Recovery of the seabed following marine aggregate dredging on the Hastings Shingle Bank off the southeast coast of England

The aim of this study was to investigate the effect of dredging intensity on the physical and biological recovery times of the seabed following marine aggregate dredging. Two areas of seabed, previously subject to, respectively, relatively high and lower levels of dredging intensity, were identified on the Hastings Shingle Bank. Two reference areas were also selected for comparative purposes. All four sites were monitored annually over the period 2001–2004, using a combination of acoustic, video and grab sampling techniques. Since the site was last dredged in 1996, this was intended to provide a sequence of data 5–8 years after cessation of dredging. However, an unexpected resumption of dredging within the high intensity site, during 2002 and 2003, allowed an additional assessment of the immediate effects and aftermath of renewed dredging at the seabed. The early stages of recovery could then be assessed after dredging ceased in 2003. Results from both dredged sites provide a useful insight into the early and latter stages of physical and biological recovery. A comparison of recent and historic dredge track features provided evidence of track erosion. However, tracks were still visible 8 years after the cessation of dredging. Within the high dredging intensity site, recolonisation was relatively rapid after the cessation of dredging in 2003. Rather than indicating a full recovery, we suggest that this initial ‘colonization community’ may enter a transition phase before eventually reaching equilibrium. This hypothesis is supported by results from the low intensity site, where biological recovery was judged to have taken 7 years. Further monitoring is needed in order to test this. An alternative explanation is that the rapid recovery may be explained by the settlement of large numbers of Sabellaria spinulosa. As the resumption of dredging within the high intensity site limited our assessment of longer-term recovery it is not yet possible to assume that a 7-year biological recovery period will be applicable to other, more intensively dredged areas at this or more distant locations.     

The Pleistocene palaeoenvironment and stratigraphy of Flatey island (northern Iceland): a preliminary assessment based on palynomorphs

For the first time, palynological data (pollen, spores and dinoflagellate cysts) from Pleistocene outcrop and core deposits from Flatey, off northern Iceland, are presented. The study provides a reconnaissance stratigraphical assessment of the deposits, the correlation between the Pleistocene of Flatey and the well‐known Tjörnes section on the mainland, and the reconstruction of the Pleistocene palaeoenvironment. The preservation of the assemblages is poor to moderate, whereas the diversity and richness are moderate to high. The pollen and spores indicate vegetation composed mainly of sedges and heath, comparable to the vegetation in the Tjörnes area during deposition of the Pleistocene Breidavík Group. Analysis of dinoflagellate cyst assemblages from an outcrop sample allowed their assignment to the Lower Pleistocene dinoflagellate cyst zone DAZ5, previously defined in the Tjörnes section. A core sample yielded a dinoflagellate cyst assemblage unknown from the Tjörnes area. The dinoflagellate cyst data combined with the palaeomagnetic record, the radiometric dating of lavas, and the position of the diamictites and tuff layers allowed refinement of the previously proposed stratigraphical correlations between the Tjörnes area and the Flatey core. The deposition of the sequence from the Flatey core took place during the Matuyama, i.e. between 2.59 and 0.78 Ma, and several hiatuses are postulated. The Lower Pleistocene dinoflagellate cyst assemblages from the outcrop sample bear similarities to those recovered from fully marine subsurface samples on the northern shelf of Iceland within the Polar Front realm. Similar oceanographical conditions influenced by the Irminger Current and the East Icelandic Current can be inferred for Early Pleistocene times.     

Confidence intervals for the correlation between the gamma-ray burst peak energy and the associated supernova peak brightness

A strong correlation between the gamma-ray burster peak energy and the peak luminosity of the associated supernova was discovered by Li for four GRBs. Despite the fact that the formal significance level of the correlation is 0.3 per cent, the smallness of the data set requires careful further evaluation of the result. Subject to the assumption that the data are bivariate Gaussian, a 95 per cent confidence interval of  (−0.9972, 0.02)  for the correlation is derived. Using data from the literature, it is shown that the distribution of known peak GRB energies is not Gaussian if X-ray flashes are included in the sample. This leads to a proposed alternative to the bivariate Gaussian model, which entails describing the dependence between the two variables by a Gaussian copula. The copula is still characterized by a correlation coefficient. The Bayesian posterior distribution of the correlation coefficient is evaluated using a Markov chain Monte Carlo method. The mean values of the posterior distributions range from −0.33 to about zero, depending on the specifics of the supernova (SN) peak brightness distribution. The implication is that the existing data favour a modest correlation between the GRB peak energy and the SN peak brightness; confidence intervals are very wide and include zero.     

Fitting power-law distributions to data with measurement errors

If X , which follows a power-law distribution, is observed subject to Gaussian measurement error e , then   X + e   is distributed as the convolution of the power-law and Gaussian distributions. Maximum-likelihood estimation of the parameters of the two distributions is considered. Large-sample formulae are given for the covariance matrix of the estimated parameters, and implementation of a small-sample method (the jackknife) is also described. Other topics dealt with are tests for goodness of fit of the posited distribution, and tests whether special cases (no measurement errors or an infinite upper limit to the power-law distribution) may be preferred. The application of the methodology is illustrated by fitting convolved distributions to masses of giant molecular clouds in M33 and the Large Magellanic Cloud (LMC), and to H  i cloud masses in the LMC.     

The rapidly pulsating sdO star, SDSS J160043.6+074802.9

A spectroscopic analysis of Sloan Digital Sky Survey (SDSS) J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields   T _eff= 70 000 ± 5000 K  and  log  g = 5.25 ± 0.30  , together with a most likely type of K3 V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. Therefore, it seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having  log  g > 5.3  to be more likely to be unstable and capable of driving pulsation in the observed frequency range.     

The Impact of Vegetation Cover on Sahelian Drought Persistence

The sensitivity of the potential for convective precipitation to land surface characteristics in the West African Sudano-Sahelian zone is studied. Simulations are performed with a one-dimensional version of a mesoscale atmospheric model coupled to a detailed land surface model to investigate the influence of the land surface on rain infiltration, surface evapotranspiration, and their impact on the convective available potential energy. The simulations show that the presence of a densely vegetated surface acts as a catalyst in the hydrological cycle, creating a positive feedback and enhancing precipitation recycling. It is argued that this is due to the relation between the characteristic drying-out time of the soil and the return frequency of the rain-triggering African easterly waves.     

The analysis of irregularly observed stochastic astronomical time-series – I. Basics of linear stochastic differential equations

The theory of low-order linear stochastic differential equations is reviewed. Solutions to these equations give the continuous time analogues of discrete time autoregressive time-series. Explicit forms for the power spectra and covariance functions of first- and second-order forms are given. A conceptually simple method is described for fitting continuous time autoregressive models to data. Formulae giving the standard errors of the parameter estimates are derived. Simulated data are used to verify the performance of the methods. Irregularly spaced observations of the two hydrogen-deficient stars FQ Aqr and NO Ser are analysed. In the case of FQ Aqr the best-fitting model is of second order, and describes a quasi-periodicity of about 20 d with an e-folding time of 3.7 d. The NO Ser data are best fitted by a first-order model with an e-folding time of 7.2 d.     

A search for short time-scale I-band variability in ultracool dwarfs

76 hours of CCD photometry of 11 L dwarfs and one T dwarf is described. Three objects, including the T dwarf ε Indi B, showed convincing evidence for variability. A periodicity of 8 h is clearly visible in the light curve of 2MASS J1155395−372735.