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Turbulent nitrate fluxes in the Amundsen Gulf during ice-covered conditions

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Turbulence and nitrate measurements collected in the Amundsen Gulf during ice-covered conditions in fall 2007 are combined to provide mean vertical profiles of eddy diffusivity K̄ and diffusive nitrate fluxes F̄. The mean diffusivity (with 95% confidence intervals) was maximum near the uppermost sampling depth (10 m) with K̄max = 3(2, 5) × 10 -3 m2 s-1 and decreased exponentially to a depth of ∼50 m, below which it was roughly constant at the background value K$\overline{b = 3(2, 5) × 10-6 m2 s-1. The nitracline, centered around 62 m depth, was subject to an eddy diffusivity close to the background value K̄b and the mean diffusive nitrate flux across the nitracline was F̄nit = 0.5(0.3, 0.8) mmol m-2 d-1. These observations are compared with other regions and the role of vertical mixing on primary production in the Amundsen Gulf is discussed.
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Turbulent nitrate fluxes in the Amundsen Gulf
during icecovered conditions
D. Bourgault,
1
C. Hamel,
2
F. Cyr,
1
J.É. Tremblay,
3
P. S. Galbraith,
4
D. Dumont,
1
and Y. Gratton
5
Received 28 April 2011; revised 21 June 2011; accepted 23 June 2011; published 2 August 2011.
[1] Turbulence and nitrate measurements collected in the
Amundsen Gulf during ice covered conditions in fall 2007
are combi ned to prov ide mean v ertical pr ofiles of e ddy
diffusivity
K and diffusive nitrate fluxes F.Themean
diffusivity (with 95% confidence intervals) was maximum
near the uppermost sampling depth (10 m) w ith
K
max
=
3(2, 5) × 10
3
m
2
s
1
and decreased exponentially to a
depth of 50 m, below which it was roughly constant at
the background value
K
b
=3(2,5)×10
6
m
2
s
1
.The
nitraclin e, centered around 62 m de pth, was subject to an
eddy diffusivity close to the background value
K
b
and the
mean diffusive nitrate flux across the nitracline was
F
nit
=
0.5(0.3, 0.8) mmol m
2
d
1
. These observations are
compared with other regions and the role of vertical mixing
on primary production in the Amundsen Gulf is discussed.
Citation: Bourgault, D., C. Hamel, F. Cyr, J.É. Tremblay, P. S.
Galbraith, D. Dumont, and Y. Gratton (2011), Turbulent nitrate
fluxes in the Amundsen G ulf during ice covered cond itions,
Geophys. Res. Lett., 38, L15602, doi:10.1029/2011GL047936.
1. Introduction
[2] Nutrient replenishment of the surface layer in the
Amundsen Gulf occurs principally during icecovered con-
ditions (November to May), when primary production is at its
minimum [Tremblay et al., 2008]. Since nitrogen is the lim-
iting element for primary producers in this region, the
strength of its replenishment is thought to control overall
annual productivity [Tremblay and Gagnon, 2009]. Nitrogen
supply to the surface layer may come horizontally, from
rivers and surrounding marginal seas, vertically, from up-
wellings and turbulent diffusion of nutrientrich bottom
layers [Lewis et al., 1986], as well as from in situ biological
processes such as ammonification and nitrification [Tremblay
et al. , 2008]. While observations exist about the total con-
tribution of these sources to nitrate uptake in the Amundsen
Gulf [Tremblay et al., 2008], little is known about each
process taken separately. This is particularly true for the
contribution of turbulent diffusion due to past difficulties of
obtaining coincident turbulence and nitrate gradient mea-
surements. While such observations now become more
commonly available in icefree environments (see references
in Table 1), they are still rare in mobile icecovered arctic
environments.
[
3] In 20072008, during the International Polar Year, a
large multidisciplinary research initiative called the Cir-
cumpolar Flaw Lead (CFL) System Study was conducted to
study the general oceanographic conditions in the Amund-
sen Gulf through an annual cycle [Barber et al., 2010]. One
novelty of the sampling strategy consisted in keeping the
Canadian research icebreaker CCGS Amundsen mobile
throughout fall and winter in the Amundsen Gulf, in contrast
to the overwintering fixed within the fastice of Franklin
Bay during the Canadian Arctic Shelf Exchange Study
(CASES) [Fortier et al., 2008]. We synthesize here about
five weeks of almost coincident turbulence and nitrate
measurements collected during the CFL campaign to esti-
mate the turbulent nitrate flux in the Amundsen Gulf during
icecovered conditions.
2. Sampling Conditions and Methods
[4] Measurements were collected aboard the CCGS
Amundsen while drifting with large mobile ice floes in
the Amundsen Gulf (Figure 1) between 16 November and
19 December, 2007, the year with the recordlow ice cover
minimum for the Arctic as a whole [Maslanik et al., 2007].
The mean and standard deviation drift speed during sampling
was 0.08 ± 0.07 m s
1
, according to GPS records. During the
sampling period ice coverage was greater than 90% and made
principally of firstyear ice, 210 km wide vast floes, 50 to
70 cm thick (Canadian Ice Service, Environment Canada,
http://iceglaces.ec.gc.ca/).
[
5] Dissipation rates of turbulent kinetic energy (in
Wkg
1
) were calculated as = (15/2)nhu
z
2
i, where n is
kinematic viscosity and u
z
is the microscale (cm) vertical
shear [e.g., Sundfjord et al., 2007; Martin et al., 2010;
Schafstall et al., 2010]. The latter is measured with an airfoil
shear probe SPM381 and pressure sensor mounted on a
looselytethered freefall vertical microstructure profiler
(VMP500) manufactured by Rockland Scientific Interna-
tional. The hisymbol represents 4m scale averaging with
instrumental noise removed prior to averaging. Such dissi-
pation measurements are generally accepted to be accurate
to within a factor of 2 [Oakey, 1982]. The VMP is also
equipped with SeaBird SBE3F and SBE4C sensors for
finescale (dm) TS measurements from which the density
r is calculated using the equation of state of seawater. The
VMP was deployed through the CCGS Amundsen moon
pool. Since the ship draft is 7.18 m, the top 10 m of profiles
are discarded from the analysis to avoid ship contamination.
1
Institut des Sciences de le Mer de Rimouski, Université du Québec
à Rimouski, Rimouski, Quebec, Canada.
2
De
´
partement de Physique, Universi de Sherbrooke, Sherbrooke,
Quebec, Canada.
3
De
´
partement de Biologie, Université Laval, Quebec, Quebec,
Canada.
4
Maurice Lamontagne Institute, Fisheries and Oceans Canada,
MontJoli, Quebec, Canada.
5
Centre Eau Terre Environnement, Quebec, Quebec, Canada.
Copyright 2011 by the American Geophysical Union.
00948276/11/2011GL047936
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L15602, doi:10.1029/2011GL047936, 2011
L15602 1of5
[6] The turbulent diffusivity of nitrate was calculated as
K = G/N
2
[Osborn, 1980], with G = 0.2, N
2
= (g/r)r
z
and
g =9.8m
2
s
1
[e.g., Sharples e t al.,2007;Rippeth et al.,
2009; Martin et al., 2010; Schafstall et al., 2010]. Note
that although this model for K was initially derived for shear
mixing it has proven adequate by Sundfjord et al. [2007,
Figure 12] in a similar icecovered coastal arctic environ-
ment (Barents Sea) where doublediffusion mixing may
have been present. Furthermore, the Kelley [1990] parame-
terization for double diffusion of salt applied to our CTD
data suggests that doublediffusion mixing was negligible in
the Amundsen Gulf during the sampling period (not shown).
[
7] Overall, 175 VMP profiles collected at 26 different
stations within the Amundsen Gulf were used in the analysis
(Figure 1). Generally, four consecutive VMP profiles were
collected at every station except for two where 55 and 24
profiles were collected. Sampling periods roughly spanned
the range of wind speed conditions occurring during the 5
week survey period presented here (Figure 2) with a slight
bias towards low wind conditions; the mean wind speed
during turbulence sampling was 80% of the mean occurring
during the entire survey.
[
8] Nitrate (NO
3
) concentrations N (in mmol m
3
) were
measured with a Satlantic Isus V3 nitrate sensor fixed under-
neath a rosette sampler. At some stations, bottle samples
were also collected and nitrate concentration determined
from la boratory an alyses as detailed by Tremblay et al.
[2008]. The Isus sensor measurements were then post
calibrated by linear regression against 151 samples from
12 stations. The calibration yielded a correlation coefficient
R = 0.91 and a standard error ±2.2 mmol m
3
(P. Guillot,
QuébecOcéan, personal communication, 2011), consistent
with the manufacturer accuracy specification of ±2 mmol m
3
.
The minimum concentration that could be detected with
this sensor is 0.5 mmol m
3
. The measurements were then
averaged into 4m bins matching the dissipation rate data.
Figure 3 shows a typical 4m scale calibrated profile com-
pared with bottled samples. Although the Isus sensor is
characterized by some systematic deviations relative to the
bottle profile, it captures satisfactorily well the position and
strength of the nitracline, which is essential for the analysis
presented here, with around 15% difference in its maximum
gradient and 10% difference in its position as defined by the
maximum gradient. Note also that very low concentrations
(<0.5 mmol m
3
) near the surface (1020 m, Figure 3) are
below the reliable detection capacity of the Isus sensor.
Overall, 71 nitrate profiles were grouped and averaged per
station, providing 26 nitrate profiles.
[
9] Profiles of vertical turbulent nitrate fluxes (in
mmol m
2
d
1
) were calculated by combining VMP and
nitrate observations through F = KN
z
× 86 400 s d
1
. Each
of the 175 K profiles was associated with the closest of the
26 nitrate profiles, providing 175 profiles of nitrate fluxes F.
[
10] Measurements were synthesized into averaged pro-
files considered to be representative of conditions prevailing
in the Amundsen Gulf throughout the sampling period. As
Table 1. Comparisons of Nitrate Turbulent Fluxes Reported in the
Literature From Different Regions of the World Ocean
a
Reference Region F (mmol m
2
d
1
)
Martin et al. [2010] Porcupine Abyssal Plain 0.09
Horne et al. [1996] Georges Bank 0.0470.18
Lewis et al. [1986] Subtropical North Atlantic 0.14
Law et al. [2003] Antarctic Circumpolar Current 0.17
This study Amundsen Gulf (>90% i.c.) 0.5
Carr et al. [1995] Equatorial Pacific 0.11
Sundfjord et al. [2007] Barents Sea (4090% i.c.) 0.12
Rippeth et al. [2009] Irish Sea 1.5
Law et al. [2001] Northern North Atlantic 1.8
Sharples et al. [2007] Celtic Sea Shelf Edge 1.33.5
Hales et al. [2009] New England Shelf Break 0.85
Hales et al. [2005] Oregon shelf O(10
1
)
Sharples et al. [2001] New Zealand Shelf 12
Schafstall et al. [2010] Mauritanian Upwelling Region 120
a
The v alues reported may represent, depending on studies, the flux
through the nitracline, through the base of the euphotic zone or through
the base of the mixed layer. Values are sorted from lowest to highest. A
molarmassof62gmol
1
was used for converting nitrate concentration
frommgtommolasusedinsomeofthereferenceslisted.The
abbreviation i.c. stands for ice coverage.
Figure 1. Map and bathymetry (m) of the Amundsen Gulf
and positions of the 26 stations where turbulence and nitrate
profiles were collected in NovemberDecember 2007.
Figure 2. Wind speed observations in the Amundsen Gulf
during the sampling period. NCEPNARR reanalysis (solid
curve), CCGS Amundsen navigati on anemomet er reporte d
in the science log book during sampling operations (black
dots), and meteorological science station aboard the CCGS
Amundsen (circles). The black lines at the to p ind icate the
sampling periods reported here.
BOURGAULT ET AL.: NITRATE FLUXES IN THE AMUNDSEN GULF L15602L15602
2of5
recommended by Baker and Gibson [1987], the mean (X )of
small data sets (10100 samples) of turbulence variables
that are approximately lognormally distributed in the ocean,
such as and K, was estimated using the maximum likeli-
hood estimator
X = exp(m + s
2
/2), where m and s
2
are the
arithmetic mean and variance of ln(X), respectively. The
mean of other variables (
N
2
, N , F ) was estimated using
the standard arithmetic mean.
[
11] Ninetyfive percent confidence intervals on means
were determined by bootstrap analysis. Unless otherwise
specified the numbers in parentheses provided next to mean
values represent the lower 2.5% and upper 97.5% confi-
dence intervals on the mean. For symmetric confidence
intervals the symbol ± is used.
3. Observations
[12] On average over the sampling period and region,
nitrate concentrations exhibited a twolayer structure with
minimum concentration of
N
min
= 2.7 ± 0.5 mmol m
3
near the surface (1020 m) and maximum N
max
= 15.6 ±
0.5 mmol m
3
at 150 m depth (Figure 4). Note however that
very low concentration near the sea surface were not
captured reliably with the Isus sensor (see section 2 and
Figure 3) and bottled samples rather average to
N
min
bot
= 0.5 ±
0.3 mmol m
3
. The mean position, and standard deviation,
of the nitracline was
z
nit
= 62 ± 12 m, which is roughly 15 m
below the pycnocline (Figure 4), and was characterized with
a mean gradient (
N
z
)
nit
= 2.0 ± 0.1 mmol m
4
.
[
13] Mean dissipation rates of turbulent kinetic energy
were maximum at 10 m depth with
max
= 4.3(2.6, 7.3) ×
10
7
Wkg
1
and decreased approximately exponentially
with depth to the minimum value
min
= 1.5(1.1, 2.1) ×
10
9
Wkg
1
around 100 m (Figure 4). Note that the gray
zones in Figure 4 show confidence intervals on the means
and thus provide no information on the variability. The
dissipation rate varies by 5 orders of magnitude at 10m
depth and are lognormally distributed in the range 10
10
10
5
Wkg
1
. At greater depth (>50 m) the variability spans
about 3 orders of magnitude in the range 10
10
10
7
Wkg
1
.
[
14] Eddy diffusivity was maximum at 10 m, with K
max
=
5(3, 9) × 10
3
m
2
s
1
, and decayed exponentially to about
50 m depth, below which it stayed approximately constant
down to 150 m (Figure 4). A least squares fit with boot-
strapped 95% confidence intervals yielded the following
piecewise analytical function for the depthdependance of
the mean eddy diffusivity:
KzðÞ¼
K
0
e
z
for 46 z 10 m
K
b
for 150 z < 46 m;
ð1Þ
with K
0
= 1.1(0.7, 1.8) × 10
2
m
2
s
1
, d = 0.17 ± 0.02 m
1
and K
b
= 3.4(2.3, 5.1) × 10
6
m
2
s
1
(Figure 4). The
nitracline, being typically located at a greater depth than
46 m, was therefore subject to an eddy diffusivity close to
the background value
K
b
.
[
15] The mean nitrate turbulent diffusive flux was in the
range 4<
F < 6 mmol m
2
d
1
throughout the profile
(Figure 4). Through the nitracline it was
F
nit
= 0.5(0.3, 0.8)
mmol m
2
d
1
.
4. Discussion
[16] In comparison to other estimates available throughout
the world ocean, the mean turbulent nitrate flux reported
here for the Amundsen Gulf during icecovered conditions
Figure 3. Example of (left) a typical nitrate profile and
(right) its gradient (3 December 2007). The triangles repre-
sent measurements from laboratory analyses of bottled water
samples a nd the solid line s r epresent th e 4m scale cali-
brated Isus V3 sensor measurements.
Figure 4. Averages of buoyancy frequency squared N
2
, dissipation rate , eddy diffusivity K, nitrate concentration N and
vertical nitrate flux
F for the Amundsen Gulf. Gray shadows are 95% confidence intervals. Only positive values greater than
10
2
mmol m
2
d
1
of nitrate fluxes are presented. The black solid line in the third plot is the best fit provided by equation (1)
along with the 95% confidence intervals (dashed lines). Note that the depth scale starts at 9 m.
BOURGAULT ET AL.: NITRATE FLUXES IN THE AMUNDSEN GULF L15602L15602
3of5
is comparable, in terms of order of magnitude, to most ice
free offshore ocean regions such as the Porcupine Abyssal
Plain, the Northern North Atlantic, the Southern Ocean or
the Equatorial Pacific as well as to the icecovered Barents
Sea (Table 1). On the other hand, this mean flux is about an
order of magnitude smaller than values reported for more
energetic environments like shelves, and two orders of
magnitude smaller compared to the Mauritanian upwelling
region (Table 1).
[
17] While some of the studies listed in Table 1 concluded
that vertical turbulent mixing may dominate nitrate uptake
in surface layers and may control the net community pro-
ductivity [e.g., Lewis et al., 1986; Sharples et al., 2007;
Hales et al., 2009; Rippeth et al., 2009], other studies have
reached opposite conclusions. For example, while Schafstall
et al. [2010] reported one of the highest turbulent nitrate
flux to date (Table 1), they concluded that it only represented
10%25% of what was required to support the net com-
munity production along the Mauritanian Upwelling Region.
They suggested that vertical advection and lateral eddy fluxes
may provide the missing nitrate supply. Similarly, Martin
et al. [2010] concluded that vertical mixing contributed
little (about 2%) to the total nitrate uptake in the euphotic zone
in the Porcupine Abyssal Plain. They considered other phys-
ical processes, such as those associated with mesoscale phe-
nomena, but also hypothesized that biological nitrification
could provide as much, or perhaps even more, nitrate to the
euphotic zone than turbulent mixing.
[
18] Returning to the context of the Amundsen Gulf of fall
2007, we found that the total rate of nitrate supply above the
nitracline was
N
t
tot
= 14.6 ± 2.7 mmol m
3
d
1
, as deter-
mined from a linear fit to 8 groups of bottle samples col-
lected from 5 November 2007 to 9 January 2008 (not shown
but with R
2
= 0.81, p < 0.001 and following Tremblay et al.
[2008]).
[
19] The contribution from turbulent mixing to this rate
could be estimated by dividing the turbulent flux through
the nitracline F
nit
by the thickness of the nitracline z
nit
and averaging over all samples. This gives
N
t
= F
nit
= z
nit
jj=
9(5, 14) mmol m
3
d
1
from turbulent diffusion alone.
Comparing these values (i.e.
N
t
/N
t
tot
) suggests that turbu-
lent mixing contributed on average to 60% of the total
nitrate uptake in the Amundsen Gulf during fall 2007.
[
20] This conclusion contrasts with that of Tremblay et al.
[2008] who hypothesized that vertical mixing contributed
little, compared to nitrification processes, to nitrate uptake
under the immobile landfast ice of Franklin Bay (Figure 1)
during fallwinter 2003/04. However, we note that the
nitracline in Franklin Bay during fallwinter 2003/04 was up
to an order of magnitude weaker than during fall 2007 with
gradients in the range 0.4 (N
z
)
nit
≤−0.2 mmol m
4
(determined from Tremblay et al. [2008, Figure 3]), com-
pared to (
N
z
)
nit
= 2.0 ± 0.1 mmol m
4
for fall 2007.
Assuming that the nitracline in 2003 was subject to the same
background diffusivity as in 2007 of
K
b
= 3.4 × 10
6
m
2
s
1
and letting the nitracline depth to be 40 m during fall 2003
[Tremblay et al., 2008] yields (
N
t
)
03
= [K
b
(N
z
)
nit
/40 m] ×
86 400 s d
1
=1to3mmol m
3
d
1
, from turbulent diffu-
sion alone. Comparing this with the total rate reported by
Tremblay et al. [2008] for fall 2003 of (
N
t
tot
)
03
= 13.2 ±
2.5 mmol m
3
d
1
suggests that 8% to 20% of the nitrate
supply in 2003 may have been from vertical turbulent dif-
fusion. This supports Tremblay et al.s [2008] hypothesis
that renewal of nitrate from vertical mixing in fall 2003 in
Franklin Bay may not have been the dominant mechanism.
5. Conclusions
[21] Our observations show that while the surface layer
may be subject to large diffusivities, the nitracline is
essentially subject to the low background diffusivity
K
b
.
For twolayer models where the interface is set around the
nitracline, or the pycnocline [e.g., Shadwick et al., 2011], it
appears reasonable to use a constant background diffusivity
K
b
to model turbulent diffusive fluxes of tracers across the
layers. However, if mean turbulent diffusivity is needed
within the surface layer [e.g., Else et al., 2011] the empirical
relationship given by equation (1) is proposed.
[
22] Our analysis supports the conclusion of Tremblay
et al. [2008] that turbulent diffusion may have played a
secondary role in nitrate uptake in Franklin Bay in 2003.
However, we reached an opposite conclusion for the
Amundsen Gulf for fall 2007 where 60% of total nitrate
supply may have come from turbulent diffusion. It is unclear
at this point whether these opposite conclusions were
reached because we are comparing two different regions or
years. However, the observations presented here provide a
source for testing sea iceoceanbiological models [e.g.,
Lavoie et al., 2009] from which insights regarding the
functioning of the coastal arctic ecosystem in a changing
climate could be gained. Such model assessments against
field measurements are critically needed because turbulence
parameterization remains one of the greatest uncertainties in
climate prediction. The information provided here may help
to progress rapidly in that direction.
[
23] Acknowledgments. Th is compon ent of th e CFL p rogr am was
funded by the Canadian International Polar Year (IPY) program office,
the Natura l Scie nces and Engineering Resea rch Council ( NSERC), a nd
the Canada Foundation for Innovation (CFI) and is a contribution to the sci-
entific program of QuébecOcéan. Thanks to Pascal Guillot for processing
the nitrate data, Matthew Asplin and Brent Else for providing us with GPS
and wind data and Dan Kelley for fruitful discussion on statistics and dou-
blediffusion.
[24] The Editor wishes to acknowledge Tom Rippeth and an anony-
mous reviewer for their assistance in evaluating this paper.
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P. S. Galbraith, Maurice Lamontagne Institute, Fisheries and Oceans
Canada, 850 route de la Mer, MontJoli, QC G5H 3Z4, Canada.
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J.É. Tremblay, Département de Biologie, Université Laval, Pavillon
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BOURGAULT ET AL.: NITRATE FLUXES IN THE AMUNDSEN GULF L15602L15602
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... In the weakly turbulent, strongly stratified Arctic region, direct measurements of turbulent dissipation have been extremely scarce (e.g. Padman & Dillon 1987;Bourgault et al. 2011;Shroyer 2012;Shaw & Stanton 2014), until very recently. The increasing importance of the Arctic region from the perspective of global climate and the role of the oceans in climate change processes in general has led to an increasingly sharp focus on Arctic Ocean mixing processes. ...
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Recent progress in the direct measurement of turbulent dissipation in the Arctic Ocean has highlighted the need for an improved parametrization of the turbulent diapycnal diffusivities of heat and salt that is suitable for application in the turbulent environment characteristic of this polar region. In support of this goal we describe herein a series of direct numerical simulations of the turbulence generated in the process of growth and breaking of Kelvin–Helmholtz billows. These simulations provide the data sets needed to serve as basis for a study of the stratified turbulent mixing processes that are expected to exist in the Arctic Ocean environment. The mixing properties of the turbulence are studied using a previously formulated procedure in which the temperature and salinity fields are sorted separately in order to enable the separation of irreversible Arctic mixing from reversible stirring processes and thus the definition of turbulent diffusivities for both heat and salt that depend solely upon irreversible mixing. These analyses allow us to demonstrate that the irreversible diapycnal diffusivities for heat and salt are both solely dependent on the buoyancy Reynolds number in the Arctic Ocean environment. These are found to be in close agreement with the functional forms inferred for these turbulent diffusivities in the previous work of Bouffard & Boegman ( Dyn. Atmos. Oceans , vol. 61, 2013, pp. 14–34). Based on a detailed comparison of our simulation data with this previous empirical work, we propose an algorithm that can be used for inferring the diapycnal diffusivities from turbulent dissipation measurements in the Arctic Ocean.
... These values are higher than previously used by Arrigo & Sullivan, 1994) to model nutrient-replete ice algae growth in McMurdo Sound, but lower than that observed under landfast ice in the Canadian Archipelago (Cota et al., 1987;Lavoie et al., 2005). At the height of the modeled ice algae growth at the low snow site, the nutrient demand was 3 mmol m −2 d −1 which is also greater than fluxes measured in situ under landfast ice in the Barrow Strait (Cota et al., 1987) and the Barents Sea (Bourgault et al., 2011, 2 mmol m −2 day −1 ). We hypothesize that the ice algae bloom at the low snow site was supplied with nutrients through a combination of turbulence generated by ice-ocean stress and/or brine convection eroding the subsurface nutricline and mixing nutrients up to the base of the ice (Skyllingstad & Denbo, 2001) plus movement of the buoy into nutrient-rich water masses. ...
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Novel observations of the seasonal evolution of an ice algal bloom on the Chukchi shelf were collected by two autonomous buoys deployed 180 km apart in first‐year drifting sea ice. High attenuation of blue light in the bottom of the ice indicated considerable accumulation of ice algae biomass with derived Chlorophyll‐a concentrations (Chl a) up to 184 mg m⁻². Differences in the magnitude and persistence of ice algae biomass under each buoy appear to have been driven by differences in snow thickness, as ice thickness was similar between the sites. Minimal snow cover (0.02 m) around one buoy was associated with algae growth beginning in mid‐May and lasting 70 days. The second buoy had notably more snow (0.4 m), causing ice algae production to lag behind the first site by approximately 4 weeks. The delay in growth diminished the peak of ice algae Chl a and duration compared to the first site. Light attenuation through the ice was intense enough at both buoys to have a potentially inhibiting impact on water column phytoplankton Chl a. Modeling ice algae growth with observed light intensities determined that nutrients were the limiting resource at the low snow site. In contrast, the algae at the high snow site were light‐limited and never nutrient‐limited. These data point toward changes in ice algae phenology with an earlier and longer window for growth; and nutrients rather than light determining the longevity and magnitude of production.
... The studies described above, however, predominantly focus their analysis on depth or horizontal spatial variability of turbulent mixing due to limitations on the temporal scope and/or resolution of measurements, features that are also common to many other studies that use direct measurements of turbulence in the Arctic Ocean (e.g., Bourgault et al., 2011;Fer et al., 2010;Fine et al., 2018;Lenn et al., 2009Lenn et al., , 2011Lincoln et al., 2016;Padman & Dillon, 1987, 1991Rainville & Winsor, 2008;Rippeth et al., 2015;Scheifele et al., 2018;Sévigny, 2013;Shroyer, 2012). Typically, time series of turbulence measurements resolve periods on the order of days or longer and/or have short durations often restricted to several hours and rarely exceeding a few months (the SHEBA experiment being a notable exception). ...
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This work investigates how internal wave‐driven turbulence varies in time, from hourly to yearly timescales, and in space, across two distinct regions of the Arctic Ocean. We apply a shear‐based fine‐scale parameterization to mooring records in Nares Strait and on the Beaufort Sea shelf‐slope that sampled the upper stratified water column every 30–45 min and span 2003–2006 and 2003–2004, respectively. In doing so, we generate over 600,000 estimates of the internal wave‐driven dissipation rate. These estimates exhibit large temporal variability in both regions, spanning over 3 orders of magnitude. Despite these wide ranges, we find distinct distributions at each site. In Nares Strait, the time series of dissipation shows systematic variation at tidal frequencies, and tidal forcing appears to influence dissipation more strongly than winds, sea ice, and stratification on daily timescales. On longer timescales, dissipation exhibits a weak seasonal cycle, being elevated when the stratification is high and during the ice melt season. In the Beaufort Sea, we detect no dominant timescales or significant relationships with forcing metrics, but note that the dissipation rate is typically 2 orders of magnitude lower than that in Nares Strait. This region is characterized as being in a turbulent mixing regime for only 2% of the record, compared to 73% of the Nares Strait record, implying that turbulence here is rarely energetic enough relative to the stratification to drive a turbulent heat flux. Inferred Beaufort Sea heat fluxes are an order of magnitude lower than the O(1) W m⁻² average value found in Nares Strait.
... Winter nutrient inventories show a strong linear relationship with annual NPP via their role in regulating phytoplankton biomass, except for areas where wind-driven and topographically enhanced mixing (that is, driving resuspension and upwelling events) resupply nutrients during the growing season 31 . Locally, depending on the interplay between atmospheric forcing (that is, intensity, duration and direction of the wind stress) and the strength of vertical stratification, the injection of nutrients into surface layers can easily vary by two orders of magnitude across the Arctic Ocean 31,34,35 . Stratification is expected to increase in the Canada Basin 36 but decrease in other regions (for example, in the Eurasian sector 6,37 ; Fig. 4). ...
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Changes in the Arctic atmosphere, cryosphere and Ocean are drastically altering the dynamics of phytoplankton, the base of marine ecosystems. This Review addresses four major complementary questions of ongoing Arctic Ocean changes and associated impacts on phytoplankton productivity, phenology and assemblage composition. We highlight trends in primary production over the last two decades while considering how multiple environmental drivers shape Arctic biogeography. Further, we consider changes to Arctic phenology by borealization and hidden under-ice blooms, and how the diversity of phytoplankton assemblages might evolve in a novel Arctic ‘biogeochemical landscape’. It is critical to understand these aspects of changing Arctic phytoplankton dynamics as they exert pressure on marine Arctic ecosystems in addition to direct effects from rapid environmental changes.
... These observations are on the order of the maximum values found in the open Chukchi Sea during storm-driven mixing (3.4 mmol m −2 day −1 Nishino et al., 2015). Similar levels of turbulent nitrate fluxes have been reported in other shallow regions in the Arctic (Bourgault et al., 2011;. Turbulent nitrate flux in the central Arctic are 2 orders of magnitude lower (1.4 × 10 −2 mmol m −2 day −1 ; Randelhoff & Guthrie, 2016). ...
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The distribution of nutrients influences the Chukchi Sea's rich ecosystems and affectsthe biogeochemistry of the central Arctic. Nutrients that become limiting in the late summer can be replenished episodically by physical processes that are likely to change in concert with Arctic climate trends. Here we report on unique, simultaneous, physical, and biogeochemical measurements of one such process: upwelling in Barrow Canyon. High-resolution transects of a towed vehicle, which was equipped with physical, turbulence, and biogeochemical sensors, captured the upwelling of dense, salty, and nitrate-rich waters into the shallow regions of Barrow Canyon. Upwelling drastically modifies the nitrate distribution of Barrow Canyon through the vertical advection of Atlantic Water from the Canada Basin and through turbulent flux across the nutricline. Upwelled waters form a highly sheared gravity current that is susceptible to both baroclinic and symmetric instabilities. The current exhibits transverse circulation common to frictional gravity currents. Our observations suggest that rapid and dramatic nutrient changes in Barrow Canyon can be accomplished by upwelling; the subsequent formation of unstable jets and fronts enhances the irreversible flux of upwelled nutrients to less dense waters in the Chukchi sea.
... Vertical gradients in turbulent di↵usivity also have consequences for nitrate supply to the surface. In the Admunsen Gulf, Bourgault et al. (2011) measured high di↵usivities at 10 m (K max = 3 x 10 3 m 2 s 1 ), but di↵usivity decreased exponentially with depth to a background value K b = 3 x 10 6 m 2 s 1 by 50 m. The nitracline was observed at 62 m depth so the mean di↵usive nitrate flux across the nitracline was low (0.5 mmolm 2 d 1 ). ...
Thesis
This thesis examines productivity in the Arctic Ocean and its response to a future ice free Arctic. Phytoplankton produce atmospheric oxygen, regulate atmospheric carbon dioxide and underpin ocean ecosystems. Production dynamics and distributions in the Arctic are poorly understood — especially the extent of growth under ice and in prevalent subsurface chlorophyll maxima is unknown — and therefore the perturbation under anthropogenic sea ice retreat is poorly understood. We relate the vertical distribution of production to the ratio of nitrate limitation to light limitation limitation across the Arctic. Depth-integrated production in each water column is then easily related to the vertical distribution because light-governed production rates decrease exponentially with depth. The scaling elucidates under ice and subsurface production magnitudes, and works equally well across the diverse hydrographic (shelves, inflows, central basin) and biogeochemical provinces of the Arctic. Further, the scaling is shown to elucidate biogeochemical transformations of water masses as they transit the Arctic and to be time-invariant. The latter fact is used to predict perturbations to plankton dynamics under anthropogenic ice retreat. Further, unique boundary conditions make the Arctic a powerful place to study general (global) plankton responses to environmental perturbations. We explore this by deriving oceanic photosynthesis across the globe from the theory developed in the Arctic Ocean. The major implication of our results are that oceanic photosynthesis is explicable in terms of a coherent dependence on ocean nutrient and light conditions, and this entails minor productivity increases in an ice-free Arctic Ocean and in the global ocean over the coming century.
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Difficulties to quantify ocean turbulence have limited our knowledge about the magnitude and variability of nitrate turbulent diffusion, which constitutes one of the main processes responsible for the supply of nitrogen to phytoplankton inhabiting the euphotic zone. We use an extensive dataset of microturbulence observations collected in contrasting oceanic regions, to build a model for nitrate diffusion into the euphotic zone, and obtain the first global map for the distribution of this process. A model including two predictors (surface temperature and nitrate vertical gradient) explained 50% of the variance in the nitrate diffusive flux. This model was applied to climatological data to predict nitrate diffusion in oligotrophic mid and low latitude regions. Mean nitrate diffusion (~ 20 Tmol N y ⁻¹ ) was comparable to nitrate entrainment due to seasonal mixed-layer deepening between 40°N–40ºS, and to the sum of global estimates of nitrogen fixation, fluvial fluxes and atmospheric deposition. These results indicate that nitrate diffusion represents one of the major sources of new nitrogen into the surface ocean in these regions.
Thesis
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Oligotrophic regions are characterized by a shortage of nutrients in surface waters, with nitrogen being the main limiting nutrient in most tropical and subtropical regions of the open ocean, as well as in temperate and polar seas during periods of seasonal stratification. Since some of the biological production in the photic layer is exported to the deep ocean (export), the maintenance of biological production will depend on the input of nutrients into the system. Mechanisms contributing to new production include biological nitrogen fixation, atmospheric deposition, and diffusive and advective vertical and horizontal transport of organic and inorganic forms of nitrogen. Calculation of vertical diffusive transport requires estimation of diffusivity (Kz). The methodological difficulties in obtaining Kz estimates led to the use of constant Kz values, and empirical parameterizations of vertical diffusivity. However, the commercialization of microstructure turbulence profilers has facilitated the obtaining of microstructure turbulence observations, which revealed an important Kz variability in the upper layer. Alternatively, the concentration of dissolved inorganic nutrients in the photic layer has been used as an estimator of nutrient availability to planktonic communities. However, under steady-state conditions, such as subtropical gyres, where nutrient supply by diffusion into the euphotic zone is slow, there may be no relationship between nutrient concentration in the photic layer and nutrient supply. The picoplankton refers to the fraction of plankton smaller than 2 µm and consists of Synechococcus, picoeukaryotes, Prochlorococcus and heterotrophic bacteria. The latter can be divided into bacteria with high (HNA) or low (LNA) nucleic acid content. Photosynthetic picoplankton generally dominate biomass and primary production in tropical and subtropical oligotrophic regions, while their contribution is less in nutrient-rich coastal regions. In marine ecosystems, a major source of environmental heterogeneity lies in the temporal fluctuation of nutrient supplies, which controls the diversity of the phytoplankton community. Under steady state conditions, the minimum level of resources that can sustain a population determines competition. Experimental studies and numerical models of competition support this theoretical basis for large phytoplankton. While numerous studies have investigated the effect of nutrient supply dynamics on interspecific competition of large phytoplankton species, their effect on the groups that make up phytoplankton has received much less attention. The main hypothesis of this thesis is that the dynamics of nutrient supply controls the composition of marine picoplankton communities. To achieve this goal, a multidisciplinary approach will be used, combining field observations made during 17 oceanographic campaigns in the Atlantic, Pacific and Indian tropical and subtropical oceans, the Northwest Mediterranean Sea, the Galician coastal upwelling ecosystem and the Antarctic Peninsula with laboratory experiments and ecological modeling of competitive interactions.
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Arctic Ocean primary productivity is limited by light and inorganic nutrients. With sea ice cover declining in recent decades, nitrate limitation has been speculated to become more prominent. Although much has been learned about nitrate supply from general patterns of ocean circulation and water column stability, a quantitative analysis requires dedicated turbulence measurements that have only started to accumulate in the last dozen years. Here we present new observations of the turbulent vertical nitrate flux in the Laptev Sea, Baffin Bay, and Young Sound (North-East Greenland), supplementing a compilation of 13 published estimates throughout the Arctic Ocean. Combining all flux estimates with a Pan-Arctic database of in situ measurements of nitrate concentration and density, we found the annual nitrate inventory to be largely determined by the strength of stratification and by bathymetry. Nitrate fluxes explained the observed regional patterns and magnitudes of both new primary production and particle export on annual scales. We argue that with few regional exceptions, vertical turbulent nitrate fluxes can be a reliable proxy of Arctic primary production accessible through autonomous and large-scale measurements. They may also provide a framework to assess nutrient limitation scenarios based on clear energetic and mass budget constraints resulting from turbulent mixing and freshwater flows.
Article
Our understanding of ocean mixing is challenged by its patchy, episodic nature and a scarcity of direct measurements, especially in the Arctic Ocean. In this study, we exploit a historical record of nearly 3,000 conductivity-temperature-depth profiles collected in the shelf and shelf-slope waters of the Canadian Arctic Ocean from 2002 to 2016 to characterize the variability of 28,872 internal wave-driven turbulent dissipation and mixing rate estimates from a finescale parameterization. We find that these estimates of wave-driven dissipation rates and associated diapycnal diffusivities are generally low, but exhibit wide variability, each spanning several orders of magnitude. We further find that stratification plays a significant role in modulating the mixing rate both vertically and regionally within the study domain. Dissipation rate and diffusivity estimates display a weak seasonal cycle, but no evidence of statistically significant interannual trends over this period. Exceptionally large localized temporal variability appears to dominate other potential underlying patterns. The presence of strong upper ocean stratification combined with predominately weak dissipation rate estimates implies that many regions in the Canadian Arctic Ocean are likely in a molecular or buoyancy-controlled mixing regime. Even when the concept of a turbulent-enhanced diffusivity is potentially relevant, most turbulent heat flux estimates out of the Atlantic Water thermocline are smaller than the average value required to close the standardly assumed Arctic Ocean heat budget. In contrast, we find evidence for isolated occurrences of anomalously large heat fluxes, which may disproportionately contribute to the liberation of Atlantic Water heat toward the surface sea ice pack.
Article
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The first quasi-annual time series of nutrients and chlorophyll fluorescence in the southeast Beaufort Sea showed that mixing, whether driven by wind, local convection, or brine rejection, and the ensuing replenishment of nutrients at the surface were minimal during autumn and winter. Anomalously high inventories of nutrients were observed briefly in late December, coinciding with the passage of an eddy generated offshore. The concentrations of NO3− in the upper mixed layer were otherwise low and increased slowly from January to April. The coincident decline of NO2− suggested nitrification near the surface. The vernal drawdown of NO3− in 2004 began at the ice-water interface during May, leaving as little as 0.9 μM of NO3− when the ice broke up. A subsurface chlorophyll maximum (SCM) developed promptly and deepened with the nitracline until early August. The diatom-dominated SCM possibly mediated half of the seasonal NO3− consumption while generating the primary NO2− maximum. Dissolved inorganic carbon and soluble reactive phosphorus above the SCM continued to decline after NO3− was depleted, indicating that net community production (NCP) exceeded NO3− -based new production. These dynamics contrast with those of productive Arctic waters where nutrient replenishment in the upper euphotic zone is extensive and NCP is fueled primarily by allochthonous NO3−. The projected increase in the supply of heat and freshwater to the Arctic should bolster vertical stability, further reduce NO3− -based new production, and increase the relative contribution of the SCM. This trend might be reversed locally or regionally by the physical forcing events that episodically deliver nutrients to the upper euphotic zone.
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Between Nov. 1 2007 and Jan. 31 2008, we calculated the air-sea flux of CO2, sensible heat, and water vapor in an Arctic polynya system (Amundsen Gulf, Canada) using eddy covariance equipment deployed on the research icebreaker CCGS Amundsen. During this time period, Amundsen Gulf was a dynamic sea ice environment composed primarily of first year ice with open water coverage varying between 1-14%. In all cases where measurements were influenced by open water we measured CO2 fluxes that were 1-2 orders of magnitude higher than those expected under similar conditions in the open ocean. Fluxes were typically directed toward the water surface with a mean flux of -4.88 μmol m-2 s-1 and a maximum of -27.95 μmol m-2 s-1. One case of rapid outgassing (mean value +2.10 μmol m-2 s-1) was also observed. The consistent patten of enhanced gas exchange over open water allows us to hypothesize that high water-side turbulence is the main cause of these events. Modification of the physical and chemical properties of the surface seawater by cooling and brine rejection may also play a role. A rough calculation using an estimate of open water coverage suggests that the contribution of these events to the annual regional air-sea CO2 exchange budget may make the winter months as important as the open water months. Although high, the uptake of CO2 fits within mixed layer dissolved inorganic carbon budgets derived for the region by other investigators.
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Empirical oceanic cummulative distribution functions (CDF) and their observed departures from lognormality are compared to CDFs generated by Monte Carlo methods which include simulated noise. A specific procedure is given to estimate the degree of noise contamination in the measured CDFs. Confidence intervals used by other workers (Elliott and Oakley 1979, 1980 and 1985) are examined to study their accuracy. New confidence intervals are derived theoretically and tested by comparison to intervals generated by Monte Carlo Methods. Several microstructure data sets from the Atlantic and Pacific equatorial undercurrents, the seasonal thermocline and the main thermocline are reviewed to test the validity and consequences of the hypothesis of lognomality.
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The Mauritanian coastal area is one of the most biologically productive upwelling regions in the world ocean. Shipboard observations carried out during maximum upwelling season and short-term moored observations are used to investigate diapycnal mixing processes and to quantify diapycnal fluxes of nutrients. The observations indicate strong tide-topography interactions that are favored by near-critical angles occurring on large parts of the continental slope. Moored velocity observations reveal the existence of highly nonlinear internal waves and bores and levels of internal wave spectra are strongly elevated near the buoyancy frequency. Dissipation rates of turbulent kinetic energy at the slope and shelf determined from microstructure measurements in the upper 200 m averages to $\varepsilon$ = 5 × 10−8 W kg−1. Particularly elevated dissipation rates were found at the continental slope close to the shelf break, being enhanced by a factor of 100 to 1000 compared to dissipation rates farther offshore. Vertically integrated dissipation rates per unit volume are strongest at the upper continental slope reaching values of up to 30 mW m−2. A comparison of fine-scale parameterizations of turbulent dissipation rates for shelf regions and the open ocean to the measured dissipation rates indicates deficiencies in reproducing the observations. Diapycnal nitrate fluxes above the continental slope at the base of the mixed layer yielding a mean value of 12 × 10−2μmol m−2 s−1 are amongst the largest published to date. However, they seem to only represent a minor contribution (10% to 25%) to the net community production in the upwelling region.
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The Circumpolar Flaw Lead (CFL) system study is a Canadian‐led International Polar Year (IPY) initiative with over 350 participants from 27 countries. The study is multidisciplinary in nature, integrating physical sciences, biological sciences and Inuvialuit traditional knowledge. The CFL study is designed to investigate the importance of changing climate processes in the flaw lead system of the northern hemisphere on the physical, biogeochemical and biological components of the Arctic marine system. The circumpolar flaw lead is a perennial characteristic of the Arctic throughout the winter season and forms when the mobile multi‐year (MY) pack ice moves away from coastal fast ice, creating recurrent and interconnected polynyas in the Norwegian, Icelandic, North American and Siberian sectors of the Arctic. The CFL study was 293 days in duration and involved the overwintering of the Canadian research icebreaker CCGS Amundsen in the Cape Bathurst flaw lead throughout the annual sea‐ice cycle of 2007–2008.In this paper we provide an introduction to the CFL project and then use preliminary data from the field season to describe the physical flaw lead system, as observed during the CFL overwintering project. Preliminary data show that ocean circulation is affected by eddy propagation into Amundsen Gulf (AG). Upwelling features arising along the ice edge and along abrupt topography are also detected and identified as important processes that bring nutrient rich waters up to the euphotic zone. Analysis of sea‐ice relative vorticity and sea‐ice area by ice type in the AG during the CFL study illustrates increased variability in ice vorticity in late autumn 2007 and an increase in new and young ice areas in the AG during winter. Analysis of atmospheric data show that a strong northeast–southwest pressure gradient present over the AG in autumn may be a synoptic‐scale atmospheric response to sensible and latent heat fluxes arising from areas of open water persisting into late November 2007. The median atmospheric boundary layer temperature profile over the Cape Bathurst flaw lead during the winter season was stable but much less so when compared to Russian ice island stations.
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The reduction in ice cover observed in the late 1980s and early 1990s has been attributed to the strongly positive Arctic Oscillation (AO) phase during that time. However, despite a change in the AO to more neutral conditions since then, ice extent and the fraction of old ice have continued to decrease. This mismatch between the AO index and loss of ice can be explained by the frequency of three main sea level pressure (SLP) patterns that yield overall variability in SLP, rather than the presence of a single, coherent physical pattern of SLP reduction associated with the positive mode of the AO. These three patterns were in phase during the peak AO period but their frequency has varied differently since then, with two of the patterns continuing to contribute to reduced ice cover in the western Arctic. Hence, regional atmospheric circulation remains a significant factor in recent reductions in ice cover.
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New production was estimated for the equatorial Pacific by quantifjling the supply of nitrate into the euphotic zone. The turbulent flux of nitrate, estimated by assuming gradient transport and by direct mea- surements of dissipation and nitrate concentration along a cross-equatorial transect, was maximum (1-3 mmol m^(-3) d^(-l)) at 0deg and 1deg S. Poleward of the equatorial region, there was little vertical diffusion of nitrate (< 10^(-3) mmol m^(-2) d^(-l)). An estimate was made of the two-dimensional advective balance of nitrate between upwelling and the meridional divergence in the equatorial region
Article
As part of a multidisciplinary cruise to the Porcupine Abyssal Plain (PAP) study site (49°00′N 16°30′W), in June and July 2006, observations were made of the vertical nitrate flux due to turbulent mixing. Daily profiles of nitrate and turbulent mixing, at the central PAP site, give a mean nitrate flux into the euphotic zone of 0.09 (95% confidence intervals: 0.05–0.16) mmol Nm−2d−1. This is a factor of 50 lower than the mean observed rate of nitrate uptake within the euphotic zone (5.1±1.3mmolNm−2d−1). By using our direct observations to ‘validate’ a previously published parameterisation for turbulent mixing, we further quantify the variability in the vertical turbulent flux across a roughly 100×100km region centred on the PAP site, using hydrographic data. The flux is uniformly low (0.08±0.26mmolNm−2d−1, the large standard deviation being due to a strongly non-Gaussian distribution) and is consistent with direct measurements at the central site. It is demonstrated that on an annual basis convective mixing supplies at least 40-fold more nitrate to the euphotic zone than turbulent mixing at this location. Other processes, such as those related with mesoscale phenomena, may also contribute significantly.
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Satellite-derived estimates of sea-ice age and thickness are combined to produce a proxy ice thickness record for 1982 to the present. These data show that in addition to the well-documented loss of perennial ice cover as a whole, the amount of oldest and thickest ice within the remaining multiyear ice pack has declined significantly. The oldest ice types have essentially disappeared, and 58% of the multiyear ice now consists of relatively young 2- and 3-year-old ice compared to 35% in the mid-1980s. Ice coverage in summer 2007 reached a record minimum, with ice extent declining by 42% compared to conditions in the 1980s. The much-reduced extent of the oldest and thickest ice, in combination with other factors such as ice transport that assist the ice-albedo feedback by exposing more open water, help explain this large and abrupt ice loss.