Short-term variability in the sedimentary BIT index of Lake Challa, East Africa over the past 2200 years : validating the precipitation proxy

Abstract. The branched vs. isoprenoid index of tetraethers (BIT index) in Lake Challa sediments has been applied as a monsoon precipitation proxy on the assumption that the primary source of branched tetraether lipids (brGDGTs) was soil washed in from the lake's catchment. However, water column production has since been identified as the primary source of brGDGTs in Lake Challa, meaning that there is no longer a clear mechanism linking BIT index variation and precipitation. Here we investigate BIT index variation and GDGT concentrations at a decadal resolution over the past 2200 years, in combination with GDGT data from profundal surface sediments and 45 months of sediment-trap deployment. The 2200 year record reveals high-frequency variability in GDGT concentrations, and therefore the BIT index. Also surface sediments collected in January 2010 show a distinct shift in GDGT composition relative to those collected in August 2007. Increased bulk flux of settling particles with high Ti / Al ratios during March–April 2008 reflect an event of high detrital input to Lake Challa, concurrent with intense precipitation at the onset of the principal rain season that year. Although brGDGT distributions in the settling material are initially unaffected, this soil erosion event is succeeded by a large diatom bloom in July–August 2008 and a concurrent increase in GDGT-0 fluxes. Near-zero crenarchaeol fluxes indicate that no thaumarchaeotal bloom developed during the subsequent austral summer season; instead a peak in brGDGT fluxes is observed in December 2008. We suggest that increased nutrient availability, derived from eroded soil washed into the lake, stimulated both diatom productivity and the GDGT-0 producing archaea which help decompose dead diatoms passing through the suboxic zone of the water column. This disadvantaged the Thaumarchaeota that normally prosper during the following austral summer. Instead, a bloom of supposedly heterotrophic brGDGT-producing bacteria occurred. Episodic recurrence of such high soil-erosion events, integrated over multi-decadal and longer timescales and possibly enhanced by other mechanisms generating low BIT index values in dry years, can explain the positive relationship between the sedimentary BIT index and monsoon precipitation at Lake Challa. However, application elsewhere requires ascertaining the local situation of lacustrine brGDGT production and of variables affecting the productivity of Thaumarchaeota. </jats:p

蓮華寺池と西湖 : 石野雲嶺の風景

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern1. Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia1, 3 were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming4. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies5 to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase

The Impact of Simulated Sulfate Deposition on Peatland Testate Amoebae

Peatlands subjected to sulfate deposition have been shown to produce less methane, believed to be due to competitive exclusion of methanogenic archaea by sulfate-reducing bacteria. Here, we address whether sulfate deposition produces impacts on a higher microbial group, the testate amoebae. Sodium sulfate was applied to experimental plots on a Scottish peatland and samples extracted after a period of more than 10 years. Impacts on testate amoebae were tested using redundancy analysis and Mann-Whitney tests. Results showed statistically significant impacts on amoebae communities particularly noted by decreased abundance of Trinema lineare, Corythion dubium, and Euglypha rotunda. As the species most reduced in abundance are all small bacterivores we suggest that our results support the hypothesis of a shift in dominant prokaryotes, although other explanations are possible. Our results demonstrate the sensitivity of peatland microbial communities to sulfate deposition and suggest sulfate may be a potentially important secondary control on testate amoebae communities

Discovery of anaerobic lithoheterotrophic haloarchaea, ubiquitous in hypersaline habitats

Hypersaline anoxic habitats harbour numerous novel uncultured archaea whose metabolic and ecological roles remain to be elucidated. Until recently, it was believed that energy generation via dissimilatory reduction of sulfur compounds is not functional at salt saturation conditions. Recent discovery of the strictly anaerobic acetotrophic Halanaeroarchaeum compels to change both this assumption and the traditional view on haloarchaea as aerobic heterotrophs. Here we report on isolation and characterization of a novel group of strictly anaerobic lithoheterotrophic haloarchaea, which we propose to classify as a new genus Halodesulfurarchaeum. Members of this previously unknown physiological group are capable of utilising formate or hydrogen as electron donors and elemental sulfur, thiosulfate or dimethylsulfoxide as electron acceptors. Using genome-wide proteomic analysis we have detected the full set of enzymes required for anaerobic respiration and analysed their substrate-specific expression. Such advanced metabolic plasticity and type of respiration, never seen before in haloarchaea, empower the wide distribution of Halodesulfurarchaeum in hypersaline inland lakes, solar salterns, lagoons and deep submarine anoxic brines. The discovery of this novel functional group of sulfur-respiring haloarchaea strengthens the evidence of their possible role in biogeochemical sulfur cycling linked to the terminal anaerobic carbon mineralisation in so far overlooked hypersaline anoxic habitats.</p