The Vérard et al. (2015) method for 3D palaeogeographic reconstructions: How solid is its base?

A. J. (Tom) van Loon*

Geocom, Valle del Portet 17, 03726 Benitachell, Spain

1 Introduction

Reconstruction of the worldwide palaeogeography for all successive time slices may be considered as the primary objective of geology as a fundamental science. In its most essential meaning palaeogeography presents the spatial configuration of all rock types; for sediments this implies the spatial distribution of all sedimentary, erosional and other environments. The palaeogeography of a sedimentary area at a specific phase of the geological pasty is commonly presented in the form of a map. If the map is sufficiently detailed, the processes involved of the sedimentation in the various sedimentary environments can be deduced from it. This helps to predict how the palaeogeography will most probably develop in the next time-slice.

One of the most important features that determine the ongoing palaeogeographic development is the relief. The palaeogeographic development of an area consequently would be much easier to reconstruct if data about the relief were available, not only in a qualitative sense, but also in a quantitative sense. There were, apart from some very specific situations, no reliable methods available until now, however, to reconstruct the relief with some accuracy. This holds particularly for the altitude at which sediment accumulated on the continents; for oceans this holds in general as well, although the depth of the sea bottom can sometimes, at least in shallow-marine environments,be determined on the basis of fossil remains. Exactly that was the reason why the Editor-in-Chief of the present journal welcomed the contribution by Vérardet al.(2015), who proposed a method to reconstruct the marine depth and continental altitudes at which sediments built up. I fully agree that a reliable method to determine hypsometry and bathymetry would be a great leap forward in geology, possibly even the greatest leap since the theory of ocean-floor spreading and plate tectonics.

It must, however, be emphasized that a method with potentially enormous significance for the Earth sciences requires a critical look before common acceptance — one should first find out whether some flaws may be present. I am myself not an expert in the field of isotope-ratio variations, which are one of the foundations of the method, but in my opinion this is not truly relevant here, as the presentation of the method by Vérard and co-workers raises some questions (and raises some doubts) regarding some of the assumptions on which this method for reconstructing the marine and continental 3D palaeogeography is based.I want to express explicitly that my comments are not intended to diminish the applicability of the method; on the contrary, I hope that Vérardet al.will be able to take away my doubts and to reassure the Earth science community of the validity of this method. Then, indeed, this would mean a great leap forwards in the Earth sciences.

2 Foundation of the method

It seems that at least three points need more clarification. The first concerns the foundation of the method used.As the authors explain, the method followed is a heuristicbased one (p. 65). It may be true that it is difficult to imagine that the present-day knowledge of the topic could use another approach that might give similar (crude) results,but a heuristic approach is, by definition, fairly dangerous:it may easily involve some vicarious reasoning, so that the outcome may be biased. The authors do not really make clear why the outcomes of their approach would be more reliable than those of, for instance, the numerous climate models that have also a large heuristic component. The model would consequently be much more convincing if not only were mentioned that the outcomes fit in general fairly well with the present-day interpretations, but if the authors would also make clear why the approach is inherently different from the current interpretations about, particularly, the depositional depths of marine sediments (which are essentially based on the principle of uniformitarianism— the present is the key to the past).

The second, related, problem is that the authors do not reveal what type of statistics are used and how they are applied (“Using statistical analyses, we converted the features constituting the model ... into depth or elevation ...”:p. 65; “... after global statistics on present-day Earth ...”:p. 67). It is well known that the application of different types of statistics commonly lead to different outcomes.Consequently, the reader should be informed in detail not only about the statistical methods that were applied, but also whether different types of statistics were applied for different analyses.

The third, fairly different, problem that is not touched at all concerns the types of sediment. All examples and details provided by Vérard and co-workers deal with clastic rocks, but how would (and could) the method be applied to sediments with another origin (chemical, biogenic, organic,pyroclastic)? It seems to me that the method cannot be well applied if non-clastic sediments prevail, and I would certainly welcome additional information from the authors regarding this aspect that would take away any scepticism.

3 Lacking information

The results of the method are claimed to provide hypsometric or bathymetric data for any place on Earth for any time. This conclusion must, by definition, be taken with a fairly large degree of uncertainty/inaccuracy, because of the difference in the rate of change of palaeogeography as a consequence of the difference in the speed with which the various geological processes take place. Mountain building, for instance, is a relatively slow process, but erosion is still slower, as can easily be deduced from the existence of high mountain ranges. This must have consequences for the87Sr/86Sr ratio in deposits built by erosion products of rising mountain chains. Not all uplift is slow, however, glacio-isostatic movements change the altitude/depth of large areas quickly (within a few thousands of years).

A good example is the Yoldia Sea in the Baltic area,which existed for a thousand years (11,700-10,700 BP) and then changed into the Ancylus Lake by rapid glacio-isostatic uplift (Mörner, 1995; Raukas, 1995). It seems highly unlikely that the palaeogeographic development of an area affected by such rapid transitions from marine to continental can be traced by the method proposed by Vérardet al.(2015). Yet,comparable transitions due to climate fluctuations in the Phanerozoic must have taken frequently, considering the Ordovician (Brenchley, 1994; Poussartet al., 1999; Younget al., 2010) and Permo-Carboniferous ice ages (Beerling,2002), and the relatively sudden time-spans of high temperature such as the Paleocene-Eocene Thermal Maximum(Sluijset al., 2006; Zhouet al., 2014). It may be true that large glacial ice caps were not truly common in the geological past and that they left relatively rare traces (Van Loon,2000), but they resulted in important and rapid changes in the configuration of the land masses and the depths of the seas, and it should be made clear by Vérard and co-workers how they think that such rapid changes can be discovered by their method.

An entirely different problem that is not faced in the article by Vérard and his team is that they make not clear what the role is of the accuracy of datings of rocks. In the field it is, as a rule, not possible to establish the age of the rock units under investigation. The method is, obviously,not suitable for igneous rocks, and the question thus arises how rocks under study must be dated, and what accuracy the dating must have for reliable results. It should be noted in this context that a palaeogeographic reconstruction can be made only if the various rock units shown on a map have all the same age (or, at least, the sedimentary or erosional surfaces of all rock units should date from the same time). Of course, index fossils can help to determine an age, sometimes even quite exactly, but they are commonly restricted to specific environments, so that they cannot be used for an area characterized by different (sub)environments, whereas it is just the objective of palaeogeographic maps to show such simultaneous environments. What are the implications for the reliability of the outcome of the method described by Vérard?

4 Problem of the water volume

The depth of the oceans depends primarily on the volume of the oceanic water (which is taken by Vérard and co-workers as the volume under 0 m: p. 73). It is roughly 1.335 x 1018m3(p. 73). During the Phanerozoic hardly any oceanic water has gone lost. Changes in the sea-water level that are significantly enough to affect the palaeogeography of the coastline therefore depend on other parameters.One of them is the temperature of the sea bottom. If it is high (like in the Late Cretaceous; it is likely that the Late Cretaceous highstand was due to increased activity of upwelling magma through the mid-ocean ridges (Sourkhabi,2009; Thompson and Turk, 2009), this must have resulted in both vertical expansion of the sea floor and heating, with consequent expansion, of the ocean water: Summerfield,2014, p. 440). This led to flooding of coastal lowlands, but this effect is volumetrically limited, so that it may not have affected the depth of the deep-sea significantly.

Another factor is the volume of ocean water that is trapped on land in the form of continental ice sheets. Melting of all ice on Antarctica and Greenland might lead to a rise in the sea level of some 70 m (Fretwellet al., 2013;Kusky, 2005, p. 223). During the maximum of the last ice age (LGM), the sea-level was some 120 lower than nowadays. Combination of these data indicates possible sea-level variations as a result of climate fluctuations of the order of 200 m. This order of magnitude of the variation in the height of the sea level was probably not exceeded for the Phanerozoic (Kominz, 2004, p. 2613), even though other processes such as increased magmatic output via mid-oceanic ridges (resulting in high temperatures with expansion of the oceanic crust and the ocean water as a consequence:Sahagian, 1988), mountain building (Harrison, 1990) and continental break-up or assembly (Herrmannet al., 2004)may influence the sea-level height; the various processes affecting sea-level height should, however, not be superimposed, as they did not all take place simultaneously.

In spite of this, Vérardet al.cites (p. 64) sea-level stands of 225±42 m (above present sea level) mentioned by EPC(1988) for 82 Ma ago, 242 m for 86 Ma mentioned by Hallam and Cohen (1989) and even 266 m for 91 Ma mentioned by Haqet al.(1987) and Haq and Schutter, (2008). Considering not only the data in the previous paragraph but also the controversies about the magnitudes of the sea-level fluctuations, it would be interesting to know whether Vérard and his team consider these extremely high sea-level stands as reasonable (they mention themselves the doubts concerning such magnitudes of the sea-level fluctuations expressed by Carter, 1998), whether they investigated rocks that date from the above times, whether they found, indeed, such extreme depths for the marine sediments deposited on the continental flat, and in how far lower values for the extreme highstands would have influenced their model, and thus the outcomes for the bathymetry and/or hypsometry of other sediments.

5 Discussion

It is beyond doubt that palaeogeographic reconstructions would greatly benefit from a tool that would allow to find out the depositional depth or altitude of specific deposits. It is also beyond doubt that tectonic activity, particularly in the form of uplift, must have affected the palaeogeography in the geological past, just like it does now.That the presence of mountain belts results in erosion is also beyond doubt, and I cannot deny that it is most likely that changes in the Sr-isotope ratio reflect changes in the erosion rate, or at least changes in tectonic activity (cf.,Mülleret al., 2008).

Erosion and sedimentation are, however, not processes that occur as exclusive and continuous processes. Just because the palaeogeography tends to change with time,erosion and sedimentation alternate, commonly both on the short and on the long term. This implies that, in the course of geological time, sediments of different origin and of different ages and of different altitudes/depths become mixed up, forming new sedimentary units of a younger age and, most commonly, formed in a different environment and most likely at a different altitude/depth. This must necessarily imply that the newly formed sedimentary units contain particles with different inherited Sr-isotope ratios.This is, for instance, well comparable with the heavy-mineral composition of fluvial sediments deposited in a river with a catchment in which rocks of different ages are exposed; and if several rivers are responsible for the sediment input in a lake or a sea, the lacustrine or marine sediments will have a heavy-mineral composition that is based on even more mixing (Do Nascimentoet al., 2015; Li, 2015).Even the surfaces of quartz grains can reveal the different origins of sediment (Woronkoet al., 2015). Would a similar mixing not occur of sediments with Sr-containing minerals derived from different sources? It seems that Vérardet al.did not take this into consideration in their model. In how far do they think that this might affect the reliability of their model?

It is true that palaeogeography has made enormous steps forward in the past decades, thanks to different approaches,but in practice we could until now reconstruct the palaeogeography of an area (however large or small, however detailed or crude) only in two dimensions. I would welcome,indeed, the possibility to add a third dimension, and I am very willing to replace the practices that are commonly followed nowadays by a 3D tool if it were available.

In modern society it is common use now to apply new devices and new techniques, just because they are new; I do not: I want to apply a new technique only if I am convinced that it is better than the old ones. Regarding the model described by Vérard and co-workers, I must admit that I’m not convinced; let’s hope, not convinced yet. I certainly do not exclude the possibility that the method could give fairly accurate results, but I cannot judge as long as the developers do not answer several critical questions, or if they do not provide (perhaps because of restrictions related to the copyright and/or the licensing) all underlying data.

6 Conclusions

The model proposed by Vérardet al.(2015) to enable 3D reconstruction of the palaeogeography in the geological past might prove to be an enormous step forwards. It is, however, impossible to judge the reliability of the outcomes already now because many aspects of the model remain unclear. It also seems unlikely that the model can provide a reliable 3D palaeogeography for thin time slices in areas where the conditions changed rapidly. It is therefore in my opinion, as long as no more convincing details are provided, too early to state, as the authors do, even in their abstract, that the model has “the advantage of being applicable anywhere on the globe and at any geological time”. The foundations of the model are not yet solid enough.

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