Developing a methodology for carbon isotope analysis of lacustrine diatoms

Abstract

Stable isotope analysis of sedimentary carbon in lakes can help reveal changes in terrestrial and aquatic carbon cycles. A method based on a single, photosynthetic organism, where host effects are minimised, should offer more precision than carbon isotope studies of bulk lake sediments. Here we systematically develop a method for use on fossil lacustrine diatom frustules, adapted from previous studies in marine environments. A step-wise cleaning experiment on diatomaceous lake sediments from Lake Challa, Mt, Kilimanjaro, was made to demonstrate the necessary treatment stages to remove external sedimentary carbon. Changes in soluble carbon compounds during these cleaning experiments were measured using GC/MS. Mass spectrometry methods were refined to measure the small percentage carbon of these samples and details of these methods are presented.Samples of cleaned diatoms containing <1% carbon yielded robust results. Carbon isotope analysis of diatom samples containing different species mixtures were performed and showed suggestive differences, although the effects lay within current experimental error and require further work.Unlike work on oxygen and silicon isotopes from diatom frustules, mineral contamination had no discernable impact on the diatom carbon isotope ratios from these sediments. The range of values found in the lakes investigated thus far can be interpreted with reference to the supply and nature of carbon from the catchment as well as demand generated from lake primary productivity.3

Taxon-specific δ13C analysis of chitinous invertebrate remains in sediments from Strandsjön, Sweden

“…These include remains of primary producers such as Potamogeton (Herzschuh et al 2010;Turney 1999) and traces of organic matter within diatom valves (Barker et al 2013;Hurrell et al 2011) as well as of secondary producers such as aquatic invertebrates (for example, cladocerans (Perga 2010) and chironomids (Wooller et al 2008)). Chitinous invertebrate remains largely consist of proteins and chitin and are chemically robust and relatively resistant to microbial degradation, especially if buried under anoxic conditions (Verbruggen et al 2010).…”

Climate, catchment runoff and limnological drivers of carbon and oxygen isotope composition of diatom frustules from the central Andean Altiplano during the Lateglacial and Early Holocene

“…Thus,  13 C bulk is very much dominated by source effects, to such an extent that the 101 One way to overcome the problem of the heterogeneous nature of sedimentary 104 organic carbon is by investigating it in single ubiquitous photosynthetic organisms such 105 as diatoms (Hurrell et al, 2011). Diatom frustules contain polysaccharides and proteins 106 enclosed within the silica cell wall structure (Kroger and Polusen, 2008).…”

Carbon isotope alteration during the thermal maturation of non‐flowering plant species representative of those found within the geological record

“…The samples were weighed into tin capsules and placed into a Carlo Erba 1500 elemental analyser (Thermo Scientific, Waltham, MA, USA) furnace at 1020°C under a continuous flow of helium carrier gas. The gases produced by exothermal flash oxidation of the tin were further oxidized by chromium and cobaltous oxide in the lower part of the furnace . After removal of excess oxygen and water (by passage through hot copper and magnesium perchlorate), the remaining N 2 and CO 2 then passed through a gas chromatography column and past a thermal conductivity detector.…”

“…After removal of excess oxygen and water (by passage through hot copper and magnesium perchlorate), the remaining N 2 and CO 2 then passed through a gas chromatography column and past a thermal conductivity detector. This generated an electrical signal proportional to the concentrations of N 2 and CO 2 present in the helium stream, producing %N and %C data for the sample . The helium stream then carried the CO 2 through a trap at −90°C (for complete removal of water), before reaching a VG Triple‐Trap (IsoPrime, Cheadle Hulme, UK) isotope ratio mass spectrometer held at −196°C.…”

“…The Optima mass spectrometer has triple collectors allowing simultaneous monitoring of CO 2 ion beams at m / z 44, 45 and 46; and a dual inlet allowing rapid comparison of sample CO 2 with a reference CO 2 . The 45/44 ratios were converted into 13 C/ 12 C ratios after correction for common ion effects (‘Craig’ correction) . The δ 13 C values were calibrated to the VPDB (Vienna Pee Dee Belemnite) scale using an in‐house laboratory standard (BROC2, a sample of Brassica oleracea (broccoli) grown in Nottingham University field trials at Sutton Bonington, UK and a secondary low %C standard SOILB, an international soil standard from LECO Corporation, St Joseph, MI, USA); BROC2 was calibrated against NBS‐19 and NBS‐22 (held and distributed by the International Atomic Energy Agency, Vienna, Austria).…”