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Browsing by Subject "carbonate"

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    The Holocene History of Lake Kivu (East Africa): New perspectives from new cores
    (2014-07) Votava, Jillian Emilia
    Lake Kivu lies at the heart of East Africa&lsquos rift lakes in a volcanically active region. Hydrothermal seeps impose a complex stratification regime with heated, high-salinity waters entering below 280 m water depth. Previous detailed studies of fossil diatoms and mineralogy of the sediment record suggest this hydrothermal activity began 5,000 years BP. Unfortunately, dating bulk organic matter of these original cores was problematic due to dissolved volcanogenic CO2. This study offers a new chronology and a detailed perspective on the limnologic history of Lake Kivu through investigation of carbonates and bulk organic matter from sediment cores recovered in 2012 and 2013. A Holocene history was compiled by 14C dating of recovered terrestrial macro fossils from a deep, central basin core, and by 210Pb geochronology of recent sediments from a near shore core. Water levels in Lake Kivu rose during the African Humid Period (AHP) pluvial from 12 ka to 5 ka. Authigenic CaCO3 deposition began around 4.2 ka in the deep, main basin with all subsequent carbonate intervals composed of endogenic aragonite. A solute budget reveals that most of the Ca2+ ion is supplied at depth via the hydrothermal seeps and suggests that this sub-lacustrine input was initiated just prior to onset of carbonate deposition. Stable isotopic analyses of δ13Caragonite and δ13COM both indicate slight enrichment beyond the expected kinetic fractionation and above other East African lakes suggesting volcanogenic influence on water column DIC began around 4.2 ka. Some of the alternating intervals of carbonate deposition and cessation in the late-Holocene, and associated δ18Oaragonite enrichment, coincide with records of drought from nearby Lake Edward, such as at the AHP termination, at 2 ka, and during the Little Ice Age. This suggests a climate overprint on the predominantly volcanogenic record of carbonate sedimentation in Lake Kivu.
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    Sulfur bacteria promote dissolution of authigenic carbonates at marine methane seeps
    (2021-01-27) Leprich, Dalton J.; Flood, Beverly E.; Schroedl, Peter R.; Ricci, Elizabeth; Marlow, Jeffery J.; Girguis, Peter R.; Bailey, Jake V.; beflood@umn.edu; Flood, Beverly E.; University of Minnesota Bailey Geobiology Lab
    Carbonate rocks at marine methane seeps are commonly colonized by sulfur-oxidizing bacteria that co-occur with etch pits that suggest active dissolution. We show that sulfur-oxidizing bacteria are abundant on the surface of an exemplar seep carbonate collected from Del Mar East Methane Seep Field, USA. We then used bioreactors containing aragonite mineral coupons that simulate certain seep conditions to investigate plausible in situ rates of carbonate dissolution associated with sulfur-oxidizing bacteria. Bioreactors inoculated with a sulfur-oxidizing bacterial strain, Celeribacter baekdonensis LH4, growing on aragonite coupons induced dissolution rates in sulfidic, heterotrophic, and abiotic conditions of 1773.97 (±324.35), 152.81 (±123.27), and 272.99 (±249.96) Mol CaCO3  cm-2  yr-1, respectively. Steep gradients in pH were also measured within carbonate-attached biofilms using pH-sensitive fluorophores. Together, these results show that the production of acidic microenvironments in biofilms of sulfur-oxidizing bacteria are capable of dissolving carbonate rocks, even under well-buffered marine conditions. Our results support the hypothesis that authigenic carbonate rock dissolution driven by lithotrophic sulfur-oxidation constitutes a previously unknown carbon flux from the rock reservoir to the ocean and atmosphere.