Victims of Avicularia

Abstract. .The function of the sessile "trap-door" avicularia of cheilostome bryozoans has been the subject of much speculation but relatively little observation. During studies of reef dwelling cheilostomes, avicularia of this type were seen to capture syllid polychaetes. This diverse group of small predatory worms, commonly found in the same habitats as bryozoan colonies, may be important factors in bryozoan ecology and evolution.     

Use of geochemical mass balance modelling to evaluate the role of weathering in determining stream chemistry in five mid-atlantic watersheds on different lithologies

The importance of mineral weathering was assessed and compared for five mid-Atlantic watersheds receiving similar atmospheric inputs but underlain by differing bedrock. Annual solute mass balances and volume-weighted mean solute concentrations were calculated for each watershed for each year of record. In addition, primary and secondary mineralogy were determined for each of the watersheds through analysis of soil samples and thin sections using petrographic, scanning electron microscope, electron microprobe and X-ray diffraction techniques. Mineralogical data were also compiled from the literature. These data were input to NETPATH, a geochemical program that calculates the masses of minerals that react with precipitation to produce stream water chemistry. The feasibilities of the weathering scenarios calculated by NETPATH were evaluated based on relative abundances and reactivities of minerals in the watershed. In watersheds underlain by reactive bedrocks, weathering reactions explained the stream base cation loading. In the acid-sensitive watersheds on unreactive bedrock, calculated weathering scenarios were not consistent with the abundance of reactive minerals in the underlying bedrock, and alternative sources of base cations are discussed. © 1997 John Wiley & Sons, Ltd.     

A Lateglacial archaeological site in the far north‐west of Europe at Rubha Port an t‐Seilich,Isle of Islay,western Scotland: Ahrensburgian‐style artefacts,absolute dating and geoarchaeology

The exact pattern, process and timing of the human re‐colonization of northern Europe after the end of the last Ice Age remain controversial. Recent research has provided increasingly early dates for at least pioneer explorations of latitudes above 54°N in many regions, yet the far north‐west of the European landmass, Scotland, has remained an unexplained exception to this pattern. Although the recently described Hamburgian artefacts from Howburn and an assemblage belonging to the arch‐backed point complex from Kilmelfort Cave have established at least a sporadic human presence during earlier stages of the Lateglacial Interstadial, we currently lack evidence for Younger Dryas/Greenland Stadial 1 (GS‐1) activity other than rare stray finds that have been claimed to be of Ahrensburgian affiliation but are difficult to interpret in isolation. We here report the discovery of chipped stone artefacts with technological and typological characteristics similar to those of the continental Ahrensburgian at a locality in western Scotland. A preliminary analysis of associated tephra, pollen and phytoliths, along with microstratigraphic analysis, suggest the artefacts represent one or more episodes of human activity that fall within the second half of GS‐1 and the Preboreal period. © 2015 The Authors. Journal of Quaternary Science published by John Wiley & Sons, Ltd.     

Evidence for carbonate platform failure during rapid sea‐level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south‐west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse‐grained and either ungraded or poorly graded, and were deposited by non‐cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub‐units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub‐units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi‐stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea‐level change.     

Origin of Phenocrysts and Compositional Diversity in Pre-Mazama Rhyodacite Lavas, Crater Lake, Oregon

Phenocrysts in porphyritic volcanic rocks may originate in avariety of ways in addition to nucleation and growth in thematrix in which they are found. Porphyritic rhyodacite lavasthat underlie the eastern half of Mount Mazama, the High Cascadeandesite/dacite volcano that contains Crater Lake caldera, containevidence that bears on the general problem of phenocryst origin.Phenocrysts in these lavas apparently formed by crystallizationnear the margins of a magma chamber and were admixed into convectingmagma before eruption. About 20 km³ of pre-Mazama rhyodacite magma erupted during arelatively short period between400 and 500 ka; exposed pre-Mazamadacites are older and less voluminous. The rhyodacites formedas many as 40 lava domes and flows that can be assigned to threeeruptive groups on the basis of composition and phenocryst content.Phenocryst abundance decreases (from 32 to 8 vol.%) and SiO₂content increases (from 68 to 73 wt.%) in the apparent orderof eruption. Phenocrysts (plagioclase, orthopyroxene, augite,and Fe-Ti oxides) are commonly fragmental or form polycrystallineaggregates with interstitial glass. Discrete phenocrysts withcomplete euhedral outlines are rare except for small elongatedcrystals. The abundance of discrete phenocrysts increases withthat of aggregates. The grain-size of minerals in the aggregatescovers the range of discrete phenocrysts (0.2–4.2 mm).Rim compositions of phenocrysts and the range of chemical zoningare almost uniform among the three rhyodacite groups, regardlessof whether crystals are discrete or in aggregates. However,a small fraction of phenocrysts, especially small elongatedcrystals, have different compositions: plagioclase with Fe-richcores and augite with Wo-poor cores, both of which are characteristicof crystals in undercooled andesite enclaves in the rhyodacites.The majority of phenocrysts were derived by disintegration ofpolycrystalline aggregates; rare, small phenocrysts crystallizedin andesitic magma similar to that represented by the andesiteenclaves. The modal and chemical compositions of the rhyodacites can beexplained by different degrees of admixing of crystals, representedby the aggregates, into magma having 4 vol.% ‘true’phenocrysts, mainly plagioclase. The aggregates may be partsof the rind formed by in situ crystallization near the walland roof of the magma chamber. The rind was disrupted duringor just before eruption, and pieces were variably disaggregatedand incorporated into erupting magma. The amount of rind incorporateddeclined during the sequence of eruptions. Owing to vesiculationof interstitial liquid and shearing during flow, crystals inthe aggregates were separated and became phenocrysts. Pre-Mazamarhyodacite was erupted dominantly as lava, as opposed to thecompositionally similar rhyodacite pumice of the Holocene caldera-formingeruption of Mount Mazama, apparently because its source chamberwas crystallizing inward rather than actively growing.     

Partial Melt Distributions from Inversion of Rare Earth Element Concentrations

Inverse theory is used to calculate the melt distribution requiredto produce the rare earth element concentrations in a wide varietyof terrestrial and extra-terrestrial magmas. The concentrationsof the major and minor elements in the source regions are assumedto be the same as those for the bulk Earth, and the peridotitemineralogy calculated from the mineral compositions by leastsquares. Rare earth element partition coefficients are thenused for inversion, assuming the melt generation is by fractionalmelting. The mean composition of the magmas is taken to be anestimate of the average composition of the melt. For n-typcand e-type MORB the results agree well with the adiabatic decompressioncalculations if the potential temperatures are 1300 and 1500?Crespectively. The major and minor element compositions calculatedfrom the melt distribution obtained from the inversion alsoagree well with those observed. The observations are consistentwith a melt fraction that increases monotonically towards thesurface, starting at 80 km and producing 9 km of melt in thecase of n-type MORB, and at 120 km to produce 23 km in thecase of e-type MORB. The inversion calculations show that the melt fractions producedbeneath an intact plate by a plume like that beneath Hawaiiare smaller, and are also in agreement with the adiabatic calculationsif the potential temperature of the plume is 1500?C. Much ofthe melt is produced in the depth and temperature range of thetransition from garnet to spinel peridotite, in agreement withlaboratory experiments and with the full convective models ofthe Hawaiian plume. The inversion calculations show that thesource region for Hawaiian tholeiites changes with time fromprimitive to depleted mantle. This behaviour is likely to resultfrom percolation, and the processes involved can be understoodwith the help of a simple analytic model. The last, post-erosional,magmas produced on Oahu come from a source that has been uniformlyenriched in all rare earth elements by a factor of about two.Magmas associated with island arcs come from two sources. Oneresembles that of n-type MORB, and probably is produced by adiabaticupwelling. The other generates calc-alkaline basalt stronglyenriched in light rare earth elements, but with a smaller constantenrichment between Gd and Lu. This composition is consistentwith the extraction of a melt fraction of 1% from a source containing9% of amphibole. Such a source region can also account for thelow values of Ti and Nb, and perhaps also of Ta, observed inisland arc magmas. Basaltic andesites and andesites from islandarcs show the same amphibole signature, and can be producedfrom the calc-alkaline basalts by fractional crystallizationif amphibole separates with olivine and orthopyroxene. The percolationof a small melt fraction through a mantle wedge that containsconsiderable amounts of amphibole can only transport very incompatibleelements, such as He, U, Th, and Rb, towards the Earth's surface.Sr and Nd are likely to be too compatible to move against thematrix flow, but Pb may do so locally. These results have importantimplications for the isotopic systematics of the upper mantle. The melt distributions obtained from ophiolites are like thosefor island arc tholeiites, though a potential temperature of1400 ?C fits the results better than does one of 1300?C. Archaeantholeiites and basaltic komatiites give melt distributions similarto that of e-type MORB from Iceland, and can be produced byadiabatic decompression if the mantle potential temperatureis 1500cC, with tholeiites having lost more material by fractionalcrystallization. The melt distribution obtained from komatiitesrequires the melt fraction to reach 60% at the surface. Thoughthe calculated compositions agree with those observed, decompressionis unable to generate such large melt fractions. Inversion shows that plateau basalts can be produced from theupper mantle beneath the plates by adiabatic upwelling beneatha mechanical boundary layer 60 km thick. Many of the variedalkali-rich continental magmas are generated by melting an enrichedsource in the stability field of garnet peridotite. The averageenrichment required, by a factor of between two and five, canbe produced by the addition of a small melt fraction. Carbonatitesshow no evidence of amphibole involvement at any stage, a resultthat is consistent with their formation by liquid immiscibility.Inversion of the rare earth element concentrations in shalesgives a melt distribution similar to that from calc-alkalinebasalts from island arcs, with a strong amphibole signature.Generation of the continental crust by separation of calc-alkalinemagma from 40% of the mantle can account for the differencebetween primitive and depleted mantle. Low-K highland basalts from the Moon can be produced directlyfrom the average primitive lunar mantle if the melt fractioninvolved is ?0-5%, and if they were generated in the stabilityfield of plagioclase and spinel peridotite. Intermediate-K highlandbasalts come from a source that has been enriched by a factorof about two, and show no evidence of amphibole involvement.The rare earth concentrations in mare basalts require melt fractionsof up to 7% in the spinel peridotite stability field, and canbe generated by adiabatic upwelling of mantle whose potentialtemperature is 1300?C beneath a mechanical boundary layer thatis 150 km thick. Because lunar gravity is only one-sixth ofthat of the Earth, the thickness of the melting zone and thevolume of melt produced are six times greater for the Moon thanfor the Earth for the same value of Tp. Both low-Ti and high-Timare basalts may have lost as much as 70 and 85% respectivelyof their original material through crystal fractionation. Itis, however, difficult to understand how such an origin canaccount for the high magnesium concentrations. Basaltic achondritesinvolve melt fractions of 10-15%, generated in the spinel orplagioclase stability field.