Effects of organic removal techniques prior to carbonate stable isotope analysis of lacustrine marls: A case study from palaeo‐lake Fucino (central Italy)

Abstract

Rationale The suitability of organic matter (OM) removal pre‐treatments in isotopic studies of lacustrine carbonates is currently under debate. Naturally occurring OM seems to have a negligible effect on the bulk isotopic composition of carbonates compared with changes induced by pre‐treatments. This study provides further insights into the possible effects induced by commonly used pre‐treatments on natural lacustrine carbonates. Methods Sixteen samples from the Fucino F1–F3 lacustrine succession (Abruzzo, central Italy) were characterised for their mineralogical and geochemical composition and each was split into three identical aliquots. One aliquot was left untreated while the remaining two were treated with NaOCl and H2O2 dilutions. The same treatment was applied to an internal standard consisting of pure Carrara marble. The treated and untreated samples were analysed for their carbon (δ13C values) and oxygen (δ18O values) isotope compositions using an Analytical Precision AP2003 isotope ratio mass spectrometer. Results The samples had variable proportions of endogenic and detrital components, the detrital portion being more (less) abundant during colder (warmer) climate phases. We observed that neither the NaOCl nor the H2O2 treatment was able to completely remove OM and therefore there was selective removal of compounds within the OM pool. A possible effect of pre‐treatment is the loss of carbonates intimately interspersed within the OM, as suggested by the evolution of isotopic ratios towards the local detrital array. Conclusions Our study highlights sample‐specific changes in geochemistry associated with sample pre‐treatments; however, such changes do not seem to lead to either systematic and/or predictable isotopic shifts. We suggest that the suitability of NaOCl or H2O2 pre‐treatments for OM removal should be evaluated on a case‐by‐case basis. In the specific case of lacustrine marls from palaeo‐lake Fucino containing relatively low amounts of OM and in which both detrital and endogenic carbonates occur, both pre‐treatments should be avoided.

Insight into summer drought in southern Italy: palaeohydrological evolution of Lake Pergusa (Sicily) in the last 6700 years

“…(2013) and Mannella et al . (2020). The isotopic ratios are reported in the well‐known δ‐notation ‰ (per mill).…”

“…Due to the high ratio between carbonate content and organic matter (total inorganic carbon/total organic carbon, i.e. TIC/TOC: 0.96 ± 0.61), no pretreatment was performed to remove sedimentary organic matter, following the general results of Wierzbowski (2007), Oehlerich et al (2013) and Mannella et al (2020). The isotopic ratios are reported in the well-known δ-notation ‰ (per mill).…”

Climate and atmospheric circulation during the Early and Mid‐Holocene inferred from lake‐carbonate oxygen‐isotope records from western Ireland

“…Bulk sediment samples were treated with Clorox to remove organic material (Mannella et al, 2020), and then sieved through a 125-µm mesh to remove shell material and isolate the fine endogenic fraction, which was then analysed using a ThermoFinnegan Delta Plus XP mass spectrometer connected to a GasBench. Results are reported in standard delta notation relative to VPDB with analytical precision of ±0.08‰ (1σ).…”

The patterns of elemental concentration (Ca, Na, Sr, Mg, Mn, Ba, Cu, Pb, V, Y, U and Cd) in shells of invertebrates representing different CaCO<sub>3</sub> polymorphs: a case study from the brackish Gulf of Gdańsk (the Baltic Sea)

“…Chemical cleaning of carbonate skeletons prior to chemical analysis and both improvement of the data quality and potential artefacts associated with this are widely discussed in the literature (Barker et al, 2003;Holcomb et al, 2015;Loxton et al, 2017), but a plausible pre-treatment method for the removal of organics still needs to be found (Inoue et al, 2004). Mannella et al (2020) showed that the suitability of chemical pre-treatments for organic matter removal from carbonate matrices should be evaluated on a case-by-case basis and, in case of relatively low organic content, should be avoided. In addition to CaCO 3 crystal lattice, shells of barnacles and bivalves usually contain up to 5 % of organic matter (Bourget, 1987;Wolowicz and Goulletquer, 1999;Marin and Luquet, 2004;Rueda and Smaal, 2004).…”

“…In the last few decades, increasing attention has been paid to the relationship between the composition of shells and external environmental factors. It has been repeatedly observed that the mineral type and accumulated trace element concentrations in CaCO 3 shells of marine invertebrates are integrated records of environmental conditions (Marchitto et al, 2000;Rodland et al, 2006). The elemental composition of the carbonate skeleton can provide records of seawater chemistry and has a significant potential for the fields of palaeoceanography and palaeoclimatology (Freitas et al, 2006;Gillikin et al, 2006;Khim et al, 2003;Ponnurangam et al, 2016;Vander Putten et al, 2000).…”