Origin of the Willaumez-Manus Rise,Papua New Guinea

The aseismic Willaumez-Manus Rise on the Bismarck Sea floor separates the Manus Basin from the New Guinea Basin. The rise does not appear to be an extinct spreading axis, or a remnant arc, but may be the result of excess magmatism possibly related to an inferred mantle hot spot beneath St. Andrew Strait. A preferred interpretation is that the rise is the raised edge of the New Guinea Basin, formed in response to a thermal anomaly beneath the extensional Manus Basin which formed later than the New Guinea Basin.     

Late-glacial and postglacial sedimentation in Lake Superior based on seismic-reflection profiles

Seismic reflection profiles (3.5 kHz) were obtained along more than 3500 km of shiptrack in Lake Superior within the last 2 yr. The acoustic character of profiles is categorized as: (I) a single, strong reflector at the lake floor, (II) a thick, acoustically transparent layer overlying a strong reflector, and (III) relatively thick sediment with internal acoustic reflectors. These profiles, in conjunction with sediment cores from the area, reveal that varved glacial-lacustrine sediment settled out preferentially in a trough between Isle Royale and the north shore, and to a lesser extent in other topographic depressions; bottom currents generated by storm waves prevent clay accumulation on till or bedrock in the open lake wherever the bottom is shallower than 100 m; bottom currents prevent deposition or erode bottom sediment in certain deep-water (> 200 m) valleys; and lacustrine sediment is disturbed by creep or slumping off Grand Portage, Minnesota, and by other processes such as dewatering in many other areas. These factors complicate sedimentation in Lake Superior, and must be considered when investigating any aspect of the lake sediment.     

First-order analysis of deformation of a thrust sheet moving over a ramp

John L. Rich introduced the revolutionary concept that many folds in the Appalachian Mountains can be explained as superficial structures formed by passive translation of thrust blocks over ramps in detachment surfaces. The amount of layer-parallel shortening can be negligible in the formation of these folds. Rich primarily was concerned with an explanation for the Powell Valley anticline, in the southern Appalachians, but the essential kinematic features of his model of folding have been verified in other folds in the Appalachians, in the Canadian Rockies, in the Idaho-Wyoming thrust belt, and in the Pyrenees. In this paper we solve the boundary-value problem for an idealized thrust block moving over a detachment surface and ramp with zero drag, and produce theoretical fold forms in the thrust block that closely resemble those in Rich's idealized model. The anticline is narrow and rounded if the translation is small, and broad and flat-topped if the translation is large. The limbs of the anticline are symmetric. We also incorporate drag along the ramp part of the detachment surface in order to derive a possible explanation for the asymmetry of dips of the two limbs of the Powell Valley anticline. We show that drag can explain the asymmetry, particularly if drag between relatively competent rocks in opposition at the ramp caused an initial anticline to form as the thrust block began to move, and then drag reduced markedly as relatively soft shales at the base of the block were thrust over competent rocks in the ramp. The existence of the initial anticline should be reflected in asymmetry of the two limbs and in a bulge at the distal edge of the broad anticline.     

Mussels (Mytilus edulis) as ‘point source’ indicators of trace metal pollution

Using X-ray Energy Spectroscopy trace metal concentrations including Pb, Cu and Zn were studied in the common blue mussel, Mytilus edulis. These organisms, sampled at intervals of only 1 km and less for a line transect study of the port of Vancouver, Canada, exhibited dramatic concentration variations. In particular, mussels only 30 m from a point source of lead displayed essentially base-line concentrations, representing a decline of two orders of magnitude.     

Mitigating Agricultural Emissions of Methane

Agricultural crop and animal production systems are important sources and sinks for atmospheric methane (CH4). The major CH4 sources from this sector are ruminant animals, flooded rice fields, animal waste and biomass burning which total about one third of all global emissions. This paper discusses the factors that influence CH4 production and emission from these sources and the aerobic soil sink for atmospheric CH4 and assesses the magnitude of each source. Potential methods of mitigating CH4 emissions from the major sources could lead to improved crop and animal productivity. The global impact of using the mitigation options suggested could potentially decrease agricultural CH4 emissions by about 30%.     

A model for Northern Hemisphere continental ice sheet variation

The marine record shows that over the last 350 ka Northern Hemisphere ice sheet volumes have fluctuated widely and only on rare short occasions have they been reduced to the present interglacial state. The fluctuations are well synchronized with hemispheric average summer insolation variations of 20 ka periodicity caused by changing orbital parameters. The development of a model which explains the varied amplitudes of the fluctuations and is consistent with the geological record embodies the following arguments: The transition from an interglacial state like today's to a glacial state is initiated when a summer insolation deficit causes a southerly extension of the North Atlantic-Arctic pack ice to 60°N latitude. The extension alters the subpolar low pressure patterns and thus causes a southward diversion of the European Gulf Stream flow. It also produces an enhanced warm West Greenland current. This current causes open seas as far north as Baffin Bay which provides moisture for rapid northern Laurentide ice sheet growth. After several glacial fluctuations driven by insolation variations, the southern Laurentide ice front may reach an extreme extension. This diverts the westerlies and the Gulf Stream thus weakening a dominant subpolar North Atlantic gyre and consequently producing a prolonged cutoff of the West Greenland current and a reduction of high latitude glacial precipitation. The subsequent high insolation can then melt back the eastern pack ice and restore the northern European Gulf Stream. This warms the high latitudes for a time sufficient to melt the continental ice, thus causing the transition back to the interglacial state.An analysis of the record in the context of model suggests that the threshold deficit in average summer insolation that is required to initiate major glacial growth is influenced by the cooling effect of the Greenland ice cap on the seas to the east. The threshold level under conditions like today's is found to lie between ?7 and ?17 ly/day relative to the present. This threshold will not be crossed for at least 54 millenia due to an interval of smaller orbital eccentricity. Probable melting of the Greenland ice cap about 30 ka AP would ensure the extension of the present interglacial beyond 120 ka AP.