An equable glaciopluvial in the West: Pleniglacial evidence of increased precipitation on a gradient from the Great Basin to the Sonoran and Chihuahuan Deserts

Dated macrofossil evidence documents the widespread occurrence of woodland in what are now desert lowlands of southwestern North America from the last pleniglacial (ca. 20,000 yr B.P.) to late glacial/Holocene transition (12,000–8000 yr B.P.). The composition of the Pleistocene woodlands indicates that they had already differentiated geographically in modern form, though immensely more extensive than today. The pinyon-juniper woodland (Pinus monophylla, Juniperus osteosperma) of the Mohave Desert province had not yet penetrated the central Great Basin, but extended from southern Nevada south through the vast lowlands of the Mohave and westernmost Sonoran Deserts to southeastern California and Baja California. The strongly xerophytic Mohavean woodland was characterized by a very well-marked altitudinal and latitudinal zonation with juniper-Joshua tree (Yucca brevifolia) sorting out below pinyon-juniper woodland, and with live oaks restricted to the upper level along the lower Colorado River drainage. Southeastward, the Sonoran Desert province was similarly zoned, but with the more slender-leaved Pinus edulis var. fallax as pinyon and with more live oaks in the upper zone. However, the pleniglacial woodland of the Chihuahuan Desert province was almost unzoned, inasmuch as the less xerophytic species of pinyon and live oaks prevailed over the entire span of available elevation; the pinyon was the very slender-leaved P. cembroides var. remota.The overall paleozonation indicates a strong northwest-to-southeast gradient of increasing summer rain with decreasing distance from the monsoonal source area over the Gulf of Mexico, as at present, but augmented pluvially along the same gradient. A key piece of evidence is the counterintuitive latitudinal-zonational anomaly between about 30 and 40° N in southwestern North America; the lower limits of modern vegetational zones are depressed with decreasing latitude (e.g., ca. 500 m lower at 34° than at 36° N). The axis of the gradient actually extends from northwest to southeast, paralleling the monsoonal gradient of increasing summer rain, which no doubt causes the apparent anomaly. During the Wisconsinan glacial, the latitudinal anomaly was greatly steepened, a fact requiring a pluvial increase in precipitation over the Southwest. The monsoonalpluvial pattern is supported by the Neotoma record of a northwest-to-southeast gradient of increasing diversity of evergreen oaks requiring summer rain, and by a parallel segregation of pinyon species. Equability of seasons during the last glacial is also suggested by the Neotoma macrofossil data.     

Carbonaceous chondrites—I. Characterization and significance of carbonaceous chondrite (CM) xenoliths in the Jodzie howardite

Mineralogical, chemical, textural, and isotopic studies of the abundant carbonaceous inclusions in the Jodzie howardite are consistent with CM characteristics. These CM xenoliths show regolith alteration on a level comparable to the Murray and Murchison meteorites but less than Nogoya, flow-oriented development of phyllosilicates and ‘poorly characterized phases’, and partial oxidation of sulfides. Temperature-programmed pyrolysis mass spectrometry (25°–1400°C) indicates that gas release patterns of volatiles and hydrocarbon components and percent contents of N(0.15), C(2.3) and S(2.4) are typical of CM meteorites. Release of significant amounts of SO₂ is attributed to the thermal breakdown of ‘poorly characterized phases’ (Fe-Ni-C-S-O) that formed during low temperature aqueous alteration in the CM parent body.Noble gas abundances are well within the reported range of CM meteorites. The fact that the Ne composition is typical for ‘solar’ values and the isotopic structure of Xe is ‘planetary’ argues that these gases were entrapped by different mechanisms. Cosmic ray exposure ages for the xenoliths (³He, 5 × 10⁶; ²¹Ne, 6.7 × 10⁶; ³⁸Ar, 6.9 × 10⁶ yr) agree with the reported exposure age for the eucritic host. Volatile abundances, presence of intact organic molecules, and phyllosilicates in the CM xenoliths preclude regolith temperatures in excess of 200°C after CM incorporation. Mixing of the host and xenoliths probably occurred during a low-velocity collision of main belt asteroids.     

Epistemology and conceptions of people and nature in geography

The paper attempts to integrate two major issues of debate in geography which are conventionally discussed separately: epistemology and conceptions of the relationship between people and Nature. Whichever issue we start with, two concepts are essential: intersubjectivity and labour. Knowledge of natural objects is tied to labour, which both subjects ideas to a kind of test and changes our position within Nature. All knowledge depends upon intersubjective understanding between people, but this implies that in social science, and hence in human geography, the traditional subject-object relationship should be recast as a form of communication between ‘knowing-subjects’. These relationships are not limited to some separate sphere of epistemology but are constitutive of society in Nature itself. Labour, though traditionally overlooked in geography, is the most active and transformative process in changing Nature and society, and intersubjectivity is an essential element of the irreducibly social character of human life. In this attempt, the paper outlines and justifies a historical materialist perspective through critical engagement with positivist and humanist approaches. It is then argued that false conceptions of people and Nature in geography are grounded in the structure and selfunderstanding of our own society such that a critique of these ideas becomes a critique of our society.     

Carbonatization and mobility of Ti,Y, and Zr in Ascot Formation metabasalts,SE Quebec

The middle Ordovician Ascot Formation of southeastern Quebec consists of greenschist facies metamorphosed silicic to mafic pyroclastic rocks and lava flows and associated metasediments. Chemical analyses of lavas reveal a preponderance of metarhyolites and metabasalts, together with some porphyritic rocks with intermediate SiO₂ contents. The metabasalts exhibit wide ranges in concentrations of TiO₂ (0.25–2.0 wt.°), Y (9–46 ppm), and Zr (5–135 ppm). The extent of the ranges, and unusual interelement ratios, suggest that the concentrations of these normally immobile elements have been affected by secondary processes.There is a strong correlation between trace-element concentrations and the degree of carbonatization of the metabasalts. Low carbonate rocks are severely depleted in Ti, Y, and Zr whereas high carbonate rocks are depleted in Y and Zr and enriched in Ti. The differing movement of Ti can be explained in terms of variable chemical potential due to the various carbonatization reactions affecting titaniferous phases. Overall mobility of these generally immobile/rd elements is attributed to high CO₂ levels in the fluid phase during metamorphism.Extrapolation of the two alteration trends to a common origin enables one to infer primary concentrations of the trace-elements. Primary inter-element ratios arrived at in this way are compatible with an island-arc origin for the Ascot Formation although TiO₂ concentrations are a little high (1.5 wt/%).     

Regional metamorphism of some Dalradian pelites in the Buchan area,N.E. Scotland

Regional metamorphic zones, based on mineral assemblages in pelites, are presented for the Dalradian rocks of Aberdeenshire and Banffshire, in the type area of Buchan metamorphism; electron microprobe analyses of cordierite (C), staurolite (S), chlorite (Chl), biotite (B) and white mica (Ms) are reported for rocks from the classic sections of the Banffshire coast and the valley of the Ythan.A low grade biotite zone, separates two NE-SW trending sets of higher grade zones, in which the sequence s defined by the entry of cordierite, andalusite (A) and (in the west only) staurolite. The zones are characterised by the assemblages (with quartz and muscovite) B-Chl, C-B-Chl, A-C-B and S-A-B.The western sequence contains a transition towards higher pressure, Barrovian type, metamorphism. The isograds arise from continuous reactions affecting rocks of restricted bulk composition (M/FM). With increasing grade, there is a regular trend towards more magnesian ranges of composition for the assemblages C-B-Chl, A-C-B and finally (as P increases in the west), S-A-B. The isograds form when these assemblages intersect the most Fe⁺⁺-rich rock compositions present which occurs in each case when the biotite M/FM=40. A complex of divariant equilibria, derived for the system KFMASH, is used to model the natural reactions.     

Nontidal circulation and mixing processes in the lower Potomac estuary

An analysis of the vertical structure of nontidal longitudinal currents and salinity in a reach of the lower Potomac River Estuary suggests that values for vertical eddy viscosity and eddy diffusivity scale with water depth H, tidal current amplitude U and bulk Richardson number according to conventional empirical formulas. However, the constant which relates the vertical eddy coefficients under conditions of neutral stability to UH is found to be an order of magnitude less than that expected for tidal conditions. Analyses also suggest that the degree of enhancement of longitudinal dispersion by the shear effect associated with the nontidal currents is a strong function of bulk Richardson number.     

The measurement of dissolved organic carbon released by phytoplankton

The effects of applied filtration vacuum and incubation time on the release of dissolved organic carbon (DOC) by natural phytoplankton populations dominated by diatoms were examined. The rate of primary production and release of DOC remained reasonably constant during a 5 hour incubation period. The measured release of DOC was found to be quite sensitive to the applied filtration vacuum, ranging from near 2 percent of fixed carbon (neglecting respiration) for gravity-filtered samples to 36 percent for samples filtered at 40 cm Hg vacuum.     

Late Wisconsinan glaciation models of northern Maine and adjacent Canada

Surficial stratigraphic units of Aroostook County, Maine, have been mapped and formal stratigraphic names for these units are proposed. Evidence exists for at least two distinct glacial phases which are represented by three tills. Two of these tills were deposited penecontemporaneously either as the result of coalescing ice sheets or as the result of the thermal regime existing within a single ice sheet. The oldest till is named the St. Francis and is correlated with the Chaudière Till of southeastern Quebec. The other tills are named the Mars Hill and Van Buren tills, respectively, and are correlated with the Lennoxville till of southeastern Quebec. Interbedded stratified sediments associated with the St. Francis till are correlated with the Gayhurst Formation. Stratified sediments associated with Van Buren and Mars Hill tills are correlated with post Lennoxville sediments of Quebec. Granite-gneiss erratics of Canadian Shield provenance in the Van Buren till indicate advance of the Laurentide ice into northern Maine during late Wisconsinan time. Moraines in southern Aroostook County with associated outwash and eskers record general recession from coastal Maine. Recession occurred after the formation of the Pineo Ridge moraine in Maine and the St. Antonin-Highland Front moraine complex in Quebec. The Caribou-Winterville moraine complex in northern Maine marks the boundary between the penecontemporaneously deposited Van Buren and Mars Hill surface tills and is correlated with the Grand Falls moraine at Grand Falls, New Brunswick.