COMPARATIVE ANALYSIS OF TWO LOWER CAMBRIAN PHOSPHORITES IN CENTRAL ASIA: RELATING PALEOENVIRONMENT, PALEOECOLOGY, AND PHOSPHORITE FORMATION IN DEEP TIME

Phosphate-rich sedimentary deposits (phosphorites) formed in shallow seaways around the world during the Ediacaran and Cambrian periods. This “phosphogenetic episode” represents a major perturbation to the biogeochemical phosphorus cycle, coincident with major changes in global temperature and climate, oxygen availability, and biodiversity. These global processes are almost certainly interconnected, but local processes — possibly at the finest, micrometer scales — may have exerted a strong control on where and how phosphate was retained in sediments. By comparing and contrasting phosphorites across the Precambrian-Cambrian boundary, we can identify common associations between early phosphate mineralization and certain biosignatures and paleoredox proxies. This talk will make such a comparison, between the Lower Cambrian phosphorites of Southwestern Mongolia and Southern Kazakhstan, both of which exhibit a wide range of styles of phosphate mineralization: hardgrounds, stromatolites, chemical sediments, and the body fossils of early diversifying multicellular organisms. Field observations combined with optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and Raman microscopy provide proxies for paleoenvironmental reconstruction which are spatially resolved with the microbial and animal fossil record. These techniques reveal a complex relationship between environment and organism, as well as between organisms, in the Cambrian oceans. Thus, we pose the questions: are sedimentary phosphorites a consequence of the evolution of ecological relationships in response to transitioning environments? Or are transitioning environments simply conducive to phosphate mineralization, and the record of life within phosphorites merely incidental?