Histograms of iceqauke counts and source levels.

<p>(A) Number of icequakes per day recorded within the Bransfield Strait (left) and Scotia Sea (right). The Scotia Sea exhibits roughly four times the number of icequakes as observed in the Bransfield Strait. Bransfield icequakes show clear seasonal distribution, peaking roughly during the austral summer months and minimum during austral winter. Scotia icequakes also show seasonality, however peak in acitivty in early 2008 is due to breakup of A53a (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g005" target="_blank">Fig 5</a>). For reference, gray background denotes begin and end times of seasons, with summer (Su), fall (F), winter (W) and spring (Sp) labeled. (B) Histogram showing estimates of Bransfield and Scotia icequakes source levels, which range between 190–242 and 203–247 dBrms re 1 μPa @ 1 m, respectively.</p

Cross-correlation functions of mean noise level in Bransfield Strait with temperature and wind speeds measured at King Sejong Station on King George Island.

<p>Noise levels were recorded on hydrophone deployed in the Bransfield Strait in the 51–90 Hz band. Noise levels show a direct correlation with wind speed, whereas noise levels lag an increase in air temperature by 1–2 days.</p

Histograms of iceqauke counts and source levels.

<p>(A) Number of icequakes per day recorded within the Bransfield Strait (left) and Scotia Sea (right). The Scotia Sea exhibits roughly four times the number of icequakes as observed in the Bransfield Strait. Bransfield icequakes show clear seasonal distribution, peaking roughly during the austral summer months and minimum during austral winter. Scotia icequakes also show seasonality, however peak in acitivty in early 2008 is due to breakup of A53a (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g005" target="_blank">Fig 5</a>). For reference, gray background denotes begin and end times of seasons, with summer (Su), fall (F), winter (W) and spring (Sp) labeled. (B) Histogram showing estimates of Bransfield and Scotia icequakes source levels, which range between 190–242 and 203–247 dBrms re 1 μPa @ 1 m, respectively.</p

Geographic location of the Antarctic Peninsula (AP), Bransfield Strait (BS), Drake Passage (DP), and Scotia Sea.

<p>Yellow triangles show 2005–2007 and 2008–2009 hydrophone mooring deployments within Bransfield Strait (elongated basin northwest of AP) and Drake Passage (B), as well as the 2007–2008 deployments within Scotia Sea (A). Circles M2 and M4 show locations of hydrophone moorings used to make spectrograms in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g006" target="_blank">Fig 6</a>. Black dots show regional icequakes located using both hydrophone arrays. All events shown were located using 4–5 hydrophones. Red line shows 8-month track of iceberg A53a from Weddell to Scotia seas derived from NASA’s Quick-Scatterometer (QuicScat) satellite (BYU Scatterometer Climate Record Pathfinder; <a href="http://www.scp.byu.edu" target="_blank">www.scp.byu.edu</a>). White polygon shows approximate shape and size of A53a when near South Georgia Island on 12 February 2008 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g005" target="_blank">Fig 5</a>). Positions “1” and “2” in red show iceberg locations where harmonic tremor (IHT) was produced. Bathymetry is from Sandwell and Smith [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.ref049" target="_blank">49</a>] satellite altimetry.</p

Satellite image and breakup of iceberg A53a in Scotia Sea near South Georgia Island.

<p>(A) Red dots show daily position of A53a as it rounded southeast tip of South Georgia. The dates of four positions of A53a are shown. The images of A53a are polygon representations made based on satellite images [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.ref050" target="_blank">50</a>]. Satellite bathymetry shown is from Sandwell and Smith [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.ref049" target="_blank">49</a>] and displayed using Geomapapp (<a href="http://www.geomapapp.org/" target="_blank">http://www.geomapapp.org/</a>). The first major breakup event occurs on 2 February 2008. Green dots show hydrophone derived locations of icequakes associated with disintegration of iceberg. Yellows stars are hydrophone locations. Scatter of icequakes on 21 February likely due to iceberg being outside array aperture. (B) Satellite image showing smaller scale views of disintegration of A53a as it rounded South Georgia following major breakup events on 2–21 February 2008[<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.ref050" target="_blank">50</a>]</p

Spectrograms of the varied acoustics sources recorded by the Bransfield and Scotia Sea hydrophones.

<p>Panel (A) shows record of emergent, broadband, icequake acoustic arrival; (B) shows record of impulsive, short duration icequake signal, indicating that icequake may be closer to source, experiencing less attenuation than emergent record; (C) shows fundamental and harmonic overtones of iceberg tremor; (D) shows Antarctic blue whale vocalization; (E) is an earthquake (T-phase) signal packet with broadband airgun arrivals in background; and (F) is noise from a ship, generated by cavitation from the propeller. The sound energy level is roughly equivalent for all sources; however, each varies in prevalence through the year.</p

Bransfield ambient noise levels compared to wind, temperature, ice cover and blue whale vocalizations.

<p>(A) Examples of ambient noise levels (averaged over 51–90 Hz) in Bransfield Strait (yellow) and Drake Passage (light blue), air temperature (dark blue), and wind speed (green) from December 2005 to December 2007. Wind speed and air temperature were measured at King Sejong Station, on King George Island (red dot on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g001" target="_blank">Fig 1B</a>). Gray background marks begin and end of seasons as in Figs <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g004" target="_blank">4</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g007" target="_blank">7</a>. (B) Shows seasonal variation of 27–28 Hz energy (blue whale occurrence) in Bransfield (yellow cross), Drake Passage (blue x), and sea-ice coverage (purple triangle). Sea-ice concentration taken from National Snow and Ice Data Center [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.ref051" target="_blank">51</a>]. High overall noise levels correlate with lowering temperatures, average winds, low ice cover, but high whale vocalization energy.</p

Spectrograms of acoustic power spectral at the Bransfield Strait (A) and Scotia Sea (B).

<p>Spectrograms are calculated from 2 second FFT windows with each vertical time slice representing the cumulative energy over a 1 day interval. Near-constant, broad-band, short-duration impulsive signals are likely sounds from wind-driven waves and ice breakup. Clustered energy in the 15–25 and 89 Hz bands are fin whale calls. Blue whale calls are the steady tone at 27 Hz. The several examples of fundamental and overtone energy <10 Hz in both spectra are interpreted to be from a combination of sources, including broadband energy created by sea-state (storms, waves, and wind), iceberg grounding tremor, and tonal “strumming” of mooring line caused by fast moving ocean currents. White areas represent times with no data.</p

Diagram of selected percentiles from the cumulative distribution of spectral energy (daily average).

<p>Spectrograms shown are hydrophone M2 in Bransfield Strait (A-B; 2005–2007) and M4 in Scotia Sea (C-D; 2008–2009). The noise levels are separated into select frequency bands to examine the influence of different sound sources on overall levels. Solid line shows the dB levels of 50% of the cumulative spectral energy in the given frequency band; lower dashed line shows the dB level of 5%; and upper dashed line shows the dB level of 95% of the cumulative spectral energy. Gray background marks begin and end of seasons as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123425#pone.0123425.g004" target="_blank">Fig 4</a>.</p

Glider track (colored line) and surfacing positions of tagged Cuvier's beaked whale (black dots) on 3 November 2009.

<p>Glider depth is color-coded. Black stars indicate position of glider at times of beaked whale surfacing events. Red star indicates position of glider when Cuvier's beaked whale clicks were acoustically detected by the glider during the mission (detections were verified in the post deployment analysis). The acoustic system was operated at depth between 500 m and 1000 m indicated by greenish/bluish colors. Times are UTC.</p