Previous   |    Next

doi:10.2204/iodp.proc.308.208.2008

Results

Organic carbon, total nitrogen, and C/N ratios

TOC content was generally low throughout all sites, averaging 0.7 wt% with a maximum concentration of 2.2 wt% (Table T1; Fig. F1). Minimal downcore variations were observed in TOC of Brazos-Trinity Basin IV. TOC concentrations (~1.5 wt%) at Ursa Basin Sites U1322 and U1324 exhibited a systematic decrease in organic carbon to ~0.5 wt% at terminal depth. TN concentrations were largely uniform across all basins and sites at <0.2 wt%.

The remarkably low C/N values at Sites U1319, U1320, and U1324 (average = 4.7 tive of sedimentary organic matter derived from algal marine sources (Table T1). Substantially elevated C/N values at Ursa Basin Site U1322 (average C/N = 13.1) imply a more significant input from terrestrially derived organic matter.

The presence of inorganic nitrogen bound to the sediment matrix may explain the C/N values less than the Redfield ratio. The positive y-intercepts in plots of TOC versus TN indicate that a significant fraction of the bulk nitrogen pool is inorganic (Fig. F2). Estimates calculated from TOC/TN regressions suggest the bulk nitrogen content from Brazos-Trinity Basin IV is at least 70% to completely comprised of inorganic nitrogen (Table T2). The inorganic nitrogen fraction was also a significant contribution to Ursa Basin sedimentary nitrogen, ranging from 56% to 90% in Holes U1322B, U1322C, and U1322D and from 9% to 48% in Holes U1324B and U1324C (Table T2). It should be noted that the accuracy of these estimates is prone to a considerable amount of error given the scatter within the data (see correlation coefficients, Fig. F2). Whether observed differences in atomic ratios are driven by changes in the relative contributions of terrestrial or marine end-members or other processes such as preferential loss of nitrogenous compounds during early diagenesis or incorporation of bound nitrogen were further addressed through dual isotopic analysis of bulk organic carbon and total nitrogen contained in the sediments.

Comparison of C/N with organic carbon and total nitrogen isotopic compositions

The carbon isotopic composition of organic matter isolated from Brazos-Trinity Basin IV sediments is widely variable (from –27‰ to –20‰; Table T1) and approximates the range expected of respectively isotopically depleted or enriched terrestrial and marine end-members (Macko et al., 1984). The pronounced shift toward isotopically lighter values (by up to 6‰) observed in lithologic Unit V of cores from Holes U1319A and U1320A records a transition from marine to terrestrially derived organic matter within Brazos-Trinity Basin IV sediments (Fig. F1). In contrast, δ¹³C values of Ursa Basin TOC are more constrained (average δ¹³C = –25.7‰ gest significant input of terrestrial organic material. The proximity of Ursa Basin to the Mississippi River delta and the high supply of riverine sediment likely promote a near-continuous record of terrestrially sourced organic matter. Within both study locations, there is no clear relationship between isotopic excursions and the occurrence of turbidite deposits (Fig. F1).

C/N ratios plotted as a function of δ¹³C reveal that Sites U1319, U1320, and U1324 fall well below the expected mixing line for an admixture of marine and terrestrial end-members (Fig. F3). Low C/N values (<5) suggest a marine end-member and are inconsistent with the broad ranges observed in δ¹³C. Paired isotopic and elemental data from Site U1322 more closely fit the proposed mixing model between isotopically light terrigenous debris and isotopically heavy marine organic matter. Alternative explanations for shifts observed in δ¹³C and C/N include algal temperature-dependent carbon isotopic effects, diagenetic alteration, or substantial input from terrestrial C₄ vascular plants (δ¹³C ≈ –14‰) (Jasper and Gagosian, 1990). Most likely, the observed trends are a reflection of low organic matter accumulation in deepwater sediments of the Gulf of Mexico coupled with significant incorporation of inorganic nitrogen during early diagenesis, which would artificially drive low C/N values (Bouloubassi et al., 1999; Schubert and Calvert, 2001). Shipboard analyses indicated significant production of ammonium in excess of 4 mM within Brazos-Trinity Basin IV and Ursa Basin pore fluids. Comparison of TOC/TN plots across all sites suggests a significant incorporation of inorganic nitrogen to the bulk nitrogen content (Fig. F2). Averaged across all sites, the bound nitrogen fraction is ~75% (Table T2). The presence of bound nitrogen coupled with the shift in C/N to lower values may explain discrepancies in the interpretation of the origins of sedimentary organic matter inferred from molar and isotopic data.

Nitrogen isotopic compositions across all sites ranged from –2.7‰ to 8.2‰ (Table T1). Average δ¹⁵N values (3.7‰) are consistent with surficial marine sediments (Macko et al., 1984; Altabet and Francois, 1994). As with δ¹³C values, Site U1322 exhibited a narrower range in δ¹⁵N values (3.9‰ tive to the other sites. A comparison of C/N values relative to δ¹⁵N indicate Site U1322 sediments are more terrestrial in origin compared to inferred marine origins for Sites U1319, U1320, and U1324 (Fig. F4). Ammonium adsorbed to the sediment matrix may have shifted the δ¹⁵N signature to isotopically lighter values (Schubert and Calvert, 2001). This potentially mixed nitrogen signal could be apportioned by isolation techniques, which operationally define the organic nitrogen fraction (Schubert and Calvert, 2001), or by direct extraction of the organics.

Top of page   |    Previous   |    Next