Ichnofossils as Environmental Proxies

点击翻译按钮获取中文摘要

An ichnofossil - also referred to as trace fossil - is fossil evidence, such as fossil-burrows or -tracks, of an animal's living behaviours. Ichnofossils are used to understand animal (vertebrate and invertebrate) distributions in the rock record and for sedimentary rocks to better understand environments of sediment deposition. This proposal outlines research that will establish relationships between ichnofossil characteristics (e.g. size, diversity and distributions of trace fossils) and sedimentary environment parameters. We will seek to provide semiquantitative and quantitative proxies for sedimentary parameters including salinity, oxygenation and sedimentation rate. Salinity Stress: Brackish water represents a continuum between fresh and normal marine water. Trace fossil assemblages associated with sediments deposited in brackish water are characterized by specific characteristics: (1) low diversity of trace fossil forms; (2) preponderance of morphologically simple burrow structures; (3) diminutive trace fossils; and, (4) some ichnospecies present in high densities. The use of these criteria has been repeatedly demonstrated and confirmed in modern studies, but only a few of these have attempted to associate absolute salinity with  ichnological characteristics. Additional research is essential to refine our understanding of the influence of salinity on infaunal communities. Oxygen Stress: Oxygen deficiency is most characteristic in deep, and/or sheltered environments. Normally, oxygen stress is accompanied by the slow accumulation of fine-grained sediments. Some modern and many rock-record studies have shown that low dissolved-oxygen concentrations in depositional waters induce a noticeable reduction in the size of the burrows observed and the diversity of ichnogenera decreases markedly. Sedimentation Stress: Conditions related to the stresses induced by high sedimentation rates include high current velocities (limiting filter feeding) and episodic sedimentation. In modern settings, these sedimentary challenges limit bioturbation to motile worm-generated structures and, more rarely, anenomes, crustaceans and robust bivalves dominate energetic settings. Turbid conditions, which may accompany high sedimentation rates, also exert a significant environmental stress on filter-feeding animals. Providing quantified ichnological proxies for the above three physico-chemical parameters is the basis of my Discovery Grant. The research projects used to address the proposed research  span geological time using datasets from the Cambrian through to modern and laboratory-based studies. Multiple methods are proposed, including visualization techniques (X-ray, CT, hyperspectral imaging), compositional assessments (XRF, XRD, ICPMS, petrography), and field techniques (outcrop analysis, modern sedimentary environments).