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Methods and materials

All sedimentary organic matter samples were analyzed in the Southeast Environmental Research Center’s Stable Isotope Laboratory (SERC SIL) at Florida International University (USA). All samples were dried and ground until fine before being homogenized in a SPEX 8000M mixer/mill for 5 min. Samples prepared for carbon isotope analysis were decarbonated by acidifying ~0.1 g of ground sediment sample in a 10% HCl bath for 24 h three times followed by decantation in deionized water to rinse away acid residue. An aliquot of 5 mg of decarbonated sediment for %TOC and δ13C analysis and 10 mg of nondecarbonated sediment for %N and δ15N analysis were compacted in 5 mm × 9 mm tin capsules and analyzed in a NA 1500 elemental analyzer coupled to a Thermo Delta C isotope ratio mass spectrometer.

All isotope data are expressed as delta notation (δ‰), using the following equation:

δR = [(RsampleRstandard)/Rstandard] × 1000,

where Rsample is the ratio of the heavy to light isotope in the sample and Rstandard is the ratio of the heavy to light isotope in the standard (Coplen, 2011). The δ13C and δ15N data are reported in permil vs. Vienna Peedee belemnite and air, respectively.

The procedure for processing inorganic carbon (%IC) samples followed a similar method to shipboard analyses (see the “Methods” chapter [Tada et al., 2015b]). An aliquot of 15 mg of ground, homogenized sample was acidified in 10 mL of 10% HClO4 in a UIC CM140 coulometer for %IC analysis. %CaCO3 was calculated from %IC by multiplying by a factor of 8.33 under the assumption that all inorganic carbon acts as CaCO3.

All b* data were collected shipboard during Expedition 346 using an Ocean Optics USB4000 spectrometer at 5 mm resolution (see the “Methods” chapter [Tada et al., 2015b]).