Source,timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis

Haapaniemi, A.I., Scourse, J.D., Peck, V.L., Kennedy, H., Kennedy, P., Hemming, S.R., Furze, M.F.A., Pieńkowski, A.J., Austin, W.E.N., Walden, J., Wadsworth, E. & Hall, I.R. 2010: Source, timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis. Boreas, Vol. 39, pp. 576–591. 10.1111/j.1502‐3885.2010.00141.x. ISSN 0300‐9483. Increased fluxes of ice‐rafted detritus (IRD) from European ice sheets have been documented some 1000–1500 years before the arrival of Laurentide Ice Sheet (LIS)‐sourced IRD during Heinrich (H) events. These early fluxes have become known as ‘precursor events’, and it has been suggested that they have mechanistic significance in the propagation of H events. Here we present a re‐analysis of one of the main cores used to generate the precursor concept, OMEX‐2K from the Goban Spur covering the last 30 ka, in order to identify whether the British–Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers. IRD characterization and planktonic foraminiferal δ¹⁸O measurements constrained by a new age model have enabled the generation of a continuous record of IRD sources, timing, frequency and flux, and of local contemporary hydrographic conditions. The evidence indicates that BIIS IRD precursors are not uniquely, or mechanistically, linked to H events, but are part of the pervasive millennial‐scale cyclicity. Our results support an LIS source for the IRD comprising H layers, but the ambient glacial sections are dominated by assemblages typical of the Irish Sea Ice Stream. Light isotope excursions associated with H events are interpreted as resulting from the melting of the BIIS, with ice‐sheet destabilization attributed to eustatic jumps generated by LIS discharge during H events. This positive‐feedback mechanism probably caused similar responses in all circum‐Atlantic ice‐sheet margins, and the resulting gross freshwater flux contributed to the perturbation of the Atlantic Meridional Overturning Circulation during H events.     

Architecture of a coarse‐grained channel–levée system: the Rosario Formation,Baja California,Mexico

Seafloor images of coarse‐grained submarine channel–levée systems commonly reveal complex braid‐plain patterns of low‐amplitude bedforms and zones of apparent bypass; however, mechanisms of channel evolution and the resultant channel‐fill architecture are poorly understood. At Playa Esqueleto the lateral relationships between various elements of a deep‐marine slope channel system are well‐exposed. Specifically, the transition from gravel‐dominated axial thalwegs to laterally persistent marginal sandstones and isolated gravel‐filled scours is revealed. Marginal sandstones pass into a monotonous thin‐bedded succession which built to form relatively low‐relief levées bounding the channel belt; in turn, the levées onlap the canyon walls. Three orders of confinement were important during the evolution of the channel system: (i) first‐order confinement was provided by the erosional canyon which confined the entire system; (ii) confined levées built of turbidite sandstones and mudstones formed the second‐order confinement, and it is demonstrated that these built from overspill at thalweg margins; and (iii) third‐order confinement describes the erosional confinement of coarse‐grained thalwegs and scours. Finer‐grained sediment was transported in suspension and largely was unaffected by topography at the scale of individual thalwegs. Facies and clast analyses of conglomerate overlying channel‐marginal scours reveal that they were deposited by composite gravity flows, which were non‐cohesive, grain‐dominant debris flows with more fluidal cores. These flows were capable of basal erosion but were strongly depositional; frictional freezing at flow margins built gravel levées, while the core maintained a more fluidal transport regime. The resultant architecture consists of matrix‐rich, poorly sorted levées bounding better‐sorted, traction‐dominated cores. The planform geometry is interpreted to have consisted of a low‐sinuosity gravel braid‐plain built by accretion around mid‐channel and bank‐attached bars. This part of the system may be analogous to fluvial systems; however, the finer‐grained sediment load formed thick suspension clouds, probably several orders of magnitude thicker than the relief of braid‐plain topography and therefore controlled by the levées and canyon wall confinement.     

HYDROGRAPHIES*

ABSTRACT. Drawing on the work of the theoretical biologist Humberto Maturana, I offer in this essay a way of conceptualizing the forms of identity that survive the various death pronouncements of our postidentitarian moment. I associate the work of imagining those strange new forms of identity with what Frederic Jameson calls “cognitive mapping,” arguing that these “afteridentities” can emerge from a “hydrographic” charting of the fluid cultural territories of the modern and the postmodern. In conclusion, I indicate how this approach may be of use in contemporary attempts to “map” the diasporic territories of the Black Atlantic.     

AN ANALYTICAL HYSTERESIS MODEL FOR ELASTOMERIC SEISMIC ISOLATION BEARINGS

For the purpose of accurately predicting the seismic response of base-isolated structures, an analytical hysteresis model for elastomeric seismic isolation bearings is proposed. An extensive series of experimental tests of four types of seismic isolation bearings—two types of high-damping rubber bearings, one type of lead-rubber bearing and one type of silicon rubber bearing—was carried out with the objective of fully identifying their mechanical characteristics. The proposed model is capable of well-predicting the mechanical properties of each type of elastomeric bearing into the large strain range. Earthquake simulator tests were also conducted after the loading tests of the individual bearings. In order to show the validity of the proposed model, non-linear dynamic analyses were conducted to simulate the earthquake simulator test results. Good agreement between the experimental and analytical results shows that the model can be an effective numerical tool to predict not only the peak response value but also the force–displacement relationship of the isolators and floor response spectra for isolated structures. © 1997 by John Wiley & Sons, Ltd.     

Temporal trends in grain-size measures on a linear sand dune

Within aeolian dune systems spatial patterns of grain-size variation have been recognized, but little has been said about temporal changes. Increasingly it is becoming clear that linear dunes are associated with bi-directional wind regimes which are often seasonal. In the Namib Sand Sea, where linear dunes are aligned roughly north-south, winds blow from the west in summer and from the east in winter. In response to this regime, sand is eroded from the west slopes and deposited on the east slopes in summer, and eroded from the east slopes and deposited on the west slopes in winter. Preliminary evidence from a study of a single Namib linear dune reported here confirms that this seasonal aeolian regime induces seasonal responses in some grain-size measurements due to the dynamics of sand transport on the dune, the characteristics of the sand source immediately upwind of the sample point and the nature of the deposit. Thus, time of sampling is crucial to the results obtained.     

A TEST OF SIGNIFICANCE FOR PARTIAL LEAST SQUARES REGRESSION

Partial least squares (PLS) regression is a commonly used statistical technique for performingmultivariate calibration, especially in situations where there are more variables than samples. Choosingthe number of factors to include in a model is a decision that all users of PLS must make, but iscomplicated by the large number of empirical tests available. In most instances predictive ability is themost desired property of a PLS model and so interest has centred on making this choice based on aninternal validation process. A popular approach is the calculation of a cross-validated r~2 to gauge howmuch variance in the dependent variable can be explained from leave-one-out predictions. Using MonteCarlo simulations for different sizes of data set, the influence of chance effects on the cross-validationprocess is investigated. The results are presented as tables of critical values which are compared againstthe values of cross-validated r~2 obtained from the user's own data set. This gives a formal test forpredictive ability of a PLS model with a given number of dimensions.     

Mineralogy, Textures and P-T Relationships of a Suite of Xenoliths from the Monaro Volcanic Province, New South Wales, Australia

Intraplate basalts of the Eocene–Oligocene Monaro VolcanicProvince (MVP), in southeastern New South Wales, include lower-crustaland refractory to weakly metasomatized upper-mantle xenoliths.Lower-crustal-derived xenoliths appear to be all two-pyroxeneplagioclase granulites (Cpx_{Fe:Mg:Ca 0·17–0·56:0·63–0·77:0·28–0·89}Opx_{Fe:Mg:Ca 0·39–0·52:1·37–1·47:0·02}An_72–86 and An_48–50) but may also include garnetpyroxenites at depth. Mantle-derived xenoliths are principallyspinel-bearing lherzolites (Fo_{89·8–90·6}Cpx_{Fe:Mg:Ca 0·07–0·45:0·70–1·70:0·01–0·94}Opx_{Fe:Mg:Ca 0·16–0·19:1·62–1·75:0·01–0·10})but also include amphibole ± spinel-bearing lherzolite(Fo_{88·7–89·1} Cpx_{Fe:Mg:Ca 0·09–0·21:0·61–0·91:0·73–0·93}Opx_{Fe:Mg:Ca 0·09–0·31:0·70–1·54:0·03–0·91}),spinel-bearing harzburgite (Fo_{90·5–90·7}Cpx_{Fe:Mg:Ca 0·08:0·91–0·93:0·74–0·84}Opx_{Fe:Mg:Ca 0·16–0·18:1·73–1·79:0·00–0·02}),wehrlite, pyroxenite (Cpx_{Fe:Mg:Ca 0·08–0·10:0·84–0·90:0·80–0·85}Opx_{Fe:Mg:Ca 0·16–0·33:1·51–1·73:0·02–0·03})and rare garnet pyroxenite (Gt_{Fe:Mg:Ca 0·83–0·95:1·60–1·70:0·45–0·48}Cpx_{Fe:Mg:Ca 0·14–0·21:0·69–0·77:0·78–0·86}Opx _{Fe:Mg:Ca 0·31–0·42:1·43–1·56:0·02–0·03})and amphibole–apatite composites. Xenolith textures aregenerally weakly to moderately foliated, a few are mosaic-porphyroblasticand rare samples are veined or highly strained. MVP xenolithsappear to have equilibrated under similar pressure–temperature(P–T) conditions to other southeastern Australian xenolithsequivalent to the South Eastern Australia (SEA) palaeogeotherm.P–T estimates for the MVP suite of xenoliths reveal aheterogeneous lower crust and upper mantle that is thickly underplatedto c. 1·8 GPa or c. 50 km depth. MVP xenolith P–Tdata are compared with those used to derive the SEA palaeogeotherm,which is shown to be in need of revision using more modern geothermometersand geobarometers and new xenolith coexisting mineral data. KEY WORDS: xenolith; petrography; texture; geotherm; Monaro; eastern Australia     

Carbonate cementation of some Pleistocene temperate marine sediments

Carbonate cementation of some carbonate and quartz sands in three raised beaches of temperate origins was investigated. The carbonate of the cements was found to have been derived from the dissolution of skeletal debris. The sandstones, so produced, now possess only low-magnesium calcite, but the original sediments, like adjacent modern beach and blown sands, probably contained low-magnesium calcite, aragonite and some high-magnesium calcite, all of skeletal origin. In meteoric water the dissolution has occurred of all carbonate within minute, tubulelike, volumes of sand. Concurrent deposition in adjacent volumes of sand of low-magnesium calcite formed cements that are irregularly nodular or uneven on a small scale. Aragonite within the minute nodules has been replaced paramorphically by low-magnesium calcite. Additional local carbonate cements were formed at later dates, around and within solution pipes.     

Phonolitic Diatremes within the Dunedin Volcano, South Island, New Zealand

The Port Chalmers Breccia is a vent-filling, clastic volcanicunit exposed within the Miocene Dunedin Volcano of South Island,New Zealand. Clasts (up to in excess of 1 m but generally <20cm) are supported in ash and fine lapilli of phonolitic (ne-benmoreiteor tephro-phonolite) composition and the dominant clast type(55 to almost 100%) is also phonolitic. Less abundant lithologiesinclude ne-normative basalt (basanite), hawaiite, mugeariteand trachyandesite, syenites and microsyenites, coarse-grainedmafic (gabbros) and ultramafic rocks (pyroxenites, hornblendites),schists and sediments. The breccias were emplaced as diatremesassociated with localized, but highly explosive, eruptive eventsin which mantle-derived CO₂ was an important component. Thesyenitic and ultramafic clasts could represent intrusive suitesproduced by crystal fractionation acting on parental ne-benmoreitemagmas that may themselves have been derived by crystal fractionationfrom basanitic precursors. An alternative variation on thismodel is that the parental ne-benmoreites were generated throughpartial melting of an alkalic igneous underplate. Sr, Nd andPb isotopic compositions are strikingly similar to those ofintraplate igneous rocks, ranging in age from 100 to less than10 Ma, from elsewhere in the South Island, and New Zealand'ssub-Antarctic islands, the south Tasman Sea and the Ross Searegion. This regional, HIMU-influenced, isotopic signature isbelieved to be derived from within the lithospheric mantle. KEY WORDS: phonolite; diatreme; nepheline syenite; Dunedin Volcano; alkalic rocks; fractional crystallization