Measurement of suspended particulate matter under sea ice using ADCP and LISST

1Departement of Ocean Sciences, Inha University, Incheon 402-751, South Korea2RPS Applied Science Associates, Inc., South Kingstown, RI 02879, USA3Scottish Association for Marine Science, Oban PA37 1QA, UK

Ho Kyung Ha1, Yong Hoon Kim2, Phil Hwang3

Abstract

Using a mooring package comprising an acoustic Doppler current profiler (ADCP) and

holographic imaging system, a 1-day ice camp study was performed under the Arctic

sea ice in the northern Chukchi Plateau to estimate vertical and temporal variations in

total suspended particulate matter (SPM). In early August, the SPM in upper mixed

layer (~15 m and above) under sea ice reached up to about 100 mg l-1 even under the

offshore regime. Results of both holographic and microscopic analyses showed that

dominant constituents of this increased SPM were biogenic rather than lithogenic

materials. Due to highest melt and break-up rates of sea ice during the summertime, the

export of particulate materials and ice algal communities embedded in the sea ice might

significantly contribute to the increase in SPM. This study suggests that combined

effects of the increase in ice algal production and the decrease in ice and snow cover

and multi-year sea ice extent could create favorable conditions for enhancing the

concentration and flux of SPM during summer time.

Fig. 1. (a) Map of study area in the Chukchi Plateau, Arctic Ocean. Color represents sea ice

concentration on August 7, 2011. Contour lines are isobaths (in meters). (b) Progressive vector

diagram of sea ice where the mooring package was installed. Black solid dots indicate the GPS

location of SIMBA at midnight of each day. Red line indicates the track of sea ice during a one-day

measuring campaign.

Objectives

So far, many studies have been conducted to investigate the variability in sea ice

coverage, thickness and motion in response to atmospheric and oceanic forcing

conditions. Only a few studies, however, have been carried out on the variability of SPM

under Arctic sea ice. This makes it difficult to accurately quantify the concentration and

flux of SPM and subsequent effects to the marine system. At this moment, the primary

unknowns in the Arctic particle dynamics are the spatial and temporal variations in SPM

under sea ice and the role of rapidly-melting summer sea ice as a source of SPM. In

order to address these unanswered questions, we carried out a one-day field experiment

using a mooring package including holographic and acoustic sensors. Despite the short

duration of the experiment, the holographic imaging technology, which has never been

deployed under sea ice, provides novel insights into under-ice particle dynamics.

Fig. 2. Profiling and time-series data from

beneath the ice floe: (a) potential temperature,

(b) salinity, (c) potential density, (d) GPS-

corrected velocity profiles, (e) current vector at

the LISST-Holo sampling level (5.1 m), (f)

SPMadcp profiles, (g) SPMadcp at 5.1 m, and (h)

flux of SPMadcp. CTD profiles are averages of

all casts. The gray and white areas in panel (e)

indicate the periods of northward and

southward flow, respectively. Thick green line

and red dots in panel (g) are low-pass filtered

and SPMsam concentration, respectively.

One-day Moorings

Figure 2 presents the temporal variability of water column structure during a one-day

mooring. CTD profiles showed a distinct pycnocline around 10–15 m below the surface.

The salinity near the ice-seawater interface was minimum (25.3 psu) influenced by the

input of ice meltwater, and gradually increased with depth until it reached 26.2 psu at 10

m, and then sharply increased to 28.0 psu at 15 m.

ADCP velocities at the near-surface layer showed reasonable agreement with ice drift

motion, and typical inertial motion was well captured in the corrected velocity. It is noted

that the measurements are the currents of water mass relative to the moving ice floe. A

strong current (> 0.3 m s-1) was observed at near-surface layer, being steered by

topographic effects of the base of the ice floe. At deeper depths exceeding about 13 m,

below the mixed layer, the water movement was relatively slow (< 0.1 m s -1). While water

was flowing toward the North within the periods of 1–7.5 and 13.5–20 h (see gray areas

in Fig. 2e), the current speed exhibited a two-layer structure identified near 12–13 m

where the vertical gradient of velocity was highest within the ADCP sensing range (Fig.

2d).

Variation in SPM under drifting sea ice

SPMadcp profiles converted from acoustic backscatter intensities showed distinct vertical

movements (Fig. 2f). During the first 4 h, SPMadcp gradually increased, producing a high-

concentration patch near 5 m. SPMadcp reached the maximum of 101.2 mg l-1 at the

uppermost bin (4.4–4.6 m) at 4 h. Abruptly, at 5 h, most of highly-concentrated materials

disappeared beyond 4.4 m, probably because it was entrained by the overlying water

mass. This upward transport of SPMadcp is also apparent in the upflux signature between

4 and 8 h (Fig. 2h). SPMadcp at the LISST-Holo sampling level (5.1 m) varied in the range

of 69.4–94.6 mg l-1. Although water samples taken were limited in number, SPMsam and

SPMadcp showed good agreement (Fig. 2g) with correlation r=0.75. The vertical flux was

highly variable with time, because it was determined by the competition between upflux

by turbulent diffusion and downflux by settling. Vertical velocities were about two orders

of magnitude larger than ws LISST (~ 0.2 mm s-1). This suggests that the flux of SPM, at

least in the under-ice boundary layer, was controlled mainly by under-ice turbulent motion

and topographic effects rather than settling.

Fig. 3. Near-inertial motion of sea ice floe and acoustic backscatter intensity during the mooring

campaign with the camera view from south to north. Thick black line at the surface show the track of

ice floe and red arrows represent ice movement. Thin vertical lines are time tick for every hour with

numbers representing hours after 00:00 GMT, August 7, 2011.

 

Near-inertial motion of drifting sea ice

Conclusions

In early August 2011, the SPM concentration under the sea ice reached up to the

order of 100 mg l-1, which is several times higher than typical SPM in open ocean

environments. The SPM flux was significantly fluctuated upward or downward by

the under-ice topographic effects. The export of particulate materials and ice algal

communities embedded in the sea ice significantly contributed to the increase in

SPM. The advection of increased biomass after phytoplankton bloom might also

contribute to the increase in SPM.

Combined effects of the increase in insolation, ice algal production, and the

decrease in ice and snow cover and multi-year sea ice extent could create

favorable conditions for enhancing the concentration and flux of SPM during the

rapidly-melting summer season. With the thinning and retreat trend of Arctic sea

ice, it is expected that under-ice seawater will in future receive a still higher rate of

discharge of particulate matters from melting sea ice.

AcknowledgementsThis study was supported by KOPRI grants (PM13020) and Inha University Research Grant

(INHA-49861).

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