Periodic Reporting for period 4 - SPADE (Speleothems paleoclimate: accounting for isotopic disequilibrium)
Reporting period: 2022-03-01 to 2023-08-31
The project examines the geochemistry of speleothems (cave deposits) in the context of the kinetics of their formation, in order to understand how do they record the climatic conditions in which they form.
Speleothems record the conditions in the cave and can serve as archives for regional climatic conditions. Using these archives to understand what was the climate of the past in different regions help in predicting regional climate of the future and therefore enable improved societal adaptation.
The research objectives are to develop a combination of 3 isotopic parameters, one classic and 2 novel, to understand the geochemical behavior of speleothems. For one of the new parameter, triple oxygen isotopes (D17O), this requires development of the basic understanding of how it behaves in different kinds of carbonates. For the other novel parameter, clumped isotopes (D47), basic understanding exists that needs to be extended.
Speleothems record the conditions in the cave and can serve as archives for regional climatic conditions. Using these archives to understand what was the climate of the past in different regions help in predicting regional climate of the future and therefore enable improved societal adaptation.
The research objectives are to develop a combination of 3 isotopic parameters, one classic and 2 novel, to understand the geochemical behavior of speleothems. For one of the new parameter, triple oxygen isotopes (D17O), this requires development of the basic understanding of how it behaves in different kinds of carbonates. For the other novel parameter, clumped isotopes (D47), basic understanding exists that needs to be extended.
The project started with installation of a mass spectrometer for clumped isotopes (D47) and its calibration. As part of this work we discovered that the mass spectrometer does not perform properly. Multiple attempts were made, together with the company that produced it, to bring the mass spectrometer to good working status. This has taken a lot of time and the instrument was finally sent back to the company to be fixed. As a remedy, we purchased a new mass spectrometer that was partially installed at the end of the previous reporting period. Installation has been prolonged due to COVID-19 related travel restrictions and was completed during the previous reporting period. Instrument calibration is now in progress.
Due to these delays in D47 measurements we focused on the questions related to D17O and on revised analysis of D47 speleothems data (that was preliminary results in the proposal). The work within the project consisted these sub-projects:
1. The calibration of D17O in international standards. In the past, strong inter-laboratory discrepancies were observed for the two commonly used CaCO3 standards. Our results of D17O values for these two standards, which are based on much more thorough analysis then previously measured, are approximately consistent between our laboratory and the data set previously reported by another laboratory. Using these standards as the basis for comparison of interlaboratory data on the 17O fractionation between water and CaCO3 (see #2 and figure) indicates consistency among most published data sets. The lab work in this sub-project was performed by the laboratory manager, together with a postdoc and a student and is published as Barkan et al. (2019) in the journal Rapid Communications in Mass Spectrometry.
2. Characterizing the 17O fractionation between CaCO3 and H2O was performed by a series of laboratory precipitation experiments, performed by a postdoc. We found that the 17O fractionation slope, which is the ratio between the fractionations of 17O and 18O is about 0.523 consistent with the fractionation slope we observed in freshwater mollusks in another project. We observed that this fractionation slope is independent of temperature. This sub-project included experiments for precipitation of either calcite or aragonite, finding that the fractionation slope is indistinguishable between the two polymorphs. This sub-project is published as Voarintsoa et al. (2020) in Chemical Geology.
3. D17O in speleothems. We measured D17O in rain water collected near Soreq cave as well as in cave drip and pool water. Results indicate that large rain events near Soreq cave are characterized by a D17O value that reflects water vapor formation over the Mediterranean Sea and is consistent with spring water in different locations in Israel. Events with a small amount of rain deviate from that value, and reflect partial evaporation of raindrops. We analyzed CaCO3 from modern and late Holocene stalagmites that grew in Soreq cave and found that the fractionation slope between speleothems carbonate and water in Soreq cave is indistinguishable from that we observed in laboratory precipitation experiments and in biogenic carbonates. This work were performed by a postdoc and 2 students. A paper for publication was written within this reporting period (Affek et al., 2023) and is now in press in the journal EPSL.
4. The calibration work of clumped isotopes (mentioned above as part of instrument installation) was used in an inter-laboratory comparison project that aims to create a set of standards that is shared by the clumped isotopes community. This work was performed by the PI and laboratory manager and was contributed to the publication Bernasconi et al., 2021 in Geochemistry, Geophysics, Geosystems.
5. Clumped isotopes (D47) were measured in a set of ancient speleothems from Soreq Cave covering in details the last two glacial terminations. These data were preliminary results in the proposed project but has been re-interpreted as temperature using a D47 thermometer calibration that is based of speleothem analog experiments that were performed previously by the PI. This thermometer was revised within this project to account for recent modifications in the calculations of D47. Results suggested that the speleothem analog thermometer fits well the D47 data of modern speleothems in Soreq cave and allowed a good estimate of glacial-interglacial temperature difference. The newly derived temperatures also enables the reconstruction of d18O in rainfall over the glacial-interglacial transition. This was performed by the PI and published as Matthews et al. (2021) in the journal QSR.
6. Within the current reporting period measurements of D17O in ancient speleothems from Soreq cave (the same samples as measured in #5 for clumped isotopes) has been done. This work was performed by a student.
7. Within the current reporting period measurements of D17O in calcite deposits from Devils Hole has been done. This work was done by a student.
Due to these delays in D47 measurements we focused on the questions related to D17O and on revised analysis of D47 speleothems data (that was preliminary results in the proposal). The work within the project consisted these sub-projects:
1. The calibration of D17O in international standards. In the past, strong inter-laboratory discrepancies were observed for the two commonly used CaCO3 standards. Our results of D17O values for these two standards, which are based on much more thorough analysis then previously measured, are approximately consistent between our laboratory and the data set previously reported by another laboratory. Using these standards as the basis for comparison of interlaboratory data on the 17O fractionation between water and CaCO3 (see #2 and figure) indicates consistency among most published data sets. The lab work in this sub-project was performed by the laboratory manager, together with a postdoc and a student and is published as Barkan et al. (2019) in the journal Rapid Communications in Mass Spectrometry.
2. Characterizing the 17O fractionation between CaCO3 and H2O was performed by a series of laboratory precipitation experiments, performed by a postdoc. We found that the 17O fractionation slope, which is the ratio between the fractionations of 17O and 18O is about 0.523 consistent with the fractionation slope we observed in freshwater mollusks in another project. We observed that this fractionation slope is independent of temperature. This sub-project included experiments for precipitation of either calcite or aragonite, finding that the fractionation slope is indistinguishable between the two polymorphs. This sub-project is published as Voarintsoa et al. (2020) in Chemical Geology.
3. D17O in speleothems. We measured D17O in rain water collected near Soreq cave as well as in cave drip and pool water. Results indicate that large rain events near Soreq cave are characterized by a D17O value that reflects water vapor formation over the Mediterranean Sea and is consistent with spring water in different locations in Israel. Events with a small amount of rain deviate from that value, and reflect partial evaporation of raindrops. We analyzed CaCO3 from modern and late Holocene stalagmites that grew in Soreq cave and found that the fractionation slope between speleothems carbonate and water in Soreq cave is indistinguishable from that we observed in laboratory precipitation experiments and in biogenic carbonates. This work were performed by a postdoc and 2 students. A paper for publication was written within this reporting period (Affek et al., 2023) and is now in press in the journal EPSL.
4. The calibration work of clumped isotopes (mentioned above as part of instrument installation) was used in an inter-laboratory comparison project that aims to create a set of standards that is shared by the clumped isotopes community. This work was performed by the PI and laboratory manager and was contributed to the publication Bernasconi et al., 2021 in Geochemistry, Geophysics, Geosystems.
5. Clumped isotopes (D47) were measured in a set of ancient speleothems from Soreq Cave covering in details the last two glacial terminations. These data were preliminary results in the proposed project but has been re-interpreted as temperature using a D47 thermometer calibration that is based of speleothem analog experiments that were performed previously by the PI. This thermometer was revised within this project to account for recent modifications in the calculations of D47. Results suggested that the speleothem analog thermometer fits well the D47 data of modern speleothems in Soreq cave and allowed a good estimate of glacial-interglacial temperature difference. The newly derived temperatures also enables the reconstruction of d18O in rainfall over the glacial-interglacial transition. This was performed by the PI and published as Matthews et al. (2021) in the journal QSR.
6. Within the current reporting period measurements of D17O in ancient speleothems from Soreq cave (the same samples as measured in #5 for clumped isotopes) has been done. This work was performed by a student.
7. Within the current reporting period measurements of D17O in calcite deposits from Devils Hole has been done. This work was done by a student.
D17O in CaCO3 is a new isotopic tracer. As a result, all our results are beyond the state of the art. The calibration of standards enable inter-laboratory comparison of data and open the way for growth of the community of scientists measuring D17O in carbonates. 17O fractionation between water and CaCO3 is now characterized in several ways, enabling the use of CaCO3 as archives for D17O in paleo-water. Our observation that this fractionation in speleothems is consistent with other CaCO3 systems is surprising, but facilitates future use of speleothems as archives for rainfall D17O. It also makes some of the experiments originally planned in this project unnecessary. We therefore focus instead on understanding rainfall D17O, where the results were more complex than anticipated. Together these will enable the understanding of D17O in paleo-rainfall, on which we will continue working on until the end of the project.