Abstract
The sediment record of Lake Kivu reveals a complex volcanogenic and climatic Holocene history. Investigation of the inorganic carbonate record dates the onset of carbonate deposition in the mid-Holocene in Kivu's deep northern and eastern basins and identifies conditions enabling deposition. The magnitude and timing of carbonate-rich sedimentation is not so much controlled by climate but, instead, linked strongly to hydrothermal activity in the basin. Sublacustrine springs supply the vast majority of the calcium and carbonate ions required for supersaturation with respect to aragonite. This major hydrothermal activity that permanently stratifies Lake Kivu today was initiated ∼3,100 y before present (3.1 ka), when carbonate-rich sediments first appeared in the Holocene record. Aragonite is the dominant CaCO3 mineral present in the lake deposits. Both δ13C and δ18O of the aragonite are enriched above the expected kinetic fractionation of meteoric waters, suggesting a volcanogenic influence on the formation waters. Repeated major fluctuations in the carbonate record after 3.1 ka therefore most likely reflect the historical variation in hydrothermal inputs.
Highlights
The sediment record of Lake Kivu reveals a complex volcanogenic and climatic Holocene history
Upper 25 m of Lake Kivu presently have Mg and Ca concentrations of 84 mg/L and 7 mg/L (3.5 mM and 0.175 mM), respectively [15], with the molar ratio being well in excess of 12:1 required for CaCO3 precipitation in the form of aragonite [16]. Such high Mg:Ca ratios are unusual in dilute freshwater lakes; geochemical evolution in closed basin lakes can result in calcium carbonate deposition removing Ca while Mg becomes increasingly concentrated [17], accompanied by a shift from calcite to aragonite deposition
The δ13Caragonite values are more enriched in comparison with primary carbonates from other African lakes, indicating an isotopically enriched source of dissolved inorganic carbon (DIC) in Lake Kivu since the beginning of carbonate deposition and continuing to the present
Summary
The sediment record of Lake Kivu reveals a complex volcanogenic and climatic Holocene history. Sublacustrine springs supply the vast majority of the calcium and carbonate ions required for supersaturation with respect to aragonite This major hydrothermal activity that permanently stratifies Lake Kivu today was initiated ∼3,100 y before present (3.1 ka), when carbonate-rich sediments first appeared in the Holocene record. Ross et al [10] correctly pointed out that there were at least two earlier periods of abrupt onset and cessation of aragonite sedimentation in Lake Kivu over the past few thousand years [11] that could not be attributed to human influence but, instead, most likely reflected the influence of variable hydrothermal input to the lake If this is true, the Kivu record of carbonate sedimentation provides important insight into the history of hydrothermal influx to the deep northern basin. The sporadic injection of hydrothermal fluids and gases to the lake signals the difficulty of predicting future trends and potential for catastrophic degassing, which would be potentially fatal to >2 million people living near the lake shore
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