Fur seal diving behaviour in relation to vertical distribution of krill

first_imgQuantitative studies of predator-prey interactions depend on a knowledge of their spatial dynamics and behaviour. Studies on marine vertebrates have hitherto been precluded by the difficulty of acquiring the relevant data. (2) Continuous records of diving depths of female Antarctic fur seals on 3-8 day feeding trips to sea from South Georgia were analysed in conjunction with data on diel changes in the abundance and distribution of their main prey, krill. (3) In 36 complete days foraging by seven seals, 75% of 4273 dives were at night. Dives then were consistently shallower (dive depth <30 m) than in daytime (mostly 40-75 m). (4) This closely matched changes in the vertical distribution of krill, nearly all of which was below a depth of 50 m from 09.00-15.00 h, with substantial quantities above 40 m only between 21.00-06.00 h. (5) Although over 40% of krill in the water column at any time of day was below 75 m, only 3% of dives exceeded this depth. We suggest that because krill migrate vertically fur seals are able to exploit them most efficiently during shallow dives at night.last_img read more

The Bouvet triple junction, 20 to 10 Ma, and extensive transtensional deformation adjacent to the Bouvet and Conrad transforms

first_imgThe Bouvet triple junction has been proposed to have evolved as a ridge-fault-fault (RFF) type between 20 and 10 Ma, connecting the southernmost Mid-Atlantic Ridge (SMAR) with the Bouvet and Conrad transforms, to the east and west, respectively. We surveyed immediately north of these two transforms with side-scan and multibeam sonars, on seafloor that would have originally been created at the SMAR close to its junction with the two transforms. The sonar data reveal that SMAR fabrics on the Bouvet and Conrad sides, when corrected for plate rotation, are parallel to each other, so they were most likely formed at the same spreading ridge and confirm that the triple junction was indeed RFF. Our second major result is that the SMAR fabrics are extensively crosscut by normal faults, over most of the 400 km surveyed along both transforms and most intensely north of the Bouvet transform. Growth faults and faults affecting the sediment surface in multichannel seismic images show that the deformation has been long-lived and is probably ongoing. Since the orientations of the crosscutting faults are similar to those of shear zone tension fractures, we interpret these areas to be transtensional zones. This extent of deformation adjacent to major oceanic transforms is rare, and we develop a number of ideas to explain its origin.last_img read more

ULF Doppler oscillation of L = 2.5 flux tubes

first_imgPlasmaspheric ducts may execute Doppler oscillations driven by propagating ultra-low frequency (ULF) waves. We examined about 100 such events recorded over 1 year under magnetically quiet conditions at L = 2.5 using artificially generated whistler-mode VLF signals and ground magnetometers. Joint peaks in the VLF Doppler and magnetometer spectra occurred at the frequency expected for ULF waves generated by the ion-cyclotron instability in the upstream solar wind. The VLF Doppler shifts are most likely due to radial motion of flux tubes of a few kilometers, driven by the east-west electric field of propagating ULF waves. When the frequencies match, the incoming wave energy also couples to standing poloidal and azimuthal field line oscillations, producing field line resonance signatures in both the D and H components on ground-based magnetometers. The phases of the VLF and ULF oscillations are consistent with ionospheric rotation of the downgoing ULF wave field. Since the scale size of VLF flux tubes is significantly smaller than for ULF flux tubes, VLF Doppler observations can provide more precise spatial information on ULF wave fields in the plasmasphere. Furthermore, it should be possible to use ULF oscillations to monitor the formation of quarter wavelength mode standing field line oscillations when the conjugate ionospheres have different conductivities.last_img read more

Bathymetry of the Amundsen Sea continental shelf: implications for geology, oceanography, and glaciology

first_imgThe Amundsen Sea continental shelf is one of the most remote areas of coastal Antarctica and was relatively unexplored until the late 1980s. Over the last two decades, increased oceanographic and geological interest has led to several cruises that resulted in sufficient bathymetric data to compile a fairly detailed regional map of the Amundsen continental shelf. We have combined available multibeam and single-beam bathymetry data from various sources and created a new regional bathymetry of the Amundsen Sea continental shelf and margin. Deep trough systems that dominate the inner shelf are aligned with present glaciers and separated by shallower ridges. Shaped by paleo-ice streams, these features merge into a small number of broader troughs on the middle shelf and shoal seaward. They now serve as conduits and reservoirs for relatively warm Circumpolar Deep Water. This new compilation is a major improvement over previously available regional maps and should aid the numerical modeling of ocean circulation, the reconstructions of paleo-ice streams, and the refinement of ice sheet models.last_img read more

Modeling the properties of plasmaspheric hiss: 1. Dependence on chorus wave emission

first_imgThere is increasing evidence that plasmaspheric hiss is formed by the evolution of a portion of chorus waves that are excited in the plasmatrough and propagate into the plasmasphere. Comparison between the statistical spatial distributions of these two emissions in the morning sector during active times from THEMIS over ∼3 years shows that the two emissions have comparable peak intensities but are distinct in their spatial distributions. We present a modeling study of the hiss spectrum, based on ray tracing, by taking the observed chorus source region as an input in the magnetosphere, which contains cold and suprathermal electrons. Our modeling results show that we are able to reproduce the main features of typical hiss, including the frequency spectrum, wave normal angle and spatial distribution. However, the simulated hiss intensity is weaker (∼15 dB less) than the observed intensity, which suggests some modest internal amplification inside the plasmasphere. The responses of hiss to variations in the spatial distribution, wave normal angle distribution and frequency distribution of the source chorus are examined. We find that the majority of hiss formation is due to a small portion of chorus emission originating within ∼3 RE from the plasmapause, with wave normal directions pointing toward the Earth at an angle of 30°–60°, and over a frequency range of 0.1–0.3 fce. If the chorus power is made to increase closer to the plasmapause, the hiss intensity and the peak frequency also increases, which roughly mimics active geomagnetic conditions. Variations of the chorus source distribution do not significantly affect the wave normal angle distribution and frequency distribution of hiss, but does impact the absolute intensity of the resulting hiss.last_img read more

Sustained retreat of the Pine Island Glacier

first_imgWe use satellite observations to show that, between 1992 and 2011, the Pine Island Glacier hinge line retreated at a rate of 0.95 ± 0.09 km yr−1 despite a progressive steepening and shoaling of the glacier surface and bedrock slopes, respectively, which ought to impede retreat. The retreat has remained constant because the glacier terminus has thinned at an accelerating rate of 0.53 ± 0.15 m yr−2, with comparable changes upstream. This acceleration is consistent with an intensification of ocean-driven melting in the cavity beneath the floating section of the glacier. The pattern of hinge-line retreat meanders and is concentrated in isolated regions until ice becomes locally buoyant. Because the glacier-ocean system does not appear to have reached a position of relative stability, the lower limit of sea level projections may be too conservative.last_img read more

A method for continuous239Pu determinations in Arctic and Antarctic ice cores

first_imgAtmospheric nuclear weapons testing (NWT) resulted in the injection of plutonium (Pu) into the atmosphere and subsequent global deposition. We present a new method for continuous semi-quantitative measurement of 239Pu in ice cores, which was used to develop annual records of fallout from NWT in ten ice cores from Greenland and Antarctica. The 239Pu was measured directly using an Inductively Coupled Plasma – Sector Field Mass Spectrometer, thereby reducing analysis time and increasing depth-resolution with respect to previous methods. To validate this method, we compared our one year averaged results to published 239Pu records and other records of NWT. The 239Pu profiles from four Arctic ice cores reflected global trends in NWT and were in agreement with discrete Pu profiles from lower latitude ice cores. The 239Pu measurements in the Antarctic ice cores tracked low latitude NWT, consistent with previously published discrete records from Antarctica. Advantages of the continuous 239Pu measurement method are (1) reduced sample preparation and analysis time; (2) no requirement for additional ice samples for NWT fallout determinations; (3) measurements are exactly co-registered with all other chemical, elemental, isotopic, and gas measurements from the continuous analytical system; and (4) the long half-life means the 239Pu record is stable through time.last_img read more

Effects of field warming on high arctic soil bacterial community: a metagenomic analysis

first_imgSoil microbial communities in the Arctic, one of themost rapidly warming regions on Earth, play animportant role in a range of ecological processes. Thisreport describes initial studies of natural soil bacterialdiversity at a High Arctic site on Svalbard, as part ofa long-term field environmental manipulation study.The impact of increased soil temperature and wateravailability on soil microbial communities was investigated.The manipulation experiment, using open-topchambers, was installed in late summer 2014, and thesoils were sampled soon after snow melt in July 2015.High throughput sequencing of 16S rRNA genes showedrelatively uniform diversity across the study area andrevealed no significant initial effect of treatmentson bacterial communities over the first 10-monthautumn–winter–spring manipulation period.last_img read more

The uppermost mantle seismic velocity structure of West Antarctica from Rayleigh wave tomography: insights into tectonic structure and geothermal heat flow

first_imgWe present a shear wave model of the West Antarctic upper mantle to ∼200 km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25 s) and earthquake data by two-plane wave analysis (periods 20-143 s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (<180 km). On the northern margin of the WARS, a pronounced low velocity anomaly extends eastward from beneath the Marie Byrd Land dome toward Pine Island Bay, underlying Thwaites Glacier, but not Pine Island Glacier. If of plume-related thermal origin, the velocity contrast of ∼5% between this anomaly and the inner WARS translates to a temperature difference of ∼125-200 . However, the strike of the anomaly parallels the paleo-Pacific convergent margin of Gondwana, so it may reflect subduction-related melt and volatiles rather than anomalously elevated temperatures, or a combination thereof. Motivated by xenolith analyses, we speculate that high velocity zones imaged south of the Marie Byrd Land dome and in the eastern Ross Sea Embayment might reflect the compositional signature of ancient continental fragments. A pronounced low velocity anomaly underlying the southern Transantarctic Mountains (TAM) is consistent with a published lithospheric foundering hypothesis. Taken together with a magnetotelluric study advocating flexural support of the central TAM by thick, stable lithosphere, this points to along-strike variation in the tectonic history of the TAM. A high velocity anomaly located in the southern Weddell Sea Rift System might reflect depleted mantle lithosphere following the extraction of voluminous melt related to Gondwana fragmentation. Lithospheric thickness estimates extracted from 1D shear wave velocity profiles representative of tectonic domains in West Antarctica indicate an average lithospheric thickness of ∼85 km for the WARS, Marie Byrd Land, and Thurston Island block. This increases to ∼96 km in the Ellsworth Mountains. A surface heat flow of and attendant geotherm best explains lithospheric mantle shear wave velocities in the central WARS and in the Thurston Island block adjacent to Pine Island Glacier; a geotherm best explains the velocities in the Ellsworth Mountains, and a geotherm best explains a less well-constrained velocity profile on the southern Antarctic Peninsula. We emphasise that these are regional average (many hundreds of km) heat flow estimates constrained by seismic data with limited sensitivity to upper crustal composition.last_img read more

Intra-specific niche partitioning in Antarctic fur seals, Arctocephalus gazella

first_imgCompetition for resources within a population can lead to niche partitioning between sexes, throughout ontogeny and among individuals, allowing con-specifics to co-exist. We aimed to quantify such partitioning in Antarctic fur seals, Arctocephalus gazella, breeding at South Georgia, which hosts ~95% of the world’s population. Whiskers were collected from 20 adult males and 20 adult females and stable isotope ratios were quantified every 5 mm along the length of each whisker. Nitrogen isotope ratios (δ15N) were used as proxies for trophic position and carbon isotope ratios (δ13C) indicated foraging habitat. Sexual segregation was evident: δ13C values were significantly lower in males than females, indicating males spent more time foraging south of the Polar Front in maritime Antarctica. In males δ13C values declined with age, suggesting males spent more time foraging south throughout ontogeny. In females δ13C values revealed two main foraging strategies: 70% of females spent most time foraging south of the Polar Front and had similar δ15N values to males, while 30% of females spent most time foraging north of the Polar Front and had significantly higher δ15N values. This niche partitioning may relax competition and ultimately elevate population carrying capacity with implications for ecology, evolution and conservation.last_img read more