The stable isotope composition of organic and inorganic fossils in lake sediment records: current understanding, challenges, and future directions

This paper provides an overview of stable isotope analysis (H, C, N, O, Si) of the macro and microscopic remains from aquatic organisms found in lake sediment records and their application in (palaeo)environmental science. Aquatic organisms, including diatoms, macrophytes, invertebrates, and fish, can produce sufficiently robust remains that preserve well as fossils and can be identified in lake sediment records. Stable isotope analyses of these remains can then provide valuable insights into habitat-specific biogeochemistry, feeding ecology, but also on climatic and hydrological changes in and around lakes. Since these analyses focus on the remains of known and identified organisms, they can provide more specific and detailed information on past ecosystem, food web and environmental changes affecting different compartments of lake ecosystems than analyses on bulk sedimentary organic matter or carbonate samples. We review applications of these types of analyses in palaeoclimatology, palaeohydrology, and palaeoecology. Interpretation of the environmental ‘signal’ provided by taxon-specific stable isotope analysis requires a thorough understanding of the ecology and phenology of the organism groups involved. Growth, metabolism, diet, feeding strategy, migration, taphonomy and several other processes can lead to isotope fractionation or otherwise influence the stable isotope signatures of the remains from aquatic organisms. This paper includes a review of the (modern) calibration, culturing and modeling studies used to quantify the extent to which these factors influence stable isotope values and provides an outlook for future research and methodological developments for the different examined fossil groups

A new terrestrial palaeoenvironmental record from the Bering Land Bridge and context for human dispersal

Palaeoenvironmental records from the now-submerged Bering Land Bridge (BLB) covering the Last Glacial Maximum (LGM) to the present are needed to document changing environments and connections with the dispersal of humans into North America. Moreover, terrestrially based records of environmental changes are needed in close proximity to the re-establishment of circulation between Pacific and Atlantic Oceans following the end of the last glaciation to test palaeo-climate models for the high latitudes. We present the first terrestrial temperature and hydrologic reconstructions from the LGM to the present from the BLB’s south-central margin. We find that the timing of the earliest unequivocal human dispersals into Alaska, based on archaeological evidence, corresponds with a shift to warmer/wetter conditions on the BLB between 14 700 and 13 500 years ago associated with the early Bølling/Allerød interstadial (BA). These environmental changes could have provided the impetus for eastward human dispersal at that time, from Western or central Beringia after a protracted human population standstill. Our data indicate substantial climate-induced environmental changes on the BLB since the LGM, which would potentially have had significant influences on megafaunal and human biogeography in the region. © 2018 The Authors

Digging and soil turnover by a mycophagous marsupial

The woylie Bettongia penicillata is a small (1 kg) kangaroo-like marsupial that digs to obtain the fruiting bodies of fungi. The number of woylies in a 60 ha area of sclerophyll woodland in south-western Australia was estimated using mark-recapture at 3 month intervals over 3 successive years. The number of new diggings by woylies, determined at the same intervals, allowed an assessment of the rate of digging per individual. This varied three-fold from 38 to 114 diggings per individual per night, with no consistent seasonality. On average, each woylie displaced 4.8 tonnes of soil annually

Digging by vertebrates as an activity promoting the development of water-repellent patches in sub-surface soil

Dry sclerophyll woodlands in south-western Australia are refugia for remnant populations of woyliesBettongia penicillata . These marsupials create holes as they forage for the fruiting bodies of hypogeous fungi. The effect of these holes on the water-repellent woodland soils was evaluated using simulated diggings. Water repellency was significantly higher in surface than in sub-surface soils, although patches of moderately water-repellent sub-soils did occur. In situ assessments of simulated diggings that were allowed to decay showed a five-fold increase in water repellency in sub-surface soil once they had become filled-in. Buried organic material was found in many decayed diggings that were severely water-repellent, and very severe water repellency occurred where masses of fungal hyphae were present. This suggests that vertebrate diggings in which surface litter and organic debris become trapped can provide a site for the development of sub-surface water repellency

The effects of Woylie (Bettongia penicillata) foraging on soil water repellency and water infiltration in heavy textured soils in southwestern Australia

In the wheatbelt region of Western Australia, brush-tailed bettongs or woylies, Bettongia penicillata, occur in remnant woodlands that have highly water repellent soils. As these marsupials dig for the fruiting bodies of hypogeous fungi they disturb the soil surface. The effect of these diggings was evaluated by laboratory and in situ assessments of soil water repellency. The undisturbed woodland soil surface showed severe water repellence whereas diggings had low water repellence, and appear to act as preferential water infiltration paths after autumn rainfall events. This indicates that Bettongia penicillata has an impact on the non-wetting property of soils in this region

Stable isotopes in chitinous fossils of aquatic invertebrates

Current stable isotope techniques permit the development of new approaches for reconstructing past climateand aquatic food webs based on chitinous invertebrate fossils from lake sediments

Late Quaternary vegetation reconstruction from the Eastern Arc Mountains, Tanzania

Pollen, spore, macrofossil and stable isotope (C and N) analyses from a 266-cm sediment core collected from a swamp on the Eastern Arc Mountains, Tanzania, are used to reconstruct vegetation and environmental history. An estimated time scale based on five 14C ages records approximately 38,000 yr. This palaeorecord is the first from this biodiversity hotspot and importantly extends through the last glacial maximum (LGM). The altitudinal transition from montane to upper montane forest shifted from 1700-1800 m (38,000 14C yr BP) to 1800-1900 m (35,000-29,000 14C yr BP). From 29,000 to 10,000 14C yr BP, it shifted from 1850-1950 m across the LGM to 1750-1800 m (during 10,000-3500 14C yr BP), and to present-day elevations at 2000 m during the last 3500 14C yr BP. The relative ecosystem stability across the LGM may be explained by the Indian Ocean's influence in maintaining continuous moist forest cover during a period of East African regional climate aridity. During the late Holocene, presence of abundant coprophilous fungi and algal blooms demonstrates increasing human impact. Neurospora spores indicate frequent fires, coinciding with clear signals of decline in Podocarpus and Psychotria trees that possibly represent selective logging

The stable isotopic composition of Daphnia ephippia reflects changes in d13C and d18O values of food and water

The stable isotopic composition of fossil resting eggs (ephippia) of Daphnia spp. is being used to reconstruct past environmental conditions in lake ecosystems. However, the underlying assumption that the stable isotopic composition of the ephippia reflects the stable isotopic composition of the parent Daphnia, of their diet and of the environmental water have yet to be confirmed in a controlled experimental setting. We performed experiments with Daphnia pulicaria cultures, which included a control treatment conducted at 12 °C in filtered lake water and with a diet of fresh algae and three treatments in which we manipulated the stable carbon isotopic composition (?13C value) of the algae, stable oxygen isotopic composition (?18O value) of the water and the water temperature, respectively. The stable nitrogen isotopic composition (?15N value) of the algae was similar for all treatments. At 12 °C, differences in algal ?13C values and in ?18O values of water were reflected in those of Daphnia. The differences between ephippia and Daphnia stable isotope ratios were similar in the different treatments (?13C: +0.2 ± 0.4 ‰ (standard deviation); ?15N: ?1.6 ± 0.4 ‰; ?18O: ?0.9 ± 0.4 ‰), indicating that changes in dietary ?13C values and in ?18O values of water are passed on to these fossilizing structures. A higher water temperature (20 °C) resulted in lower ?13C values in Daphnia and ephippia than in the other treatments with the same food source and in a minor change in the difference between ?13C values of ephippia and Daphnia (to ?1.3 ± 0.3 ‰). This may have been due to microbial processes or increased algal respiration rates in the experimental containers, which may not affect Daphnia in natural environments. There was no significant difference in the offset between ?18O and ?15N values of ephippia and Daphnia between the 12 and 20 °C treatments, but the ?18O values of Daphnia and ephippia were on average 1.2 ‰ lower at 20 °C than at 12 °C. We conclude that the stable isotopic composition of Daphnia ephippia provides information on that of the parent Daphnia and of the food and water they were exposed to, with small offsets between Daphnia and ephippia relative to variations in Daphnia stable isotopic composition reported from downcore studies. However, our experiments also indicate that temperature may have a minor influence on the ?13C, ?15N and ?18O values of Daphnia body tissue and ephippia. This aspect deserves attention in further controlled experiment

Stable Carbon isotope compositions of Eastern Beringian grasses and sedges: investigating their potential as paleoenvironmental indicators

The nature of vegetation cover present in Beringia during the last glaciation remains unclear. Uncertainty rests partly with the limitations of conventional paleoecological methods. A lack of sufficient taxonomic resolution most notably associated with the grasses and sedges restricts the paleoecological inferences that can be made. Stable isotope measurements of subfossil plants are frequently used to enhance paleoenvironmental reconstructions. We present an investigation of the stable carbon isotope composition (d13C) of modern and subfossil grasses and sedges (graminoids) from Eastern Beringia. Modern grasses from wet habitats had a mean d13C of 229.1% (standard deviation [SD] 5 2.1%, n 5 75), while those from dry habitats had a mean of 226.9% (SD 5 1.19, n 5 27). Sedges (n 5 ,50) from dry, wet, marsh, and sand dune habitats had specific habitat ranges. Four modern C4 grasses had d13C values typical of C4 plants. Analyses were also conducted using subfossil graminoid remains from several sedimentary paleoecological contexts (e.g.,arctic ground squirrel nests, loess, permafrost, and paleosols) in Eastern Beringia. Results from these subfossil samples, ranging in age from .40,000 to ca. 11,000 cal. yr BP, illustrate that the d13C of graminoid remains has altered during the past. The range of variation in the subfossil samples is within the range from modern graminoid specimens from dry and wet habitats. The results indicate that stable isotopes could contribute to a comprehensive and multiproxy reconstruction of Beringian paleoenvironments