Download In situ measurements of thermal diffusivity in sediments of the

In situ measurements of thermal diffusivity
in sediments of the methane-rich zone of
Cascadia Margin, NE Pacific Ocean
Kira Homola 1,2* • H. Paul Johnson 1 • Casey Hearn 1,2
School of Oceanography, University of Washington, Seattle, Washington, United States
Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, United States
1
2
*[email protected]
Abstract
Thermal diffusivity (TD) is a measure of the temperature response of a material to external thermal ­forcing. In
this study, TD values for marine sediments were determined in situ at two locations on the Cascadia Margin
using an instrumented sediment probe deployed by a remotely operated vehicle. TD measurements in this area
of the NE Pacific Ocean are important for characterizing the upslope edge of the methane hydrate stability
zone, which is the climate-sensitive boundary of a global-scale carbon reservoir. The probe was deployed on
the Cascadia Margin at water depths of 552 and 1049 m for a total of 6 days at each site. The instrumented
probe consisted of four thermistors aligned vertically, one sensor exposed to the bottom water and one each
at 5, 10, and 15 cm within the sediment. Results from each deployment were analyzed using a thermal conduction model applying a range of TD values to obtain the best fit with the experimental data. TD values
corresponding to the lowest standard deviations from the numerical model runs were selected as the best
approximations. Overall TDs of Cascadia Margin sediments of 4.33 and 1.15 × 10–7 m2 s–1 were calculated
for the two deployments. These values, the first of their kind to be determined from in situ measurements
on a methane hydrate-rich continental margin, are expected to be useful in the development of models of
bottom-water temperature increases and their implications on a global scale.
Domain Editor-in-Chief
Jody W. Deming University of
Washington
Associate Editor
Laurenz Thomsen, Jacobs
University Bremen, Germany
Knowledge Domains
Earth and Environmental Science
Ocean Science
Article Type
Research Article
Received: May 29, 2014
Accepted: December 18, 2014
Published:February 11, 2015
1. Introduction
Thermal diffusivity (TD) is defined as the ratio of the ability of a solid to conduct thermal energy to its
­capacity for storing thermal energy (Incropera et al., 2013); this ratio best describes how the solid responds
to time-varying external thermal forcing. The TD of marine sediments is very poorly characterized due to the
challenge of acquiring quantitative in situ values in an extreme environment. Previous measurements have used
laboratory fabricated sediments or sediments recovered from coring (Hurwitz et al., 2012); methods which
do not reproduce the in situ environment due to either mechanical disturbance or the ephemeral nature of
pore water hydrate (liquid-gas) phases. Furthermore, a time-series of measurements on the order of days or
longer is required to accurately compute TD for penetration depths of adequate length to be representative
of the sediment column. Acquisition of such data is possible, however, with the use of a remotely operated
vehicle (ROV) or similar submersible. To our knowledge, in situ thermal diffusivity measurements have not
been obtained previously from any portion of the Cascadia Margin (Canadian, Washington, Oregon, northern
California), although they have been made in other environments and in other sediment types ( Jackson and
Richardson, 2000;Wheatcroft et al., 2007). Near shore measurements of sediment TD are also uncommon,
although the acquisition procedures are far less demanding, as described by Thomson (2010) for the collection
of sediment TD values in the very shallow coastal waters of Washington State.
The goal of this experiment was to determine the range of TD values for marine sediments in 500 to 1100
m of water, which includes the upslope boundary of the methane hydrate stability zone on the Cascadia ­Margin
in the NE Pacific. The Washington portion of the Cascadia North American margin is a focus area for two
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elementascience.org
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Thermal diffusivity of marine sediments
US National Science Foundation programs (GeoPRISM and EarthScope). The accretionary wedge on this
margin has been described at considerable length in the literature (e.g. Schmalzle et al., 2014; McCrory
et al., 2014 and references within), as have the sediment types and physical properties for the alternating
layers of pelagic clays and sandy silt turbidites on the Washington margin (see Davis and Hyndman, 1989;
Atwater et al., 2014 and references within). Heat flow on the Washington margin varies from 120 mW m–2
at the deformation front to less than 50 mW m–2 near the edge of the shallow shelf ( Johnson et al., 2013).
Characterizing the methane hydrate stability zone at mid-latitude continental margins has global
­significance, as methane hydrate deposits are the most climate-sensitive reservoirs of carbon on Earth.
­Globally, an e­ stimated 99% of the hydrate reservoir is held in continental margin sediments at depths below
500 m (Collett et al., 2009; Ruppel, 2011). Slight increases in near-bottom water temperatures can ­potentially
­destabilize large quantities of hydrate, releasing bubbles of methane gas with 30 times the greenhouse potency
of CO2 (Denman et al., 2007; Riedel et al., 2010). One consequence of global warming over the past 100 years
is the impact of rising temperatures of the near-bottom seawater that is in physical contact with the large
deposits of methane hydrate present on continental slopes. Because the stability of methane ­hydrate within
these massive carbon reservoirs depends on both temperature and pressure, warming seawater will cause
­dissociation of the hydrate phase and release of CH4 gas on a time scale of only decades (Boswell and Collett,
2011; Ruppel, 2011; Phrampus and Hornbach, 2012). The time scale over which these thermal disturbances
propagate into the sediment-hosted methane hydrate deposits depends directly on the thermal diffusivity of
the uppermost seafloor. Sediment thermal diffusivity is a physical property that is only poorly known from
laboratory studies; very few measurements have been made in situ.
Previous studies of contemporary methane hydrate decomposition have focused on high-latitude Arctic
regions (Berndt et al., 2014; Westbrook et al., 2009), although more recent studies have shown that similar
hydrate dissociation is occurring at mid-latitudes, with large volumes of methane currently being released on
the Cascadia portion of the North American continent (Hautala et al., 2014). This release of oxidized carbon
from hydrate decomposition can lower the pH of surrounding seawater, threatening local marine organisms
sensitive to acidification. Key to understanding the dynamic upslope limit of methane hydrate stability is the
rate of heat transfer from warming bottom water into the overlying sediments; accurate values of thermal
diffusivity are required to advance this field of inquiry.
2. Methods
TD measurements were acquired during the R/V ATLANTIS cruise AT26-04, where the primary research
goal was to study heat flow and fluid flux on the Washington continental margin ( Johnson et al., 2013).
During this cruise, an instrumented fiberglass probe was inserted into the seafloor for approximately 6 days
at each of two sites on the continental slope; one deployment was at 552 m depth and the other at 1049 m.
Site depths were selected to include the upper limit of methane hydrate stability and an additional region
well below this depth to provide a control for future hydrate stabilities. Prior to deployment, high-resolution
swath bathymetry and water column data were inspected to select sites with uniform low-slope bathymetry
and no methane bubbles detected in the water-column data. In addition, a visual inspection of the selected
deployment locations were performed via the high-resolution camera of the ROV Jason II; the probe was
deployed only in flat, heavily sedimented regions with no visible evidence of hard surface material exposures
(carbonate or hydrate) or fluid discharge.
The TD probe was approximately one meter in length and instrumented with four temperature sensors;
it was inserted using the manipulator of the ROV Jason II. The probe was designed to hold four Onset
TidbiT v2 temperature loggers at three sub-surface spacings: 5 cm, 10 cm, and 15 cm. To reduce vertical
resistance during insertion and provide support during deployment, a 5.1 × 5.1 cm fiberglass L-channel with
low thermal conductivity served as a rigid backbone for the probe. Mounting configuration ensured that no
two consecutive sensors would lie directly above another to minimize insertion disturbance to the sediment.
A schematic of the probe is shown in Figure 1.
The solid epoxy-encased TidbiT Version 2 sensors had a rated absolute accuracy of ± 0.21 °C, resolution
of 0.02 °C, and a factory-rated depth limit of only 305 m. As deployment depths would exceed this limit, a
pre-cruise pressure test was necessary to identify sensors that might fail. All four sensors were pressure-tested
to a simulated depth of 700 m in the Pressure Test Vessel of the School of Oceanography at the University
of Washington. All sensors passed the pressure test and were certified for use in the field.
All temperature sensors were calibrated prior to the cruise, as required for marine temperature ­sensors
( Johnson et al., 2010). This laboratory calibration procedure included measurement of the offset of
each ­thermistor with respect to a primary sensor (an Antares thermistor) by long-term soaking in a stirred
­ice-bath in a sediment core cold room (4°C). This calibration was done after the sensors had been placed
in the high pressure vessel to accurately determine their sensitivity when deployed at water depths deeper
than their ­factory certification. These offset values are reported in Table S1. For the calculation of thermal
diffusivity, the need for highly accurate relative temperatures is much greater than the absolute temperatures
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Thermal diffusivity of marine sediments
Figure 1
Line drawing of the assembled
probe.
The sensors and their brackets
are configured with 5 cm spacing.
The gray bar, 30 cm long, is used
to mark sediment-water interface
during insertion. The scale bar on
the probe is 20 cm long with 1 cm
intervals.
doi: 10.12952/journal.elementa.000039.f001
measured, as it is the difference between sensor temperatures over the different measurement depths that
determines the thermal diffusivity.
3. Data
The first probe deployment, at a water depth of 552 m, began on August 4, 2013 and lasted 147 h; the
second deployment, at a depth of 1049 m, began on August 15 and lasted 140 h (Figure 2). The specifics of
both deployments are listed in Table 1. All four thermistors logged temperature at 1 minute intervals from
their respective positions on the probe. This configuration ensured that one sensor remained 5 cm above the
sediment-water interface (gray bar in Figure 1), that the first buried sensor was 5 cm below the interface, and
the last two sensors were at consecutive 5 cm depth intervals deeper within the sediment. Upon recovery of
the probe, the calibration offsets were applied to the data acquired from each individual sensor.
The maximum observed temperature variation for a single sensor during the first deployment was ± 0.4°C,
more than 25 times the resolution of the sensor. This bottom water temperature variation was sufficient to
cause a fluctuation of ± 0.15°C at 15 cm, the maximum depth sampled. This amplitude variation is 7.5 times
the resolution of the calibrated sensor, resulting in a dataset with statistically significant values for calculating
TD at this site. A series of oscillations in bottom water temperature with a clear tidal period of approximately
24 h were observed in these data. There was also a systematic decrease in bottom water temperature observed
over the full duration of the deployment (Figure 3). This observed long-term trend could be due to seasonal
variability or other long time scale phenomena not fully captured in the six day window.
Table 1. Specifics of TD probe deployments
Deployment
Latitude
Longitude
Depth (m)
Durationa (h)
1
46.84954
2
46.78218
-124.9574
552
147
-125.2637
1049
140
Duration indicates total hours the probe was deployed at each location.
a
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Thermal diffusivity of marine sediments
Figure 2
Locations (yellow circles) of
probe deployments on the
Cascadia Margin.
The eastern location (Deployment
1) was at a depth of 552 m; the
western location (Deployment 2),
at a depth of 1049 m.
doi: 10.12952/journal.elementa.000039.f002
The maximum temperature range for the diurnal variation of ± 0.25°C observed during the second,
deeper deployment was similar to the first dataset. This 0.15°C loss of variation between sites only reduced
the ratio between sensor resolution and magnitude of water temperature variation from a factor of 25 to 20.
Although smaller in amplitude, the frequency of the bottom water temperature oscillations at the deeper
site was unexpectedly higher than at the shallower site, with an average period of 7 h (Figure 4). The smaller
amplitude temperature variation at the second site, combined with the much higher frequency of temperature
oscillations, resulted in a considerably attenuated signal at greater depth in the sediment, with the 15 cm-deep
sensor only observing two temperature steps through the entire deployment.
Figure 3
Temperature versus time for
Deployment 1 at a depth of
552 m.
The measured temperatures, with
offsets applied (x markers), and
the best fits of each model set
(solid lines) are shown for each
depth. The bottom water data set
has no model fit, as it provides
the forcing function for the other
models.
doi: 10.12952/journal.elementa.000039.f003
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Thermal diffusivity of marine sediments
Figure 4
Temperature versus time for
Deployment 2 at a depth of 1049 m.
See Figure 3 for legend and
explanation of symbols.
doi: 10.12952/journal.elementa.000039.f004
4. Results and discussion
To estimate temperatures within the sediment column at depth z and time t due to bottom water thermal forcing, a range of TD values was modeled in Matlab Version R2011a using the differences between t­ emperature
at a given time from the bottom water sensor and the sensor at the depth of interest; see Text S1 for details of
method. The program was custom-written and is also included in the Text S1. The analysis was run with 100
input TD values; the standard deviation (SD) between the model and true temperature values at each depth
was calculated for each of the input TDs. These SDs were then plotted for the two deployments (Figure 5).
For each depth, the TD value corresponding to the lowest standard deviation between the modeled and
observed temperature variations was chosen as the best approximation for the true TD. This process was
repeated for the data from the second deployment, resulting in a total of six TD values; two for each sensor
depth within the sediment column (Table 2). The model fit, calculated using the chosen TD value, is plotted
for each sediment depth on Figures 3 and 4. The discretization of input TD values introduced a maximum
error of 3.26% for each final TD value, as the nature of an input matrix cannot resolve an exact output value.
In order to compare the overall variation in thermal diffusivity between the two deployment depths, a
second analysis was performed. This computation followed the same procedure as for the previous analysis,
but the datasets from all three sensor depths at each site were analyzed using the same TD value. For each
artificial TD value produced by the thermal model, the standard deviations were summed for all three depth
intervals. Thus, for each artificial TD value, a measure of its fit across the full dataset was obtained. The TD
value with the best fit (lowest summed deviation) for the full dataset was then chosen as the best ­representative
value for the depth-integrated profile for each site. The purpose of this final technique was to reduce the noise
for a series of calculations in what is assumed to be a 15-cm thick sediment layer of homogenous thermal
diffusivity. The results yielded a TD of 4.33 × 10–7 m2 s–1 for the first shallower location and 1.15 × 10–7 m2 s–1
for the second, deeper location.
Table 2. TD values that correspond to the lowest standard deviation for each model run
Deployment
1
Depth (cm)
a
2
Thermal diffusivity (x 10–7 m2 s–1)
5
7.05
10
2.66
1.00
1.63
15
2.54
0.215
Overalla
4.33
1.15
The last row (overall depth) shows the results from the overall analysis (see text).
doi: 10.12952/journal.elementa.000039.t002
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Thermal diffusivity of marine sediments
Figure 5
Standard deviation of measured
minus modeled values versus
(log) TD used in the model
calculations.
Colors are used to indicate sensor
depth in the sediments. Top
panel shows the results from the
first deployment (water depth of
554 m); bottom panel shows the
results from the second deployment
(water depth of 1049 m).
doi: 10.12952/journal.elementa.000039.f005
The magnitude of the temperature change recorded by the sensors has a direct effect on the accuracy of
our final result. Thus, for the model runs where the corresponding dataset had greater variability, the range of
calculated standard deviation values is likewise greater, making possible the selection of a minimum value with
a high degree of confidence (see Figure 5). However, for the runs where the variability of the corresponding
dataset was low (specifically the 15-cm depth of the second deployment), there was almost no difference in
the standard deviation values, producing an unreliable TD estimate. For the two deployments conducted
during this project, only the 15-cm sensor from the second deployment fell into this unreliable category, as
all other sensors recorded significant variability and produced distinct standard deviation curves. Here, our
significance criterion was a variation in temperature at least three times the resolution of the sensor over a
temperature oscillation cycle. For our experiment, this translates to a change in temperature of at least 0.06°C
in 14 h or less. The accuracy of our results were also affected by the insertion of the probe, as differences
between the true and model sensor depths have the potential to introduce errors. A full sensitivity analysis
was performed to address this and is included in the Text S1.
Turcotte and Schubert (1982) described a simple method for estimating penetration depth (also called
diffusion length) of external thermal forcing:
——
L=√α * ∆t (1)
Here L is the diffusion length, a is the overall thermall diffusivity found at each deployment locations,
and ∆t is the time interval of interest. Calculations using overall diffusivity values from each deployment were
­performed for six different time intervals of interest (listed in Table 3). Penetration depths of 0.10, 3.69, and
11.68 m for bottom water forcing periods of ∆t = 7 h, 1 y, and 10 y using the overall diffusivity value were
calculated from the first deployment (552 m water depth). The estimates for the second deployment (1049 m
water depth) found diffusion lengths of 0.05, 1.90, and 6.02 m for time intervals of 7 h, 1 y, and 10 y. Though
the diffusion length solutions to this equation are low in accuracy and do not account for expected variations
in physical properties with depth due to compaction, they provide a reasonable first-order approximation
based on actual in situ thermal diffusivity measurements and highlight the potential for deeper penetration
of warming bottom waters at the upper boundary of this methane hydrate zone.
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Thermal diffusivity of marine sediments
Table 3. Diffusion lengths for time intervals of interesta
Deployment
1
Time interval
2
Diffusion length (m)
7h
0.10
0.05
24 h
0.19
0.10
1y
3.69
1.90
10 y
11.68
6.02
40 y
23.37
12.04
100 y
36.95
19.04
Lengths were calculated using the method outlined in Turcotte and Schubert (1982) with the overall TD values from Deployments
1 and 2 (Table 2).
a
doi: 10.12952/journal.elementa.000039.t003
Recent studies (Pohlman et al., 2009; Phrampus and Hornbach, 2012; Brothers et al., 2014; Berndt
et al., 2014; Hautala et al., 2014) have shown that increasing seawater temperatures at mid-water depths
on continental margins globally are releasing methane hydrate-derived carbon at flux rates that can provide
positive feedback to the present level of global warming. In some cases, hydrate-induced slope failures on
the continental margin slopes can produce tsunamis capable of inundating coastal communities (Rao et al.,
2002; Lopez et al., 2010). The Cascadia Margin in the NE Pacific has both over-steepened slopes and
methane-rich sediments containing abundant solid hydrate that is vulnerable to warming-induced slope
failure (Riedel et al., 2002; Booth-Rea et al., 2008; Torres et al., 2009). There is considerable societal need to
examine the tsunami hazards associated with the present and on-going phase dissociation associated with
ocean warming: our current experimental values of in situ thermal diffusivity can contribute to the boundary
conditions of these models.
5. Conclusions
This study aimed to provide estimates of the thermal diffusivity of marine sediments on the Cascadia Margin
through in-situ measurements acquired using a simple, yet highly functional device. The resulting, new in situ
TD data for sediments located on a methane-rich continental margin span the relevant ocean depth interval
where gas hydrates are known to be dissociating (Torres et al., 2009; Johnson et al., 2013). Eight TD values
were obtained: three depth-sptecific values for each of two deployment locations, and two depth-independent
values that characterize the surface sediments of the two deployment sites. The first deployment, at a water
depth of 552 m, had an overall TD of 4.33 × 10–7 m2 s–1, while the second deployment, at a water depth of
1049 m, had an overall TD value of 1.15 × 10–7 m2 s–1. Using a simple analytical model for heat transfer,
these diffusivity values were used to predict 1/e thermal penetration depths. These penetration depths can be
improved using more complex, realistic numerical models, but are the first estimates based on data obtained
in situ from a hydrate and methane-rich continental margin. These new data can be used to improve the
­ability to model and predict how rising seawater temperatures in the future may impact the relatively unstable
reservoir of methane–derived carbon, which has the potential to provide a strong positive feedback to the
already substantial anthropogenic greenhouse gas emissions.
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Author contributions
•
Contributed to conception and design: KLH, HPJ
•
Contributed to acquisition of data: KLH, HPJ, CKH
•
Contributed to analysis and interpretation of data: KLH, HPJ
•
Drafted and/or revised the article: KLH, HPJ, CKH
•
Approved the submitted version for publication: KLH, HPJ, CKH
Acknowledgments
The crew of the R/V Atlantis and operating personnel for the ROV Jason II were essential to the success of this study.
Special thanks are given to Tor Bjorklund for assistance with design and construction of the temperature probe, and to
Robert Harris, OSU, for his assistance and advice during the acquisition and processing phases of this project.
Funding information
Support for this project was provided by NSF Grant 1339635 to H. P. Johnson & E. A. Solomon.
Competing interests
The authors have no competing interests.
Supplementary material
•
Text S1. Detailed model analysis.
•
Table S1. Calibration offsets for temperature sensors.
Data accessibility statement
Prior to publication, all data used in this study, including site location and thermistor logs, will be posted on the
­GeoPRISM data archives and the National Geophysical Data Center database, as part of the NSF Open Access policy
for all data from this cruise.
Copyright
© 2015 Homola, Paul Johnson and Hearn. This is an open-access article distributed under the terms of the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided
the original author and source are credited.
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