Conflo 2

Sediment samples were analyzed for stable isotopes of nitrogen, as well as for %C and %N, at the Ján Veizer Stable Isotope Laboratory (University of Ottawa, ON). The molar ratio of C:N can be used as an approximate measure to assess whether sedimentary organic matter is largely of aquatic (C:N < 10) or terrestrial (C:N > 20) origin (Meyers & Ishiwatari, 1993 ). For elemental %C and %N analysis, sediment samples and standards were analyzed using a Vario EL III Elemental Analyzer (Elementar, Germany). Sediment amounts needed for the δ¹⁵N isotopic analyses were determined based on the results of the elemental analysis and weighed accordingly into tin capsules with two parts tungsten trioxide (WO₃). The isotopic composition of nitrogen was determined by the analysis of N₂, produced by combustion on a VarioEL III Elemental Analyzer (Elementar, Germany) followed by “trap and purge” separation and online analysis by continuous‐flow with a DeltaPlus XP Plus Advantage Isotope Ratio Mass Spectrometer coupled with a ConFlo II (Thermo, Germany). Our δ¹⁵N data were reported using delta (δ) notation in parts per thousand (‰) enrichments or depletions relative to common standards (AIR for δ¹⁵N). Isotope data were normalized using previously calibrated internal standards, and analytical precision was ±0.2%. δ¹⁵N data was not obtained for A085 and C:N data for D004, D016, and A085.

All beaks were dried at 60 °C for 24–48 hours and ground into a fine powder. Powder sub-samples were weighed (to the nearest 0.3 mg) with a micro-balance and sterile-packed in tin containers. Stable isotope values were determined by a Flash EA 1112 Series elemental analyser coupled online via a Finnigan ConFlo II interface to a Delta VS mass spectrometer (Thermo Scientific) and expressed as: δ¹³C and δ¹⁵N = [(R_sample/R_standard) − 1] * 1000, where R = ¹³C/¹²C and ¹⁵N/¹⁴N, respectively. The carbon and nitrogen isotope ratios were expressed in delta (δ) notation relative to Vienna-PeeDee Belemnite limestone (V-PDB) for δ¹³C and atmospheric nitrogen (AIR) for δ¹⁵N. Replicate measurements of internal laboratory standards (acetanilide STD: Thermo Scientific PN 338 36700) in every batch (n = 14) indicated precision <0.2‰ for both δ¹³C and δ¹⁵N values. Mean mass C:N ratio of all samples was 3.19 ± 0.02.