Richard D. Ricketts

Late Pleistocene Desiccation of Lake Victoria and Rapid Evolution of Cichlid Fishes

Lake Victoria is the largest lake in Africa and harbors more than 300 endemic species of haplochromine cichlid fish. Seismic reflection profiles and piston cores show that the lake not only was at a low stand but dried up completely during the Late Pleistocene, before 12,400 carbon-14 years before the present. These results imply that the rate of speciation of cichlid fish in this tropical lake has been extremely rapid.

The Holocene paleolimnology of Lake Issyk-Kul, Kyrgyzstan: trace element and stable isotope composition of ostracodes

The stable isotope and trace element geochemistry of ostracode shells from Lake Issyk-Kul, Kyrgyzstan, provide an important new paleoclimate record for central Asia. The N 18 O, N 13 C, Sr/Ca, Mg/Ca and U/Ca composition of shells from the species Candona neglecta found in two piston cores constrains the paleohydrology of the lake from 8700 to 1000 calendar yr before present. These data, combined with robust chronologies based on 16 AMS 14 C dates, suggest that the region went from a relatively moist climate in the early Holocene to a more arid climate in the late Holocene. Intense freshwater input, presumably glacial runoff, flowed into the lake from 8700 to 8300 cal yr BP as evident from the N 18 O and N 13 C values. From 8300 to 6900 cal yr BP N 18 O and Sr/Ca values indicate that the lake was relatively fresh and an open basin, and U/Ca values suggest that the lake was well-mixed with well-oxygenated bottom waters. There is a transition zone between 6900 and 4900 cal yr BP when N 18 O and Sr/Ca values rapidly increase and Mg/Ca and U/Ca values rapidly decrease. During this period the lake evolved from an open, freshwater, well-mixed lake to a closed, more saline, less well-mixed lake. For the remainder of the record N 18 O, Sr/Ca, Mg/Ca and U/Ca are relatively constant, implying conditions similar to modern conditions, although the slight increasing trend in Sr/Ca may suggest an increase in salinity over this period. The N 13 C values were approximately in equilibrium with atmospheric CO2 in the early part of the record but became relatively negative in the later part of the record. This indicates that the dominant mechanism controlling the isotopic composition of dissolved inorganic carbon shifted from exchange with the atmosphere to variations in biological productivity. fl 2001 Elsevier Science B.V. All rights reserved.

Pelagic barite precipitation at micromolar ambient sulfate

Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.The question of how significant barite deposits were able to form from early Earth’s low-sulfate seas remains controversial. Here, the authors show pelagic barite precipitation within a strongly barite-undersaturated ecosystem, highlighting the importance of particle-associated microenvironments.

A Comparison Between the Stable Isotopic Composition of Early Holocene and Late Pleistocene Carbonates from Lake Malawi, East Africa

Late Pleistocene carbonates in two cores from Lake Malawi were analyzed for stable isotopic composition. A comparison is made between carbonates deposited during the late Pleistocene (ca. 31–40 ka) and the early Holocene (ca. 5–10 ka). Differences in δ13C values record changes in the composition of atmospheric CO2 and lake productivity. The δ18O values of the late Pleistocene carbonates record either spatial variability in the composition of lake water or a change in climatic conditions in the basin. This change could have been caused by at most a 3.7 °C decrease in temperature, but other climatic parameters may also have contributed to the observed change in the isotopic composition of the carbonates.

Publisher Correction : Pelagic barite precipitation at micromolar ambient sulfate

The original version of this Article contained an error in the barite saturation state equation in the fourth paragraph of the Introduction and incorrectly read ‘Ωbarite=({134Ba2+}⋅{SO42−})/Ksp)’. The correct version removes the superscript 134 next to ‘Ba2+’. This error has now been corrected in both the PDF and HTML versions of the Article.