How Mantle Compositional Heterogeneity Determines Energy Availability in Hydrothermal Ecosystems: A Global Perspective.

Abstract:

Significant quantities of energy and mass are exchanged between the mantle and the hydrosphere along Mid-Ocean Ridges (MOR), where magma emplacement is continuously tempered by cold seawater, generating a diverse array of Hydrothermal Fluids (HF) in the process. The compositional diversity of MOR basalt and the mantle regions they are drawn from are reflected in these HF, as are the bioavailable energy landscapes established when HF mix into seawater. These energy landscapes are explored here computationally, at a large scale, with results that are consequential to extant life, its evolution, and perhaps its origins. Thirty-eight million energy calculations across ranges in basalt composition, venting temperatures (T), water-to-rock mass ratios, fluid mixing extents, and redox reactions reveal globally distributed mantle-derived features of ecological competitive landscapes that were previously undetectable.

As an example, a dichotomy is revealed between high-T and low-T communities preforming aerobic H₂ and CH₄ oxidations. Variations in basalt composition are irrelevant to hyperthermophiles and thermophiles pursuing these metabolisms, as the energy available to them is limited by seawater O₂ and not the electron donors in HF. Conversely, energy supplies from these reactions are highly sensitive to basalt composition at conditions where mesophiles and psychrophiles live (HF-limiting), suggesting evolution and proliferation under the selective pressure of mantle and basalt compositional heterogeneities. In contrast, this high-T/low-T dichotomy does not exist for anaerobic oxidations using SO₄^2- and CO₂, which present globally heterogeneous, basalt-dependent energy to communities across T. The most homogeneously distributed energy supply across T is from aerobic oxidation of H₂S, suggesting few barriers to dispersal along-ridge, and independence from mantle and basalt heterogeneity. These patterns suggest that the first life would have enjoyed barrier-free dispersal along early MOR spreading centers when reducing seawater oxidants of moderate concentration at high T (analogous to modern O₂), and would have struggled against heterogeneous energy distributions if operating at low T, or operating on seawater oxidations at high concentrations (analogous to modern sulfate and CO₂).