Concentration Patterns of Redox Sensitive Metals in Lake Sediments - Indicators for Wind and Temperature Regimes

Tobias Schaller Limnological Research Center EAWAG/ETH, CH-6047 Kastanienbaum, Switzerland

Bernhard Wehrli Limnological Research Center EAWAG/ETH, CH-6047 Kastanienbaum, Switzerland


Early diagenesis of redoxsensitive transition metals in lake sediments depends strongly on the oxygen conditions in the deep water and sediment. Different transition metals such as Fe, Mn, V, Cr, As, Mo, Sb, W show a broad range of behaviour with respect to adsorption, complexation, and solubility, depending on oxygen conditions. Their concentration and distribution patterns in lake sediments offer some potential as indicators for deep water oxygen conditions and local climatic conditions, such as wind and temperature distribution.

Study site and methods

Lake Baldegg is a eutrophic hardwater lake situated in the swiss prealpine region. Permanent anoxic layers existed in the deeper parts of the lake from the beginning of this century. Since the installation of an artifical aeration in 1982 the deep water has been an oxic environment, while the
sediment itself has remained anoxic. In October 1992, 38 short cores have been taken to investigate the horizontal distribution of redox sensitive metals. In October 1993 a freeze core has been taken at the deepest site of the lake: sampling was performed with an annual resolution (back to 1886).


Oxygen boundary conditions strongly influence the
horizontal distributions of the measured redox sensitive metals: In case an anoxic sediment is in contact with an oxic bottom water Fe and Mn concentrations in the sediment are correlated with water depth. Taking into account typical half lifes of Fe(II) and Mn(II) in oxygenated water, vertical mass transport can be neglected but horizontal mass transport
can occur over a range of 10m to 1000m. The difference in transport distances leads to a depletion of the near shore regions in Fe- and Mn-sediment concentrations, whereas the deepest parts are enriched (focussing effect). Reductive dissolution of Fe- and Mn-oxyhydroxides at the sediment-water interface and subsequent diffusion of dissolved Fe(II) and Mn(II) into the water column intensifies the internal cycling of Fe and Mn in the lake. The intensified internal cycling has strong effects on the distribution of trace metals: In case of an anoxic sediment Fe-oxydation products in the water column seem to act as carriers for the oxoanions vandate, arsenate and tungstanate to the sediment. Mn-oxydation products seem to act as carriers for molybdate and hydroxo complexes of antimony under anoxic conditions in the bottom water.

The metal profiles of the freeze core were compared with oxygen time series in the deep water since 1950. Manganese is a very sensitive tracer for the dynamic behavior of the oxycline: Permanent anoxic conditions are correlated with lowest concentration values, periodic up- and downward movement of the oxycline, i.e. oxic conditions in the bottom water during spring turnover and anoxic conditions in late summer and fall leads to high pulselike concentration values in the Mn profile. Periodes with pulselike concentration values of manganese are correlated with decreasing annual mean temperatures.


Horizontal distribution in combination with concentration profiles of redox sensitive metals in deep lakes are promising indicators for the reconstruction of deep-water mixing from the sedimentary record. The deep water mixing potential can be estimated by comparing wind speed with the stability of the water column (Brunt-Väisälä frequency). Work is in progress to correlate the metal concentrations profiles with wind force and the stability frequency.