South Of Polar Front Research Articles

The Royal Society and the South Seas

Almost from its inception The Royal Society has had a particular interest in the seas of the Southern Hemisphere. The Endeavour voyage of circumnavigation in southern waters by James Cook and his naturalist Joseph Banks, which was initiated by the Society, had repercussions—far beyond its original astronomical purpose—in oceanography, biology, exploration and world politics. It left a tradition, which still continues in the Society, of promoting wide-ranging expeditions such as those of the Erebus and Terror , the Challenger and, more recently, those to the Great Barrier Reef, the Solomon Islands, the New Hebrides and the island of Aldabra. The sea areas covered are those lying between the South Polar Front and, approximately, the Equator. Small islands and inshore waters are included but not land-based expeditions, such as those to Southern Chile and the Matto Grosso. The contributions of both the Society and its Fellows acting individually have been numerous and varied but here attention is restricted to three interconnected topics: physical and geological oceanography, biogeography and the genesis of coral reefs.

Movement of hydrological fronts and widespread erosional events in the southwestern Tasman sea during the Late Quaternary*

The planktonic foraminiferal palaeotemperatures and oxygen isotopes, radiocarbon dates and calcareous nanno‐plankton biostratigraphy of six deep‐sea cores in the southwestern Tasman Sea are presented, along with the nannoplankton biostratigraphy of the tops of 23 other cores in the region. A complex history of deposition and erosion is indicated, with development of hiatuses at 10 ka, and widespread absence of sediments younger than 73 ka. The history of movement of the major hydrological fronts over the areas west to southwest of Tasmania has been deduced from palaeotemperature records of the undisturbed core segments. During the Last Glacial Maximum, the Subtropical Convergence was positioned north of 42°S, 145°E. During the subsequent deglaciation and Holocene the front rapidly moved southward, with its southern limits established south of 47°S. Presently the northern limit of the Polar Front south of Tasmania is positioned south of ∼50cS, and palaeotemperatures and the absence of silica‐rich sediments in all cores north of there, suggests that the front has been south of there during the past 500 000 years.

Estimating Southern Ocean eddy flux of heat and salt from satellite altimetry

An outstanding problem in the oceanic and atmospheric sciences is the rate of heat and fresh water transport from the equator to the poles, for it is this transport which powers the Earth's weather and climate system1. Closing the Southern Hemisphere balances has proven particularly difficult because of the dominance of eddy fluxes across the Antarctic Circumpolar Current (ACC). Their relatively small space, but long time scales, make any in situ measurement program prohibitively expensive. Here we use satel-lite data to estimate the overall eddy activity and a turbulence closure scheme to relate this to eddy flux. The resulting estimated heat flux is sufficient to supply the heat lost to the atmosphere south of the Antarctic Polar Front. Estimated salinity flux is found to be poleward at high latitude—in the wrong direction to com-pensate for the observed excess of precipitaton over evaporation south of the Polar Front.

A model of Weichselian glacier variation in the North Atlantic region

The evidence for the extent and timing of Weichselian glaciation in Arctic regions shows that: (1) there were no major marine ice domes in the Arctic at 18,000 B.P. but that glaciers were relatively limited in extent; (2) there were no extensive ice shelves at 18,000 B.P. as envisaged by Hughes, Denton & Grosswald(1977); (3) the major periods of glacier expansion were between 125,000 and 80,000 B.P., just prior to 45,000 B.P., and between 11,000 and 8,000 B.P., and thus that glacier fluctuations at the southern margins of the Laurentide and Fennoscandian ice sheets were out of phase with those in the Arctic which advanced during southern interstadials.Phases of glacier advance in the Atlantic sector of the Arctic can be identified in deep sea cores by the peaks in concentration of iceberg‐dropped detritus and an increase in sedimentation rates, which are highest when sub‐polar water penetrates to the north. The key to the temporal pattern of Arctic glaciation and its association with oceanic changes is given by the intimate association of present‐day Arctic glacierisation with the two major low pressure troughs which penetrate the Arctic in the Atlantic sector and in Baffin Bay. The chronology of glaciation in the Atlantic sector is associated with the activity of these troughs and the related oceanic circulation. Cooling of the Arctic due to reduction in solar radiation at the end of the last interglacial, when the pack ice lay north of 75d̀N in the Atlantic, produced ideal conditions for Arctic glacier growth, with moisture transported by a strong cyclonic flux into a cooling Arctic from a strong North Atlantic Drift current.A positive feedback loop involving ocean and atmospheric circulation and pack ice, caused movement of the polar front to the south, thus slowly cutting off the supply of moisture to the Arctic. Further cooling at 75,000 B.P. caused a rapid extension of the polar front south of 45d̀N, effectively cut off the northward movement of surface currents on the North Atlantic, and produced a strong zonal oceanic and atmospheric circulation which starved Arctic glaciers of nourishment and caused their retreat, and initiated rapid build up of the Fennoscandian and Laurentide ice sheets. Subsequent extensions of Arctic glaciers were associated with limited northward movement of sub‐polar water and associated Atlantic depressions. The expansion of glaciers within the Arctic between 11,000 and 8,000 B.P. was associated with the first and diachronous penetration of moisture into a still cool Arctic during decay of the two great ice sheets.

Late Tertiary and Quaternary Events as Interpreted from Radiolaria in Antarctic Sediments: ABSTRACT

A study of the radiolaria from the tops of more than 80 cores spaced around the Antarctic continent between 35° and 77°S. has revealed two distinct faunas, the boundary between them corresponding closely with the position of the South Polar Front (Antarctic Convergence). End_Page 530------------------------------ An examination of samples taken at 40-cm. intervals down cores from both north and south of the Polar Front shows that several cores located north of the Polar Front contain alternating layers of the two faunas, whereas cores located at the south show no such faunal fluctuations. If the alternating faunal zones observed in the northern cores are caused by a shift in the position of the Polar Front, then the data collected so far suggest that the Polar Front today may be as far south as it has been during the time period represented by the cores sampled. Other changes are exhibited by 10 cores from the area. In these cores there is an abrupt change from a fauna composed predominantly of recent species, the majority of which are not found living in Antarctic waters today. This change in the radiolarian assemblage is accompanied in 80 cores by a large decrease in other siliceous or calcareous organisms. In two cores the carbonate content drops from 40-60 per cent to zero. In one core the older assemblage is associated with abundant discoasters and on this basis is tentatively assigned a Late Tertiary age. The cause of the change in planktonic assemblages from Late Tertiary to Quaternary is not known, but it is suggested that it may be due to a change from a relatively stable stratification of the Antarctic Ocean during Late Tertiary to a stronger degree of vertical convection in the Quaternary associated with the cooling of this part of the world during the growth of the Antarctic ice cap. End_of_Article - Last_Page 531------------