Understanding the Role of Terrestrial and Marine Carbon in the Mid‐Latitude Fjords of Scotland

Abstract

The sediments within fjords are critical components of the mid‐ to high‐latitude coastal carbon (C) cycle, trapping and storing more organic carbon (OC) per unit area than other marine sedimentary environments. Located at the land‐ocean transition, fjord sediments receive OC from both marine and terrestrial environments; globally, it has been estimated that 55%–62% of the OC held within modern fjord sediments originates from terrestrial environments. However, the mid‐latitude fjords of the Northern Hemisphere have largely been omitted from these global compilations. Here we investigate the mechanism driving the distribution of OC originating from different sources within the sediments of 38 Scottish fjords. From an array of fjord characteristics, the tidal range and outer sill depth were identified as the main drivers governing the proportions of marine and terrestrial OC in the sediments. Utilizing this relationship, we estimate that on average 52% ± 10% of the OC held within the sediments of all Scotland's fjords is terrestrial in origin. These findings show that the Scottish fjords hold equivalent quantities of terrestrial OC as other global fjord systems. However, the analysis also highlights that the sediments within 29% of Scottish fjords are dominated by marine derived OC, which is driven by local fjord geomorphology and oceanography.

Sediment Resuspension Accelerates the Recycling of Terrestrial Organic Carbon at a Large River‐Coastal Ocean Interface

“…The estimated burial efficiency of terrestrial POC in the delta-shelf of the Yangtze River (∼25% ± 17%) is comparable to those in global continental margins (∼33% ± 18%) and the Amazon (∼20% ± 4%) (Blair & Aller, 2012). However, these preservation efficiencies are lower than those observed in other river systems such as the Mackenzie (∼65% ± 12%; Hilton et al, 2015;Kim et al, 2022;Vonk et al, 2015) and the Ganges-Brahmaputra (up to 100%; Galy et al, 2007), as well as in Steep-Mountainous River systems (SMRs) (∼73% ± 21%; Blair & Aller, 2012;Hilton & West, 2020) and fjords (∼80%; Smeaton & Austin, 2022b;Smith et al, 2015) (Figure 9e).…”

The Global Turbidity Current Pump and Its Implications for Organic Carbon Cycling

“…Efficient burial of both terrestrial and marine OC occurs within fjords, which are often characterized by high TOC (average 2.6%), rapid sediment accumulation, and poorly oxygenated seabed conditions (Smith et al 2015;Bianchi et al 2018Bianchi et al , 2020. Rapid transport of terrestrial OC from forests and soils, as well as episodic sediment supply from mountainous rivers, may also lead to a high percentage of fresh (biospheric) carbon (Smith et al 2015;Cui et al 2016;Bianchi et al 2018Bianchi et al , 2020Smeaton & Austin 2022). Thus, even though their surface areas are ∼40 times smaller than those of deltas and continental shelves, fjords represent ∼17% of the global terrestrial OC burial and ∼11% of the TOC burial in marine sediments (Smith et al 2015).…”

Burial of Organic Carbon in Swedish Fjord Sediments: Highlighting the Importance of Sediment Accumulation Rate in Relation to Fjord Redox Conditions

“…The global heterogeneity of fjords presents challenges in establishing universal drivers of biogeochemical cycles in these systems, such as redox control, dynamics of bottom water exchange, sources of OC, and sill depth, just to name a few (Cui, Bianchi, Savage, & Smith, 2016; Faust & Knies, 2019; Hinojosa et al., 2014; Smeaton & Austin, 2022). In this study, we utilized the close proximity (approximately 40 km) of three fjords with distinctly different redox regimes (e.g., long‐term anoxic, seasonally hypoxic, and long‐term oxic) to study the role of redox and OC composition on OC burial in fjord sediments.…”