Cold glacial oceans would have inhibited phyllosilicate sedimentation on early Mars

Phyllosilicate minerals are commonly found in marine sediments on Earth(1,2). Accordingly, the presence of an ocean in the northern lowlands of Mars during the Noachian period would be expected to lead to the presence of abundant phyllosilicates in crust of the same age. However, mineralogical data from orbiting spectrometers show that phyllosilicates are rare in the Noachian-aged crust that is exposed in impact craters in the northern lowlands(3). In contrast, phyllosilicate minerals are abundant in the equatorial and tropical highlands(4,5), raising doubts about the presence of an ocean. Here we use climatic and geochemical model calculations and palaeohydrological reconstructions to assess the factors that control phyllosilicate synthesis and sedimentation on early Mars. Our model results show that temperatures in an ocean confined to latitudes poleward of 30 degrees N would have been near freezing, which would have hindered the formation of phyllosilicate minerals in the ocean basin. In addition, the presence of cold-based glaciers surrounding the ocean would have limited the delivery of phyllosilicates from the highlands to the ocean basin. We therefore suggest that the presence of a cold, Noachian ocean could explain the paucity of phyllosilicates in the Noachian-aged crust of the northern lowlands.