Abstract
Oxygen isotope (δ18O) measurements on the exoskeletons of aquatic insects can be used to reconstruct changes in the δ18O of ambient water and, indirectly, to infer the climate and environmental conditions at the time of tissue synthesis. Prior to stable isotope analysis, it is often necessary to chemically pretreat insect remains to remove allochthonous organic and inorganic compounds without altering the δ18O signature. We tested the effectiveness and impact of duration of exposure to a buffered 2 M ammonium chloride (NH4Cl) solution for removing carbonates at neutral pH from chironomid head capsules, water beetle sclerites and marine crab remains prior to stable isotope analysis. Immersion in NH4Cl for 24 h efficiently removed the effect of carbonates with no long-term effects of prolonged exposure observed. Furthermore, we assessed the variability in δ18O values within and between individual sclerites (exoskeleton parts) of both modern and fossil water beetle remains. Both modern and fossil specimens had similar intra-sclerite variability in δ18O values (~ 2‰ range). In contrast, modern specimens had much smaller inter-sclerite variability (< 0.9‰ range) compared with fossil specimens from the same sample (up to 10‰ range). The high inter-sclerite variability observed in fossil material likely results from the nature of fossil material: a mix of sclerites from a 1–2–L sample bin, originating from different individuals that may have existed at different times and under different environmental conditions. We therefore recommend that material to be analysed for stable isotopes be sampled at high temporal resolution to reduce uncertainties in paleotemperature estimates derived from water beetle δ18O records.
Highlights
Oxygen isotope ratios in the water of hydrologically open lakes (d18Olakewater) dominantly reflect the isotopic composition of precipitation (d18Oprecipitation) received by the lake via rainfall, streams and groundwater and thereby, indirectly, record a climate signal (Dansgaard 1964; Clark and Fritz 1997; Leng and Marshall 2004)
The artificial addition of CaCO3 was found to significantly change the d18O of chironomid and beetle samples only compared to controls (Fig. 1), whereas the CaCO3 and crab chitin samples had similar d18O values
After 2 h and 24 h of treatment with NH4Cl, the d18O values of the aquatic insect samples did not indicate an effect of CaCO3 anymore, as the samples were not statistically different from the untreated controls (ESM Tables 4b and 4c)
Summary
Oxygen isotope ratios in the water of hydrologically open lakes (d18Olakewater) dominantly reflect the isotopic composition of precipitation (d18Oprecipitation) received by the lake via rainfall, streams and groundwater and thereby, indirectly, record a climate signal (Dansgaard 1964; Clark and Fritz 1997; Leng and Marshall 2004). The d18O of the exoskeleton of aquatic insects has been shown to reflect the d18Olakewater at the time of cuticle formation (Wooller et al 2004; Verbruggen et al 2011), with around 70% of the oxygen required for biosynthesis acquired from the host lake water itself (Wang et al 2009; Soto et al 2013). Sclerotized chitinous remains of aquatic insects are generally abundant and well-preserved within lake sediments, with d18O values on their fossil remains having the potential to provide information on paleoclimate (Wooller et al 2004; Verbruggen et al 2010b; Lasher et al 2017; van Hardenbroek et al 2018). The ability to identify insect remains under a microscope minimizes the possibility of including terrestrial contamination commonly faced by records derived for aquatic cellulose and algal lipids (Sauer et al 2001; Leng and Barker 2006)
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