The pelagic distribution of marine birds in a heterogeneous environment

In this paper I review recent, small-scale, process-oriented studies of the pelagic distribution of marine birds. I examine the roles played by a variety of hydrographic features, including steep flow gradients (fronts) and water column stratification, in determining the abundance or availability of avian prey. The ice edge/marginal ice zone is a frontal area of particular interest in polar regions. In most oceanic systems we have examples of avian use of a feature, hut only poor information on the importance of the feature for supporting the population as a whole. I review recent studies of the spatial and numerical concordance of marine birds and their prey, and find that these studies have yielded mixed results, with correlations stronger for piscivorous birds, particularly murres foraging on capelin, than for planktivores. Review of investigations of multispecies interactions during foraging shows that flock foraging has both beneficial and negative aspects. Interspecific facilitation of foraging occurs in numerous interactions involving both other species of birds and marine mammals. However, co-occurrence of predators may occur because both seek the same prey, and caution must be used in assessing mutualistic interactions between predatory species.     

Aeolian to marine transition in Cambro—Ordovician cratonic sheet sandstones of the northern Mississippi valley, U.S.A.

Cratonic quartz sandstones have presented several intractable problems. Besides their extreme textural and compositional maturity and paucity of shale, their sheet-like geometry is particularly notable. If the sandstones were entirely marine, as long supposed, such geometry is difficult to explain in terms of modern shelf sediments, which are generally held to be either relict or only slightly reworked by the Holocene transgression (palimpsest). Re-study of two quartz sandstones in the northern Mississippi Valley region reveals evidence for significant non-marine deposition followed by varying degrees of marine reworking during transgressions. Facies patterns are similar in the Cambrian Wonewoc and Ordovician St Peter sandstones, both of which overlie unconformities. In both, a large-scale cross-stratified facies believed to represent aeolian ergs passes laterally into a planar-and-channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits. Criteria of aeolian deposition in both facies include adhesion structures, large ripple index, fine climbing translatent lamination, grainfall and grainflow stratification. Criteria of braided fluvial deposition include shallow channels containing sequences of thinning-upward sets of trough cross-stratification, reactivation surfaces, low-index ripples, and polygonal cracks. Probable aeolian sand sheets contain flat bedding punctuated by small channels, adhesion structures, and coarse-sand ripples with large index. There is a conspicuous absence of trace and body fossils from these inferred non-marine deposits. In contrast is a burrowed and trough cross-stratified facies characterized by medium-scale cross-bedding alternating with bioturbated intervals and rare brachiopod or trilobite-mould coquinas, which is interpreted as shallow marine. In both formations, this last facies replaces laterally and overlaps the other two, reflecting transgression and variable reworking. The main areas of non-marine deposits in both formations are capped by a thin, burrowed subfacies that represents the culmination of each transgression; that is, a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense. It is suggested that the sheet-like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes. That such processes must have been orders-of-magnitude more important on pre-Devonian, vegetation-free landscapes than they have been since, not only helps to explain the sheet-like character but also the exceptional maturity of the older cratonic sandstones.     

Submerged forest off Nyuzen, Kurobegawa alluvial fan, Toyama Bay, Central Japan

Remnants of a submerged ancient forest have been found on the continental shelf of NW Japan off the Kurobe alluvial fan in Toyama Bay. The remains, standing tree stumps and roots, are located at depths between 20 and 40 m and ages of 8,000 years B.P. to 10,000 years B.P. have been determined by the ¹⁴C method. Alnus and Salix are the most commonly preserved genera and the trees grew on the now sub-merged seaward margin of the alluvial fan. Pollen analysis permits an estimate of palaeo-temperatures 2°C to 4°C lower than at present. Diatoms give some indication of salinity variations at the time of forest development and suggest brackish water, possible lagoonal conditions. The distribution of the stumps and roots suggests that there were probably two separate forests which were overwhelmed by rising sea-level and flash flood deposits. The present day submarine exposures may be due to recent submarine erosion related to the seaward movement of flood waters which passed from the fan into the upper reaches of the Toyama submarine canyon system.