Sources, Sinks, and Subsidies: Terrestrial Carbon Storage in Mid‐latitude Fjords

Craig Smeaton,William E N Austin

doi:10.1002/2017jg003952

Abstract

AbstractFjords are recognized as globally important sites for the burial and long‐term storage of carbon (C) within sediments. The proximity of fjords to the terrestrial environment in combination with their geomorphology and hydrography results in the fjordic sediments being subsidized with organic carbon (OC) from the terrestrial environment. It has been well documented that terrestrial OC (OCterr) is an important component of coastal sediments, yet our understanding of the quantity of OCterr stored in these sediments remains poorly constrained. Utilizing Bayesian isotopic sediment fingerprinting techniques to the surface sediments of Loch Sunart, we estimate that 42.0 ± 10.1% of the OC is terrestrial in origin. Through combining these outputs with sedimentary OC stock estimates, we have calculated that the surface sediments (0–15 cm) hold 0.1 megaton (Mt) OCterr and estimate that the postglacial sediment held within the fjord contains 3.96 Mt OCterr. When these totals are compared to the quantity of OC stored in the adjacent terrestrial environment, it is clear that the fjord's catchment stores a greater amount of OCterr in the form of vegetation and soil. Though when normalized for area the results suggest that the marine sediments are a more effective long‐term store of OCterr than the adjacent terrestrial environment. This striking result highlights the importance of the terrestrial environment as a source of OC to the coastal ocean and that the OCterr subsidy to the marine sediments is a significant mechanism for the long‐term storage of OC in coastal marine sediments.

Highlights

The burial and storage of organic carbon in the coastal ocean is a key component of the carbon cycle at both local and global scales (Bauer et al, 2013)
Though when we consider the difference in size of the catchment versus the fjord, the results show that the fjord is a more effective long-term store of terrestrial organic carbon (OC) than its adjacent catchment
Bulk elemental data show a clear distinction between the quantities of OC and inorganic carbon (IC) held in the sediment of each subbasin of the fjord

Summary

Introduction

The burial and storage of organic carbon in the coastal ocean is a key component of the carbon cycle at both local and global scales (Bauer et al, 2013). An improved understanding of the factors regulating these C fluxes and their sources and an ability to account for the carbon stored in these coastal sediments remain important global challenges, with far-reaching implications for integrating the coastal ocean into global carbon budgets (Bauer et al, 2013). A key component of our understanding of the coastal carbon cycle is the growing recognition of significant terrestrial OC subsidies into coastal marine sediments. The terrestrial environment is an important source of both particulate (POC) and dissolved (DOC) organic carbon to the coastal ocean (Bianchi, 2011). Current global fluvial OC input to the coastal ocean is estimated to be SMEATON AND AUSTIN

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Discussion

Conclusion