Thursday, November 24, 2022

Mid Oceanic Ridges: Geodiversity And Biodiversity

Mid Oceanic Ridges. Unlike continental mountain chains, these undersea mountain ranges are invisible to our day to day gaze. 

Yet, they are among the most dynamic of geologic features. They form where tectonic plates split and move apart, and new ocean floor is generated by upwelling magma. Sea water percolates through the cracks in this new crust, heats up in the subsurface and then rises carrying with it gases and metals. These vigorous hydrothermal circulation systems provide a link to exchange chemicals between the mantle and the crust. Varied microbial and macrofaunal communities colonize these environments depending on proximity to magma, the strength and chemistry of hydrothermal systems, and the nature of bedrock composition and fault controlled topography. 

Some specific geologic settings, those with serpentinite rocks,  have recently attracted great interest because they are thought to have provided the right combination of heat and chemicals to stimulate pre-biotic chemistry and the origin of life.

Gretchen L. Früh-Green and colleagues review this fascinating underworld, bringing out, both, the diversity of geologic processes at work and the resulting biodiversity that depends on this varied geology and energy supply. The introductory paragraph shared below gives an idea of the importance of this geological environment.

"Mid-ocean ridge (MOR) systems extend approximately 60,000 km around the globe and are the most dynamic and continuous tectonic feature on the planet (Fig. 1). On average, about 3.3 km2 yr−1 of new oceanic crust is generated at global spreading centres, which account for >70% of the total volcanism, and where about 60–70% of the Earth’s surface has been produced over the past 160–180 Myr (ref.2). Mantle melting, volcanism and faulting at MORs drive hydrothermal circulation that allows the transfer of heat, chemical compounds, metals and volatiles from the asthenosphere to the hydrosphere and biosphere. Approximately 75–80% of the Earth’s total heat flux occurs as the oceanic crust ages, and it is most pronounced at ridge flanks, where low-temperature fluid flow continues off axis for millions of years and contributes to global biogeochemical cycles. It is estimated that the volume of the ocean circulates through the oceanic ridge system in much less than 1 Myr (ref.6).

Spreading centres are one of the most extreme environments on Earth that can support oases of life at high temperatures and thriving in perpetual darkness. Microorganisms obtain energy from magmatic gases and chemical compounds of altered oceanic crust, rather than from light, through a process called chemosynthesis. In turn, many of these microorganisms symbiotically sustain macrofaunal communities that populate hydrothermal vent environments. The microorganisms with the highest known growth temperature on Earth are found within MOR hydrothermal systems and investigation of their genetic diversity has changed the current view of the tree of life".

I have put the last sentence of the first paragraph in bold to highlight the scale of this geological system. This is rich and rewarding reading. The paper is open access.

Gretchen L. Früh-Green, Deborah S. Kelley, Marvin D. Lilley, Mathilde Cannat, Valérie Chavagnac & John A. Baross: Nature Reviews Earth & Environment- Diversity of magmatism, hydrothermal processes and microbial interactions at mid-ocean ridges.

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