Jaco H. Baas

Turbulence Modulation in Clay-Rich Sediment-Laden Flows and Some Implications for Sediment Deposition

Clay-rich flows are common in many sedimentary environments, ranging from rivers with high suspended sediment concentrations and fluid muds on the continental shelves to deep-ocean density currents. Such sediment-laden flows have fluid-dynamic characteristics which are radically different from their clear-water counterparts. Laboratory experiments demonstrate that an increasing concentration of kaolinite results in a distinct change in velocity and turbulence structure in flows transitional between Newtonian and non-Newtonian behavior. Such transitional flows develop a lower region of reduced velocity that is separated from the overlying flow by a distinct shear layer. Large-scale Kelvin-Helmholtz instabilities are generated along this shear layer and dominate both turbulence production and fluid mixing. These experimental results are used to interpret the formation of parallel laminae in turbiditic muds and propose a model for sediment sorting in a range of clay-rich flows. Non-Newtonian flows with low concentrations of kaolinite (∼ 4% by volume) are devoid of both turbulent and cohesive strength at the shear velocities investigated. Deposition of coarser, noncohesive particles in such flows would be expected to be controlled entirely by differences in settling velocity, and the deposits would therefore be expected to be graded and massive.

Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products

The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment gravity cores. Hemipelagic sedimentation (lithofacies A) was dominant during glacial times. It was interrupted periodically by deposition of shelf- and upper-slope-derived silty and sandy terrigenous material by dilute turbidity currents (lithofacies B and C), ice-rafted debris during distinct periods of breakdown of North Atlantic ice sheets (Heinrich events, lithofacies D) and large amounts of pteropods (lithofacies F). Settling of biogenic particulate material (lithofacies E) prevailed during the Holocene, when sea level and sea surface temperatures were high and terrigenous shelf-input was low. Downslope transport was dominant on the northern part of the Portuguese margin, culminating in frequent turbidity current transport between 35 and 70 ka. This may be due to a humid climate and a high fluvial input. Pteropod muds are confined to cores south of 41°N. Prominent peaks in pteropod concentration were radiocarbon dated at 17.8 and 24.6 ka. Layers rich in ice-rafted debris (IRD) were found along the entire margin. The base of these layers have been dated at 13.6–15.9 14C ka, 21.0–22.0 14C ka, 33.8 14C ka and ±64.5 ka, which correspond well with the ages of Heinrich events 1, 2, 4 and 6 in the central North Atlantic. Heinrich events 0 (10.5 ka), 3 (27 ka) and 5 (50 ka) rarely influenced sedimentation on the Portuguese slope. A mineralogical study of the IRD within Heinrich layers suggests that most icebergs were derived from the Laurentide Ice Sheet in the Hudson Strait and Hudson Bay area through the Labrador Current and the Canary Current and flowed in a southward direction along the margin. IRD from European ice sheets may have been mixed in during Heinrich event 6. On their way along the margin the icebergs lost much of their sediment load due to melting of the ice in a progressively warmer climate. The southernmost latitude studied (37°N) may be close to the southeastern extension of iceberg transport during Heinrich events.

EZ-ROSE: a computer program for equal-area circular histograms and statistical analysis of two-dimensional vectorial data

Abstract EZ-ROSE 1.0 is a computer program for the statistical analysis of populations of two-dimensional vectorial data and their presentation in equal-area rose diagrams. The program is implemented as a Microsoft® Excel workbook containing worksheets for the input of directional (circular) or lineational (semi-circular) data and their automatic processing, which includes the calculation of a frequency distribution for a selected class width, statistical analysis, and the construction of a rose diagram in CorelDraw™. The statistical analysis involves tests of uniformity for the vectorial population distribution, such as the nonparametric Kuiper and Watson tests and the parametric Rayleigh test. The statistics calculated include the vector mean, its magnitude (length) and strength (data concentration); the Batschelet circular standard deviation as an alternative measure of vectorial concentration; and a confidence sector for the vector mean. The statistics together with the frequency data are used to prepare a Corel Script™ file that contains all the necessary instructions to draw automatically an equal-area circular frequency histogram (rose diagram) in CorelDraw™. The advantages of EZ-ROSE, compared to other software for circular statistics, are: (1) the ability to use an equal-area scale in rose diagrams; (2) the wide range of tools for a comprehensive statistical analysis; (3) the ease of use, as Microsoft® Excel and CorelDraw™ are widely known to users of Microsoft® Windows; and (4) the high degree of flexibility due to the application of Microsoft® Excel and CorelDraw™, which offer a whole range of tools for possible addition of other statistical methods and changes of the rose-diagram layout.

The pervasive role of biological cohesion in bedform development

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS.

Sticky stuff: Redefining bedform prediction in modern and ancient environments

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

The use of sedimentary structures as indicators of flow and sediment morphodynamics in ancient sediments lies at the very heart of sedimentology, and allows reconstruction of formative flow conditions generated in a wide range of grain sizes and sedimentary environments. However, the vast majority of past research has documented and detailed the range of bedforms generated in essentially cohesionless sediments that lack the presence of mud within the flow and within the sediment bed itself. Yet most sedimentary environments possess fine-grained sediments and recent work has shown how the presence of this fine sediment may substantially modify the fluid dynamics of such flows. It is increasingly evident that understanding the influence of mud, and the presence of cohesive forces, is essential to permit a fuller interpretation of many modern and ancient sedimentary successions. In this paper, the present state of knowledge on the stability of current- and wave-generated bedforms and their primary current stratification is reviewed, and a new extended bedform phase diagram is presented that summarizes the bedforms generated in mixtures of sand and mud under rapidly decelerated flows. This diagram provides a phase space using the variables of yield strength and grain mobility as the abscissa and ordinate axes, respectively, and defines the stability fields of a range of bedforms generated under flows that have modified fluid dynamics owing to the presence of suspended sediment within the flow. Our results also present unique data on a range of bedforms generated in such flows, whose recognition is essential to help interpret such deposits in the ancient sedimentary record, including the following: (1) heterolithic stratification, comprising alternating laminae or layers of sand and mud; (2) the preservation of low-amplitude bed-waves, large current ripples and bed scours with intrascour composite bedforms; (3) low-angle cross-lamination and long lenses and streaks of sand and mud formed by bed-waves; (4) complex stacking of reverse bedforms, mud layers and low-angle cross-lamination on the upstream face of bed scours; (5) planar bedding comprising stacked mud–sand couplets. Furthermore, the results shown herein demonstrate that flow variability is not required to produce deposits consisting of interbedded sand and muds, and that the nature of flaser, wavy and lenticular bedding (sensu Reineck & Wunderlich 1968) may also need reconsideration in the deposits of such sediment-laden flows.

Sediment fluxes along the northeastern European Margin: inferring hydrological changes between 20 and 8 kyr

Abstract In an attempt to delineate past hydrographic conditions on the northeastern Atlantic Ocean Margin along a latitudinal transect (37°–63°N) and link the eastern North Atlantic Ocean with the Norwegian Sea, we have investigated planktonic and benthic stable oxygen isotopes, as well as abundances and fluxes of planktonic and benthic foraminifera, calcium carbonate, organic carbon, and detrital grains in six sediment cores studied by the ENAM project. In our joint approach, we focused on the major climatic events of the last 20 kyr, that is, the Last Glacial Maximum (LGM), the Heinrich event 1 (H1), the Younger Dryas (YD) and the Holocene Climatic Optimum (HCO). SIMMAX temperature estimates suggest glacial SSTs off Portugal, similar to Holocene values, contradicting the proposed polar front position of CLIMAP Project Members (1976) . Besides, increased planktonic and benthic foraminiferal fluxes in the Norwegian Sea and the Faeroe Islands cores point to Nordic Seas partially ice-free with seasonally productive polynas. Multiple IRD spikes observed at the Faeroe Islands and Norwegian Sea sites, coeval with H1, suggest multiple pulses of the Fenno-Scandian ice sheet. At the same time the presence of IRD on the Portuguese Margin cores confirms the southward influence of icebergs. The increase in foraminiferal abundances and fluxes that parallel IRD spikes at all sites suggests that times of increased productivity either succeed or occur simultaneously with the iceberg surges in the open North Atlantic. Productivity conditions similar to those observed during H1 are also registered during the YD even though planktonic foraminiferal assemblages suggest less cold conditions.

The role of biophysical cohesion on subaqueous bed form size

Abstract Biologically active, fine‐grained sediment forms abundant sedimentary deposits on Earths surface, and mixed mud‐sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross‐stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record.

Duration of deposition from decelerating high-density turbidity currents

Using recent advances in the stability analysis of current ripples, a new model for the calculation of duration of sediment deposition from decelerating high-density turbidity currents is proposed. The model, named TDURE, refines a duration model proposed by Allen (1991) (J. Sediment. Petrol. 61, 291‐295) by calculating the accumulation time of the rippled Tc-division separately from the massive and plane parallel-laminated Tab-divisions in Bouma-type turbidites. TDURE consists of three modules. In the first module, the accumulation time for theTa- and Tb-divisions is approximated by assuming a linear decrease in sedimentation rate with height in the turbidite. As in the original model, a straight line is fitted through inferred sedimentation rates at the Tab- and Tbc-boundaries. In the second module, angle of climb of ripples, thickness of the Tc-division and grain size distribution are used in empirical relationships between ripple migration rate and a grain-related mobility parameter to estimate the accumulation time of the Tc-division. In the third module, the expected development of ripple size across the Tc-division is calculated using empirical relationships between rate of development of ripple height and grain-related mobility parameter, and subsequently compared with the observed development of ripple size in the turbidite. In this way, the accuracy of the accumulation time calculated in module 2 can be verified independently. TDURE was tested using Bouma-type turbidites from the Doheny Channel (Capistrano Fm., CA, USA) and the Flysch di Motta (Calabria, Italy). Accumulation times of 12 and 10.75 min for the Tc-division, and 19 and 16 min for the Tabc-sequence were calculated for the Doheny Channel and the Flysch di Motta turbidites, respectively. Although module 3 underestimates the rate of development of current ripples near the Tbcboundary in both beds, ripple size at the Tcd-boundary is calculated accurately. The underestimation of development rate may be caused by differences between flow conditions in experimental flumes on which the model calculations are based and turbiditycurrent dynamics. q 2000 Elsevier Science B.V. All rights reserved.

Washed-Out Ripples: Their Equilibrium Dimensions, Migration Rate, and Relation to Suspended-Sediment Concentration in Very Fine Sand

The transition from linguoid ripples to upper-stage plane bed under steady, uniform flow conditions in very fine sand (D 50 = 0.108 mm) was studied quantitatively using a flume. The transitional ripples, referred to as washed-out ripples, are symmetrical to slightly asymmetrical bed forms with a convex-up profile and low-angle foresets in the direction of the main flow. The washed-out ripples are stable between 10 degrees C-equivalent depth-averaged flow velocities of 0.75 m/s and 0.91 m/s. With increasing velocity in this range the 10 degrees C-equivalent average equilibrium height of the bed forms decreases from 12 mm to zero, the 10 degrees C-equivalent average equilibrium spacing decreases slightly from 138 mm to 103 mm, and the 10 degrees C-equivalent average migration rate increases from 1 mm/s to 3 mm/s. The changes in morphology and migration rate with increasing flow velocity from linguoid ripples to washed-out ripples result from a spatial shift of the locus of maximum sediment flux from the bed-form crest to the downstream wake region. The shift follows the increasing concentration of sediment particles in a near-bed traction carpet above the sediment surface. With increasing flow velocity the volumetric suspended sediment concentration at 0.01 m above ripple crests increases from 0.9% at the lower boundary to 1.6% at the upper boundary of the washed-out-ripple stability field.