World's Continental Shelves Research Articles

Morphodynamics of a field of crescent-shaped rippled scour depressions: Northern Monterey Bay, CA

Despite the prevalence of rippled scour depression (RSD) on the world's continental shelves and their importance as nursery habitats for many commercially-important species, the processes responsible for their formation and geomorphic evolution are still not well understood. Most studies that focused on RSD evolution have been based on data acquired over multiple years to decades, and often during calmer summer months. Here, multiple geophysical seafloor mapping surveys of a field of RSD's off Santa Cruz, California, USA, were conducted over the course of two winters with distinctly different oceanographic forcing; these results were compared to time series oceanographic measurements made within the survey area, and put into the context of long-term trends from survey data collected during previous decades. Although the migration of these features across the seafloor occurs on decadal time scales, significant change was detected within a given year, depending on the wave climate and fluvial sediment input. RSDs shrank up to 16% or grew up to 28% in a span of a few months. Such change was predominantly the result of redistribution of fine-grained sediment input to the system by local rivers. Migration of the seafloor features appears to be the result of accumulation of fine-grained sediment at the boundary between the RSD and the surrounding fine-grained cap.

Seismic Oceanography's Failure to Flourish: A Possible Solution

AbstractA recent paper in Journal of Geophysical Research: Oceans used multichannel seismic observations to map estimates of internal wave mixing in the Gulf of Mexico, finding greatly enhanced mixing over the slope region. These results suggest that the ocean margins may supply the mixing required to close the global thermohaline circulation, and the techniques demonstrated here might be used to map mixing over much of the world's continental shelves. The use of multichannel seismics to image ocean phenomena is nearly 15 years old, and despite the initial promise, the techniques have not become as broadly used as initially expected. We discuss possible reasons for this, and suggest an alternative approach that might gain broader success.

Chapter 1 An overview of the continental shelves of the world

This Memoir explores the variability and controlling processes of sedimentation, morphology and tectonics on the world's continental shelves, with emphasis on their evolution during the last glacio-eustatic cycle. This work builds on some earlier volumes on continental shelves, notably those by Trumbul et al. (1958), Boillot (1978), De Batist & Jacobs (1996), Nittrouer et al. (2007) and Li et al. (2012) among others, although there has not been a previous comprehensive global-scale synthesis. The subject material presented here was developed in several International Geoscience Programmes (formerly the International Geological Correlation Programme, IGCP) concerned with shelves. Project 396 ‘Continental Shelves in the Quaternary’ (1996–2000) was followed by IGCP 464 ‘Continental Shelves during the Last Glacial Cycle: Knowledge and Applications’ (2001–2007) and finally by IGCP 526 ‘Risks, Resources and Record of the Past on the Continental Shelf’ (2007–2011). In this Memoir, 23 papers are devoted to the description of different aspects of shelves from all seven continents and which are representative of the world's shelves. However, there remain significant gaps in our compilation. A glance at Figure 1.1 indicates the sparse coverage of data, particularly using modern techniques of investigation, for Africa, Russia and the Polar regions. However, new data are presented from some frontier provinces, such as Myanmar, Morocco and Ecuador. Fig. 1.1. The location of the shelf areas presented in this volume. Base map from Smith, W. H. F. & Sandwell, D. T. 1997. Global sea floor topography from satellite altimetry and ship depth soundings. Science , 277 , 1956–1962. Reprinted with permission from AAAS. The continental shelf is the part of the seafloor most used by society. Although shelf areas account for little more than 8% of the marine areas of the world, they are the part of the sea most used for navigation, recreation, fishing and …

About this title ‐ Continental Shelves of the World: Their Evolution During the Last Glacio-Eustatic Cycle

The world's continental shelves are the sites of vast resources of food, energy and minerals, the exploitation of which is continuously increasing. Fluctuating global sea levels throughout the Quaternary period produced multiple transgressive and regressive cycles that profoundly affected and shaped these shelves. The complex interactions among climate, sea level, tectonics, oceanography and sediment input have formed distinctive sediment packages on each shelf and provide a guide to the interpretation of older shelf sequences throughout the geological record. This Memoir compiles studies on 23 selected shelves from all the continents, focusing on their evolution and examining the patterns of sedimentation during the past approximately 125 000 years. In addition to providing basic background information for each area, the chapters consider specific aspects of continental shelf research, from seismic stratigraphy to geomorphology, from palaeoceanography to palaeo sea-level reconstruction and from palaeontology to geochemistry.

Air–sea exchanges of CO<sub>2</sub> in the world's coastal seas

Abstract. The air–sea exchanges of CO2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO2 (39 ± 56 mol C m−2 yr−1, positive flux indicates that the water is losing CO2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m−2 yr−1) and lower estuaries with salinities of more than 25 are weak sources (8.4 ± 14 mol C m−2 yr−1). With respect to latitude, estuaries between 23.5 and 50° N have the largest flux per unit area (63 ± 101 mmol C m−2 d−1); these are followed by lower-latitude estuaries (23.5–0° S: 44 ± 29 mmol C m−2 d−1; 0–23.5° N: 39 ± 55 mmol C m−2 d−1), and then regions north of 50° N (36 ± 91 mmol C m−2 d−1). Estuaries south of 50° S have the smallest flux per unit area (9.5 ± 12 mmol C m−2 d−1). Mixing with low-pCO2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO2 in estuaries, but wind speed, seldom discussed, is critical to controlling the air–water exchanges of CO2. The total annual release of CO2 from the world's estuaries is now estimated to be 0.10 Pg C yr−1, which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high pCO2, are low in sea-to-air fluxes because of low wind speeds. Previously determined flux values rely heavily on data from Europe and North America, where pCO2 is lower but wind speeds are much higher, such that the CO2 fluxes are higher than in Asia. Newly emerged CO2 flux data in the Arctic reveal that estuaries there mostly absorb rather than release CO2. Most continental shelves, and especially those at high latitude, are undersaturated in terms of CO2 and absorb CO2 from the atmosphere in all seasons. Shelves between 0 and 23.5° S are on average a weak source and have a small flux per unit area of CO2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO2 values are low, the air-to-sea fluxes are high in regions above 50° N and below 50° S. At low latitudes, the winds tend to be weak, so the sea-to-air CO2 flux is small. Overall, the world's continental shelves absorb 0.4 Pg C yr−1 from the atmosphere.

Impact of self‐attraction and loading effects induced by shelf mass loading on projected regional sea level rise

We investigate the effect of self‐attraction and loading (SAL) induced by the projected accumulation of sea water on shallow continental shelf areas. Using output from a climate model, we compute 21st century changes in regional steric sea surface height and find that steric changes are largest over the deep ocean and relatively small on the shallow continental shelves. The resulting redistribution of sea water towards the shelf areas leads to mass accumulation on the shelves and therefore to increased gravitational attraction as well as increased loading on the sea floor. We find that, depending on the scenario and region, SAL effects may result in an additional sea level rise of 1–3 cm on the world's continental shelf areas by the end of the 21st century. These estimates are at most 15% of the combined changes in sea surface height induced by redistribution of water masses and steric expansion.

Seasonal and spatial patterns of sedimentary denitrification rates in the Chukchi sea

Abstract Recent constructions of the global nitrogen budget estimate that at least half of the ocean's fixed nitrogen is lost by sedimentary denitrification, the majority of which occurs in continental shelves. The Arctic contains approximately 20% of the world's continental shelf, suggesting it is a substantial contributor to the global sedimentary denitrification rate. During two cruises in the summer and spring of 2002 and 2004, respectively, denitrification rates were calculated from the downward diffusive flux of nitrate in the shelf and slope sediments of the Chukchi Sea in the western Arctic. Additionally, in the spring of 2004, denitrification rates were determined by whole-core incubations in which the flux of nitrogen gas out of the sediments was measured. Measurements were made along three transects crossing the shelf and slope (50–3000 m), each transect having different overlying water characteristics. Denitrification rates generally decreased with increasing water depth: rates varied from about 1.6 mmol N m−2 d−1 for the shallow-water sediments to undetectable in deep-water sediments. Rates showed little variation between the two seasons. However, rates were found to correspond with differences in annual overlying primary productivities and particulate organic carbon export fluxes. An extrapolation to the whole Arctic yielded an average Arctic sedimentary denitrification rate of 13 Tg N yr−1. Taken in the context of the global nitrogen budget, it is about 4–13% of the total sink of fixed nitrogen in the ocean.

Constraints on the amplitude of Mid-Pliocene (3.6–2.4 Ma) eustatic sea-level fluctuations from the New Zealand shallow-marine sediment record

Ice-volume calibrations of the deep-ocean foraminiferal delta18O record imply orbitally influenced sea-level fluctuations of up to 30m amplitude during the Mid-Pliocene, and up to 30 per cent loss of the present-day mass of the East Antarctic Ice Sheet (EAIS) assuming complete deglaciation of the West Antarctic Ice Sheet (WAIS) and Greenland. These sea-level oscillations have driven recurrent transgressions and regressions across the world's continental shelves. Wanganui Basin, New Zealand, contains the most complete shallow-marine Late Neogene stratigraphic record in the form of a continuous cyclostratigraphy representing every 41 and 100ka sea-level cycle since ca 3.6Ma. This paper presents a synthesis of faunally derived palaeobathymetric data for shallow-marine sedimentary cycles corresponding to marine isotope stages M2-100 (ca 3.4-2.4Ma). Our approach estimates the eustatic sea-level contribution to the palaeobathymetry curve by placing constraints on total subsidence and decompacted sediment accumulation. The sea-level estimates are consistent with those from delta18O curves and numerical ice sheet models, and imply a significant sensitivity of the WAIS and the coastal margins of the EAIS to orbital oscillations in insolation during the Mid-Pliocene period of relative global warmth. Sea-level oscillations of 10-30m were paced by obliquity.

Seasonal and diurnal activity of sixgill sharks (Hexanchus griseus) on a shallow water reef in the Strait of Georgia, British Columbia

The bluntnose sixgill shark (Hexanchus griseus) is a large, deepwater, cosmopolitan species whose range includes the Strait of Georgia, British Columbia. Although this species is probably the top predator on most of the world's continental shelves, there is little information on its population biology or behaviour. At a unique site off Flora Islets, in the Strait of Georgia, a number of H. griseus individuals show unexpected diurnal activity on a shallow reef. Regular movements of these sharks along the base of the reef wall allowed us to use an automated, time-lapse video system to make the first extensive observations of their behaviour between June 2001 and July 2002. Sixgill shark frequency (no. of sharks observed/h) increased in June and reached a peak between mid-June and mid-July. Sixgill sharks were not observed between October and May. Sixgill shark frequency was highest in the afternoon and the sharks tended to move as individuals rather than in groups. The observations suggest that this shallow-water activity is not related to either reproduction or feeding and its purpose remains unclear.

A sediment and organic carbon budget for the Canadian Beaufort Shelf

The Arctic Ocean accounts for 20% of the world's continental shelves. Because the Arctic is sensitive to global change, budgets of organic carbon for its shelves are of immediate interest. The Mackenzie Shelf of the Canadian Beaufort Sea is the best North American proxy for the enormous Eurasian Shelves (large area, large river input), and the only site for which a complete organic carbon budget can be attempted, due to an extensive data base. A mass balance for the Mackenzie Shelf has been constructed for sediments, terrestrial organic carbon, and primary produced carbon. We have considered allochthonous inputs from the Mackenzie River, from coastal erosion, from smaller rivers, from groundwater, from the atmosphere and import by ice. The Mackenzie River dominates the supply to the Beaufort shelf of inorganic sediment (127 Mt a −1) and paniculate and dissolved terrestrial carbon (2.1 Mt a −1 POC, 1.3 Mt a −1 DOC). The combined input from all other sources contributes only about 5% of the Mackenzie load. Using sediment accumulation data we estimate that about half of the sediment supply is trapped in the delta, about 40% on the shelf and the remainder escapes the shelf edge by various processes. Autochthonous primary production in the delta and on the shelf adds a further 3.3 Mt a −1 of particulate organic carbon. A box model has been constructed to account for sediment, terrestrial organic carbon and primary produced carbon. Whereas about 60% of the terrestrial POC is preserved in delta and shelf sediments, it appears that most (97%) of the primary produced carbon is recycled and not preserved in sediments. Confidence in the budget should be improved by focusing future research on the determination of modern sedimentation rates on the delta and shelf, measurement of organic carbon content of deltaic sediments, determination of primary production on the shelf, and determination of the relative proportions of terrestrial and marine organic carbon preserved in sediments.

The global flux of methane from shallow submarine sediments

The seepage of methane through the seabed of the world's continental shelves is considered. Methane may be generated by both bacterial and thermogenic processes acting on the complex organic matter stored in the seabed. Because these processes are ubiquitous, occurrences of gas and gas seepages are probably widespread. Although numerous observations of methane seepages from shallow marine sources have been reported, there are few documented flux measurements. Whereas a prolific 400 g CH 4 m −2 year −1 has been calculated for an 18 km 2 area in the Santa Barbara Channel, California, a more modest 26 g CH 4 m −2 year −1 has been estimated for a site in the North Sea. Descriptions of these and other areas of gas seepage, from a variety of geological and oceanographic environments, are presented. A rough estimation of the global flux suggests that between 8 and 65 Tg of methane passes through the seabed of the continental shelves annually. If a significant proportion of this passes through the water column and into the atmosphere, the oceans may represent a more significant source of methane than has been suggested previously. As much of this methane is 14C-depleted, it may at least partially account for the shortfall of fossil methane in current atmospheric methane budgets.

Ancient charts and modern mariners

Abstract The paper seeks to demonstrate that the antique nature of most nautical charts available to commercial shipping is not by any means the principal cause of maritime accidents and pollution. The enormous task facing hydrographers surveying the world's continental shelves, many areas of which have never been surveyed, is described. Examples are given of the disappearance of a number of tankers and other ships, which is explained by episodic waves. The lethal nature of these waves is described, and the geographical areas in which they occur and recur, discounting the appellation “freak”; waves, which do not recur. The necessity of precise topographical surveys of areas prone to these waves, enabling hydrographers and thus commercial users of the ocean to avoid such dangerous areas, is discussed. The urgent need for much quicker legal processes following “accidents”; at sea is touched on, and the German Sea Courts system is advocated, effective there since 1871, for use in U.S. coastal areas and, in fact, in all the littoral areas of the world. The paper concludes that unless manning standards are improved by the guilty being shown up shortly after accidents have occurred when public interest is still aroused, no improvement to the steadily increasing rise in ship losses will occur. The increase in ship losses over the year preceding 1978 was 40%, and in oil pollution of varying viscosities, 300%.

Tidal dissipation in the oceans: astronomical, geophysical and oceanographic consequences

The most precise way of estimating the dissipation of tidal energy in the oceans is by evaluating the rate at which work is done by the tidal forces and this quantity is completely described by the fundamental harmonic in the ocean tide expansion that has the same degree and order as the forcing function. The contribution of all other harmonics to the work integral must vanish. These harmonics have been estimated for the principal M 2 tide using several available numerical models and despite the often significant difference in the detail of the models, in the treatment of the boundary conditions and in the way dissipating forces are introduced, the results for the rate at which energy is dissipated are in good agreement. Equivalent phase lags, representing the global ocean-solid Earth response to the tidal forces and the rates of energy dissipation have been computed for other tidal frequencies, including the atmospheric tide, by using available tide models, age of tide observations and equilibrium theory. Orbits of close Earth satellites are periodically perturbed by the combined solid Earth and ocean tide and the delay of these perturbations compared with the tide potential defines the same terms as enter into the tidal dissipation problem. They provide, therefore, an independent estimate of dissipation. The results agree with the tide calculations and with the astronomical estimates. The satellite results are independent of dissipation in the Moon and a comparison of astronomical, satellite and tidal estimates of dissipation permits a separation of energy sinks in the solid Earth, the Moon and in the oceans. A precise separation is not yet possible since dissipation in the oceans dominates the other two sinks: dissipation occurs almost exclusively in the oceans and neither the solid Earth nor the Moon are important energy sinks. Lower limits to the Q of the solid Earth can be estimated by comparing the satellite results with the ocean calculations and by comparing the astronomical results with the latter. They result in Q > 120. The lunar acceleration n , the Earth’s tidal acceleration O T and the total rate of energy dissipation E estimated by the three methods give astronomical based estimate —1.36 —28±3 —7.2 ± 0.7 4.1±0.4 satellite based estimate —1.03 —24 ±5 — 6.4 ± 1.5 3.6±0.8 numerical tide model — 1.49 —30 ±3 —7.5± 0.8 4.5±0.5 The mean value for O T corresponds to an increase in the length of day of 2.7 ms cy -1 . The non-tidal acceleration of the Earth is (1.8 ± 1.0) 10 -22 s ~2 , resulting in a decrease in the length of day of 0.7 ± 0.4 ms cy -1 and is barely significant. This quantity remains the most unsatisfactory of the accelerations. The nature of the dissipating mechanism remains unclear but whatever it is it must also control the phase of the second degree harmonic in the ocean expansion. It is this harmonic that permits the transfer of angular momentum from the Earth to the Moon but the energy dissipation occurs at frequencies at the other end of the tide’s spatial spectrum. The efficacity of the break-up of the second degree term into the higher modes governs the amount of energy that is eventually dissipated. It appears that the break-up is controlled by global ocean characteristics such as the ocean­-continent geometry and sea floor topography. Friction in a few shallow seas does not appear to be as important as previously thought: New estimates for dissipation in the Bering Sea being almost an order of magnitude smaller than earlier estimates. If bottom friction is important then it must be more uniformly distributed over the world's continental shelves. Likewise, if turbulence provides an important dissipation mechanism it must be fairly uniformly distributed along, for example, coastlines or along continental margins. Such a global distribution of the dissipation makes it improbable that there has been a change in the rate of dissipation during the last few millennium as there is no evidence of changes in ocean volume, or ocean geometry or sea level beyond a few metres. It also suggests that the time scale problem can be resolved if past ocean-continent geometries led to a less efficient breakdown of the second degree harmonic into higher degree harmonics.

Floating-Type Offshore Drilling Vessels

After two decades of oil exploration on the world's continental shelf, various design types of mobile drilling rigs have evolved, namely the submersible, the jack-up, and the floater. The authors consider that the current stage of development of these rigs has sufficiently established design patterns which are worthy of investigation and review. The authors have selected the floater type for discussion and presentation as being of germane interest to the naval architect and marine engineer. Section 1 of the paper reviews the design aspects of floating drilling vessels by considering them in two groups: "surface hull" type and "column stabilized" type. Functional design requirements are discussed, including some fundamentals on rotary oil drilling to familiarize the naval architect with its basic techniques. Requirements and recommendations for deck areas and volumes are given, together with representative weights for drilling equipment and vessel weight summaries. Hydrostatic, stability, and motion characteristics are compared between the two types. Structural design, safety features, and logistic support systems are reviewed. Section 2 describes the offshore drilling vessel Reforma as typical of one of the large surface-hull-type rigs incorporating the latest drilling techniques and marine systems. The rig, one of a fleet of three, is owned and operated by Perforadora Mexico, S. A., a private drilling contractor under contract to Petroleos Mexicanos, off the east coast of Mexico.

Oil and Gas Licensing and the North Sea

MAN has dreamt for centuries of exploiting the wealth of the oceans but, except for fisheries, little progress had been made until recently in realizing those dreams. Technology is now translating those dreams into reality. The future is already upon us in petroleum where offshore wells provide 14 per cent of the world's production and 15 per cent of the world's estimated reserves.' There is reason, moreover, to believe that further exploration will show that the world's continental shelves contain a good deal more petroleum than its land areas.2 On these expectations more than $6 billion has been invested in offshore exploration and production.3 Technology may provide the opportunity sooner than is generally appreciated to extract from the oceans many other things, ranging from fresh water to several kinds of minerals. With an ever greater share of wealth-producing activities occurring at sea, policy decisions are being taken by governments which will affect profoundly the allocation of world resources and the relationship between the public and private sectors. It is widely assumed that institutions such as the price system and private property, which have been cornerstones of the economic system in Western developed countries, can play only a minor role in offshore developments. Whether this assumption is tenable may prove to be one of the crucial questions of political economy in the last third of this century. It is therefore appropriate to make a preliminary examination of this question in the context of a particular contemporary development. Chosen for this purpose is the United Kingdom licensing program for oil and gas production in the North Sea. The choice has been dictated by the twin considerations that oil and gas exploitation is far in advance of other forms of