Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis☆
Introduction
The fate of organic matter during early diagenesis is an important concern for many oceanographic and limnological studies (e.g., Meyers and Eadie 1993, Dean et al 1994, Bernasconi et al 1997, Ostrom et al 1998, Sachs and Repeta 1999, Hedges et al 2001. Estimates of organic carbon and nitrogen fluxes to marine and lake sediments are essential for balancing the global carbon budget, as well as for quantifying the importance of organic matter burial as one of the nitrogen removal mechanisms in eutrophic lakes. Marine and lacustrine sediment-trap studies reveal that only 1 to 35% of the organic carbon synthesized in the photic zone reaches the sediment surface Eadie et al 1984, Bloesch and Uehlinger 1990, Bernasconi et al 1997, Hernes et al 2001. Further mineralization during early diagenesis leads to burial of only an estimated 0.1% of the global net marine primary production (e.g., Berner, 1989).
Organic matter decomposition is mediated by a variety of aerobic and anaerobic microbial processes, which can progressively modify the bulk composition of the organic substrate because different fractions of organic matter degrade at different rates Skopintsev 1981, Henrichs and Doyle 1986, Hedges et al 1988, Harvey et al 1995, Meyers and Eadie 1993. In addition, a contribution from in situ bacterial biomass may also change the bulk biogeochemical signal. Previous laboratory experiments have shown that 5 to 25% of the degraded algal carbon is converted to bacterial carbon (Harvey et al., 1995), whereby the bacterial matter itself is subsequently modified or destroyed (Meyers and Ishiwatari, 1993). Data compiled by Harvey et al. (1995) indicate that substantial variations of bacterial biomass over the course of the incubations significantly contributed to the observed changes of the relative abundance of carbohydrates, proteins, and lipids in the residual organic matter pool.
In summary, multiple processes acting together result in sedimentary organic matter with a markedly different distribution of biochemical species with respect to the original biogenic material. Hence, it is reasonable to expect that these processes could also affect the primary carbon and nitrogen isotope signals produced in the photic zones of aquatic environments.
The carbon and nitrogen isotope composition of organic matter has been widely used to trace biogeochemical cycling in marine and lacustrine environments (e.g., Bernasconi et al 1997, Ostrom et al 1997, Hodell and Schelske 1998. The δ13COM has proven to be a proxy indicator of paleoproductivity and atmospheric pCO2 levels (e.g., Hollander and McKenzie 1991, Schelske and Hodell 1991, Fontugne and Calvert 1992, Brenner et al 1999, whereas nitrogen isotopic ratios have been used as a recorder of changes in the degree of nitrate utilization (e.g., Calvert et al 1992, Francois et al 1992, Altabet and Francois 1994, Holmes et al 1997, Teranes and Bernasconi 2000, denitrification (e.g., Altabet et al 1995, Ganeshram et al 1995, Altabet et al 1999, Emmer and Thunell 2000, Ganeshram et al 2000 and N2-fixation (e.g., Haug et al., 1998). The isotopic composition of sinking or sedimented organic matter may be altered during oxidation in the water column and in the sediments, possibly obscuring the primary signal. While some studies have shown that selective loss of specific fractions of the total organic carbon, which have different composition than the bulk, can create diagenetic shifts in δ13C (Benner et al., 1987), other studies indicate that the δ13C of organic matter is resistent to isotopic alteration during water-column or postburial diagenesis Meyers and Eadie 1993, Schelske and Hodell 1995.
In many marine and lacustrine sediment trap studies, microbial degradation of phytoplankton has been associated with an increase in the δ15N value of the residual organic matter (up to 6‰) as a result of discrimination against 15N during metabolic reactions Saino and Hattori 1980, Saino and Hattori 1987, Altabet 1998, Fry et al 1991, Schaefer and Ittekkot 1993, Altabet and Francois 1994, Ostrom et al 1997, Sachs and Repeta 1999. An increase in sedimentary δ15N with depth in sediments of the eastern subtropical Atlantic has recently been related to organic matter loss during early diagenesis (Freudenthal et al., 2001). In contrast, only minor changes or depletions in the 15N content of settling particles have been observed in other studies Saino and Hattori 1987, Libes and Deuser 1988, Altabet et al 1991, Meyers and Eadie 1993, Altabet et al 1999. Also, contrasting reports of oxygenation effects on the magnitude and direction of N-isotope shifts have been reported. Sachs and Repeta (1999) suggested that, under anoxic conditions, N-isotopic alteration during organic matter degradation is minimal and the severity of the 15N-enrichment during organic matter decay is proportional to bottom-water oxygen concentrations. Libes and Deuser (1988) reported a 15N-enrichment under oxic and a 15N-depletion under anoxic conditions and attributed this difference to the type and degree of microbial activity.
Experiments conducted to study the changes of the C- and N-isotopic composition of organic matter with microbial degradation have produced contrasting results Wada et al 1980, Zieman et al 1984, Holmes et al 1999. Therefore, more laboratory studies are necessary to understand the mechanisms that cause isotope effects during organic matter degradation. Here, we report on a series of incubation experiments simulating phytoplankton decay under oxic and anoxic conditions. In addition, we evaluate the impact of microbial degradation on the C- and N-isotope composition of bulk sedimentary organic matter by comparing a long-term sediment trap data set with sediment core isotopic data from the southern basin of Lake Lugano (Switzerland). The southern basin of Lake Lugano is eutrophic, with a mean annual primary productivity (in 1990 to 2000) of ∼340 g C m−2 yr−1. As a result of thermal stratification and concomitant water-column stagnation, anaerobic conditions prevail in near-bottom waters between May and December/January. Due to the high organic carbon content, oxygen penetration in the sediments during oxic conditions is probably minor as indicated by well-preserved annual laminations.
The purpose of this study is to obtain information on the isotopic alteration of organic matter during early sedimentary diagenesis. A detailed understanding of isotope effects during decomposition will enhance our ability to use organic matter stable isotope abundances from sedimentary records as proxy indicators for past changes of environmental conditions. In addition, it may allow us to use C and N stable isotope ratios as tracers for organic matter transformation processes and, therefore, help to assess the origin of diagenetically altered material.
Section snippets
Experimental system and material
To simulate the microbial degradation of algae during early diagenesis at the water-sediment interface, lacustrine biomass was incubated in 5-L bottles for 111 d in three experiments with different redox conditions/electron acceptor concentrations. Algae, primarily diatom cells, were collected in July 2000 from the photic zone of Lake Lugano, using a 20 μm-plankton net. Three separate aliquots were incubated in bottles on an orbital shaker (60 rpm) in darkness under constant (25°C) temperature.
Decomposition kinetics
POC and PON concentrations decreased rapidly within the first few days of the experiments (Fig. 1) indicating rapid cell death and onset of degradation. After approximately 20 d of oxic incubation, the POC and PON concentrations were more or less stable at 13±1% of the initial particulate organic matter (POM) concentration. The fraction of POM remaining after 111 d, which degrades either at a very slow rate or is in fact undegradable, was about twice as high in the anoxic as in the oxic
Decomposition kinetics
The decay constants for reactive organic matter degradation found in this study are high compared to most degradation rates for phytoplankton reported in earlier studies Foree and McCarty 1970, Jewell 1971, Emerson and Hedges 1988, Harvey et al 1995, Kristensen and Holmer 2001. The large variability of reported decay rates, ranging over three orders of magnitude, is partially due to the differing algal material and incubation temperatures used. In addition, the use of double- vs.
Conclusion
The carbon and nitrogen isotope ratio of organic matter in sediments results from several complex processes including biosynthesis in the photic zone, organic matter degradation and bacterial growth in the water column and in the sediment, and the input from allochthonous sources. Oxygenation conditions have a direct effect on the preservation of the organic matter, with enhanced preservation under anoxic conditions. The isotopic composition of bulk organic matter undergoes significant
Acknowledgements
We thank M. Simona and M. Veronesi for providing sediment trap material as well as sediment flux and primary productivity data. H. Paul, N. Andersen, J. Lehmann, and three anonymous reviewers made constructive comments on earlier versions of the manuscript. This study was supported by Swiss National Science Foundation Grant NF 21-5232.97.
Associate editor: J. I. Hedges
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†Present address: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08540, USA.