Archean mafic–ultramafic volcanic landmasses and their effect on ocean–atmosphere chemistry

Ancient sediments that precipitated from seawater contain elemental and isotopic records that have yielded mutually exclusive conclusions regarding the supply of elements. Certain isotopic data (e.g. Sr and Nd) appear to imply a preponderance of hydrothermal flux while a number of elemental abundances (e.g. Eu and Y) apparently require a much greater flux from land. Refined insight into the origin of the marine rare earth element pattern is shown to strengthen the requirement for a significant elemental flux from the early Earth landmass. Here it is proposed that the Archean landmass included a significant fraction of emerged oceanic plateaus in addition to the continents. Alternatively, repeated volcanic resurfacing of the continents with mantle-derived basalt increased continental element flux.Model calculations for the Eo- to Neo-Archean marine rare earth element pattern require that ca. 20% of the planet was occupied by volcanic plateaus (compared to the 3% of today). Not only were the plateaus more extensive but they were also thicker. A tectonic picture emerges in which the Archean ocean basins may have consisted of lithosphere of lesser (similar to present day) thickness populated by relatively short-lived (several 100 Ma at most) plateaus of considerable thickness.One important feature that distinguishes the Archean from the modern land surface is the preponderance of freshly erupted volcanic rocks. Due to the instability of high-Mg rocks under weathering, a substantial land surface of this type would have had importance both for the supply of nutrients to the ocean, as well as a potential sink for water and atmospheric gases. For example, the disappearance of high-MgO land towards the end of the Archean may have reduced the supply of Ni to the ocean and brought to an end the supremacy of methanogens, ultimately leading to the great oxidation event. Furthermore, both the submarine and subaerial volcanic resurfacing of the continents and oceanic plateaus may have led to high 18O/16O alteration products and isotopically light seawater, removing the need for an unreasonably hot early Earth surface temperature.Hot mantle plumes capable of producing vast volcanic plateaus may thus have played a major role in regulating the composition of the early hydrosphere and biosphere and indirectly also the evolution of the atmosphere.