Phosphorus mobilization and speciation in modern hot springs as analogues for life’s earliest habitats

Biogeochemical Cycles

IAP2-21-220

Overview

Phosphorus is an essential nutrient for all life on Earth and possibly elsewhere. It forms the backbone of DNA and RNA, it is found in membrane lipids, and it is part of life’s energy currency ATP. However, the major form of phosphorus at planetary surfaces is the relatively insoluble mineral apatite, followed by phosphorus substitutions in silicate minerals [1]. The release of phosphorus into surface environments by weathering is therefore slow, and dissolved phosphate in seawater is rapidly scavenged into new authigenic mineral phases [2]. For these reasons it is generally thought that phosphorus would have been the most limiting ingredient for prebiotic chemistry and the origin of life.

One potential solution to the early phosphorus crisis on Earth and other habitable planets may have been enhanced dissolution of apatite and phosphatic silicates under hydrothermal conditions. Mafic volcanic rocks, which are subject to hydrothermal alteration along mid-ocean ridges and around hotspots on the seafloor, are relatively enriched in phosphorus [3], and hydrothermally altered volcanic rocks have been shown to be apatite-depleted [4]. In the modern ocean, black smokers are considered to be a phosphorus sink, because phosphate adsorbs to proximal FeMn-oxide deposits [5]; however, this sink may have been weaker on the anoxic Archean Earth. Furthermore, it was recently demonstrated that modern hot springs can harbor phosphite – a reduced form of phosphorus that is more soluble in seawater and that was potentially crucial in prebiotic phosphorus chemistry [6, 7]. It is therefore conceivable that hot springs and deep-marine hydrothermal vents were key point sources of phosphorus for early life
.
The aim of this project is to test this hypothesis with new analyses of phosphorus mobilization and speciation in hot springs and weathering profiles in Iceland and in an alkaline spring in Italy. These two settings offer a unique opportunity to explore the Eh-pH conditions under which phosphate and phosphite are most enriched in hydrothermal fluids. While the Icelandic systems are mostly magma-heated, the Italian spring is driven by serpentinization and therefore offers an extreme endmember in pH and reducing potential [8]. Spring data will be compared to low-temperature weathering profiles to test if hydrothermal springs are indeed more efficient sources of dissolved phosphorus. Lastly, modern carbonate precipitates will be explored as archives of the dissolved phosphate and phosphite reservoirs. If time permits, these modern carbonate deposits will be compared to Carboniferous hydrothermal vein carbonates that are associated with volcanic necks along the coast of Fife near St Andrews.

Methodology

Samples will be collected in Iceland and Italy from a range of hydrothermally active localities. Basic water properties (T, pH, Eh, salinity) will be measured in the field. Water samples and rock samples will be collected and analysed in the laboratory at the University of St Andrews. Analyses of the fluid will be conducted by ion chromatography to separate phosphate from phosphite ions. Rock samples, including unaltered and altered basalt, as well as samples along weathering profiles, will be dissolved and analysed by ICP-MS. Thin sections will be prepared for optical microscopy and SEM analyses to determine the major P-host minerals.

Project Timeline

Year 1

Field trip to Iceland, sample collection along hydrothermal streams and weathering profiles; analyses of the Icelandic samples in the laboratory.

Year 2

Field trip to Italy, sample collection along the alkaline spring and of carbonate deposits; analyses of the Italian samples in the laboratory.

Year 3

Collection and analysis of Carboniferous carbonate veins. Completion of analyses; compilation of publications about phosphorus in hydrothermal springs, basaltic weathering profiles and in carbonate precipitates.

Year 3.5

Completion of publications and conference presentations; preparation of the dissertation

Training
& Skills

The student will obtain training in technical and transferrable skills, including:
– Field trip preparation
– Geological sample collection
– Geochemical analyses by IC and ICP-MS
– Data interpretation and presentation
– Geochemical modelling for advanced data analysis
– Thin section preparation, petrography and SEM analyses
– Writing of scientific papers
– Oral presentations to a general scientific audience

References & further reading

[1] Walton, C.R., Shorttle, O., Jenner, F.E., Williams, H.M., Golden, J., Morrison, S.M., Downs, R.T., Zerkle, A., Hazen, R.M. and Pasek, M., 2021. Phosphorus mineral evolution and prebiotic chemistry: from minerals to microbes. Earth-Science Reviews, p.103806.[2] Reinhard, C.T., Planavsky, N.J., Gill, B.C., Ozaki, K., Robbins, L.J., Lyons, T.W., Fischer, W.W., Wang, C., Cole, D.B. and Konhauser, K.O., 2017. Evolution of the global phosphorus cycle. Nature, 541(7637), pp.386-389.[3] Horton, F., 2015. Did phosphorus derived from the weathering of large igneous provinces fertilize the Neoproterozoic ocean?. Geochemistry, Geophysics, Geosystems, 16(6), pp.1723-1738.[4] Agangi, A., Hofmann, A., Ossa, F.O., Paprika, D. and Bekker, A., 2021. Mesoarchaean acidic volcanic lakes: A critical ecological niche in early land colonisation. Earth and Planetary Science Letters, 556, p.116725.[5] Wheat, C.G., McManus, J., Mottl, M.J. and Giambalvo, E., 2003. Oceanic phosphorus imbalance: Magnitude of the mid‐ocean ridge flank hydrothermal sink. Geophysical research letters, 30(17).[6] Pech, H., Henry, A., Khachikian, C.S., Salmassi, T.M., Hanrahan, G. and Foster, K.L., 2009. Detection of geothermal phosphite using high-performance liquid chromatography. Environmental science & technology, 43(20), pp.7671-7675.[7] Pasek, M.A., Gull, M. and Herschy, B., 2017. Phosphorylation on the early earth. Chemical Geology, 475, pp.149-170.[8] Schwarzenbach, E.M., Lang, S.Q., Früh-Green, G.L., Lilley, M.D., Bernasconi, S.M. and Mehay, S., 2013. Sources and cycling of carbon in continental, serpentinite-hosted alkaline springs in the Voltri Massif, Italy. Lithos, 177, pp.226-244.

Further Information

For further information feel free to contact Dr Eva Stueeken (ees4@st-andrews.ac.uk)

Apply Now