Hydrothermal Vent Systems and the Emergence of Life

Hydrothermal systems (HSs) transfer heat from the Earth’s interior to its surface. In addition to the magmatic heat at mid-oceanic ridges and the natural geothermal gradient elsewhere, exothermic heat released by serpentinization provides an additional heat source, although the latter process alone is considered not to be able to provide a significant temperature change (Allen and Seyfried 2004). Serpentinization also produces reduced compounds, as by water reduction to H2. Hence, these systems deliver both, hot water and reduced compounds, towards the carbonic Hadean ocean—conditions that supply chemical free energy to porous mineral mounds on the Hadean ocean floor and that offer a plausible mechanism for the emergence of life (Shock 1992, Martin and Russell 2007).

Here we ask how the interplay between chemical kinetics, interior heating and fluid convection shapes the magnitude of chemical free energy generation and under which conditions this generation is maximized. We argue that the origin of life would be most plausible at conditions where abiotic chemical free energy generation is greatest. If these conditions correspond to those found at off-axis HSs, we would have further indication for where life possibly emerged.

A Simple Model of the Interaction of Fluid Dynamics and Chemical Kinetics

The HS can be described as a heat engine, driven by heating from below and the cold sink of the ocean, as well as a chemical reactor in which crust is hydrated by exothermic reactions. These two processes—fluid dynamics and chemical kinetics—interact in that motion advects reactants to the reaction sites and removes the products, and in that both processes affect, and are affected by, the temperature T s of the base of the HS. Hence, T s is central to the coupled dynamics and our model, as shown in Fig. 1. This temperature is shaped by the heating of the heat flux from the interior J h,i and the heat released by the chemical reaction J h,r and by the cooling by radiative-diffusive loss J h,d and convection J h,c .

Fig. 1
figure 1

Conceptual model of the main processes that result in abiotic chemical free energy generation in hydrothermal vent systems. The +/− signs indicate the kind of effect that one variable has on another

Full size image

The convective heat flux J h,c is related to the strength of convective motion. This motion is generated by the heat engine that is driven by the temperature gradient T s – T o (with T o being the temperature at the ocean floor) and the heat flux J h,c extracted from the energy balance: \( G(KE) = {J_{{h,c}}}\left( {{T_s} - {T_o}} \right)/{T_s} \). The resulting motion reflects the balance of generation and frictional dissipation, which depends, among other factors, on the fluid friction of the rock matrix. The strength of motion determines the mass exchange J m,c . This mass exchange affects the concentrations of reactants and products at the reaction site, and the advection of the products relates to the chemical free energy generation of the HS.

The chemical reaction rate v r and the associated heat release J h,r = v r ΔH are affected by T s and the concentration of reactants and products. The reaction rate is greater when the concentrations are in greater chemical disequilibrium. The equilibrium is set by the equilibrium constant ln K eq = ΔH/R T s and shifts the equilibrium concentrations towards reactants with greater T s . By supplying reactants and removing products, the convective motion maintains the concentrations out of equilibrium and thereby enables the maintenance of the reaction in a steady state.

Chemical Free Energy Generation and Feedbacks

Chemical free energy generation depends on the strength of convection and on how much the equilibrium of the chemical reaction favors products (which is the case for colder T s ). Both of these are affected by a series of feedbacks:

  • the strength of convection is shaped by an immediate positive and negative feedback. With the generation of motion, more heat is taken from the energy balance, which results in a greater G(KE). On the other hand, the greater J h,c , T s – T o is depleted, resulting in a reduced G(KE) and hence weakened convection, which constitutes a negative feedback. These two feedbacks together result in a state of maximum power which sets an upper limit on the strength of convection;

  • the rate of the chemical reaction is shaped by an immediate, negative feedback. As the reaction releases heat, the equilibrium constant shifts towards reactants, thereby resisting further heating (i.e. Le Chatelier’s principle). On the other hand, a positive feedback results from the heat release by the chemical reaction: it increases T s , strengthens convection and mass exchange, which brings the concentrations of reactants and products out of equilibrium thereby positively feeding back to the rate of the chemical reaction.

Conclusions and Outlook

Our model shows that fluid convection plays a critical role in enhancing geochemical reactions and the generation of abiotic chemical free energy that could fuel the emergence of metabolism and early life. Confirming many experimental results (Allen and Seyfried 2004), our model suggests that a HS that generates its heat by chemical reactions (such as off-axis HSs) favors greater rates of chemical free energy generation than those that are mostly driven by the interior heat flux (such as black smoker type HSs).

What we have not included yet is that the precipitation of products at the ocean floor results in the formation of structure and that the utilization of the chemical free energy at the ocean floor would transform the chemistry of the water that is advected to the reaction. Both of these would likely result in additional feedbacks that could further enhance geochemical free energy generation. Our conceptual model allows us to include these and evaluate these feedbacks more quantitatively in the future.