Magnetic studies of erosion in a Scottish lake catchment. 1. Core chronology and correlation1

Abstract

Magnetic susceptibility, *'OPb, 137Cs, 14C and paleomagnetic secular variation are used to establish the chronology of sedimentation in a set of cores from Loch Frisa in western Scotland. The 14C dates obtained are not compatible with the chronology derived from all the other techniques and this is ascribed to inwash of "old" particulate carbon from the watershed. Central cores show little evidence of changes in sedimentation rate over the last 150 years whereas marginal cores contain evidence of major increases arising from ploughing and from drainage associated with catchment afforestation since 1935. The results illustrate the value of a multiple core approach to sedimentation and erosion studies even where complex lake morphometry precludes calculation of sediment budgets.For a paleolimnological study of Loch Frisa, on the Island of Mull in western Scotland, we used data obtained from short cores of largely allochthonous, minerogenic sediment to reconstruct the recent history of erosion within the lake catchment. The requirements for such a study include cores from sites representing the full range of depositional environments within the lake and some means of dating and correlating the record in these cores. We have used mineral magnetic measurements both for core correlation (cf. Bloemendal et al. 1979; Dearing et al. 198 1; Dear-mg 198 3) and for detailed characterization of sediments and catchment sources (cf. Oldfield et al. 1979;Walling et al. 1979;Oldfield 1983). We used 14C, 210Pb, and 137Cs assay and paleomagnetic secular variation as dating techniques.Loch Frisa was chosen for study largely for reasons of lithology and recent land use. Like Lough Neagh in Northern Ireland ( Thompson et al. 1975) it is surrounded by basalt rocks rich in titanomagnetite.It thus provided a context within which to explore ' John Dearing's work was financed by an NERC studentship. Funds for radiometric dating were provided under NERC research grant 3424. and extend the application of mineral magnetic studies to lake watersheds characterized by strongly ferrimagnetic parent material. Loch Frisa has also experienced a dramatic land use change in recent times through the extensive commercial afforestation of the slopes surrounding the lake. The physical settingLoch Frisa lies in a glacially deepened depression running NW-SE within tertiary basalt in the parish of Milninian and Kilmore in the northern half of Mull (Fig. 1) (grid ref. NM 490485; 6"04'W long, 56"34'N lat). It is a long, narrow deep lake (ca. 7 km x 0.5-0.75 km; max depth 62.4 m; mean depth 23.3 m; volume 102 x 1 O6 m3) (Murray and Pullar 19 10) with a catchment rising to 444 m at Speinne Mor on the NE side and 273 m at Tam nam Fitheach on the SW side (Fig. 2). The surface of the lake lies 75 m above sea level and has an area of 409 ha. The catchment surface area of 1,396 ha thus gives a ratio of lake area : catchment area of about 1: 3.5. The morphometry of the lake bottom is complex and irregular (Fig. 3), probably as a result of the presence of ...

The late-Holocene history of Gormire Lake (NE England) and its catchment: a multiproxy reconstruction of past human impact

“…Other studies show that the relatively more dense sediment resulting from minerogenic inwash is more likely to be retained in marginal situations than to be preferentially resuspended into the centre of the lake (cf. Appleby et al, 1985). Blackham et al 's (1981) reference to marginal minerogenic inwash layers at the site reinforces this interpretation.…”

Development and erosion in the Brazilian Amazon: A geochronological case study

“…The assumption of a constant rate of supply for 210pb is frequently violated (Oldfield and Appleby 1984;Appleby et al 1985;Murchie 1985). The assumption of a constant rate of supply for 210pb is frequently violated (Oldfield and Appleby 1984;Appleby et al 1985;Murchie 1985).…”

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“…Other similar work in marine and lacustrine environments, involving sourc-ing of sediments and core correlation include those of Somayajulu eta!. (1975), Thompson& Morton (1978, Bloemendal et al ( 1979), Oldfield et al ( 1983), Appleby et al (1986), Dearing (l 986), Robinson (1990) and Peck et al (1994). The work of Peck et al (1994) is the only previous paper presenting core correlations (of ca.…”