soaked in 1.2 N HCl for 1 h, rinsed with distilled water,
dried at 60 °C, weighed, powdered and used for total organic carbon,
total nitrogen and isotopic analyses. A mass spectrometer (MAT 252,
Finnigan MAT) and coupled online via a Finnigan ConFlo II interface
with the elemental analyzer were used for the elemental and isotopic
analyses. Results of the isotopic analysis are expressed in the
standard δ unit notation as δX=((Rsamples /Rreference)−1)×103,
where X is 13C or 15N and R=13C/12C for carbon and 15N/14N for nitrogen.
The standard reference materials were Pee Dee Belemnite
standard (PDB) for carbon and atmospheric N2 for nitrogen.