[Paper Review] Nutrient Supply to Planetary Biospheres from Anoxic Weathering of Mafic Oceanic Crust

Drew D. Syverson, Christopher T. Reinhard+7 more|arXiv (Cornell University)|Feb 18, 2020

Paleontology and Stratigraphy of Fossils64 references44 citations

TL;DR

This study demonstrates that anoxic weathering of mafic oceanic crust releases significant bioavailable phosphorus, with a mobility ratio of ∑PO₄³⁻/∑CO₂ = 3.99 ± 1.03 µmol mmol⁻¹—comparable to modern rivers. This challenges the long-held assumption that continental weathering is the sole source of bioavailable P, implying that ocean-covered, continentless exoplanets could still sustain robust biospheres capable of generating detectable atmospheric biosignatures.

ABSTRACT

Phosphorus is an essential element for life, and the phosphorous cycle is widely believed to be a key factor limiting the extent of Earth's biosphere and its impact on remotely detectable features of Earth's atmospheric chemistry. Continental weathering is conventionally considered to be the only source of bioavailable phosphorus to the marine biosphere, with submarine hydrothermal processes acting as a phosphorus sink. Here, we use a novel 29Si tracer technique to demonstrate that alteration of submarine basalt under anoxic conditions leads to significant soluble phosphorus release, with an estimated ratio between phosphorus release and CO2 consumption (P/CO2) of 3.99+/-1.03 umol/mmol. This ratio is comparable to that of modern rivers, suggesting that submarine weathering under anoxic conditions is potentially a significant source of bioavailable phosphorus to planetary oceans and that volatile-rich Earth-like planets lacking exposed continents could develop robust biospheres capable of sustaining remotely detectable atmospheric biosignatures.

Motivation & Objective

• To test whether anoxic weathering of submarine basalt can release bioavailable phosphorus, challenging the assumption that continental weathering is the only significant P source.
• To quantify the ratio of phosphorus release to CO₂ consumption during anoxic basalt weathering using a novel ²⁹Si tracer technique.
• To assess the biogeochemical implications of this process for early Earth and habitable exoplanets lacking exposed continents.
• To evaluate the potential for such weathering to sustain significant biospheric O₂ fluxes, even in the absence of subaerial landmasses.
• To re-evaluate existing models of planetary redox evolution and biosignature potential by incorporating anoxic submarine P release as a key process.

Proposed method

• Employed a ²⁹SiO₂-enriched tracer to directly correlate the extent of primary silicate mineral dissolution with PO₄³⁻ mobilization in anoxic seawater experiments.
• Conducted controlled laboratory experiments on fresh and reductively pre-treated mid-ocean ridge basalt (MORB) at 15–75°C to simulate early Earth conditions.
• Used time-series ICP-MS and pH electrode measurements to track dissolved Si, P, Fe, and other cations over time, enabling precise calculation of P release and CO₂ consumption.
• Applied synchrotron X-ray fluorescence (SXRF) and XANES imaging to characterize Fe speciation and P distribution in reacted basalt, confirming Fe²⁺ oxidation and P mobilization.
• Calculated the ∑PO₄³⁻/∑CO₂ mobility ratio from linear regression of time-series data, yielding a key quantitative estimate of P release efficiency.
• Used a simple mass balance model to estimate global biospheric O₂ fluxes based on P release, assuming 50% scavenging efficiency and a C/P burial ratio of 300.

Experimental results

Research questions

• RQ1 Can anoxic weathering of mafic oceanic crust release bioavailable phosphorus at rates comparable to continental weathering?
• RQ2 What is the quantitative ratio of phosphorus release to CO₂ consumption during anoxic basalt weathering under early Earth-like conditions?
• RQ3 How does this process affect the potential for biosignature gas production on ocean-covered exoplanets without exposed continents?
• RQ4 What is the impact of Fe²⁺ oxidation on P mobility and scavenging in anoxic marine systems?
• RQ5 How does this mechanism alter our understanding of the long-term redox evolution of terrestrial planets and the persistence of biosignatures?

Key findings

• Anoxic weathering of submarine basalt releases bioavailable phosphorus at a rate of 3.99 ± 1.03 µmol mmol⁻¹ of CO₂ consumed, comparable to modern riverine inputs.
• The P release ratio is sustained even after pre-treatment to remove Fe³⁺-oxides, indicating that P is sourced from primary silicate minerals rather than surface oxides.
• Time-series data show a progressive increase in dissolved Fe²⁺ and PO₄³⁻, confirming co-release during basalt dissolution under anoxic conditions.
• The mobility ratio is consistent across multiple experimental runs, indicating robust and reproducible P mobilization under anoxic conditions.
• Modeling suggests that such weathering could sustain biospheric O₂ fluxes of ~10 Tmol y⁻¹, rivaling or exceeding those of the modern Earth.
• The results imply that volatile-rich, ocean-covered exoplanets without continents could still support biosignature-producing biospheres via submarine P release.