Shallow erosion beneath turbidity currents and its impact on the architectural development of turbidite sheet systems

Erosion by turbidity currents changes the morphology of the sea floor. The relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition; this could, in turn, affect flow and deposition patterns in subsequent events. This study investigates shallow, centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the Oligocene Macigno Formation of North‐west Italy, where erosion and deposition are variably coupled at the bed scale in a net‐aggradational setting. The research focus was on: (i) the recognition of scour edges and erosive surfaces; (ii) quantification of spatial differences in the amount of erosion; and (iii) an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces. Where they can be observed, scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded. A consequence of this de‐laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges, giving the appearance of normal conformable bed bases. Despite their cryptic nature, such bedding‐parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs (here, 16 bed‐parallel scour surfaces were identified in a succession comprising 95 beds). Different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief. It is found that differential erosion is on average under‐compensated by differential sedimentation. In some cases, the overlying deposits anti‐compensate, being thinner at the location where more erosion has occurred. Unequal spatial distribution of differential erosion in the study area combines with sedimentary under‐compensation to result in a trend of accumulating section thickness differences over multiple beds. In one ca 25 m thick package, the maximum cumulative change in lateral gradient during some 20 events reached 0·17°, before being reset by a single event. This process can be interpreted either as a lobe compensation effect, or as a scour enhancement effect, depending on the orientation of the palaeohorizontal datum. If allowed to proceed, the latter process could force the system past a channellization threshold, prompting a change from sheet to channelled architecture. This type of shallow substrate scouring and differential deposition is likely to be an important process in the build‐up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea‐floor gradients.     

Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle

The phase diagram for lherzolite–CO₂–H₂O providesa framework for interpreting the distribution of phase assemblagesin the upper mantle with various thermal structures, in differenttectonic settings. Experiments show that at depths >80 km,the near-solidus partial melts from lherzolite–CO₂–H₂Oare dolomitic, changing through carbonate–silicate liquidswith rising temperatures to mafic liquids; vapor, if it coexists,is aqueous. Experimental data from simple systems suggest thata critical end-point (K) occurs on the mantle solidus at anundetermined depth. Isobaric (T–X) phase diagrams forvolatile-bearing systems with K elucidate the contrasting phaserelationships for lherzolite–CO₂–H₂O at depths belowand above a critical end-point, arbitrarily placed at 250 km.At levels deeper than K, lherzolite can exist with dolomiticmelt, aqueous vapor, or with critical fluids varying continuouslybetween these end-members. Analyses of fluids in microinclusionsof fibrous diamonds reveal this same range of compositions,supporting the occurrence of a critical end-point. Other evidencefrom diamonds indicates that the minimum depth for this end-pointis 125 km; maximum depth is not constrained. Constructed cross-sectionsshowing diagrammatically the phase fields intersected by upwellingmantle indicate how rising trace melts may influence trace elementconcentrations within a mantle plume. KEY WORDS: mantle solidus; critical end-point; dolomitic magma; diamond inclusions; critical fluids     

ASSESSMENT OF CLIMATE CHANGE AND FRESHWATER ECOSYSTEMS OF THE ROCKY MOUNTAINS,USA AND CANADA

The Rocky Mountains in the USA and Canada encompass the interior cordillera of western North America, from the southern Yukon to northern New Mexico. Annual weather patterns are cold in winter and mild in summer. Precipitation has high seasonal and interannual variation and may differ by an order of magnitude between geographically close locales, depending on slope, aspect and local climatic and orographic conditions. The region's hydrology is characterized by the accumulation of winter snow, spring snowmelt and autumnal baseflows. During the 2–3-month ‘spring runoff’ period, rivers frequently discharge > 70% of their annual water budget and have instantaneous discharges 10–100 times mean low flow. Complex weather patterns characterized by high spatial and temporal variability make predictions of future conditions tenuous. However, general patterns are identifiable; northern and western portions of the region are dominated by maritime weather patterns from the North Pacific, central areas and eastern slopes are dominated by continental air masses and southern portions receive seasonally variable atmospheric circulation from the Pacific and the Gulf of Mexico. Significant interannual variations occur in these general patterns, possibly related to ENSO (El Niño–Southern Oscillation) forcing. Changes in precipitation and temperature regimes or patterns have significant potential effects on the distribution and abundance of plants and animals. For example, elevation of the timber-line is principally a function of temperature. Palaeolimnological investigations have shown significant shifts in phyto- and zoo-plankton populations as alpine lakes shift between being above or below the timber-line. Likewise, streamside vegetation has a significant effect on stream ecosystem structure and function. Changes in stream temperature regimes result in significant changes in community composition as a consequence of bioenergetic factors. Stenothermic species could be extirpated as appropriate thermal criteria disappear. Warming temperatures may geographically isolate cold water stream fishes in increasingly confined headwaters. The heat budgets of large lakes may be affected resulting in a change of state between dimictic and warm monomictic character. Uncertainties associated with prediction are increased by the planting of fish in historically fishless, high mountain lakes and the introduction of non-native species of fishes and invertebrates into often previously simple food-webs of large valley bottom lakes and streams. Many of the streams and rivers suffer from the anthropogenic effects of abstraction and regulation. Likewise, many of the large lakes receive nutrient loads from a growing human population. We concluded that: (1) regional climate models are required to resolve adequately the complexities of the high gradient landscapes; (2) extensive wilderness preserves and national park lands, so prevalent in the Rocky Mountain Region, provide sensitive areas for differentiation of anthropogenic effects from climate effects; and (3) future research should encompass both short-term intensive studies and long-term monitoring studies developed within comprehensive experimental arrays of streams and lakes specifically designed to address the issue of anthropogenic versus climatic effects. © 1997 John Wiley & Sons, Ltd.     

HOBBY RANCHING AND CHILE'S LAND‐REFORM LEGACY*

ABSTRACT. Twentieth‐century land reform in Chile reflects familiar themes. The state subdivided large land monopolies, ostensibly for social justice reasons: to provide opportunities for campesinos to earn a living from agriculture. In southern Chile's Tierra del Fuego a combination of social justice, nationalism, and geopolitical concerns stimulated land reform between 1924 and 1978. The effort succeeded in creating a new material landscape oriented around smaller ranches but failed to meet its socioeconomic goals. In a scenario that resonates with reforms elsewhere in Latin America, an elitist cultural landscape dominated by large‐holding corporations was replaced with another elitist landscape, though in a different form. Politically well‐connected and essentially absentee landholders acquired subdivided land and began to practice “hobby ranching.” The history of land reform in Tierra del Fuego demonstrates how new socioeconomic conditions created over the course of many decades have left their imprint on land use and the biophysical landscape. It also contributes to larger regional debates about the role of land degradation in stocking‐rate decline.     

Surge mark formation and morphology

Surge marks have previously been described as hieroglyphs and more recently as erosional rills caused by the turbulent bank-swashing of storm waters upstream of obstructions. This paper presents evidence that surge marks have various forms directly related to the steepness of sediment slope. These features are due, at least in part, to deformation and not necessarily to erosion by surging water. The previous term dendritic surge mark may now only apply to one member of the surge mark formation.     

The Cenozoic and Late Cretaceous evolution of the Indian Ocean Basin: uncertainties in the reconstructed positions of the Indian, African and Antarctic plates

Abstract Reconstructions of the relative positions of the Indian, African, and Antarctic plates and their uncertainties are given for the times of selected magnetic anomalies that could be identified on adjacent pairs of these plates. Among the most certain reconstructions are those for the Antarctic and African plates, which can be determined directly from recently published magnetic anomalies from both sides of the Southwest Indian Ridge. As Patriat and his colleagues reported, there was an important change in direction and a decrease in rate of separation between Africa and Antarctica between the times of anomalies 33 and 20. India moved rapidly away from both Africa and Antarctica in the Late Cretaceous and early Tertiary periods, but slowed markedly near the time of anomaly 20 (≅ 45 Myr). The positions of the Indian plate with respect to the others are poorly constrained between the times of anomaly 5 (≅ 10 Myr) and anomaly 23 (≅ 54 Myr), but using the reconstructions of the African and Antarctic plates, the uncertainties can be reduced. Despite the relatively large uncertainties, the positions of anomalies 5, 6, and 13 on the Antarctic and Indian plates apparently cannot be described by the same parameters that describe the history of separation of Australia and Antarctica. Therefore, Stein and Okal's contention that Australia and India lie on separate plates appears to be valid not only for the present, but for the last 35 Myr.     

Palsa and peat plateau development in the Hudson Bay Lowlands, Canada: timing, pathways and causes

The Holocene development of a treed palsa bog and a peat plateau bog, located near the railroad to Churchill in the Hudson Bay Lowlands of northeastern Manitoba, was traced using peat macrofossil and radiocarbon analyses. Both sites first developed as wet rich fens through paludification of forested uplands around 6800 cal. yr BP. Results show a 20th-century age for the palsa formation and repeated periods of permafrost aggradation and collapse at the peat plateau site during the late Holocene. This timing of permafrost dynamics corroborates well with that inferred from previous studies on other permafrost peatlands in the same region. The developmental history of the palsa and peat plateau bogs is similar to that of adjacent permafrost-free fens, except for the specific frost heave and collapse features associated with permafrost dynamics. Permafrost aggradation and degradation is ascribed to regional climatic, local autogenic and other factors. Particularly the very recent palsa development can be assessed in terms of climatic changes as inferred from meteorological data and surface hydrological changes related to construction of the railroad. The results indicate that cold years with limited snowfall as well as altered drainage patterns associated with infrastructure development may have contributed to the recent palsa formation.     

Cosmogenic 10 Be ages on the Pomeranian Moraine, Poland

Rinterknecht, V. R., Marks, L., Piotrowski, J. A., Raisbeck, G. M., Yiou, F., Brook, E. J. & Clark, P. U. 2005 (May): Cosmogenic ¹⁰Be ages on the Pomeranian Moraine, Poland. Boreas , Vol. 34, pp. 186–191. Oslo. ISSN 0300–9483.
We measured the ¹⁰Be concentrations in boulders collected from the Pomeranian Moraine in Poland, providing the first direct dating of the southern margin of the Scandinavian Ice Sheet (SIS) in the Polish Lowland. The mean age of 8 ¹⁰Be ages of the Pomeranian Moraine in northwestern Poland is 14.30.8 ¹⁰Be ka, while in northeastern Poland the mean age of 19 ¹⁰Be ages of the moraine is 15.00.5 ¹⁰Be ka. Given the excellent agreement between the two age groups, we calculate a mean age of 14.80.4 ¹⁰Be ka for final deposition of the Pomeranian Moraine of northern Poland. The age of the Pomeranian Moraine suggests that the southern margin of the SIS was near its maximum extent in Poland at a younger time than previously inferred, and that retreat from the moraine at 14.80.4 ¹⁰Be ka probably occurred in response to the onset of the Bølling interstade.     

Last Interglacial stratigraphy at Ristinge Klint, South Denmark

Past environmental changes in the Baltic area are discussed on the basis of foraminifera and ostracods as well as pollen and spores in marine sediments in cliff sections at Ristinge Klint, Langeland, southern Denmark. The sediment succession represents Jessen & Milthers' (1928) pollen zones d-g or Andersen's (1961, 1975) zones E2-E5, and a correlation with the annually laminated Bispingen sequence indicates that the sequence spans about 3400 years. Marine conditions seem to have occurred at c. 300-365 years after the beginning of the Eemian Interglacial, close to fully marine conditions developing by c. 2500 years. This early date of the marine ingression pre-dates that of most previous studies in the region by several hundred years, but it post-dates the initial marine ingression in the easternmost Baltic. A marked change in salinity at c. 650 years after the beginning of the Eemian was presumably caused by an opening of the Danish Belts. An indication of a major alteration in current activity is registered at c. 3000 years after the beginning of the interglacial. The recognition of the relative timing of these events may be significant for the understanding of the opening of connections between the North Sea, the Baltic and the White Sea.     

Composition and source of the clay-sized fraction of Saalian till in The Netherlands

The clay mineral composition of a number of Saalian till samples from The Netherlands shows a large variation in smectite and illite abundances. The samples were selected according to their source area, as inferred from coarse erratic components. Specimens containing primarily rock fragments indicative of East and East-Central Baltic areas are associated with no or low smectite percentages in their clay fractions. Those with mainly flint and rock fragments indicative of the West Baltic areas have high smectite contents in the clay. In general the smectite-rich material is found in the lower portions of the Saalian till bed, whereas the smectite-poor occurs in the upper portions. The linkage between very fine and coarse till components in the upper smectite-free parts of the till suggests that this entire till has been subjected to englacial transport over distances of at least 1000 km.