Chapter 5

Map Showing the Distribution of Total Organic Carbon in Long Island Sound

Lawrence J. Poppe1, Harley J., Knebel1, Zofia J. Mlodzinska2, and Barbara A. Seekins1
Table of Contents
Distribution of Total Organic Carbon
Figure Captions
Digital Data and Metadata
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The Coastal and Marine Geology Program of the U.S. Geological Survey has produced detailed geologic maps of the sea floor in Long Island Sound, a major U.S. east-coast estuary surrounded by the most densely populated region of the United States.  These studies build upon cooperative research with the State of Connecticut Department of Environmental Protection, that was initiated in 1982.  The current phase of the program emphasizes studies of sediment attributes, processes that control sediment distribution, nearshore environmental concerns, and the relationship of benthic communities to sea-floor geology.  The map presented here, which shows the distribution of total organic carbon (TOC), serves as a base map for subsequent sedimentological, geochemical and biological observations because precise information on environmental setting is important for selection of sampling sites and for accurate interpretation of point measurements.

Although sedimentary organic matter is usually only a quantitatively minor component of marine sediments, it affects many biologic, chemical, and geologic processes and, ultimately, the character of the sediments themselves.  Analyses of TOC are commonly conducted as a measure of the total organic material, however, previous studies have not outlined the regional distribution of TOC within the Sound.


Figure 1 - TOC Sample Locations Surficial sediments (0-2 cm below the sediment-water interface) were sampled at 580 locations within the Sound aboard the research vessels Asterias, John Dempsey, and Seaward Explorer (Fig. 1). These samples were placed in sealed containers aboard ship, and frozen for later analysis.  In the laboratory, these samples were thawed and homogenized.  A 0.5-g split was removed; large animals and shell fragments were eliminated during sub-sampling.  The samples were dried at 60oC, ground to a fine (<62 µm) homogeneous powder,  placed in a desiccator containing concentrated HCL, and allowed to digest for 24-48 hours to remove the carbonates.  This vapor-phase acidification converted the calcium carbonate in the sample to water vapor, CO2, and calcium chloride (Zimmermann and others, 1992).  After digestion, the samples were disaggregated, re-dried at 60oC, and stored in a desiccator until analysis.  Analysis was performed using a Perkin Elmer 2400 CHN Elemental Analyzer.  About 40% of the CHN analyses were standards, conducted to calibrate the instrument and check precision.  Precision was always better than one standard deviation. All TOC data were salt corrected.

The TOC data from this project (Poppe and others, 1998d) were combined with information from previous studies to produce a map and interpretation (Poppe and others, in press).  The preliminary map was manually contoured on paper and photographed.  The resultant negative was scanned to create a TIF image file that was brought into ArcInfo. This image file was geo-referenced using a projected tic coverage and the "register" and "rectify" commands.  Conversion of the geo-referenced image to a grid was performed to Figure 2 - Sidescan coveragegenerate a Universal Transverse Mercator (UTM) projection. Digitizing of the TOC contours was performed on-screen in ArcView. This vector file was merged with a coastline file to form a polygon shapefile and the polygons were tagged with the TOC weight percentages as attributes.  This shapefile was converted to an ARCInfo coverage using the "shapearc", "clean", and "regionpoly" commands.   Bathymetry, backscatter data from continuous and regional sidescan-sonar surveys (Fig. 2), and bottom videocamera data were used to extrapolate between stations.  Units on the TOC maps represent predominant concentrations; small-scale heterogeneity is common.  All contacts are inferred because the transitions between the various map units are gradational and lateral changes in TOC content are seldom abrupt.


Figure 3 - Map showing TOC distributionThe map showing regional TOC concentrations contoured in weight percent of the surficial sediments in Long Island Sound is presented in Figure 3.  To open a georeferenced display of this theme in ESRI's ArcView program make sure the application is loaded on your computer.  Users should go to the lisound directory located on the top level of this CD-ROM and double click on the lisound.apr project file.  The individual ArcView shapefiles may also be opened directly with any Arc application (e.g. ArcInfo, ArcExplorer) and can also be found on the data page.  Further detailed information can be found on the ArcView Project File page.

The highest TOC values exceed 3% and occur north of Hempstead Harbor  in the western Sound; the lowest values are less than 0.1% and occur along the north shore of Long Island in the eastern Sound (Poppe and others, in press).  TOC concentrations in Long Island Sound are generally elevated relative to open-marine environments, but similar to TOC concentrations in the bottom sediments of most other U.S. east-coast estuaries (Gorsline, 1963; Froelich and others, 1971; Folger, 1972;  Poppe and others, 1990).

Our TOC results generally agree with other previously reported concentrations in the Sound.  For western Long Island Sound, Hathaway (1971) and Reid (1982) reported average TOC values of 1.79% and 2.77%, respectively. In the central Sound, Krishnaswami and others (1984) found average TOC values of 1.44%.

Figure 4 - Plot of TOC/FinesThe TOC concentrations in Long Island Sound vary inversely with grain size (Fig. 4).  Coarser-grained sediments, such as gravelly sediments and sands, typically contain less organic carbon than finer grained sediments, such as clayey silts.  TOC concentrations average 1.91% in clayey silts from the Sound, but only 0.37% in the sands. This inverse relationship has been observed elsewhere along the United States east coast (Hulsemann, 1967; Froelich and others, 1971; Emery and Uchupi, 1972; Maciolek and others, 1986; Poppe and others, 1990),

The inverse correlation between the amount of organic carbon and sediment texture is dependent on the fine-grained nature of the organic carbon, the adsorption of organic particles onto the charged surfaces of clay minerals, and the grain-surface area available for adsorption (Froelich and others, 1971; Mayer, 1994). Because of the relatively shallow nature of the Sound and because the finer-grained sediments tend to accumulate in lower energy environments, the depth or redox state of the overlying water column do not appear to be limiting factors in the distribution of TOC.

Figure 5 - TOC/LongitudeThe data from this study (Fig. 5) and earlier work (Hathaway, 1971; Reid, 1982) suggest that TOC concentrations generally increase westward within the Sound.  For example, TOC concentrations in clayey silts and sand off Stratford Point average only 1.76% and 0.80%, respectively, but average 2.11% and 0.94%, respectively, off the Norwalk Islands (Poppe and others, 1996).  Similarly, TOC concentrations from all samples analyzed in the western Sound average 1.46%, but only 0.61% in the eastern Sound.  This westward increase in TOC in the sediments is probably related to higher nutrient inputs, to seasonal stratification of the water column, and to the east-to-west progression of sedimentary environments.  Higher nutrient inputs result in a high production rate of organic matter, whereas seasonal stratification promotes hypoxia in the bottom waters, which increases preservation by limiting macro- and microbiologic scavenging (Stein, 1990; Long Island Sound Study, 1994). Lower energy environments in the central and western Sound allow the deposition of fine-grained sediment, which contains more organic matter (Knebel and others, 1999; Knebel and Poppe, in press; Poppe and others, in press).

Data collected seasonally during 1995 as part of this study suggest a decrease in the amount of organic matter in the surficial sediments between spring and late summer.  TOC concentrations average 1.73% in the samples collected during April, but average only 1.42% in samples collected at the same locations during August.  This seasonal variability in the TOC, which has been noted by earlier studies along the United States mid-Atlantic slope and rise (Maciolek and others, 1986), is probably related to increased oxidation and macro-biologic reworking of the organic matter during the late spring and early summer.

Figure 6 - Bottom photoSedimentary organic matter in the marine environment is primarily derived from phytoplankton and, to a lesser degree, from continental sources (Fig. 6).  Because marine and land plants contain different amounts of carbon and nitrogen, molar elemental carbon/nitrogen (C/N) ratios can be used as a rough means of differentiating between algal and terrigenous organic matter (Premuzic and others, 1982; Meyers, 1994).  Aquatic (marine and lacustrine) algae typically have atomic C/N ratios of less than ten, whereas vascular land plants have C/N ratios greater than 20.  This difference arises from the absence of cellulose in algae and its abundance in vascular plant material (Meyers, 1994).  The C/N molar elemental ratios from sediments near the Stratford shoal complex average 10.775 (Poppe and others, 1996) and suggest that marine algae are the primary source of sedimentary organic matter in this area.  Although similar C/N ratios are present in the sediments from most of the southern and central parts of the Sound off Norwalk, those from near the northern shore are much higher.  The high ratios from this shoreward area commonly exceed 20 and suggest a more terrigenous source for the sedimentary organic matter.


Some of the figures and the text presented herein were previously published in the Journal of Coastal Research (Poppe and others, in press) and are now released in digital form with this journal's permission. We thank M. Moffett and Sarah Pratt, the analysts who performed the sample preparation; P. Simpson, M. Peterle, and D. Simpson, who crewed the Connecticut Department of Environmental Protection research vessel John Dempsey; and D. Olmsted, who captained the Woods Hole Oceanographic Institution research vessel Asterias. ArcView-formatted shape files of the sediment and TOC maps are available at the Woods Hole Field Center of the Coastal and Marine Geology Program, U.S. Geological Survey.


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Figure 1.  Index map showing the TOC sample stations (solid circles). The raw data are presented in Poppe and others (1998d).

Figure 2.   Index map showing the locations of continuous-coverage (hatched areas; Poppe and others, 1997, 1998a, 1998b, 1998c, 1999a, 1999b; Twichell and others, 1997, 1998) and reconnaissance (dashed lines; Knebel and others, 1999) sidescan sonar surveys used to extrapolate the textural data.

Figure 3.  Map showing the distribution of TOC in the sediments of Long Island Sound. Contours are in weight percent; block diagrams explain map units.

Figure 4.  Plot showing the general relationship between increasing TOC percentages and decreasing grain size.

Figure 5.  Plot showing the westward increase in TOC concentrations in Long Island Sound.

Figure 6.   Bottom photograph from near the mouth of the Connecticut River. Note the coarse terrigenous organic debris collecting in the troughs of current ripples.

1 U.S. Geological Survey, Coastal and Marine Geology Team, Woods Hole, MA 02543
2 Woods Hole Oceanographic Institution, Woods Hole, MA 02543 [an error occurred while processing this directive]