒，可能反映了泻湖内早期的海底岩化作用。我们的研究表明，在全新世，海平面和气候变化影响了沉积物的进口，组成和分布在波拉波拉泻湖。环境对外部变化的敏感反应表明，热带珊瑚礁泻湖有可能成为气候和海平面变化的档案。
Five mixed carbonate-siliciclastic sedimentary facies were identified in the barrier-reef lagoon of Bora Bora using microfacies and statistical analyses of 70 sediment samples taken at high resolution from two vibrocores. Facies and facies successions were interpreted with respect to Holocene sea-level and climate changes. The windward lagoon core is characterized by sideritic marly wackestones and foraminifera-sideritic wackestones, deposited around 7700 years BP (years before present) during the early-mid Holocene transgression. At that time, extensive weathering and erosion of iron-bearing minerals from the volcanic island, due to a wetter climate, were expressed in the formation of synsedimentary siderite in lagoonal sediments. The enrichment in δ18O (+ 0.32 to + 0.54‰) in the siderite grains indicates marine to mixed marine-meteoric conditions during precipitation. Siderite formation resulted from microbial degradation of organic material, indicated by depleted δ13C values (− 13.61 to − 14.48‰) that led to reducing conditions in lagoonal sediments, and resulted in iron reduction in the presence of dissolved bicarbonate. The chemical compositions of the siderites changes upcore, from relatively high Fe (91–95 mol%) and low Mn (5–6 mol%) at the core base to relatively low Fe (83–88 mol%) and high Mn (11–16 mol%) at the core top. The substitutions of Fe by Mn, Ca and Mg at grain margins illustrate changes in pore-water chemistry towards more oxygenated conditions and reflect sea-level rise and elevated rainfall during the early-mid Holocene. A drier climate during the mid-late Holocene was accompanied by reduced iron input and the proportion of siderite decreased, approaching zero in the upper section of the core. In the leeward lagoon core, siderite is again common in the lower section, and decreases in abundance upcore. Mollusc-foraminifera marly packstones and mollusc wackestones accumulated ~ 5400–3500 years BP during the mid-Holocene in the windward core. Early in this period rotalid and miliolid foraminifera dominated. These are tolerant of environmental stress such as changes in water quality, nutrients or salinity. From the mid-Holocene to the present, textularid foraminifera are common in both cores, and indicate normal marine lagoonal conditions. Since the mid-late Holocene sea-level highstand and fall to modern level, mudstones have dominated in both cores. During the last 1000 years coral fragments have increased in abundance in the windward lagoon, presumably as a result of lagoonward progradation of fringing reefs in the mid-late Holocene. Since the late Holocene, motus on the windward side of Bora Bora have hampered sediment transport and lagoonward progradation of sand aprons. Increasing numbers of peloids, largely hardened faecal pellets, in the windward core in the last 1000 years may reflect early submarine lithification within the lagoon. Our study shows that during the Holocene, sea-level and climate change have influenced sediment import, composition and distribution in the Bora Bora lagoon. The sensitive response of the environment to external changes demonstrates the potential of tropical reef lagoons as archives of climate and sea-level changes.