versión On-line ISSN 0717-7178
Investig. mar. v.33 n.2 Valparaíso nov. 2005
Invest. Mar., Valparaíso, 33(2): 183-194, 2005
Early diagenesis and vertical distribution of organic carbon and total nitrogen in recent sediments from southern Chilean fjords (Boca del Guafo to Pulluche Channel)
Diagénesis temprana y distribución vertical de carbono orgánico y nitrógeno total en sedimentos recientes de los fiordos del sur de Chile (Boca del Guafo hasta canal Pulluche)
Nora Rojas & Nelson Silva
Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso Casilla 1020, Valparaíso, Chile
ABSTRACT. Eleven surface sediment cores were taken with a Rumohr corer during the oceanographic cruise Cimar 8 Fiordos (July 2002; between the Corcovado Gulf and Pulluche Channel). These cores were used to determine the vertical distribution of organic carbon, total nitrogen, and their atomic ratio (C/N) for use as a diagenesis indicator. The grains observed were mostly clay-silt in four of the sediment cores and more heterogeneous in the other seven cores. Organic carbon and total nitrogen concentrations were higher in the four clay-silt cores than in the sandy cores, although in terms of their vertical distribution, both concentrations were relatively homogenous at most stations. Nevertheless, exponential decreases characteristic of first-order diagenetic degradation were observed in cores from three stations. The C:N ratio fluctuated between 7 and 10, indicating that the organic material in the sediments was mostly marine in origin. Values were lower at more oceanic stations and greater at more coastal stations; the contribution of terrigenous materials was greater at the latter. We inferred a slower break down of organic carbon as compared to total nitrogen from a steady-state first-order degradation kinetics model, that was applied to stations where both fractions had exponential vertical distributions. Remineralization percentages were between 23 and 34% (organic carbon) and 33 and 43% (total nitrogen) and accumulation percentages were between 77 and 66% (organic carbon) and 67 and 57% (total nitrogen).
Key words: sediments, grain size, carbon, nitrogen, C:N, diagenesis, estuaries, fjords, Chile.
RESUMEN. Durante el crucero oceanográfico Cimar 8 Fiordos (julio 2002), entre golfo Corcovado y canal Pulluche, con un Rumohr corer se obtuvo 11 testigos de sedimentos superficiales. Con éstos se determinó la distribución vertical de carbono orgánico, nitrógeno total y sus relaciones atómicas (C:N) para ser usados como indicadores de la presencia de procesos diagenéticos. En cuatro testigos o "cores", la textura se caracterizó por estar compuesta principalmente por sedimento limo-arcilloso y en los otros siete la composición de textura fue más heterogénea. Las muestras limo-arcillosas presentaron mayores concentraciones de C-org y N-tot, que aquellas más arenosas. En cuanto a la distribución vertical de C-org y N-tot, sus concentraciones fueron relativamente homogéneas en la mayoría de las estaciones. Sin embargo, tres estaciones presentaron disminuciones de tipo exponencial características de procesos de degradación diagenética de primer orden. Valores de la razón C:N, entre 7 a 10, indican, en una primera aproximación, que la procedencia de la materia orgánica del sedimento es mayormente de origen marino. Los menores valores se registraron hacia el océano y los mayores valores hacia el continente, donde el aporte fluvial y terrígeno, es mayor. La aplicación de un modelo de diagénesis de primer orden a estaciones con distribuciones verticales exponenciales de C-org y N-tot, permitió inferir que el C-org se degrada más lentamente que el N-tot, obteniéndose porcentajes de remineralización entre 23 y 34%, y 33 y 43% para el C-org y N-tot y de acumulación entre 77 y 66%, y 67 y 57%, respectivamente.
Palabras clave: sedimentos, textura, carbono, nitrógeno, C:N, diagénesis, fiordos, Chile.
Estuarine systems are areas that are subject to a large contribution of both autochthonous organic material from planktonic productivity and allochthonous particulate organic material transported by rivers and surface runoff. These coastal areas, including the estuarine ones, can be differentiated from oceanic areas by, amongst others, their high sedimentation rates (0.1-10 cm·year-1 v/s 0.1-10 cm·1000 year-1) (Berner, 1980). The gradual accumulation of sedimentary material, leaves the organic matter that is deposited on the sediment surface exposed to bacterial degradation in an oxidizing environment for long periods of time. Some authors suggest that processes such as adsorption into the sediments of material in the mineral phase, favors the preservation of compounds that, otherwise, might be easily degradable (Müller & Suess, 1979; Hedges & Keil, 1995).
During sedimentation in the water column, autochthonous and/or allochthonous particulate organic material undergoes intense degradation and only a small fraction reaches the surface sediments (Wakeham & Lee, 1993). Having reached the bottom, the sediment is again submitted to decomposition as a result of a series of diagenetic reactions, which can either release or affix elements and compounds between the interstitial water and the overlying water column (Berner, 1980; Klump & Martens, 1987; Farías et al., 1994, 1996). A variety of factors determine the quantity, vertical distribution, and chemical composition of the organic material found in the sediments. These factors include primary production, contributions of allochthonous material, adsorption of organic material in mineral phases, water column depth, sedimentation rates, bioperturbation, and the concentration of dissolved oxygen (Stumm & Morgan, 1981; Jorgensen et al., 1990; Cowie & Hedges, 1992; Hedges & Keil, 1995; Fütterer, 2000).
Studies of the vertical distribution of components like organic carbon (C-org), total nitrogen (N-tot), and their atomic ratio (C/N) could be used to indicate diagenesis (Wakeham, 2002). C-org and N-tot decrease with increasing depth of burial as organic matter is being remineralized.
Eleven surface sediment cores, taken between Boca del Guafo and Pulluche Channel, were analyzed in order to determine the vertical distribution of C-org and N-tot and to elucidate the presence of diagenetic processes and the intensity of their degradation based on a steady-state first-order degradation kinetics model.
MATERIALS AND METHODS
Leg one of the Cimar 8 Fiordos cruise was carried out between Boca del Guafo (43°47'S) and Pulluche Channel (45°49'S), from 5 to 20 July 2002, on board of the research vessel AGOR Vidal Gormaz.
Physical and chemical analysis
Sediment samples were taken from 11 stations with a Rumohr corer (70 cm long) (Fig. 1). The cores (max. length 50 cm) were stored vertically and frozen at -20°C until their analysis in the laboratory. The cores were cut into sections: 1 cm thick for the first three cm, 2 cm thick for the next 14 cm, and 3 cm thick from 17 cm to the core bottom. Each section was divided in two subsamples for physical (grain size) and chemical (carbon and nitrogen) analysis.
Sediment grain size (granulometry) was determined by wet sieving each section, separating the fractions according to the Udden Wentworth scale (F).
For the chemical analysis, each section was dried at 60°C in a vacuum oven and then ground to a very fine dust with an agate mortar and pestle. The analyses were done for C-org and N-tot. Total carbon was determined with a carbon elemental analyzer (LECO CR-12). C-org by burning in the LECO CR-12 a sample that had been pretreated with hydrochloric acid to eliminate the inorganic carbon. N-tot was determined by using the Kjeldahl technique indicated in Willard et al. (1956) and adapting it to a Micro-Kjeldahl device.
The equipment used in both, carbon and nitrogen analyses, was calibrated with soil standards with the carbon and nitrogen concentrations within the ranges of the samples. These standards are certified by the National Institute of Standards and Technology (NIST).
In the stations where the vertical C-org and N-tot distribution decreased exponentially with depth, a steady-state first-order degradation kinetics model was applied (Berner, 1980). This model separates sedimented organic material (Go) into two fractions: the labile or metabolizable fraction (Gm) that is degraded and the residual refractory fraction (G¥) that is buried, where:
According to the Berner model (1980), sedimentation and burial rates are assumed to be constant over time and in steady state.
The remineralization of labile organic material is estimated according to:
where: GZ is the concentration of C-org or N-tot (mg·g-1) at depth Z (cm); Gmo is the labile fraction that is deposited initially (mg·g-1); G¥ is the quantity of the residual refractory material (mg·g-1); k is a constant representing the rate of first-order decay (year-1); and w is the sedimentation rate (cm·year-1). The constant k was obtained from the slope of equation (2), which involves the sedimentation rate, in which decay is a function of time.
The recycled or degraded fraction is equal to the difference between the total concentration at the sediment surface, Go (mg·g-1), and the concentration at depth where the remaining organic material is residual or refractory, G¥(mg·g-1):
In a stationary state, the fractions Go, Gmo, and G¥ are constants and the remineralized and accumulated percentages are given by:
Distribution of physical and chemical sediment characteristics
Grain sizes in four cores were nearly homogenous, with clay-silt fractions over 80% (St. 8, 9, 17a and 53; Fig. 2). Four other cores were somewhat heterogeneous, with a preponderance (50-60%) of one fraction, usually very fine sand or clay-silt (St. 6, 14, 51 and 54; Fig. 2). The remaining three cores were highly heterogeneous, having more dispersed grain sizes and no single fraction over 50% (St. 11, 72, and 76; Fig. 2).
Vertical distribution values for C-org and N-tot were greatest (> 2.0% and > 0.3%, respectively) throughout the entire sediment column at stations 8 and 9, whereas the lowest values (< 0.5% and < 0.1%) throughout the column were found at stations 11, 14, and 72 (Fig. 3).
The greatest variations in the vertical distribution of the C:N ratio were observed at stations 14 and 76 (6.8-14.1); lesser variations were observed at stations 8 and 9 (7.1-8.6) (Fig. 4).
In general terms, the vertical C-org and N-tot distribution was almost homogenous in some cores (St. 14, 54) and abruptly varied in others (St. 51). In a reduced group, the vertical distribution of the C-org and N-tot concentration resembled an exponential distribution (St. 6, 9, and 53); because of this, a steady- state first-order degradation kinetics model was applied (Berner, 1980) in order to estimate the respective decay rates and remineralization and accumulation percentages. This model requires a sedimentation rate, which had to be estimated since the cores used were not dated. In order to estimate the sedimentation rate, the different values known for the area were considered: Rojas (2002) estimated sedimentation rates between 0.26 and 0.33 cm·year-1 for Aysén Fjord and 0.28 cm·year-1 for Costa Channel; Salamanca & Jara (2003) estimated 0.15 cm·year-1 for Cupquelán Fjord, between 0.29 and 0.36 cm·year-1 for Aysén Fjord, and 0.67 cm·year-1 for Quitralco Fjord; and Sepúlveda et al. (2005) estimated 0.25 cm·year-1 for Puyuhuapi Channel. Unfortunately, sedimentation rates were not available for oceanic channels (St. 53, King Channel), although these rates could differ from those of the interior channels.
Based on the previous sedimentation rates for Aysén Fjord and the Costa and Puyuguapi channels, and taking into account that these channels showed oceanographic characteristics similar to those of Moraleda Channel (Silva et al., 1998), an average sedimentation rate of 0.29 cm·year-1 was calculated as representative of all the inner channels. Since information was not available for areas similar to the oceanic station 53, this station was assigned the same sedimentation rate as the others. C-org and N-tot remineralization and accumulation percentages (Table 1) were estimated using this sedimentation rate and applying Berner's (1980) steady-state first-order degradation kinetic model.
Relationship between grain size and concentrations of C-org and N-tot
Stations 8, 9 and 53 are characterized by being made up of a high percentage of clay-silts (> 80%), which were the sediments with the greatest concentrations of C-org and N-tot (> 2% and > 0.2%) (Figs. 2 and 3). This well known characteristic (Hedges & Keil, 1995; Fütterer, 2000; Hedges, 2002; Wakeham, 2002) has also been observed in the Chilean fjord zone where the organic material content was higher in fine than in coarse sediments (Silva et al., 1998; Silva & Ortiz, 2002; Rojas & Silva, 2003).
This can be explained by the fact that fine, clay silt particles have larger surface areas than sand particles and tend to have associated negative charges. Therefore, upon entering the sea water, their surfaces are quickly coated with a layer of organic material. The ratio C-org/surface area is higher for finer than for coarser sediments, such as sand particles (Hedges & Keil, 1995; Hedges, 2002; Wakeham, 2002). Therefore, the finer sediments should have greater concentrations of organic material.
Station 17a is an exception to the above pattern, consisting largely of clay silt sediments but with intermediate concentrations of C-org and N-tot (ª1% and ª0.1%). This could be due to a greater inorganic fraction brought into the fjord's fine sediment by adjacent rivers (Aysén, Cuervo, and Cóndor). If the inorganic fine sediment fraction input is large and fast enough to reach the bottom quickly, it could increase the presence of very fine particles without necessarily increasing the sediment's organic content.
Silva et al. (1998) observed a similar situation in the more southern Quitralco and Cupquelán fjords, where sediments were 100% clay-silt and had less organic content (averaging 0.4% C-org and 0.04% N-tot) than did Station 17a. These authors explained the low concentration of organic matter by the huge presence of clay-silt sediments originating in the weathering of rocks from the Campo de Hielo Norte glacier. This material, which appears milky in the fjord waters, lowering the light penetration to a few centimeters, causes organic carbon to be "diluted" upon reaching the sediments with the results indicated above. The situation of Aysén Fjord is different because no large glaciers are found in the area and so no milky material is observed suspended in the fjord. Thus the contribution of fine inorganic material from the weathering of rocks in its hydrographic basin should be lower than Quitralco and Cupquelán fjords, resulting in less "dilution" of the organic components and, therefore, intermediate C-org and N-tot concentrations.
Stations 11, 14, 51, 54, 72, and 76 had more heterogeneous grain size, with sandy fractions predominating over clays and silts (Fig. 2). Thus, these stations had low concentrations of C-org and N-tot (< 1% and < 0.1%; Figs. 2 and 3).
Vertical distribution of C-org and N-tot
In general, vertical C-org and N-tot distributions did not show exponential type decay in the first 20-30 cm (Fig. 3). At times, the profiles of these variables were relatively homogenous, insinuating that the sediments may be affected by mixing from physical and/or biological processes (St. 14 and 54). This is possible because bacterial and animal activities are most intense and electron acceptors more energetic in the surface layer of the sediments (Aller, 1982). This uniform distribution could also be the result of rapid sedimentary deposition (i.e., high sedimentation rates), in which organic material is quickly moved below the more diagenetically active zone and, therefore, suffers less degradation. Moreover, rapid deposition quickly protects the sedimented organic material from contact with the main oxidizing agents found in the overlying sea water such as oxygen, nitrate, and sulfate (Hedges & Keil, 1995).
The situation at stations 11, 51, and 72 is more complex. The column presented nearly homogenous strata of C-org and N-tot in the upper sections, typical of bioperturbed activity. C-org and N-tot increased at depth along with clay-silts (Fig. 2). These changes in grain size and C-org and N-tot concentrations with depth can be related to different chemical and physical compositions of the sediments as opposed to those previously accumulated or with changes in the material's degradation.
At stations 6, 9 and 53, C-org and N-tot concentrations were similar to an exponential distribution with depth. This type of distribution allows us to assume scarce or null bioperturbation activity and diagenetic decomposition of the organic material with a first-order kinetic (Berner, 1980).
The C:N ratio for marine phytoplankton is 6.6, for fresh marine sediments is 7 to 10, and for terrestrial plants is over 20 (Deevy, 1973; Rullkötter, 2000). Because of this, it is possible to use the C:N ratio as a "proxy" for an initial inference as to the origin of the material that composes the sediments. The combined use of the C:N ration and the stable carbon isotope ratio (d13C), with higher values in marine (-15 to -19º/oo; Emerson & Hedges, 1988), than in terrestrial material (-19 to -22º/oo; Fontugne & Jouanneau, 1987), has also been used frequently to the same purpose (Meyers, 1994, 1997; Thornton & McManus, 1994; Müller, 2001; Ruiz-Fernández et al., 2002).
When using only the C:N ratio, it is important to keep in mind that, as the organic material falls to the bottom, the nitrogen-containing compounds are preferentially used relative to carbon-containing compounds (Burdige & Martens, 1988; Jorgensen, 1996; Wakeham, 2002). This can partially alter the relationship's indication as to the organic material's origin.
The C:N ratio observed in the study area (Fig. 4) fluctuated between 6 and 8 (station 72) and between 7 and 10 (stations 6, 8, 9, 11, 17a, 53 and 54). Stations 14, 51, and 76 had a few C:N values over 10, although in general, values remained between 7 and 10. Similar C:N values were observed by Silva & Prego (2002), throughout the entire Chilean fjord area, from Puerto Montt (41°S) to Cape Horn (56°S).
However, this southern Chilean fjord area has abundant vegetation with native forests, and an important part of the organic residues and terrigenous material is transported by rivers and runoff from the nearby coast (the Aysén River basin drainage provides an average annual flow of 5000 m3·s-1 and has an average annual rainfall of 3000 mm; Guzmán, 2004). Thus, allochthonous organic material is eventually deposited in the sediments along with autochthonous marine material.
Pinto & Bonert (2005), using the d13C measurements from sediment samples taken in Moraleda Channel and Aysén Fjord (Cimar 4 Fiordos cruise, September 1998), observed that Moraleda had high values (-15 to -17º/oo) characteristic of marine material, whereas the head of Aysén had low values (-22 to -25º/oo), which were associated with increased allochthonous content toward the head of the fjord. Stations 6, 11, 14, and 17a (Cimar 8 Fiordos), coincided in position with those of Cimar 4 Fiordos and, therefore, the sediment's chemical characteristics should be similar. In this area, the d13C of the surface sediments fluctuated between -15.9 and -21.7º/oo (Pinto & Bonert, 2005) and the C:N ratio fluctuated between 7 and 9 (Fig. 4).
Therefore, using the database of calculated C:N values from this study and the d13C observations of Pinto & Bonert (2005), it can be concluded that the surface sediments in the study area are mostly marine in origin with contributions from terrestrial materials that increase towards the continental extremes of the fjords.
The only station where the vertical distribution of the C:N ratio increased gradually with depth (from 7.1 to 8.6) throughout the column was station 53. This increase in the C:N ratio (Fig. 4) and decrease of C-org and N-tot (Fig. 3) was caused by the selective degradation of the nitrogen-containing compounds, as previously indicated by Klump & Martens (1987), Farías & Salamanca (1990); and Jorgensen et al. (1990), amongst others.
In the cores from stations 6, 9 and 53, the C-org and N-tot concentrations decreased practically exponentially with depth. This was attributed to diagenetic degradation and, therefore, the steady-state first-order degradation kinetics model proposed by Berner (1980) could be applied. Of these three cores, only the core from station 53 smelled of H2S, indicating anaerobic conditions in the sediments. Berner (1980) and Boudreau (1997) indicated that the exponential diagenetic profile tends to be more defined with anaerobic degradation than with aerobic degradation, which agrees with the observations of station 53.
The decay constants (k) or first-order kinetic decompositions for stations 6, 9 and 53 were estimated to be 0.02, 0.04 and 0.05 year-1 for C-org and 0.04, 0.05, and 0.06 year-1 for N-tot, respectively (Table 1). Their estimated half lives were 37, 18 and 15 years (C-org) and 20, 15 and 12 years (N-tot). These results indicate that, in general, C-org decomposes more slowly than N-tot in the sediment column.
The low remineralization and high accumulation percentages of C-org and N-tot calculated for stations 6, 9, and 53 (Table 1) could be explained by a rapid sedimentation rate that did not favor bacterial degradation of organic material, but rather its accumulation in the sediments.
The differences observed in these percentages indicate, furthermore, that the remineralization of N-tot is faster than that of C-org, so the buried organic material contains more C-org than N-tot. This is due to the fact that the nitrogen-containing compounds (e.g., proteins) are more labile than carbon compounds (e.g., fats) and therefore break down more rapidly (Burdige & Martens, 1988; Wakeham, 2002). Louchouarn et al. (1997) also showed, in the St. Lawrence Estuary (Canada), a diagenetic selectivity in the sediments, obtaining a k = 0.035 year-1 and a half-life of 20 years for C-org and a k = 0.054 year-1 and a half life of 13 years forN-tot. Although this selective remineralization between C-org and N-tot is quite common in sediments, it is not always the case. Louchouarn et al. (1997), in the same St. Lawrence Estuary, observed equal reactivity rates for C-org and N-tot at one station. The selective degradation and decay rates observed in this Chilean fjord area are similar to those of the Canadian fjords.
The authors wish to thank the Ministerio de Hacienda, Servicio Hidrográfico y Oceanográfico de la Armada (SHOA), and the Comité Oceanográfico Nacional (CONA) for the partial financing of project CONA-C8F 02-20. We are also grateful to project FONDEF 2-41 for the contribution of the instrument used for chemical analyses. We express our appreciation, moreover, to the captain, officers, and crew of the AGOR Vidal Gormaz, Dr. Laura Farías, Mr. Julio Sepúlveda and three anonymous reviewers for comments that helped to improve this paper. The late Mr. Reinaldo Rehhof and Mr. Francisco Gallardo helped with sampling and Ms. Paola Reinoso assisted in the sediment analyses.
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Corresponding author: Nelson Silva (email@example.com)Recibido: 7 abril 2005; Aceptado: 28 octubre 2005