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doi:10.2204/iodp.proc.307.206.2009

Results

The δ¹³C_DIC values range from –18.3‰ to +3.9‰ VPDB (Table T1). At all three sites, the middle Miocene/upper Pliocene unconformity corresponds to an inflection in the slope of downcore profiles in δ¹³C_DIC values (Fig. F1). At Site U1317, δ¹³C_DIC values decrease from –2.5‰ near the top of the core to –8.5‰ to the unconformity at ~148 meters below seafloor (mbsf) (base of lithostratigraphic Unit 1). Below this depth, values increase gradually downcore through Unit 2 to a maximum of +3.9‰ at 257.80 mbsf. The overall trend through Unit 2 is interrupted by two outliers to lower values of –13.8‰ at 207.80 mbsf and –18.5% at 266.01 mbsf. The maximum and minimum δ¹³C_DIC values at this site occur in adjacent samples that represent the two deepest samples recovered from Site U1317. These extremes correspond to a dissolved methane concentration of ~6500 µM, the highest concentration encountered during Expedition 307 (see the “Site U1317” chapter).

Downcore trends in δ¹³C_DIC values are very similar at Sites U1316 and U1318 (Fig. F1). Within the upper ~10 m of Unit 1, δ¹³C_DIC values decrease by ~8‰ to reach minimum values for these sites of –15‰ or lower. Below this depth, δ¹³C_DIC values increase to approximately –6‰ at the middle Miocene/​upper Pliocene unconformity (base of Unit 2), with the rate of change increasing with depth. In Unit 3, δ¹³C_DIC values increase with depth toward maxima of +0.4‰ at Site U1316 and +1.6‰ at Site U1318.

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