Collaborative Research: Tropical Taphonomy and Porewater Geochemistry: Comparative Analysis of Molluscan Preservation in Siliciclastic and Carbonate Sediments, Western Carribean

碳酸盐岩软体动物死后的命运受到生物和地球化学循环过程的影响，但人们对热带地区这些过程的速度和选择性知之甚少，也不知道它们对现存化石记录的净影响是什么。了解这些现象是利用地学损伤重建古环境条件（如地学相标准）和控制古生态、生物地理和进化分析偏差的先决条件。这些问题也具有地球化学意义，因为壳材料的Sr同位素组成可能受到有机质呼吸作用和孔隙水中碳酸盐和硅酸盐矿物溶解作用的影响。这项拟议的西加勒比热带硅屑和碳沉积物合作实地研究将主要集中在巴拿马的圣布拉斯群岛，其次是洪都拉斯的洛斯科奇诺斯大陆架；它建立在巴拿马Bocas del Toro地区成功的地层学试点研究和佛罗里达湾碳酸盐沉积物中广泛的孔隙水工作的基础上。这项研究将产生关于热带硅塑料的分类模式和过程的第一个信息，热带硅塑料约占低纬度现代陆架区域的一半，并将允许在单一区域内对富铁（硅塑料）和贫铁沉积物（碳酸盐）的碳酸盐反应和孔隙水演化进行第一次测试。本研究将量化：1。死后双壳类死壳的损伤与物理环境（如水深、颗粒大小、由Pb-210决定的沉降速率）有关，包括钻孔、结壳、磨圆、表面降解和破碎的频率；2. 死后损伤是孔隙-水化学的函数，特别是碳酸、主要元素和次要元素，以及控制碳酸盐溶解和再结晶速率的硫系统参数；样品还将允许测定孔隙水中的稳定同位素（C &amp； S）和87Sr/86Sr组成，以评估孔隙水化学如何影响有和没有微观结构改变的壳材料的同位素特征；3. 分类、生活习性、矿物学和微观结构对双壳类生物灭种选择性的影响，以及这些因素相对于环境力量在决定死亡组合条件和偏向的地理变化中的重要性；4. 平均时间尺度的环境变化、地层紊乱、贝壳损伤与贝壳死亡年龄之间的关系（即对埋藏时钟的测试），特别是作为沉积速率、溶解速率、当地贝壳输入成分和其他环境变量的函数；和5。在特定条件下，使用长达3年的贝壳实验阵列，研究贝壳降解的速率和途径；这是测试壳破坏的选择性和控制的确证手段，对化石组合的成分保真度有影响。

9628369 Walter The post-mortem fate of molluscan carbonate is influenced by processes of biological and geochemical recycling, but little is known about the rates and selectivities of these processes in the tropics, nor what their net effect on the surviving fossil record might be. Understanding these phenomena is pre-requisite both in using taphonomic damage to reconstruct paleoenvironmental conditions (e.g. taphofacies criteria) and in controlling for bias in paleoecologic, biogeographic, and evolutionary analyses. These issues also have geochemical significance, because Sr isotope compositions of shell material may be affected by compositional changes driven by organic matter respiration and by carbonate and silicate mineral dissolution in pore waters. This proposed collaborative field study of tropical siliciiclastic and carbon sediments of the western Caribbean will focus primarily on the San Blas Archipelago of Panama, and secondarily on the Los Cochinos shelf of Honduras; it builds on a successful taphonomic pilot study in the Bocas del Toro area of Panama, and extensive pore-water work in carbonate sediments of Florida Bay. This research will produce the first information on taphonomic patterns and processes in tropical siliciclastics, which constitute about half of modern shelf areas in low latitudes, and will permit the first test of carbonate reactivies and pore-water evolution in both Fe-rich (siliciclastic) and Fe-poor sediments (carbonates) within a single region. The research will quantify: 1. post-mortem damage on dead bivalve shells as a function of physical environment (e.g., water depth, grain size, sedimentation rate as determined by Pb-210), including frequency of boring, encrustation, rounding, surface degradation, and fragmentation; 2. post-mortem damage as a function of pore-water chemistry, particularly carbonic acid, major and minor element, and sulfur system parameters that govern rates of carbonate dissolution and recrystallization; samples will also permit determination of stable isotope (C & S) and 87Sr/86Sr compositions in pore-waters to assess how pore-water chemistry influences the isotopic signatures of shell material with and without microstructural alteration; 3. selectivity in taphonomic destruction as a function of taxonomy, life habit, mineralogy and microstructure of bivalves, and the importance of these factors relative to environmental forces in determining geographic variation in death assemblage condition and bias; 4. environmental variation in scales of time-averaging, stratigraphic disordering, and relationships between shell damage and shell age-since-death (i.e., test for a taphonomic clock), especially as a function of sedimentation rate, dissolution rate, composition of local shell input, and other environmentally arrayed variables; and 5. rates and pathways of shell degradation under specific conditions, using experimental arrays of shells deployed over periods up to 3 years; this a corroborative means of testing for selecitivities in and controls on shell destruction, with implications for compositional fidelity of fossil assemblages.