Large burial flux of modern organic carbon in the St. Lawrence estuarine system indicates a substantial atmospheric carbon sink

Abstract

Estuarine and fjord systems host large amounts of buried organic carbon with highly heterogeneous sources in their sediments. The age of this buried carbon is important because it determines to what extent it represents a short-term atmospheric carbon sink on decadal to centennial timescales. Here, we utilized molecular (fatty acids and n-alkanes) and bulk radiocarbon (¹⁴C), stable isotope, and elemental analyses and a mixing model to apportion the source of organic carbon buried in the Lower St. Lawrence Estuary, the world's largest estuary, and the linked Saguenay Fjord, differentiating between modern, millennial aged, and fossil carbon sources. The ¹⁴C ages of long-chain (C₂₄₊₂₆) fatty acids indicate an average terrestrial storage time of ∼1700 ± 284 yr (before present) for soil organic carbon prior to re-deposition in these sediments. A three-tracer source model for bulk organic carbon indicates that 64 ± 0.8 % of organic carbon buried in the Saguenay Fjord and Lower St. Lawrence Estuary was modern marine- and terrestrially-derived carbon, representing direct atmospheric carbon sinks, while 36 ± 4 % was pre-aged soil and fossil petrogenic organic carbon. Comparison with a similar dataset from the subtropical Pearl River Estuary in China indicates that burial of soil and petrogenic organic carbon is significantly lower in the Lower St. Lawrence Estuary on both a fractional and flux basis, probably as a result of greater topographic relief and human land use in the Pearl River catchment, which generates greater erosional inputs of soil and petrogenic carbon.