T15 New Ideas on Studying Exceptionally Preserved Fossils: What to Do Next?

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room 205CD

GEOCHEMICAL CONTROLS ON PHOSPHATIZATION TAPHONOMY IN THE MIDDLE CAMBRIAN

Paper number: T15-2
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Much of our knowledge of early animal diversification comes from small shelly fossils preserved through phosphate mineralization. To understand the temporal and environmental dimensions of this taphonomic window and, therefore, how it influences our perceptions of Cambrian evolution, we must understand how phosphorus enters the sediment column and how it is redistributed during early diagenesis. Here we report integrated sedimentological, petrographic and geochemical data from the early Middle Cambrian Thorntonia Formation, Georgina Basin, Australia (NTGS 99/1 drill core). With these samples, we explore why the window for phosphogenesis remained open in this location, even as it began to close in coeval basins around the globe. In particular, we report decimeter-scale sequential phosphorus and iron extraction data to test the hypothesis that iron-oxides enhanced phosphorus delivery to, and retention within, the sediment column.

In the Thorntonia Formation, phosphorus enrichment is confined to meter to sub-meter scale subtidal to intertidal shallowing-upward carbonate cycles. Sequential phosphorus extractions indicate that P is hosted predominately by authigenic apatite, and petrography confirms that much of this apatite precipitates as steinkerns of small shelly fossils. Organic phosphorus (Porg) is low throughout the P-enriched interval (< 0.01 wt %), likely due to the conversion to, and retention by, early diagenetic apatite (1 +/- 1 wt %). Sequential iron extraction shows that iron-sulfides dominate the highly reactive Fe pool and that iron oxides are negligible (< 0.2 wt %). With a majority of the reactive iron residing in sulfides, however, one cannot place firm constraints on Fe-oxide associated P delivery. These observations do not preclude a component of Fe-oxide P delivery, but do place a limit on this vector and show that they do not represent a significant repository for P in the sediment column. Thus, we look to early diagenetic processes, which influence the saturation state of apatite and source P from organic remineralization, as primary mechanisms controlling phosphatization.