scholarly journals Antarctic and Southern Ocean dust transport pathways: Forward-trajectory modeling and rare earth element source constraints from the RICE ice core

2021 ◽  
Author(s):  
◽  
Peter David Neff
Keyword(s):  
Rare Earth ◽  
New Zealand ◽  
South America ◽  
Southern Ocean ◽  
Mineral Dust ◽  
Ice Core ◽  
Ice Cores ◽  
Potential Contribution ◽  
Anthropogenic Aerosols ◽  
Dust Transport

<p>Mineral dust fertilization of Southern Ocean surface waters, and mixing with Antarctic deep-water, influences oceanic uptake of carbon dioxide and draws down global atmospheric concentrations during glacial periods. Quantifying modern variability in dust source and transport strength, especially with respect to high- and low-latitude climate phenomena (e.g. the Southern Annular Mode, El Niño Southern Oscillation), will improve understanding of this important aspect of the global carbon cycle. Using high-order geochemical provenance techniques can also reveal in greater detail what aspects of dust transport are recorded in Antarctic ice core records, allowing for better interpretation of glacial-interglacial dust records at individual sites.  First, using forward trajectories and climate reanalysis data, this work explores modern variability (1979-2013) in atmospheric transport of mineral dust from Southern Hemisphere potential source areas (PSA)—primarily Australia, southern South America and southern Africa. Estimates of the relative source and transport strength of New Zealand are also discussed, and compared with other dust PSA to evaluate New Zealand’s potential contribution to Southern Ocean and Antarctic dust deposition. Extra-Antarctic dust PSA distributions are detailed for individual ice core sites, including the newly recovered Roosevelt Island Climate Evolution (RICE) ice core (79.36ºS, 161.71ºW, 550 m a.s.l.). This approach—applicable to many types of aerosol—reveals persistent, strong transport from New Zealand and Patagonia to the southern high-latitudes during all seasons. It also demonstrates that southward transport of air masses from pan-Pacific dust sources is affected by circulation variability initiated in the central tropical Pacific Ocean.  High-resolution discrete sampling of the RICE core allows for unprecedented analysis of trace elements at sub-annual to annual scales. The rare earth elements (REE, lanthanide elements Lanthanum to Lutetium) can preserve the signature of their original source material and thus provide provenance constraints for dust preserved in Antarctic snow and ice. While challenging, measurements of REE concentration to the single femtogram per gram (10-15 g g-1) level have been made by combining efficient sample introduction and a jet-interface sector-field inductively coupled plasma mass spectrometer. The methodology and fidelity of these measurements are presented, in addition to results for other low-concentration elements associated with natural and anthropogenic aerosols.  REE data from the RICE ice core are then used to explore possible modern sources of dust in the Ross Sea sector of Antarctica, testing hypothesized trajectory model distributions. Twentieth-century and late-Holocene (2.3 ka – present) REE data from the RICE ice core represent the first measurements of this kind from the Pacific sector of Antarctica. RICE data are compared with Holocene REE data from the Drønning Maud Land and Dome C ice cores, with consideration of REE signatures in dust samples from PSA. Data from the RICE ice core indicate fewer than 5% contributions of dust from South America, and show strong negative trends in crustal-normalized REE signatures suggesting contributions from local Antarctic dust.</p>

scholarly journals Antarctic and Southern Ocean dust transport pathways: Forward-trajectory modeling and rare earth element source constraints from the RICE ice core

2021 ◽  
Author(s):  
◽  
Peter David Neff
Keyword(s):  
Rare Earth ◽  
New Zealand ◽  
South America ◽  
Southern Ocean ◽  
Mineral Dust ◽  
Ice Core ◽  
Ice Cores ◽  
Potential Contribution ◽  
Anthropogenic Aerosols ◽  
Dust Transport

<p>Mineral dust fertilization of Southern Ocean surface waters, and mixing with Antarctic deep-water, influences oceanic uptake of carbon dioxide and draws down global atmospheric concentrations during glacial periods. Quantifying modern variability in dust source and transport strength, especially with respect to high- and low-latitude climate phenomena (e.g. the Southern Annular Mode, El Niño Southern Oscillation), will improve understanding of this important aspect of the global carbon cycle. Using high-order geochemical provenance techniques can also reveal in greater detail what aspects of dust transport are recorded in Antarctic ice core records, allowing for better interpretation of glacial-interglacial dust records at individual sites.  First, using forward trajectories and climate reanalysis data, this work explores modern variability (1979-2013) in atmospheric transport of mineral dust from Southern Hemisphere potential source areas (PSA)—primarily Australia, southern South America and southern Africa. Estimates of the relative source and transport strength of New Zealand are also discussed, and compared with other dust PSA to evaluate New Zealand’s potential contribution to Southern Ocean and Antarctic dust deposition. Extra-Antarctic dust PSA distributions are detailed for individual ice core sites, including the newly recovered Roosevelt Island Climate Evolution (RICE) ice core (79.36ºS, 161.71ºW, 550 m a.s.l.). This approach—applicable to many types of aerosol—reveals persistent, strong transport from New Zealand and Patagonia to the southern high-latitudes during all seasons. It also demonstrates that southward transport of air masses from pan-Pacific dust sources is affected by circulation variability initiated in the central tropical Pacific Ocean.  High-resolution discrete sampling of the RICE core allows for unprecedented analysis of trace elements at sub-annual to annual scales. The rare earth elements (REE, lanthanide elements Lanthanum to Lutetium) can preserve the signature of their original source material and thus provide provenance constraints for dust preserved in Antarctic snow and ice. While challenging, measurements of REE concentration to the single femtogram per gram (10-15 g g-1) level have been made by combining efficient sample introduction and a jet-interface sector-field inductively coupled plasma mass spectrometer. The methodology and fidelity of these measurements are presented, in addition to results for other low-concentration elements associated with natural and anthropogenic aerosols.  REE data from the RICE ice core are then used to explore possible modern sources of dust in the Ross Sea sector of Antarctica, testing hypothesized trajectory model distributions. Twentieth-century and late-Holocene (2.3 ka – present) REE data from the RICE ice core represent the first measurements of this kind from the Pacific sector of Antarctica. RICE data are compared with Holocene REE data from the Drønning Maud Land and Dome C ice cores, with consideration of REE signatures in dust samples from PSA. Data from the RICE ice core indicate fewer than 5% contributions of dust from South America, and show strong negative trends in crustal-normalized REE signatures suggesting contributions from local Antarctic dust.</p>

scholarly journals High-resolution mineral dust and sea ice proxy records from the Talos Dome ice core

2013 ◽  
Vol 9 (6) ◽  
pp. 2789-2807 ◽  
Author(s):  
S. Schüpbach ◽  
U. Federer ◽  
P. R. Kaufmann ◽  
S. Albani ◽  
C. Barbante ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ross Sea ◽  
Transport Model ◽  
Mineral Dust ◽  
Ice Core ◽  
Ice Cores ◽  
Last Interglacial ◽  
The Holocene ◽  
Proxy Records

Abstract. In this study we report on new non-sea salt calcium (nssCa2+, mineral dust proxy) and sea salt sodium (ssNa+, sea ice proxy) records along the East Antarctic Talos Dome deep ice core in centennial resolution reaching back 150 thousand years (ka) before present. During glacial conditions nssCa2+ fluxes in Talos Dome are strongly related to temperature as has been observed before in other deep Antarctic ice core records, and has been associated with synchronous changes in the main source region (southern South America) during climate variations in the last glacial. However, during warmer climate conditions Talos Dome mineral dust input is clearly elevated compared to other records mainly due to the contribution of additional local dust sources in the Ross Sea area. Based on a simple transport model, we compare nssCa2+ fluxes of different East Antarctic ice cores. From this multi-site comparison we conclude that changes in transport efficiency or atmospheric lifetime of dust particles do have a minor effect compared to source strength changes on the large-scale concentration changes observed in Antarctic ice cores during climate variations of the past 150 ka. Our transport model applied on ice core data is further validated by climate model data. The availability of multiple East Antarctic nssCa2+ records also allows for a revision of a former estimate on the atmospheric CO2 sensitivity to reduced dust induced iron fertilisation in the Southern Ocean during the transition from the Last Glacial Maximum to the Holocene (T1). While a former estimate based on the EPICA Dome C (EDC) record only suggested 20 ppm, we find that reduced dust induced iron fertilisation in the Southern Ocean may be responsible for up to 40 ppm of the total atmospheric CO2 increase during T1. During the last interglacial, ssNa+ levels of EDC and EPICA Dronning Maud Land (EDML) are only half of the Holocene levels, in line with higher temperatures during that period, indicating much reduced sea ice extent in the Atlantic as well as the Indian Ocean sector of the Southern Ocean. In contrast, Holocene ssNa+ flux in Talos Dome is about the same as during the last interglacial, indicating that there was similar ice cover present in the Ross Sea area during MIS 5.5 as during the Holocene.

Soluble/insoluble fractionation of elements in mineral dust from Antarctic samples

2020 ◽  
Author(s):  
Elena Di Stefano ◽  
Giovanni Baccolo ◽  
Paolo Gabrielli ◽  
Aja Ellis ◽  
Barbara Delmonte ◽  
...  
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Mineral Dust ◽  
Ice Core ◽  
Ice Cores ◽  
Dust Particles ◽  
Aeolian Dust ◽  
The Past ◽  
Icp Ms

&lt;p&gt;Deposition of dust on the Antarctic continent is controlled by many factors, such as the primary supply of dust particles from the continents [1], the long range transport, the hydrological cycle and the snow accumulation rate [2, 3]. Thus, the study of mineral dust in ice cores gives the possibility to reconstruct past climatic and environmental conditions.&lt;/p&gt;&lt;p&gt;Generally, when an ice core sample is melted, soluble elements dissolve in water, while insoluble elements remain in the solid phase. Other elements, such as iron, calcium, potassium and sulfur, typically partition between the soluble and the insoluble fractions. However recent studies have shown how the dust record may be chemically and physically altered in deep ice cores [4, 5], posing a challenge in the interpretation of the climatic signal that may lie within such samples. In particular, relative abundance of specific elements was shown to be different when comparing shallow and deep dust samples, suggesting that post depositional processes are taking place.&lt;/p&gt;&lt;p&gt;In this study we present a comparison between samples belonging to the Talos Dome ice core analyzed through two different techniques: instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). While the former is used to investigate only the insoluble fraction of dust, as it can only be applied to solid samples, the latter is used to assess the elemental composition of both the total and the soluble fraction of dust. We determined 45 elements through ICP-MS and 39 through INAA, with a good overlapping of the elements between the two techniques. Besides the determination of major elements, the high sensibility of both techniques also permitted the determination of trace elements. Among these, rare earth elements (REE) are of particular importance as they have been widely used as a geochemical tracer of aeolian dust sources [6]. We here present depth profiles for each analysed element, covering discrete portions of the entire ice core.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Bibliography&lt;/p&gt;&lt;p&gt;[1] Petit, Jean-Robert, et al. &quot;Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica.&quot; Nature 399.6735 (1999): 429-436.&lt;/p&gt;&lt;p&gt;[2] Lambert, Fabrice, et al. &quot;Dust-climate couplings over the past 800,000 years from the EPICA Dome C ice core.&quot; Nature 452.7187 (2008): 616.&lt;/p&gt;&lt;p&gt;[3] Wegner, Anna, et al. &quot;The role of seasonality of mineral dust concentration and size on glacial/interglacial dust changes in the EPICA Dronning Maud Land ice core.&quot; Journal of Geophysical Research: Atmospheres 120.19 (2015): 9916-9931.&lt;/p&gt;&lt;p&gt;[4] Baccolo, Giovanni, et al. &amp;#8220;The contribution of synchrotron light for the characterization of atmospheric mineral dust in deep ice cores: Preliminary results from the Talos Dome ice core (East Antarctica).&amp;#8221; Condensed Matter 3, no. 3 (2018): 25.&lt;/p&gt;&lt;p&gt;[5] De Angelis, Martine, et al. &amp;#8220;Micro-investigation of EPICA Dome C bottom ice: Evidence of long term in situ processes involving acid-salt interactions, mineral dust, and organic matter.&amp;#8221; Quaternary Science Reviews 78 (2013): 248-265.&lt;/p&gt;&lt;p&gt;[6] Gabrielli, Paolo, et al. &amp;#8220;A major glacial-interglacial change in aeolian dust composition inferred from Rare Earth Elements in Antarctic ice.&amp;#8221; Quaternary Science Reviews 29, no. 1-2 (2010): 265-273.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;/p&gt;

scholarly journals Centennial mineral dust variability in high-resolution ice core data from Dome C, Antarctica

2012 ◽  
Vol 8 (2) ◽  
pp. 609-623 ◽  
Author(s):  
F. Lambert ◽  
M. Bigler ◽  
J. P. Steffensen ◽  
M. Hutterli ◽  
H. Fischer
Keyword(s):  
High Resolution ◽  
Southern Ocean ◽  
Mineral Dust ◽  
Ice Core ◽  
Principal Component ◽  
Dominant Factor ◽  
Core Data ◽  
The Past ◽  
Dust Flux ◽  
Soluble Calcium

Abstract. Ice core data from Antarctica provide detailed insights into the characteristics of past climate, atmospheric circulation, as well as changes in the aerosol load of the atmosphere. We present high-resolution records of soluble calcium (Ca2+), non-sea-salt soluble calcium (nssCa2+), and particulate mineral dust aerosol from the East Antarctic Plateau at a depth resolution of 1 cm, spanning the past 800 000 years. Despite the fact that all three parameters are largely dust-derived, the ratio of nssCa2+ to particulate dust is dependent on the particulate dust concentration itself. We used principal component analysis to extract the joint climatic signal and produce a common high-resolution record of dust flux. This new record is used to identify Antarctic warming events during the past eight glacial periods. The phasing of dust flux and CO2 changes during glacial-interglacial transitions reveals that iron fertilization of the Southern Ocean during the past nine glacial terminations was not the dominant factor in the deglacial rise of CO2 concentrations. Rapid changes in dust flux during glacial terminations and Antarctic warming events point to a rapid response of the southern westerly wind belt in the region of southern South American dust sources on changing climate conditions. The clear lead of these dust changes on temperature rise suggests that an atmospheric reorganization occurred in the Southern Hemisphere before the Southern Ocean warmed significantly.

scholarly journals Temperature and mineral dust variability recorded in two low-accumulation Alpine ice cores over the last millennium

2018 ◽  
Vol 14 (1) ◽  
pp. 21-37 ◽  
Author(s):  
Pascal Bohleber ◽  
Tobias Erhardt ◽  
Nicole Spaulding ◽  
Helene Hoffmann ◽  
Hubertus Fischer ◽  
...  
Keyword(s):  
Time Series ◽  
Mineral Dust ◽  
Ice Core ◽  
Ice Cores ◽  
Temperature Reconstruction ◽  
Relative Increase ◽  
Ice Age ◽  
European Alps ◽  
Stable Water Isotopes ◽  
Annual Layer

Abstract. Among ice core drilling sites in the European Alps, Colle Gnifetti (CG) is the only non-temperate glacier to offer climate records dating back at least 1000 years. This unique long-term archive is the result of an exceptionally low net accumulation driven by wind erosion and rapid annual layer thinning. However, the full exploitation of the CG time series has been hampered by considerable dating uncertainties and the seasonal summer bias in snow preservation. Using a new core drilled in 2013 we extend annual layer counting, for the first time at CG, over the last 1000 years and add additional constraints to the resulting age scale from radiocarbon dating. Based on this improved age scale, and using a multi-core approach with a neighbouring ice core, we explore the time series of stable water isotopes and the mineral dust proxies Ca2+ and insoluble particles. Also in our latest ice core we face the already known limitation to the quantitative use of the stable isotope variability based on a high and potentially non-stationary isotope/temperature sensitivity at CG. Decadal trends in Ca2+ reveal substantial agreement with instrumental temperature and are explored here as a potential site-specific supplement to the isotope-based temperature reconstruction. The observed coupling between temperature and Ca2+ trends likely results from snow preservation effects and the advection of dust-rich air masses coinciding with warm temperatures. We find that if calibrated against instrumental data, the Ca2+-based temperature reconstruction is in robust agreement with the latest proxy-based summer temperature reconstruction, including a “Little Ice Age” cold period as well as a medieval climate anomaly. Part of the medieval climate period around AD 1100–1200 clearly stands out through an increased occurrence of dust events, potentially resulting from a relative increase in meridional flow and/or dry conditions over the Mediterranean.

scholarly journals A circumpolar dust conveyor in the glacial Southern Ocean

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Torben Struve ◽  
Katharina Pahnke ◽  
Frank Lamy ◽  
Marc Wengler ◽  
Philipp Böning ◽  
...  
Keyword(s):  
South America ◽  
Southern Ocean ◽  
Net Primary Production ◽  
South Pacific ◽  
Dust Deposition ◽  
Carbon Export ◽  
Transport Pathways ◽  
Dust Transport ◽  
Glacial Periods ◽  
Central South

Abstract The increased flux of soluble iron (Fe) to the Fe-deficient Southern Ocean by atmospheric dust is considered to have stimulated the net primary production and carbon export, thus promoting atmospheric CO2 drawdown during glacial periods. Yet, little is known about the sources and transport pathways of Southern Hemisphere dust during the Last Glacial Maximum (LGM). Here we show that Central South America (~24‒32°S) contributed up to ~80% of the dust deposition in the South Pacific Subantarctic Zone via efficient circum-Antarctic dust transport during the LGM, whereas the Antarctic Zone was dominated by dust from Australia. This pattern is in contrast to the modern/Holocene pattern, when South Pacific dust fluxes are thought to be primarily supported by Australian sources. Our findings reveal that in the glacial Southern Ocean, Fe fertilization critically relies on the dynamic interaction of changes in dust-Fe sources in Central South America with the circumpolar westerly wind system.

Arsenic record from a 3 m snow pit at Dome Argus, Antarctica

2016 ◽  
Vol 28 (4) ◽  
pp. 305-312 ◽  
Author(s):  
Hua Rong ◽  
Hou Shugui ◽  
Li Yuansheng ◽  
Pang Hongxi ◽  
Paul Mayewski ◽  
...  
Keyword(s):  
South America ◽  
Climate Models ◽  
Source Region ◽  
Ice Core ◽  
Arsenic Concentration ◽  
Copper Smelting ◽  
Sudden Increase ◽  
Transport Pathways ◽  
Dust Transport ◽  
Arsenic Distribution

AbstractThis study presents an arsenic concentration time series from 1964–2009 at Dome Argus, Antarctica. The data show a very large increase in arsenic concentration from the mid-1980s to the late-1990s (by a factor of~22) compared with the values before the mid-1980s. This increase is likely to be related to the increased copper smelting in South America. Arsenic concentration then decreased in the late-1990s, most probably as a result of environmental regulations in South America. The sudden increase in arsenic concentration observed at Dome Argus coincides with similar increases observed at Dome Fuji and in Antarctica Ice Core-6 (IC-6) at the same time, suggesting that arsenic pollution during the period from the mid-1980s to the late-1990s was a regional phenomenon in Antarctica. Investigations of arsenic concentrations at these three Antarctic locations show that, during this time, regional arsenic distribution followed dust transport pathways associated with general climate models with South America as a major source region for the half of Antarctica facing the Atlantic and Indian oceans.

scholarly journals Temperature and mineral dust variability recorded in two low accumulation Alpine ice cores over the last millennium

2017 ◽  
Author(s):  
Pascal Bohleber ◽  
Tobias Erhardt ◽  
Nicole Spaulding ◽  
Helene Hoffmann ◽  
Hubertus Fischer ◽  
...  
Keyword(s):  
Time Series ◽  
Mineral Dust ◽  
Ice Core ◽  
Ice Cores ◽  
Temperature Reconstruction ◽  
Ice Age ◽  
European Alps ◽  
Annual Layer ◽  
Instrumental Temperature

Abstract. Among ice core drilling sites in the European Alps, the Colle Gnifetti (CG) glacier saddle is the only one to offer climate records back to at least 1000 years. This unique long-term archive is the result of an exceptionally low net accumulation driven by wind erosion and rapid annual layer thinning. To-date, however, the full exploitation of the CG time series has been hampered by considerable dating uncertainties and the seasonal summer bias in snow preservation. Using a new core drilled in 2013 we extend annual layer counting, for the first time at CG, over the last 1000 years and add additional constraints to the resulting age scale from radiocarbon dating. Based on this improved age scale, and using a multi-core approach with a neighboring ice core, we explore the potential for reconstructing long-term temperature variability from the stable water isotope and mineral dust proxy time series. A high and potentially non-stationary isotope/temperature sensitivity limits the quantitative use of the stable isotope variability thus far. However, we find substantial agreement comparing the mineral dust proxy Ca2+ with instrumental temperature. The temperature-related variability in the Ca2+ record is explained based on the temperature-dependent snow preservation bias combined with the advection of dust-rich air masses coinciding with warm temperatures. We show that using the Ca2+ trends for a quantitative temperature reconstruction results in good agreement with instrumental temperature and the latest summer temperature reconstruction derived from other archives covering the last 1000 years. This includes a Little Ice Age cold period as well as a medieval climate anomaly. In particular, part of the medieval climate period around 1100–1200 AD stands out through an increased occurrence of dust events, potentially resulting from a relative increase in meridional flow and dry conditions over the Mediterranean.

scholarly journals Iron in ice cores from Law Dome, East Antarctica: implications for past deposition of aerosol iron

1998 ◽  
Vol 27 ◽  
pp. 365-370 ◽  
Author(s):  
R. Edwards ◽  
P. N. Sedwick ◽  
Vin Morgan ◽  
C. F. Boutron ◽  
S. Hong
Keyword(s):  
Southern Ocean ◽  
Late Holocene ◽  
Ice Core ◽  
Iron Concentration ◽  
Ice Cores ◽  
Snow Accumulation ◽  
East Antarctica ◽  
Iron Deposition ◽  
Past Century ◽  
Preliminary Estimate

Total-dissolvable iron has been measured in sections of three ice cores from Law Dome, East Antarctica, and the results used to calculate atmospheric iron deposition over this region during the late Holocene and to provide a preliminary estimate of aerosol iron deposition during the Last Glaciol Maximum I LGM). Ice-core sections dating from 56-2730 BP (late Holocene) and ~18000 BP (LGM) were decontaminated using trace-metal clean techniques, and total-dissolvable iron was determined in the acidified meltwatcrs by flow-injection analysis. Our results suggest that the atmospheric iron flux onto the Law Dome region has varied significantly over time-scales ranging from seasonal to Glaciol-interglaciol. The iron concentrations in ice-core sections from the past century suggest (1) a 2 4-fold variation in the atmospheric iron flux over a single annual cycle, with the highest flux occurring during the spring and summer, and (2) a nearly 7-fold variation in the annual maximum atmospheric iron flux over a 14 year period. The average estimated atmospheric iron flux calculated from our late-Holocene samples is 0.056-0.14 mg m a−1, which agrees well with Holocene flux estimates derived from aluminium measurements in inland Antarctic ice cores and a recent order-of-magnitude estimate of present-day atmospheric iron deposition over the Southern Ocean. The iron concentration of an ice-corc section dating from the LGM was more than 50 times higher than in the late-Holocene ice samples. Using a snow-accumulation rate estimate of 130 kg m −2 a−1 for this period, we calculate 0.87 mgm −2 a−1 as a preliminary estimate of atmospheric iron deposition during the LGM, which is 6-16 times greater than our average late-Holocene iron flux. Our data are consistent with the suggestion that there was a significantly greater flux of atmospheric iron onto the Southern Ocean during the LGM than during then Holocene.

The pattern of distribution of the antarctic fauna

1960 ◽  
Vol 152 (949) ◽  
pp. 624-631 ◽  
Keyword(s):  
New Zealand ◽  
South America ◽  
Southern Ocean ◽  
Stepping Stones ◽  
Physical Conditions ◽  
Effective Barrier ◽  
Antarctic Continent ◽  
Scotia Arc ◽  
Living Organisms ◽  
The Antarctic

The antarctic climate is unfavourable to the development of a land flora, and the true land fauna is meagre and inconspicuous, consisting of little more than a few insects and fresh water Crustacea; but the water circulation of the southern ocean allows for a rich production of phytoplankton, and we have a very abundant fauna living in or on the sea. Topographical and oceanographical conditions The Antarctic continent (figure 74) is surrounded by a belt of deep, cold ocean, generally very wide, which constitutes a rather effective barrier to shelf-living organisms, but within which the physical conditions tend to be uniform in a circumpolar direction. There are, however, certain submarine ridges which radiate from the continent and which may offer routes or stepping stones for dispersal. Thus the Scotia Arc connects South America with Graham Land and has several island groups. The Kerguelen Gaussberg Ridge (about 70° - 90° E) has fewer islands but no wider gaps of abyssal depths, and south of New Zealand the deep belt is narrower than at most other points.

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