Holocene carbonate record of Lake Kivu reflects the history of hydrothermal activity

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.

Missing atmospheric noble gases in a large, tropical lake: The case of Lake Kivu, East-Africa

“…Scenario 3 assumes at least one large outgassing event to have happened in the past, which is supported by observations in sediment cores (8,32,33). During such an event, not only large parts of the main gases CO2…”

Lacustrine dolomite in deep time: What really matters in early dolomite formation and accumulation?

“…Therefore, the moderately positive carbon isotopes of laminated dolomudstones (+2 to +10‰) cannot be solely controlled by methanogenesis. Modern carbonate minerals of Lake Kivu show positive carbon isotopes (+5 to +10‰), which have been suggested a volcanogenic influence on the formation waters (Votava et al, 2017). Lower Cretaceous dolomite (+1.4 to +8.6‰) and associated minerals in the Erlian Basin was likely triggered by changes in the proportion of lake water in the brine-rich mixture and/or CO 2 degassing of hydrothermal fluids (Yang et al, 2020(Yang et al, , 2021a(Yang et al, , 2021b.…”

“…+3‰) (Warren, 2010;Meister et al, 2011;McCormack et al, 2019). Moderately positive carbon isotopes (+2 to +10‰) of lacustrine carbonates can be attributed to bacterial methanogenesis (Sun et al, 2021) or hydrothermal influence (Votava et al, 2017). As organogenic dolomite is mainly formed directly from porewater and/or by replacing preexisting calcite and aragonite in anoxic organic-rich sediments (Compton, 1988;Slaughter and Hill, 1991;Mazzullo, 2000), it seldomly forms pure dolomite laminae like those in the alkaline brackish lakes, previously expounded above.…”

Intrusions of sediment laden rivers into density stratified water columns could be an unrecognized source of mixing in many lakes and coastal oceans

“…However, there is evidence in the sedimentary record that lake overturn has occurred in the past, and that it has been associated with turbidity currents (Zhang et al, 2014). The lake is thought to have become chemically stratified 3100 years ago (Votava et al, 2016), but prior to that this deep lake was likely permanently thermally stratified below 150 m depth similar to Lake Tanganyika for several thousand years (Votava et al, 2016;Zhang et al, 2014). Turbidites are present over the past 10 000 year sediment record, recurring every few hundred years (Zhang et al, 2014).…”