the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Climate changes and the formation of fluvial terraces in central Amazonia
- 1Institute of Geological Sciences, University of Bern, Bern, 3012, Switzerland
- 2Instituto de Geociências, Universidade de São Paulo, São Paulo, Brazil
- 3Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, Brazil
- 4School of Earth and Environmental Sciences, Seoul National University, Seoul 21990, Republic of Korea
- 1Institute of Geological Sciences, University of Bern, Bern, 3012, Switzerland
- 2Instituto de Geociências, Universidade de São Paulo, São Paulo, Brazil
- 3Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, Brazil
- 4School of Earth and Environmental Sciences, Seoul National University, Seoul 21990, Republic of Korea
Abstract. Climate changes have been considered as an essential factor controlling the shaping of the recent alluvial landscapes in central Amazonia, with implications for explaining the biogeographic patterns in the region. This landscape is characterized by wide floodplains and various terrace levels at different elevations. A set of older terraces with ages between 50’000 and > 200’000 yrs occupy the higher portions of central Amanzonia, whereas multiple terraces next to floodplains occur at lower elevations and display ages of a few thousands of years. These lower terraces, referred to as mid-lower terraces, reveal what can be perceived as a stochastic pattern both in space and time. Despite the widespread occurrence of these geomorphic features, no process-oriented analysis has been conducted to explain their formation. Here, we develop a landscape evolution model referred to as SPASE to explicitly account for fluvial erosion and deposition in combination with lateral channel migration to explore the controls on terrace development. The model results show that the higher terraces were deposited under the condition of a higher base-level for the basins upstream of the confluence between the Solimões and Negro Rivers. The subsequent decrease in the base level initiated a phase of gradual incision, thereby resulting in the current fluvial configuration. The model also predicts that high-frequency climate changes yielded in the construction of mid-lower terraces at various elevations, which however, are all situated at lower elevation than the higher terrace levels. Our model shows that dry-to-wet shifts in climate, in relation to the modern situation, yield a landscape architecture where mid-lower terrace levels are better preserved than wet-to-dry changes in climate, again if the current situation is considered as reference. Finally, our results show that fast and widespread landscape changes possibly occurred in response to high-frequency climate changes in central Amazonia, at least since the Late Pleistocene, with great implications for the distribution and connectivity of different biotic environments in the region. Because of this short time scale of response to external perturbations, we suggest that the streams in Central Amazonia possibly also respond in rapid and sensitive ways to human perturbations.
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Ariel Henrique do Prado et al.
Status: closed
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RC1: 'Comment on esurf-2021-63', Anonymous Referee #1, 07 Oct 2021
General comments
This is an interesting and well written paper addressing the conditions under which river terraces are formed – both a ‘classic’ problem as well as one with specific relevance to the Amazon basin.
At the outset I should say that I am not a modeller and not really qualified to comment on the model. Like me, others may appreciate a change to the title that clearly indicates that this is a modelling paper, not a paper with new field evidence.
Specific comments
The authors model three scenarios and how they may impact terrace formation (aggradation and degradation of the channel and floodplain). Two of those scenarios explore changing climate conditions: one where conditions become wetter and one where conditions become drier. The third scenario explores base level change. The results are interesting and provide some food for thought, particularly in how these results may (or may not) be applicable to other lowland river systems.
I wonder if the authors considered exploring a broader range of conditions (in effect, a sensitivity analysis)? For example they use modern Amazonian precipitation as an initial condition and very it by +/-30%. What are the model results if 2000 +/- 30%, 1000 +/- 30% etc are used?
Second, sediment discharge is dependent on water discharge using a regression based on modern observational data (line 265) but I am left wondering how valid this is under past climate regimes, especially where sediment delivery to rivers may be significantly different to today (line 480-490)?
Finally, with regard to the larger, higher terraces compared with the smaller, lower terraces: I wonder if different mechanisms are required or if this is just a product of their differential preservation as a function of their age? If you like, an example of the Sadler effect where there is more preserved accumulation (and small scale detail) in recent sediments where older sediments have been ‘edited’ to leave only the broader/larger signals. Just an idea.
Technical corrections
See PDF
-
AC1: 'Reply on RC1', Ariel do Prado, 03 Nov 2021
General comments
Our answer:
We thank the reviewer for the constructive comments and suggestions on this study. We address the comments one-by as explained one below.
This is an interesting and well written paper addressing the conditions under which river terraces are formed – both a ‘classic’ problem as well as one with specific relevance to the Amazon basin.
At the outset I should say that I am not a modeller and not really qualified to comment on the model. Like me, others may appreciate a change to the title that clearly indicates that this is a modelling paper, not a paper with new field evidence.
Our answer:
We will change the title to: "Climate changes and the formation of fluvial terraces in central Amazonia inferred from landscape evolution modelling"
Specific comments
The authors model three scenarios and how they may impact terrace formation (aggradation and degradation of the channel and floodplain). Two of those scenarios explore changing climate conditions: one where conditions become wetter and one where conditions become drier. The third scenario explores base level change. The results are interesting and provide some food for thought, particularly in how these results may (or may not) be applicable to other lowland river systems.
I wonder if the authors considered exploring a broader range of conditions (in effect, a sensitivity analysis)? For example they use modern Amazonian precipitation as an initial condition and vary it by +/-30%. What are the model results if 2000 +/- 30%, 1000 +/- 30% etc are used?
Our answer:
Thank you very much for these suggestions. We actually tested many different conditions for the parameters that we used in the models in order to calibrate the model itself. In Section 5.1 we already present a general description of how the model responds to different environmental conditions. We take the opportunity, motivated by the suggestion of the reviewer, to conduct further model runs where we change the initial precipitation conditions. The model indeed predicts that the erosion of the elevated plateaus adjacent to the trunk rivers becomes larger as we increase the precipitation rates (1000 mm to 3000 mm as initial condition). However, the patterns at which the low-elevated terraces form will not change. Therefore, a change of the initial precipitation rates will not alter the main conclusions. We will include these additional model runs in a supplementary file.
Second, sediment discharge is dependent on water discharge using a regression based on modern observational data (line 265) but I am left wondering how valid this is under past climate regimes, especially where sediment delivery to rivers may be significantly different to today (line 480-490)?
Our answer:
We assumed that all the rivers entering from outside of the model grid are at capacity under the modern conditions. Before modelling the basin response to climate change, we generated a stable – or near steady state - initial scenario using the at-capacity condition as a-prior constraint, where the slope of these rivers are stationary for the actual (i.e., measured) values of water and sediment discharge. We then simulated variations of the water discharge for each one of these rivers using Eq. 5 to calculate the variations in sediment flux. By applying Eq. 5 we indeed assume, as noted by the reviewer, that these rivers are at capacity upstream the model grid during the model runs. Since most of these rivers have their sediment sources either in the high Andes or in the elevated plateaus with a generally thick regolith cover (an exception is the Rio Negro), we anticipate that sufficient sediment is available to be eroded by overland flow erosion so that sediment flux of the major streams is at capacity. We will explain these points more carefully in the revised version of our paper.
Finally, with regard to the larger, higher terraces compared with the smaller, lower terraces: I wonder if different mechanisms are required or if this is just a product of their differential preservation as a function of their age? If you like, an example of the Sadler effect where there is more preserved accumulation (and small scale detail) in recent sediments where older sediments have been ‘edited’ to leave only the broader/larger signals. Just an idea.
Our answer:
The ages of terrace deposition that are provided by Pupim et. al (2019) shows a stochastic pattern for the lower terraces. From the results of our model this pattern can be explained as a response to the climatic variations within tens of thousands of years. However, the ages and the elevation of the higher terraces could only be explained by a previously higher base level. However, we can indeed expect that at such a stage of a higher base level, climate cycles would also cause the formation of cut-and-fill terrace sequences with a similar stochastic distribution of ages. In fact, we can see such a pattern in Figure 2 by looking at the ages of the higher terraces. We acknowledge that this has not become clear in the manuscript, and we will update the text accordingly.
Technical corrections
See PDF
Our answer:
Thank you for the suggestions of technical corrections. We will incorporate them in the main text.
-
AC1: 'Reply on RC1', Ariel do Prado, 03 Nov 2021
-
RC2: 'Comment on esurf-2021-63', Anonymous Referee #2, 17 Mar 2022
The comment was uploaded in the form of a supplement: https://esurf.copernicus.org/preprints/esurf-2021-63/esurf-2021-63-RC2-supplement.pdf
Status: closed
-
RC1: 'Comment on esurf-2021-63', Anonymous Referee #1, 07 Oct 2021
General comments
This is an interesting and well written paper addressing the conditions under which river terraces are formed – both a ‘classic’ problem as well as one with specific relevance to the Amazon basin.
At the outset I should say that I am not a modeller and not really qualified to comment on the model. Like me, others may appreciate a change to the title that clearly indicates that this is a modelling paper, not a paper with new field evidence.
Specific comments
The authors model three scenarios and how they may impact terrace formation (aggradation and degradation of the channel and floodplain). Two of those scenarios explore changing climate conditions: one where conditions become wetter and one where conditions become drier. The third scenario explores base level change. The results are interesting and provide some food for thought, particularly in how these results may (or may not) be applicable to other lowland river systems.
I wonder if the authors considered exploring a broader range of conditions (in effect, a sensitivity analysis)? For example they use modern Amazonian precipitation as an initial condition and very it by +/-30%. What are the model results if 2000 +/- 30%, 1000 +/- 30% etc are used?
Second, sediment discharge is dependent on water discharge using a regression based on modern observational data (line 265) but I am left wondering how valid this is under past climate regimes, especially where sediment delivery to rivers may be significantly different to today (line 480-490)?
Finally, with regard to the larger, higher terraces compared with the smaller, lower terraces: I wonder if different mechanisms are required or if this is just a product of their differential preservation as a function of their age? If you like, an example of the Sadler effect where there is more preserved accumulation (and small scale detail) in recent sediments where older sediments have been ‘edited’ to leave only the broader/larger signals. Just an idea.
Technical corrections
See PDF
-
AC1: 'Reply on RC1', Ariel do Prado, 03 Nov 2021
General comments
Our answer:
We thank the reviewer for the constructive comments and suggestions on this study. We address the comments one-by as explained one below.
This is an interesting and well written paper addressing the conditions under which river terraces are formed – both a ‘classic’ problem as well as one with specific relevance to the Amazon basin.
At the outset I should say that I am not a modeller and not really qualified to comment on the model. Like me, others may appreciate a change to the title that clearly indicates that this is a modelling paper, not a paper with new field evidence.
Our answer:
We will change the title to: "Climate changes and the formation of fluvial terraces in central Amazonia inferred from landscape evolution modelling"
Specific comments
The authors model three scenarios and how they may impact terrace formation (aggradation and degradation of the channel and floodplain). Two of those scenarios explore changing climate conditions: one where conditions become wetter and one where conditions become drier. The third scenario explores base level change. The results are interesting and provide some food for thought, particularly in how these results may (or may not) be applicable to other lowland river systems.
I wonder if the authors considered exploring a broader range of conditions (in effect, a sensitivity analysis)? For example they use modern Amazonian precipitation as an initial condition and vary it by +/-30%. What are the model results if 2000 +/- 30%, 1000 +/- 30% etc are used?
Our answer:
Thank you very much for these suggestions. We actually tested many different conditions for the parameters that we used in the models in order to calibrate the model itself. In Section 5.1 we already present a general description of how the model responds to different environmental conditions. We take the opportunity, motivated by the suggestion of the reviewer, to conduct further model runs where we change the initial precipitation conditions. The model indeed predicts that the erosion of the elevated plateaus adjacent to the trunk rivers becomes larger as we increase the precipitation rates (1000 mm to 3000 mm as initial condition). However, the patterns at which the low-elevated terraces form will not change. Therefore, a change of the initial precipitation rates will not alter the main conclusions. We will include these additional model runs in a supplementary file.
Second, sediment discharge is dependent on water discharge using a regression based on modern observational data (line 265) but I am left wondering how valid this is under past climate regimes, especially where sediment delivery to rivers may be significantly different to today (line 480-490)?
Our answer:
We assumed that all the rivers entering from outside of the model grid are at capacity under the modern conditions. Before modelling the basin response to climate change, we generated a stable – or near steady state - initial scenario using the at-capacity condition as a-prior constraint, where the slope of these rivers are stationary for the actual (i.e., measured) values of water and sediment discharge. We then simulated variations of the water discharge for each one of these rivers using Eq. 5 to calculate the variations in sediment flux. By applying Eq. 5 we indeed assume, as noted by the reviewer, that these rivers are at capacity upstream the model grid during the model runs. Since most of these rivers have their sediment sources either in the high Andes or in the elevated plateaus with a generally thick regolith cover (an exception is the Rio Negro), we anticipate that sufficient sediment is available to be eroded by overland flow erosion so that sediment flux of the major streams is at capacity. We will explain these points more carefully in the revised version of our paper.
Finally, with regard to the larger, higher terraces compared with the smaller, lower terraces: I wonder if different mechanisms are required or if this is just a product of their differential preservation as a function of their age? If you like, an example of the Sadler effect where there is more preserved accumulation (and small scale detail) in recent sediments where older sediments have been ‘edited’ to leave only the broader/larger signals. Just an idea.
Our answer:
The ages of terrace deposition that are provided by Pupim et. al (2019) shows a stochastic pattern for the lower terraces. From the results of our model this pattern can be explained as a response to the climatic variations within tens of thousands of years. However, the ages and the elevation of the higher terraces could only be explained by a previously higher base level. However, we can indeed expect that at such a stage of a higher base level, climate cycles would also cause the formation of cut-and-fill terrace sequences with a similar stochastic distribution of ages. In fact, we can see such a pattern in Figure 2 by looking at the ages of the higher terraces. We acknowledge that this has not become clear in the manuscript, and we will update the text accordingly.
Technical corrections
See PDF
Our answer:
Thank you for the suggestions of technical corrections. We will incorporate them in the main text.
-
AC1: 'Reply on RC1', Ariel do Prado, 03 Nov 2021
-
RC2: 'Comment on esurf-2021-63', Anonymous Referee #2, 17 Mar 2022
The comment was uploaded in the form of a supplement: https://esurf.copernicus.org/preprints/esurf-2021-63/esurf-2021-63-RC2-supplement.pdf
Ariel Henrique do Prado et al.
Model code and software
SPASE_Model Ariel Henrique do Prado https://github.com/Ariel-H-Prado/SPASE_Model.git
Ariel Henrique do Prado et al.
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