Periodic Reporting for period 4 - OOID (The Ocean's Oxygen Isotopes Deciphered: Combining Observations, Experiments and Models)
Reporting period: 2023-03-01 to 2023-08-31
The geologic history of the oxygen isotopic composition of seawater has been a topic of intense study and debate for more than 60 years. An increase of 1-2% in the abundance of the heavy isotope of oxygen, 18O, in marine sedimentary rocks over the past ~3.5 billion years has been interpreted to represent either much higher early temperatures than today (~70-80°C) or a gradual increase in the 18O content of the ocean through time. Whichever of these hypotheses is correct, the implications for origins-of-life scenarios, planetary habitability, long-timescale climate regulation and the evolution of ocean chemistry are far-reaching. However, to explore these implications, we must distinguish between the two hypotheses. This has not been possible due to the control of both temperature and the oxygen isotope composition of the ocean on the oxygen isotope composition of marine sedimentary rocks. Other means to constrain the temperature independently of the oxygen isotope composition of seawater have fallen short, and the use of dual oxygen isotope records to deconvolve the effects of the geologic evolution of temperature and seawater oxygen isotopes has failed due to similar temperature dependences of mineral-water oxygen isotope fractionations of the minerals that appear in marine sedimentary rocks.
Iron oxides appear to be an exception, in that their mineral-water oxygen isotope fractionation is relatively insensitive to temperature. We identified this possible advantage, and planned to experimentally calibrate the temperature-dependent oxide-water oxygen isotope fractionation, and to construct a record of the oxygen isotope composition of marine iron oxides. The overall objectives of the OOID project are to identify the reason(s) for the observed increase in the 18O abundance in marine sedimentary rocks, and to explore the implications for various aspects of Earth history.
Iron oxides appear to be an exception, in that their mineral-water oxygen isotope fractionation is relatively insensitive to temperature. We identified this possible advantage, and planned to experimentally calibrate the temperature-dependent oxide-water oxygen isotope fractionation, and to construct a record of the oxygen isotope composition of marine iron oxides. The overall objectives of the OOID project are to identify the reason(s) for the observed increase in the 18O abundance in marine sedimentary rocks, and to explore the implications for various aspects of Earth history.
To date, we have:
1. Developed methods for the extraction of iron oxides from a rock matrix and their oxygen isotopic analysis by laser fluorination gas-source isotope-ratio mass spectrometry. We verified that the extraction and purification protocols do not affect the isotopic composition of the oxides.
2. Experimentally calibrated the temperature-dependent fractionation of oxygen isotopes between the common iron oxide minerals, goethite and hematite, and their parent fluids. We have done this in both freshwater and seawater.
3. Checked this calibration against iron oxides forming in the modern ocean, at a known temperature and seawater isotopic composition.
4. Collected and analyzed the oxygen isotopic composition of iron oxides from more than 80 locations, spanning the last 2000 million years of Earth history.
5. Used the results of items 2 and 4 above to produce a novel record of the oxygen isotopic composition of seawater over Earth history. The new record suggests that the increase in the abundance of 18O in marine sedimentary rocks reflects a parallel increase in the 18O content of seawater, rather than a gradual cooling from very warm early temperatures to cool recent temperatures.
6. Explored mechanistic explanations for the increase in the 18O content of seawater, with implications for climate regulation and ocean chemistry, among other topics.
7. Collected and begun analyzing the oxygen isotopic composition of marine iron-bearing clays spanning the last 2000 million years of Earth history, which will complement the iron oxide record (item 2). Iron-bearing clays provide built-in constraints on the post-depositional alteration history of the samples, which is a strength of this complementary record.
8. Developed cathodoluminescence methods mounted on a scanning electron microscope (SEM-CL) for petrographic characterization iron-bearing minerals, which are difficult to study by other microscopic techniques. Together with other imaging and geochemical analyses, the SEM-CL methods are used to constrain the post-depositional burial and alteration history of our samples, allowing us to avoid altered/compromised samples.
9. Identified a time interval, approximately 750 million years ago, during which the Earth is thought to have experienced a global glaciation (the Sturtian snowball Earth event), and during which our samples suggest that the ocean was more 18O-enriched than before or after. We collected and analyzed additional samples, which we then used to observationally constrain, for the first time, the volume of ice and the hydrological cycle during this extreme climatic event.
10. Measured the triple oxygen isotope fractionation between seawater and ice for the purposes of constructing an isotopic mass balance to constrain the amount and type of ice present during the Sturtian snowball Earth event.
1. Developed methods for the extraction of iron oxides from a rock matrix and their oxygen isotopic analysis by laser fluorination gas-source isotope-ratio mass spectrometry. We verified that the extraction and purification protocols do not affect the isotopic composition of the oxides.
2. Experimentally calibrated the temperature-dependent fractionation of oxygen isotopes between the common iron oxide minerals, goethite and hematite, and their parent fluids. We have done this in both freshwater and seawater.
3. Checked this calibration against iron oxides forming in the modern ocean, at a known temperature and seawater isotopic composition.
4. Collected and analyzed the oxygen isotopic composition of iron oxides from more than 80 locations, spanning the last 2000 million years of Earth history.
5. Used the results of items 2 and 4 above to produce a novel record of the oxygen isotopic composition of seawater over Earth history. The new record suggests that the increase in the abundance of 18O in marine sedimentary rocks reflects a parallel increase in the 18O content of seawater, rather than a gradual cooling from very warm early temperatures to cool recent temperatures.
6. Explored mechanistic explanations for the increase in the 18O content of seawater, with implications for climate regulation and ocean chemistry, among other topics.
7. Collected and begun analyzing the oxygen isotopic composition of marine iron-bearing clays spanning the last 2000 million years of Earth history, which will complement the iron oxide record (item 2). Iron-bearing clays provide built-in constraints on the post-depositional alteration history of the samples, which is a strength of this complementary record.
8. Developed cathodoluminescence methods mounted on a scanning electron microscope (SEM-CL) for petrographic characterization iron-bearing minerals, which are difficult to study by other microscopic techniques. Together with other imaging and geochemical analyses, the SEM-CL methods are used to constrain the post-depositional burial and alteration history of our samples, allowing us to avoid altered/compromised samples.
9. Identified a time interval, approximately 750 million years ago, during which the Earth is thought to have experienced a global glaciation (the Sturtian snowball Earth event), and during which our samples suggest that the ocean was more 18O-enriched than before or after. We collected and analyzed additional samples, which we then used to observationally constrain, for the first time, the volume of ice and the hydrological cycle during this extreme climatic event.
10. Measured the triple oxygen isotope fractionation between seawater and ice for the purposes of constructing an isotopic mass balance to constrain the amount and type of ice present during the Sturtian snowball Earth event.
We provided the first temperature calibration of the goethite-water and hematite-water oxygen isotopic fractionation under environmentally relevant pH and temperature.
We produced a new record of oxygen isotopes in marine iron oxides, the first of its kind, and used it to solve the longstanding problem of the oxygen isotopic evolution of seawater.
We are using our new record together with the existing record of oxygen isotopes in marine carbonate rocks to provide robust constraints on Earth's long-timescale climate.
We observationally constrained, for the first time, the volume and type of ice that formed during one of the most extreme climatic episodes in Earth history, the Sturtian snowball Earth event.
To constrain the snowball hydrology, we provided the first experimental determination of the ice-water triple oxygen isotopic fractionation in freshwater and seawater.
We developed new SEM-CL techniques for the study of iron oxides and iron sulfides, which are not amenable to study by traditional CL techniques.
We produced a new record of oxygen isotopes in marine iron oxides, the first of its kind, and used it to solve the longstanding problem of the oxygen isotopic evolution of seawater.
We are using our new record together with the existing record of oxygen isotopes in marine carbonate rocks to provide robust constraints on Earth's long-timescale climate.
We observationally constrained, for the first time, the volume and type of ice that formed during one of the most extreme climatic episodes in Earth history, the Sturtian snowball Earth event.
To constrain the snowball hydrology, we provided the first experimental determination of the ice-water triple oxygen isotopic fractionation in freshwater and seawater.
We developed new SEM-CL techniques for the study of iron oxides and iron sulfides, which are not amenable to study by traditional CL techniques.