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In our opinion the amino acid data are consistent with the till/nonglacial stratigraphy. We reject Dyke's proposal that the plotting of data in 0.04 increments is appropriate as an unwarranted interpretation that errors are cumulative. We also see no grounds for accepting his alternative interpretation that the groups of amino acid ratios reflect various transport (read temperature) histories of a single population of Bell Sea shells. It is our opinion that the relative sequence of marine incursions in Hudson Bay is reliable and we repeat that the evidence favors one or more deglacial events. We stress that the ages of the units between the Tyrrell Sea and Bell sea end membrers are interpolated and that the chronology of events is currently based on the assumption that the Bell Sea represents marine incursion at the onset of marine isotope stage 5.Dyke has raised a number of points which have concerned us since we started our joint research on the aminostratigraphy of the Hudson Bay Lowlands. The answer to many questions will come, not from the amino acid results per se, but from detailed litho-, and biostratigraphic logging of the thousands of kilometers exposed along the large rivers that drain into James Bay and southwestern Hudson Bay. This work is presently going on. Let us say in conclusion that analysis of a further 63 shells and shell fragments resulted in a virtually identical frequency distribution to that discussed in our paper. We are currently evaluating the stratigraphic integrity of these results. Field expeditions by the Geological Survey of Canada in 1982 and 1983 into the Hudson Bay Lowlands were specifically designed to log new sections and make additional shell collections. We hope to report on these new data in due course. 相似文献
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The stratigraphic record in the James and Hudson Bay Lowlands indicates that the sequence of glacial events at the geographical center of the 12.6 × 106 km2 Laurentide Ice Sheet may have been more complex than hitherto imagined. Isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups. Two alternative explanations of the data are offered, of which we strongly favor the first. Hypothesis 1: Setting the age of the “last interglacial” marine incursion, the Bell Sea, at 130,000 yr B.P. results in a long-term average diagenetic temperature for the lowlands of +0.6°C. Using this temperature enables us to predict the age of shells intermediate in age between the “last interglaciation” and the incursion of the Tyrrell Sea 8000 yr ago. Between these two interglacial marine inundations, Hudson Bay is predicted to have been free of ice along its southern shore about 35,000, 75,000, and 105,000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments. These results suggest (1) that traditional concepts of ice-sheet build-up and decay must be reexamined; (2) that “high” sea levels may have occurred during the Wisconsin Glaciation; and (3) that a critical reappraisal is required of the open ocean δ18O record as a simple indicator of global ice volume. An alternative, Hypothesis 2, is also examined. It is based on the assumption that the 35,000-yr-old deposits calculated on the basis of Hypothesis 1 date from the “last interglaciation”; this, in effect, indicates that the Missinaibi Formation, commonly accepted as sediments of the “last interglaciation,” are about 500,000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130,000 yr has been close to ?6°C. We argue for rejection of this alternative hypothesis. 相似文献
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Mercury in lake sediments of the Precambrian Shield near Huntsville, Ontario, Canada 总被引：10，自引：0，他引：10
P. E. Rasmussen D. J. Villard H. D. Gardner J. A. C. Fortescue S. L. Schiff W. W. Shilts 《Environmental Geology》1998,33(2-3):170-182
Long sediment cores (>1 m) were collected from eight Precambrian Shield lakes in southern Ontario, Canada and analyzed for
mercury (Hg), loss-on-ignition (LOI), and a suite of 36 other elements. Results indicated at least 100-fold variation in sediment
Hg concentrations between lakes in close proximity (from 450 ppb), comparable to the variation reported for lakes across the
whole of Canada. Strong areal correlations between Hg concentrations and LOI (r
2
=0.77), between Hg and other trace element concentrations (Pb, Zn, Cd, Sb, As, Br), and similarities in the vertical concentration
profiles of Hg and LOI, all point to the importance of organic matter in the release, transport and redistribution of metals
in watershed systems. The spatial pattern of Hg concentrations in deep, precolonial sediments (>20 cm) was found to mirror
the pattern of Hg concentrations in modern surface sediments, an observation that was confirmed in a follow-up survey (r
2
=0.85;n=25 lakes), indicating that natural processes govern the unequal distribution of Hg among these lakes. Between-lake differences
in surface sediment Hg concentrations normalized to organic carbon (Hg/C) were also reflected by Hg concentrations in smallmouth
bass normalized to 35 cm length (R
2
=0.63;n=15 lakes). The latter relationship suggests that smallmouth bass and lake sediment indicators provide mutually supportive
information regarding Hg loading to the lacustrine environment from geological sources in the watershed system.
Received: 31 October 1996 · Accepted: 27 May 1997 相似文献
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