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From November 2006 to January 2010, a sediment trap that was cleared monthly was deployed in Lake Challa, a deep stratified freshwater lake on the eastern slope of Mt. Kilimanjaro in southern Kenya. Geochemical data from sediment trap samples were compared with a broad range of limnological and meteorological parameters to characterize the effect of single parameters on productivity and sedimentation processes in the crater basin. During the southern hemisphere summer (November—March), when the water temperature is high and the lake is biologically productive (nondiatom algae), calcite predominated in the sediment trap samples. During the “long rain” season (March—May) a small amount of organic matter and lithogenic material caused by rainfall appeared. This was followed by the cool and windy months of the southern hemisphere winter (June—October) when diatoms were the main component, indicating a diatom bloom initiated by improvement of nutrient availability related to upwelling processes. The sediment trap data support the hypothesis that the light—dark lamination couplets, which are abundant in Lake Challa cores, reflect seasonal delivery to the sediments of diatom‐rich particulates during the windy months and diatom‐poor material during the wet season. However, interannual and spatial variability in upwelling and productivity patterns, as well as El Niño—Southern Oscillation (ENSO)‐related rainfall and drought cycles, exert a strong influence on the magnitude and geochemical composition of particle export to the hypolimnion of Lake Challa.

The hypersaline crater lake and its catchment on seabird island Isabel (Pacific, off Mexico) was studied to explore the influence of strong seasonal variations in rainfall/evaporation and guano contribution on its limnology. The hypersaline lake water (HSW, 78 ‰) is up to 2. 2-times enriched in inert ions relative to mean seawater. Rainfall during summer dilutes the HSW to form a less saline rainwater body (RWB) above a chemolimnion between 2 and 4 m water depth. The RWB is inhabited first by diatoms and ostracods followed later on by cyanobacteria and ciliates. Evaporation of > 1. 5 m depth of lake water over the dry season increases the salinity of the RWB until the water column becomes isohaline at HSW concentrations in the late dry season. Differences in the stable isotope composition of water and primary producers in RWB and HSW reflect this development. Introduction of seabird guano and the decrease of salinity fuel a high primary production in the RWB with higher d1 CDIC and d13 Corg of particulate organic matter than in the HSW. The high N supply leads to high d1 NNH4 values (+39 ‰ in the HSW) as the consequence of ammonia volatilization that is strongest during guano maturation and with evaporative salinity increase from the HSW. Precipitation of carbonate (calcite and aragonite) from the RWB and the HSW is hindered by the high concentration of guano-derived P. This inhibition may be overcome with evaporative supersaturation during particularly dry conditions. Carbonate may also precipitate during particularly wet conditions from the dilute RWB, where the P-concentration is reduced during an active phytoplankton production that raises the pH. Differences in the stable isotope signatures of carbon and oxygen in HSW and RWB (+5 ‰ d1. CDIC and –3 ‰ d1. OH2O) suggest the processes of carbonate precipitation can be distinguished based on the isotope signature of the carbonates deposited. Changes in the lake system are indicated when lower temperatures and higher rainfall in the 2006 wet season introduced more and less mature guano to the lake. The lower pH was accompanied by lower ammonia volatilization and carbonate precipitation as indicated by an increased concentration of NH4, Ca, Sr and DIC, while d2 H, d1. NNH4, and salinity were lower. According to our results, the observed sediment laminations should reflect the introduction of catchment material (including guano) with runoff, the RWB plankton production, and the carbonate precipitation in relation to its origin and seasonality.

A series of synthetic apatite crystals along the fluorapatite-chlorapatite Ca5(PO4)3(F,Cl) join have been synthesized at 1220 to 1375 °C from Ca3(PO4)2dissolved in a CaF2-CaCl2melt. The solid solutions have then been characterized both chemically and structurally. Because of well-known difficulties in measuring the chemical composition of apatites containing F and Cl by electron microprobe a new semi-micro wet chemical method has been developed. Apatite is relatively well dissolved in mineral acids including HNO3. Nitric acid digestion of apatite for analysis of F and Cl has not been applied in the past to our knowledge. One reason is the potential risk of losing gaseous HF and HCl during acid decomposition. We present an analytical procedure that enables the analyses of F, Cl, Ca, P, and trace elements after digestion of 10 mg apatite with a small amount of 1 N HNO3in gas-tight PTFE tubes at 80 °C. Analytical results from three independent closed acid digestions of 20 synthetic fluor-chlorapatites are presented. The reliability of this method was tested on the basis of charge balance, the Ca/P, Ca/(F+Cl), and P/(F+Cl) ratios, and the total cation sum. Independently derived Cl/F ratios by XRD using Rietveld refinement of apatite crystals from the same synthesis experiments are additionally presented.