Dynamic Sea Level Research Articles

Production, preservation and prediction of source-rock facies in deltaic settings

This article hypothesizes that production and preservation of source-rock type facies in deltaic systems, both landward and seaward of a coastline, is mutually exclusive, time-successive and related to the dynamics of relative sea level. Sedimentologic research in the Holocene Mississippi and Rhône delta complexes suggests that rheotrophic peats and associated organic-rich beds preferably accumulate in the accommodation space created behind landward stepping shorelines in a transgressive systems tract (TST). Such a setting also allows for a sufficient supply of recharging fresh nutrient-rich groundwater into the peat forming mires [Kosters, E.C., Suter, J.R., 1993. Facies relationships and systems tracts in the late Holocene Mississippi Delta Plain. Journal of Sedimentary Petrology 63 (4) 727–733.]. Independently carried-out quantitative paleoecological studies in the same delta systems (and in addition in the Orinoco and Po deltas) suggest that in a progradational setting (highstand systems tract/HST), seasonally discharged nutrient- and sediment-laden river waters on the shelf may give rise to anoxia or dysoxia. Subsequent overfertilization of the shelf leads to accumulation of organic-rich mud belts on the shelf [VanderZwaan, G.J., Jorissen, F.J., 1991. Biofacial patterns in river-induced anoxia. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. Geological Society of London, Special Publication no. 58, pp. 65–82.]. Thus, production and preservation of source-rock type facies landward of a shoreline (as peats and related sediments) is preferred in a TST, when accommodation space and nutrient supply are landward of the shoreline. Vice versa, production and preservation of such facies seaward of a shoreline (as organic-rich shelf muds) occurs preferentially in a highstand systems tract. In that situation, accommodation space is on the shelf, where river-fed nutrients are supplied as well. This hypothesis suggests further potential for application of sequence stratigraphic concepts for improved understanding of the occurrence of source-rock type facies.

The Wave-Driven Ocean Circulation

Oceanic surface gravity waves have a mean Lagrangian motion, the Stokes drift. The dynamics of wind-driven, basin-scale oceanic currents in the presence of Stokes drift are modified by the addition of so-called vortex forces and wave-induced material advection, as well by wave-averaged effects in the surface boundary conditions for the dynamic pressure, sea level, and vertical velocity. Some theoretical analyses previously have been made for the gravity wave influences on boundary-layer motions, including the Ekman currents. The present paper extends this theory to the basin-scale, depth-integrated circulation in a bounded domain. It is shown that the Sverdrup circulation relation, with the meridional transport proportional to the curl of the surface wind stress, applies to Lagrangian transport, while the associated Eulerian transport is shown to have a component opposite to the Stokes-drift transport. A wave-induced correction to the relation between sea level and surface dynamic pressure is also derived. Preliminary assessments are made of the relative importance of these influences using a global wind climatology and an empirical relationship between the wind and wave fields. Recommendations are made for further development and testing of this theory and for its inclusion in general circulation models.

Regional analysis of the inverted barometer effect over the global ocean using TOPEX/POSEIDON data and model results

Crossover sea level differences from almost 5 years of TOPEX/POSEIDON data are regressed against corresponding differences in atmospheric pressure (−Δpa), and the regression coefficient (α) is examined for deviations from the value of ∼1 cm/mbar expected under a pure inverted barometer (IB) signal. Only crossovers within each 10‐day repeat cycle are used. We focus on variability at the shortest periods where non‐IB response is more likely. Results indicate a marked tendency to have α < 1 cm/mbar, with values in the general range of 0.8–1 cm/mbar in middle and high latitudes and 0.4–0.8 cm/mbar in the tropics. Effects of errors in Δpa and altimeter data seem small to explain all of the observed IB deviations, which imply then positive correlations between Δpa and dynamic sea level Δη′ (sea level adjusted for an IB signal). A simple constant‐density ocean model is used to help interpret the regression results. Only effects of winds and pa on Δη′ are considered. Model‐based α estimates agree qualitatively with the data estimates. On the basis of the model results, wind‐driven Δη′ signals contribute importantly to the spatial variability of α observed at middle and high latitudes. This is particularly evident in the Southern Ocean, where strongest wind‐driven effects on α coincide with regions of anomalous ambient potential vorticity gradients and different vorticity dynamics. In contrast, the observed decrease in α toward low latitudes is due to a true dynamic response to pa, mostly at periods shorter than 10 days. The stronger non‐IB signals suggest a response closer to resonance and may be due to the richer resonance spectrum in the tropics. The non‐IB signals are largely remotely driven and therefore weakly correlated with local pa. Thus regression results tend to underestimate the importance of the nonisostatic response in the tropics but also at higher latitudes.

Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the Early Mesozoic time–scale and the long–term behaviour of the planets

During the Late Triassic and Early Jurassic the Newark rift basin of the northeastern US accumulated in excess 5 km of continental, mostly lacustrine strata that show a profound cyclicity previousl...

Modeling Carbonate Microfacies in the Context of High-Frequency Dynamic Relative Sea-Level and Environmental Changes

Modeling Carbonate Microfacies in the Context of High-Frequency Dynamic Relative Sea-Level and Environmental Changes

Fossil wood in Middle–Upper Jurassic marine sedimentary cycles of France: relations with climate, sea-level dynamics, and carbonate-platform environments

Vegetation, climate, and sea-level oscillations interact in complex ways. Intuition suggests that climatic variations are recorded in geological deposits by both fossil phytoclasts and sediments. Therefore investigation of palaeobotanical remains within a precise sedimentological framework may help to decipher this much-debated interaction. A database of 129 Jurassic (Bathonian–Oxfordian) fossil wood samples from France is used here as a case study. The palaeoenvironment (freshwater swamp, carbonate lagoon, shoreface, argillaceous offshore) and the second-order relative sea-level phases were determined for each datum. There is no correlation between the wood genera distribution and second-order relative sea-level cycles. However, 66% of the fossil wood samples were found within transgressive parts of cycles or at maximum flooding surfaces versus 34% in their regressive counterparts. By contrast, there is a close correlation between wood distribution and palaeoenvironment for Agathoxylon and Brachyoxylon, the two most common genera. Although both genera occur in the four main depositional systems, Agathoxylon is the more common in offshore facies, whereas Brachyoxylon predominates in protected lagoon environments. A model of wood deposition/preservation is proposed. It is shown that (1) the distribution of wood is not controlled by climate but by environmental processes related to relative sea-level changes; (2) the abundance of wood in transgressive deposits may be related to ravinement of previously emerged areas and/or to the high preservational potential associated with increased accommodation space; (3) carbonate platforms were probably inhabited by low-diversity Brachyoxylon xylofloras, whereas offshore environments received a more diversified, probably multi-source flora from emergent land areas. Periods of Bathonian, Callovian and Oxfordian subaerial exposure of carbonate platforms may not have been conducive to the establishment of a diverse, stable climax xyloflora.

Sequence architecture and sea-level dynamics of Upper Permian shelfal facies, Guadalupe Mountains, southern New Mexico

ABSTRACT A 3.2 km profile of mixed carbonates and clastics is superbly exposed in Slaughter Canyon in the Guadalupe Mountains of southern New Mexico and provides a seismic-scale panorama of stratal architecture and cyclic facies relationships across the Upper Permian shelf margin of the Delaware basin. The upper Seven Rivers and Yates Formations exposed in the mapped profile behind the Capitan reef margin are characterized by extreme seaward thickening and strongly progradational architecture. Meter-scale cycles exhibit systematic lithofacies changes in a seaward direction, spanning pisolitic shelf-crest to skeletal outer-shelf facies tracts. Retrogradational, aggradational, and progradational stacking patterns of meter-scale cycles in the Yates define four complete high-frequency sequences (Y1-Y4) and the lower half of a fifth (Y5) that continues into the overlying Tansill Formation. Individual Yates high-frequency sequences are fundamentally macroscale versions of Yates meter-scale cycles, on the basis of comparable internal arrangements of lithofacies and their seaward-thickening geometry. The evolution of the Yates-Capitan shelf margin from Y1 through Y4 is expressed by systematic trends in downdip thickness changes, lateral extent of facies tracts relative to the Capitan reef margin, aspect ratios of facies tracts, progradation:aggradation ratios and derived offlap angles, and progradation rates. These trends reflect the position of individual high-frequency sequences in the larger scale Yates-Tansill composite sequence. Long-term changes in aspect ratios record the progressive seaward migration and lateral expansion of the shelf-crest facies tract from Y1 through Y4 and the reciprocal seaward-stepping architecture and lateral contraction of the outer-shelf facies tract. The four Yates high-frequency sequences in Slaughter Canyon are characterized by an average offlap angle of 3.6°, whereas the corresponding average growth angle for the time-equivalent Capitan reef is 5.2°. The higher reef growth angle reflects the greater amount of accommodation available near the outer shelf-to-reef transition as well as limitations to seaward growth imposed by the steepness of the reef front. The Yates shelf prograded at an average rate of 2.1 m/k.y., whereas the time-equivalent Capitan reef prograded at a rate of 1.7 m/k.y. The strongly progradational architecture of the Yates shelf resulted in a progressive decrease in the depth of the Capitan reef through time from a maximum of ~ 65 m during early Yates time to near sea level during the latest stages of the Yates platform. The abrupt seaward expansion within each Yates high-frequency sequence occurs directly above the terminal reef margin of the preceding high-frequency sequence. The underlying reef likely acted as a foundation to localize the basinward shift in deposition associated with sea-level fall along high-frequency sequence boundaries, contributing to the seaward thickening and progradational step-out of the outer shelf. Architectural changes controlled by the interaction between relative sea level and antecedent depositional topography may be a fundamental characteristic of many progradational shelf margins.

Sequence Architecture and Sea-Level Dynamics of Upper Permian Shelfal Facies, Guadalupe Mountains, Southern New Mexico

Sequence Architecture and Sea-Level Dynamics of Upper Permian Shelfal Facies, Guadalupe Mountains, Southern New Mexico

The Depositional History of the Younger Dryas-Preboreal Búdi Moraines in South-Central Iceland

Detailed sedimentological analysis of the Búdi morainal complex in south-central Iceland records a rapid accumulation within marine-, coastal, and glacial environments during a recessional phase of the glaciers within the Preboreal chronozone. Three principal outcrop areas are characterized by stratified diamictite, fine-grained silty-sandstone, and diamictite with dropstones; gravel-and sandfills of channels are also important. Elsewhere, the morainal complex is characterized by deltaic sedimentary structures that reflect an interweaving of marine and terrestrial processes. The Búdi morainal complex indicates a recession of the glacier margin, possibly since the Younger Dryas stadial. Sea-level changes, jökulhlaups, and glacier dynamics were responsible for fluctuation of the glacier margin and for the differential rate of calving during the accumulation of the morainal complex. Earlier conclusions on two separated glacier readvances are disputed.

Ichnologic Applications in Paleoceanographic, Paleoclimatic, and Sea-Level Studies

Continued growth of ichnology over the last decade is reflected not only by an expanded realm of applications but also by the emergence of more holistic approaches involving the ichnofabric concept. Both of these trends are exemplified by recent ichnologic applications in paleoceanographic, paleoclimatologic, and sequence stratigraphic studies. Ichnofabrics and representative ichnocoenoses in pelagic/hemipelagic strata provide the basis for reconstructing benthic oxygenation histories of marine basins. Recent studies demonstrate that oxygen-related ichnocoenoses, when mapped within lithochronostratigraphic units, can help recognize spatial paleo-oxygenation gradients that reflect intrabasinal variations in circulation and govern organic matter accumulation. When cointerpreted with sedimentologic data, ichnology-based paleooxygenation histories also can be used to recognize and interpret paleoceanographic cycles and events, particularly those linked to climatic perturbations such as Milankovitch rhythms and El Nino-like phenomena. As demonstrated by recent investigations in the context of marine sequence stratigraphy, potential applications of ichnology in paleobathymetric studies extend well beyond the use of generalized ichnofacies models. Ichnofabric parameters can facilitate reconstructions of relative sea-level dynamics by providing supplementary or primary evidence used to delineate key sequence stratigraphic surfaces. Sequence boundaries and transgressive surfaces, which record erosional exhumation of underlying strata, typically are marked by well-expressed substrate-controlled ichnocoenoses (e.g., those representative of the firmground Glossifungites ichnofacies). Maximum flooding surfaces and other parasequence-bounding marine flooding surfaces can be recognized on the basis of abrupt to subtle vertical ichnofabric successions that signal sea-level-mediated paleoenvironmental change. INTRODUCTION

Comparison of dynamic height variations in the tropical Atlantic during 1987–1989 as viewed in sections Hydrography and Geosat altimetry

During the 1980s the Soviet Union conducted a series of 25 hydrographic cruises in the tropical Atlantic under the Sections (acronym for Energetically Active Zones of the Ocean and Climate Variability) program. As one of its objectives, Sections has produced the most extensive hydrographic data set ever collected in the tropical Atlantic. Here we examine dynamic topography from seven cruises during the years 1987 through 1989 and compare them to corresponding maps of sea level from the Geosat satellite mission. The two independent estimates of sea level variation are well correlated, with the sea level fields explaining approximately half of the variance of dynamic height fields. There is a seasonal variation to the agreement, so that the correlation increases somewhat in summer and fall, as the variability of the height field increases. Wavenumber spectra of both dynamic height and sea level are quite red for wavelengths greater than 400–600 km. Smaller wavelengths cannot be resolved by the hydrographic sampling program. A comparison of the dynamic height and sea level spectra indicates that sea level has somewhat less variability than dynamic height at all wavenumbers.

Variations of zonal currents in the central tropical Pacific during 1970 to 1987: Sea level and dynamic height measurements

The annual and interannual variations of the major zonal currents of the central tropical Pacific for the period 1970–1987 are described by analyzing two data sets: sea level data from a few selected island stations and ship‐of‐opportunity expendable bathythermograph (XBT) observations. The annual cycles of geostrophic surface current indices (dynamic height or sea level difference between the ridge and the trough defining the current) in the North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC) are found to be in phase (maxima in November), to be strong, and to explain about 50% of the total variance. The annual cycle of volume transport relative to 400 dbar in the NECC is also strong, but NEC volume transport has a weak annual cycle. The discrepancy between the NEC surface current and the transport is due to the variation of the surface current being larger than that of the deep currents. The annual cycle of the transport of the South Equatorial Current (SEC) is 6 months out of phase with that of the NEC and NECC. Phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general, but there are significant differences between the annual amplitudes of the two types of measurements. These differences arise from a variety of sources: dynamic height reference level assumptions, longitudinal spacing of sea level gauges, meridional resolution of sea level array, and different signal to noise ratios for the two sets of indices. The NECC shows marked flow maxima during the peaks of the strong Niños of 1972 and 1982, which were followed by near disappearance of the current the following year; these fluctuations are clearly present in both the surface current indices and the volume transport. In 1976 and 1986–1987 (moderate Niños) the same pattern is not marked. The El Niño signal in NEC transport is weak. There was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 Niños followed by a fall the next year. This resulted in a decrease of the transport of the SEC in the first year followed by an increase to a maximum during the second year; the maximum coincides with a minimum in NECC transport.

Teredolites, wood substrates, and sea-level dynamics

Allochthonous logs and/or Teredolites , clavate borings produced within xylic (wood) substrates, occur in extraordinary abundance as sedimentary components in transgressive marine shelf deposits of the lower Paleocene Clayton Formation in western Alabama. It is pro-posed herein that the abundance and preservational state of these components were controlled by (1) an influx pulse of xylic substrates into marine and marginal-marine environments, (2) hydraulic concentration of substrates during ravinement, and (3) condensation associated with sediment starvation, all three of which are induced by sea-level rise. The proposed relations among fossil wood, ichnofossils produced therein, and sea-level dynamics may be of use in the discrimination of sequence stratigraphic packages and their bounding surfaces; these relations also have implications for paleobotanical prospecting and the biological evolution and stratigraphic distribution of marine organisms that inhabit xylic substrates.

Sensitivity of wind-driven tropical Pacific Ocean simulations on seasonal and interannual time scales

The purposes of this study are (1) to characterize differences in the time/space structure of various multiyear surface wind products for the tropical Pacific; and (2) to quantify the impact these differences may have on our ability to model oceanic wind-forced variability on seasonal and interannual time scales. Three coincident wind field analyses are used, viz, the Florida State University (FSU) subjective analysis, the University of Hawaii (SAWIN) subjective analysis and the Fleet Numerical Oceanography Center (FNOC) operational analysis. The five years chosen for study, 1979–1983, encompass three years of a fairly regular seasonal cycle leading up to the 1982–1983 El Niño. A linear multi-vertical model model is forced with these analyses; model dynamic height and sea level are then compared with observations based on expendable bathythermograph and island tide gauge data. The mean seasonal cycle prior to El Niño (1979–1981) is considered first, which then serves as a self-consistent basis for analyzing the interannual variability, particularly the significant anomalies about the mean in 1982–1983. The impact of discrepancies in the forcing functions is discussed relative to the dominant seasonal and interannual scales of variability for the wind-driven oceanic response. The analyses of the wind products and model solutions indicate the need for special attention to the wind stress curl fields when evaluating wind products for use in tropical oceanographic applications. On seasonal time scales, critical differences in the wind stress products, of order 0.2–0.4 dyn cm −2, are in wind regimes of surface convergence and significant gradients such as the ITCZ and SPCZ. These uncertainties in the wind fields, or more appropriately the wind stress curl distributions, are manifested in model sea level solutions as 6–12 cm discrepancies near the NECC Trough and east of New Guinea. On average, the seasonal amplitude of the wind-forced sea level response in any one simulation is of the same order as the differences between any two sea level simulations. The interannual variability is dominated by the anomalies associated with the 1982–1983 El Niño. Root mean square differences between the product versus product wind stress anomalies range from 0.1–0.2 dyn cm −2 along the major ship tracks and up to 0.5 dyn cm −2 away from the shipping lanes. The basin-wide average of the rms differences, approximately 0.25 dyn cm −2, is of similar magnitude to the average wind stress anomaly. Discrepancies in the interannual variability of the wind products lead to large-scale differences in the model sea level anomalies of up to 9–21 cm. The anomalous year to year variability of model sea level is of similar order in each of the simulations. Results for the FSU and SAWIN forced simulations are generally in better agreement with the observations than the FNOC simulation, especially during the 1982–1983 El Niño.

Observations and wind‐forced model simulations of the mean seasonal cycle in tropical Pacific sea surface topography

We examine simulations of the mean seasonal cycle in the tropical Pacific using a multiple vertical mode linear numerical model forced with three different surface wind stress products averaged over the period 1979–1981. The model is run to equilibrium for each of four vertical modes, and results are summed. Simulated mean seasonal cycles in dynamic height and sea level are then compared with observed variations based on expendable bathythermograph and island tide gauge data averaged over the same 1979–1981 period. All simulations show characteristic features of the mean meridional ridge‐trough structure in surface topography. However, north and south equatorial ridges at 20°N and 20°S are much higher than those observed, only weak equatorial ridges are generated near 4°N, and none of the simulations exhibits a significant equatorial trough. These discrepancies are due principally to limitations in model physics and in the wind forcing. Observed and modeled mean seasonal variations in surface height are of the order of a few centimeters. Coherence estimates of 0.5–0.7 are found between the model simulations and the observations for the 1 cycle per year harmonic, which dominates the seasonal cycle over most of the tropical Pacific. This suggests that about 25–50% of the variance in the observed annual surface height is accounted for by the linear model, given current estimates of the surface wind field. Harmonics higher than the annual are less well modeled because of their weaker signal levels. Regional patterns are observed in coherence levels between modeled and observed variability; i.e., longitudinally, the eastern Pacific is most poorly modeled, while latitudinally, the equatorial band (5°N to 5°S) is best modeled. However, no wind stress product is clearly superior to the others for simulating the mean seasonal cycle. Thus uncertainty in the surface stress field remains a fundamental obstacle to more accurate modeling of the variability in tropical Pacific sea surface topography.

Relations between sea level, thermocline depth, heat content, and dynamic height in the tropical Pacific Ocean

The use of combined information from expendable bathythermograph and sea level observations for ocean monitoring requires the establishment of relations between sea level, thermocline depth, heat content, and dynamic height. Sea level fluctuations are a good measure of thermocline depth fluctuations in the tropical Pacific between about 15°N and 15°S and allow the determination of changes of upper‐layer volume. Sea level is also a good measure of heat content, and useful correlations extend to higher latitudes. Dynamic height and sea level fluctuations agree only in those areas where the thermal structure resembles a two‐layer system very well, and good correlations are restricted to a narrower area. The combination of bathythermograph and sea level observations will allow a better mapping of the changes of thermocline topography, heat content, and dynamic height for the monitoring of climatic changes in the tropical Pacific.

Geoidal topography: Origin and time consistency

The geoid topography is a direct measure of the irregular density distribution within the Earth. Hence, the geoid is an important tool in deciphering the internal structures and conditions. Deformations of the geoid — i.e. the mean sea level — with time, therefore, give evidence of density changes within the Earth. Past sea level changes that can be shown not to originate from tectonic processes, changes in the oceans’ water volume, changes in the oceans’ basin volumes or local dynamic sea level effects are found to represent paleo-geoid changes (Morner, 1976, 1981a, 1983a). Such paleogeoid changes have now been identified for time units ranging from a few years to several million years (Morner, op. cit.). This gives evidence of rapid processes within the Earth changing the density distribution; i.e. it gives evidence of a high-dynamic Earth.

Deadly Surges in the Bay of Bengal: Dynamics and Storm-tide Tables

DURING the 1960s alone about 400,000 people died in the exceptionally high tides which swept in along the low lying northern coast of the Bay of Bengal under the force of tropical cyclones. Over half of these lives were lost in the single surge caused by the storm of November 19701. Such loss of life can be prevented only by depopulation of the area, construction of permanent defences, or by prediction of and protection against individual storms. The latter involves the identification and survey of a severe cyclone, forecast of the development and course of the storm, prediction of the sea level response, warning of danger, and protection of the survivors. We have considered the problem of sea level dynamics and prediction, and using simple mathematical models have studied the characteristics of the surge and found the sea level response to a number of model cyclones moving across the Bay along a variety of paths. Our results are summarized in a form which is handy for immediate incorporation into a warning programme.