Holocene evolution of a rock‐bounded barrier‐lagoon system,Cíes Islands,northwest Iberia

Coastal geomorphology results from the combined effects of contemporary dynamics, sea‐level rise and the inherited geological framework, yet the relative importance of these driving mechanisms may change throughout the evolutionary history of coastal deposits. In this contribution, we analyse the depositional history of the Cíes Islands barrier‐lagoon system, based on lithofacies, radiocarbon ages, and pollen analysis. Our results reveal a sedimentary sequence that provides evidence for striking changes in the dynamical functioning of this complex since the mid‐Holocene. The sedimentary sequence commenced about 7700 cal years bp by fresh‐water ponding of an upland depression located about 4 m below present mean sea‐level. Fresh‐water ponds were infilled by aeolian sediments following a gradual lowering of the water‐table 4000 cal years bp . Post‐3600 cal years bp sea‐level rise allowed water oscillations to reach the elevation of the bedrock causing the inundation of fresh‐water ponds and subsequent lagoonal and marine sedimentation. Subsequently, landward and upward migration of a sand‐barrier led to overwash and deposition of sand in the newly formed lagoon. The resultant sedimentary sequence suggests that climatic conditions played an important role controlling the sedimentation regime during the entire history of the basin; changing water‐table levels during early stages of evolution and increasing storminess during more recent times. In addition, background sea‐level rise related to the Holocene transgression was a key factor in controlling the evolution of the system, yet its influence depended to an extent on the relative elevation of the bedrock topography. Copyright © 2009 John Wiley & Sons, Ltd.     

A Bayesian analysis of the Gumbel distribution: an application to extreme rainfall data

With the objective of modelling annual rainfall maximum intensities in different geographical zones of Chile, we have created a Bayesian inference method for the generalized extreme value type I distribution (Gumbel distribution). We considered an uninformative prior distribution for the location parameter, μ, and three different prior distributions for the scale parameter, σ. Under these conditions we obtained the posterior distribution of (μ, σ) and associated summary statistics such as modes, expected values, quantiles and credibility intervals. In order to predict and estimate return periods, we obtained the posterior distribution of future observations, its expected value, quantiles and credibility intervals. To obtain several of these posterior summary measures it was necessary to utilize both numerical and Laplace approximations. Furthermore we estimate return period curves and intensity–duration–frequency curves.     

Baseline of butyltin pollution in coastal sediments within the Basque Country (northern Spain), in 2007-2008

Tributyltin (TBT), dibutyltin (DBT) and monobutyltin (MBT) were measured in surficial sediments at, the ports of Pasaia and Bilbao, together with other mid- and small-size harbours of the Basque Country (northern Spain), in 2007-2008. The highest values of the sum of the three measured butyltin species (3523-3640 ng g⁻¹, as Sn) were found at sampling stations near to shipyards located within the port of Pasaia. The highest value of TBT concentration (3143 ng g⁻¹, as Sn) was found at the marina of Getxo, in the port of Bilbao. The degree of TBT degradation varied greatly between sampling stations, being found to be generally higher in those sediments with higher values of redox potential and lower values of TBT concentration (normalized by organic matter content).     

Petrology, magnetostratigraphy and geochronology of the Miocene volcaniclastic Tepoztlán Formation: implications for the initiation of the Transmexican Volcanic Belt (Central Mexico)

The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite to rhyolite. Thick gravity-driven deposits and large-scale alluvial fan environments document the buildup of steep volcanic edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time intervall 22.8–18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant mode of deposition: (1) the fluvial dominated Malinalco Member (22.8–22.2 Ma), (2) the volcanic dominated San Andrés Member (22.2–21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3–18.8 Ma).     

Saturn's rings: II. Condensations of light and optical thickness of Cassini's division

“Condensations” of light have been observed when Saturn's rings are seen almost edge on, and the Sun and the Earth are on opposite sides of the ring plane. These condensations are associated with ring C and Cassini's division. If the relative brightness between the two condensations and the optical thickness of ring C are known, we can calculate the optical thickness of Cassini's division, τ_CASS. Using Barnard's and Sekiguchi's measurements, we have obtained 0.01 ? τ_CASS ? 0.05. A brightness profile of the condensations which agrees well with visual observations is also presented.We are able to set an upper limit of about 0.01 for the optical thickness of any hypothetical outer ring. This rules out a ring observed by C. Cragg in 1954, but does not eliminate the D′ ring observed by Feibelman in 1967.It is known that the outer edge of ring B is almost at the position of the 1/2 resonance with Mimas. Franklin, Colombo, and Cook explained this fact in 1971, postulating a total mass of ring B of 10^?6M_SATURN. We have derived a formula for the mass of the rings, which is a linear function of the mean particle size. We find that 10^?6M_SATURN implies large particles (～70m). If the particles are small (～10cm), as currently believed, the total mass of ring B is not enough to shift the outer edge. We conclude that the above explanation and current size estimates are inconsistent.     

Overpassing and armouring phenomena on gravel beaches

Armouring phenomena are common in river beds, gravel beaches and spits. The gravel segregation recognized at depth in three different beaches of Patagonia and Tierra del Fuego, Argentina, allows us to propose a mechanism for formation.

A mixed population of gravel set into motion by waves is deposited progressively. Granules and fine gravels with higher pivoting angles in the beach slope are easily trapped within the bed, while rounded pebbles continue rolling over (overpassing) it. Finally, decreasing flow velocity allows the deposition of the larger pebbles, thus armouring the beach.

A carpet of round clasts of uniform size is more stable than the same clasts in a mixed population armouring the bed and covering a uniform layer of rounded granules and fine gravels.