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 assumed preserved in the shells of foraminifera and in the shells of many other seawater organisms that have lived during the past 120 million years, or more.
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 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.
Our work and its results have demonstrated that the fundamental hypothesis of the initial proposal, namely that biocarbonates (examined primarily with tests of benthic foraminifera) are highly susceptible to isotope exchange. This process can proceed without leaving any observable changes to the structure of the biocarbonate.
The main mechanism for this process is grain boundary diffusion and isotopic exchange on the inter-particle surfaces within the biocarbonate structure, which represents an enormous surface area, so that even low level of isotopic exchange can have a significant effect on the bulk isotopic composition of the biocarbonate.
We have shown that this mechanism is effective in both pristine, i.e. Modern biocarbonates, as well as in fossil biocarbonates (albeit to a lesser extend). In more detail, the grain boundary diffusion is related to the distribution of organic matrixes within biocarbonates, and we have characterized these organic matrixes at the nanoscale. We have shown a direct correlation between the distribution or organic matrixes and the patterns of isotope exchange that happen during the fossilisation process. Together, these results questions the accuracy of the current paleo-environmental records based on, in particular, the oxygen isotopic compositions of fossil biocarbonates (primarily calcites) on geological timescales because these processes are not currently taken into account. On the other hand, understanding these diagenetic processes opens the door to possibly correcting for their impact on the paleo-reconstructions, which will lead be better and more robust understanding of how Earth's climate has evolved over at least the last 100 million years.
