Dei Huws

Holocene evolution of seasonal stratification in the Celtic Sea: refined age model, mixing depths and foraminiferal stratigraphy

Published stable isotopic (oxygen, carbon) and preliminary foraminiferal data from a Holocene vibrocore from the Celtic Sea (Northwest European continental shelf) have been interpreted in terms of the progressive replacement of tidally mixed by seasonally stratified water, the first study of the long-term dynamics of seasonal stratification. This study was hampered by poor age control and the foraminiferal data were based on processing with a 125-μm sieve which has been shown not to recover critical small taxa. We present here a new age model for this vibrocore (BGS 51/-07/199) based on 12 accelerator mass spectrometry 14C dates on foraminifera and molluscs, radionuclide (137Cs, 40K) profiles from the vibrocore and juxtaposed multicores, and a complete foraminiferal stratigraphy based on sieving at 63 μm. Together, the three datasets enable a mixing model to be proposed which clarifies the resolution of the record which is confirmed as extending from the Late-Glacial to the late Holocene. Correction for autocompaction reveals an increase in sedimentation rate and mixing depth from the early to the late Holocene. A temporary increase in sedimentation rate at 6650 yr cal BP is attributed to migration of the productive frontal zone across the core site. Some time after 3000 yr cal BP sedimentation either slowed abruptly or ceased completely, giving the modern mixed layer an apparent age of ∼3000 yr cal BP. The mixed layer depth indicates strongly that the apparent transition to stratification during the early Holocene is primarily a threshold change attenuated by bioturbation; secondary attentuation is related to reworking. The mixed layer model adopted suggests that this isotopic transition occured between 8990 and 8440 yr cal BP (8720±2σ). The foraminiferal analyses, in the light of modern foraminiferal distributional data, support the inference that the succession can be interpreted as a response to the progressive seasonal stratification of the Celtic Sea during the Holocene. The data highlight the value of key taxa as indicators of shelf palaeostratification in the geological record, notably of Textularia bocki and Stainforthia fusiformis as mixed-frontal and frontal-stratified indicators, respectively. The critical change to this frontal assemblage occurs at the same depth as the isotopic threshold, based on the mixing model. These data affirm the earlier interpretation of the isotopic record as registering bottom water temperature and water column productivity changes driven by the evolution of seasonal stratification. The clear association between grain size and sedimentation, as well as these isotopic and faunal data, indicates that the shelf record of highstand sedimentation is preferentially biased towards the preservation of sequences deposited under seasonally stratified rather than mixed water masses.

Climatic variability, plasticity, and dispersal: A case study from Lake Tana, Ethiopia

The numerous dispersal events that have occurred during the prehistory of hominin lineages are the subject of longstanding and increasingly active debate in evolutionary anthropology. As well as research into the dating and geographic extent of such dispersals, there is an increasing focus on the factors that may have been responsible for dispersal. The growing body of detailed regional palaeoclimatic data is invaluable in demonstrating the often close relationship between changes in prehistoric environments and the movements of hominin populations. The scenarios constructed from such data are often overly simplistic, however, concentrating on the dynamics of cyclical contraction and expansion during severe and ameliorated conditions respectively. This contribution proposes a two-stage hypothesis of hominin dispersal in which populations (1) accumulate high levels of climatic tolerance during highly variable climatic phases, and (2) express such heightened tolerance via dispersal in subsequent low-variability phases. Likely dispersal phases are thus proposed to occur during stable climatic phases that immediately follow phases of high climatic variability. Employing high resolution palaeoclimatic data from Lake Tana, Ethiopia, the hypothesis is examined in relation to the early dispersal of Homo sapiens out of East Africa and into the Levant. A dispersal phase is identified in the Lake Tana record between c. 112,550 and c. 96,975 years ago, a date bracket that accords well with the dating evidence for H. sapiens occupation at the sites of Qafzeh and Skhul. Results are discussed in relation to the complex pattern of H. sapiens dispersal out of East Africa, with particular attention paid to the implications of recent genetic chronologies for the origin of non-African modern humans.

Surficial seabed sediment properties derived from seismic profiler responses

Abstract As a result of the complex and variable nature of sea floor sedimentary environments and the problems associated with obtaining representative samples of sea floor materials, it has long been accepted that geophysical methods hold the greatest potential for rapid assessment of sea floor sediment physical property variability. This paper presents results of two experimental geophysical studies aimed at developing in situ sea floor sediment classification methodologies applicable to siliciclastic environments. At one of the two study sites, Irvine Bay in the Clyde Sea, an area of approximately 10 km2 was intensively surveyed using a boomer seismic profiler and digital data acquisition system. Three core samples recovered in the area were used to calibrate seabed reflection responses at spot locations, and once calibrated, the entire geophysical dataset was inverted to produce maps showing the spatial distribution of sediment physical properties (porosity, density and grain size distribution) at the seabed surface. Very good agreement was found between seismic predictions of sediment properties and physical measurements made on independent core and grab samples. Similar levels of agreement were found for seismo-acoustic predictions made after surveying a transect in the Southern Baltic Sea. On the basis of these experimental studies it appears that, with only limited ground truth information, it is possible to invert high-resolution seismic reflection data to produce quantitative information on the spatial distribution of sea floor sediment physical properties.

Development of a seafloor geophysical sledge

Abstract With the increasing exploitation of the worlds coastal zone and continental shelves, covering a range of applications (waste disposal, civil engineering, resource extraction), new techniques of evaluating the seafloor are constantly in demand. For any new technique to prove economically viable and be widely used it must offer a rapid, inexpensive, and efficient method of data collection. To this end a seafloor geophysical sledge has been developed to measure in situ the seismic wave velocities (shear and compressional) and the electrical formation factor. Data are collected while the sledge is towed along the seafloor with measurements made at discrete points along a profile line. The collected velocity values may be used directly to input into acoustic models or compute the elasticity of the sediment, or the three geophysical parameters can be applied indirectly via empirical relations to predict other in situ physical parameters.

Geophysical ground-truthing experiments in Eckernfrde Bay

During the 1994 Coastal Benthic Boundary Layer Project research cruise in Eckernförde Bay, multichannel digital seismic and electrical resistivity data were collected using surface- and bottom-towed arrays. Profiling with a bottom-towed sled yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from < 10 m s−1 at the surface to around 16 m s−1 at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground-truthing.

150,000-year palaeoclimate record from northern Ethiopia supports early, multiple dispersals of modern humans from Africa

Climatic change is widely acknowledged to have played a role in the dispersal of modern humans out of Africa, but the timing is contentious. Genetic evidence links dispersal to climatic change ~60,000 years ago, despite increasing evidence for earlier modern human presence in Asia. We report a deep seismic and near-continuous core record of the last 150,000 years from Lake Tana, Ethiopia, close to early modern human fossil sites and to postulated dispersal routes. The record shows varied climate towards the end of the penultimate glacial, followed by an abrupt change to relatively stable moist climate during the last interglacial. These conditions could have favoured selection for behavioural versatility, population growth and range expansion, supporting models of early, multiple dispersals of modern humans from Africa.

Geological settings and seafloor morphodynamic evolution linked to methane seepage

Methane seeps have been shown to be a powerful agent in modifying seabed morphology, amongst others by cementation processes such as the formation of methane-derived authigenic carbonates (MDACs). The cements stabilise mobile sediment particles and thereby promote the formation of edifices such as mounds on various scales. The release of methane from shallow subsurface sources, when concentrated in seeps, has proven hazardous to offshore construction activities. In this paper, methane cycling and MDAC precipitation is explored as a potential “finger on the pulse” for the recognition of shallow gas pockets and active gas seepage. This would provide a valuable planning tool for seabed engineering developments in areas of potential gas seepage. Measurements of methane concentrations in the Irish Sea are correlated with a unique record of longer-term morphological evolution (up to 11 years) of MDAC structures and subsurface geological settings which would favour the build-up of shallow gas. It was found that gas seepage activity associated with fault zones correlates with carbonate mound steepness. Cessation of gas seepage results in a relatively slow process of erosion and burial of the mounds, eventually producing a subdued carbonate mound morphology after several decades. The Quaternary glacial legacy equally seems to define the distribution and geometry of the MDAC structures. In this case, methane gas locally concentrated in sands and gravels capped by clayey glacial sediments may percolate upwards to the seafloor. A link between methane seeps and the formation of unusually large, trochoidally shaped sediment waves observed on continental shelves worldwide is deemed unlikely. However, the observations suggest that gas percolating through sediment waves may be capped by muddy sediments which have deposited on the sediment waves due to anoxic conditions or eroded from a neighbouring cliff. Other sediment waves in the Irish Sea were found to have a step-like morphology similar to that documented in the neighbouring MDAC cemented seafloor. These processes may influence sediment waves dynamics and warrant further investigation.

A Nondestructive Technique for Predicting the In Situ Void Ratio for Marine Sediments

Abstract This study tests the hypothesis that the in situ void ratio of surficial marine sediments may be predicted from shear wave velocity-depth data with a reliability equal to that of other methods currently available. Shear wave velocity is fundamentally controlled by the number of grain-to-grain contacts per unit volume of material and by the effective stress across those contacts. In this study, three previously established empirical formulae are used to predict void ratio from velocity-depth data. Field data were acquired along a transect off the northern Californian coast across which water depth increased from 35 to 70 m and seafloor sediment type varied from sand to silty-sand, respectively. A towed seafloor sled device was used to collect shear wave refraction data, and a marked, systematic decrease in velocity was observed along the line, ranging from 35–70 m/s for the coarse, near-shore material to 25–40 m/s for the finer, offshore deposits. Void ratios predicted from these velocities were compared with data measured directly from box-core samples. Of the formulae used for prediction, two agree remarkably well with the control data. Both predicted and control values increase from 0.6–0.8 for the sandy material to 1.1–1.5 for the silty-sand. Thus, this study does not disprove the hypothesis set and demonstrates the potential of field shear wave velocity-depth data as a means of delineating spatial variation in void ratio for surficial marine sediments in a remote, nondestructive manner.

A combined geotechnical/geophysical method for the prediction of liquefaction, with particular reference to the Fraser River Delta, British Columbia

Abstract Over the past decade or so, much work has been undertaken into the development of various techniques for the measurement of seismic shear wave velocity, offshore, onshore and in the laboratory. Previous research has suggested that the shear wave velocity of an uncemented sand may be a useful index to its liquefaction potential. The research described in this paper has combined the concepts of critical/steady-state soil mechanics and the measurement of shear wave velocity in order to provide a new index of sand consolidation state, enabling the prediction of flow liquefaction in laboratory-prepared samples. Results obtained from the laboratory are then applied to the field situation, in this case, the Fraser River Delta, British Columbia, which is an example of a site currently undergoing a major earthquake stability evaluation. A preliminary analysis of the data indicates that the sediments around Roberts Bank, on the southwestern portion of the delta, are potentially liquefiable. This conclusion is confirmed by other, more conventional, investigative techniques.

Geophysical ground‐truthing experiments in Eckernfoerde Bay

During the 1994 Coastal Benthic Boundary Layer Special Research Program’s (CBBLSRP) experiment in Eckernfoerde Bay, multichannel digital seismic and electrical resistivity data were collected using surface and bottom towed arrays. Profiling with a bottom towed sledge yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the sediment’s pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from <10 at the surface to around 16 m/s at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground‐truthing.