RESULTS

Both sites are characterized by moderate to low contents of organic carbon and nitrogen (Fig. F2), due to the high degree of dilution with siliciclastic terrestrial material. At Site 1234, total organic carbon (TOC) ranges between 0.6 and 2.5 wt%, TN is between <0.1 and 0.3 wt% (Fig. F2A), and the TOC/TN (atomic) ratios (Fig. F2C) vary between 4 and 11, with an average of 8.3 ± 0.3. Both TOC and TN concentrations exhibit a maximum between 60 and 80 mcd. There is a slight decrease in TOC/TN downcore.

High pore water ammonium concentrations (~10 mM) and relatively low porosity (65%) may cause a significant amount of NH₄⁺ to adsorb to clays. Assuming an ammonium partition coefficient between 1.8 and 2 (cm³/g) for similar types of coastal sediments (Mackin and Aller, 1984), we calculated that as much as 20% of the total nitrogen in the bulk sediment may be the in the pool of the adsorbed ammonium. Figure F2C shows TOC/TN ratios and, for intervals where pore water ammonium concentrations were measured, TOC/TN_org (total nitrogen _organic) ratios, which are corrected for nitrogen contributed by the adsorbed ammonium pool. At Site 1234, the average TOC/TN_org ratio corrected for ammonium is ~10.3 ± 1.1. The uncertainty largely results from the possible nonlinearity of ammonium partition coefficients over a range of dissolved ammonium concentrations.

At Site 1235 (Fig. F2B), both TOC and TN concentrations are lower than at Site 1234, ranging between 0.5 and 1 wt% and 0.05 and 0.12 wt%, respectively; total atomic TOC/TN ratios vary between 4 and 11, with an average of 8.2 ± 0.3 (Fig. F2D). The contribution of adsorbed ammonium to the TN pool at Site 1235 is comparable to that at Site 1234, ~20%. The average value of TOC/TN_org ratios corrected for adsorbed ammonium is 9.8 ± 1.1, identical to the TOC/TN_org at Site 1234, when the uncertainty is taken into account. As at Site 1234, there are maxima in the TOC and TN concentrations, but at shallower depths, ~30–40 mcd. As at Site 1234, the TOC/TN_org decreases slightly downcore.

The depth profiles of major metabolite concentrations are similar at the two sites as well (Fig. F3; Tables T1, T2) (dissolved inorganic carbon [DIC] data are not shown; see Shipboard Scientific Party, 2003a, 2003b). At Site 1234, DIC concentrations reach 71–72 mM at a depth of ~30 mcd. Ammonium concentrations reach the maximum of ~12 mM slightly deeper into the sediments, in the 40- to 50-mcd interval. Below 9 mcd, sulfate concentrations are less than the detection limit. At Site 1235, the DIC maximum of ~60 mM is located in the interval of 30–40 mcd, the zone where ammonium concentrations reach their highest values of ~8 mM. Sulfate concentrations are less than the detection limit below 19 mcd. At both sites, the ammonium maxima in pore water are roughly coincident with maxima in TN and TOC (Figs. F2, F3).

The depth profiles of ¹⁵N of TN, assumed to represent mostly N_org, are similar at Sites 1234 and 1235, although the isotopic ratios at Site 1235 are ~1 lighter than those at Site 1234. At Site 1234, the uppermost interval measured (9.65 mcd) has ¹⁵N of 11 (Table T1; Fig. F3A). The values decrease to ~9 at 50 mcd and remain constant from 50 to 140 mcd. At 150 mcd, the ¹⁵N of N_org increases to ~11 and then gradually returns to 9.5 downcore. The average isotopic composition of N_org at Site 1234 is 9.8 ± 0.2 (Table T3).

At Site 1235 (Table T2; Fig. F3B), ¹⁵N changes gradually from ~10 at 1.45 mcd (the uppermost interval measured) to ~8 at a depth of ~70 mcd. A horizon at 9.5 mcd, a possible turbidite deposit, has anomalously low ¹⁵N, where N_org is ~6. With the exception of two horizons of small (up to ~9.5) enrichment, the ¹⁵N remains constant at 8 through the rest of the sediment column downcore. The average ¹⁵N of N_org at Site 1235 is 9.0 ± 0.2.

The two sites exhibit similar trends in isotopic composition of pore water ammonium (Fig. F3) as well. At Site 1234 (Fig. F3A), ¹⁵N of ammonium decreases from 10.5 to ~9.5 within the upper 60–70 mcd then increases by ~0.5 from 70 to 120 mcd. Between 120 mcd and the bottom of the sediment column, ¹⁵N declines from 10 to 8.5. The average ¹⁵N of ammonium at Site 1234 is 9.4 ± 0.1, which is 0.4 ± 0.3 lighter than the solid-phase isotopic composition (Table T3).

The shallowest interval where ¹⁵N of ammonium was measured at Site 1235 (Fig. F3B) is 9.5 mcd (coincident with the possible turbidite deposit) (Shipboard Scientific Party, 2003a, 2003b). At this depth, ¹⁵N of ammonium is 5.3. Downhole, the ammonium ¹⁵N is 9.5 at 19.7 mcd, decreases to 8.5 at 70 mcd, increases by ~1 within the next 80 mcd, and oscillates between 9.3 and 7.8 between 150 and 210 mcd. At Site 1235, the average difference ¹⁵N of the pore water ammonium and N_org is 0.2 ± 0.3 (Table T3), which is within the limits of the analytical precision.