Mangrove Lake Research Articles

Historical deposition of trace metals in a marine sapropel from Mangrove Lake, Bermuda with emphasis on mercury, lead, and their isotopic composition

Sapropel is an organic-rich sediment formed under conditions that can result in sequestration of trace metals. Here, we determined the concentration of total mercury (Hg), lead (Pb), and 16 other metals (Al, Ba, Ca, Cd, Cr, Cs, Cu, Fe, Li, Mn, Ni, Sr, Tl, U, V, Zn) in a sapropel core from Mangrove Lake, Bermuda that dated back nearly two millennia. The purpose was to assess historical patterns of metal deposition on this geographically remote island in the North Atlantic Ocean. Two sediment cores were collected from Mangrove Lake using a modified piston corer. Cores were age-dated with a Bayesian statistical age-depth model using a multiproxy approach relying on 210Pb and radiocarbon dates. Total Hg was determined following US EPA Method 7473 using a direct mercury analyzer. Other metals were determined by sector field ICP-MS following a microwave-assisted strong-acid digestion. Stable isotope measurements of Hg and Pb were used to better track sources of these contaminants. Sapropel Hg concentrations were low (~ 50 ng g−1) from about 1000 AD to 1600 AD, followed by a slow rise in concentration until a high point of 209 ng g−1 in the early nineteenth century. Hg levels then returned to ~ 70 ng g−1, still elevated above baseline levels, before a final sharp rise in the mid-1900s to 430 ng g−1. Pb, Zn, Cu, Cr, and Ba had similar patterns, with Pb isotope ratios showing early natural fluctuations followed by the greater influence of pollution sources. Mercury stable isotope data also show the influence of anthropogenic sources during the 1800s and suggest a mix of atmospheric, terrestrial, and possibly marine-derived Hg inputs to the lake. The vertical distribution of elements and isotopes reveals changes in deposition through time associated with pollution from industrialization, and, possibly, volcanic activity, seawater intrusion, intense hurricane events, and local pollution from ship and fortress building. Overall, this study demonstrates that organic-rich sapropel provides a good historical record of metal contamination, and that Bermuda, despite its remote location, had metal deposition profiles much like the records of other sediment cores from around the world.

A new approach for molecular characterisation of sediments with Fourier transform ion cyclotron resonance mass spectrometry: Extraction optimisation

Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) can begin to tease apart the molecular character of sedimentary organic matter (SOM). We therefore tested five different solvents (aqueous base, CHCl3, MeOH, pyridine and water) for their ability to extract a representative fraction from two organic rich lacustrine sediments, Mangrove Lake, Bermuda (MLB) and Mud Lake, Florida (MLF). Following comparison using liquid state nuclear magnetic resonance spectroscopy (NMR) and negative ion mode electrospray ionization mass spectrometry (FTICRMS) we found that pyridine was the optimal solvent, extracting a more diverse (10–100× greater integration for carbonyl, amide and amine groups) and a larger number of peaks on average (1375–1450 vs. 380–1450). Comparison of the pyridine extracts between MLB, MLF and two organic poor sediments from the Mississippi River Delta and Bayou Grande (Pensacola, FL) showed that only 4.9% of the molecular formulae were common to all four and that unique formulae made up the highest proportion of the assignments. The use of pyridine for extracting immature (Holocene) SOM for FTICRMS analysis can therefore be widely applied to immature sediments and produce representative spectra.

Mass-dependent sulfur isotope fractionation during reoxidative sulfur cycling: A case study from Mangrove Lake, Bermuda

The multiple sulfur isotope composition of porewater sulfate from the anoxic marine sapropel of Mangrove Lake, Bermuda was measured in order to establish how multiple sulfur isotopes are fractionated during reoxidative sulfur cycling. The porewater-sulfate δ34S and Δ33S dataset exhibits the distinct isotopic signatures of microbial sulfate reduction and sulfur reoxidation. We reproduced the measurements with a simple diagenetic model that yielded fractionation factors for net sulfate removal of between −29.2‰ and −32.5‰. A new approach to isotopic modeling of the sulfate profiles, informed by the chemistry of sulfur intermediate compounds in Mangrove Lake, reveals that sulfate reduction produces a relatively small intrinsic fractionation and that an active reoxidative sulfur cycle increases the fractionation of the measured values. Based on the model results, the reoxidative cycle of Mangrove Lake appears to include sulfide oxidation to elemental sulfur followed by the disproportionation of the elemental sulfur to sulfate and sulfide. This model also indicates that the reoxidative sulfur cycle of Mangrove Lake turns over from 50 to 80% of the sulfide produced by microbial sulfate reduction. The Mangrove Lake case study shows how sulfur isotope fractionations can be separated into three different “domains” in Δ33S–δ34S space based on their ability to resolve reductive and reoxidative sulfur transformations. The first domain that differentiates reductive and reoxidative sulfur cycling is well illustrated by previous studies and requires 34S–32S fractionations more negative than ≈−70‰, beyond the fractionation limit of microbial sulfate reduction at earth surface temperatures. The second domain that distinguishes reductive and reoxidative processes is between 34S–32S fractionations of −40‰ and 0‰, where the 33S–32S fractionations of sulfate reduction and reoxidation are significantly different. In the remaining domain (between 34S–32S fractionations −70‰ and −40‰), the similarity of the multiple sulfur isotope signals from microbial sulfate reduction and disproportionation means that the two processes cannot be discriminated from each other.

Deep danger: intra-specific predation risk influences habitat use and aggregation formation of juvenile lemon sharks Negaprion brevirostris

Non-consumptive or risk effects imposed by predators can influence prey behaviour over different spatio-temporal scales. Prey vulnerability to predation can also be dependent on abiotic conditions, such as tidal height. We conducted direct field observations of juvenile lemon sharks Negaprion brevirostris in a tidally influenced mangrove-inlet. We also used acoustic tracking to determine the movement patterns of juvenile lemon sharks and their predators (sub-adult lemon sharks) across the tidal cycle. Results showed that greater numbers of juvenile lemon sharks used the mangrove-inlet for longer time periods at deeper and warmer high tide depths. This coincided with an increased presence of potential predators (sub-adult lemon sharks) in the surrounding areas. Furthermore, in accordance with body-size dependent anti-predatory investment, smaller juvenile lemon sharks visited the mangrove inlet more often, spent longer there and left latest on average. Our acoustic tracking data also revealed a tidally-influenced pattern, with both juvenile and sub-adult lemon sharks detected at locations inshore over the high tide and offshore during the low tide. We concluded that the mangrove lake served as a 'refuge' for juvenile lemon sharks over the high tide, providing safe habitat when inshore areas become accessible to large predators, such as sub-adult lemon sharks. We suggest that these decisions are updated through ontogeny and also with daily fluctuations in abiotic factors, such as water depth. This study provides evidence for how intra-specific predatorprey interactions in a top predator species influence juvenile habitat selection, with potential implications for population structure and regulation.

Alkyl amides in two organic-rich anoxic sediments: A possible new abiotic route for N sequestration

Fixed nitrogen (N) plays an integral role in global cycling; while most is recycled to refuel primary productivity, a small fraction escapes to be preserved and stabilised in sediments. Despite decades of research, the functionality and reactivity of this sequestered organic N has been poorly understood. This study was undertaken to shed light on this problem by characterising the bulk sediment organic matter using nuclear magnetic resonance (NMR), followed by molecular level analysis using ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT–ICR–MS). We studied two organic-rich anaerobic sediments, one from a freshwater system and another from a marine paralic basin. Mangrove Lake, Bermuda (marine) and Mud Lake, Florida (freshwater) have been shown in past studies to contain high levels of N-containing organic matter. Our resulting multidimensional NMR data suggests the existence of a new type of amide functionality in both these lake sediments, and we investigated this further using FT–ICR–MS and gas chromatography mass spectrometry (GC–MS). FT–ICR–MS confirmed the existence of homologous series of CHNO containing compounds, whose structures are verified using GC–MS as alkyl amides. Model reactions involving naturally occurring esters and ammonia suggest the source of alkyl amides to be amidation of esters with sedimentary ammonia derived from anaerobic degradation of organic matter. This expands upon previous hypotheses for preservation of amide containing compounds that call upon biological/abiological protection of proteins and peptides or the formation of refractory nitrogenous adducts.

Reproductive seasonal periodicity of the endemic Bermuda killifish Fundulus bermudae in an anchialine pond

A total of 245 individuals from a population of the endemic Bermuda killifish Fundulus bermudae in an isolated anchialine pond (Mangrove Lake) were trapped between November 2004 and November 2005. Laboratory analysis of gonad development allowed determination of the seasonal reproductive cycles of both females and males. A distinctive annual pattern was evident, with female and male gonadal cycles synchronous throughout the study period. The results indicate that the F. bermudae males and females began their spawning season in February, but reached primary peaks in May and June respectively. Gonadal indices abruptly fell after June and continued to fall at a steady rate until September, marking the end of the spawning season. Gonad recrudescence, as indicated by basal gonad indices, occurred in September and lasted throughout the autumn and early winter months.

Formation of Heteroaromatic Nitrogen after Prolonged Humification of Vascular Plant Remains as Revealed by Nuclear Magnetic Resonance Spectroscopy

In the search for the mechanisms involved in the immobilization of organic nitrogen in humified remains of vascular plants, the efforts of the present investigation were directed toward the examination of the transformation of nitrogenous compounds during the peat and coal stage by means of solid-state nuclear magnetic resonance (NMR) spectroscopy. While accumulation of heteroaromatic-N is not detected in most of the studied peat layers, a clear shoulder in the chemical shift region of pyrrole- or indole-N is observed in the solid-state 15N NMR spectrum of material from the deepest (and thus oldest) peat layer underlying the sapropel from Mangrove Lake, Bermuda (10000 years). This points to the assumption that transformation of nitrogen occurs between an advanced stage of peatification and an early stage of coalification. The observed sudden alteration in nitrogen functionality indicates that continuous accumulation of newly synthesized or selectively preserved biogenic structures is not responsible for the presence of heteroaromatic-N in these fossilized deposits. It seems rather likely that abiotic conditions, occurring during advanced sediment maturation, have an effect on the observed N transformation. With increasing coalification, pyrrole-type-N becomes the dominant form in the macromolecular coal network. Pyridine-type-N was only detected in a coal of anthracite rank.

Formation of Heteroaromatic Nitrogen after Prolonged Humification of Vascular Plant Remains as Revealed by Nuclear Magnetic Resonance Spectroscopy

In the search for the mechanisms involved in the immobilization of organic nitrogen in humified remains of vascular plants, the efforts of the present investigation were directed toward the examination of the transformation of nitrogenous compounds during the peat and coal stage by means of solid-state nuclear magnetic resonance (NMR) spectroscopy. While accumulation of heteroaromatic-N is not detected in most of the studied peat layers, a clear shoulder in the chemical shift region of pyrrole- or indole-N is observed in the solid-state 15N NMR spectrum of material from the deepest (and thus oldest) peat layer underlying the sapropel from Mangrove Lake, Bermuda (10000 years). This points to the assumption that transformation of nitrogen occurs between an advanced stage of peatification and an early stage of coalification. The observed sudden alteration in nitrogen functionality indicates that continuous accumulation of newly synthesized or selectively preserved biogenic structures is not responsible for the presence of heteroaromatic-N in these fossilized deposits. It seems rather likely that abiotic conditions, occurring during advanced sediment maturation, have an effect on the observed N transformation. With increasing coalification, pyrrole-type-N becomes the dominant form in the macromolecular coal network. Pyridine-type-N was only detected in a coal of anthracite rank.

A Py–GC–MS and NMR spectroscopy study of organic nitrogen in Mangrove Lake sediments

Solid-state CP–MAS 13C and 15N nuclear magnetic resonance (NMR) spectroscopy and flash pyrolysis–gas chromatography–mass spectrometry (Py-GC–MS) techniques have been used to characterize the organic-rich sedimentary deposit from Mangrove Lake, Bermuda. The chemical nature of organic nitrogen in sediment samples from different depths was studied as it relates to the early diagenesis of proteinaceous material in the reducing marine environment. Although the majority of proteins were lost during early diagenesis, part of the proteinaceous material became resistant to proteolysis and chemical hydrolysis. The proteinaceous material is proposed to have survived early diagenesis via encapsulation by refractory, aliphatic organic matter.

Sequestration of organic nitrogen in the sapropel from Mangrove Lake, Bermuda

Humic isolates from different layers of an organic-matter-rich algal sapropel from Mangrove Lake, Bermuda, were subjected to solid-state 15N nuclear magnetic resonance (NMR) spectroscopy in order to reveal chemical structural interrelationships that allow delineation of diagenetic pathways. Amide-N, most likely from peptide-like material, was found to represent the main organic nitrogen form in all humic fractions at all depths of the sapropel. Amides were also identified in the residues from 6 M HCl hydrolysis of the alkaline insoluble extract (formerly called humin). These results indicate that some peptide-like material resists both alkaline and acid hydrolysis. We suggest that the chemical, and also the biological recalcitrance of peptide-like material, is related to encapsulation into the paraffinic network of algaenans. Being hydrophobic in nature, algaenan biomacromolecules may represent an effective means of protection against the attack of hydrophilic enzymes and aqueous extraction solutions.

Preservation of protein in marine systems: Hydrophobic and other noncovalent associations as major stabilizing forces

—The fate of proteins during early diagenesis was investigated in environments with low mineral content to assess preservation mechanisms other than mineral sorption. Preservation was examined in anoxic, organic-rich sediments of Mangrove Lake, a marine environment located in Bermuda, and for particulate material generated during oxic decay of diatoms. N-phenacylthiazolium bromide (PTB) treatment tested the hypothesis that proteins may undergo modification reactions with glucose to form advanced-glycation end products (AGEs). A small but significant release (additional 14%) of proteins was observed after PTB treatment in surficial sediments, indicating that some aggregations can proceed through an α-dicarbonyl intermediate of the AGE pathway. Size-exclusion high-pressure liquid chromatography with protein fluorescence, absorbance, and evaporative light-scattering detector measurements under native (phosphate or bicarbonate buffers) and denaturing (guanidine · HCl, urea, or acetonitrile) conditions point to the importance of hydrophobic and other noncovalent interactions in the stabilization of proteinaceous material in the environment. Soluble aggregates of substantial, relative molecular mass ( M r ≳ 10 6) appear to be formed early in the diagenetic sequence. The preferential preservation of very high M r, multisubunit phytoplankton proteins in sediments suggests that such aggregations confer resistance to degradation. Alternatively, some of the proteinaceous material may represent that fraction of organic matter that is highly prone to aggregations. Extended incubations (18 h; 37°C) with trypsin and proteinase-K showed that much of the aggregates that could be extracted are receptive to proteolytic cleavage. Buffer-, surfactant-, and NaOH-extractable aggregates comprised most of the acid-hydrolyzable proteinaceous material in detritus and surficial sediments but <35% in 9.7-m-deep sediments, suggesting additional mechanisms for preservation might be in operation. The results are direct evidence for the preservation of peptide linkages in sediments as old as 4000 yr and for noncovalent associations (hydrophobic interaction, hydrogen bonding) of protein as important mechanisms in long-term preservation.

Taxonomic diversity, origin, and conservation status of Bermuda killifishes (Fundulus) based on mitochondrial cytochrome b phylogenies

Sequence variation in the mitochondrialCytochrome b (Cytb) gene was assayed infour of six extant Bermuda killifishpopulations, representing two endemic species,to test taxonomic and phylogenetic hypothesesand reconstruct colonization history. Twodivergent (4.6%) haplotypes were detected; oneis identical to the Georgia 2c haplotype ofF. heteroclitus and is fixed in threeeastern populations: Lover's Lake (F.relictus), Mangrove Lake (F. bermudae),and Walsingham Pond. The second is fixed andrestricted to a western population, Evan'sPond. Likelihood and parsimony cytochrome btrees recognize a Bermuda Fundulus / F. heteroclitus clade in which the Evan's Pondhaplotype is basal. Phylogenies and haplotypedivergence indicate at least two Bermudacolonizations, the more recent involvingtransfer of the Georgia 2c haplotype. Enforcing Bermuda killifish monophyly, aspredicted from a single colonization event,does not increase tree length significantly;i.e., the trees also are consistent with asingle colonization. However, divergencebetween the Evan's Pond haplotype and the F. bermudae / F. relictus / Georgia 2chaplotype (4.6%) far exceeds the maximumdivergence among all F. heteroclitus,F. bermudae, and F. heteroclitushaplotypes (1.2%) and argues for independentcolonizations. Alternatively, recentintroduction of F. heteroclitus couldaccount for occurrence of the GA2c haplotype inBermuda but does not explain the presence ofthe genetically divergent Evans' Pondhaplotype. Cytb sequences areuninformative of the taxonomic status ofBermuda endemics, F. bermudae and F.relictus, but support recognition of theEvan's Pond population as an evolutionarilysignificant unit within the F. heteroclitus group.

The Early Diagenetic Formation of Organic Sulfur in the Sediments of Mangrove Lake, Bermuda

Due to a low mineral content, the sapropelic sediments depositing in Mangrove Lake, Bermuda, provide an excellent opportunity to explore for possible additions of sulfur to organic matter during the early stages of diagenesis. We evaluated early diagenetic organic sulfur transformations by monitoring the concentrations and stable isotopic compositions of a number of inorganic and organic sulfur pools, thereby accounting for all of the sulfur in the sediments. We have identified and quantified the following sulfur pools: porewater sulfate, porewater sulfide, elemental sulfur, pyrite sulfur, hydrolyzable organic sulfur (HYOS), chromium-reducible organic sulfur (CROS), and nonchromium-reducible organic sulfur (Non-CROS). Of the organic sulfur pools, the Non-CROS pool is by far the largest, followed by CROS, and finally HYOS. By 60 cm depth these pools contribute, respectively, to 85, 7.9, and 3.6% of the total solid phase sulfur. The HYOS pool is probably of biological origin and shows no interaction with the sulfur compounds produced during diagenesis. By contrast, CROS is produced, most likely, from the diagenetic addition of polysulfides to functionalized lipids in the upper, H 2S-poor, elemental sulfur-rich, region of the sediment. A portion of this sulfur pool is unstable and decomposes on contact with the H 2S-rich porewaters. The portion of CROS that remains in the sulfidic waters appears to readily exchange sulfur isotopes with H 2S. While some of the Non-CROS pool is of biological origin, some is also formed by the diagenetic addition of sulfur to organic compounds in the upper H 2S-poor region of the sediment. By contrast with CROS, Non-CROS is not diagenetically active in the H 2S-rich porewaters. Overall, somewhere between 27 and 53 % of the organic sulfur buried in Mangrove Lake sediments is of diagenetic origin, with the remaining organic sulfur derived from biosynthesis. We extrapolate our Mangrove Lake results and calculate that in typical coastal marine sediments between 11 and 29 μmol g −1 of organic sulfur will form during early diagenesis, of which 2–5 μmol g −1 will be chromium reducible.

13C and 15N NMR spectroscopic investigation on the formation of fossil algal residues

13C and 15N NMR spectroscopy was applied to modern (a mixed algal culture, its algaenan and its compost), ancient (algal derived sediments from Mangrove Lake, Bermuda) and fossilized algal residues (Torbanite, Green River Shale) for the purpose of establishing the forms of nitrogen algal remains and evaluating their long-term stabilities. The results indicate that proteinaceous material can resist microbial degradation in sediments as old as 4000 yr, possibly in refractory biopolymers, by encapsulation into their macromolecular network. The 15N NMR spectra of the Torbanite and the Green River Shale show a relative enrichment of heterocyclic-N, which may derive from selective preservation of biogenic heterocyclic compounds or by rearrangement of peptide chains during diagenesis.

Light and scanning electron microscopic studies on Trichodina labrisomi n.sp. from Labrisomas nuchipinnis (Osteichthyes: Labrisomidae) from Mangrove Lake, Bermuda

Trichodina labrisomi n.sp. from the gills of the hairy blenny, Labrisomas nuchipinnis (Quoy and Gaimard), in Mangrove Lake, Bermuda, is described using light microscopy and scanning electron microscopy (SEM). Light microscopy reveals that the dimensions and morphology of T. labrisomi are similar to those given for only 13 previously described species of Trichodina (T. acuta Lom, 1961; T. baicalensis Dogiel, 1957; T. cottomephori Stein, 1979; T. dalli Zhukov, 1964; Trichodina domerguei domerguei Lom and Shtein 1966; T. elegans Stein, 1979; T. fultoni Davis, 1947; T. jadranica Haider, 1964; T. jadranica noblei Lom, 1970; T. partidisci Haider, 1964; T. reticulata Hirschmann et Partsch, 1955; T. tenuidens Faure-Fremiet, 1943; and T. tenuiformis Stein, 1979). However, compared with these similar forms, T. labrisomi n.sp. is distinguished by the dimensions of its body and denticulate ring and (or) the appearance of its silver-impregnated adhesive disc and denticles. Features of T. labrisomi examined by SEM include the body shape, pellicle, aboral pellicular pores, border membrane, aboral ciliary complex, and adoral ciliature, which are described and compared with those of other trichodinids studied using SEM. SEM also revealed that bacilli were distributed circumequatorially over the oral surface of T. labrisomi.

Chemical composition of pseudo-phlobaphinite precursors: implications for the presence of aliphatic biopolymers in vitrinite from coal

Pseudo-phlobaphinite cell inclusions in a buried stem of angiospermous wood of Rhizophora mangle from Mangrove Lake, Bermuda have been investigated petrographically and chemically. Optical and scanning electron microscopy reveal that the light wood from inner sections of the stem consists predominantly of cell walls with few cell inclusions, whereas the dark wood from outer portion of the stem is composed of both cell walls and cell inclusions which are believed to represent pseudo-phlobaphinite precursors. 13C NMR spectroscopy reveals the presence of lignin and tannin structures within the cell walls. The wood also displays resonances suggestive of aliphatic components. The NMR results are further supplemented and confirmed by flash pyrolysis gas chromatography and mass spectrometry analyses. The pyrogram of the cell-wall dominated light wood is characterized by peaks related to both syringyl and guaiacyl units. The major pyrolysis products from the dark wood, however, are a homologous series of long-chain n-alkalines, n-alk-l-enes and α, ω- alkadienes extending from chain length C 10 to C 30. These aliphatic components arise from the material associated with the pseudo-phlobaphinite cell inclusions. Py-gc/ms analysis of a humic acid extracted from the sediments surrounding the studied wood consists almost exclusively of a homologous series of straight-chain alkanes, alkenes and alkadienes up to C 30. The striking resemblance between pyrograms of the pseudo-phlobaphinite precursor and humic acid suggests that the possible mechanism for the formation of the pseudo-phlobaphinite precursor migration and deposition of humic acid into cell lumina.

Early diagenesis in a marine sapropel, Mangrove Lake, Bermuda

The pore waters of a marine sapropel in Mangrove Lake (Bermuda) have been analyzed for a suite of diagenetically sensitive species, i.e. pH, SO42−, ∑H2S, Ca2+, ∑CO2, ∑NH4, ∑P04i Fe2+, and Mn2+. We have also measured a 210Pb depth profile (suggesting a burial velocity of 0.94 cm yr−1) and experimentally determined the rate constant for sulfate reduction (0.062 yr−1).Waves and turbulence have mixed sediment and pore waters to a depth of 15 cm. However, both stoichiometric and transport‐reaction models argue that molecular diffusion dominates solute transport beneath this mixed zone and that the pore waters are open to diffusion despite rapid burial. Diagenetic modeling shows further that diffusion accounts for 95% of the SO42−, ∑H2S, and ∑CO2 fluxes below the mixed layer. Comparison of model results with sulfur burial fluxes indicates that at least 94% of the generated ∑H2S diffuses out of the sediments. The pore‐water ∑CO2 is also within 5% of that predicted by our models. The : N ratio of the decaying organic matter is calculated to be between 3.86 and 6.87, which is less than half that obtained by analysis of the solids.The carbonate chemistry of the pore waters suggests that they are nearly saturated with respect to aragonite in the mixed zone, even though this mineral has not been identified in the solids. Precipitation in the mixed zone is confirmed by comparing the observed dissolved Ca profile with that expected for conservative behavior.

Dissolved organic matter in anoxic pore waters from Mangrove Lake, Bermuda

Dissolved organic matter and dissolved inorganic chemical species in anoxic pore water from Mangrove Lake, Bermuda sediments were studied to evaluate the role of pore water in the early diagenesis of organic matter. Dissolved sulphate, titration alkalinity, phosphate, and ammonia concentration versus depth profiles were typical of many nearshore clastic sediments and indicated sulphate reduction in the upper 100 cm of sediment. The dissolved organic matter in the pore water was made up predominantly of large molecules, was concentrated from large quantities of pore water by using ultrafiltration and was extensively tudied by using elemental and stable carbon isotope analysis and high-resolution, solid state 13C nuclear magnetic resonance and infrared spectroscopy. The results indicate that this material has a predominantly polysaccharide-like structure and in addition contains a large amount of oxygen-containing functional groups ( e.g., carboxyl groups). The 13C nulcear magnetic resonance spectra of the high-molecular-weight dissolved organic matter resemble those of the organic matter in the surface sediments of Mangrove Lake. We propose that this high-molecular-weight organic matter in pore waters represents the partially degraded, labile organic components of the sedimentary organic matter and that pore waters serve as a conduit for removal of these labile organic components from the sediments. The more refractory components are, thus, selectively preserved in the sediments as humic substances (primarily humin).

Carbon isotope fractionation of sapropelic organic matter during early diagenesis

Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of 13C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of 13C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ 13C values of the organic matter is observed as a result of early diagenesis.

Selective preservation and origin of petroleum-forming aquatic kerogen

Studies of a marine algal sapropel from Mangrove Lake, Bermuda, by 13C NMR and stable carbon isotopic methods show that precursors of aquatic kerogen (insoluble, macromolecular, paraffinic humic substances) are primary components of algae and possibly associated bacteria and that these substances survive microbial decomposition and are selectively preserved during early diagenesis.