Extensive delays in our research were caused by the Covid-19 pandemic, therefore an extension of 18 months was granted to compensate for the time lost to complete this project.
We have searched extensively for diatoms and/or sponge spicules in the early fossil record. This has been one of the biggest challenges we have faced and is one of the high-risk aspects of this research project. We continue to request materials from archives and also to go on field trips to search for siliceous fossils as travel restrictions ease.
We participated in an Integrated Ocean Drilling Program Expedition 391: The Walvis Ridge Hotspot (December 2021 - February 2022). Although the cruise focused on deep drilling to study mantle geodynamics, we were able to collect 2 cores that contain siliceous fossils that will potentially provide the records across the last 80 million years.
Our studies of diatom phylogenies have not progressed until recently because of travel restrictions. We are currently working on reconstructing the evolution of silica transporters (SITs) and its link to the marine-freshwater diatom transition.
We have been testing methods for extracting siliceous fossils (sponges and radiolarians) embedded in cherts in a method that has previously been used by taxonomists. The procedure involves dissolving cherts in 5% HF for 10 minutes, then collecting and washing the materials that are released. Our preliminary analysis suggests that this method my only recover material that has been transformed from opal-A to opal-CT. This change in silica necessitates a dissolution-reprecipitation reaction that is known to fractionate the Si isotopes. Therefore, these materials can not be used to obtain accurate measurements for Si isotopes.
We have observed that many of the siliceous sponge spicules we have collected from the Mesozoic era have been diagenetically altered. We have initiated a study to determine the impacts of varying extents of diagenesis on Si isotopes by comparing modern and fossil sponge spicules. Based on a library of 12 modern sponges and 12 Mesozoic sponge spicules, we have used different techniques to determine extents of diagenesis from opal-A to opal-CT of these samples (Raman spectroscopy, FTIR (on-going), XRD and SEM coupled with element mapping). We also are measuring Si isotopes of these samples using SIMS for individual spicules and liquid mass spectrometry for bulk Si isotopes. This will be a tremendous resource to the geochemical community to verify the status of the materials from the geologic record used to determine their Si isotopic composition.
Biogeochemical models of past ocean chemistry and biology are being used to evaluate the mechanistic links between variations in the global Si and C cycles. We have hired a knowledgeable and highly capable young scientist to carry out this research. We are using the current version of the cGENIE Earth System model that has been configured for Si cycling and Si isotope systematics.
