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

9628369沃尔特软体动物碳酸盐的死后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可能是什么。了解这些现象既是利用埋藏学破坏重建古环境条件(例如，埋藏相标准)的先决条件，也是控制古生态、生物地理和进化分析中的偏差的先决条件。这些问题也具有地球化学意义，因为壳体物质的锶同位素组成可能受到有机质呼吸和孔隙水中碳酸盐和硅酸盐矿物溶解作用的成分变化的影响。这项拟议的西加勒比热带硅碎屑和碳沉积野外合作研究将主要侧重于巴拿马的San Blas群岛，其次是洪都拉斯的Los Cochinos陆架；它建立在巴拿马Bocas del Toro地区一项成功的挖掘学试点研究和佛罗里达州海湾碳酸盐沉积物的广泛孔隙水研究的基础上。这项研究将首次提供关于热带硅质碎屑岩的挖掘模式和过程的信息，热带硅质碎屑岩约占低纬现代陆架区的一半，并将首次在单一区域内对富铁(硅屑)和贫铁(碳酸盐)沉积物中的碳酸盐反应和孔隙水演化进行测试。这项研究将量化：1.死双壳贝壳的死后损害作为物理环境(例如，水深、颗粒大小、由铅-210确定的沉积速率)的函数，包括钻孔、结壳、圆形、表面退化和破碎的频率；2.死后损害作为孔隙水化学的函数，特别是碳酸、主要和次要元素，以及控制碳酸盐溶解和重结晶速率的硫系参数；样品还将允许测定稳定同位素(C&Amp；S)和~(87)Sr/~(86)Sr组分，以评价孔隙水化学如何影响微结构改变前后贝壳物质的同位素特征：3.双壳类分类、生活习性、矿物学和微结构对化石破坏的选择性，以及这些因素在决定死亡聚集条件和偏向的地理变化中的重要性；4.时间平均、地层无序和贝壳损坏与贝壳死亡年龄之间的关系的环境变化(即，测试攻击学时钟)，特别是作为沉积速率、溶解速率、当地贝壳投入的组成和其他环境排列变量的函数；以及5.利用在长达3年的时间内部署的贝壳的实验阵列，在特定条件下贝壳退化的速率和途径；这是测试贝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.