The temporal evolution of the carbon isotope composition of calcite in the presence of cyanobacteria

Abstract

Quantifying the link between cyanobacterial activity and the carbon isotope signature of precipitated carbonate minerals is crucial for reconstructing the environmental conditions present at the time of carbonate mineral formation. In this study, calcite was precipitated in the presence and absence of Synechococcus sp. cyanobacteria in batch reactors. The temporal evolution of the carbon isotope composition of calcite ( 13 CCalcite) and dissolved inorganic carbon ( 13 CDIC) was monitored to evaluate the rate and degree to which the carbon isotope compositions in calcite are modified during and after its precipitation. The presence of cyanobacteria promoted calcium carbonate formation by increasing fluid pH and the CaCO3 saturation state. It also changed significantly the carbon isotope composition of dissolved inorganic carbon due to the preferential incorporation of 12 C into the biomass. This generated an isotope disequilibrium between the calcite and the aqueous fluid phase over time after the calcite precipitated. The carbon isotope composition of the calcite evolved continuously towards mineral-fluid isotope equilibrium after its precipitation, at geometric surface area normalized rates ranging from 1.75×10 -14 to 1.71×10 -13 mol 13 C/m 2 /s. These rates are sufficiently fast such that the  13 CDIC value of aqueous fluids in calcite-rich rocks would be buffered by the  13 CCalcite value of the co-existing calcite. Mass balance calculations suggest that the carbon isotope composition of calcite could change noticeably when the calcite is in isotope disequilibrium with its co-existing fluid, for example through the presence of a local 12 C sink such as photosynthetic microorganisms or 12 C source such as decomposition of organic material. KeywordsCarbon isotopes, calcite, cyanobacteria, isotope re-equilibration 13 ) as summarized in Fig. 1. The δ C DIC 13values in these experiments are initially controlled by the carbon isotope composition of the carbonate and bicarbonate powder added to water to produce the initial reactive fluids. This value is subsequently influenced by 1) carbon exchange with the atmospheric CO 2 that was continuously bubbled in the reactors, 2) photosynthetic uptake of DIC by growing cyanobacteria, 3) live cell respiration and dead cell lysis together with the subsequent heterotrophic degradation of organic compounds to dissolved carbon species, and 4) DIC removal from the fluid by calcite precipitation. These processes are described in detail in the following section. Equilibrium fractionation between atmosphericCO2 and DIC An estimate of δ C DIC 13 values in equilibrium with the atmosphere can be made by taking account of the distinct equilibrium fractionation factors of each major carbon species present in the aqueous fluids using (Myrttinen et al., 2012): δ 13 C DIC = ∑ (𝑥 𝑖 • (δ 13 C CO 2 + 10 3 ln α 13 C 𝑖−CO 2(g) )) 𝑖 ,where 𝑥 𝑖 designates the mole fraction of the ith aqueous carbon species and δ 13 C CO 2 designates the carbon isotope composition of atmospheric CO 2 , which wa...

Rates of carbon and oxygen isotope exchange between calcite and fluid at chemical equilibrium

“…In our experiments, the moles of C in the solid and fluid were nearly equal. On the other hand, in a marine sediment, there is a far greater mass of C in the solid than the fluid, suggesting that the fluid isotopic composition is likely to be dictated by the composition of the solid post-deposition in the absence of substantial fluid flow (Grimm et al, 2021). In the absence of fluid flow, however, the decomposition of organic carbon, if present in the system, could maintain C isotope disequilibrium between calcite and fluid.…”

“…In this process, it is assumed that the net reaction rate at equilibrium is zero, yet continued transfer of mass may occur between solid and fluid with equal forward (dissolution) and backward (precipitation) rates (Che et al 2021Liu et al, 2016;Pearce et al, 2012;Mavromatis et al, 2015). Recent experimental studies have utilized the principle of simultaneous forward and backward reactions (though not necessarily TST) to estimate near-to-equilibrium reaction rates of a variety of minerals including calcite (Subhas et al, 2017;Zhu et al, 2016), to account for equilibration of isotopic compositions between fluid and solid after initial precipitation (Fernandez et al, 2019;Mavromatis et al, 2016;Pearce et al, 2012) and to explain isotope exchange at or near chemical equilibrium (Chanda et al, 2019;Grimm et al, 2021;Oelkers et al, 2019Oelkers et al, , 2018. However, the driving force for dynamic equilibrium as a mechanism for isotope exchange is not clear, but could be related to isotopic disequilibrium.…”

Laboratory growth capacity of an invasive cyanobacterium (Microcystis aeruginosa) on organic substrates from surface waters of permafrost peatlands

“…This translates into 1.5 to 2 g wet L −1 produced during 10 to 15 days of incubation. Using the results of the present laboratory experiments and considering that the wet: dry biomass ratio of cyanobacteria is 8 to 10, 75,76 and that the carbon proportion in dry biomass is about 50%, the C uptake rate during cyanobacterial growth ranges between 5 and 8.3 mg C L −1 d −1 . By applying these results obtained under laboratory conditions to observations made in a natural environment and considering that the photic layer depth of surface waters in organic-rich permafrost peatlands is 0.5 to 1.0 m, 6,77 cyanobacterial growth over 2 weeks of bloom period in the lake could create a C sink ranging between 2.5 and 8.3 g C–CO 2 m −2 d −1 .…”

Kinetics and mechanisms of cyanobacterially induced precipitation of magnesium silicate

“…Cyanobacterial biomass was calculated via optical density measurement using a Helios Epsilon spectrophotometer at a wavelength of 680 nm (OD 680nm ), which corresponds to the absorption peak of chlorophyll‐ a , based on methodology developed for experiments with single cyanobacterial species (Mavromatis et al, 2012; Shirokova et al, 2013; Bundeleva et al, 2014; Grimm et al, 2021). This wavelength is preferred for the estimation of cyanobacterial biomass if the solution is turbid due to mineral particles (Grimm et al, 2019; Myers et al, 2013).…”

“…The filtered samples were stored in the refrigerator (+4°C) pending analysis. Cyanobacterial biomass was calculated via optical density measurement using a Helios Epsilon spectrophotometer at a wavelength of 680 nm (OD 680nm ), which corresponds to the absorption peak of chlorophyll-a, based on methodology developed for experiments with single cyanobacterial species (Mavromatis et al, 2012;Shirokova et al, 2013;Bundeleva et al, 2014;Grimm et al, 2021).…”