Isotopic variations of dissolved inorganic carbon (original) (raw)
Related papers
Marine Chemistry, 1997
In order to investigate sources and turnover rates of dissolved organic matter from Chesapeake Bay and Galveston Bay, colloidal organic matter (COM) was isolated using cross-flow ultrafiltration and subsequently characterized for its elemental (C, N, and S) and isotopic (13C and 14C) composition. Distributions of dissolved organic carbon (DOC) in Chesapeake Bay showed a non-systematic variation, while in Galveston Bay, a non-conservative behavior of DOC with source inputs in the low salinity region was observed. Results of size fractionation of total organic carbon (TOC) revealed that, on average. particulate organic carbon (POC) comprised -12% and 39% of the TOC pool in Galveston Bay and the Chesapeake Bay, respectively.
Limnology and Oceanography, 2006
Seasonal and interannual variation of the stable carbon (C) and nitrogen (N) isotope composition of suspended particulate organic matter (POM) was measured in the brackish and tidal freshwater regions of the Mattaponi River, a tributary of the York River, Virginia, and a pristine end member on a continuum of anthropogenic modification within Chesapeake Bay. A principal components analysis indicated that seasonal variation was related to physical mixing and river discharge. Freshwater POM had high C : N (.12), depleted particulate organic carbon isotopic composition (d 13 C POC , 226% to 230%), and depleted particulate nitrogen isotopic composition (d 15 N PN , 2-10%) compared to brackish water POM, which had lower C : N and enriched d 13 C POC (224% to 227%) and d 15 N PN (7-15%). During high discharge events, the d 13 C POC was enriched, the d 15 N PN depleted, and the C : N high relative to low discharge periods, indicating a large contribution from terrestrial-derived material. Within tidal freshwater, POM was comprised of humic-rich sediment, vascular plant matter, and phytoplankton produced in situ. Nonconservative mixing behavior was observed. Endogenously produced phytoplankton increased POC concentrations in tidal freshwater and oligohaline portions during base flows. Where estuarine and riverine POM mixed, the isotopic composition of the POM was homogenized, blurring source-specific characters observed upriver and thereby emphasizing the need to characterize the freshwater end member of estuaries carefully in order to identify POM sources.
Stable carbon and nitrogen isotope biogeochemistry in the Delaware estuary
Limnology and Oceanography, 1988
Seasonal variability in stable carbon (S'XZ) and nitrogen (b15N) isotope ratios was observed in suspended particulate matter of the Delaware estuary. Two major pools of organic matter were found in the estuary-phytoplankton growing in situ and a mixture of planktonic and terrestrial detritus. In general, the 6°C and 615N of suspended particulate matter reflected planktonic dominance. With the background chemical and physical information available for the estuary, it is evident that biogeochemical processes influence isotopic distributions in the estuary to a greater extent than does physical mixing. During spring, we postulate that isotopic fractionation of ammonium assimilated at concentrations >20 PM resulted in more negative 615N values for organic matter fixed by phytoplankton. As algal growth proceeded, the 615N of seston reached a maximum (+ 1 SY&) because phytoplankton were using a pool of NH,+ enriched in 15N as a result of previous fractionation during assimilation. Similarly, maximal 813C values were related to high rates of primary productivity associated with algal growth. Decreased isotopic fractionation occurred at high rates of production, implying that diffusion of CO, across the cell membrane became increasingly rate limiting.
Organic carbon in estuarine sediments can have many different sources. Terrestrial, riverine, estuarine and marine C pools may all contribute to and influence the organic C (C org ) inventory of the estuarine sediments and the differing stable isotope signatures of the sources are reflected in the sediment's overall 13 C content. Ecological interpretations of sedimentary isotope data may, however, be limited by the fact the total C org inventory of a sediment may not be an accurate representation of the fraction that is labile and being actively turned over by the sedimentary community. To gain a better understanding of sedimentary C org dynamics in estuaries and the relationship between the sedimentary C pool and the C org undergoing mineralisation, we studied three components of an estuarine system: (1) the sedimentary C org inventory on a transect from the mouth to the upper end of the estuary, (2) temporal changes of sedimentary C org at one station throughout a year, and (3) the d 13 C of respired CO 2 compared to the d 13 C of available source material and sedimentary C org in a novel application of methods developed for soil science. Our experiments demonstrated that material of marine origin dominated the studied estuary. At the time-series station, material of marine origin dominated the sedimentary C org throughout the 1-yr study period. d 13 C values of CO 2 released from the sediment differed significantly from the sedimentary C org inventory at all study sites, but also clearly reflected differences between the main sections of the estuary. These results suggest that d 13 C measurements of respired CO 2 are promising as a tool to advance our understanding of C cycling in estuaries, and highlight that the sedimentary C org pool alone may not be a satisfactory indicator of OM utilisation in estuarine sediments.
Estuarine, Coastal and Shelf Science, 2003
In this study, we examine the distribution and carbon stable isotope signature of dissolved organic carbon (DOC) and humic substances (HS) along a salinity gradient in the Altamaha and Satilla River estuaries. The maximum DOC concentrations in the Altamaha and Satilla were 10 and 29 mg C l ÿ1 , respectively, though concentrations were similar at the mouth of both estuaries. There was a decrease in HS content of DOC from 50 to 80% at the head of the estuaries to 10% at salinities higher than 30&. The d 13 C DOC varied between ÿ25.5 and ÿ19& and between ÿ27 and ÿ21& in the Altamaha and Satilla estuaries, respectively. The tendency towards more depleted d 13 C DOC in the Satilla, especially in the lower salinity portion of this estuary, suggests greater terrestrial inputs in the Satilla than in the Altamaha. Seasonal fluctuations were observed in the form of increased (two to three times) range in DOC concentration, heavier d 13 C DOC and increased proportion of estuarine-marine-derived DOC (average enrichment of d 13 C DOC from +1 to +2) during low river flow (July-October). The d 13 C HS in both rivers showed a similar trend, but was consistently more depleted than DOC, with an average range from ÿ28 to ÿ24.5&. This suggests that HS have larger proportions of terrestrial components (a maximum of >60% at the mouth of the estuary) than DOC. The less depleted d 13 C values of DOC in comparison with HS indicate a different source for the non-humic (non-HS) component of DOC (range in d 13 C non-HS, ÿ22 to ÿ16&). That source could either be the decomposition of detrital material derived from saltmarsh environments or microalgalderived DOC of estuarine or marine origins.
Dissolved and Particulate Organic Carbon in Chesapeake Bay
Estuaries, 1998
We measured dissolved and particulate organic carbon (DOC and POC) in samples collected along 13 transects of the salinity gradient of Chesapeake Bay. Biverine DOC and POC end-members averaged 232 + 19 pM and 151 + 53 p.M, respectively, and coastal DOC and POC end-members averaged 172 + 19 pM and 43 f 6 pM, respectively. Within the chlorophyll maximmn, POC accumulated to concentrations 50-150 pM above those expected from conservative mixing and it was significantly correlated with chlorophyll a, hulicating phytoplankton origin. POC accumulated primarily in bottom waters in spring, and primarily in surface waters in summer. Net DOC accumulation (60-120 pM) was observed within and downstream of the chlorophyll maxbnum, prhnarily during spring and summer in both surface and bottom waters, and it also appeared to be derived from phytoplankton. In the turbidity maximmn, there were also net decreases in chlorophyll a (-3 pg 1-l to-22 pg 1-l) and POC concentrations (-2 pM to-89 PM) and transient DOC increases (9-38 pM), primarily in summer. These occurred as freshwater plankton blooms mixed with turbid, low salinity seawater, and we attribute the observed POC and DOC changes to lysis and sedimentation of freshwater plankton. DOC accumulation in both regions of Chesapeake Bay was estimated to be greater than atmospheric or terrestrial organic carbon inputs and was equivalent to-10% of estuarine prhuary production.
Geo-marine Letters, 2009
Three sediment cores were collected off the Mississippi River delta on the Louisiana Shelf at sites that are variably influenced by recurring, summer-time water-column hypoxia and fluvial loadings. The cores, with established chronology, were analyzed for their respective carbon, nitrogen, and sulfur elemental and isotopic composition to examine variable organic matter inputs, and to assess the sediment record for possible evidence of hypoxic events. Sediment from site MRJ03-3, which is located close to the Mississippi Canyon and generally not influenced by summer-time hypoxia, is typical of marine sediment in that it contains mostly marine algae and fine-grained material from the erosion of terrestrial C4 plants. Sediment from site MRJ03-2, located closer to the mouth of the Mississippi River and at the periphery of the hypoxic zone (annual recurrence of summer-time hypoxia >50%), is similar in composition to core MRJ03-3, but exhibits more isotopic and elemental variability down-core, suggesting that this site is more directly influenced by river discharge. Site MRJ03-5 is located in an area of recurring hypoxia (annual recurrence >75%), and is isotopically and elementally distinct from the other two cores. The carbon and nitrogen isotopic composition of this core prior to 1960 is similar to average particulate organic matter from the lower Mississippi River, and approaches the composition of C3 plants. This site likely receives a greater input of local terrestrial organic matter to the sediment. After 1960 and to the present, a gradual shift to higher values of δ13C and δ15N and lower C:N ratios suggests that algal input to these shelf sediments increased as a result of increased productivity and hypoxia. The values of C:S and δ34S reflect site-specific processes that may be influenced by the higher likelihood of recurring seasonal hypoxia. In particular, the temporal variations in the C:S and δ34S down-core are likely caused by changes in the rate of sulfate reduction, and hence the degree of hypoxia in the overlying water column. Based principally on the down-core C:N and C:S ratios and δ13C and δ34S profiles, sites MRJ03-3 and MRJ03-2 generally reflect more marine organic matter inputs, while site MRJ03-5 appears to be more influenced by terrestrial deposition.
Limnology and oceanography, 2000
Particulate organic carbon (POC) is an important component in the carbon cycle of estuarine systems. Many studies have utilized carbon isotopic composition (␦ 13 C-POC) to estimate the origin of POC. Most of the early studies were restricted to areas of minimum terrestrial inputs of POC, whereas later studies used multiple stable isotope analysis. In our study, we assess the proportion of POC from C 3 and C 4 plants and microalgae in two terrestrially affected, contrasting estuaries of the southeastern U.S.A.: the Altamaha (Piedmont) and the Satilla (blackwater) Rivers. Our analysis was based on a mass balance equation, taking into consideration the estimated amounts of microalgae and C 3 and C 4 plant material. Analysis of ␦ 13 C of extracted chlorophyll a (Chl a) was used to estimate the isotopic signature of microalgae to constrain the three endmember mass balance. The maximum concentration of POC fluctuated between 9 and 11 mg C L Ϫ1 , coinciding with the period of maximum riverine flow. The POC represented 50-70% of the total organic carbon. However, large fluctuations were observed, suggesting important tidal-and wind-driven resuspension. The ␦ 13 C-POC ranged from Ϫ21 to Ϫ28‰, being more depleted at lower salinities. This indicates rapid dilution of terrestrially derived material with estuarine-or marine-derived material. The Chl a concentration was maximal during summer in upstream stations of the Altamaha estuary, while the Satilla generally showed a midestuary maximum. The ␦ 13 C of Chl a ranged from Ϫ20.7 to Ϫ31.43‰, indicating isotopically depleted inputs of microalgal-derived material (MaDM). Our results suggest that MaDM is the dominant component of POC in the Altamaha River during summer, whereas the C 3 component is dominant during periods of lower productivity and high flushing. Although MaDM is significant in the Satilla estuary, most of the POC consists of material derived from C 3 and C 4 plants.
Marine Chemistry, 2002
In this study, we examined the temporal and spatial variability of terrestrial organic carbon sources in lower Mississippi River and Louisiana shelf sediments (during 11 cruises over a 22-month period) to further understand the sorting dynamics and selective transport of vascular plant materials within the primary dispersal system of the river. Bulk d 13 C values in lower river sediments ranged from À21.90xto À24.64x(mean = À23.20 F 1.09x ), these values were generally more depleted than those found in shelf sediments (À22.5xto À21.2x ). The L 8 (L 8 = sum of vanillyl, syringyl and cinnamyl phenols produced from the oxidation of 100 mg of organic carbon) values in the lower river ranged from 0.71 to 3.74 (mean = 1.78F0.23). While there was no significant relationship between L 8 and river discharge ( p > 0.05), the highest value occurred during peak discharge in April 1999-which corresponded to the highest observed C/N value of 17.41. The L 8 values on the shelf ranged from 0.68 to 1.36 (mean = 0.54 F 0.30) and were significantly lower ( p < 0.05) than the average value for lower river sediments. The range of S/V (syringyl/vanillyl) and C/V (cinnamyl/vanillyl) ratios on the shelf, 0.11 to 0.95 and 0.01 to 0.08, respectively, were similar to that found in the lower river. These low C/V ratios are indicative a mixture of woody and non-woody carbon sources. Recent work by Goni et al. [Nature 389 (1997) 275; Geochim. Cosmochim. Acta 62 (1998) 3055], which did not include sampling transects within the primary dispersal system of the Mississippi River, showed a non-woody vascular plant signature on the Louisiana shelf. This suggests that riverine-derived woody tissues preferentially settle out of the water column, in the lower river and inner shelf, prior to the selective dispersal of C 3 versus C 4 non-woody materials in other regions the shelf and slope. This works further demonstrates the importance of differential settlement of particles, sampling location within the dispersal system, and river discharge, when examining biogeochemical cycles in river-dominated margins. D