Palaeoecology and palaeoenvironment of Mississippian coastal lakes and marshes during the early terrestrialisation of tetrapods
Palaeoecology and palaeoenvironment of Mississippian coastal lakes and marshes during the early terrestrialisation of tetrapods
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DOI：
10.1016/j.palaeo.2020.110194
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发表时间：
2021-02
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通讯作者：
C. Bennett;T. Kearsey;S. Davies;M. Leng;D. Millward;T. Smithson;P. Brand;M. Browne;D. Carpenter;J. Marshall;H. Dulson;L. Curry
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作者：
C. Bennett;T. Kearsey;S. Davies;M. Leng;D. Millward;T. Smithson;P. Brand;M. Browne;D. Carpenter;J. Marshall;H. Dulson;L. Curry
英国北部的巴拉甘组提供了早期密西西比生态系统的独特记录，这些生态系统是随着四足动物出现在陆地上而发展起来的。在本文中，我们研究了Berwick-upon-Tweed附近两个500米的地层剖面，其特征是丰富的铁质白云岩层。确定了胶结粉砂岩与砂岩、均质白云岩、白云岩与粉砂岩混合、方解石与白云岩混合、白云岩与蒸发岩矿物混合5种岩相。胶结沉积物具有非平面到平面的半面状白云岩晶体，尺寸可达40 μm，而其他相主要由白云岩或15 μm大小的平面自面状白云岩菱形组成，其中较大的菱形表明部分再结晶。白云岩的宏观和微观化石含量以肉翅目（根茎类）和放光鳍目鱼类、双壳类、蛇形类、介形虫和球粒石类化石为主；与罕见的螺旋体，软骨鱼(Ageleodus， hybodonts和？棘棘鱼、异种棘棘鱼、非棘棘鱼、腹足类、泛足类、腕足类、植物残骸、木材、石松根、木炭、大孢子、藻叶状洞穴、植藻？andRhizocorallium。白云岩的氧、碳同位素组成范围分别为-3.6‰~ -1.7‰（δ18O）和-2.6‰~ +1.6‰（δ13C），表明白云岩生长于混合盐度水域。频繁的海洋风暴潮事件将海水和动物带入泛滥平原湖泊，在那里，蒸发、间隙硫酸盐还原细菌、铁还原和甲烷生成使白云岩在浅层地下生长。次生成土作用（通过根、角化、干燥和土壤形成过程）是常见的，代表湖泊蒸发，在某些情况下，盐碱地的发展。白云岩相是一个复杂的环境马赛克的一部分，包括空中干洪泛平原、潮湿的沼泽洪泛平原、河流和湖泊，盐度从淡水到高盐不等。海洋的影响在地层底部最强，随着时间的推移而减弱，因为洪泛区变得更加干燥，森林地区变得更加成熟。沿海湖泊是泥盆纪末汉根堡危机中恢复的动物的重要栖息地，可能是泛盐鱼类、软体动物和节肢动物进入淡水环境的途径。
The Ballagan Formation of northern Britain provides an exceptional record of Early Mississippian ecosystems that developed as tetrapods emerged onto land. In this paper, we study two 500-metre sections of the formation near Berwick-upon-Tweed, which are characterised by abundant ferroan dolostone beds. Five lithofacies are identified: cemented siltstone and sandstone, homogeneous dolomicrite, mixed dolomite and siltstone, mixed calcite and dolomite, and dolomite with evaporite minerals. Cemented sediments have non-planar to planar subhedral dolomite crystals, up to 40 μm in size, whereas other facies predominantly comprise dolomicrite or planar euhedral dolomite rhombs 15 μm in size, with patches of larger rhombs indicating partial recrystallisation. The macro- and microfossil content of the dolostones is dominated by sarcopterygian (rhizodont) and actinopterygian fish, bivalves,Serpula,ostracods andChondritestrace fossils; with rarerSpirorbis,chondrichthyans (Ageleodus, hybodonts and ?ctenacanths, xenacanths), non-gyracanth acanthodians, gastropods, eurypterids, brachiopods, plant debris, wood, lycopsid roots, charcoal, megaspores, phycosiphoniform burrows,Zoophycos? andRhizocorallium. The oxygen and carbon isotope composition of dolomites range from –3.6‰ to –1.7‰ (for δ18O) and –2.6‰ to +1.6‰ (for δ13C) respectively indicating dolomite growth in mixed salinity waters. Frequent marine storm-surge events transported marine waters and animals into floodplain lakes, where evaporation, interstitial sulphate-reducing bacteria, iron reduction and methanogenesis allowed dolomite growth in the shallow sub-surface. Secondary pedogenic modification (by roots, brecciation, desiccation, and soil forming processes) is common and represents lake evaporation with, in some cases, saline marsh development. The dolostone facies are part of a complex environmental mosaic of sub-aerial dry floodplain, wet marshy floodplains, rivers, and lakes ranging in salinity from freshwater to hypersaline. Marine influence is strongest at the base of the formation and decreases over time, as the floodplain became drier, and forested areas became more established. Coastal lakes were an important habitat for animals recovering from the end-Devonian Hangenberg Crisis and may have acted as a pathway for euryhaline fishes, molluscs and arthropods to access freshwater environments.