MODELING ARCHAEAN HYDROTHERMAL SYSTEMS

Seafloor hydrothermal systems are an integral component of a complex, dynamic heat engine by which energy brought towards the seafloor by mantle convection is transferred to the ocean by a combination of thermal conduction and hydrothermal venting. Modern seafloor hydrothermal circulation accounts for approximately 33% of the heat transferred through the seafloor and plays an important role in global geochemical cycling. Moreover, the discovery of chemotrophic hyper-thermophilic microbes living at seafloor hydrothermal vents has led to the idea that life on Earth, and perhaps on other planetary bodies, originated in hydrothermal environments characteristic of the early Earth. Here I explore some likely similarities and differences between modern and early Archean hydrothermal systems. An appreciation of these differences may lead to different perspectives regarding the role of Archean hydrothermal activity in the evolution of ocean chemistry and the origin of life.

Greater seafloor spreading rates, ridge length and volume, heat flow and volcanic activity during the Archean all suggest more vigorous hydrothermal activity than during the Phanerozoic. The absence of large continental land masses, coupled with the presence younger, smaller lithospheric plates suggests that open hydrothermal circulation would have been nearly ubiquitous and that ocean water would have been rapidly cycled through Archean oceanic lithosphere of all ages. Archean ocean chemistry would have thus been dominated by hydrothermal inputs. Although the basic nature of this vigorous circulation may have been similar to the present, there are two significant differences. First, the Archean ocean was probably much warmer than present (~ 100°C); secondly it was much shallower (~ 1-2 km). As a result anhydrite precipitation would have rapidly removed sulfate from seawater, and because the subsurface would have remained warmer, the precipitate would have been sequestered in the crust until the ocean and upper crust cooled over geologic time. Moreover, the lower pressures of high-temperature magma-driven circulation at ridge axes would have resulted in much more prevalent sub-critical two-phase flow there than under present conditions.