Abstract
Fjords are recognized as globally significant hotspots for the burial and long-term storage of marine and terrestrially derived organic carbon (OC). By trapping and locking away OC over geological timescales, fjord sediments provide a potentially important yet largely overlooked climate regulation service. Currently, our understanding of the spatial distribution of OC within the surficial sediments of fjords is limited and this potentially implies an overestimation in the global estimates of OC buried in fjords as current calculation methods assume a homogeneous seabed. Using the mid-latitude fjords of Scotland and Ireland as a natural laboratory, we have developed a multi-tiered methodological approach utilizing a spectrum of data ranging from freely available chart data to the latest multibeam geophysics to determine and map the seabed sediment type. Targeted sampling of fjord sediments was undertaken to establish a calibration of sediment type against OC content. The results show that fjord sediments are highly heterogeneous both in sediment type and OC content. Utilizing the tiered mapping outputs, first order estimates of the surficial (top 10 cm) sediment OC stock within Scottish fjords (4.16 ± 0.5 Mt OC) and Irish systems (2.09 ± 0.26 Mt OC), when normalized for area the surficial sediments of Scottish and Irish fjords hold 2027 ± 367 and 1844 ± 94 respectively far exceed estimates for the continental shelf, again highlighting fjord sediments as hotspots for the capture of OC. This tiered approach to mapping sediment type is ideally suited to areas of the marine environment where data availability and quality is a limiting issue. Further understanding of the spatial heterogeneity of these sediments provides a foundation to reevaluate global fjord OC burial rates and to better understand the role of fjord sediments in regulating the global climate.
Highlights
The results indicate that the clay and silt component of the sediment and organic carbon (OC) content in Scottish and Irish fjords are poorly correlated, there is be a degree of covariance between the variables
Understanding the spatial heterogeneity of seabed sediments in fjords is a crucial step toward quantifying OC stocks, the formation of long-term OC stores and any associated rates of change in these globally significant OC burial hotspots
We have demonstrated that when combined with the tiered mapping approach, both readily available point observation and backscatter data can produce robust predictions of sediment type and surficial sediment OC content
Summary
Sediments at the land-ocean interface have been identified as key components in the global carbon (C) cycle (Berner, 1982; Bauer et al, 2013), with fjord sediments being highlighted as hotspots for high organic carbon (OC) burial (Smith et al, 2015; Cui et al, 2016) and storage (Smeaton et al, 2016, 2017). Fjords are estimated to bury 21–31 Tg OC yr−1 (Smith et al, 2015; Cui et al, 2016), which equates to approximately 11% of annual marine OC burial. It is well known that the seabed of both fjords and other coastal sedimentary systems are spatially highly variable and that this heterogeneity impacts the sedimentary OC content of the sediments. This issue can be further confounded by the choice of coring sites; the best coring sites for paleo- and geochemical studies tend to be the OC rich muddy sediments. As well as the uncertainty arising in net OC burial rates, there is uncertainty as to where the OC is buried and stored, which hinders potential policy or management actions aimed to protect these globally significant C stores
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