Vapour-phase fumigation with HCl is routinely used to remove inorganic carbon in preparation for the measurement of the concentration and delta C-13 value of organic carbon in a sample using elemental analysis coupled to an isotope ratio mass spectrometer. Acidification of the sample to be analyzed can lead to the loss of low molecular weight conjugate bases as volatile organic acids during the acidification and/or the drying steps following fumigation, through protonation of the conjugate base and volatilization. Such loss could lead to a severe bias in incubation experiments where C-13-enriched compounds such as acetate are used to trace reaction pathways or metabolites in a cultivation medium or a mesocosm for example. In this work, we enriched a carbonate-free freshwater sediment with 1-C-13 sodium acetate by 5, 10 and 20 parts per thousand relative to the delta C-13 value of the natural organic carbon of the sediment, and then tested the effects of HCl fumigation, drying at 50 degrees C and drying at room temperature, alone or in combination, on the measured delta C-13 values. We found that fumigation and drying at 50 degrees C, alone or in combination, both lead to the loss of the majority of the C-13-enriched acetate spike. [GRAPHICS] .

Sediments comprise a multitude of inorganic and organic components, with much of the composition of the organics still not fully characterized. Our research targeted n-alkanes, to determine whether compound specific carbon and hydrogen isotope analysis allows for their source identification in coastal sediments. Here, we map the current abundances and sources of straight chain n-alkanes in sediments of the St. Lawrence Estuary and Gulf using molecular (diagnostic ratios) and isotopic fingerprinting (813C, 82H). n-Alkane abundances (117.11 +/- 1.61 to 418.64 +/- 70.20 mu g/g OC), carbon preference index (CPI; 1.95 +/- 0.05 to 5.09 +/- 0.10), average chain length (ACL; 28.36 +/- 0.02 to 28.97 +/- 0.01), proportion of aquatic submerged plants and terrestrial plant inputs (Paq; 0.295 +/- 0.003 to 0.377 +/- 0.002), terrigenous aquatic ratio (TAR; 3.43 +/- 0.16 to 7.99 +/- 0.05), and n-alkane ratio (NAR; 0.169 +/- 0.011 to 0.584 +/- 0.011) values varied along the terrestrial-marine continuum. Large differences in the concentration weighted average (WA) 813C and 82H for odd and even n-alkanes were found, with WA 813C ranging from -30.9 +/- 0.3 to -33.4 +/- 0.09 %o and -28.8 +/- 0.01 to -32.3 +/- 0.2 %o, respectively, and 165.6 +/- 3.6 to -200.8 +/- 2.4 %o and -96.0 +/- 2.8 to -158.7 +/- 2.1 %o for 82H. The diagnostic ratios were shown to misrepresent the input sources of organic matter (OM) and were inaccurate as source indicators when more than one OM source was present. With the addition of compound specific 813C and 82H analysis of n-alkanes, it was determined that the n-alkanes were predominantly derived from natural, rather than anthropogenic sources, with variations being driven by geographic changes in vegetation type and differing ratios of terrestrial and marine OM inputs. Importantly, compound specific isotope analysis of the even numbered n-alkanes would permit identification and tracking of petroleum-derived contaminants. Molecular data alone are ineffective for this, owing to the similarity in CPI values for petroleum-derived contaminants and highly degraded OM which is discharged by the St. Lawrence River into the estuary.

Known as the smell of earth after rain, geosmin is an odorous terpene detectable by humans at picomolar concentrations. Geosmin production is heavily conserved in actinobacteria, myxobacteria, cyanobacteria, and some fungi, but its biological activity is poorly understood. We theorized that geosmin was an aposematic signal used to indicate the unpalatability of toxin-producing microbes, discouraging predation by eukaryotes. Consistent with this hypothesis we found that geosmin and the related terpene 2-methylisoborneol reduced predation of Streptomyces coelicolor and Myxococcus xanthus by the bacteriophagous Caenorhabditis elegans. Predation was restored by the removal of both terpene biosynthetic pathways or deletion of the C. elegans ASE sensory neuron, and resulted in the death of the nematodes. Geosmin itself was non-toxic. This is the first warning chemical to be identified in bacteria or fungi, and suggests molecular signalling affects microbial predator-prey interactions in a manner similar to the well-studied visual markers of poisonous animal prey.

Faecal stanols in lake sediments have been used as a proxy for human populations in the past in a variety of contexts, with the assumption that variability in faecal stanol concentration or ratios is a reliable proxy for relative catchment-scale human populations. Despite that, the specific controls on faecal stanol concentrations and ratios in lake sediments remain poorly understood. In this study we analyse faecal stanol concentrations in lake surface sediments across Guatemala and the Yucatán Peninsula of Mexico in order to constrain geographical and biogeochemical variables controlling stanol concentrations and ratios in lake sediments in this region. We propose and test the hypothesis that the stanol ratios coprostanol:(coprostanol + stigmastanol) and coprostanol:(coprostanol + cholestanol) scale according to the proximity to and size of nearby population centres. The key controls on stanol concentrations that we identify are the proximity to human population centres and the human population within 5 km of the sampling point. Based on a transect across Lake Petén Itzá, the ratio coprostanol:(coprostanol + cholestanol) does not appear to be an accurate proxy for proximity to human population centres, nor does it correlate with catchment human population. We suggest that normalising stanol concentrations to TOC is an appropriate way to take into account the effects of mineral dilution as well as the potential effects of organic matter deposition and preservation, and that the ratio coprostanol:(coprostanol + stigmastanol) does not scale with human population but may be an effective approach to determine the relative contribution of coprostanol-producing mammals and herbivores. Further, we discuss the current limitations of the proxy as well as its future directions, including the implications of our results for sediment core siting, the use of stanol ratios in palaeolimnology, as well as the storage, transport, and diagenesis of stanols.

The analysis of faecal stanols in lake sediment cores offers a novel opportunity to reconstruct human population change, assuming that variability in faecal stanol concentration is a reliable proxy for relative human populations. The ancient lowland Maya of Mesoamerica represents an important ancient society whose demographic dynamics in many locations remain uncertain. We apply the faecal stanol proxy to a sediment core retrieved from a lake adjacent to the archaeological site of Itzan, an ancient population centre in the southwestern Maya lowlands. The sedimentary faecal stanol record from Laguna Itzan implies substantial centennial-and millennial-scale changes in local human populations from 3300 cal years BP to the present. Variability in faecal stanol concentrations is broadly consistent with archaeological evidence for regional societal change across the Maya lowlands, but also implies an earlier presence of humans at this site than is currently indicated in the Itzan archaeological record. We find evidence for high-frequency variability in coprostanol concentrations during the Maya Preclassic period, which we infer represents centennial-scale shifts in settlement patterns associated with changes in agricultural and land use patterns. Given Preclassic-period faecal peak stanol concentrations, we observe lower-than-expected Classic-period faecal peak stanol concentrations, and these may partly be a result of either use of human waste for fertiliser or reduced soil erosion or both. Three periods of inferred population decline are associated with palaeoclimate evidence for a drying climate, specifically during the Terminal Classic (1220-1050 cal yr BP) and the Protoclassic 2 (1860-1670 cal yr BP), as well as the less well-studied drought between 3330 and 2900 cal yr BP during the Early to Middle Preclassic periods. An additional decline and hiatus in coprostanol input coincides with a period of anomalously wet climate in the Late Preclassic. These linkages suggest that climatic change and variability could have played a role in demographic change at multiple points in the evolution of Maya civilisation. Our work shows that faecal stanols are valuable proxies for past human population dynamics, and their relation to climatic change, in Mesoamerica. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Excessive consumption of petroleum and crude oil for energy purposes has resulted in the contamination of many natural systems and waterways. However, determining the presence and level of contamination has been difficult due to the presence of naturally occurring hydrocarbons and to the complexity of the molecular fingerprint of petroleum and crude oils. Naturally occurring straight-chain n-alkanes and polycyclic aromatic hydrocarbons (PAHs) in sediments are both commonly used to determine organic matter sources through diagnostic and isotope ratios, and these ratios are affected by the presence of petroleum and crude oil. As such, they offer a potential avenue for determining whether crude oil contaminants are present in natural systems. The purpose of this project was to determine whether diagnostic ratios of n-alkanes and of PAHs as well as compound-specific isotope ratios of n-alkanes (∂2Halk and ∂13Calk) could be used to detect crude oil or petroleum contamination, and at what level of contamination the difference becomes significant. This was accomplished by separating the aliphatic and aromatic fractions of the natural and crude oil hydrocarbons by column chromatography, spiking natural sediment hydrocarbons with crude oil hydrocarbons at different levels, and analyzing the samples by GC-MS (Gas Chromatography-Mass Spectrometry) and by GC-IRMS (Isotope-Ratio Mass Spectrometry). The isotopic ratios and the hydrocarbon concentrations were determined by external standard calibration, and the diagnostic ratios were then calculated from the concentrations. Both ratios were then evaluated for their efficiency in detecting the presence of crude oil contamination.

The analysis of faecal stanols in lake sediment cores offer a novel opportunity to reconstruct human population change in the past, assuming that sedimentary faecal stanol concentrations can be accurately related to human population. The ancient Lowland Maya civilization of Central America represents an important ancient society where demographic change remains highly uncertain and contested. In this study we present faecal stanol concentrations in lake surface sediments from lakes across climatic and land-use gradients in Guatemala, Belize, and the Yucatan Peninsula of Mexico to constrain societal and biogeochemical drivers of stanol concentrations in sediments, including proximity to population centres and lake salinity. We then use these insights to interpret the faecal stanol record in a sediment core retrieved from a lagoon adjacent to the archaeological site of Itzan, an ancient population centre in the south-west of the Maya Lowlands. The faecal stanol record from Laguna Itzan implies highly dynamic millennial and centennial scale changes in human populations from 3300 years BP to the present. The variability in faecal stanol concentrations and ratios coincides broadly with patterns of large-scale societal change across the Maya Lowlands, but also implies an earlier presence of humans at this site than is documented by archaeological evidence. The stanol data also indicates frequent centennial scale variability in human populations, especially during the Maya Preclassic period. Our work shows that faecal stanols have a strong potential as proxies for changes in human population and land-use change through time in tropical regions, and can be used to complement archaeological datasets