Low δ18O record of the early Paleoproterozoic glaciations

Unlike the climate reconstructions of the recent times (e.g. Pleistocene), investigating the environment of the early Paleoproterozoic requires creative approaches. We used continental hydrothermal systems that formed 2.4-2.2 billion years ago to study the environmental conditions of our planet when it undergoing global cooling. This period is marked in geological record by a series of glacial diamictites deposited near the equator. This suggests that that our planet was frozen from poles to equator (or almost), a.k.a. snowball Earth glaciations. The δ18O values of rocks from the Belomorian belt in Russian Karelia are used in my studies to test the extent and timing of snowball Earth glaciations. The studied formations have δ18O values that extend the range of terrestrial rock values spanning from -25 ‰ to +10 ‰ on the scale of a single outcrop (a low δ18O rock is shown here). These rocks originally formed as Archean protoliths that were subsequently intruded by mafic magmas and hydrothermally altered by surface-derived fluids during the early Paleoproterozoic magmatism. Now, a couple billion years later, their isotope signature was explored in a magnitude of studies. Due to strong chemical bonding and abundance of oxygen in high-tempearture minerals, the original isotope signal of surface waters is still preserved. Since the recent efforts in high-precision Δ17O measurements allow to distinguish small differences in fractionations, these rocks were used to derive the δ18O values of local precipitation. The δ18O value of precipitation is a function of temperature, elevation and distance from the shore. Thus, these reconstructed values represent a unique record of the early Paleoproterozoic globally cold climate. Using Δ17O measurements I studied multiple occurrences within the Belomorian belt of Russian Karelia that provide temperature estimtimates (MAT around -40 °C at latitudes 20-0) and timing of the snowball Earth episodes (at 2.44-2.41 Ga and one at 2.29 Ga dated by TIMS).

The hydrothermal record of ancient seawater isotope composition

Fundamentally, all precipitation originates through evaporation of seawater. So, it is important to constrain the seawater δ18O value before confidently interpreting the δ18O value of precipitation recorded in continental systems. I used the high-temperature altered komatiitc basalts of the Vetreny belt, Russia, where 2.44-2.41 Ga volcanic-sedimentary succession of exceptional preservation allow for using equilibrium isotope approaches. Fluid inclusion thermometry combined with δD, δ18O and Δ17O values measured in quartz-epidote pairs provided internally consistent data set of equilibrium temperature and equilibrium fluid estimates. The study includes Δ17O measurements of quartz-epidote pairs extracted from similar lithological units of modern oceanic crust extracted by the Oceanic Drilling Program at site 504B.The results suggest that the isotope composition of least-modified equilibrium fluids are indistinguishable from the isotope composition of the Cenozoic seawater. That is to say that δ18O and Δ17O of the early Paleoproterozoic seawater were close to 0 ‰ (within ± 1.5 ‰ and ± 0.005 ‰, respectively). This study also presents one of the earliest constraints on the δD value of seawater. The large crystals of unaltered epidote with 1.9-2.0 wt. % H2O were used to define the δD value of equilibrium fluids. The δD values of reconstructed are well within 0 ± 20 ‰. This study is important because it contributes to the controversy on the δ18O value of seawater in the deep past and it provides one of the earliest record of δD value of seawater.

The Δ17O approach tested in modern submarine and continental systems

After I conducted several studies of the 2.5-2.2 billion year old hydrothermal systems, it was a good time to test the approaches in modern systems, where both temperature and fluid isotope compositions are well established. Supported by the National Geographic society, I headed to Iceland to sample modern Reykjanes and Krafla systems as well as a 6 million year old system at Geitafell. These systems offer examples of seawater- and meteoric-water-rock interaction. That means that the δ18O and Δ17O signatures of fluids and minerals can be ‘verified’ against the values measured in local ground water. We produced a δD, δ18O and Δ17O data-set for geothermal waters, quartz and epidote separates extracted by Icelandic drilling programs. The results are in good agreement with the local ground water sources and with the temperature downward profiles measured in the drilling wells. Additionally, we sampled the fossil Geitafell system that is now eroded to about 2km depth, where high-temperature alteration facies are available for direct sampling and comparison with modern meteoric systems. The δ18O values measured in Geitafell epidote and quartz plotted against distance from contact form a nice increasing trend consistent with decreasing equilibrium temperature away from the Geitafell gabbro intrusion (~150 °C about 4 km away). In this study I also analyzed Δ17O values in rhyolite glass quenched and extracted by the IDDP-1 well when it penetrated a body of magma at ~2km depth. The water-rock reaction calculations using 3 isotope ratios (δD, d17O and δ18O) suggests that the magma formed by assimilation of about 10-20 % of hydrothermally altered crust at Krafla by mantle-derived melts. This project is supported by previous measurements of epidotes and quartz extracted from modern oceanic crust at ODP site 504B. Several samples of quartz were mounted for in situ δ18O measurements. We found that single crystals of quartz can range over 5 ‰ due to combined effect of variable temperature and variable W/R (portrayed here).

Side projects

A few projects conducted in my spare time are listed below:
Adularia: A web-based application that calculates mineral formula based on EMPA analyses
A Shiny app that visualizes elemental maps in the Vetreny belt sample of hydrothermally altered komatiitic basalt (takes a while to load).
Oxygen isotope cheatsheet is a pdf/ppt file with oxygen isotope delta-notations, ranges, standardizations, fractionation factors, meteoric water cycle, etc.