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doi:10.2204/iodp.proc.327.202.2015

Results

The concentrations of DOC in sediment pore water at Site U1363 ranged from 0.10 to 0.86 mM (Fig. F2; Table T2). For comparison, similar measurements from Ocean Drilling Program Hole 856 located at Middle Valley, also on the eastern flank of Juan de Fuca Ridge, ranged from 0.01 to 7.14 mM (Simoneit and Sparrow, 2002). The lowest sedimentary DOC concentration was observed near the sediment/basement interface of Hole U1363D, but the concentration was still nearly an order of magnitude higher than that in basement fluid collected by the IODP CORK in Hole U1301A, 55 km north of Site U1363 (0.10 versus 0.012 mM; Lin et al., 2012). It should be noted that the overall procedural blank for the measurement of DOC in basement fluid and bottom seawater was only 0.002 mM. In Hole U1363G, the sedimentary DOC concentration was 0.25 mM in near-surface sediment pore water collected from a sediment core depth of 1.4 mbsf, which is significantly higher than was found in background bottom seawater from this region (0.039 mM; Lin et al., 2012). Depth profiles of DOC concentrations at Site U1363 show a mid-depth (8–11 mbsf) maximum, unlike those from Middle Valley (Simoneit and Sparrow, 2002), which showed an increase toward the basement.

Interestingly, the depth profiles of DOC concentration parallel those of alkalinity and appeared to inversely mirror depth profiles of sulfate concentration (Fig. F2), suggesting that the buildup of DOC in sediment pore water might be related to remineralization of sedimentary POM. The correlation between sulfate and DOC concentration was high and indicated that ~0.09 mM of DOC was added to the pore water with every 1 mM of sulfate removed (Fig. F3). Similarly, ~0.07 mM of DOC was added to pore water with an increase of 1 meq/L of alkalinity (i.e., ~1 mM increase of bicarbonate; Fig. F3). However, no significant correlation was found between DOC concentrations in the sediment pore water and the POC content of the sediment (Fig. F4).

No systematic variation of POC, PN, C/N, δ¹³C-POC, and δ¹⁵N-PN with depth was observed (Fig. F5), with the exception of a correspondence between POC and PN depth profiles. The POC content of sediment samples from Site U1363 ranged from 47 to 391 µmol-C/g (0.06–0.47 wt%), whereas PN ranged from 4.1 to 32.9 µmol-N/g (0.006–0.044 wt%), which were significantly lower than those in rivers and continental-shelf deposits on the Washington Coast (POC = ~3 wt%; PN = ~0.3 wt%) (Keil and Fogel, 2001). The low POC and PN contents suggested that degradation of particulate organic matter had occurred or an accumulation of inorganic material diluted the POC and PN content. POC-δ¹³C values from Site U1363 sediment samples ranged from –25.3‰ to –22.5‰ (Table T2; Fig. F5). For comparison, the organic carbon isotopic composition of fresh marine plankton were measured at approximately –20.5‰ and terrestrial debris at –26‰ (Keil and Fogel, 2001). The similarity of POC and PN profiles resulted in relatively low variation in C/N ratios (12 ± 2; n = 54) (Figs. F6, F7), which were slightly higher than the Redfield ratio (~6; Redfield, 1934) but significantly lower than terrestrial vascular plants (>20) (Meyers, 1994; Keil and Fogel, 2001). POC-δ¹³C values and C/N ratios of Site U1363 sediments suggested that the sediments at Site U1363 were likely a mixture of degraded plankton biomass and terrestrial debris. Some of the low POC and PN and high C/N ratios corresponded to sandy intervals, an indication of the influence of sedimentary composition by turbidites (Fig. F5).

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