Interpretation and application of carbon isotope ratios in freshwater diatom silica

Abstract

Carbon incorporated into diatom frustule walls is protected from degradation enabling analysis for carbon isotope composition (δ13Cdiatom). This presents potential for tracing carbon cycles via a single photosynthetic host with well‐constrained ecophysiology. Improved understanding of environmental processes controlling carbon delivery and assimilation is essential to interpret changes in freshwater δ13Cdiatom. Here relationships between water chemistry and δ13Cdiatom from contemporary regional data sets are investigated. Modern diatom and water samples were collected from river catchments within England and lake sediments from across Europe. The data suggest dissolved, biogenically produced carbon supplied proportionately to catchment productivity was critical in the rivers and soft water lakes. However, dissolved carbon from calcareous geology overwhelmed the carbon signature in hard water catchments. Both results demonstrate carbon source characteristics were the most important control on δ13Cdiatom, with a greater impact than productivity. Application of these principles was made to a sediment record from Lake Tanganyika. δ13Cdiatom co‐varied with δ13Cbulk through the last glacial and Holocene. This suggests carbon supply was again dominant and exceeded authigenic demand. This first systematic evaluation of contemporary δ13Cdiatom controls demonstrates that diatoms have the potential to supply a record of carbon cycling through lake catchments from sediment records over millennial timescales.

Modern sedimentation processes in Lake Towuti, Indonesia, revealed by the composition of surface sediments

“…However, samples with elevated δ 13 C OM values do not have higher C/N ratios, suggesting low terrigenous OM contributions. Alternatively, autochthonous OM can obtain a terrigenous δ 13 C OM value while maintaining low C/N ratios due to algal and microbial metabolism of terrestrially‐sourced dissolved inorganic and organic carbon (Webb et al ., ). Data from this study suggests that this may occur on a catchment scale in Lake Towuti.…”

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

“…Compared to d 18 O diatom studies, the application of d 30 Si diatom , d 13 C diatom and d 15 N diatom techniques are in their relative infancy, with only a few studies being applied in lacustrine systems (Alleman et al 2005;Street-Perrott et al 2008;Swann et al 2010;Hurrell et al 2011;Hernández et al 2011Hernández et al , 2013Opfergelt et al 2011;Chen et al 2012;Barker et al 2013;Cockerton et al 2015;Webb et al 2016;Panizzo et al 2016Panizzo et al , 2018a. There are several reviews of the use of isotopes in biogenic (including diatom) silica (including Leng and Barker 2006;Swann and Leng 2009;Leng et al 2009;Leng and Henderson 2013;Sutton et al 2018), which highlight, in detail, the many issues associated with such analyses.…”

“…O isotope extraction generally adopts fluorination (offline) techniques, with measurement via gas-source isotope ratio mass spectrometry (IRMS), a technique which can also be used to measure d 30 Si diatom (Leng and Sloane 2008). However, the use of multi-collector inductively-coupled-plasma mass spectrometry (MC-ICP-MS) is becoming increasingly more dominant for d 30 Si diatom analyses due to the reduced sample size needed (~ 1 mg), while carbon and nitrogen (for d 13 C diatom and d 15 N diatom ) are measured on very small quantities of organic material hosted (occluded) within the structure (Webb et al 2016). (‰) signatures are drawn, highlighting the effects of biological uptake in surface waters (e.g., lower [DSi] and higher d 30 Si DSi ).…”

Substratum‐Associated Microbiota

“…Carbon isotopes occluded in the silica of diatoms can be used to differentiate biological vs. geological sources in sedimentary C records. Analysis of C isotopes in modern diatom samples showed a close relationship between C cycling in freshwaters and their catchment geology, so that when geology is held constant (and not highly calcareous), this method can be used to interpret C cycling in sediment records over millennial timescales (Webb et al, 2016). Rapid Assessment of Periphyton Ecology in Rivers (RAPPER), a survey method for macroscopic algae, uses genus-level identification, along with categorization into stress or competitive taxa from sample and cover data collected along 10 m lengths to determine if a site is at risk from eutrophication .…”