BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-13-5771-2016Substantial stores of sedimentary carbon held in mid-latitude fjordsSmeatonCraigcs244@st-andrews.ac.ukhttps://orcid.org/0000-0003-4535-2555AustinWilliam E. N.https://orcid.org/0000-0001-6544-3468DaviesAlthea L.BaltzerAgnèsAbellRichard E.HoweJohn A.School of Geography & Geosciences, University of
St Andrews, St Andrews, KY16 9AL, UKScottish Association for Marine Science, Scottish Marine
Institute, Oban, PA37 1QA, UKInstitut de Géographie et d'Aménagement
Régional de l'Université de Nantes, BP 81 227 44312 Nantes CEDEX
3, FranceCraig Smeaton (cs244@st-andrews.ac.uk)19October20161320577157876June201617June201619September20163October2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/13/5771/2016/bg-13-5771-2016.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/13/5771/2016/bg-13-5771-2016.pdf
Quantifying marine sedimentary carbon stocks is key to improving our
understanding of long-term storage of carbon in the coastal ocean and to
further constraining the global carbon cycle. Here we present a
methodological approach which combines seismic geophysics and geochemical
measurements to quantitatively estimate the total stock of carbon held within
marine sediment. Through the application of this methodology to Loch Sunart, a
fjord on the west coast of Scotland, we have generated the first full
sedimentary carbon inventory for a fjordic system. The sediments of Loch
Sunart hold 26.9 ± 0.5 Mt of carbon split between 11.5 ± 0.2 and
15.0 ± 0.4 Mt of organic and inorganic carbon respectively. These new
quantitative estimates of carbon stored in coastal sediments are
significantly higher than previous estimates. Through an area-normalised
comparison to adjacent Scottish peatland carbon stocks, we have determined
that these mid-latitude fjords are significantly more effective as carbon
stores than their terrestrial counterparts. This initial work supports the
concept that fjords are important environments for the burial and long-term
storage of carbon and therefore should be considered and treated as unique
environments within the global carbon cycle.
Introduction
The rising prominence of blue carbon (i.e. carbon (C) which is stored in
coastal ecosystems, notably, mangroves, tidal marshes, seagrass meadows and
sediments) has forced a reassessment of our knowledge of C in the coastal
ocean (Nellemann et al., 2009). In recent years there have been a number of
reviews (Bauer et al., 2013; Cai et al., 2011; Duarte, 2016) highlighting
knowledge gaps and the limited understanding of both the C sources and sinks
in the coastal ocean (Bauer et al., 2013). Quantifying the stores of C in the
coastal ocean is the first step to a better understanding of coastal carbon
dynamics. Global C burial in the coastal zone is estimated in the region of
237.6 Tg yr-1 with approximately 126.2 Tg yr-1 of C being
buried in depositional areas, i.e. estuaries and the shelf (Duarte et al.,
2005). The lack of regional and national coastal sedimentary C inventories
means these global estimates cannot be confirmed or further constrained.
One of the rare examples of a national marine C inventory was carried out by
Burrows et al. (2014), producing initial estimates of blue carbon in Scottish
territorial waters; they calculated that these waters stored 1757 Mt C, with
coastal and offshore sediments acting as the main repositories. Burrows et
al. (2014) suggested that the majority of this organic carbon (OC) was held
in fjord sediments.
It has long been known that fjords are important stores of C (Syvitski et
al., 1987) and that C burial in sediments is the most significant mechanism
of long-term (> 1000 years) OC sequestration in the coastal ocean
setting (Hedges et al., 1995). These carbon accumulation and burial processes
have been investigated in the fjordic systems of New Zealand (Pickrill, 1993;
Knudson et al., 2011; Hinojosa et al., 2014; Smith et al., 2015), Chile
(Sepúlveda et al., 2011), Alaska (Cui et al., 2016) and the
high latitudes of NW Europe (Winkelmann and Knies, 2005; Müller, 2001;
Kulinski et al., 2014), yet the mid-latitude fjords of Scotland have been
largely overlooked, with only limited data available (Loh et al., 2008). Smith
et al. (2015) brought much of the available data together and showed that
globally fjordic systems act as a CO2 “buffer” by efficiently
capturing and burying labile terrestrially derived OC and preventing it from
entering the adjacent ocean system where it is prone to recycling. These
authors calculated that 11 % of annual global marine carbon sequestration
occurs within fjords.
Despite these findings, much of the global research to assess and quantify C
stocks is disproportionately skewed towards the terrestrial environment (e.g.
Yu et al., 2010). This trend is also found at the regional scale where there
have been multiple studies quantifying the carbon held within Scottish soils
(Aitkenhead and Coull, 2016; Bradley et al., 2005; Chapman et al., 2013) and
peats (Aitkenhead and Coull, 2016; Howard et al., 1995; Cannell et al., 1999;
Chapman et al., 2009).
In addition to the challenges of access and cost in sampling these environments
when compared to the adjacent terrestrial environment, it might also be
argued that the sparsity of marine sedimentary C inventories is due to the
lack of a robust methodology to quantify these C stores. Syvitski et
al. (1987) commented that “the development of a methodological approach to
quantify the C in the sediment of a fjord must be a priority”, yet in the
subsequent years there has been relatively little progress towards this goal.
The absence of a robust methodology to quantify the C held in marine
sediments is illustrated by Burrows et al. (2014), who estimated that there
is 0.34 Mt OC stored in the sediments of Scottish fjords. However, these
calculations only take into account an estimate of OC in the top 10 cm of
sediment, despite the fact that sediment depths of > 25 m are
common in Scottish fjords (Baltzer et al., 2010; Howe et al., 2002).
Therefore, it is likely that current best estimates (Burrows et al., 2014) of
the quantity of OC within these systems as a whole have been significantly
underestimated and that the presence of significant quantities of inorganic
carbon (IC) held within fjord sediments (Nørgaard-Pedersen et al., 2005)
has been overlooked.
This study combines geochemical, geophysical and geochronological techniques
to produce a methodology capable of delivering quantitative first-order
estimates of the mass of C stored within the sediment of a fjord and,
potentially, of achieving the goal set out by Syvitski et al. (1987). This
work provides the first carbon inventory for a fjord and further develops the
concept of these fjords as being globally important sites for the burial of C
as set out by Smith et al. (2015) and Cui et al. (2016b).
Material and methodsStudy area
Loch Sunart is a fjord on the west coast of Scotland (Fig. 1). The fjord is
30.7 km long and covers an area of 47.3 km2 with a maximum depth of
145 m. It consists of three basins separated by shallower rock sills. The
inner basin is separated from the middle basin by a sill at approximately
6 m depth, while the middle and outer basins are separated by a sill at
approximately 31 m depth (Edwards and Sharples, 1986; Gillibrand et al.,
2005). The silled nature of the bathymetry allows the fjord to act as a
natural sediment trap for both terrestrial- and marine-derived materials (e.g.
Nørgaard-Pedersen et al., 2006).
Maps of Loch Sunart illustrating (a) the three basins and the
sediment core locations and (b) Loch Sunart in a Scottish context.
Loch Sunart's catchment covers 299 km2; the main tributaries of the
fjord are the rivers Carnoch and Strontian; the latter has a mean daily
discharge of 1409 m3 (2009–2013). The mean annual precipitation in
Loch Sunart's catchment is 2632 ± 262 mm (Capell et al., 2013). The
combination of small catchment size and high precipitation means that the
flow network is sensitive to precipitation changes which can result in a
flashy flow regime (Gillibrand et al., 2005).
Map of the 34 seismic transects undertaken in Loch Sunart with
SIESTEC profile 11 highlighted.
The catchment is largely dominated by high relief and poorly developed
soils. The bedrock consists primarily of igneous and metamorphic rocks,
overlain by gley and podzol soils with limited peat in the upper catchment
(Soil Survey of Scotland, 1981). Exposed rock is common on the steep slopes.
Much of the catchment's vegetation can be found by streams or on the shore
of the fjord and is dominated by both commercial forestry and natural
woodlands; there is only very limited agriculture within the catchment. The
combination of steep, exposed slopes, poorly developed soil, a reactive
river network and poorly developed vegetation typically results in high
surface runoff and sediment transport (Hilton et al., 2011).
The characteristics of Loch Sunart and its catchment are representative of
fjords across mainland Scotland (Edwards and Sharples, 1986), with the
possible exception of Loch Etive which has a permanently hypoxic upper basin
(Friedrich et al., 2014). The fjords of the Scottish islands (Shetland,
Orkney and the Western Isles) differ from their mainland counterparts in that
they are generally shallower and have catchments characterised by lower
relief and are largely dominated by peat or peaty soil (Soil Survey of
Scotland, 1981). Syvitski and Shaw's (1995) table of generalised fjord
characteristics allows us to compare the fjords of mainland Scotland to other
fjordic systems globally. The fjords of the Norwegian mainland, Canada and
Fiordland, New Zealand (Hinojosa et al., 2014), are characterised by similar
climate, geomorphology, river discharge, basin water temperature and
sedimentation rate to the fjords of Scotland. The fjords of mainland Scotland
differ significantly from those in Greenland, Alaska, Svalbard and the
Canadian Arctic, many of which still have active glaciers, resulting in very
different sediment input regimes.
Seismic data acquisition and processingData acquisition
A seismic geophysical survey of Loch Sunart took place in 2002 aboard the RV
Envoy (Fig. 2). A SIESTEC Boomer system was used to create seismic
profile data throughout the fjord. The data were recorded using an
Elics–Delph data acquisition system coupled to the Differential Global
Positioning System (DGPS). The Boomer system operated on a frequency of 1 to
10 kHz and had a pulse duration of 75 to 250 ms at a power of 150 J. The
system has a depth resolution of 25 cm and can penetrate 100 m in soft
sediment (Simpkin and Davis, 1983). A total of 34 transects of the fjord were
acquired (Fig. 2). The survey achieved an average penetration of 50 m; gas
blanking prevented the signal from penetrating the sediment in some areas
(Baltzer et al., 2010).
Defining sedimentary horizons
Each seismic profile was combined with the DGPS data and processed with the
Petrel (Schlumberger) software package. Subsequent analysis was undertaken
using the open-source SeiSee (DMNG) software package. Initial interpolation,
following the methodology of Baltzer et al. (2010), defined the different seismic
horizons (H) and the layers between the horizons which are defined as seismic
units (U) numbered 1 to 3 from the basement horizon upwards (Fig. 3). The
compilation of the horizons and units allows the construction of an
equivalent seismic stratigraphy for each sediment core and the fjord as a
whole.
SIESTEC profile 11: a characteristic seismic profile displaying
the four seismic horizons (H1, H2, H3 and H4) and the three seismic units
(U1, U2 and U3) with depth indicated as two way travel time (TWTT), adapted from Baltzer et al. (2010).
Using SeiSee, points were picked along each of the four horizons, creating
polylines. Each polyline was split into points at 0.25 m intervals and each
point was assigned an x, y, z coordinate that represents its geographic
location and depth (relative to mean sea level).
Sediment sampling
Eight sediment cores (Table 1) were collected from Loch Sunart (Fig. 1) in
2001 using a gravity corer (GC) as part of the HOLSMEER (Late Holocene Shallow Marine Environments Of Europe) project. This was
supplemented with further sampling on a follow-up cruise on board the RV
Calanus in August 2013 where a short GC was collected to fill a gap
between the original coring sites. These cores capture the postglacial
history of sediment accumulation within the fjord, as confirmed by 14C
basal dates. Additionally, we accessed the lower sections of core MD04-2833
which was recovered using the CALYPSO giant piston corer from the R/V
Marion Dufresne in July 2004 as part of the IMAGES (International Marine Past Global Changes Study) project. Sampling
of Section VIII (1050–1200 cm) of MD04-2833 was undertaken to obtain
sediment of inferred glacial origin for geochemical analysis (Baltzer et al.,
2010).
Details of the sediment cores extracted from Loch Sunart that were
used in this study.
Detailed sediment logging was undertaken for each of the cores (Supplement). The gravity cores were sub-sampled at 10 cm intervals and high-resolution sampling at 1 cm intervals was undertaken on the short core
(GC01). Section VIII of glacial sediment core MD04-2833 was sub-sampled at
12 cm intervals. Each sub-sample was split for physical property and
geochemical analyses. The wet (WBD) and dry bulk density (DBD) of the
sediment was calculated following Dadey et al. (1992), while porosity was
calculated using the methodology of Danielson and Sutherland (1986).
Bulk elemental analysis
To quantify the total carbon (TC) content, each sub-sample was freeze-dried
and milled to a fine powder. A 20 ± 2 mg aliquot was placed in a tin
capsule and measured on a COSTECH Elemental Analyser (EA) calibrated with
acetanilide (Verardo et al., 1990; Nieuwenhuize et al., 1994). Precision of
the analysis is estimated from repeat analysis of standard reference material
B2178 (Medium Organic
content standard from Elemental Microanalysis, UK)
with C = 0.07 % and N = 0.02 % (n= 8).
To quantify OC, the process was repeated with the addition of H2SO3
to remove the IC. After acidification, vessels were placed
in a vacuum desiccator to remove any remaining CO2 and the sample was
then freeze-dried to remove the H2SO3 (Loh et al., 2008). IC was
calculated from the difference between TC and OC measurements. The mean
standard deviations of TC and OC triplicate measurements (n=10) were
0.04 % and 0.17 % respectively.
Sediment geochronology
Basal radiocarbon dates for five of the gravity cores were obtained by
accelerator mass spectrometer (AMS) radiocarbon dating of marine carbonate
material (mollusc). This was carried out at the University of Aarhus, Denmark
(AAR), Centre of Accelerator Mass Spectrometry, USA (CAMS), and the NERC
Radiocarbon Laboratory, Scotland (SUERC). The radiocarbon dating was used to
validate the Holocene chronology of the seismic stratigraphy. A single
MD04-2833 sample was processed at Laval University, Canada (UL), to confirm
that the sediment was early postglacial in age. Dates were calibrated using
OxCal 4.2.4 age modelling software (Bronk Ramsey, 2009; Bronk Ramsey and Lee,
2013), applying the Marine13 curve (Reimer et al., 2013) and the regional
marine radiocarbon reservoir age correction: ΔR value of
-26 ± 14 yr (Cage et al., 2006).
Sediment quantification and characterisationDigital terrain models (DTMs)
The points collected from each seismic horizon were connected to form a DTM
of that horizon. This was achieved using spatial modelling techniques in
ArcGIS. The compiled x, y, z data were statistically tested to
determine the gridding technique best suited to the interpolation of the
data. Eleven gridding techniques were subjected to cross validation (Chiles
and Delfiner, 1999) (Supplement). The residual Z mean value and standard
deviation were examined; the technique with the lowest residual Z mean and
standard deviation for each horizon (and the data set as a whole) was chosen
as the gridding technique best suited to the interpolation of the data.
Kriging, with linear interpolation (Cressie, 1990) and a 100 by 1000 node
structure, performed best and was chosen to create computationally efficient
DTMs for each seismic horizon.
Volumetric calculations
The horizon DTM grids were used to calculate the volume of sediment in each
seismic unit and, by extension, within the fjord as a whole. By subtracting
one DTM grid from another (e.g. surface DTM minus bedrock DTM) the volume
between the grids was calculated. Three different numerical integration
algorithms were used for this calculation (Eqs. 1, 2, 3). The net volume is
reported as the mean of these three calculations. In the following formulae
Δx represents the grid column spacing, Δy represents the grid
row spacing, Gi,j represents the grid node value in row i and
column j and Ai represents the abscissa (Press et al., 1988).
Trapezoidal rule
The pattern of coefficients is {1,2,2,2,…,2,2,1}:
Ai=Δx2[Gi,1+2Gi,2+2Gi,3…+ 2Gi,nCol-1+Gi,nCol]Volume≈Δy2[A1+ 2A2+ 2A3+… 2AnCol-1+AnCol].
Extended Simpson's rule
The pattern of coefficients is {1,4,2,4,2,4,2,…,4,2,1}:
Ai=Δx3[Gi,1+ 4Gi,2+ 2Gi,3+ 4Gi,4+…+ 2Gi,nCol-1+Gi,nCol]Volume≈Δy3[A1+ 4A2+2A3+ 4A3+…+ 2AnCol-1+AnCol].
Extended Simpson's 3/8 rule
The pattern of coefficients is {1,3,3,2,3,3,2,…,3,3,2,1}:
Ai=3Δx8[Gi,1+3Gi,2+ 3Gi,3+ 2G1,4+…+ 2Gi,nCol-1+Gi,nCol]Volume≈3Δy8[A1+ 3A2+ 3A3+ 2A3+…+ 2AnCol-1+AnCol].
Sediment mass quantification
The mean DBD for each seismic unit was calculated and
assigned to the equivalent seismic unit within each core. The spatial
distribution of the DBD for each seismic unit was modelled, again using
Kriging (with linear interpolation). The resulting contour plot was
integrated with the volumetric model for each seismic unit to calculate the
dry mass of the sediment held within that seismic unit. The integration
process calculates the volume of sediment held within each of the DBD
contours and multiplies that volume with the associated DBD value to
calculate the mass of sediment.
Sedimentary carbon quantification
The same methodology used to integrate the volume and density data was used
to combine bulk elemental data with the sediment dry mass calculations. Mean
values for TC, OC and IC in each seismic unit were assigned to the seismic
units from the available core data. Kriging (with linear interpolation) was
again used to create contour maps representing the quantity of TC, OC and IC
in each seismic unit, and the mass of sediment held between the contours was
multiplied by the percentage of OC and IC, quantifying the mass C held within
the fjord's sediment. Finally, we calculated how effectively the fjord stores
C (Ceff) as a depth-integrated average value per km2 for both
the postglacial and glacial-derived sediments. This measure allows the
fjord's C stores to be directly compared with other C stores (peatlands,
soil, etc.).
Carbon accumulation and burial
Sedimentation rates (SRs) were calculated as an approximation for the
postglacial sediment burial history using basal ages and a linear
interpolation to the core top, assuming a contemporary surface. We recognise
that the calculations will be crude and do not take into consideration
factors such as compaction and possible changes in sedimentation rate through
time, but these calculations provide initial insight into the variability of
SRs within the fjord and allow first-order C accumulation rates (CARs) to be
estimated. The SRs were converted to CARs through the use of Eq. (4). The
%OC, %IC, bulk density and porosity data used for these calculations
were based upon a mean value for the postglacial unit of each dated core.
CAR=%C×SR×(porosity-1)×bulk density
As there is no available data on how efficiently OC is buried in the sediment
of Scottish sea fjords, burial efficiencies of 64 % (Sepúlveda et
al., 2005) and 80 % (Smith et al., 2015) were used to convert CARs to
carbon burial rates (CBRs) (low and high). For the purposes of this study and in the absence of
reliable estimates of burial efficiency, we assume that the IC accumulation
rates equal the IC burial rates. These CBRs were, in turn, used to calculate
the long-term annual average burial of OC and IC; while potentially very
useful, such estimates should be treated with caution.
Radiocarbon ages from Loch Sunart cores. Ages were calibrated using
OxCal 4.2.4 (Bronk Ramsey, 2009; Bronk Ramsey and Lee, 2013) with the
Marine13 curve (Reimer et al., 2013) and regional correction of ΔR
value of -26 ± 14 yr (Cage et al., 2006) . All ages are calibrated
at 95.4 % probability and the mean age has been determined from the
minimum and maximum calibrated ages. Additionally, we list the seismic unit
assigned to each equivalent (eqv.) depth and compare this to the age-equivalent seismic unit based on Baltzer et al. (2010).
Four horizons were identified throughout the fjord (Fig. 3): these represent
the basement (H1) and the sediment water interface (H4) with two intermediate
horizons (H2 and H3). Core stratigraphies (Baltzer et al., 2010) indicate
that H2 divides the postglacial and glacial sediment, while H3 splits the
postglacial sediment into two units. The seismic data display a fifth
horizon between H1 and H2 which is only present in the inner basin and
partially in the middle basin. We interpret this as glacial sediment from the
Younger Dryas, as confirmed by radiocarbon dating (Baltzer et al., 2010;
Mokeddem et al., 2010); for the purposes of this paper, the horizon was
amalgamated with H2.
A seismic stratigraphy was developed based on these horizons (Fig. 3). U1 is
interpreted as glacial sediment based on the observation of the short,
discontinuous seismic reflections which are synonymous with poorly sorted
material; the unit varies in thickness but never drops below a minimum
thickness of 10 m. U2 is found throughout the fjord with an average
thickness of 5 to 10 m; the unit drapes over U1. U3 is the uppermost unit
and has a homogenous thickness of around 1m; it is characterised by laminated
acoustic reflections. Both U2 and U3 are interpreted as postglacial infill
of the fjord; though clear in the seismic geophysics, the boundary between U2
and U3 is poorly defined in the sediment lithology (Supplement).
Similar patterns in seismic stratigraphy have been observed throughout the
west coast of Scotland (Binns et al., 1974a, b; Boulton et al., 1981; Howe et
al., 2002).
We compared our interpretation of the seismic data to the seismic
interpretation of Baltzer et al. (2010); this exercise was designed to test
the replicability of our interpretation and allow potential uncertainties in
the seismic interpolation to be built into our future applications. The
comparison identified small differences in the depth of H1 (-0.17 m), H2
(+0.34 m) and H3 (-0.22 m). These differences were integrated into the
volumetric calculations as an error term.
Sediment geochronology
Calibrated radiocarbon dates for the gravity cores (Table 2) indicate that
these cores are comprised of sediment accumulated during the postglacial
period (Holocene). The age of the deeper basal sediment of MD04-2833
(Section VIII) was confirmed through dating of a mollusc (Pecten maximus); the calibrated age was 17041 ± 312 cal BP which, combined
with the characteristic glacial core lithology of poorly sorted sedimentary
material, indicates that this basal sediment of MD04-2833 was deposited by
the retreat of the British ice sheet (BIS) at the end of the last glacial
period 13 500 to 17 000 cal BP (Clark et al., 2010; Scourse et al., 2009;
Wilson et al., 2002).
Through comparison of the chronologies to the seismic stratigraphy we can
test the interpolation and further constrain the age of each seismic unit.
The seismic unit for the equivalent depth of each of the radiocarbon samples
has been compiled (Table 2), then compared to the seismic unit that the
sample would fall into based on age alone as per the Baltzer et al. (2010)
chronostratigraphy. Of the 18 samples tested, 15 have ages which match the
appropriate seismic units. Three samples (all from GC023) have ages which
are apparently too young for their corresponding seismic unit; this suggests
a possible problem with the dating of this particular core, rather than the
interpolation of the seismic geophysics. Close inspection of the seismic
profile suggests sediment slumping could be the cause of this dating problem
at the core site. This test signifies that our interpolation of the seismic
geophysics is accurate and that the chronostratigraphy developed for
MD04-2833 (Baltzer et al., 2010) can be applied throughout Loch Sunart. The
seismic interpolation and the dated samples confirm that both U2 and U3 are
postglacial in origin. We can further constrain the age of the seismic units
with U2 representing the early to mid-Holocene and U3 mid- to late Holocene
in age.
Sediment analysisBulk density measurement
Mean DBD was calculated for U1, U2 and U3 from each core. Figure 4 displays
the DBD results, which are arranged to mirror the spatial distribution of the
cores, from the inner basin to the outer basin. U1 sediment is characterised
by the single section of MD04-2833, which has a mean DBD of 2.19 ± 0.09 g cm-3. This is within the range of other Northern Hemisphere fjords
(Pedersen et al., 2012; Forwick et al., 2010; Baeten et al., 2010). DBD
increases down each core as a result of sediment dewatering in response to
compaction. GC011 is the only core where U3 has a higher DBD than U2, most
likely due to large quantities of shell in the upper part of the core. U1 has
the highest DBD; this reflects both the type of sediment deposited during
glacial retreat and long-term compaction over the postglacial period.
Dry bulk density values from each sediment cores corresponding to
seismic units 1, 2 and 3.
Bulk elemental analysis
The mean quantity OC and IC has been calculated for U1, U2 and U3 (Fig. 5).
Again values for U1 have been calculated using basal sediments of MD04-2833
(Section VIII). Clear trends emerge from these data, with U3 always
containing a greater quantity of OC than U2, while the proportion of
sedimentary OC generally decreases seawards away from the inner basin. The
opposite is true for sedimentary IC, which generally increases seawards away
from the inner basin.
%OC and %IC values from each sediment cores corresponding
to seismic units 1, 2 and 3.
Volumetric modelling
The interpolation of the seismic profiles led to the creation of four DTMs
(Supplement) which represent horizons H1 to H4. To determine the
accuracy of the models, the DTM for H4 was compared to an existing
high-resolution bathymetric model of the fjord (Bates et al., 2004). The
coordinates (x,y,z) of key high and low points (n=12) were compared between
surveys; the mean divergence between surveys was calculated as x=-0.56 m,
y=-0.81 m and z= 0.21 m. Although the H4 DTM slightly negatively offsets the
x and y and overestimates the z coordinates of these points, the general location
and pattern of these seabed features compare favourably.
The DTMs and numerical integration algorithms were combined to calculate the
volume of sediment held within each seismic unit. A further subdivision by
basin and according to postglacial (U2 and U3) and glacial (U1) sediment
origin has also been undertaken (Table 3). The fjord as a whole contains a
greater volume of glacial
(6.00 × 108 m3± 1.89 %) than postglacial
sediment (5.31 × 108 m3± 7.39 %). Comparison
of the three basins indicates that the middle basin contains the greatest
combined (postglacial + glacial) volume of sediment
(3.04 × 107 m3± 5.30 %) followed by the outer
(1.60 × 107.2 m3± 5.74 %) and inner basins
(4.17 × 106 m3± 4.48 %).
Sediment volume calculated as the mean of the three numerical
integration algorithms; the error is reported as relative standard deviation
(%RSD) which integrates the uncertainty in the seismic interpolation and
the standard deviation of the numerical integration algorithms. The data are
reported for the postglacial (PG) and glacial (G) sediment at the basin
level.
The mean DBD for U2 and U3 were modelled (Fig. 6) to determine the
variability in spatial distribution throughout the fjord. A similar spatial
pattern of DBD is found in both U2 and U3; the DBD is lowest in the inner
basin (U2: 0.47 g cm-3, U3: 0.59 g cm-3), rising through the
middle basin where it peaks at 1.75 and 1.67 g cm-3 for U2 and U3
respectively. The transition between the middle and outer basins is
characterised with low DBD values (U2: 0.72 g cm-3, U3:
0.91 g cm-3); from this low point the DBD rises towards the seaward
end of the fjord.
Contour maps showing the output of the spatial distribution model
for the mean dry bulk density of (a) U3 and (b) U2. Sampling locations
indicated with black diamonds.
The model output was integrated with the volumetric data to calculate the
mass of sediment held within the postglacial sequences (Table 4). Since we
have a single mean value of DBD for U1, we applied this throughout the fjord
to calculate the mass of sediment held within this unit. The fjord holds a
total of 1928.3 ± 7.3 Mt of sediment which is split into
652.1 ± 6.6 Mt of postglacial and 1276.2 ± 8.9 Mt of glacial
sediment. The inner basin holds the least sediment, followed by the outer
basin, with the middle basin acting as the main store of sediment in Loch
Sunart.
Mass of sediment held within Loch Sunart and the mass of total
carbon (TC), organic carbon (OC) and inorganic carbon (IC) held within Loch
Sunart's postglacial (PG) and glacial (G) sediment.
Using a similar approach, the mean OC and IC were spatially modelled
throughout the fjord. The output for U3 is illustrated in Fig. 7. As
before, the model outputs for U2 and U3 were integrated with the sediment
mass data in order to quantify the mass of TC, OC and IC held within the
postglacial and glacial sediments (Table 4). Single mean values for TC, OC
and IC were again used to calculate their respective mass of C within the
sediment of U1.
Output of U3 spatial distribution model for (a) organic
carbon (OC) and (b) inorganic carbon (IC). Sampling locations indicated with black diamonds.
The sediment of Loch Sunart holds a significant quantity of C
(26.9 ± 0.5 Mt) split between OC (11.5 ± 0.2 Mt) and IC
(15.0 ± 0.4 Mt). Though a greater mass of sediment is held within the
glacial component, it is the postglacial sediments which hold the largest
quantity of C (19.9 ± 0.3 Mt). The quantity of C held within each of
Loch Sunart's basins varies; the lowest amount is found in the inner basin
(2.1 ± 0.5 Mt), followed by the outer basin (6.7 ± 0.6 Mt). The
sediment of the middle basin holds significantly more C than both the inner and
outer basins combined – with 18.1 ± 0.7 Mt C stored in these sediments,
indicating that the middle basin is the main repository for sedimentary C in
Loch Sunart.
How effectively the fjord stores C is measured by the Ceff
(Table 5) and the OC : IC ratio. Loch Sunart is characterised by an
OC : IC ratio of 0.74 and has an average Ceff of 0.560 Mt
C km-2, which can be further broken down to a postglacial
Ceff of 0.412 Mt C km-2 and a glacial Ceff of
0.148 Mt C km-2. The effective C storage can also be illustrated at
the individual basin level, with the postglacial sediments of the inner,
middle and outer basins characterised by OC : IC ratios of 4, 1 and 0.42,
illustrating the transition from OC as the dominant component of the sediment
in the upper fjord to an IC-dominated sediment at the seaward end of the
fjord. The middle basin is the most effective at storing postglacial OC
followed by the inner and outer basin; similarly the middle basin is most
effective at storing IC, but in contrast to the effective storage of OC, the
outer basin ranks second followed by the inner basin for IC. The glacial
material held within the fjord as a whole is characterised by an OC : IC
ratio of 0.42 with a mean OCeff 0.044 Mt km-2 and
ICeff 0.104 Mt km-2.
The effective C storage (Ceff) of Loch Sunart's postglacial
and glacial sediment in comparison to Scottish terrestrial C stores.
CAreaTCCeffOCeffICeffReferenceInventories(km2)(Mt)(Mt km-2)(Mt km-2)(Mt km-2)Postglacial Inner basin5.51.30.2380.1910.047Middle basin24.714.10.5700.2850.284Outer basin17.14.50.2630.0770.184Glacial Inner basin5.50.80.1470.0440.104Middle basin24.74.00.1610.0470.113Outer basin17.12.20.1290.0380.091Postglacial47.319.90.4120.1990.213Glacial47.37.00.1480.0440.104Loch Sunart47.326.90.5600.2420.3182 m depth Peatlands*17 27016200.094Chapman et al. (2009)Organo-mineral soil*754Bradley et al. (2005)Mineral soil*4981 m depth Peat17 369813.90.047Aitkenhead andCoull (2016)Alluvial soil165740.80.025Alpine soil3825145.70.038Bare ground167250.50.030Brown earth15 971590.30.037Gley15963645.40.040Podzol18 159536.60.029Ranker253182.60.033Regosol43719.00.044
* Both studies calculated the soil C stocks excluding IC data; therefore the
stocks only represent the OC held within these stocks.
Carbon accumulation and burial
The SRs vary between the sedimentary basins of the fjord, with the most rapid
rates in the inner basin recorded in core GC013 (0.087 cm yr-1). The
middle and outer basins have lower SRs as shown by cores GC020
(0.025 cm yr-1) and GC011 (0.017 cm yr-1). The calculated organic carbon accumulation (OCAR) and burial (OCBR) rates for Loch Sunart are presented in Table 6 alongside rates from other
fjords globally. Our estimates are in line with the fjords of New Zealand
(Pickrill, 1993; Knudson et al., 2011; Hinojosa et al., 2014; Smith et al.,
2015), Alaska (vegetated) (Cui et al., 2016) and Chile (Sepúlveda et al.,
2011); they are somewhat lower than the glaciated fjords of NW Europe
(Winkelmann and Knies, 2005; Müller, 2001; Kulinski et al., 2014).
Although not shown in Table 6, the calculated inorganic carbon accumulation rates (ICARs) range between 0.69 and
36.89 g IC m-2 yr-1, resulting in long-term annual average
estimates of IC burial of between 56 and 1.7 × 103 t for
the fjord as a whole.
Sedimentation, OC accumulation and OC burial rates for Loch Sunart in comparison to global fjords.
a OC Burial rate calculated assuming a burial
efficiency of 63 % (Sepúlveda et al., 2005). b OC Burial
rate calculated assuming a burial efficiency of 80 % (Smith et al.,
2015).
A methodology for estimating sedimentary carbon and attributing uncertainty
estimates
The joint geophysical and geochemical methodology outlined (Fig. 8) provides
a robust approach to allow the first quantification of total sedimentary C
stocks in a fjord setting. An important part of estimating sedimentary C
stocks should be the quantification of uncertainty associated with these
estimates. There are several types of uncertainty that can influence
sedimentary carbon estimations (Fig. 8), including interpolation,
algorithmic, analytical, sampling and extrapolation uncertainty. Several of
these types of uncertainties are easily dealt with statistically: for example
the analytical uncertainties have been quantified through triplicate
measurements. The sampling uncertainty of a stratigraphic sequence (i.e.
spatial variability of C content in relation to sampling density) can be
overcome by calculating the mean and standard deviation to create composite
values that are representative of the seismic unit as a whole. We integrated
the quantifiable uncertainties at each calculation step (Fig. 8). By
calculating composite standard deviations we are able to propagate the
uncertainties throughout the C quantification process. In the interpolation
of the seismic geophysics, it is difficult to fully quantify the uncertainty
involved in the process. Bond et al. (2007) set out a five-step framework
designed to reduce uncertainty in this process. We utilised the framework of
Bond et al. (2007) and additionally integrated a validation step using
radiocarbon dating of sedimentary cores (see Sect. 3.2). This allows us to
reduce the uncertainties associated with the seismic interpretation, although
we recognise that some uncertainty remains (e.g. highly variable patterns of
sediment thickness) which cannot be fully quantified. Within this framework
of uncertainty, we consider our method to give a robust estimate for the
carbon stocks present.
Flow diagram detailing the steps towards calculating the
sedimentary C stocks within a fjord with the known uncertainties specified.
Discussion: a new sedimentary C inventory for Scottish coastal
waters
The development of this methodology has allowed the estimation of the
sedimentary C stocks stored in a mid-latitude fjord. An estimated
26.9 ± 0.5 Mt C has been accounted for within our study site (Loch
Sunart).
The only directly comparable estimation for sedimentary C stocks is the
report by Burrows et al. (2014), where they calculated that 0.3 Mt OC was
stored in all 110 Scottish fjords. In comparison, our findings estimate that
Loch Sunart alone holds 11.5 Mt OC. However, Burrows et al. (2014) focused
on the top 10 cm of sediment because data availability and the lack of a
robust methodology made it impossible to calculate the entire sedimentary C
stock; this has resulted in a significant underestimation of the quantity of
C held within the sediment of these fjords. Additionally, Burrows et
al. (2014) did not consider IC to be a major component in these sediments;
instead the authors focused on Scottish fjords largely as OC stores. In
contrast, our results demonstrate that Loch Sunart stores 15.0 Mt IC in
comparison to 11.5 Mt OC. The general lack of IC data for the coastal
environment makes it difficult to assess how representative Loch Sunart is of
these coastal sedimentary IC stores; however, our results do highlight the
potential significance of IC as a major component of sedimentary C stores in
these depositional environments. Our results also highlight that fjords in
general (Smith et al., 2015) act as an OC-rich sediment transition zone
between terrestrial and oceanic environments.
Loch Sunart's sediment currently holds 11.5 Mt OC with an additional
estimated range of between 89 and 1.2 × 103 t of OC buried
annually. This highly localised OC trapping in the coastal zone may further
reduce reworking and remineralisation of the material which would have
otherwise resulted in the release of CO2 through biotic processes (Smith
et al., 2015). This 11.5 Mt of sedimentary OC is equivalent to 40.9 Mt
CO2e (carbon dioxide equivalent). As a whole, the sediment within Loch
Sunart stores 99.6 Mt CO2e, which is equivalent to over 2 years of
Scotland's total greenhouse gas emission for 2014, estimated to have reached
46.7 Mt CO2e (Scottish Government, 2016).
Globally, the terrestrial C stores have received much more attention than
their marine counterparts, with significant focus on quantifying the forest
(Köhl et al., 2015) and soil C stocks (Köchy et al., 2015;
Scharlemann et al., 2014). The work by Duarte et al. (2005) to compile the
known stocks and burial rate of C in the coastal environment highlighted that
the coastal ocean constitutes a large store of carbon, which remains poorly
understood; from this work the concept of blue carbon arose (Nellemann et
al., 2009). The focus of Duarte et al. (2005) was to highlight that the
vegetated coastal zones (i.e. salt marsh, seagrass and mangroves) bury and
store significant quantities of C and that these stores should be further
investigated and recognised in policy outputs. However, these authors largely
overlook the importance of what they described as depositional areas
(estuaries and the shelf sea) as long-term repositories of OC detritus from
the vegetated coastal environment (Krumhansl et al., 2012), and they ignored the
terrestrial OC inputs. These authors recognised that coastal (and shelf)
depositional areas are important stores of sedimentary C globally, yet almost
no consideration is given to how these areas vary in terms of their capacity
to store C.
Furthermore, if we consider the range of estuarine environments (e.g. fjord,
delta, coastal plain, bar-built and tectonic), it is clear that the
characteristics of each type of estuary will impact the manner in which C is
buried and stored. For example, the restricted nature of fjords will be
conducive to sediment capture and effective C storage when compared to the
more open estuarine environments which experience greater flushing. Globally,
the rates at which fjords accumulate and bury OC is reasonably well defined
(Table 6). This study adds data for the under-represented mid-latitude fjords
which are comparable to other vegetated fjordic systems around the world
(Pickrill, 1993; Sepúlveda et al., 2011; Knudson et al., 2011; Hinojosa
et al., 2014; Smith et al., 2015). Additionally, for the first time, we
cautiously report IC accumulation and burial rates for a fjord. The burial of
IC is another significant mechanism of CO2 sequestration that has been
overlooked in fjordic systems and requires further investigation to quantify
its importance to the coastal C cycle as a whole.
Our initial work suggests that the depositional area category could be
further expanded upon to include fjords as a separate component, and this
concept is supported by Smith et al. (2015), who indicated that fjords are
“hot-spots for OC burial” and should be considered separately from
estuaries when investigating global ocean OC burial. Currently, there are
insufficient globally available data to advocate fjords being categorised as
a separate component in global coastal C stores; however, the standardised
methodology outlined (Fig. 8) provides a platform to investigate this concept
further.
At the national level there has been a significant focus on quantifying
Scottish soil C stocks, with much attention given to the peatlands
(Aitkenhead and Coull, 2016; Bradley et al., 2005; Chapman et al., 2009).
Peat and other organic-rich soils cover 66 % of Scotland and account for
50 % of all the United Kingdom's soil C stocks (Cummins et al., 2011).
The Scottish peatlands store 1620 Mt C (Chapman et al., 2009) over an area
of 17 270 km2, while the other soils hold 2110.9 Mt C over
60 215 km2 (Aitkenhead and Coull, 2016). In comparison to these
figures, the quantity of C stored in Loch Sunart is small, but the fjord
itself only covers an area of 47.3 km2. When the fjord's Ceff
is compared to how effectively Scotland's soils and peatlands store C
(Table 5), we can see that, when normalised as a store per unit area basis,
Loch Sunart stores significantly more C than the soils of Scotland. The fjord
has a Ceff of 0.568 Mt C km-2 compared to 0.094 and
0.035 Mt C km-2 for the peatlands and other soils of Scotland. Our
results suggest that Loch Sunart is one of the most effective stores of C in
Scotland and highlight the potential of the sediment in these mid-latitude
fjords to hold a significant quantity of C. Many of these terrestrial C
stores are, of course, vulnerable to rapid and long-term environmental
change; the Scottish terrestrial C stocks are at risk from erosion (Cummins
et al., 2011) and even fire (Davies et al., 2013), both of which are
increasing in pace and frequency because of anthropogenic activities. In comparison,
a fjord's geomorphology combined with its depth gives sedimentary C stores a
level of protection not afforded to terrestrial C stores. This does not mean
that the sedimentary C in sea lochs is invulnerable, but rather that it is
buffered from the immediate effects of chemical, biological and physical
environmental change during interglacial periods. Over longer time frames it
is known that these sedimentary stores are scoured by glacial advances,
resulting in the material being transported to the adjacent shelf and slope
(Jaeger and Koppes, 2016). Further investigation is required to better
understand the processes governing the transfer of material to the shelf and
what impact this has on the quality of OC in coastal sediment stores (Smith
et al., 2015).
The methodology outlined in this paper provides a platform from which to
calculate the carbon stocks in other fjordic systems, as well as environments
with restricted sediment exchange processes such as estuaries,
freshwater lakes and artificial systems (e.g. reservoirs and
irrigation pools).
Conclusions
The integration of the geochemical and geophysical techniques outlined
provides a robust and repeatable methodology to quantitatively calculate the
volume of sediment and make first-order estimations of carbon stored within
fjordic sediments. Using this methodology we have shown that Loch Sunart, a
fjord on the west coast of Scotland, holds 26.9 Mt C, which is equivalent to
double Scotland's CO2 emissions for 2014. While these individual fjord
stores may be small in comparison with Scotland's peatland and soil C stocks,
we show they are potentially far more effective stores of both OC and IC than
Scotland's terrestrial habitats (area-normalised comparison). The results
from this study suggest that the sediment in Scotland's 110 fjords (Edwards
and Sharples, 1986) represent a potentially significant, yet currently
largely unaccounted for repository of both OC and IC. These fjords act to
trap sediment and reduce the remineralisation of OC into the atmosphere.
Additionally, the C held within these 110 fjords is likely to represent a
significant portion of Scotland's blue carbon capital that has not yet been
considered at the marine ecosystem, global C cycle and wider policy levels.
Without a better understanding of these globally significant stores of marine
sedimentary C, it remains difficult to fully quantify the coastal C cycle.
However, evidence suggests that these fjordic environments do play an
important role in buffering the release of CO2 through the effective
burial of large quantities of C in these sediments. The future strategic
application of the methodology outlined in this study to different fjord
types and locations offers the potential through appropriate upscaling to
estimate the fjordic sedimentary C stores at regional, national and
global scales.
Data availability
The data used for this publication are publicly available and can be accessed
at the NERC National Geoscience Data Centre (NGDC)(Data Submission 3561). The
seismic geophysical data used in this study can be obtained by contacting the
fourth author (agnes.baltzer@univ-nantes.fr). For any further requests, please
contact the corresponding author.
The Supplement related to this article is available online at doi:10.5194/bg-13-5771-2016-supplement.
Craig Smeaton and William E. N. Austin conceived the research and wrote the manuscript, to which all
co-authors contributed data or provided input. Craig Smeaton conducted the research as
part of his PhD at the University of St. Andrews, supervised by William E. N. Austin, Althea L. Davies and
John A. Howe.
Acknowledgements
This work was supported by the Natural Environment Research Council (grant
number: NE/L501852/1) with additional support from the NERC Radiocarbon
Facility (Allocation 1934.1015). Seismic profiles and the CALYPSO long core
were acquired within the frame of the French ECLIPSE programme with additional
financial support from NERC, SAMS and the University of St Andrews. The
authors would like to thank Marion Dufresne's Captain J.-M. Lefevre, the
Chief Operator Y. Balut (from IPEV) and Richard Bates (University of St Andrews). Additionally, we would like to thank Colin Abernethy (Scottish
Association of Marine Science) for laboratory support. Finally, we thank
Jessica Hinojosa and one anonymous reviewer whose insightful comments
improved this paper.
Edited by: S. Pantoja
Reviewed by: J. Hinojosa and one anonymous referee
References
Aitkenhead, M. J. and Coull, M. C.: Mapping soil carbon stocks across
Scotland using a neural network model, Geoderma, 262, 187–198,
2016.
Baeten, N. J., Forwick, M., Vogt, C., and Vorren, T. O.: Late Weichselian and
Holocene sedimentary environments and glacial activity in Billefjorden,
Svalbard, Geol. Soc. London, Spec. Publ., 344, 207–223,
2010.
Baltzer, A., Bates, C. R., Mokeddem, Z., Clet-Pellerin, M., Walter-Simonnet,
A.-V., Bonnot Courtois, C., and Austin, W. E. N.: Using seismic facies and pollen
analyses to evaluate climatically driven change in a Scottish sea loch
(fjord) over the last 20 ka, Geological Society, London, Special
Publications, 344, 355–369, 2010.
Bates, C. R., Moore, C. G., Harries, D. B., Austin, W. E. N., and Lyndon, A.
R.:
Broad scale mapping of sublittoral habitats in Loch Sunart, Scotland,
Scottish Natural Heritage Commissioned, Report No. 006 (ROAME No. F01AA401C), 2004.
Bauer, J. E., Cai, W.-J., Raymond, P. A., Bianchi, T. S., Hopkinson, C. S., and
Regnier, P. A. G.: The changing carbon cycle of the coastal ocean,
Nature, 504, 61–70, 2013.
Binns, P. E., Harland, R., and Hughes, M. J.: Glacial and post glacial
sedimentation in the sea of the Hebrides, Nature, 248, 751–754, 1974a.
Binns, P. E., Mcquillin, R., and Kenolty, N.: The geology of the sea of the
Hebrides, Institute of Geological Sciences 73/14, 1974b.
Bond, C. E., Gibbs, A. D., Shipton, Z. K., and Jones, S.: What do you think
this is? Conceptual uncertainty, in: Geoscience Interpretation, GSA Today,
17, 4–10, 2007.
Boulton, G. S., Chroston, P. N., and Jarvis, J.: A marine seismic study of late
Quaternary sedimentation and inferred glacier fluctuations along western
Inverness–shire, Scotland, Boreas, 10, 39–51, 1981.
Bradley, R. I., Milne, R., Bell, J., Lilly, A., Jordan, C., and Higgins, A.:
A soil carbon and land use database for the United Kingdom, Soil Use
Manage., 21, 363–369, 2005.
Bronk Ramsey, C.: Bayesian analysis of radiocarbon dates, Radiocarbon, 51,
337–360, 2009.
Bronk Ramsey, C. and Lee, S.: Recent and planned developments of the program
OxCal, Radiocarbon, 55, 720–730, 2013.
Burrows, M. T., Kamenos, N. A., Hughes, D. J., Stahl, H., Howe, J. A., and
Tett, P.: Assessment of carbon budgets and potential blue carbon stores in
Scotland's coastal and marine environment, Scottish Natural Heritage
Commissioned Report No. 761, 2014.
Cage, A. G., Heinemeier, J., and Austin, W. E. N.: Marine radiocarbon reservoir
ages in Scottish coastal and fjordic waters, Radiocarbon, 48,
31–43, 2006.Cai, W.-J.: Estuarine and coastal ocean carbon paradox: CO2 sinks or sites
of terrestrial carbon incineration?, Ann. Rev. Mar. Sci., 3, 123–45,
2011.
Cannell, M. G. R., Milne, R., Hargreaves, K. J., Brown, T. A. W., Cruickshank,
M. M., Bradley, R. I., Spencer, T., Hope, D., Billett, M. F., Adger, W. N., and
Subak, S.: National inventories of terrestrial carbon sources and sinks: The
UK experience, Climatic Change, 42, 505–530, 1999.
Capell, R., Tetzlaff, D., and Soulsby, C.: Will catchment characteristics
moderate the projected effects of climate change on flow regimes in the
Scottish Highlands?, Hydrol. Process. 27, 687–699, 2013.
Chapman, S. J., Bell, J., Donnelly, D., and Lilly, A.: Carbon stocks in
Scottish peatlands, Soil Use Manage., 25, 105–112, 2009.
Chapman, S. J., Bell, J. S., Campbell, C. D., Hudson, G, Lilly, A., Nolan,
A. J., Robertson, A. H. J., Potts, J. M., and Towers, W.: Comparison of soil
carbon stocks in Scottish soils between 1978 and 2009, Eur. J.
Soil Sci., 64, 455–465, 2013.
Chiles, J. P. and Delfiner, P.: Geostatistics: Modeling Spatial Uncertainty,
John Wiley and Sons, New York, ISBN: 0471083151, 695 pp., 1999.Clark, C. D., Hughes, A. L. C., Greenwood, S. L., Jordan, C., and Petter, H.:
Pattern and timing of retreat of the last British-Irish Ice Sheet, Quaternary
Sci. Rev., 44, 112–146, 10.1016/j.quascirev.2010.07.019, 2010.
Cressie, N. A. C.: The Origins of Kriging, Math. Geol., 22,
239–252, 1990.
Cui, X., Bianchi, T. S., Jaeger, J. M., and Smith, R. W.: Biospheric and
petrogenic organic carbon flux along southeast Alaska, Earth Planet. Sc.
Lett., 452, 238–246, 2016a.
Cui, X., Bianchi, T. S., Savage, C., and Smith, R. W.: Organic carbon burial
in fjords?: Terrestrial versus marine inputs, Earth Planet. Sci. Lett., 451,
41–50, 2016b.
Cummins, R., Donnelly, D., Nolan, A., Towers, W., Chapman, S., Grieve, I.,
and Birnie, R. V.: Peat erosion and the management of peatland habitats.
Scottish Natural Heritage Commissioned Report No. 410, 2011.
Dadey, K. A., Janecek, T., and Klaus, A.: Dry bulk density: its use and
determination, Proceedings of the Ocean Drilling Program, Scientific Results,
126, 551–554, 1992.
Danielson, R. E. and Sutherland, P. L.: Porosity, in: Methods of
soil analysis, part 1, Physical and mineralogical methods, edited by: Klute, A., Am. Soc. Agr.,
Madison, Wisconsin, 443–461, 1986.
Davies, G. M., Gray, A., Rein, G., and Legg, C. J.: Peat consumption and
carbon loss due to smouldering wildfire in a temperate peatland, For. Ecol.
Manage., 308, 169–177, 2013.Duarte, C. M.: Reviews and syntheses?: Hidden Forests, the role of
vegetated coastal habitats on the ocean carbon budget, Biogeosciences Discuss., 10.5194/bg-2016-339, in review, 2016.Duarte, C. M., Middelburg, J. J., and Caraco, N.: Major role of marine vegetation on the oceanic carbon cycle, Biogeosciences, 2, 1–8, 10.5194/bg-2-1-2005, 2005.
Edwards, A. and Sharples, F.: Scottish Sea Lochs: A Catalogue. Scottish Marine
Biological Association/Nature Conservancy Council, Oban, 1986.
Forwick, M., Vorren, T. O., Hald, M., Korsun, S., Roh, Y., Vogt, C., and Yoo,
K.-C.: Spatial and temporal influence of glaciers and rivers on the
sedimentary environment in Sassenfjorden and Tempelfjorden, Spitsbergen,
Geol. Soc. London, Spec. Publ., 344, 163–193,
2010.Friedrich, J., Janssen, F., Aleynik, D., Bange, H. W., Boltacheva, N.,
Çagatay, M. N., Dale, A. W., Etiope, G., Erdem, Z., Geraga, M., Gilli,
A., Gomoiu, M. T., Hall, P. O. J., Hansson, D., He, Y., Holtappels, M.,
Kirf, M. K., Kononets, M., Konovalov, S., Lichtschlag, A., Livingstone, D.
M., Marinaro, G., Mazlumyan, S., Naeher, S., North, R. P., Papatheodorou,
G., Pfannkuche, O., Prien, R., Rehder, G., Schubert, C. J., Soltwedel, T.,
Sommer, S., Stahl, H., Stanev, E. V., Teaca, A., Tengberg, A., Waldmann, C.,
Wehrli, B., and Wenzhöfer, F.: Investigating hypoxia in aquatic
environments: Diverse approaches to addressing a complex phenomenon,
Biogeosciences, 11, 1215–1259, 10.5194/bg-11-1215-2014, 2014.
Gillibrand, P. A., Cage, A. G., and Austin, W. E. N.: A preliminary investigation
of basin water response to climate forcing in a Scottish fjord: evaluating
the influence of the NAO, Cont. Shelf Res., 25,
571–587, 2005.
Hedges, J. I., Keil, R. G., and Benner, R.: What happens to terrestrial organic
matter in the ocean?, Org. Geochem., 27, 195–212, 1997.
Hilton, R. G., Galy, A., Hovius, N., and Horng, M. J.: Efficient transport of
fossil organic carbon to the ocean by steep mountain rivers: An orogenic
carbon sequestration mechanism, Geology, 39, 71–74, 2011.Hinojosa, J. L., Christopher, M., Moy, C. M., Claudine, H., Stirling, C. H., Gary,
S., Wilson, G. S., and Eglinton, T. I.: Carbon cycling and burial in New Zealand's
fjords, Geochem.
Geophys. Geosyst., 15, 4047–4063, 10.1002/2014GC005433.Received, 2014.
Howard, P. J. A., Loveland, P. J., Bradley, R. I., Dry, F. T., Howard, D. M., and
Howard, D. C.: The carbon content of soil and its geographical distribution
in Great Britain, Soil Use Manage., 11, 9–15, 1995.
Howe, J. A., Shimmield, T., Austin, W. E. N., and Longva, O.: Postglacial
depositional environments in a mid-high latitude glacially-overdeepened sea
loch, inner Loch Etive, western Scotland, 185, 417–433, 2002.
Jaeger, J. M. and Koppes, M. N.: The role of the cryosphere in
source-to-sink systems, Earth-Sci. Rev., 153, 43–76,
2016.
Johnston, D. H. and Cooper, M. R.: Methods and Applications in Reservoir
Geophysics, Investigations in geophysics, Tulsa, OK, Society of Exploration
Geophysicists, 15, 2010.
Kennedy, P., Kennedy, H., and Papadimitriou, S.: The effect of acidification
on the determination of organic carbon, total nitrogen and their stable
isotopic composition in algae and marine sediment, Rapid Commun.
Mass Sp., 19, 1063–1068, 2005.
Knudson, K. P., Hendy, I. L., and Neil, H. L.: Re-examining Southern Hemisphere
westerly wind behaviour: Insights from a late Holocene precipitation
reconstruction using New Zealand fjord sediments, Quaternary Sci. Rev.,
30, 3124–3138, 2011.Köchy, M., Hiederer, R., and Freibauer, A.: Global distribution of soil organic
carbon – Part 1: Masses and frequency distributions of SOC stocks for the
tropics, permafrost regions, wetlands, and the world, Soil, 1, 351–365,
10.5194/soil-1-351-2015, 2015.
Köhl, M., Lasco, R., Cifuentes, M., Jonsson, Korhonen, K. T., Mundhenk,
P., de Jesus Navar, J., and Stinson, G.: Changes in forest production,
biomass and carbon: Results from the 2015 UN FAO Global Forest Resource
Assessment, For. Ecol. Manage., 352, 21–34,
2015
Krumhansl, K. A. and Scheibling, R. E.: Production and fate of kelp
detritus, Mar. Ecol. Prog. Ser., 467, 281–302, 2012.
Kuliński, K., Kędra, M., Legeżyńska, J., Głuchowska, M.,
and Zaborska,
A.: Particulate organic matter sinks and sources in high Arctic fjord, J.
Mar.
Syst., 139, 27–37, 2014.
Mokeddem, Z., Baltzer, A., Goubert, E., and Clet-Pellerin, M.: A multiproxy
palaeoenvironmental reconstruction of Loch Sunart (NW Scotland) since the
Last Glacial Maximum, Geological Society, London, Special Publications, 344,
341–353, 2010.
Müller, A.: Geochemical expressions of anoxic conditions in
Nordåsvannet, a land-locked fjord in western Norway, Appl. Geochem., 16,
363–374, 2001.
Nellemann, C., Corcoran, E., Duarte, C. M., Valdés, L., DeYoung, C.,
Fonseca, L., and Grimsditch, G. (Eds.): Blue Carbon: A Rapid Response
Assessment, United Nations Environment Programme, GRID-Arendal, 2009.
Nieuwenhuize, J., Maas, Y. E. M., and Middelburg, J. J.: Rapid analysis of
organic carbon and nitrogen in particu-late materials, Mar. Chem.,
45, 217–224, 1994.
Nørgaard-Pedersen, N., Austin, W. E. N., Howe, J. A., and Shimmield, T.:
The Holocene record of Loch Etive, western Scotland: Influence of catchment
and relative sea level changes, Mar. Geol., 228, 55–71,
2006.
Pedersen, J. B. T., Kroon, A., Jakobsen, B. H., Mernild, S. H., Andersen, T.
J., and Andresen, C. S.: Fluctuations of sediment accumulation rates in front
of an Arctic delta in Greenland, The Holocene, 23, 860–868,
2013.
Pickrill, R. A.: Sediment yields in Fiordland, J. Hydrol. N. Z., 31,
39–55, 1993.
Press, W. H., Flannery, B. P., Teukolsky, S. A., and Vetterling, W. T.: Numerical
Recipes in C, Cambridge University Press, 1988.
Reimer, P.: IntCal13 and Marine13 Radiocarbon Age Calibration Curves
0–50,000 Years cal BP, Radiocarbon, 55, 1869–1887, 2013.
Scharlemann, J. P., Tanner, E. V., Hiederer, R., and Kapos, V.: Global soil
carbon: understanding and managing the largest terrestrial carbon pool,
Carbon Manage., 5, 81–91, 2014.Scottish Government: Scottish Greenhouse Gas Emissions 2014,
http://www.gov.scot/Publications/2016/06/2307, last access: 9 September
2016.
Scourse, J. D., Haapaniemi, A. I., Colmenero-hidalgo, E., Peck, V. L., Hall,
I. R., Austin, W. E. N., Knutz, P. C., and Zahn, R.: Growth, dynamics and
deglaciation of the last British – Irish ice sheet?: the deep-sea
ice-rafted detritus record, Quaternary Sci. Rev., 28, 3066–3084,
2009Sepúlveda, J., Pantoja, S., Hughen, K., Lange, C., Gonzalez, F.,
Muñoz, P., Rebolledo, L., Castro, R., Contreras, S., Ávila, A.,
Rossel, P., Lorca, G., Salamanca, M., and Silva, N.: Fluctuations in export
productivity over the last century from sediments of a southern Chilean
fjord (44∘ S), Estuar. Coast. Shelf Sci., 65, 587–600,
2005.Sepúlveda, J., Pantoja, S., and Hughen, K. A.: Sources and distribution of
organic matter in northern Patagonia fjords, Chile (44–47∘ S): A
multi-tracer approach for carbon cycling assessment, Cont. Shelf Res.,
31, 315–329, 2011.
Simpkin, P. G. and Davis, A.: For seismic profiling in very shallow water, a
novel receiver, in: Sea Technology, 1983.Smith, R. W., Bianchi, T. S., Allison, M., Savage, C., and Galy, V.: High
rates of organic carbon burial in fjord sediments globally, Nat. Geosci., 8,
450–453, 10.1038/NGEO2421, 2015.Soil Survey of Scotland: (1970–1987). Soil maps of Scotland (partial
coverage) at a scale of 1:25 000, Macaulay Institute for Soil Research,
Aberdeen.St-Onge, G. and Hillaire-Marcel, C.: Isotopic constraints of sedimentary
inputs and organic carbon burial rates in the Saguenay Fjord, Quebec, Mar.
Geol., 176, 1–22, 10.1016/S0025-3227(01)00150-5, 2001.
Syvitski, J. P. M. and Shaw, J.: Sedimentology and Geomorphology of Fjords,
Geomorphology and Sedimentology of Estuaries, Dev. Sedimento., 53, 113–178,
1995.
Syvitski, J. P. M., Burrell, D. C., and Skei, J. M.: Fjords, Processes and
Products, Springer-Verlag New York, 1987.
Verardo, D. J., Froelich, P. N., and McIntyre, A.: Determination of organic
carbon and nitrogen in marine sediments using the Carlo Erba NA-1500
Analyzer, Deep-Sea Res., 37, 157–165, 1990.
Wilson, L. J., Austin, W. E. N., and Jansen, E.: The last British Ice Sheet?,
Growth, maximum extent and deglaciation, Polar Res., 21, 243–250, 2002.Winkelmann, D. and Knies, J.: Recent distribution and accumulation of organic
carbon on the continental margin west off Spitsbergen, Geochem. Geophys.
Geosyst., 6, Q09012, 10.1029/2005GC000916, 2005.Yu, Z., Loisel, J., Brosseau, D. P., Beilman, D. W., and Hunt, S. J.: Global
peatland dynamics since the Last Glacial Maximum, Geophys. Res.
Lett., 37, L13402, 10.1029/2010GL043584, 2010.