Isotopic analyses on bulk carbonates are considered a useful tool for
palaeoclimatic reconstruction assuming calcite precipitation occurring
at oxygen isotope equilibrium with local water and detrital carbonate
input being absent or insignificant. We present results from Lake
Neuchatel (western Switzerland) that demonstrate equilibrium
precipitation of calcite, except during high productivity periods, and
the presence of detrital and resuspended calcite. Mineralogy,
geochemistry and stable isotope values of Lake Neuchatel trap sediments
and adjacent rivers suspension were studied. Mineralogy of suspended
matter in the major inflowing rivers documents an important contribution
of detrital carbonates, predominantly calcite with minor amounts of
dolomite and ankerite. Using mineralogical data, the quantity of
allochthonous calcite can be estimated by comparing the ratio ankerite +
dolomite/calcite + ankerite + dolomite in the inflowing rivers and in
the traps. Material taken from sediment traps shows an evolution from
practically pure endogenic calcite in summer (10-20% detrital material)
to higher percentages of detrital material in winter (up to 20-40%).
Reflecting these mineralogical variations, delta(13)C and delta(18)O
values of calcite from sediment traps are more negative in summer than
in winter times. Since no significant variations in isotopic composition
of lake water were detected over one year, factors controlling oxygen
isotopic composition of calcite in sediment traps are the precipitation
temperature, and the percentage of resuspended and detrital calcite.
Samples taken close to the river inflow generally have higher delta
values than the others, confirming detrital influence. SEM and isotopic
studies on different size fractions (<2, 2-6, 6-20, 20-60, >60 mu m) of
winter and summer samples allowed the recognition of resuspension and to
separate new endogenic calcite from detrital calcite. Fractions >60 and
(2 mu m have the highest percentage of detritus, Fractions 2-6 and 6-20
mu m are typical for the new endogenic calcite in summer, as given by
calculations assuming isotopic equilibrium with local water. In winter
such fractions show similar values than in summer, indicating
resuspension. Using the isotopic composition of sediment traps material
and of different size fractions, as well as the isotopic composition of
lake water, the water temperature measurements and mineralogy, we
re-evaluated the bulk carbonate potential for palaeoclimatic
reconstruction in the presence of detrital and re-suspended calcite.
This re-evaluation leads to the following conclusion: (1) the endogenic
signal can be amplified by applying a particle-size separation, once the
size of endogenic calcite is known from SEM study; (2) resuspended
calcite does not alter the endogenic signal, but it lowers the time
resolution; (3) detrital input decreases at increasing distances from
the source, and it modifies the isotopic signal only when very abundant;
(4) influence of detrital calcite on bulk sediment isotopic composition
can be calculated. (C) 1998 Elsevier Science B.V. All rights reserved.