- Email: [email protected]
Continental shelf drowned landscapes: Submerged geomorphological and sedimentary record of the youngest cycles
Continental shelf drowned landscapes: Submerged geomorphological and sedimentary record of the youngest cycles Kim M. Cohen, F.J. (Paco) Lobo PII: DOI: Reference:
S0169-555X(13)00462-5 doi: 10.1016/j.geomorph.2013.09.006 GEOMOR 4485
To appear in:
Geomorphology
Please cite this article as: Cohen, Kim M., Lobo, F.J. (Paco), Continental shelf drowned landscapes: Submerged geomorphological and sedimentary record of the youngest cycles, Geomorphology (2013), doi: 10.1016/j.geomorph.2013.09.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Continental shelf drowned landscapes: submerged geomorphological and sedimentary record of the youngest cycles
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Dr. Kim M. Cohen 1, 2, 3, Dr. F.J. (Paco) Lobo 4,
[email protected],
Dept. Physical Geography, Utrecht University, Heidelberglaan 2, 3584 CS, Utrecht, The Netherlands
2
TNO Geological Survey of the Netherlands, Princetonlaan 6, Utrecht, The Netherlands
3
Deltares Research Institute BGS-TGG, Princetonlaan 6, Utrecht, The Netherlands
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[email protected], 4
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From the guest editors
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Instituto Andaluz de Ciencias de la Tierra, CSIC-University of Granada, Avenida de las Palmeras nº 4, 18100 Armilla (Granada), Spain
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Continental shelves today find themselves largely submerged as a consequence of the sea-level rise in the last 20,000 years, the time since the Last Glacial Maximum (LGM), the period of maximum ice mass and minimum ocean volume within the Last Glacial Cycle. Their geomorphology, however, is far from controlled by marine processes alone. Shallow parts of the shelves were land during most of the past 120,000 years (the last full interglacial-glacial cycle), and deeper parts of the current shelves were coastal zones for most of that time, and this goes for older cycles too. To describe and understand the geomorphology of continental shelf drowned landscapes requires combining the knowledge on marine and terrestrial processes and to take into account both climatic and sea-level cyclicities in the larger picture of shelf evolution. With this special issue, we hope to interest the Geomorphology readership in the peculiarities of Continental Shelf Drowned Landscapes and their evolution during the youngest climatic-eustatic cycles. The special issue emerges from two international meetings organised in 2011. These were (i) the CMP-commission sponsored session on ‘Drowned landscapes and continental shelves during the last glacial cycles’ at the XVIII. INQUA congress (21-27 July 2011) in Bern, Switzerland, and (ii) the 5th IGCP526 conference (13-16 Sept 2011) held in Sidney, British Columbia, Canada, on ‘Continental Shelves: Risks, Resources and Record of the Past’.
ACCEPTED MANUSCRIPT Contents of the special issue
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The issue brings together 10 papers that cover a considerable area of the shelves of the Northern and Southern Atlantic Ocean and the Mediterranean, off North America (1 paper), Europe (5 papers), North Africa (1 paper) and South America (3 papers) (Fig. 1). Mapping the shelf morphology is truly a marine geological exercise, which has experienced great developments in sonar bathymetry imaging and seismic sub-bottom profiling techniques. Collecting and logging cores and obtaining material for dating is already more a mix of ‘marine’ and ‘terrestrial’ techniques – and full integration of ‘land’ and ‘sea’ thinking is required for reconstructions of the continental shelf landscape evolution to make sense.
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The studies included in this special publication cover a wide range of climatic zones that are reflected in the nature of dominant sedimentary processes, including periglacial, siliciclastic, mixed (carbonatesiliciclastic) and aeolian coastal to shelf environments. This environmental diversity is exemplified in the variety of morphologies and deposits reported. The areas covered in typical continental shelf studies are large and much of the collected data are of graphical nature. Consequently, this special issue is rich in figures, in particular maps and seismic sections. The issue collects the following papers:
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The paper by Harris et al. presents the shelf evolution off the SE United States over the last glacial cycle, positioned in the periphery of the Laurentide ice sheet, with prominent submerged cliff-like coastlines from MIS 3 and MIS 2 and river valleys traversing the exposed shelf, in places developing deltaic deposits.
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The paper by Reis et al. also studies features produced by the basinward migration of Late Pleistocene coastlines, on the continental shelf of the Santos basin (Rio de Janeiro State, SE Brazil). Particular attention is given to bathymetric steps resulting from coastal versus submarine processes.
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The paper by Dominguez et al. addresses the much narrower shelf off central Brazil, focusing on the interaction between exposed-shelf geomorphological features and inundated-shelf carbonaceous mud accumulations. The short paper by Isla reports on the peculiar setting of local tectonic basins that are part of the Patagonian shelf in similar ways as further inland – up to the times that transgression overtops sills and inundates them. Moving back to the northern hemisphere and to the opposite side of the Atlantic, the paper by Thébaudeau et al. focusses on the rocky northern coast of Ireland and Northern Ireland. It investigates shoreline evolution and the significance of inherited morphology in the youngest 16,000 years applying mathematical modelling for various sea-level rise scenarios. The paper by Mellett et al. presents mapping and dating evidence from the peculiar geomorphology of the English Channel shelf off southern England. This shelf occupies a position in the immediate periphery of ice sheets once covering Britain and Ireland, and through the Strait of Dover connects to the North Sea Basin and the Scandinavian ice masses and the water that this produces. Erosional features from times of greatest meltwater release are contrasted with depositional and geomorphological record from younger times.
ACCEPTED MANUSCRIPT The paper by Ruiz et al. addresses the role of incidental tsunamis on the estuaries of Southwestern Spain during prehistorical and historical times. The geomorphological and sedimentological imprints are reviewed in the general context of the gulf of Cádiz. In addition, a recurrence period is tentatively suggested, taking into account existing evidences and corrected reservoir effect.
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The paper by Rodriguez-Vidal et al. addresses the shelf landscape off Gibraltar during the last glacial, exploring the vegetation, fauna and Neanderthaler population of the time. Emerged shelf geomorphological features, substrate geology and freshwater hydrology together made the area the Mediterranean refuge it was.
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The paper by Tropeano et al. addresses the Metaponto coastal plain and near shore in southern Italy, with particular attention to mapping out palaeovalley incisions marking the longer stages of lowered sea level. The paper further highlights the contrast of fills of such incision with the sequences of their interfluves, comprised of coastal plain sequences from the penultimate cycle and before mostly.
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The paper by Hanebuth et al. addresses the shelf off Mauretania and the great Sahara desert. Here, rather than fluvial phenomena that receive attention in most of the other papers, it is eolian transport from the Sahara that provides major sediment supply. Indeed, eolian deposits are preserved as part of the shelf record from critical periods within the last glacial cycle, at least to the present day.
A geomorphological view on shelves
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Geomorphological processes acting on the shelves are the physical link between the continental terrestrial and oceanic marine realms of the world. Whereas in recent decades many studies have sought linkage between continental and oceanic records and processes over millennial and longer scales, this has quite often been done by tele-correlation and distant intercomparison. The complexities that also correlating shelf realm records would add were often skipped and instead made ‘black box’ parts of the system to explain response lags. Even worse, the possibility that a shelf zone induced such a lag is not always fully considered when correlations between developments on the continent and records in ocean correlations are proposed. To phrase this most negatively: roles for the shelf are lost in simplification, or only invoked when ‘needed’. Much of this, we believe, has more to do with shelf records of the youngest few cycles not yet being very well disclosed, than true misconceptions on the role of the shelf in system earth. Various research communities now realise that the shelves and coastal zones of the world, are important elements in the source-to-sink chain, because they dynamically alternate their functioning as source and as sink through climatic and sealevel cycles. Though currently submerged and difficult to access, the shelves hold an important archive resulting from supply of sediment and throughput of discharge from surrounding terrestrial areas (Walsh and Nittrouer, 2009); and only transfer part of these fluxes to deeper waters, in some stages presumably with more delay than in others. The characterization of the morphological variability in continental shelves is of paramount importance to understand the long-term growth of continental margins, and more recently for habitat characterization (Harris, 2012). The geomorphological variability may be related either to
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marine sediment dispersal processes or to successive transgressive-regressive shoreline translations (Olariu and Steel, 2009). This last driving process is associated with shelf exposure and subsequent flooding and therefore more directly influences the interrelation between terrestrial and marine processes. The continuous shoreline migrations are thus able to generate distinctive morphological features, such as palaeo-coastlines at a range of water depths (Thébaudeau et al., this issue), and/or paleovalleys (Tropeano et al., this issue; Dominguez et al., this issue; Harris et al., this issue) as the most conspicuous elements.
Sea-level falls and rises
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The geomorphological and shallow geological records of continental shelf settings have been quite complete in recording its palaeoenvironmental evolution, due to the influence of sea-level changes between highstand and lowstand shorelines (Suter, 2006). The most recent change was driven by a rapid, dramatic postglacial sea-level rise (especially between 14.7 and 8.3 ka cal BP; e.g. Carlson and Clarke, 2012). The slower and more oscillatory sea-level fall over most of the Last Glacial Cycle (especially between 80 and 30 ka), however was also highly influential in sculpting the shelf morphology – as many of the papers in the special issue suggest (e.g., Mellett et al., this issue; Reis et al., this issue; Tropeano et al., this issue; Harris et al., this issue). To categorize encountered submerged geomorphological features in the middle sectors of the shelf as principally the product of either ‘the transgression after the LGM lowstand’ or ‘the sea-level fall towards the LGM’ is a critical step in working up observational data to scientific result in any continental shelf mapping study, and for the papers in this special issue in particular.
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Many papers in the special issue (Table 1), based on grown amounts of data, find, recall and refine earlier inferences that most of the shelf features would be the product of the last transgression. The perceived erosive nature of the youngest transgressive stage is being downplayed, the preservation potential for deposits from earlier stages in the last glacial (MIS 4 and MIS 3, besides MIS 2) is highlighted. This primarily considers the preservation potential of ‘falling stage deposits’ within the current cycle, which is not yet complete. To what extent the MIS 4, 3 and 2 deposits – and their postLGM transgressive cover – will also survive yet-to-come stages of sea-level fall (expected 50 to 100 ka in the future) can be evaluated by comparative study of deposit preserved from MIS 6 and MIS 5, from the penultimate climatic-eustatic cycle. Such comparison is started in the papers by Hanebuth et al. (this issue) and Mellett et al. (this issue). Regarding the last glacial stage, many papers in this special issue present themselves as ‘users’ of data of past sea-level positions, as a means of indirectly assigning ages to features. It should not be overlooked, however, that these same studies produce information on feature depths (e.g. Isla, this issue; Harris et al., this issue). When the new insight that falling stage features did remain preserved despite the post-LGM transgression is combined with efforts to more-directly date paleo-coastline features, this greatly enlarges opportunities to spatially diversify paleoconstraints to sea-level for the period MIS 5 to MIS 2. In the text above, the climatic and sea-level cyclicity terminology as developed in the Quaternary Geological community (such as the use of ‘marine isotope stages’ and ‘the LGM’) is mixed with eustasy-connected terminology in ‘highstand’, ‘lowstand’, ‘falling’ and ‘transgressive’ stages that
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come from the sequence stratigraphical method. That terminology originates from hydrocarbon reservoir geology, is widely in use in sedimentary geology (to integrate seismic, outcrop and core data collected at basin scale), and in the last decade has found more and more application in Quaternary shelf and coastal contexts too (Zecchin and Catuneanu, 2013). Indeed, the youngest, most-complete shelf and coastal sequences provide the interpretation templates for more ancient counterparts. Delays between Source and Sink
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Because of the additional independent time control that Quaternary dating methods provide, their relative completeness and essentially even preservation of the youngest deposits around the world, for sequence stratigraphic applications the youngest shelf records not only provide nice examples for comparison, but also the opportunity to quantify rates at which accommodation space is created and sediment fluxes are able to fill it. We are far from making maximum use of the completeness of the shelf record, because mapping them has only just started and high resolution shelf studies are still patchy (or the data are not in the public domain because of commercial value). It is clear that studies such as in this Special Issue make major contributions in unlocking the shelf record also from the ‘source to sink’ global sediment routing perspective, and as such have relevance at much longer than glacial-interglacial timescales. Under this perspective, the storage capacity of shelf settings appears to be much more variable than catchment areas (landward) and deeper environments (seaward). Given the sea-level variations over glacial-interglacial cycles, storage capacity depends on relative elevation and physiographic properties combined (e.g. Sømme et al., 2009). During highstand conditions, for example, storage capacity on shelves is mainly affected by the interplay between alongshore currents and submarine canyons – and it affects only the proportion of continental sediment that is not trapped in the coastal and deltaic plain sub-environments. In contrast, sediment delivery to the continental slope is enhanced during lowstand conditions due to the generation of larger composite river systems crossing the shelf, which carry sediment generated in permanent terrestrial parts of the continent, and additional sediment evacuated from previous coastal plain and upper shelf areas. The deeper ocean may be the ultimate sink for some of the finest sediments eroded from the continents, but a much greater amount and range of grain sizes accumulates and is preserved on continental shelves and slopes (e.g., Hanebuth et al., this issue). Indeed, a significant amount of finegrained sediment accumulates in shelf settings under various configurations, depending on the type of dispersal system which is defined by the hydrodynamic regime, shelf width and sediment load (Walsh and Nittrouer, 2009). Let us return to the geomorphological perspective and the time scales of the last few glacial cycles. The nature of the geomorphology and Quaternary record of the shelves is controlled by the alternating durations that specific areas were `terrestrial´, `coastal´ and `marine´. The drowned landscapes featured in the special issue have been subject to alternating fluvial terrestrial, coastal, transgressive marine and high-stand marine processes, and in some places also to desert, periglacial and proglacial processes. Their submerged or coastally buried landforms are complex outcomes of erosion, reworking and deposition by all these processes. Through these landscapes, fresh water, nutrients and sediment have been routed to permanently marine parts of the shelf and oceanic areas beyond.
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Past Habitat
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Now lost biomes once covered the shelf areas, traversed by rivers, fauna and prehistoric humans. Biogeographically the regions are areas that provide and have provided ephemeral dispersal opportunities for terrestrial, marine and freshwater aquatic species (Rodriguez-Vidal et al., this issue). Physical oceanographic control interacts with drowned terrestrial geomorphology in the submerged shelf landscape (Reis et al., this issue). The shelf’s dynamic role in the physical and biological transport networks of the world is easily overlooked when only the modern configuration is considered (Cohen et al., 2012), a fact more and more realized in the global climate, earth system and human dispersal modelling communities.
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Shelves are part of the global hydrological, nutrient and mineral cycles – and together with coastal plains and deltas, the shelves occupy a peculiar and important position, as they store major volumes of routed sediment and nutrients, in part permanent and in part temporarily (Laruelle et al., 2013). Furthermore, they seemed to play an important role in the delivery of important amounts of methane to the atmosphere during the deglacial flooding, due to rapid biomass decomposition (Ridgwell et al., 2012).
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Linked to climatic and sea-level cycles they do so at exactly the time scales and places that make earth system modelling complicatedly ‘complex’. Being a natural boundary zone for both marine and continental research communities the parameterization of the geomorphological functioning of shelves in the bigger earth system, is still a bit under developed compared to, for example, hydrological, glaciological and oceanographic modelling. Its record, however, is increasingly uncovered and revealed (this issue) and rather than a separating boundary zone, the shelves have become a zone of overlapping marine and terrestrial research. It is believed that in the longer term this will also be beneficial for quantitative approaches – not least because from mapping, dating and reasoning it has become clearer what the relevant time and spatial scales are at which the shelf zone affects hydrological and sedimentary cycles. Ancient humans have also used the natural shelf landscape, during the last glacial cycle, the landscapes saw rapidly growing human populations, forced to evade these areas with postglacial sealevel rise, leaving archaeological heritage behind. In cycles before, alternating shelf flooding and exposure and the coastal response constituted a major influence on human settlement and migration routes. Modern humans are increasingly using and affecting the coastal and shelf realm. Shelves represent key locations for aggregates and sand extraction that will be mined much more intensively in the coming years to meet growing demands, including that required to supply coastal defence projects, beach nourishment and in a broader sense coastal zone management. Continental shelves provide a range of present-day marine habitats, which are related to the geological characteristics of the seafloor, such as sediment size percentages and degree of heterogeneity (de Juan et al., 2013). Additionally, habitat distribution is also influenced by human activities that can pose significant threats, such as fishing and pollution (Harris and Baker, 2012). Consequently, habitat mapping is being regarded as a very useful tool to monitor changes and to mitigate threats in order to achieve a sustainable management (Dominguez et al., this issue). Finally, considerable parts of shelf areas have been affected by mass movement processes and neotectonic activity; bearing substantial risk to submarine infrastructures and conducive to tsunami generation – affecting the
ACCEPTED MANUSCRIPT ever growing global population, growth that tends to concentrate in the coastal zones (Ruiz et al., this issue).
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Conclusion
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The ‘drowned landscape’ nature of continental shelves and man’s occupancy and use of the coastal and shallow marine environment demands knowledge transfer between scientific disciplines. This objective is at the heart of IGCP 526 (Risks, Resources, and Record of the Past on the Continental Shelf; http://igcp526.io.usp.br/), the European Commission COST-action TD0902 SPLASHCOS (Submerged Prehistoric Archeology and Landscapes of the Continental Shelf; http://splashcos.org/), and INQUA projects “1202: Rapid environmental changes and human impact on continental shelves.” and 1301: “PALSEA2 (PALeo-constraints on SEA-level rise2)”, and is equally advocated through this editorial and the special issue as a whole. We feel that considerable scientific progress is made; and that this achieved through innovations in approach and methods, but especially through interdisciplinary knowledge transfer and conceptual integrations. With this publication, we hope to unlock and display this progress to the wider geomorphological research community.
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Acknowledgements
References
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We gratefully acknowledge authors, reviewers, co-chairs of the INQUA and IGCP526 sessions S. Toucanne, F. Chiocci, R. Hetherington and J. Vaughn Barrie, senior editor Adrian Harvey (overseeing Geomorphology’s special issues) and Elsevier’s production staff, in particular Karunamurthy Narasimhan for their work towards completing this issue. This is a contribution to IGCP-526, to COSTaction TD0902 SPLASHCOS, to INQUA project 1202 and to PALSEA2 (PAGES-IGBP/INQUA1301/WUN).
Carlson, A.E., Clark, P.U., 2012. Ice sheet sources of sea level rise and freshwater discharge during the last deglaciation. Reviews of Geophysics, 50, RG4007. Cohen, K.M., MacDonald, K, Joordens, J.C.A., Roebroeks, W., Gibbard, P.L. 2012 The earliest occupation of north-west Europe: a coastal perspective. Quaternary International, 271, 70-83. de Juan, S., Lo Iacono, C., Demestre, M., 2013. Benthic habitat characterisation of soft-bottom continental shelves: Integration of acoustic surveys, benthic samples and trawling disturbance intensity. Estuarine, Coastal and Shelf Science 117, 199-209. Harris, P.T., 2012. Seafloor Geomorphology – Coast, Shelf, and Abyss. In: Harris, P.T., Baker, E.K. (Eds.), Seafloor Geomorphology as Benthic Habitat. Elsevier, London, pp. 109-155. Harris, P.T., Baker, E.K., 2012. GeoHab Atlas of Seafloor Geomorphic Features and Benthic Habitats: Synthesis and Lessons Learned. In: Harris, P.T., Baker, E.K. (Eds.), Seafloor Geomorphology as Benthic Habitat. Elsevier, London, pp. 871-890.
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Laruelle, G.G., Dürr, H.H., Lauerwald, R., Hartmann, J., Slomp, C.P., Goossens, N. & Regnier, P.A.G., 2013. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins. Hydrology and Earth System Sciences 17, 2029-2051.
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Olariu, C., Steel, R.J., 2009. Influence of point-source sediment-supply on modern shelf-slope morphology: implications for interpretation of ancient shelf margins. Basin Research 21, 484-501.
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Ridgwell, A., Maslin, M., Kaplan, J.O., 2012. Flooding of the continental shelves as a contributor to deglacial CH4 rise. Journal of Quaternary Science 27, 800-806.
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Sømme, T.O., William, H.-H., Ole, J.M., John, B.T., 2009. Relationships between morphological and sedimentological parameters in source-to-sink systems: a basis for predicting semi-quantitative characteristics in subsurface systems. Basin Research 21, 361-387.
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Suter, J.R., 2006. Facies Models Revisited: Clastic Shelves. In: Posamentier, H.W., Walker, R.G. (Eds.), Facies Models Revisited. SEPM Special Publication, 84, p. 339-397. Walsh, J.P., Nittrouer, C.A., 2009. Understanding fine-grained river-sediment dispersal on continental margins: Marine Geology 263, 34-45.
Figures
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Zecchin, M., Catuneanu, O., 2013. High-resolution sequence stratigraphy of clastic shelves I: Units and bounding surfaces. Marine and Petroleum Geology 39, 1-25.
Tables
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Figure 1: Map of global shelf area - with studied areas of papers in this issue.
Table 1: Aspects of Shelf Geomorphology addressed in papers
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X X X X X
X X X X X X
Modern resources, aggregate, fishery
Submerged biomes and archaeology
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X
X X
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X
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X X
Gradual vs. punctuated development
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Shelf substrate geological diversity
Earlier glacial-interglacial cycles
Last interglacial highs stand
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Last glacial cycle regression
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Shelf off Central Brazil N Mauritania SE United States N Patagonia English Channel Rio d. J., Brazil Gibraltar SW Spain Ireland S Italy
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Continent S. Am. Africa N. Am. S. Am. Europe S. Am. Eur. Med. Europe Europe Eur. Med.
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Paper (this issue) Dominguez et al. Hanebuth et al. Harris et al. Isla Mellet et al. Reis et al. Rodriguez-Vidal et al. Ruiz et al. Thébaudeau et al. Tropeano et al.
Post-LGM transgression
TABLE 1
X
X
X X X X X
X X
X X X
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Figure 1
S0169-555X(13)00462-5 doi: 10.1016/j.geomorph.2013.09.006 GEOMOR 4485
To appear in:
Geomorphology
Please cite this article as: Cohen, Kim M., Lobo, F.J. (Paco), Continental shelf drowned landscapes: Submerged geomorphological and sedimentary record of the youngest cycles, Geomorphology (2013), doi: 10.1016/j.geomorph.2013.09.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Continental shelf drowned landscapes: submerged geomorphological and sedimentary record of the youngest cycles
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Dr. Kim M. Cohen 1, 2, 3, Dr. F.J. (Paco) Lobo 4,
[email protected],
Dept. Physical Geography, Utrecht University, Heidelberglaan 2, 3584 CS, Utrecht, The Netherlands
2
TNO Geological Survey of the Netherlands, Princetonlaan 6, Utrecht, The Netherlands
3
Deltares Research Institute BGS-TGG, Princetonlaan 6, Utrecht, The Netherlands
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[email protected], 4
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From the guest editors
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Instituto Andaluz de Ciencias de la Tierra, CSIC-University of Granada, Avenida de las Palmeras nº 4, 18100 Armilla (Granada), Spain
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Continental shelves today find themselves largely submerged as a consequence of the sea-level rise in the last 20,000 years, the time since the Last Glacial Maximum (LGM), the period of maximum ice mass and minimum ocean volume within the Last Glacial Cycle. Their geomorphology, however, is far from controlled by marine processes alone. Shallow parts of the shelves were land during most of the past 120,000 years (the last full interglacial-glacial cycle), and deeper parts of the current shelves were coastal zones for most of that time, and this goes for older cycles too. To describe and understand the geomorphology of continental shelf drowned landscapes requires combining the knowledge on marine and terrestrial processes and to take into account both climatic and sea-level cyclicities in the larger picture of shelf evolution. With this special issue, we hope to interest the Geomorphology readership in the peculiarities of Continental Shelf Drowned Landscapes and their evolution during the youngest climatic-eustatic cycles. The special issue emerges from two international meetings organised in 2011. These were (i) the CMP-commission sponsored session on ‘Drowned landscapes and continental shelves during the last glacial cycles’ at the XVIII. INQUA congress (21-27 July 2011) in Bern, Switzerland, and (ii) the 5th IGCP526 conference (13-16 Sept 2011) held in Sidney, British Columbia, Canada, on ‘Continental Shelves: Risks, Resources and Record of the Past’.
ACCEPTED MANUSCRIPT Contents of the special issue
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The issue brings together 10 papers that cover a considerable area of the shelves of the Northern and Southern Atlantic Ocean and the Mediterranean, off North America (1 paper), Europe (5 papers), North Africa (1 paper) and South America (3 papers) (Fig. 1). Mapping the shelf morphology is truly a marine geological exercise, which has experienced great developments in sonar bathymetry imaging and seismic sub-bottom profiling techniques. Collecting and logging cores and obtaining material for dating is already more a mix of ‘marine’ and ‘terrestrial’ techniques – and full integration of ‘land’ and ‘sea’ thinking is required for reconstructions of the continental shelf landscape evolution to make sense.
MA
NU
The studies included in this special publication cover a wide range of climatic zones that are reflected in the nature of dominant sedimentary processes, including periglacial, siliciclastic, mixed (carbonatesiliciclastic) and aeolian coastal to shelf environments. This environmental diversity is exemplified in the variety of morphologies and deposits reported. The areas covered in typical continental shelf studies are large and much of the collected data are of graphical nature. Consequently, this special issue is rich in figures, in particular maps and seismic sections. The issue collects the following papers:
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The paper by Harris et al. presents the shelf evolution off the SE United States over the last glacial cycle, positioned in the periphery of the Laurentide ice sheet, with prominent submerged cliff-like coastlines from MIS 3 and MIS 2 and river valleys traversing the exposed shelf, in places developing deltaic deposits.
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The paper by Reis et al. also studies features produced by the basinward migration of Late Pleistocene coastlines, on the continental shelf of the Santos basin (Rio de Janeiro State, SE Brazil). Particular attention is given to bathymetric steps resulting from coastal versus submarine processes.
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The paper by Dominguez et al. addresses the much narrower shelf off central Brazil, focusing on the interaction between exposed-shelf geomorphological features and inundated-shelf carbonaceous mud accumulations. The short paper by Isla reports on the peculiar setting of local tectonic basins that are part of the Patagonian shelf in similar ways as further inland – up to the times that transgression overtops sills and inundates them. Moving back to the northern hemisphere and to the opposite side of the Atlantic, the paper by Thébaudeau et al. focusses on the rocky northern coast of Ireland and Northern Ireland. It investigates shoreline evolution and the significance of inherited morphology in the youngest 16,000 years applying mathematical modelling for various sea-level rise scenarios. The paper by Mellett et al. presents mapping and dating evidence from the peculiar geomorphology of the English Channel shelf off southern England. This shelf occupies a position in the immediate periphery of ice sheets once covering Britain and Ireland, and through the Strait of Dover connects to the North Sea Basin and the Scandinavian ice masses and the water that this produces. Erosional features from times of greatest meltwater release are contrasted with depositional and geomorphological record from younger times.
ACCEPTED MANUSCRIPT The paper by Ruiz et al. addresses the role of incidental tsunamis on the estuaries of Southwestern Spain during prehistorical and historical times. The geomorphological and sedimentological imprints are reviewed in the general context of the gulf of Cádiz. In addition, a recurrence period is tentatively suggested, taking into account existing evidences and corrected reservoir effect.
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The paper by Rodriguez-Vidal et al. addresses the shelf landscape off Gibraltar during the last glacial, exploring the vegetation, fauna and Neanderthaler population of the time. Emerged shelf geomorphological features, substrate geology and freshwater hydrology together made the area the Mediterranean refuge it was.
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The paper by Tropeano et al. addresses the Metaponto coastal plain and near shore in southern Italy, with particular attention to mapping out palaeovalley incisions marking the longer stages of lowered sea level. The paper further highlights the contrast of fills of such incision with the sequences of their interfluves, comprised of coastal plain sequences from the penultimate cycle and before mostly.
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The paper by Hanebuth et al. addresses the shelf off Mauretania and the great Sahara desert. Here, rather than fluvial phenomena that receive attention in most of the other papers, it is eolian transport from the Sahara that provides major sediment supply. Indeed, eolian deposits are preserved as part of the shelf record from critical periods within the last glacial cycle, at least to the present day.
A geomorphological view on shelves
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Geomorphological processes acting on the shelves are the physical link between the continental terrestrial and oceanic marine realms of the world. Whereas in recent decades many studies have sought linkage between continental and oceanic records and processes over millennial and longer scales, this has quite often been done by tele-correlation and distant intercomparison. The complexities that also correlating shelf realm records would add were often skipped and instead made ‘black box’ parts of the system to explain response lags. Even worse, the possibility that a shelf zone induced such a lag is not always fully considered when correlations between developments on the continent and records in ocean correlations are proposed. To phrase this most negatively: roles for the shelf are lost in simplification, or only invoked when ‘needed’. Much of this, we believe, has more to do with shelf records of the youngest few cycles not yet being very well disclosed, than true misconceptions on the role of the shelf in system earth. Various research communities now realise that the shelves and coastal zones of the world, are important elements in the source-to-sink chain, because they dynamically alternate their functioning as source and as sink through climatic and sealevel cycles. Though currently submerged and difficult to access, the shelves hold an important archive resulting from supply of sediment and throughput of discharge from surrounding terrestrial areas (Walsh and Nittrouer, 2009); and only transfer part of these fluxes to deeper waters, in some stages presumably with more delay than in others. The characterization of the morphological variability in continental shelves is of paramount importance to understand the long-term growth of continental margins, and more recently for habitat characterization (Harris, 2012). The geomorphological variability may be related either to
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marine sediment dispersal processes or to successive transgressive-regressive shoreline translations (Olariu and Steel, 2009). This last driving process is associated with shelf exposure and subsequent flooding and therefore more directly influences the interrelation between terrestrial and marine processes. The continuous shoreline migrations are thus able to generate distinctive morphological features, such as palaeo-coastlines at a range of water depths (Thébaudeau et al., this issue), and/or paleovalleys (Tropeano et al., this issue; Dominguez et al., this issue; Harris et al., this issue) as the most conspicuous elements.
Sea-level falls and rises
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The geomorphological and shallow geological records of continental shelf settings have been quite complete in recording its palaeoenvironmental evolution, due to the influence of sea-level changes between highstand and lowstand shorelines (Suter, 2006). The most recent change was driven by a rapid, dramatic postglacial sea-level rise (especially between 14.7 and 8.3 ka cal BP; e.g. Carlson and Clarke, 2012). The slower and more oscillatory sea-level fall over most of the Last Glacial Cycle (especially between 80 and 30 ka), however was also highly influential in sculpting the shelf morphology – as many of the papers in the special issue suggest (e.g., Mellett et al., this issue; Reis et al., this issue; Tropeano et al., this issue; Harris et al., this issue). To categorize encountered submerged geomorphological features in the middle sectors of the shelf as principally the product of either ‘the transgression after the LGM lowstand’ or ‘the sea-level fall towards the LGM’ is a critical step in working up observational data to scientific result in any continental shelf mapping study, and for the papers in this special issue in particular.
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Many papers in the special issue (Table 1), based on grown amounts of data, find, recall and refine earlier inferences that most of the shelf features would be the product of the last transgression. The perceived erosive nature of the youngest transgressive stage is being downplayed, the preservation potential for deposits from earlier stages in the last glacial (MIS 4 and MIS 3, besides MIS 2) is highlighted. This primarily considers the preservation potential of ‘falling stage deposits’ within the current cycle, which is not yet complete. To what extent the MIS 4, 3 and 2 deposits – and their postLGM transgressive cover – will also survive yet-to-come stages of sea-level fall (expected 50 to 100 ka in the future) can be evaluated by comparative study of deposit preserved from MIS 6 and MIS 5, from the penultimate climatic-eustatic cycle. Such comparison is started in the papers by Hanebuth et al. (this issue) and Mellett et al. (this issue). Regarding the last glacial stage, many papers in this special issue present themselves as ‘users’ of data of past sea-level positions, as a means of indirectly assigning ages to features. It should not be overlooked, however, that these same studies produce information on feature depths (e.g. Isla, this issue; Harris et al., this issue). When the new insight that falling stage features did remain preserved despite the post-LGM transgression is combined with efforts to more-directly date paleo-coastline features, this greatly enlarges opportunities to spatially diversify paleoconstraints to sea-level for the period MIS 5 to MIS 2. In the text above, the climatic and sea-level cyclicity terminology as developed in the Quaternary Geological community (such as the use of ‘marine isotope stages’ and ‘the LGM’) is mixed with eustasy-connected terminology in ‘highstand’, ‘lowstand’, ‘falling’ and ‘transgressive’ stages that
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come from the sequence stratigraphical method. That terminology originates from hydrocarbon reservoir geology, is widely in use in sedimentary geology (to integrate seismic, outcrop and core data collected at basin scale), and in the last decade has found more and more application in Quaternary shelf and coastal contexts too (Zecchin and Catuneanu, 2013). Indeed, the youngest, most-complete shelf and coastal sequences provide the interpretation templates for more ancient counterparts. Delays between Source and Sink
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Because of the additional independent time control that Quaternary dating methods provide, their relative completeness and essentially even preservation of the youngest deposits around the world, for sequence stratigraphic applications the youngest shelf records not only provide nice examples for comparison, but also the opportunity to quantify rates at which accommodation space is created and sediment fluxes are able to fill it. We are far from making maximum use of the completeness of the shelf record, because mapping them has only just started and high resolution shelf studies are still patchy (or the data are not in the public domain because of commercial value). It is clear that studies such as in this Special Issue make major contributions in unlocking the shelf record also from the ‘source to sink’ global sediment routing perspective, and as such have relevance at much longer than glacial-interglacial timescales. Under this perspective, the storage capacity of shelf settings appears to be much more variable than catchment areas (landward) and deeper environments (seaward). Given the sea-level variations over glacial-interglacial cycles, storage capacity depends on relative elevation and physiographic properties combined (e.g. Sømme et al., 2009). During highstand conditions, for example, storage capacity on shelves is mainly affected by the interplay between alongshore currents and submarine canyons – and it affects only the proportion of continental sediment that is not trapped in the coastal and deltaic plain sub-environments. In contrast, sediment delivery to the continental slope is enhanced during lowstand conditions due to the generation of larger composite river systems crossing the shelf, which carry sediment generated in permanent terrestrial parts of the continent, and additional sediment evacuated from previous coastal plain and upper shelf areas. The deeper ocean may be the ultimate sink for some of the finest sediments eroded from the continents, but a much greater amount and range of grain sizes accumulates and is preserved on continental shelves and slopes (e.g., Hanebuth et al., this issue). Indeed, a significant amount of finegrained sediment accumulates in shelf settings under various configurations, depending on the type of dispersal system which is defined by the hydrodynamic regime, shelf width and sediment load (Walsh and Nittrouer, 2009). Let us return to the geomorphological perspective and the time scales of the last few glacial cycles. The nature of the geomorphology and Quaternary record of the shelves is controlled by the alternating durations that specific areas were `terrestrial´, `coastal´ and `marine´. The drowned landscapes featured in the special issue have been subject to alternating fluvial terrestrial, coastal, transgressive marine and high-stand marine processes, and in some places also to desert, periglacial and proglacial processes. Their submerged or coastally buried landforms are complex outcomes of erosion, reworking and deposition by all these processes. Through these landscapes, fresh water, nutrients and sediment have been routed to permanently marine parts of the shelf and oceanic areas beyond.
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Past Habitat
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Now lost biomes once covered the shelf areas, traversed by rivers, fauna and prehistoric humans. Biogeographically the regions are areas that provide and have provided ephemeral dispersal opportunities for terrestrial, marine and freshwater aquatic species (Rodriguez-Vidal et al., this issue). Physical oceanographic control interacts with drowned terrestrial geomorphology in the submerged shelf landscape (Reis et al., this issue). The shelf’s dynamic role in the physical and biological transport networks of the world is easily overlooked when only the modern configuration is considered (Cohen et al., 2012), a fact more and more realized in the global climate, earth system and human dispersal modelling communities.
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Shelves are part of the global hydrological, nutrient and mineral cycles – and together with coastal plains and deltas, the shelves occupy a peculiar and important position, as they store major volumes of routed sediment and nutrients, in part permanent and in part temporarily (Laruelle et al., 2013). Furthermore, they seemed to play an important role in the delivery of important amounts of methane to the atmosphere during the deglacial flooding, due to rapid biomass decomposition (Ridgwell et al., 2012).
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Linked to climatic and sea-level cycles they do so at exactly the time scales and places that make earth system modelling complicatedly ‘complex’. Being a natural boundary zone for both marine and continental research communities the parameterization of the geomorphological functioning of shelves in the bigger earth system, is still a bit under developed compared to, for example, hydrological, glaciological and oceanographic modelling. Its record, however, is increasingly uncovered and revealed (this issue) and rather than a separating boundary zone, the shelves have become a zone of overlapping marine and terrestrial research. It is believed that in the longer term this will also be beneficial for quantitative approaches – not least because from mapping, dating and reasoning it has become clearer what the relevant time and spatial scales are at which the shelf zone affects hydrological and sedimentary cycles. Ancient humans have also used the natural shelf landscape, during the last glacial cycle, the landscapes saw rapidly growing human populations, forced to evade these areas with postglacial sealevel rise, leaving archaeological heritage behind. In cycles before, alternating shelf flooding and exposure and the coastal response constituted a major influence on human settlement and migration routes. Modern humans are increasingly using and affecting the coastal and shelf realm. Shelves represent key locations for aggregates and sand extraction that will be mined much more intensively in the coming years to meet growing demands, including that required to supply coastal defence projects, beach nourishment and in a broader sense coastal zone management. Continental shelves provide a range of present-day marine habitats, which are related to the geological characteristics of the seafloor, such as sediment size percentages and degree of heterogeneity (de Juan et al., 2013). Additionally, habitat distribution is also influenced by human activities that can pose significant threats, such as fishing and pollution (Harris and Baker, 2012). Consequently, habitat mapping is being regarded as a very useful tool to monitor changes and to mitigate threats in order to achieve a sustainable management (Dominguez et al., this issue). Finally, considerable parts of shelf areas have been affected by mass movement processes and neotectonic activity; bearing substantial risk to submarine infrastructures and conducive to tsunami generation – affecting the
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Conclusion
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The ‘drowned landscape’ nature of continental shelves and man’s occupancy and use of the coastal and shallow marine environment demands knowledge transfer between scientific disciplines. This objective is at the heart of IGCP 526 (Risks, Resources, and Record of the Past on the Continental Shelf; http://igcp526.io.usp.br/), the European Commission COST-action TD0902 SPLASHCOS (Submerged Prehistoric Archeology and Landscapes of the Continental Shelf; http://splashcos.org/), and INQUA projects “1202: Rapid environmental changes and human impact on continental shelves.” and 1301: “PALSEA2 (PALeo-constraints on SEA-level rise2)”, and is equally advocated through this editorial and the special issue as a whole. We feel that considerable scientific progress is made; and that this achieved through innovations in approach and methods, but especially through interdisciplinary knowledge transfer and conceptual integrations. With this publication, we hope to unlock and display this progress to the wider geomorphological research community.
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Acknowledgements
References
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We gratefully acknowledge authors, reviewers, co-chairs of the INQUA and IGCP526 sessions S. Toucanne, F. Chiocci, R. Hetherington and J. Vaughn Barrie, senior editor Adrian Harvey (overseeing Geomorphology’s special issues) and Elsevier’s production staff, in particular Karunamurthy Narasimhan for their work towards completing this issue. This is a contribution to IGCP-526, to COSTaction TD0902 SPLASHCOS, to INQUA project 1202 and to PALSEA2 (PAGES-IGBP/INQUA1301/WUN).
Carlson, A.E., Clark, P.U., 2012. Ice sheet sources of sea level rise and freshwater discharge during the last deglaciation. Reviews of Geophysics, 50, RG4007. Cohen, K.M., MacDonald, K, Joordens, J.C.A., Roebroeks, W., Gibbard, P.L. 2012 The earliest occupation of north-west Europe: a coastal perspective. Quaternary International, 271, 70-83. de Juan, S., Lo Iacono, C., Demestre, M., 2013. Benthic habitat characterisation of soft-bottom continental shelves: Integration of acoustic surveys, benthic samples and trawling disturbance intensity. Estuarine, Coastal and Shelf Science 117, 199-209. Harris, P.T., 2012. Seafloor Geomorphology – Coast, Shelf, and Abyss. In: Harris, P.T., Baker, E.K. (Eds.), Seafloor Geomorphology as Benthic Habitat. Elsevier, London, pp. 109-155. Harris, P.T., Baker, E.K., 2012. GeoHab Atlas of Seafloor Geomorphic Features and Benthic Habitats: Synthesis and Lessons Learned. In: Harris, P.T., Baker, E.K. (Eds.), Seafloor Geomorphology as Benthic Habitat. Elsevier, London, pp. 871-890.
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Laruelle, G.G., Dürr, H.H., Lauerwald, R., Hartmann, J., Slomp, C.P., Goossens, N. & Regnier, P.A.G., 2013. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins. Hydrology and Earth System Sciences 17, 2029-2051.
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Olariu, C., Steel, R.J., 2009. Influence of point-source sediment-supply on modern shelf-slope morphology: implications for interpretation of ancient shelf margins. Basin Research 21, 484-501.
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Ridgwell, A., Maslin, M., Kaplan, J.O., 2012. Flooding of the continental shelves as a contributor to deglacial CH4 rise. Journal of Quaternary Science 27, 800-806.
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Sømme, T.O., William, H.-H., Ole, J.M., John, B.T., 2009. Relationships between morphological and sedimentological parameters in source-to-sink systems: a basis for predicting semi-quantitative characteristics in subsurface systems. Basin Research 21, 361-387.
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Suter, J.R., 2006. Facies Models Revisited: Clastic Shelves. In: Posamentier, H.W., Walker, R.G. (Eds.), Facies Models Revisited. SEPM Special Publication, 84, p. 339-397. Walsh, J.P., Nittrouer, C.A., 2009. Understanding fine-grained river-sediment dispersal on continental margins: Marine Geology 263, 34-45.
Figures
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Zecchin, M., Catuneanu, O., 2013. High-resolution sequence stratigraphy of clastic shelves I: Units and bounding surfaces. Marine and Petroleum Geology 39, 1-25.
Tables
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Figure 1: Map of global shelf area - with studied areas of papers in this issue.
Table 1: Aspects of Shelf Geomorphology addressed in papers
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X X X X X
X X X X X X
Modern resources, aggregate, fishery
Submerged biomes and archaeology
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Gradual vs. punctuated development
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Shelf substrate geological diversity
Earlier glacial-interglacial cycles
Last interglacial highs stand
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Last glacial cycle regression
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Shelf off Central Brazil N Mauritania SE United States N Patagonia English Channel Rio d. J., Brazil Gibraltar SW Spain Ireland S Italy
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Continent S. Am. Africa N. Am. S. Am. Europe S. Am. Eur. Med. Europe Europe Eur. Med.
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Paper (this issue) Dominguez et al. Hanebuth et al. Harris et al. Isla Mellet et al. Reis et al. Rodriguez-Vidal et al. Ruiz et al. Thébaudeau et al. Tropeano et al.
Post-LGM transgression
TABLE 1
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X
X X X X X
X X
X X X
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Figure 1