Intracrystalline Organic Matter Research Articles

Kinetics and fractionation of carbon and oxygen isotopes during the solid-phase transformation of biogenic aragonite to calcite: The effect of organic matter

The phase transformation of biogenic shells from aragonite to calcite can modify the primary carbon and oxygen isotopic values (δ13C and δ18O), and thus affect their significance as paleoenvironmental proxies. While the biasing of δ13C and δ18O by dissolution and reprecipitation phase transformation (DR) has been extensively studied, thermal solid-phase transformation (SPT) in archaeological shell middens is rarely considered. In this study, the effects of intra-crystalline organic matter on the kinetics and carbon and oxygen isotopic fractionation during thermal SPT are evaluated by comparing two sets of experiments on powdered aragonite snail shells heated in air and pure nitrogen atmospheres. The results show that during heating temperature below 400 °C, the intra-crystalline organic matter could inhibit the phase transformation due to the shielding effect of organic matter, while during heating temperature above 400 °C, the decomposition of the intra-crystalline organic matter increases the kinetic rate of the phase transformation. The activation energy for the phase transformation was determined to be +150.5 kJ·mol−1, lower than that of +247 kJ·mol−1 obtained for abiogenic polycrystalline aragonite and close to that of +158 to +163 kJ·mol−1 for abiogenic single crystal aragonite. This is attributed to the larger lattice parameter c in biogenic aragonite and the increasing porosity of calcium carbonate with the decomposition of organic matter. In a nitrogen atmosphere, the deviation of δ13C and δ18O between the heated sample (the mixture of aragonite and calcite) and the original shell aragonite (Δ13CM-A and Δ18OM-A) is negligible during heating temperature below 400 °C; however, Δ13CM-A (from −0.43‰ to 0.00‰) and Δ18OM-A (from −1.54‰ to −0.02‰) are significant during heating temperature above 400 °C. The temperature-dependence of Δ13CM-A in biogenic carbonates is much more significant than that in abiogenic carbonates, which clearly reflects the occurrence of isotopic exchange between the intra-crystalline organic matter and the carbonates. In air, the heating-induced calcium carbonate is depleted in δ13C by up to −0.48‰ and in δ18O by up to −3.46‰, compared with the original samples, which is mainly caused by isotopic exchange with the external carbon dioxide in air and the intra-crystalline organic matter, with the latter being the dominant source. Our results demonstrate that the decomposition of the intra-crystalline organic matter increases the kinetic rate and induces carbon and oxygen isotope fractionation during SPT from aragonite to calcite. This study provides direct evidence for evaluating the preservation of carbon and oxygen isotopes of biogenic calcium carbonate shells which have undergone the SPT process in archaeological shell middens.

Geochemical evidence of microbial activity within ooids

AbstractOoid formation remains elusive despite their importance as palaeoclimatic indicators and important contributors to global carbonate budget. Based on stable isotopes, nutrient and elemental analyses on solid components and ooidal leachates, this study supports the notion of microbial involvement in the development of ooids from Great Bahama Bank. Carbon and nitrogen isotopic analyses on organic fractions identified geochemical signatures of microbial activity. The δ13C values for organic carbon in the bulk (−11·9 to −16·9‰); intercrystalline/intracrystalline (−11·9 to 16·7‰); and intracrystalline phases (−12·4 to −17·7‰) were similar and, except for the more enriched values of ooids from Butterfly Beach, were within the range of photosynthesisers. The δ15N values for the bulk (+0·5 to −0·2‰); intercrystalline/intracrystalline (−0·3‰ to −0·7‰) and intracrystalline organic matter (−0·3 to −1·7‰) showed a narrow range consistent with nitrogen fixation. While positive δ15N and δ18O values of the leached from the ooids provided evidence of denitrification, the carbonate associated sulphate δ34SCAS of the bulk sediments (+19·2 to +19·6‰) and δ34S of the leachates (+16·6 to +18·3‰) provided weak indication of sulphate reduction, suggesting either that high concentrations of isotopically enriched S are overriding bio‐signatures of sulphate reduction or that microbes are preferentially using as an electron acceptor. In contrast, the elevated sulphate concentrations of the leachates suggest the occurrence of microbial sulphide oxidation within ooids. The high Mg/Ca of the leachates and scanning electron microscope analyses provide putative evidence of amorphous calcium carbonate and a formative role in CaCO3 precipitation. Together, these findings indicate that a redox dependent microbial consortium may influence CaCO3 precipitation in the form of ooid accretion, cementation and micritization. It is also inferred that ooid deposits are not suitable indicators of palaeoclimate because ooids are affected throughout their life by a complex chain of abiotic and biological processes which can lead to large geochemical offsets.

Metagenomic and stable isotopic analyses of modern freshwater microbialites in Cuatro Ciénegas, Mexico

Ancient biologically mediated sedimentary carbonate deposits, including stromatolites and other microbialites, provide insight into environmental conditions on early Earth. The primary limitation to interpreting these records is our lack of understanding regarding microbial processes and the preservation of geochemical signatures in contemporary microbialite systems. Using a combination of metagenomic sequencing and isotopic analyses, this study describes the identity, metabolic potential and chemical processes of microbial communities from living microbialites from Cuatro Ciénegas, Mexico. Metagenomic sequencing revealed a diverse, redox-dependent microbial community associated with the microbialites. The microbialite community is distinct from other marine and freshwater microbial communities, and demonstrates extensive environmental adaptation. The microbialite metagenomes contain a large number of genes involved in the production of exopolymeric substances and the formation of biofilms, creating a complex, spatially structured environment. In addition to the spatial complexity of the biofilm, microbial activity is tightly controlled by sensory and regulatory systems, which allow for coordination of autotrophic and heterotrophic processes. Isotopic measurements of the intracrystalline organic matter demonstrate the importance of heterotrophic respiration of photoautotrophic biomass in the precipitation of calcium carbonate. The genomic and stable isotopic data presented here significantly enhance our evolving knowledge of contemporary biomineralization processes, and are directly applicable to studies of ancient microbialites.

Organic matter diagenesis in shallow water carbonate sediments

Muddy carbonate deposits near the Dry Tortugas, Florida, are characterized by high organic carbon remineralization rates. However, approximately half of the total sedimentary organic matter potentially supporting remineralization is occluded in CaCO 3 minerals (intracrystalline). While a portion of nonintracrystalline organic matter appears to cycle rapidly, intracrystalline organic matter has an approximately constant concentration with depth, suggesting that as long as its protective mineral matrix is intact, it is not readily remineralized. Organic matter in excess of intracrystalline organic matter that is preserved may have a variety of mineral associations (e.g., intercrystalline, adsorbed or detrital). In surface sediment, aspartic acid contributed ∼22 mole % and ∼50 mole % to nonintracrystalline and intracrystalline pools, respectively. In deeper sediment (1.6–1.7m), the composition of hydrolyzable amino acids in both pools was similar (aspartic acid ∼40 mole %). Like amino acids, intracrystalline and nonintracrystalline fatty acids have different compositions in surface sediments, but are indistinguishable at depth. These data suggest that preserved organic matter in the nonintracrystalline pool is stabilized by its interactions with CaCO 3. Neutral lipids are present in very low abundances in the intracrystalline pool and are extensively degraded in both the intracrystalline and nonintracrystalline pools, suggesting that mineral interactions do not protect these compounds from degradation. The presence of chlorophyll- a, but absence of phytol, in the intracrystalline lipid pool demonstrates that chloropigments are present only in the nonintracrystalline pool. Sedimentary chloropigments decrease with depth at similar rates in Dry Tortugas sediments as found in alumino-silicate sediments from the Long Island Sound, suggesting that chloropigment degradation is largely unaffected by mineral interactions. Overall, however, inclusion and protection of organic matter by biominerals is a major pathway for organic matter preservation in this low-organic carbon, biomineral-rich regime.

Preservation of organic matter in mound-forming coral skeletons

This study demonstrates that intracrystalline organic matter in coral skeletons is well preserved over century timescales. The extent of preservation of organic matter in coral skeletons was investigated by measuring total organic carbon (TOC), total hydrolyzable amino acid (THAA), chloropigment, and lipid concentrations in 0–300 year old annual growth bands from Montastraea annularis (Florida Keys) and Porites lutea (Red Sea). Organic matter intrinsic to the calcium carbonate mineral (intracrystalline) was analyzed separately from total skeletal organic matter. The Red Sea coral had less TOC (0.02–0.04 wt%) than the Florida Keys coral (0.04–0.11 wt%), but a higher percent of intracrystalline organic matter in all annual bands measured. Carbon in the form of THAA, most likely from mineral-precipitating proteins, contributed 30–45% of the TOC in both corals. Carbon in lipids represented about 3% of the TOC in the coral skeletons. Chlorophyll-a and b were present in annual bands where endolithic algae were present, but these compounds were minor contributors to TOC. The distribution of specific organic compounds showed that organic matter was well preserved throughout the time period sampled in both the total and intracrystalline pools. Variations in THAA were not correlated with TOC over time, suggesting that organic matter that is involved in biomineralization, like amino acids, may be deposited in response to different environmental factors than are other components of skeletal organic matter. Differences in the quantity and composition of organic matter between the two corals investigated here were assessed using principal components analysis and suggest that location, species and skeletal structure may all influence organic matter content and possibly the degree of physical protection of organic matter by the coral skeleton. Further, our study suggests that intracrystalline organic matter may be better protected from diagenesis than non-intracrystalline organic matter and may therefore be a more reliable source of organic matter for paleoceanographic studies than total skeletal organic matter.