Probable Cold and Alkaline Surface Environment of the Hadean Earth Caused by Impact Ejecta Weathering

Abstract

Constraining the surface environment of the early Earth is essential for understanding the origin and evolution of life. The release of cations from silicate weathering depends on climatic temperature and pCO 2 , and such cations sequester CO 2 into carbonate minerals in or on the seafloor, providing a stabilizing feedback on climate. Previous studies have suggested that this carbonate-silicate cycle can keep the early Earth's surface temperature moderate by increasing pCO 2 to compensate for the faint young Sun. However, the Hadean Earth experienced a high meteorite impactor flux, which produced ejecta that is easily weathered by carbonic acid. In this study, we estimated the histories of surface temperature and ocean pH during the Hadean and early Archean using a new model that includes the weathering of impact ejecta, empirically justified seafloor weathering, and ocean carbonate chemistry. We find that relatively low pCO 2 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, log 10 (pCO 2 ) (in bar) is −2.21 +3.01 −2.54 [2 ] and temperature is 259.2 +84.1 −14.4 [2 ] K. Such a low pCO 2 would result in a circumneutral to basic pH of seawater, for example, 7.90 +1.21 −1.69 [2 ] at 4.3 Ga.A probably cold and alkaline marine environment is associated with a high impact flux. Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean. Therefore, if the origin of life occurred in the Hadean, life likely emerged in a cold global environment and probably spread into an alkaline ocean. Plain Language SummaryThe Earth's environment during the Hadean eon, 4.5 to 4 billion years ago, is obscured by a lack of geological evidence. However, life likely arose then, so improving our knowledge of the early environment is essential for understanding the origin and evolution of life. Here, we build a geological carbon cycle model that simulates the early surface environment and generates probability distributions for the level of atmospheric carbon dioxide (CO 2 ), average surface temperature, and ocean pH over time. During the Hadean, CO 2 dissolved in water is consumed by reacting with material ejected from meteorite impacts, so CO 2 levels tend to be low and the greenhouse effect weak. The consequences are low surface temperature and alkaline seawater. The probability that the surface temperature was lower than the freezing point of water and that seawater pH exceeded 7 is 70% at 4.3 billion years ago. Thus, if life began in the Hadean, it likely emerged in a cold global environment, and early life may have spread into an alkaline ocean. Key Points:• We estimate the range of pCO 2 , surface temperature, and ocean pH in the Hadean and Archean • A cold, Hadean surface environment and circumneutral to basic ocean pH is likely because of impact ejecta weathering • The origin of life may have occurred in a cold environment, and early life may have spread into a cold alkaline ocean Supporting Information:• Supporti...

The Archean atmosphere

“…Because a liquid ocean likely existed by~4.4 Ga ago, feedbacks in the geologic carbon cycle (discussed later) probably stabilized the long-term climate (34). However, consumption of CO 2 in the weathering of impact ejecta by carbonic acid suggests a cool early Hadean surface near 0°C under the faint Sun (35,36).…”

“…Ammonium substitutes for potassium, and breakdown of previously subducted ammonium-containing minerals in magmas at oceanic islands releases N 2 and radiogenic 40 Ar derived from 40 K. The ratio 40 Ar/N 2 in plume-related lavas scatters by a factor of~4 to 5, and higher values (older from more 40 Ar) correlate with smaller, Archean-like values of d 15 N, consistent with a history of ammonium subduction. Because N 2 is uncorrelated with nonradiogenic 38 Ar or 36 Ar, nitrogen in the current mantle is not primordial but recycled (154).…”

Macrobiont: Cradle for the Origin of Life and Creation of a Biosphere

“…Underwater, there is no opportunity for wet–dry or freeze–thaw cycling to promote the dehydration reactions needed to form protein and nucleic acid polymers. A colder early Earth [ 132 , 133 ], with more extensive polar caps, would provide the ice–water interface that could promote freeze–thaw conditions, unless the HTVs are simply too far away to supply key ingredients at useful concentrations. Ocean worlds such as Europa and Enceladus have similar considerations.…”

“…This is ascribed to episodic lava flows and glaciations which interrupted surface-dwelling organisms while the chemotrophs survived in networks of sub-surface hydrothermal channels. In analogy, after life got its global start, proliferated, and adapted to a variety of habitats, if global-scale volcanic resurfacing and/or climate change leading to an early version of snowball earth [ 132 ] occurred, the bottleneck refugia may have been subsurface or suboceanic hydrothermal systems, and/or impact crater hydrothermal lakes, where only thermophiles would survive and proliferate.…”

Carbonate-silicate cycle predictions of Earth-like planetary climates and testing the habitable zone concept

“…The range for α on the Earth was determined empirically from geologic constraints over the past 100 Myr 26 . We assume that this derived range for α applies to the Earth through time 21 , 45 , 46 and the Earth-like exoplanets modeled here that have a carbonate–silicate cycle. However, better proxy data for the ancient Earth or observing the carbonate–silicate cycle on habitable exoplanets 32 , 34 may be necessary to understand if the assumed range for α applies more generally to habitable planets.…”