Terrain Age Research Articles

Seed rain, seedling establishment and clonal growth strategies on a glacier foreland

Abstract. Seed production, composition of the seed rain, germination, and seedling mortality, as well as vegetative growth characteristics of common pioneer plant species were studied on the foreland of the retreating Morteratsch glacier in the Swiss Alps. The frequency of diaspores trapped in different successional stages was related to their dispersal mode and was highly skewed towards a few species. Plenty of diaspores well adapted for dispersal by wind are a precondition for the most important pioneer species. Seeds from all pioneer species investigated had a good germination success, provided that the moisture content of the soil was high enough. However, requirement for seedling establishment differed among sites of increasing terrain age and among species. Only specialized pioneers such as Cerastium pedunulatum, Linaria alpina, Oxyria digyna and Saxifraga aizoides tolerate the cold and moist conditions near the glacier. However, these species are restricted to early successional stages. Seedlings of Epilobium fleischeri are affected not only by the cold and moist conditions near the glacier but also by moderately dry conditions on older sites. Availability of safe sites becomes crucial for most species with increasing age of sites and with drier conditions. Most species playing a dominant role during early succession and persisting during later successional stages have a distinctive ability to spread clonally and have a growth form with more or less widely spaced ramets: Achillea moschata. Cerastium pedunculatum. Epilobium fleischeri and Hieraium staticifolium. The growth strategy and demography of the clonal E. fleischeri is presented as an example. The life cycle of this species is characterized in succession by (1) the colonization of safe sites by small seeds adapted for wind dispersal, (2) horizontal spread by clonal growth, and (3) the persistence through phenotypic morphological plasticity in later successional stages. Seedling establishment and clonal growth are thus complementary mechanisms in plant succession on recently deglaciated terrain.

Development of Principle B in Russian: Coindexation at LF and Coreference

This article presents evidence in the development of Russian for a theory that says that children at a terrain age know a syntactic principle (Principle B of the Binding Theory) that governs the distribution of pronouns but that they do not know a pragmatic or semantic principle (Principle P) that restricts the situations in which NPs may be contraindexed. The major result concerns Russian possessive pronouns that must be disjoint from subject antecedents. We show that this result follows from the raising of pronouns at Logical Form together with Principles B and P. The prediction thus is that children who do not know Principle P will accept this sentence. We experimentally confirm this result. We also predict that children will reject sentences with possessive pronouns that are bound by quantifiers. We also confirm this prediction. Along the way we replicate in Russian known results in English and other languages. The pattern of results provides strong support for the Principle P/modularity theory.

Single plume model for asynchronous formation of the Lamont Seamounts and adjacent Eeast Pacific Rise terrains

Sonar reflectance is used to quantify the relative ages of young volcanic terrains on a seamount chain and a nearby spreading center in the north equatorial Pacific. The reflectance is a measure both of the burial of lava by pelagic sedimentation and of the microrelief of the lava. The extent of sediment cover and the lava type distribution are calibrated with thousands of bottom photographs. The relative ages indicate that the locus of volcanic eruptions has migrated toward the East Pacific Rise axis. Zero‐age activity lies just to the west of the East Pacific Rise south of the Clipperton Transform and is probably the cause of an anomalously shallow neovolcanic zone. The evidence fits a scenario in which eruptive centers appear, evolve, and become extinct in succession, with little or no overlap in time. The eruptive centers are fed in sequence through conduits from buoyant, tightly focused magma plumes. The near identical heights of many of the centers at the time of their latest eruptions suggest that the seamounts tap the same mantle depth range. Broad summit plateaus with calderas are formed if the plume volume is such that the hydraulic head is reached and magma is trapped within the volcano in ephemeral magma chambers or conduits. If the plume volume is small and the hydraulic head is not reached, the seamount remains small with only a small crater. The plumes ascend asynchronously from a common region of nucleation in the asthenosphere. Plumes that follow in close succession build a composite seamount with a string of summit calderas. Plumes that rise less often result in a line of discrete circular volcanoes.

The Vegetation of the Storbreen Gletschervorfeld, Jotunheimen, Norway. III. Vegetation-Environment Relationships

The Storbreen glacier foreland has been the sub- ject of a spatial approach to vegetation chronosequence studies (Papers I and II of the series), providing a detailed base-line for further study. It is argued here that an under- standing of primary succession requires knowledge of the relationships among and between environmental variables and plant communities. This paper reports an attempt to quantify such relationships for an altitudinally-restricted sub-set of 108 of the 638 permanent vegetation sites located on this glacier foreland. Twelve environmental variables and four vegetational (ordination) axes are examined individually and collec- tively using non-parametric correlation and 2-D and 3-D graphical displays. Significant roles are suggested for ter- rain age, acting through disturbance regime (especially by small-scale frost action) and soil development, and for microtopography involving site moisture regime, length of snow-lie and microclimate. The explanation of vegetation structure is improved by viewing environmental variables as forming 'factor com- plexes', defined by a plexus approach. This involves in-series application of Detrended Correspondence Analysis (DCA) and Non-metric multidimensional scaling (NMDS). The results of these indirect gradient analyses are compared with those from direct gradient analysis by Canonical Correspondence Analysis (CCA), providing further support for the existence of terrain age and microtopographic factor complexes. Although there are complex spatial inter-relationships among the variables discussed, the strength of the relationship between the first ordination axis and terrain age lends credence to the application of a multivariate form of the chronosequence approach to succession on this foreland.

Ion microprobe ages of zircons from early Archaean granite pebbles and greywacke, barberton greenstone belt, Southern Africa

We report single-grain and grain-domain U-Pb zircon ages for three granitoid clasts from the Moodies conglomerate and for detrital zircons from a Fig Tree greywacke from clastic sediments in the early Archaean Barberton greenstone belt of southern Africa. The crystallization ages obtained for the Moodies clasts vary between 3570 ± 6 Ma and 3518 ± 11 Ma, while the Fig Tree zircons show a surprisingly uniform source terrain age of 3453 ± 9 Ma that may date the stratigraphically underlying Onverwacht Group felsic metavolcanics. The discordancy patterns are ascribed to a combination of variable Pb loss at unspecified times in the past and new zircon growth and/or recrystallization < 3300 Ma ago. The SHRIMP data record details of isotopic variation, even within single zircons, that result in precise definition of upper concordia intercept ages. Our results suggest an age range between ∼ 3460 and ∼ 3570 Ma for the source region of the Barberton belt clastic metasediments, presumably the Ancient Gneiss Complex of Swaziland and felsic volcanics of the Overwacht Group. This source included geochemically evolved granitoids that predate the oldest greenstones by some 100 Ma.

Comment and Reply on "Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating"

Comment and Reply on "Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating"

Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating

The authors report precise U-Pb zircon ages for single grains of a metarhyodacite from the Late Archean Dengfeng greenstone belt in Henan Province, China, near the southern margin of the North China craton. Most grains belong to an igneous population whose U-Pb isotopic systematics define a straight line intersecting concordia at 2512 +/- 12 Ma, and this is interpreted as the time of crystallization of the original greenstone volcanics. Several grains are distinctly older, between 2576 +/- 9 and 2945 +/- 44 Ma, and the authors interpret the older grains as xenocrysts of pre-greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava. The xenocrysts suggest evolution of the Dengfeng greenstone belt in a continental environment that may be represented by the Taihua high-grade gneisses bordering the Dengfeng greenstones and for which the authors obtained ages of 2806 +/- 7 and 2841 +/- 6 Ma. The data add evidence to the now widely held concept that most Archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination. In such cases, conventional zircon dating may not always record the precise age of rock formation.

Guatemala: Preliminary Zircon Ages from Central Cordillera

Concordia resolution of uranium-lead analyses of zircons from rocks of the Guatemalan Cordillera indicates a period of plutonism, and perhaps metamorphism, in late Paleozoic times (345 +/- 20 million years). Gneisses of the Chuacus Series yield an age of 1075 +/- 25 million years which may be either the age of a source terrain for the original sediments, or the age of principal metamorphism. Zircons from a Paleozoic granite intruding the gneisses appear to contain inherited or contaminating material acquired during the process of magma generation or emplacement, or both.