A COMPARISON OF EASTERN AND WESTERN HONG KONG PHYTOPLANKTON FROM WEEKLY SAMPLES (1997-1999)

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Elm bark beetle in Holocene peat deposits and the northwest European elm decline

The elm decline of 5000 ¹⁴C yr ago has been the most widely discussed phenomenon in post‐glacial vegetation history. This pan‐European reduction of elm populations, echoed in the decimation of elmwoods in Europe during the twentieth century, has attracted a series of interrelated hypotheses involving climate change, human activity, disease and soil deterioration. The elm bark beetle (Scolytus scolytus L.) is an essential component of disease explanations. We present evidence for the presence of the beetle over a prolonged period (ca. 7950–4910 yr BP [8800–5660 cal. yr BP]) from a lowland raised mire deposit in northeast Scotland, with its final appearance at this site, and the first and only appearance in another mire of a single scolytid find, around the time of the elm decline. The subfossil S. scolytus finds are not only the first from Scotland, but they also represent the most comprehensive sequence of finds anywhere. Copyright © 2004 John Wiley & Sons, Ltd.     

Denudation,vertical crustal movements and sedimentary basin infill

Denudation and vertical crustal movements are linked through the isostatic equation for conservation of lithospheric mass. Recent empirical results from a wide range of sources are presented on the rates of these processes. Denudation alone can cause non-tectonic (isostatic) surface uplift of the right magnitude predicted by theory. A case is made, following Ahnert, for a single linear relationship (although with much scatter) between denudation rate and mean drainage basin relief for large basins. Some recent determinations of denudational time constants have neglected to include tectonic and isostatic terms which oppose the reduction of relief by denudation. Application of diffusional transport models to determine the rate of sediment deposition are severely limited by inherent one-dimensionality and neglect of suspended and washload contributions. Basin infill sequences may be used to determine past sediment transfer coefficients from a hinterland, but problems arise in the apportionment of the various hinterland fluxes that occur in »real-world« three dimensional basins. Sediment yields in smaller half-graben and growth fold basins must reflect the magnitude of drainage basins and the complex local controls upon sediment yields. Examples are given from extensional basins in the Western USA and Greece. The »length« of tectonic relief produced in tilt blocks and growth folds will control the area of drainage basin developed (through the fractal form of Hack's Law) and hence the magnitude of deposition in sedimentary basins adjoining tectonic uplands.

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Zusammenfassung Denudation und vertikale Krustenbewegungen sind verbunden durch das isostatische Gesetz über den Erhalt der lithosphärischen Masse. Neuerliche empirische Ergebnisse aus einem weiten Quellenbereich werden im Verhältnis zu diesen Prozessen präsentiert. Denudation allein, kann nicht tektonisch bedingte (isostatische) Oberflächenhebungen hervorrufen, die in der richtigen Größenordnung liegen, wie sie auch theoretisch ermittelt wurden. Es wird in Anlehnung an Ahnert ein Fall geschaffen, in der eine einfach lineare Beziehung (obgleich mit vielen Streuungen) zwischen der Denudationsrate und dem durchschnittlichen Abflußbeckenrelief für große Becken vorliegt. Einige neue Ermittlungen der Denudationszeitkonstanten wurden vernachlässigt, um tektonische und isostatische Begriffe einzufügen, welche sich der Verringerung des Reliefs durch Denudation widersetzen. Die Anwendung des Verbreitungstransportmodells zum Bestimmen der Sedimentationsrate sind streng begrenzt durch die ihm innewohnende Eindimensionalität, und der Vernachlässigung der Schwebe- und Schwämmfrachtbeteiligung. Bekkenfüllungssequenzen können benutzt werden um vergangene Sedimenttransferkoeffizienten aus dem Hinterland festzulegen; aber es treten auch Probleme bei der Verteilung der verschiedenen Hinterlandeinträge auf, die bei einem realen dreidimensionalen Becken vorhanden sind. Sedimenteinträge in kleineren Halbgräben und Growth-fold Becken müssen die Größe des Abflußbeckens widerspiegeln, sowie die komplexen lokalen Steuerungen auf die Sedimenteinträge. Beispiele von Dehnungsbecken aus dem Westen der USA und Griechenlands werden angesprochen. Die Länge des tektonischen Reliefs in verkippten Blöcken und Growth-Folds wird die Fläche des entwickelten Abflußbeckens kontrollieren (durch die fraktale Form des Gesetzes von Hack), und daher die Ablagerungsmenge von Sedimentbecken in der Nähe von tektonischen Hochlagen.

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Résumé L'érosion et les mouvements crustaux verticaux sont liés par l'équation isostatique de conservation de la masse lithosphérique. Des résultats empiriques récents provenant d'un large éventail de sources sont présentés en relation avec ces processus. L'érosion seule peut être la cause d'une montée non tectonique (isostatique) dont l'ampleur est prévue exactement par la théorie. Un cas est présenté, d'après Ahnert, qui montre, pour de grands bassins, une relation linéaire simple (mais avec une forte dispersion des points) entre le taux de dénudation et le relief moyen de l'aire de drainage. Certaines déterminations récentes des constantes de temps de dénudation ont omis de tenir compte des termes tectonique et isostatique qui s'opposent à la destruction du relief par érosion. L'utilisation de modèles de transport diffus en vue de déterminer le taux d'accumulation des sédiments est sévèrement limitée par le caractère unidimensionnel de tels modèles et par le fait qu'il ne tiennent pas compte de la fraction en suspension. Les séquences de comblement d'un bassin sédimentaire peuvent être utilisées pour déterminer les anciens cfficients de transfert à partir de l'hinterland; mais des difficultés surgissent à propos de la répartition des différents flux en provenance de l'hinterland dans le cas réel d'un bassin à 3 dimensions. Les apports sédimentaires dans les bassins de demi-graben et de »growth-fold« doivent traduire la grandeur des aires de drainage et les facteurs locaux complexes qui régissent ces apports. Des exemples sont présentés, relatifs à des bassins d'extension de l'ouest des USA et de Grèce. La »longueur« des reliefs tectoniques en relation avec les blocs basculés et les »growth-folds« déterminent, par la forme fractale de la loi de Hack, la surface des aires de drainage ainsi engendrées et, en conséquence, l'importance des dépôts dans les bassins sédimentaires voisins des reliefs tectoniques.

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High resolution paleoenvironmental and chronological investigations of Norse landnám at Tasiusaq, Eastern Settlement, Greenland

High-resolution paleoenvironmental data from a peat profile with a small pollen source area are used to reconstruct the impacts of landnám on vegetation and soils at a Norse farm complex (∅2 at Tasiusaq) comprising two farms in the Eastern Settlement of Greenland. Analyses include the AMS ¹⁴C dating of plant macrofossil samples and the use of Bayesian radiocarbon calibration to construct improved age-depth models for Norse cultural horizons. The onset of a regional landnám may be indicated by the clearance of Betula pubescens woodland immediately prior to local settlement. The latter is dated to AD 950-1020 (2σ) and is characterised by possible burning of Betula glandulosa scrub to provide grassland pasture for domestic stock. Clearance and grazing resulted in accelerated levels of soil erosion at a westerly farm. This was followed by an easterly migration of settlement and agriculture. Site constraints prevent an assessment of the demise of the easterly farm, but pressures of overgrazing and land degradation may have been the major factors responsible for the abandonment of the earlier farm.     

Brine-pool deposition for the Zn–Pb–Cu massive sulphide deposits of the Bathurst mining camp, New Brunswick, Canada. II. Ocean anoxia during mineralisation

Current models of massive sulphide ore genesis in the Bathurst mining camp, New Brunswick, involve settling of sulphide particles from a stagnating, low-salinity hydrothermal plume spreading laterally in an anoxic ocean layer with minimal sulphate content. There is fragmentary evidence of ocean anoxia in the form of local fine lamination in the shales that host some of the deposits but the total organic carbon, S, Fe, U/Th, Ni/Co, V/Ni and V/Cr relationships indicate deposition under oxic or dysoxic conditions. Vanadium and Mn values range from oxic to anoxic and sulphate-reducing to non-sulphate reducing but Mn may be anomalously low due to derivation by erosion of acidic volcanic rocks. The somewhat equivocal physical and chemical data, combined with the likely disturbing effects of penecontemporaneous volcanism, considerably weaken the case for an anoxic bottom layer in a static ocean. The presence of barite with ambient seawater ³⁴S values in Brunswick no. 12 ore, and the abundance of sulphate in modern euxinic basin waters, make a sulphate-free layer unlikely, even if anoxic. Sulphate-bearing, low-salinity fluids mixing with seawater would lead to growth of barite-bearing chimneys and baritic rubble mounds, which are not observed. A model involving brine-pool deposition better explains the major features of the Bathurst ores.     

Iron isotope fractionation in subterranean estuaries

Dissolved Fe concentrations in subterranean estuaries, like their river-seawater counterparts, are strongly controlled by non-conservative behavior during mixing of groundwater and seawater in coastal aquifers. Previous studies at a subterranean estuary of Waquoit Bay on Cape Cod, USA demonstrate extensive precipitation of groundwater-borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands. Waquoit Bay is thus an excellent natural laboratory to assess the mechanisms of Fe-isotope fractionation in redox-stratified environments and determine potential Fe-isotope signatures of groundwater sources to coastal seawater. Here, we report Fe isotope compositions of iron-coated sands and porewaters beneath the intertidal zone of Waquoit Bay. The distribution of pore water Fe shows two distinct sources of Fe: one residing in the upward rising plume of Fe-rich groundwater and the second in the salt-wedge zone of pore water. The groundwater source has high Fe(II) concentration consistent with anoxic conditions and yield δ⁵⁶Fe values between 0.3 and −1.3‰. In contrast, sediment porewaters occurring in the mixing zone of the subterranean estuary have very low δ⁵⁶Fe values down to −5‰. These low δ⁵⁶Fe values reflect Fe-redox cycling and result from the preferential retention of heavy Fe-isotopes onto newly formed Fe-oxyhydroxides. Analysis of Fe-oxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong δ⁵⁶Fe and Fe concentration gradients over less than 2m, yielding an overall range of δ⁵⁶Fe values between −2 and 1.5‰. The relationship between Fe concentration and δ⁵⁶Fe of Fe-rich sands can be modeled by the progressive precipitation of Fe-oxides along fluid flow through the subterranean estuary. These results demonstrate that large-scale Fe isotope fractionation (up to 5‰) can occur in subterranean estuaries, which could lead to coastal seawater characterized by very low δ⁵⁶Fe values relative to river values.     

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS ¹⁴C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.     

Use of Continuous Plankton Recorder information in support of marine management: applications in fisheries, environmental protection, and in the study of ecosystem response to environmental change

The Continuous Plankton Recorder (CPR) survey was conceived from the outset as a programme of applied research designed to assist the fishing industry. Its survival and continuing vigour after 70 years is a testament to its utility, which has been achieved in spite of great changes in our understanding of the marine environment and in our concerns over how to manage it. The CPR has been superseded in several respects by other technologies, such as acoustics and remote sensing, but it continues to provide unrivalled seasonal and geographic information about a wide range of zooplankton and phytoplankton taxa. The value of this coverage increases with time and provides the basis for placing recent observations into the context of long-term, large-scale variability and thus suggesting what the causes are likely to be. Information from the CPR is used extensively in judging environmental impacts and producing quality status reports (QSR); it has shown the distributions of fish stocks, which had not previously been exploited; it has pointed to the extent of ungrazed phytoplankton production in the North Atlantic, which was a vital element in establishing the importance of carbon sequestration by phytoplankton.The CPR continues to be the principal source of large-scale, long-term information about the plankton ecosystem of the North Atlantic. It has recently provided extensive information about the biodiversity of the plankton and about the distribution of introduced species. It serves as a valuable example for the design of future monitoring of the marine environment and it has been essential to the design and implementation of most North Atlantic plankton research.