Periodic Reporting for period 3 - UltraPal (Ultimate Paleo-Ocean Records from Biogenic Calcites) Período documentado: 2022-01-01 hasta 2023-06-30 Resumen del contexto y de los objetivos generales del proyecto Nearly all of our knowledge about sea surface and deep ocean water temperatures of the last 120 million years comes from a single group of organisms: foraminifera. These tiny organisms tolerate all water temperatures, live at all water depths, can be found in every major saltwater body, and most importantly, they generally build their internal shells (tests) out of calcium carbonate. When foraminifera produce their carbonate tests (biomineralization), information is saved about the water chemistry during the biomineralization process. The chemistry of the tests contains information about the ratio of magnesium to calcium (Mg:Ca) or strontium to calcium (Sr:Ca) in the seawater, the pH of the seawater, and the barium, carbon and oxygen isotopic composition of the seawater. All of these chemical and isotopic details are preserved in the shells of foraminifera and in the shells of many other seawater organisms that have lived during the past 120 million years.In addition, there are fundamental relationships between the Mg:Ca ratio or the oxygen isotope composition of a carbonate biomineral shell and the temperature at which that shell was biomineralized, i.e. the seawater temperature. These relationships are known as paleothermometers. With some basic assumptions about the paleoseawater conditions during the growth of the organisms, it is thus possible to use these geochemical temperature proxies to estimate the original seawater temperatures during the growth of the organism using these paleothermometers. The first step in this process, is to ascertain that the fossil carbonate shell/test used has not been altered chemically or isotopically at some point after the organism’s death and burial, by the processes known as diagenesis. If the shell has been modified, then its isotopic or chemical composition will provide erroneous temperature data.The current way to check a foraminifera test for diagenesis is to observe it through a reflect-light microscope and determine if it looks “glassy” or transparent, or if it looks “frosty” or opaque. The former points to an test unaffected by diagenesis, and the latter suggests that it has been modified and will therefore not be used in any paleoclimate reconstruction. Unfortunately, recent studies suggest that even glassy looking tests may be affected by diagenesis. Certain diagenetic processes can modify the isotopic and chemical composition of biominerals without any corresponding changes in the ultrastructures of visible structures of those organisms. This means that many biominerals may be providing erroneous temperatures, despite looking visible pristine. The goal of the UltraPal project is to identify, characterize, assess the extent of – and eventually correct – the effects of these digenetic process of the paleotemperature record. Our focus is on foraminifera, but the research conducted herein is easily applied to most seawater carbonate biomineralizing organisms. We simulate the burial conditions present during biomineral diagenesis with carefully controlled chemical and isotopic compositions to change the chemistry of these shells without being able to visibly tell that they’ve been changed at all. Using novel techniques, we locate where these chemical changes occur within biominerals to build reliable methods of avoiding diagenetic biases and to identify it in natural samples. Then using this information, we can examine the paleotemperature record derived from foraminifera and correct it sample-by-sample to reconstruct a more accurate temperature record. Trabajo realizado desde el comienzo del proyecto hasta el final del período abarcado por el informe y los principales resultados hasta la fecha The UltraPal project is separated into 4 subprojects.The first subproject on bulk-effect characterization, consists of using high-precision mass-spectrometry to measure the isotopic and chemical shifts of foraminifera tests, induced by low temperature diagenetic experiments. We have developed experimental workflows that quickly induce diagenesis, without any textural evidence of diagenesis. This aspect of the project already confirms that visually pristine biominerals are not chemically/isotopically pristine. Having confirmed this in foraminifera tests, we are now addressing the susceptibility of other biocarbonates, used in paleoclimate reconstructions, to the same processes.The second subproject on ultrastructural characterization, is to determine where diagenesis takes place within the biomineral crystal structures. Using a NanoSIMS to image isotopically-enriched samples, we can identify how certain aspects of a biomineral ultrastructure are correlated with the uptake of certain isotopes. These samples can then be examined with SEM, FIB-SEM, AFM and EBSD, to further characterize any otherwise invisible indications of diagenesis. These studies are coupled with studies on the behaviour and role of organic material in the diagenetic process.The third subproject, is to calculate kinetic isotope exchange rates from the experiments in subprojects 1 and 2. Any extrapolation of our results into the geological history requires accurate biomineral isotope exchange rates, which have been poorly characterized compared with the innumerable studies on abiogenic minerals. Of key importance is to calculate the rates for diffusion, a process, which is extremely slow in abiogenic minerals at low temperatures but may be faster by several orders of magnitude in biogenic minerals. The fourth subproject, is the synthesis of all our knowledge from subprojects 1–3 to develop numerical models capable of correcting the paleotemperature record for the effects of diagenesis. Avances que van más allá del estado de la técnica e impacto potencial esperado (incluida la repercusión socioeconómica y las implicaciones sociales más amplias del proyecto hasta la fecha) We have developed workflows that enable us to consistently modify isotopic signatures of biominerals at multiple temperatures and water conditions. By the project’s conclusion, we will have a robust knowledge of the extent that certain organisms can be impacted by diagenesis and which organisms provide the most reliable materials for paleoclimate reconstructions. We will also be able to provide the resources, workflows, and technical knowledge for future researchers to continue this line of experimentation. The end product of this product will be greatly improved reconstructions of the Earth’s past climate.At present, no way of identifying diagenesis in texturally pristine biominerals exists. Ultimately, through our SEM, FIB-SEM, AFM, EBSD, and NanoSIMS studies we hope to provide a method or key-indicator of diagenesis. This method would hopefully be rapid, inexpensive and accessible to those working on paleoclimate reconstructions.
