4:00 PM - 4:15 PM
[HCG23-08] Reconstructing the past sea-level change on ancient Mars:
insights from flume-tank experiment of delta morphology
Keywords:Delta morphology, Mars, Flume-tank experiment
Present-day Mars has no liquid water on its surface, and an extremely cold and dry environment prevails. On the other hand, due to the existence of fluvial landforms such as valley networks, outflow channels, deltas and shoreline topography, it has been suggested that there were oceans in lowlands of the northern hemisphere about 4.1-3.7 billion years ago. However, the timing and process of the ocean disappearance remains uncertain. In this study, we attempted to elucidate the origin of delta topography and sea-level changes on ancient Mars using flume-tank experiments at Nagasaki University.
First, we examined changes of delta morphology and its relationship with water-level changes. By water-level rise experiments, distinct terraced landforms were formed, and we named them as ‘Step Deltas’. On the other hand, by water-level fall experiments, Lobe Dominant Deltas which are tongue-shaped topographies elongating seaward were formed. By constant water-level experiments, Distributary Channel Deltas, which are characterized by lateral movement and avulsion of channels were formed.
Next, we explored Martian satellite images in order to search for the distribution of delta topography similar in shape to those formed in flume-tank experiments. Then, we examined formation ages of each delta topography by the crater counting method. As a result, the Lobe Dominant Deltas formed by the water-level fall and the Distributary Channel Deltas formed by constant water-level are dated to be at about 3.6~3.5Ga and are concentrated at the elevation range of -1670m~-4116m near the paleo-shoreline. On the other hand, Step Delta formed by water-level rise are dated to be at about 3.4~3.2Ga, and they are observed only inside craters at elevations of +2698~-4211m.
Based on these lines of evidence, it is considered that a large-scale regression occurred around 3.6~3.5Ga. On the other hand, there was no vast ocean around 3.4~3.2Ga, but local lakes may have developed in craters due to the melting of glaciers and groundwater discharge caused by the temporal warming related to volcanic activities. We hypothesize that the northern lowland oceans that spread on ancient Mars may have disappeared by 3.5~3.4Ga, followed by an environment with localized lakes.
First, we examined changes of delta morphology and its relationship with water-level changes. By water-level rise experiments, distinct terraced landforms were formed, and we named them as ‘Step Deltas’. On the other hand, by water-level fall experiments, Lobe Dominant Deltas which are tongue-shaped topographies elongating seaward were formed. By constant water-level experiments, Distributary Channel Deltas, which are characterized by lateral movement and avulsion of channels were formed.
Next, we explored Martian satellite images in order to search for the distribution of delta topography similar in shape to those formed in flume-tank experiments. Then, we examined formation ages of each delta topography by the crater counting method. As a result, the Lobe Dominant Deltas formed by the water-level fall and the Distributary Channel Deltas formed by constant water-level are dated to be at about 3.6~3.5Ga and are concentrated at the elevation range of -1670m~-4116m near the paleo-shoreline. On the other hand, Step Delta formed by water-level rise are dated to be at about 3.4~3.2Ga, and they are observed only inside craters at elevations of +2698~-4211m.
Based on these lines of evidence, it is considered that a large-scale regression occurred around 3.6~3.5Ga. On the other hand, there was no vast ocean around 3.4~3.2Ga, but local lakes may have developed in craters due to the melting of glaciers and groundwater discharge caused by the temporal warming related to volcanic activities. We hypothesize that the northern lowland oceans that spread on ancient Mars may have disappeared by 3.5~3.4Ga, followed by an environment with localized lakes.