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doi:10.2204/iodp.proc.344.204.2016

Results

Elemental composition

The elemental composition of samples analyzed are listed in Table T1 and their downcore profiles are illustrated in Figures F1, F2, F3, and F4. Because not all samples from Sites U1380, U1412, and U1413 were analyzed for TC, TOC, and TN during the expedition, we completed the analyses of all samples from Expedition 344 postcruise. Most of the samples that were analyzed for TC, TOC, TIC, and TN content both during the expedition and postcruise display similar values at all sites. However, TOC content values obtained onboard from samples deeper than ~160 mbsf at Site U1414 are higher than those measured postcruise (Fig. F4).

Downcore profiles of TC, TOC, TIC, and TN vary with depth at Site U1414 (Fig. F4). TOC and TN gradually decrease from the seafloor to ∼200 mbsf and then slightly increase deeper than 270 mbsf, whereas TC and TIC contents are relatively constant from the seafloor to ∼120 mbsf and then gradually increase with depth with the exception of the interval from 215 to 230 mbsf. Deeper than 120 mbsf, the downcore profiles of TOC and TN are mirror images to those of TC and TIC (Fig. F4). TOC also decreases from seafloor to ∼120 mbsf, with a minimum (<1.0 wt%) deeper than ∼500 mbsf at Site U1413 (Fig. F3). In contrast, downcore profiles of TC, TOC, TIC, and TN from seafloor to 490 mbsf at Sites U1380 and U1412 and downcore profiles of TC, TIC, and TN at Site U1413 do not show variation with depth (Table T1; Figs. F1, F2, F3).

Organic matter derived from marine algae typically has an atomic TOC/TN ratio of 4–10; ratios derived from vascular land plants are 20 or higher (Emerson and Hedges, 1988; Meyers, 1994). Most atomic TOC/TN ratios at Sites U1380, U1412, and U1413 are 4–12 and are relatively constant with depth (Table T1; Figs. F1, F2, F3). On the other hand, TOC/TN ratios at Site U1414 are constant at 7–10 in the upper 200 mbsf and are mostly >10 deeper than 200 mbsf, varying from 9 to 38 (Table T1; Fig. F4). Deeper than 200 mbsf at Site U1414, TOC/TN ratios calculated using onboard and postcruise data show significant deviation because TOC contents are remarkably different between them (Fig. F4).

Most TS contents are <3 wt% and have a mid-maximum around 50 mbsf at Site U1414 (Table T1; Fig. F4). In addition, TS values decrease with depth at Site U1380 (Table T1; Fig. F1). TOC/TS ratios at Sites U1380 and U1412 have maximum values in the lower sections (∼584 mbsf at Site U1380 and ∼352 mbsf at U1412), where TS content is at minimum values at each site (Table T1; Figs. F1, F2).

Rock-Eval pyrolysis

Most S₂ and S₃ values are <2 mg HC/g Rock and <3 mg CO₂/g Rock, respectively, and show higher values at shallow depths at Sites U1412–U1414 (Table T1; Figs. F1, F2, F3, F4). In addition, S₂ and S₃ values display mirror images to each other at depths shallower than 100 mbsf at Site U1412, whereas these values tend to decrease with depth deeper than 200 mbsf at Site U1414 (Figs. F2, F4). S₃ values at Site U1413 also decrease with depth (Fig. F3).

Most HI and OI values range from 50 to 150 mg HC/g TOC and from 100 to 300 mg CO₂/g TOC, respectively (Table T1; Figs. F1, F2, F3, F4). OI maximum values are found at ~160 mbsf at Site U1412, ~110 mbsf at Site U1413, and ~200 mbsf at Site U1414. Maximum OI values are in lithostratigraphic unit Unit I at Site U1412, Unit II at Site U1413, and at the Subunit IIA/IIB boundary at Site U1414 (Figs. F2, F3, and F4).

Plots of modified van Krevelen–type and S₂ versus TOC diagrams using Rock-Eval data show that most data correspond to the Type III evolution field (Fig. F5). In addition, there is a strong positive correlation between S₂ and TOC at Sites U1380, U1413, and U1414 (R² > 0.80) and a moderate positive correlation at Site U1412 (R² = 0.68) (Fig. F5).

T_max values are mostly lower than 435°C, suggesting that organic matter is at a thermally immature stage (Nali et al., 2000). T_max generally does not show vertical variation with depth at Sites U1380, U1412, U1413, and U1414 (Table T1; Figs. F1, F2, F3, F4). However, T_max values rapidly decrease (<400°C) between 175 and 200 mbsf in Subunit IIA at Site U1414 (Fig. F4).

Isotopic composition

δ¹³C_org and δ¹⁵N_org values at Site U1380 display significant different trends at depths shallower than 500 mbsf (Table T1; Fig. F4). Their values at shallow depths are relatively constant (δ¹³C_org = 27.01‰ ± 0.48‰ and δ¹⁵N_org = 6.70‰ ± 0.21‰; N = 5), whereas δ¹³C_org increases and δ¹⁵N_org decreases with depth deeper than 500 mbsf. In addition, the downcore profile of δ¹³C_org decreases in the intervals from seafloor to 75 mbsf and from 450 mbsf to the bottom of the hole and is relatively constant between 75 and 450 mbsf at Site U1413. δ¹⁵N_org decreases from 150 to 225 mbsf and then gradually increases with depth at Site U1413 (Fig. F3). The variation of δ¹³C_org and δ¹⁵N_org at Sites U1380 and U1413 do not correlate with lithostratigraphic units (Figs. F1, F3).

Organic matter produced by land plants through the C₃ pathway has an average δ¹³C_org of approximately –27‰ (range = –32‰ to –21‰ VPDB) (Deines, 1980); through the C₄ pathway, average δ¹³C_org is approximately –14‰ (range = –17‰ to –9‰ VPDB) (Deines, 1980). Marine organic matter typically has δ¹³C values from –22‰ to –20‰ (Jasper and Gagosian, 1990; Meyers, 1994). Measured δ¹³C_org values do not clearly identify the source of organic matter, rather they reveal that the organic matter in the sediment could be derived from an admixture of marine algae and C₃ land plants (Table T1; Figs. F1, F2, F3, and F4). However, the relationship between TOC/TN ratios and δ¹³C_org values leads to a different interpretation. As shown in Figure F6, most organic matter at Sites U1380 and U1413 lies in the marine/fresh algal admixture origin, whereas organic matter at Site U1412 predominantly originated from marine algae. There is no correlation of organic matter source with lithostratigraphic units.

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