For granulometric, geochemical and mineralogical analyses, approximately 25 ml of each surface sample was frozen for 24 hours and subsequently lyophilized using a Christ BETA 1-8 LDplus (Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). The freeze-dried samples were homogenized and split into two aliquots. The other aliquot of the freeze-dried surface samples was ground to <63 µm with a Planetary Mill Pulverisette 5 (FRITSCH GmbH, Idar-Oberstein, Germany) and used for mineralogical and geochemical analyses.
Total organic carbon (TOC) as well as total carbon (TC), total nitrogen (TN) and total sulfur (TS) were measured with a vario MICRO cube and vario EL cube combustion elemental analyzers (Elementar Analysesysteme Corp., Langensebold, Germany), respectively. For the TOC measurements, 15 mg of sediment powder was placed into metallic silver containers, heated to 100 to 120°C, and treated three times with a few drops of HCl (32 %) to dissolve carbonates. The metallic silver containers were then wrapped and pressed into silver paper, and the resulting pellets were analyzed for their TOC concentration using the vario EL cube. All concentrations are given as mean values of duplicate measurements. For TC, TN and TS measurements with the vario MICRO cube, 10 mg of sediment powder was placed in zinc containers, with 20 mg of tungsten (VI) oxide (WO2) added to catalyze oxidation. The total inorganic carbon (TIC) was calculated as the difference between TC and TOC. Analytical errors were determined on internal and external reference material. The C/N ratio is calculated as the weight ratio of TOC and TN. The carbon isotopic composition of bulk OM (δ13COM) in the sediment was measured on a set of 42 subsamples at Brown University, Providence, RI, USA. For that purpose, ca 50 mg of sediment was acidified in HCl (2 N) for one hour at 80ºC to remove carbonate minerals. The acid-treated samples were subsequently rinsed in deionized water and centrifuged four times to remove any excess HCl. The samples were then freeze-dried and homogenized prior to isotopic analysis. The δ13COM values were measured using a Carlo Erba Elemental Analyzer coupled to a Thermo DeltaV Plus isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The analytical precision determined through replicate measurements of internal sediment standards was 0.16 ‰. All results are reported relative to the Vienna PeeDee Belemnite (VPDB) standard.
