Early Earth Surface Environment

Geochemical environments on the early Earth (before the Great Oxidation Event, GOE) are crucially important for the origin and evolution of life. Previous studies through various proxies or simulations, such as detrital minerals, sulfur isotope fractionations, paleosols and other sedimentary records, and atmospheric models, have provided various constraints on the redox state of the surface environment in the Archean. However, the consistency of these results and the corresponding surface geochemical progresses, e.g., weathering and river water chemistry, is not addressed yet.

1. Redox states of the Archean surfacial environments

Together with my JHU advisor Dimitri Sverjensky and GL mentor Robert Hazen, I examined the consistency between various lines of evidence on the redox state of the Archean surface environment through our thermodynamic calculations on stability of detrital minerals. We found H2,g instead of O2,g is the more reasonable redox proxy before the GOE (Hao et al., 2019 CG).

2. Archean continental weathering

With the redox level controlled by pH2,g, We developed thermodynamic models to simulate the weathering and river chemistry on the late Archean (Hao et al., 2017a EPSL). With the framework of Archean weathering and riverine transport, we have studied the speciation and mobility of nutrients and trace metals on the Archean (Hao et al., 2017b EPSL). Our study can provide important implications on various geochemical processes on the Archean surface environment, such as the formation of paleosols, stability of detrital minerals, whiffs of oxygen recorded by abnormal enrichments of trace metals and their isotopic fractionations, and haze formations.

After graduation, I continued working on simulating weathering of nutrients and transition metals under the early Earth conditions. For example, Hao et al. (2020 GCA) and Hao et al. (in revision) systematically examined various important processes of phosphorus cycle and investigated the availability of P for the bio-productivity on the Archean Earth. Together with my collaborator, we linked the availability of transition metals (e.g., Co, V) with bio-function of enzymes through the geological history (Moore, Hao, et al., 2018, JGR-Biogeosci; Hao et al., 2020 Geobiology).

This model will be modified to study water-rock interactions under various T-P and/or redox conditions. Moreover, water-rock interactions in other planets, such as the early Mars, will be simulated to be compared with geological observations.

High T-P Aqueous Geochemistry

Solubility and speciation of trace metals in the crustal fluids at elevated temperatures and pressures are one of the controlling factors for the habitability of life in hydrothermal systems. This research could also provide some implications for the effects of diagenesis and metamorphism on preservation of depositional signals in sedimentary records. During my thesis research, I have studied the dissolution and speciation of Cr in the aqueous environments at elevated T-P and found that aqueous Cr(II) could be dominant species under hydrothermal and subduction-zone conditions (Hao et al., in preparation; Huang & Hao et al., 2019 GPL). Further works will explore other interesting elements.

Origin of Life & Astrobiology

1. Interaction between mineral surfaces and organic molecules

In the summer of 2013, I started a series of batch adsorption experiment in the Surface Lab, Geophysical Laboratory as the predoctoral fellow working with Dr. Cecile Fuille and Dr. Robert Hazen. I investigated the adsorption of amino acids onto various mineral surfaces including pyrite and magnetite to explore the concentration effects for biomolecules through adsorption onto mineral surfaces. Since December, 2016, I started working on the adsorption of nucleotides onto clay minerals at Lyon, France under various T-P conditions under the supervision of Prof. Isabelle Daniel at University of Lyon1 and École normale supérieure de Lyon. Together with colleagues at Lyon, I conducted adsorption experiment of nucleotides at elevated temperature and pressure using our newly designed facility. In January, 2017, I mentored an undergraduate intern of Ens Lyon together with Isabelle. I designed a project for the intern student to examine the roles of divalent transition metals which are presumably abundant in the primitive seawater due to the water-ultramafic rock interaction under the reducing and acidic conditions, in the adsorption of nucleotides onto clay minerals. We conducted the experiment together and found the presence of transition metals, Zn and Ni, could significantly increase the adsorption of nucleotides on clay surface compared with traditionally used alkali and alkaline earth salts environment (Na, Ca, Mg). Our work is published recently (Hao et al., 2019 ACS ESC).

Right now, I'm conducting experiment to test if this enhancing adsorption by transition metals is true or not onto non-swelling clay minerals. Also, the experimental system at elevated temperatures and pressures relevant with hydrothermal conditions is also investigated.

2. Stability of organic compounds in hydrothermal environment

While doing the predoc in Geophysical Lab, I conducted hydrothermal experiments using teflon autoclave reactors to study the stability of amino acids under simulated hydrothermal conditions. We focused on exploring the effects of aqueous cations and mineral surfaces in naturally hydrothermal environmnets on the thermal stability of amino acids at elevated temperatures and pressures.

3. Partitioning behavior of amino acids in planetary aqueous environments

Since December, 2016, I worked with Isabelle and three intern students from Ens Lyon on measuring the partitioning coefficients of amino acids in ice Ih and high-pressure phases. We found that non-negligible amounts of glycine, alanine, proline, and phenylalanine could be incorporated into ice Ih during the equilibrium freezing. These results invalidate the hypothesis that massive freezing of primitive ocean could concentrate amino acids in the remaining fluids, but support the extraterrestrial transport of amino acids by icy comets. Moreover, we found that incorporation of amino acids into high-pressure phases of ice (VI and VII) could favor the crystallization of aqueous fluid which may have some implications for the mantle structure of icy moons and large ocean exo-planets. The results of ice Ih have been published in Astrobiology (Hao et al., 2018).

Right now, I'm preparing a manuscript on the partitioning of biomolecules into high-pressure ices under conditions relevant with the icy mantle of exoplanets (Hao et al., submitted).

Thermodynamics: theories & tools

1. Thermodynamic properties of the minerals and aqueous species

Based on early works by Sverjensky & Molling (1992, Nature), I and collaborators further developed the Linear Free Energy Relationship (LFER) method to estimate the Gibbs free energies of isostructural, end-member silicate and oxide minerals (Hao et al., 2017a EPSL).

2. Thermodynamic model of surface complexation and adsorption

Since October 2016, I started getting training on modeling surface complexation and adsorption in JHU with Prof. Dimitri Sverjensky. Future study includes applying this method to simulate adsorption experiment results at elevated temperatures and pressures.