34 potential base-level change knickpoints were identified in 12 major channels spanning the full width of the planet and covering elevations from 900 to −3,500 m, as shown in Figs 2 and 3. None of the channels had equilibrium long profiles, all base-level change knickpoints are located along the Southern edge of the Northern lowlands and every channel contains (at least) one potential base-level change knickpoint.

Figure 2 Topography, distribution of base-level change knickpoints and elevation of knickpoint zones on Mars. The lower map shows the topography of Mars with superimposed possible base-level change knickpoints categorized by the knickpoint zone in which are included. The upper part of the figure comprises, on the right, the Kernel Density Estimate of base-level change knickpoints by elevation and, on the left, a graph displaying the distribution by elevation of the possible base-level change knickpoints against their longitude. Both graphs share the same elevation axis (Y-axis). In the upper-left graph each knickpoint is associated with a number indicating the channel system within is located. The upper-left graph and lower map share the same longitude axis (X-axis). The knickpoints contained within each knickpoint zones have been represented by a point with the same colour than the knickpoint zone. Knickpoints represented by the black colour are not contained within any knickpoint zone. Some of the 34 potential base-level change knickpoints are located too close to each other and thus overlap at this map scale (see Supplementary Table 1 for the complete list of knickpoints). Full size image

Figure 3 (1) View of the circum-Chryse highland-lowland boundary region, which consists of Chryse Planitia, Western Arabia Terra, Tempe Terra, Margaritifer Terra and Xanthe Terra. This boundary is breached by the circum-Chryse outflow channels, where Kasei Valles Maja Valles Ares Valles and Mawrth Valles exhibit base-level change knickpoint (red points) at −3,500 m. The shoreline mapped by tsunami deposits, and the body of water comprised, is represented in blue color. (2) View of the circum-Chryse and Sirenum Terra highland-lowland boundary region, where a Northern body of water at −2,550 m (defined by the average elevation of deltaic deposits) is represented by color. Kasei Valles, Maja Valles and Mangala region channels exhibit base-level change knickpoints (blue points) at −2,500 m. The images for both panels (1 & 2) are color-coded shaded-relief MOLA digital elevation models (460 m/pixel). Credit: MOLA Science Team, MSS, JPL, NASA. We produced the mosaic and maps in this figure using Esri’s ArcGIS 10.6 software (http://www.esri.com/software/arcgis). Full size image

Our analysis shows four clear elevation zones where there are more than one base-level change knickpoint with the lower two zones having frequencies greater than 2 and 3 respectively, indicating they are unlikely to be random (Fig. 2). Importantly, the lowest two zones have knickpoints from geographically distant channels – thereby making it less likely that the common knickpoint elevation was due to a common geological control (e.g. resistant lithology at the same elevation). Therefore, we suggest that the knickpoints were formed as the non-equilibrium channels adjusted their long profiles to a common – planet wide – base level, as sketched in Fig. 1 and evidenced in Fig. 3. Whilst we cannot completely rule out that the knickpoint zones are due to other controls, we argue that the most likely explanation is that the common knickpoints capture past ocean/sea levels.

Importantly, in support of this explanation, the elevations of our knickpoint zones correspond with ocean levels identified in previous research. The detection of deposits likely emplaced by tsunami waves31 at an elevation of ca. −3,795 m suggesting an ancient shoreline (during early stages of Late Hesperian) in the same regions where knickpoints in zone 4 are located at the elevation of −3,462 ± 20 m. The identification of deltaic deposits32 found in widespread locations provides evidence for a large standing body of water (during the latter stages of the Late Hesperian) at an elevation of ca. −2,500 m, thus fully consistent with the past ocean/sea level inferred from the knickpoint zone 3 at −2,485 ± 40 m. Whilst this latter ocean level has been questioned by recent investigations around the Gale Crater34, the similarity in elevation of our knickpoint zone 3 and deltas within other regions closer to them32 (e.g. Tempe Terra or circum-Chryse Region) suggests that (at least) some of these deltaic deposits were most likely formed by global ocean controls. Importantly, our methodology of using knickpoints, for the first time, enables the independent identification of multiple base/ocean levels within the same record.

Further supporting our argument that knickpoints zones 3 and 4 represent past ocean levels, the timing of flows within channels where knickpoints for zones 3 and 4 occur correspond with the dates of corresponding elevation shoreline/tsunami deposits. For knickpoint zone 4, studies indicate the Kasei Valles, Ares Valles, Maja Valles and Mawrth Vallis formed or were active around 3.6 Ga ago35,36,37,38 concurrent (in geological time) with the emplacement of tsunami deposits35 and the Deuteronilus Shoreline39. For our knickpoint zone 3, studies show periods of flow in the Kasei Valles and Maja valles35,40, as well as channel formation in Mangala Region23,35 around 3.4 Ga ago.

Therefore, we suggest the following scenario led to the formation of these major channels and their knickpoints. (1) At around 3.6Ga an ocean or major water body was in place at c. −3,500 m (2) Major flows occurred in the Kasei, Ares, Maja and Mawrth Valles, carving both the channels and also incising into the non-equilibrium channel long profile generating knickpoints at or close to the base level (ocean level) (3) Flow stopped in these channels and over the next 0.2Ga ocean levels rose to c. −2,500 m (4) A second major period of channel flow occurred in the Kasei and Maja Valles, as well as in the Mangala channels, leading to channel incision and the development of new higher knickpoints close to the new base level.

Morphological evidence in the channels suggests that flows were (relatively) short lived. Firstly, the elevation of the bottom of knickpoints can show a small rise in elevation as they retreat upstream from the originating base level (e.g. Fig. 1). In our examples the relatively small difference in elevation between the knickpoint zones and the corresponding shorelines/ocean levels suggests that these knickpoints did not retreat long distances (compared with the scale of these channels), which indicates their forming events may have been short lived or ephemeral. Secondly, most terrestrial knickpoint research is based on the deviation of a channel long profile from an equilibrium profile41 and, visually, our long profiles are clearly not at equilibrium (Supp. Fig. 8), as confirmed using long profile/drainage area analysis (see Supp. Material). This may be explained by the Martian water being sourced differently from Earth channels42, but can also show that such channels flowed for a period insufficient to achieve any equilibrium, thereby supporting episodic or short periods of flow. It is important to note that an equilibrium channel is not a precondition for the presence of knickpoints generated by base level change, as evidenced by those observed by Mackey et al.43, Germanoski & Ritter44 and in the laboratory experiments of Baynes et al.18. Thirdly, the preservation of the lower knickpoints following base level rise suggests that the base/ocean level rose very rapidly, or more likely during a period of no channel flows. Therefore, our results suggest major channel and ocean activity during the Late Hesperian, with two distinct phases of significant channel flows at 3.6 and 3.4 Ga interacting with a Northern ocean at −3,500 m (3.6Ga) rising to −2,500 m (3.4Ga).

A fundamental requirement for separate channel base-level knickpoints to have preserved the signal of the same base level fall is for there to have been no uplift or subsidence of the surface after the knickpoint formation. However, the emplacement of the Tharsis complex has likely caused a major impact on the Martian topography29,30 as can be seen in the Supp. Fig. 7. Our knickpoint zones 1 & 2 are within areas of negligible/no surface elevation changes, allowing them to precede (or be concurrent with the early stages of) Tharsis emplacement and still preserve their initial elevation. In contrast, our knickpoint zones 3 & 4 are within the area affected by Tharsis, but their geomorphology (e.g. Kasei Valles incises back into regions constructed by Tharsis volcanism) suggests that they formed after Tharsis emplacement (or during the latter stages of). The topographic distribution of the knickpoints within the knickpoint zone 3, being consistent with deltaic deposits32, implies no significant modification of the Martian topography since their formation. Therefore, they suggest a post-Tharsis ocean level consistent with a large portion of the Arabia shoreline28,29,30, which (in turn) indicates a re-occupation – during the late Hesperian – of the older Arabia ocean level (presumed Noachian in age29). However, variations in such shoreline topography requires further studies on the relationship between the evolution of a Northern ocean and the Tharsis volcanism.

At its simplest level, knickpoints are markers of (any) disequilibrium within a channel profile, and can be caused by many reasons (e.g., lithological boundaries, base level fall, periodic flow, etc.). Here, we show (i) there are knickpoints on Mars, (ii) the identified knickpoints are at consistent elevations across the Northern part of the planet and not randomly grouped, (iii) they are located close to known past ocean levels, both in elevation and location, (iv) channels with corresponding knickpoints have flowed at the same time as shoreline records were deposited and (v) the knickpoints are not associated with lithological boundaries. This evidence leads to two options for interpretation: knickpoints could either have been formed by (1) a drop in ocean level and the channels grading to that new base level or (2) periodic switching on of the fluvial network and grading to whatever the ocean level is at that moment in time. Given the known chronology of the ocean elevations (i.e., a rise), the fact the channels are not in equilibrium, and the known occurrence of short periods of channel activity (e.g. outburst floods), this favours interpretation 2.

We quite deliberately avoid discussing mechanisms for channel flow or ocean formation and acknowledge that our findings may complement45 or contradict46 other research. However, our results are consistent with a warmer and wetter early Mars climate and suggest that, at stages in Mars’ history, massive channelized flows interacted with Northern oceans at two distinct levels (as proposed in previous investigations47). Furthermore, these findings indicate a complex, dynamic hydrosphere with an active hydrological cycle and an ocean exerting a global control on channel systems.