The quantification of rates of total sediment influx to Lynn Goddionduon, Gwynedd.
(i) This study uses mineral magnetic parameters (e.g. magnetic susceptibility,
saturation isothermal remanence and anhysteretic remanence) and
lithostratigraphical description to correlate a suite of sed~ent cores
sampled from a closely-meshed grid network covering the whole of a small
upland lake basin in Gwynedd, North Waleso
(ii) The core correlations are used to define three horizons within the
lake sediment deposits: (1) the Loch Lomond Stadia I Clay - early Flandrian
clay-mud contact, at the base of the Post-glacial sediment sequence. (2) A
peak in magnetic susceptibility which is identified as a concentration of
paramagnetic iron and manganese which are tentatively assumed to have entered
the lake basin in solution. (3) A peak in magnetic susceptibility
and magnetic remanence in the uppermost sections of the cores which is
identified as a concentration of ferrimagnetic material. The correlated
horizons define three contiguous layers of sediment within the lake basin.
Contour maps of the thicknesses of the sediment layers are constructed and
USed to estimate the volume of each layer.
(iii) Radiometric dating (C-14 and Cs-137) is used to estimate the ages
of the boundaries of the sediment layers, thereby enabling rates of total
sediment influx and sediment yield per unit area of the terrestrial part
of the watershed to be obtained. Sediment yield estimates of 36 kg/ha/yr,
1~6 kg/ha/yr and 263-326 kg/ha/yr were obtained for the periods (represent1ng
the duration of the deposition times of the sediment layers) 10 400-
800 bp; 800 bp-AD 1951 and AD 1951-1977, respectively.
(iv) Pollen analyses are used together with documented regional and local
?nvironmental history to provide a ccmtext for the eva'Lua.tdon of the est-
~mated rates of sediment yield. During the majority of the 10 400-800 bp
period the watershed is likely to have been occupied by relatively undisturbed
deciduous woodland, and during the 800 bp-AD 1951 period by grazed
and possibly burnt-over moorland. The sediment yield for the AD 1951-1977
period is ascribed predominantly to soil erosion resulting from a forest
:ire within the watershed in AD 1951. Comparison of the estimates of sed-
~~ent yield with those obtained using conventional geomorphological techn1ques
in watersheds under comparable forms of land-use suggests some accordance.
(v) The core correlations and sediment thickness contour maps depict the
history of the spatial pattern of sediment accumulation within the lake
baSin. The lake basin appears to have infilled asymmetrj.cal1y, with the
f?CUS of sediment accumulation having moved progressively northwards with
t1me. Marked variations in rates of sediment accumulation over short dist~
ces (tens of metres) are also evident. In contrast to the results ob-
~a~ed from other lake basins, the currently deepest part of the lake bas-
1n 1S virtually devoid of sediment. The observed patterns of sediment
accumulation are tentatively assumed to be a response to wind stress.