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Use of a two-frequency algorithm to determine size and abundance of plankton in three widely spaced locations

first_imgThe application of a two-frequency algorithm to the in situ estimation of large zooplankton is explored in an effort to use information from more than one acoustic frequency. To accomplish this, 38 and 120 kHz echo-sounders, found on most fisheries research vessels, were utilized. A two-frequency method using a highpass fluid sphere scattering model was employed to estimate the mean size (length) and the density per m3 of the dominant acoustic scatterers (mainly euphausiids and shrimps), by selected depth layer. Three locations were surveyed: the Gulf of St Lawrence, the Antarctic, and the Irish Sea. Ground-truth samples were collected from various plankton net systems, the catches being compared to the acoustic estimates. Results are discussed with regard to conditions that apply to the method, the ground-truth performance and the relative complexity of the zooplankton composition at the different study sites. The conclusion is that this method has provided satisfactory results but still has a capacity for refinement.last_img read more

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Status of southern elephant seals at South Georgia

first_imgApproximately 54% of the world population of southern elephant seals (Mirounga leonina) breeds at South Georgia. A partial survey in 1951 and a complete survey in 1985, together with counts at specific sites between these times, suggested that the population (around 100 000 breeding females) had not changed significantly in 34 years. This was in contrast to marked declines in most other populations. To examine this further, we conducted a third survey in 1995. This produced an estimate of 113 444 (se = 4902) breeding females. Taking into account improved information about the behaviour of female elephant seals since the survey in 1985, there was no significant change in the number of breeding female elephant seals between 1985 and 1995. When combined with information from the 1951 survey, this supports the view that the total population size has not changed significantly during the past 45 years. Evidence for regulation of the population by environmental factors is equivocal. We hypothesize that the lack of any net change in population size may be linked to a limited availability of high quality breeding habitat.last_img read more

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Gondwanian age dextral transpression and spatial kinematic partitioning within the Heritage Range, Ellsworth Mountains, West Antarctica

first_imgThe Ellsworth Mountains, West Antarctica, consist of two mountain ranges; the Sentinel, and Heritage ranges. The more southerly Heritage Range is composed of a lower Paleozoic sedimentary and volcanic rock sequence, deformed during a single major deformation event in the early Mesozoic. This Gondwanian Orogeny possibly resulted from Andean-style convergence along the southern margin of Gondwana, prior to break-up of the super-continent, and the subsequent translation of the Ellsworth Mountains from a position close to the Natal embayment of southern Africa to that of the present day. Rocks of the Heritage Range are intensely folded, with close to tight, upright to inclined folds, plunging gently about a horizontal axis trending NNW-SSE. Locally, folds plunge moderately to subvertical toward the NNW, possessing asymmetries consistent with a dextral sense of shear. Cleavage is generally axial planar displaying downdip and strike-parallel stretching lineations that are frequently associated with domains of reverse, and dextral shear striking parallel to the regional structural grain. The spatial and temporal relationship of fractures developed within these domains as a result of noncoaxial shear, in addition to the progressive incremental strain histories derived from mineral fibres in strain shadows, indicate the contemporaneous nature of these shear domains. Strain analysis of deformed tuffaceous diamictites and oncolithic limestones reveal k-values <1 (mean 0.59) throughout the Heritage Range. The coexistence of strike-parallel dextral, oblique and reverse-shear domains, abrupt reorientation of progressive strain axes, steep cleavage dips, and k-values <1 are all consistent with a dextral transpressive deformation regime, previously unrecognized in the Ellsworth Mountains. A model of highly oblique (pure-shear dominated) transpression, associated with efficient spatial partitioning of the strike-slip component of shear is proposed to describe the structural relationships observed.last_img read more

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Evidence for acceleration of outer zone electrons to relativistic energies by whistler mode chorus

first_imgWe use plasma wave and electron data from the Combined Release and Radiation Effects Satellite (CRRES) to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by gyroresonant interactions with whistler mode chorus. In particular, we examine the temporal evolution of the spectral response of the electrons and the waves during the 9 October 1990 geomagnetic storm. The observed hardening of the electron energy spectra over about 3 days in the recovery phase is coincident with prolonged substorm activity, as monitored by the A(E) index and enhanced levels of whistler mode chorus waves. The observed spectral hardening is observed to take place over a range of energies appropriate to the resonant energies associated with Doppler-shifted cyclotron resonance, as supported by the construction of realistic resonance curves and resonant diffusion surfaces. Furthermore, we show that the observed spectral hardening is not consistent with energy-independent radial diffusion models. These results provide strong circumstantial evidence for a local stochastic acceleration mechanism, involving the energisation of a seed population of electrons with energies of the order of a few hundred keV to relativistic energies, driven by wave-particle interactions involving whistler mode chorus. The results suggest that this mechanism contributes to the reformation of the relativistic outer zone population during geomagnetic storms, and is most effective when the recovery phase is characterised by prolonged substorm activity. An additional significant result of this paper is that we demonstrate that the lower energy part of the storm-time electron distribution is in steady-state balance, in accordance with the Kennel and Petschek (1966) theory of limited stably-trapped particle fluxes.last_img read more

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A test of the magnetospheric source of travelling convection vortices

first_img[1] Traveling convection vortices (TCVs) are a powerful tool for probing the nature of the coupling between the solar wind, the magnetosphere, and the ionosphere. There is no reliable model of the plasma concentration in the magnetosphere, resulting in uncertainties about the factors controlling the scale size, the motion, and the numbers of field-aligned currents associated with TCV events. There is also uncertainty about whether TCV generation is current driven, voltage driven, or even driven by some more complex source. We use conjugate ground-based magnetometer data from the Greenland magnetometer chain and Antarctica to test the nature of the magnetospheric source of 18 TCV events associated with changes in the magnetopause dynamic pressure. This is achieved by statistically comparing two groups of TCV events: for one group the conjugate ionospheres are of similar conductivity, and for the other group the conductivities of the conjugate ionospheres differ by an order of magnitude. Statistically, we find that conjugate TCV events are of similar intensity in both hemispheres regardless of any difference in conductivity between the two hemispheres. We propose that this is evidence in favor of a constant current source for TCVs where the amplitude of a TCV is controlled by the local plasma concentration, the magnetic field strength, and the acceleration of the plasma.last_img read more

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The Mediterranean hydrologic budget from a Late Miocene global climate simulation

first_imgDuring the Late Miocene the Mediterranean experienced a period of extreme salinity fluctuations known as the Messinian Salinity Crisis (MSC). The causes of these high amplitude changes in salinity are not fully understood but are thought to be the result of restriction of flow between the Mediterranean and Atlantic, eustatic sea level change and climate. Results from a new Atmospheric General Circulation Model (AGCM) simulation of Late Miocene climate for the Mediterranean and adjacent regions are presented here. The model, HadAM3, was forced by a Late Miocene global palaeogeography, higher CO2 concentrations and prescribed sea surface temperatures. The results show that fluvial freshwater fluxes to the Mediterranean in the Late Miocene were around 3 times greater than for the present day. Most of this water was derived from North African rivers, which fed the Eastern Mediterranean. This increase in runoff arises from a northward shift in the intertropical convergence zone caused by a reduced latitudinal gradient in global sea surface temperatures. The northwards drainage of the Late Miocene Chad Basin also contributes. Numerical models designed to explore Late Miocene salt precipitation regimes in the Mediterranean, which typically make use of river discharge fluxes within a few tens of percent of present-day values, may therefore be grossly underestimating these fluxes. Although the AGCM simulated Late Miocene river discharge is high, the model predicts a smaller net hydrologic budget (river discharge plus precipitation minus evaporation) than for present day. We discuss a possible mechanism by which this change in the hydrologic budget, coupled with a reduced connection between the Mediterranean and the global ocean, could cause the salinity fluctuations of the MSC.last_img read more

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The fieldwork during the “Polarstern” cruise ANT XVI/2 as a contribution to the study of bottom water formation and sea ice transport in the Weddell Sea

first_imgThe field work during ”Polarstern” cruise ANT XVI/2 aimed to measure circulation and water mass properties in the Atlantic sector of the Southern Ocean, to investigate the contribution of the Weddell Sea to the ocean’s role in climate. In this respect water mass modification plays animportant role, because it leads to the formation of Antarctic Bottom Water which is an essential part of the global thermohaline circulation.last_img

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The Arctic cryosphere in the Mid-Pliocene and the future

first_imgThe Mid-Pliocene (ca 3 Myr ago) was a relatively warm period, with increased atmospheric CO2 relative to pre-industrial. It has therefore been highlighted as a possible palaeo-analogue for the future. However, changed vegetation patterns, orography and smaller ice sheets also influenced the Mid-Pliocene climate. Here, using a general circulation model and ice-sheet model, we determine the relative contribution of vegetation and soils, orography and ice, and CO2 to the Mid-Pliocene Arctic climate and cryosphere. Compared with pre-industrial, we find that increased Mid-Pliocene CO2 contributes 35 per cent, lower orography and ice-sheet feedbacks contribute 42 per cent, and vegetation changes contribute 23 per cent of Arctic temperature change. The simulated Mid-Pliocene Greenland ice sheet is substantially smaller than that of modern, mostly due to the higher CO2. However, our simulations of future climate change indicate that the same increase in CO2 is not sufficient to melt the modern ice sheet substantially. We conclude that, although the Mid-Pliocene resembles the future in some respects, care must be taken when interpreting it as an exact analogue due to vegetation and ice-sheet feedbacks. These act to intensify Mid-Pliocene Arctic climate change, and act on a longer time scale than the century scale usually addressed in future climate prediction.last_img read more

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Oscillations in the southern extent of the Indo-Pacific Warm Pool during the mid-Holocene

first_imgThe Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between similar to 5500 and 4300 years BP (similar to 1.2 degrees C +/- 0.3 degrees C) and were similarly cool before similar to 6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between similar to 6600 and 6300 years BP (similar to 1.3 degrees C +/- 0.3 degrees C), while similarly warm conditions may have also existed after similar to 4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesting that the Asian monsoon-IOD interaction that exists at interannual time scales also persists over centennial to millennial scales. Associated mean changes in the Pacific ENSO modes may have also occurred during the mid-Holocene. The dynamic and inter-connected behaviour of the IPWP with tropical climate systems during the mid-Holocene highlights the fundamental importance of the warm pool region for understanding climate change throughout the tropics and beyond. (C) 2009 Elsevier Ltd. All rights reserved.last_img read more

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Sea ice extent and seasonality for the Early Pliocene northern Weddell Sea

first_imgGrowth increment analysis coupled with stable isotopic data (δ18O/δ13C) from Early Pliocene (ca 4.7 Ma)Austrochlamys anderssoni from shallow marine sediments of the Cockburn Island Formation, northernAntarctic Peninsula, suggest these bivalves grew through much of the year, even during the coldest parts ofwinter recorded in the shells. The high frequency fluctuation in growth increment width of A. anderssoniappears to reflect periodic, but year-round, agitation of the water column enhancing benthic food supplyfrom organic detritus. This suggests that Austrochlamys favoured waters that were largely sea ice free. Ourdata support interpretation of the Cockburn Island Formation as an interglacial marine deposit and theprevious hypothesis that Austrochlamys retreated from the Antarctic as sea ice extent expanded, thistransition occurring during climate cooling in the Late Pliocene.last_img read more

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