High Orbital Obliquity Promotes Planetary Oxygenation

Next-generation telescopes such as the LUVOIR or HabEx concepts will soon be able to characterize distant exoplanets better than ever before. However, while these telescopes will primarily be limited to observations of exoplanet atmospheres, a planet’s habitability potential relies on additional surface environment conditions as well. As potential habitats for life, the characteristics of a planet’s ocean are particularly impactful on planetary habitability. Indeed, life on Earth spent the majority of its evolutionary history in the ocean. The majority of previous exo-ocean research has focused on ocean circulation and associated heat transport and oceans effect on habitability via climate. However, the effect of planetary properties on marine biogeochemical cycles and biological activity have not yet been fully explored. As a first step, we explore these relationships for differing orbital obliquities. We begin by using the ROCKE-3D general circulation model (GCM) to simulate atmosphere and ocean dynamics for varying orbital obliquity. We then use wind fields from those simulations as input data for the biogeochemical cycling software cGENIE in order to characterize the spatiotemporal patterns of nutrients and photosynthetic activity for each obliquity scenario. We additionally consider the effects of differing oceanic phosphate inventories and remineralization length scales for settling organic particulates.We find that export of particulate organic carbon (POC) and sea-to-air fluxes of oxygen (fO₂) increase with increasing obliquity in all model scenarios, given sufficient nutrient availability. We also find, similar to on Earth, that biological activity increases with increasing phosphate levels and increases with decreasing remineralization depth. We find that while nutrient inventory (in this case phosphate) has a first order control on biological activity and oxygen production, remineralization depth and obliquity also exhibit significant effects on both biological activity and oxygenation potential. These results suggest that life on planets with higher obliquity may be easier to detect without needing a larger biosphere. Additionally, our results imply that present-day Earth may not be the optimal environment for the evolution of complex life as it is sometimes considered.