CARBON ISOTOPE RATIOS OF PARTICULATE ORGANIC-MATTER IN THE GULF OF ST-LAWRENCE

Sixteen offshore surface samples within the Gulf of St. Lawrence show low δ 13C values and are similar to eight offshore surface samples collected seaward of the Gulf of St. Lawrence. The δ 13C surface values are consistent with δ 13C values in plankton produced at the temperature found in the euphotic zone in the study area. Higher values are observed in four surface samples from the mouth of St. Lawrence Estuary and probably result from high carbon demand during periods of high biological productivity. Lower values found in seven deep POC samples indicate changes in the nature of the POC caused by biological degradation of the organic matter. Significant differences (2-6‰) between the uniformly high δ 13C values of the organic carbon in surface sediments and the low values of near bottom water POC have been observed. The similarity between the δ 13C values of surface water POC and the surface sediments suggest that surface water POC is an important source of organic carbon in surface sediments. Several observations of large vertical δ 13C gradients in deep water POC suggest the presence of suspended sediments 30-60 m above the sediment water interface.