Sea‐level rise,depth‐dependent carbonate sedimentation and the paradox of drowned platforms

A mathematical model of carbonate platform evolution is presented in which depth‐dependent carbonate growth rates determine platform‐top accumulation patterns in response to rising relative sea‐level. This model predicts that carbonate platform evolution is controlled primarily by the water depth and sediment accumulation rate conditions at the onset of relative sea‐level rise. The long‐standing ‘paradox of a drowned platform’ arose from the observation that maximum growth rate potentials of healthy platforms are faster than those of relative sea‐level rise. The model presented here demonstrates that a carbonate platform could be drowned during a constant relative sea‐level rise whose rate remains less than the maximum carbonate production potential. This scenario does not require environmental changes, such as increases in nutrient supply or siliciclastic sedimentation, to have taken place. A rate of relative sea‐level rise that is higher than the carbonate accumulation rate at the initial water depth is the only necessary condition to cause continuous negative feedbacks to the sediment accumulation rates. Under these conditions, the top of the carbonate platform gradually deepens until it is below the active photic zone and drowns despite the strong maximum growth potential of the carbonate production factory. This result effectively resolves the paradox of a drowned carbonate platform. Test modelling runs conducted with 2·5 m and 15 m initial sea water depths at bracketed rates of relative sea‐level rise have determined how fast the system catches up and maintains the ‘keep‐up’ phase. This is the measure of time necessary for the basin to respond fully to external forcing mechanisms. The duration of the ‘catch‐up’ phase of platform response (termed ‘carbonate response time’) scales with the initial sea water depth and the platform‐top aggradation rate. The catch‐up duration can be significantly elongated with an increase in the rate of relative sea‐level rise. The transition from the catch‐up to the keep‐up phases can also be delayed by a time interval associated with ecological re‐establishment after platform flooding. The carbonate model here employs a logistical equation to model the colonization of carbonate‐producing marine organisms and captures the initial time interval for full ecological re‐establishment. This mechanism prevents the full extent of carbonate production to be achieved at the incipient stage of relative sea‐level rise. The increase in delay time due to the carbonate response time and self‐organized processes associated with biological colonization increase the chances for platform drowning due to deepening of water depth (> ca 10 m). Furthermore this implies a greater likelihood for an autogenic origin for high‐frequency cyclic strata than has been estimated previously.     

Luminescence dating of Holocene spit deposits: An example from Skagen Odde, Denmark

Clemmensen, L. B. & Murray, A. S. 2009: Luminescence dating of Holocene spit deposits: An example from Skagen Odde, Denmark. Boreas , 10.1111/j.1502-3885.2009.00110.x. ISSN 0300-9483.
Skagen Odde is a large, active spit system in northern Denmark that started to form about 7200 years ago. Models for spit growth have usually relied on radiocarbon-dating of swale peat (Martørv). In this study, we date the spit deposits at three sites directly using Optically Stimulated Luminescence (OSL) to obtain supplementary age control on spit development. The spit deposits consist of a lowermost succession of shoreface, beach and backshore aeolian deposits topped by a swale peat and followed by an uppermost succession of aeolian sand sheet and dune deposits. The ages of the shallow marine, beach and backshore aeolian deposits at the main study site are indistinguishable, implying good resetting of the shallow marine deposits; the average age of 4640±250 years compares well with earlier model predictions based on radiocarbon-dating of swale peat. Aeolian sand extracted from the uppermost part of the swale peat at this site provides OSL ages of between 1600 and 2500 years, in good agreement with a calibrated AMS age from the same level of 2330–2200 years. The uppermost aeolian succession consists of two units separated by a thin palaeosol, and the aeolian units have OSL ages of about 1500 years and younger than 130 years. Lowermost spit deposits at the two additional sites have average ages of 5010±240 and 3730±190, respectively, supporting the existing chronology for spit growth based on radiocarbon-dating.     

The hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels

ABSTRACT Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane-beds. The influence of clusters is a major determinant of sedimentary sorting.     

Zirconium Determination by ED-XRF: a Critical Evaluation of Silicate Reference Materials as Calibration Standards

An assessment of the difficulties in determining zirconium by various techniques indicates that X-ray fluorescence analysis has several favourable characteristics, provided the technique can be calibrated reliably. After critical evaluation, a group of silicate reference materials was selected to calibrate energy dispersive X-ray fluorescence instrumentation. The accuracy of the resultant calibration was verified by a standards addition procedure. New zirconium data were obtained on a wide range of reference samples and compared with values from recent compilations.     

The effect of species on lacustrine δ18Odiatom and its implications for palaeoenvironmental reconstructions

The oxygen isotope composition of diatom silica (δ¹⁸O_diatom) is increasingly being used to reconstruct climate from marine and lacustrine sedimentary archives. Although diatoms are assumed to precipitate their frustule in isotopic equilibrium with their surrounding water, it is unclear whether internal processes of a given species affect the fractionation of oxygen between the water and the diatom. We present δ¹⁸O_diatom data from two diatom size fractions (3–38 and >38 µm) characterized by different species in a sediment core from Heart Lake, Alaska. Differences in δ¹⁸O_diatom between the two size fractions varies from 0 to 1.2‰, with a mean offset of 0.01‰ (n = 20). Fourier transform infrared spectroscopy confirms our samples consist of pure biogenic silica (SiO₂) and δ¹⁸O_diatom trends are not driven by contamination. The maximum offset is outside the range of error, but the mean is within analytical error of the technique (± 1.06‰), demonstrating no discernible species‐dependent fractionation in δ¹⁸O_diatom. We conclude that lacustrine δ¹⁸O_diatom measurements offer a reliable and valuable method for reconstructing δ¹⁸O_water. Considering the presence of small offsets in our two records, we advise interpreting shifts in δ¹⁸O_diatom only where the magnitude of change is greater than the combined analytical error.     

Coastal dune conservation on an Irish commonage: community-based management or tragedy of the commons?

In Ireland 'commonage' refers to lands jointly owned by several individuals who have grazing rights. Commonage can provide the low-intensity grazing regime regarded as optimal for habitat conservation, and it is also unlikely to suffer the negative impacts of building development or coastal engineering. Today, however, the traditional control systems of coastal commonage are generally moribund, leading to habitat degradation. The only viable future management model is likely to be one based on local community control. Community management would have the legitimacy to counter the negative perceptions of external authority that generate environmental degradation.     

A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South-Central Alaska

The Early to Middle Jurassic Talkeetna Arc section exposed inthe Chugach Mountains of south–central Alaska is 5–18km wide and extends for over 150 km. This accreted island arcincludes exposures of upper mantle to volcanic upper crust.The section comprises six lithological units, in order of decreasingdepth: (1) residual upper mantle harzburgite (with lesser proportionsof dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lowercrustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanicrocks. The pyroxenites overlie residual mantle peridotite, withsome interfingering of the two along the contact. The basalgabbronorite overlies pyroxenite, again with some interfingeringof the two units along their contact. Lower crustal gabbronorite(10 km thick) includes abundant rocks with well-developed modallayering. The mid-crustal plutonic rocks include a heterogeneousassemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40%area), and concentrations of mafic dikes and chilled mafic inclusions.The volcanic rocks (7 km thick) range from basalt to rhyolite.Many of the evolved volcanic compositions are a result of fractionalcrystallization processes whose cumulate products are directlyobservable in the lower crustal gabbronorites. For example,Ti and Eu enrichments in lower crustal gabbronorites are mirroredby Ti and Eu depletions in evolved volcanic rocks. In addition,calculated parental liquids from ion microprobe analyses ofclinopyroxene in lower crustal gabbronorites indicate that theclinopyroxenes crystallized in equilibrium with liquids whosecompositions were the same as those of the volcanic rocks. Thecompositional variation of the main series of volcanic and chilledmafic rocks can be modeled through fractionation of observedphase compositions and phase proportions in lower crustal gabbronorite(i.e. cumulates). Primary, mantle-derived melts in the TalkeetnaArc underwent fractionation of pyroxenite at the base of thecrust. Our calculations suggest that more than 25 wt % of theprimary melts crystallized as pyroxenites at the base of thecrust. The discrepancy between the observed proportion of pyroxenites(less than 5% of the arc section) and the proportion requiredby crystal fractionation modeling (more than 25%) may be bestunderstood as the result of gravitational instability, withdense ultramafic cumulates, probably together with dense garnetgranulites, foundering into the underlying mantle during thetime when the Talkeetna Arc was magmatically active, or in theinitial phases of slow cooling (and sub-solidus garnet growth)immediately after the cessation of arc activity. KEY WORDS: island arc crust; layered gabbro; Alaska geology; island arc magmatism; lower crust     

A scale of dissolution for quartz and its implications for diagenetic processes in sandstones

During diagenesis quartz grains undergo selective dissolution, controlled in location by the surface energy characteristics of the individual grains. Experimental etching in HF of isolated quartz grains reproduces comparable textures to those of natural occurrences. Some experimental results illustrate the specific effects of surface textures on their initial dissolution rates, so demonstrating the control surface energy variation has over dissolution. A hierarchy of grain surface characteristics, according to surface energies, provides a useful guide to the relative rates of dissolution during decomposition.