Geochemical cycling of redox-sensitive metals in sediments from Lake Malawi: a diagnostic paleotracer for episodic changes in mixing depth

Brown, E.T., Le Callonnec, L. and German, C.R. (2000) Geochemical cycling of redox-sensitive metals in sediments from Lake Malawi: a diagnostic paleotracer for episodic changes in mixing depth. Geochimica et Cosmochimica Acta, 64 (20), 3515-3523. (doi:10.1016/S0016-7037(00)00460-9).

Abstract

Changes in heat balance, wind stress, and lake level can affect the efficiency of mixing and depth of oxygen penetration in deep tropical lakes. This study examines how transient changes in these parameters, as reflected by the redox state of bottom waters, are recorded in the sedimentary record of Lake Malawi. We measured concentration profiles of redox-sensitive metals across a layer of oxidized sediment deposited within a sequence composed of reduced sediments in a core taken at 298 m water depth, ~100 m below the present-day chemocline. By examining a range of metals that exhibit different geochemical behaviors—mobilization under reducing (Fe, Mn) versus. oxidizing (U) conditions; formation of insoluble sulfides (Cd, Mo, Pb, Zn); susceptibility to scavenging onto Fe or Mn oxyhydroxide precipitates (Co, Mo, Pb, V, Zn)—we are able to outline a sequence of events consistent with the formation of this feature: (1) At some time before 11 ky BP there was an episode of penetration of oxygenated waters to depths close to or greater than that of this core, i.e., substantially deeper in the lake basin than at present. At this time diagenetically remobilized Fe and Mn were oxidized and precipitated in a near-surface sediment horizon. (2) Fe and Mn oxyhydroxides that precipitated from the water column as metal-rich anoxic waters were mixed upward into oxygenated waters, were deposited in an immediately overlying layer of the sediment. Such material continued to accumulate in surface sediments for a few hundred years until oxic conditions no longer prevailed in overlying waters. (3) As the depth of the chemocline rose (relative to the present-day lake surface) this authigenic material was, in turn, overlain by reducing sediments. (4) Subsequent diagenetic remobilization at the top and bottom of this "redox sandwich" resulted in formation of horizons enriched in, U, Mo, and sulfide-forming metals (notably Cd) along the redox gradients both above and, to a lesser extent, below the layer of oxidized sediment. These enrichments, immobilized under reducing conditions, will remain as permanent markers of the episode of deepening of the chemocline even after reduction and remobilization of the oxidized Fe and Mn solids. In addition, such markers may be useful in determining whether observed episodic interruptions in deposition of varved sediments in the deep anoxic basins of Lake Malawi are due to bioturbation brought on by episodes of deeper oxygen penetration in the water column.

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