Quality not quantity : prioritizing the management of sedimentary organic matter across continental shelf seas

The Scottish Blue Carbon Forum (Scottish Government) funded this research.Disturbance of marine sediments results in the remineralization of sedimentary organic matter (OM) and impacts upon natural burial processes. Management interventions which restrict or remove activities that cause seabed disturbance may offer effective strategies to protect the most vulnerable of these shelf sea OM stores, offering new opportunities to deliver climate mitigation actions. While the largest quantities of OM are often stored in the expansive offshore regions of continental shelves and might therefore suggest appropriate zones for management interventions to protect vulnerable OM stores, our results highlight that these offshore regions generally contain OM of low reactivity. Conversely, inshore and coastal sediments store significant quantities of highly reactive OM that is at greater risk of remineralization when disturbed. The marked spatial disparities between OM reactivity across shelf sea sedimentary environments highlights the need to focus emergent policy and future management interventions towards the protection of inshore and coastal sediments.Publisher PDFPeer reviewe

Distribution of particulate and dissolved organic carbon in surface waters of northern Scottish fjords

This project was supported by the Scottish Blue Carbon Forum.Coastal waters can act concurrently as a source and sink of carbon (C), with coastal sediments trapping and storing significant quantities of C, while C is simultaneously released to the atmosphere through biogeochemical processes in the water column. The mid-to high-latitudes fjords are recognised as hotspots for the burial and storage of OC in their sediments and for their potential to provide a long-term climate regulation service. Yet the distribution of particulate (POC) and dissolved organic carbon (DOC) in fjord water columns remains a significant knowledge gap, that is particularly acute in temperate vegetated systems such as those in the North Atlantic. Here we present POC and DOC data from surface waters across four Scottish fjords with differing meteorological, catchment and submarine geomorphological characteristics to both quantify and understand the factors governing the spatial distribution of POC and DOC in fjords. The measured POC and DOC concentrations in the surface waters of the four fjords are broadly analogous to other temperate vegetated fjord systems around the world. Within these systems, local factors such as submarine geomorphology is the primary factor driving POC and DOC dispersal in fjord surface waters. The data highlights that fjords are important pathways for the transport and storage of POC and DOC in the coastal ocean and that a greater focus is required on the water column OC to allow the role fjords play in near- (water column) and long-term (sediment) climate regulation to be quantified.Publisher PDFPeer reviewe

Sources, sinks and subsidies : carbon in Scotland's coastal environments

The rise of anthropogenic carbon dioxide concentrations in the atmosphere has forced a revaluation of our current understanding of the magnitude and mechanisms which govern natural carbon pools around the world. A largely overlooked carbon pool is held within the sediments of the world’s oceans with the coastal oceans potentially playing a globally significant role in climate regulation. These marine sedimentary environments and fjords in particular are recognised as hotspots for the burial of carbon, yet little is known about the quantity of carbon held within such environments. In this study, we use the mid-latitude fjords of Scotland as a natural laboratory to develop new methodologies to quantify marine sedimentary C stores and better understand how these stores develop through time with a specific focus on the long-term linkages with the terrestrial environment. The newly developed methodology has allowed for the first time the quantification of a national marine sedimentary carbon stock. The sediments within these mid-latitude fjords hold a quantity of carbon comparable in magnitude to most terrestrial environments in Scotland. However, when area-normalised comparisons are made, these mid-latitude fjords are significantly more effective as C stores than their terrestrial counterparts, including Scottish peatlands. Additionally; our understanding of the long-term role of the terrestrial environment in the development of these systems has been significantly improved with it being estimate that approximately half the carbon held within Scottish fjords is terrestrial in origin. Through the Holocene fjordic sediments have been shown too adapted to increased carbon input through increasing the rate at which carbon in buried, going forward this will be highly significant in mitigating the impact of predicated climatic and human induced environmental change. This project has highlighted the importance of understanding carbon held within marine and coastal sediments. By increasing our understanding of such coastal sedimentary carbon stores we will be better able to estimate the global carbon burial rates and storage further constraining the their role in the global carbon cycle

Carbon burial in the mid-latitude fjords of Scotland

This project was supported by funding from the Scottish Blue Carbon Forum and BBSRC/NERC (ref. BB/M026620/01).Fjord sediments are recognized global hotspots for the burial of organic carbon (OC) and as an integral part of the global carbon (C) cycle. Relative to their spatial extent, more OC is trapped and stored in the sediments of fjords than any other marine sedimentary environment. Until recently, our understanding of the rate at which OC accumulates and is buried in mid-latitude fjord sediments was poor, as these systems have largely been overlooked in favour of their high latitude counterparts. In this study, we quantify and explore the drivers of OC burial in the mid-latitude fjords of Scotland. By examining fifteen sediment cores from ten fjords, it is estimated that on average 57.1 ± 10.9 g C m−2 yr−1 accumulates in the sediments of Scottish fjords, exceeding observed OC burial in other vegetated fjord systems. When combined with an understanding of the spatial heterogeneity of the fjord sediments, it is estimated that Scottish fjords bury 84,000 t of OC annually, which is equivalent to the whole North Sea sedimentary system, despite the area of the latter being approximately 190 times larger. These findings highlight that mid-latitude fjords play a more significant role in global carbon cycling than previously thought, providing highly effective burial and storage of OC in fjord sediments.PostprintPeer reviewe

An assessment of the tea bag index method as a proxy for organic matter decomposition in intertidal environments

This work was supported by the Natural Environment Research Council (grant NE/R010846/1) Carbon in Storage in Intertidal Environments (C-SIDE) project.Intertidal wetlands capture and store carbon (C) for long periods of time, helping to reduce the concentration of CO2 in the atmosphere. Yet the processes which govern the decomposition and subsequent long‐term storage of organic matter (OM) and C in these habitats remains poorly understood. The Tea Bag Index (TBI) uses a standardized OM (green and Rooibos tea) and has the potential to shed light on OM decomposition across habitats, including saltmarshes. Here, we apply the TBI method at two saltmarshes within the same estuary with the aim of (i) reducing the influence of climatic variables and (ii) determining the role of the environment, including the soil characteristics, in the decomposition of OM. We extended the standard (3 months) incubation period over a full year in order to investigate the longer‐term decomposition processes at each site. The initial results partially support previous studies that the early stages of decomposition (leaching of the water‐soluble fraction) is governed by climatic conditions, but may be further enhanced by tidal flushing in saltmarshes. By extending the incubation period, we observed the initiation of mid‐stage OM decomposition (Cellulose degradation) upon which the soil characteristics appear to be the dominant control. These results highlight the importance of long‐term TBI incubations to understand early‐stage OM decomposition. The relationship between tea mass (OM) loss and C loss in these intertidal environments is not straightforward and we would caution the use of the TBI as a direct universal proxy for soil C degradation in such intertidal wetlands.Publisher PDFPeer reviewe

Carbon accumulation and storage across contrasting saltmarshes of Scotland

This research was finically supported by the Scottish Blue Carbon Forum and the Natural Environment Research Council funded Carbon Storage in Intertidal Environments (C-SIDE) project (grant NE/R010846/1).Saltmarshes are acknowledged to be “carbon hotspots” due to their capacity to trap and store large quantities of carbon (C) within their soils and potentially have the ability to regulated climate over different timescales. In-turn governments and international organizations are now recognizing the need to include these intertidal ecosystems in national and global C accounting. Yet, in many regions, estimates of organic carbon (OC) storage and the rate at which OC is buried in saltmarsh soils either do not exist or at not at the scale necessary for inclusion in national C budgets. Here we bring together tools from across the geosciences to investigate the quantity of OC held within the soil and above/belowground biomass alongside estimates of the rate at which OC accumulates and the source of the OC within the soils of four contrasting Scottish saltmarshes. Using radiometric dating techniques it is estimated that OC accumulates at a rate of between 29.1 and 198.1 g C m⁻² yr⁻¹ across the different study sites. In contrast, the source of the OC varies little across the sites with 73%–99% of the OC within the saltmarsh soil originating from terrestrial/in situ sources; marine-derived OC plays a minor role in the development of the saltmarsh OC stocks. Using average values derived from the four sites it is possible to make first-order estimates of saltmarsh OC stocks and accumulation rates for all Scotland's 240 mapped saltmarshes. It is estimated that across Scotland saltmarsh habitat stores 1.15 ± 0.21 Mt OC which is supplemented by an additional 4385 ± 481 tonnes of OC each year.Publisher PDFPeer reviewe

UAV approaches for improved mapping of vegetation cover and estimation of carbon storage of small saltmarshes examples from Loch Fleet, northeast Scotland

Saltmarsh environments are recognised as key components of many biophysical and biochemical processes at the local and global scale. Accurately mapping these environments, and understanding how they are changing over time, is crucial for better understanding these systems. However, traditional surveying techniques are time-consuming and are inadequate for understanding how these dynamic systems may be changing temporally and spatially. The development of Uncrewed Aerial Vehicle (UAV) technology presents an opportunity for efficiently mapping saltmarsh extent. Here we develop a methodology which combines field vegetation surveys with multispectral UAV data collected at two scales to estimate saltmarsh area and organic carbon storage at three saltmarshes in Loch Fleet (Scotland). We find that the Normalised Difference Vegetation Index (NDVI) values for surveyed saltmarsh vegetation communities, in combination with local tidal data, can be used to reliably estimate saltmarsh area. Using these area estimates, together with known plant community and soil organic carbon relationships, saltmarsh soil organic carbon storage is modelled. Based on our most reliable UAV-derived saltmarsh area estimates, we find that organic carbon storage is 15-20% lower than previous area estimates would indicate. The methodology presented here potentially provides a cheap, affordable, and rapid method for saltmarsh mapping which could be implemented more widely to test and refine existing estimates of saltmarsh extent and is particularly well-suited to the mapping of small areas of saltmarsh environments.Peer reviewe

Marine sedimentary carbon stocks of the United Kingdom’s Exclusive Economic Zone

This work was supported by Marine Scotland through the Blue Carbon Forum. Additional support from the NERC Life Sciences Mass Spectrometry Facility (CEH_L_115_05_2018) allowed additional analytical work to be undertaken. BGS provided access to samples through their In-kind sample loan scheme (Loan: 273227).Continental shelf sediments are recognized as long-term stores of globally significant quantities of carbon (C) and potentially provide an important, yet largely overlooked climate regulation service via the Earth’s C cycle. Current understanding of the spatial distribution of sedimentary C across continental shelves remains poor, inhibiting the targeted management and potential inclusion of these globally significant C stores into national C budgets. Further understanding of the spatial heterogeneity of continental shelf sediments and associated C provides a foundation to quantify the organic carbon (OC) stock and better understand the role that marine sediments play in regulating the global climate and the potential for CO2 to be released through anthropogenic disturbance of these C stores. Utilizing a spectrum of available marine data, we have created bespoke sediment maps that quantify the surficial (top 10 cm) OC stock and highlight significant spatial heterogeneity in the distribution of sediments and their associated C content across the United Kingdom’s Exclusive Economic Zone (EEZ). The surficial sediments within the UK EEZ are estimated to store 524 ± 68 Mt of organic carbon (OC) and 2,582 ± 168 Mt of inorganic carbon (IC). The spatial mapping of this C highlights well-defined OC accumulation hotspots in fjords, estuaries and coastal muds, while large accumulations of IC are found in the tidally swept areas around Orkney, Shetland and the South West of England. Within the well-defined OC hotspots, muddy sediments store the greatest quantity of OC; the muds offer potentially valuable opportunities for targeted future management and protection of sedimentary C stores within the UK EEZ. In the future, if areas of the seafloor were to be managed to include the protection of these valuable sedimentary C resources, we recommend an initial focus on hotspots of high sedimentary OC density.Publisher PDFPeer reviewe

Blue carbon stock in Scottish saltmarsh soils

In this study, we provide an estimate of the quantity of organic carbon (OC) held within the surficial soils (top 10 cm and 15 cm) of Scotland’s extensive saltmarsh habitats. This work builds upon the Scottish Saltmarsh Survey (Haynes, 2016) by integrating the mapped extent of saltmarsh vegetation types with field-derived measurements of soil dry bulk density and OC obtained from a diverse range of saltmarsh soils across Scotland (Ruranska et al., 2020). This approach has enabled a new surficial OC stock estimation to be made for the soils within Scotland’s saltmarshes.Publisher PD

Where's the carbon : exploring the spatial heterogeneity of sedimentary carbon in mid-latitude fjords

Funding: UK NERC NE/L501852/1, NERC/BBSRC BB/M026620/1.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.Publisher PDFPeer reviewe