Subantarctic Surface Water Research Articles

Oceanographic and climatic changes over the past 160,000 years at Deep Sea Drilling Project Site 594 off southeastern New Zealand, southwest Pacific Ocean

High‐resolution textural, carbonate, microfossil, and oxygen (δ18O) and carbon (δ13C) stable isotopic analyses are presented for the late Quaternary (isotopic stages 1 to 6) interval of a core at Deep Sea Drilling Project site 594, situated just south of the present Subtropical Convergence in northernmost Subantarctic surface waters on the southern flank of Chatham Rise in the southwest Pacific. Downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes, respectively. Interglacial oozes contain a northern Subantarctic assemblage of planktonic foraminifera, with rare cool subtropical species, while glacial oozes are characterized by species typical of southern(most) Subantarctic waters and include radiolaria with affinity for Antarctic waters. The planktonic δ18O record for the site supports a 3°–6°C temperature change in near‐surface waters between interglacial and glacial stages and indicates that during stage 5e, the near‐surface waters were about 1°C warmer than at present. A pronounced cooling during stage 5d matches that of the Vostok ice core δD record, and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation. Benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when δ18O results suggest a temperature drop in glacial stage bottom waters of 2°–4°C, possibly a result of upward displacement of Antarctic Intermediate Water by Circumpolar Deep Water at the site. The foraminiferal δ13C records support a reduced influence of North Atlantic Deep Water in the southwest Pacific during glacial stages, when nutrient enhancement occurred in both bottom and surface waters. Despite the pronounced changes between interglacial and glacial conditions inferred at site 594, contributed to by a substantial northward shift by at least 5° of latitude in the position of the Antarctic Convergence south of New Zealand during glacial episodes, we conclude that the Subtropical Convergence remained locked to Chatham Rise (approximately 44°S) throughout stages 1 to 6. Major compression of the intervening belt of Subantarctic water during glacial episodes, and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest Pacific, account for the harsh, frigid environment reported for on‐land southern New Zealand at these times.

Brazil‐Malvinas Confluence: Water mass composition

A quantitative analysis of water masses in the Brazil‐Malvinas Confluence zone is performed with a least squares multiple tracer analysis using data from the confluence winter 1989 cruise. The purpose is to find the mixture of source water types that best describes the composition of a given water sample. This method is valuable in regions involving strong mixing among various source water types, as is the Brazil‐Malvinas Confluence zone. Seven main core layers are identified in this region, and all are retained for the analysis: the Thermocline Water (TW), the Subantarctic Surface Water (SASW), the Antarctic Intermediate Water (AAIW), the Upper Circumpolar Deep Water (UCDW), the North Atlantic Deep Water (NADW), the Lower Circumpolar Deep Water (LCDW), and the Weddell Sea Deep Water (WSDW). Tracers selected are temperature, salinity, dissolved nutrients, and oxygen. The results show the proportion of each source water type along four east‐west sections (35.4°S, 36.5°S, 37.9°S, 39°S). They are accurate to within 20% for all sources. The solution presents evidence of local recirculation of AAIW largely influenced by the two strong currents, Brazil and Malvinas. Southward TW and NADW separate from the coast, NADW turning eastward at a higher latitude than TW.

Early life cycle of cephalopods in relation to the major oceanographic features of the southwest Atlantic Ocean

Cephalopod paralarvae and juveniles were sampled with RMT8 and Bongo nets during 2 cruises of the M V 'Falklands Protector' in the southwest Atlantic Ocean in October/November 1990 and 1991 Each cruise had 2 phases, a northern phase over the Patagonian Shelf edge and adjacent waters, and a southern phase over the shelf around the Falkland Islands. Vertical temperature profiles were used to identify the major fronts and water masses. The subtropical water of the Brazil Current was characterized by the tropical/subtropical Abraliopsis pfefferi and Histioteuthis corona corona, the subtropical/subantarctic H. atlantica and the widespread Taningia danae, species of Brachioteuthis, Chiroteuthis and Galiteuthis, a single specimen of Illex argentinus and an ommastrephid rhynchoteuthion type B. The subantarctic surface water of the Falkland Current contained the richest assemblage of species and was characterised by the subtropicaVsubantarctic H. atlantica, the subantarctic Batoteuthis skolops, H. eltaninae, H. rnacrohista and the subantarctic/antarctic Gonatus antarcticus. With the exception of some small Gonatus antarcticus, the polar frontal zone water of the Falkland Current was relatively poor in species. The Falkland Islands Shelf yielded Loligo gahi, Gonatus antarcticus, Martialia hyadesi, Moroteuthis knipovitchi, Batoteuthis skolops. Semirossia patagonica and an Octopus sp. The data provide new information on the life cycles of Martialia hyadesi and Gonatus antarcticus and are consistent with the view that L. gahi reproduces in shallow water around the Falkland Islands with 1 major brood spawning in the late austral winter.

Hydrology and near‐surface nutrient distribution along the South Otago continental shelf, New Zealand, in summer and winter 1986

Abstract The 3 Otago coastal water masses (Neritic water, subtropical Southland Current water, and Subantarctic Surface Water) defined in the literature were identified on 1 cruise in early summer and 1 in mid winter. Winter results showed that Neritic water is derived from the Clutha River. During winter the Southland Current was characterised by temperatures between 9.0 and 10.6°C and a salinity range of 34.4 to 34.9 × 10‐3. During summer a thermocline and pycnocline were observed at 30–50 m depth sloping down toward the coast. Low‐salinity Neritic water was less apparent than in winter. Summer nutrient levels were depleted over all of the continental shelf, relative to winter. This is probably caused by the enclosure of coastal surface waters inshore of the pycnocline. Reactive Si was conservative in winter regardless of water mass. Reactive P was approximately constant in Neritic water and Southland Current water. In summer, neither nutrient was conservative.

Late Neogene erosion of the Agulhas Moat and the Oligocene position of Subantarctic Surface Water

The Agulhas Ridge Moat is a scoured depression up to 100 m deep and over 20 km wide flanking the northern side of the Agulhas Ridge in the southeastern Atlantic. A core from the moat has been assigned a late Oligocene age (Zone NP 24) based on the nannoplankton present. Scouring was probably caused by westward-flowing Antarctic Bottom Water, moving in excess of 15–20 cm s −1, possibly during the middle to late Pleistocene. Oxygen-isotope analyses indicate a late Oligocene surface-water temperature between 6.9 and 16.6°C over the core site. These paleotemperatures suggest that the Subantarctic Surface Water mass extended farther north during the Oligocene. Modern sea-surface temperatures over the site are about 21°C.

Pliocene variations in the position of the Antarctic convergence in the southwest Atlantic

Middle to late Pliocene (4.1–1.9 Ma) variations in the positions of surface water masses and migrations of the Polar Front in the southwest Atlantic are inferred from a factor analysis of radiolarian assemblages from DSDP site 514. Faunal results suggest that surface water masses underwent progressive cooling as the Polar Front Zone (PFZ) advanced northward during the latest Gilbert to late Gauss chron. The late Gilbert chron (4.12–3.86 Ma) was the waning phase of a mid‐Gilbert warm interval. Northern Subantarctic surface water prevailed between 4.12 and 3.97 Ma, indicative of surface water temperatures warmer than those during the succeeding Pliocene. A long‐term sea surface cooling trend began between 3.97 and 3.86 Ma which moved the PFZ northward and closer to site 514, bringing cooler southern Subantarctic surface waters to the site. This movement northward of cooler surface waters is inferred to be related to the initial growth of sea ice and ice shelves throughout regions of West Antarctica which were deglaciated or unglaciated during the prior warming interval of the Gilbert chron. A major late Gilbert/early Gauss chron increase in the production of Antarctic Bottom Water (AABW) resulted in an erosional or nondepositional hiatus at site 514, spanning 3.86 to 3.18 Ma. Increased AABW activity at this time is linked to continued growth of expansive ice shelves and sea ice in the West Antarctic region, in the East Antarctic margin, and possibly in the marine embayments of East Antarctica. Decreased AABW production led to a 3.2 Ma resumption of sedimentation at site 514 and elsewhere in the southern ocean. Antarctic surface water assemblages above the hiatus (3.18–3.16 Ma) suggest that the PFZ first passed over site 514 during the 3.86–3.18 Ma interval of the hiatus. Following this initial invasion of Antarctic surface waters, between 3.16 and 2.94 Ma the PFZ retreated to a position near or south of the site. This slight retreat of the PFZ, with a corresponding decrease in AABW activity, is possibly related to a significant reduction in the area of shelf and sea ice as grounded ice expanded in the West Antarctic region. We interpret the widespread global enrichment of δ18O at approximately 3.2 Ma to be partially related to a cooling of surface waters, the production of cooler bottom waters, and an expansion of West Antarctic ice volume associated with its grounding phase between 3.5 and 2.7 Ma. Based on surface water mass migrations at site 514, the West Antarctic ice volume increased significantly slightly prior to or at approximately 3.2 Ma, decreased between 3.14 and 2.7 Ma, and then increased by 2.67 Ma to a more stable configuration with a volume at least as great as that at 3.2 Ma. During the late Gauss (2.94–2.47 Ma), four northward advances of the PFZ led to the migration of Antarctic surface waters over the site by 2.67 Ma and continued dominance of Antarctic surface water through the remainder of the Pliocene. We suggest that this permanent change from Subantarctic to Antarctic surface water mass dominance was linked indirectly to the initial growth of northern hemisphere ice and a reduction of sea level. We propose a model of simultaneous northern and southern hemisphere ice volume growth induced by interhemisphere positive feedback which is responsible for the major late Gauss advance of Antarctic surface waters and the global enrichment of δ18O at 2.5 Ma.

The stratification and water masses at Drake Passage

The waters at Drake Passage are known to be separated horizontally into four distinct zones of relatively small geostrophic shear by three fronts in which the shear and eastward flow are large. The two northern fronts, Subantarctic Front and Polar Front, that bound the Polar Frontal Zone comprise most of the transport of the Antarctic Circumpolar Current. Vertical sections of closely spaced (∼50 km) oceanographic stations made across Drake Passage in the summers of 1975 and 1976 produced quality measurements of temperature, salinity, oxygen, silicate, nitrate, and phosphate. Based on these data, a traditional description of characteristic distributions is presented. Good sampling resolution in the vertical allows maxima in hydrostatic stability to be traced laterally (approximately along isopycnals) between individual stations. Eleven strata of maximum stability are identified. Most vertically separate distinct water masses. The most pronounced stability stratum is associated with the seasonal pycnocline; north of the Polar Front, it separates the Subantarctic Surface Waters from Subantarctic Mode Water. To the south of the Polar Front the seasonal pycnocline is found slightly above the temperature minimum characteristic of the Antarctic Surface Waters in austral summer. This stability stratum extends northward near the base of the Subantarctic Mode Water. Another stability stratum extending across the entire passage forms the upper boundary for the Circumpolar Deep Water. South of the Polar Front, this stratum is at the base of the Antarctic Surface Water; to the north it is the lower boundary of the Antarctic Intermediate Water. Although Subantarctic Mode Water and Antarctic Intermediate Water are separated by a stability stratum north of the Polar Front, there is no such separation between Antarctic Intermediate Water and the temperature minimum of the Antarctic Surface Water south of the front. This suggests that Subantarctic Mode Water and Antarctic Intermediate Water are distinct and that the latter derives from the surface waters south of the Polar Front. At greater depths a horizontally discontinuous stratum is found between the Upper and Lower Circumpolar Deep Water. Two horizontally continuous stability strata are found near the base of the Circumpolar Deep Water, marking the transition to a high‐silicate variety of denser deep water from the Southeast Pacific Basin. The densest water mass found is Weddell Sea Deep Water, which flows through a gap in the South Scotia Ridge and along the southern Scotia Sea to fill the trough in the southern Drake Passage. It is overlain by a laterally continuous stratum of relative maximum stability.

Geographic variations in the relationship between silicic acid and nitrate in the South Pacific Ocean

Scatter plots of silicic acid vs nitrate concentration from stations in the South Pacific Ocean and the Pacific Sector of the Southern Ocean divide the area into zones of nutrient balance, excess silicic acid, or excess nitrate. The boundaries of the zones are similar to published surface-water mass maps. A T-S analysis of stations divided on the basis of nutrient character show that: (1) South Pacific Central Water is in nutrient balance; (2) Subantarctic Surface Water has actual excess nitrate and Antarctic Circumpolar Water (Surface) has potential excess nitrate; and (3) Ross Sea Water exhibits potential excess silicic acid. The actual nitrate excess in Subantarctic Surface Water is attributed to the circumpolar character of the water mass, to the extensive diatom growth in southern source water, and to the different mechanisms of regeneration of silicic acid and nitrate. The nutrient character of this water mass probably effects food web dynamics in its resident area and in coastal areas on which it impinges. The potential silicic acid excess of Ross Sea Water is attributed to the upwelling of silicic acid-enriched deep water at the Antarctic Divergence (≈ 70°S), which probably contributes to excess silicic acid in the North Pacific Ocean through Antarctic Bottom Water formation.

Surface lithofacies, biofacies, and diatom diversity patterns as models for delineation of climatic change in the southeast Atlantic Ocean

Distribution of diatom species in surface sediments of the southeast Atlantic Ocean is regulated by present-day oceanographic and hydrodynamic processes. Five assemblages (vectors) defined by factor-vector analysis, reflect different environments and conditions. Assemblage A is a high diversity flora associated with the nutrient-rich, relatively cold waters south of the Polar Front in the diatom ooze belt. Occurrence of this flora on Maud Rise and vicinity may reflect periodic occurrence of a polynya in that area. Assemblage B is a reworked assemblage that dominates the southern portion of the study area. Productivity there is low, reflecting sea ice cover during most of the year. The area north of the Polar Front is dominated by Assemblage C, whose characteristic species reflect the relatively warm Subantarctic Surface Water. Winnowing and frustule breakage have altered Assemblage D (found in three isolated samples) by removing relatively delicate forms leaving lag deposits of more robust species. Assemblage E is a low diversity stress flora, reflecting unstable, unpredictable environments along the Polar Front, Antarctic slope, and the northern boundary of winter sea ice. These sites are characterized by the sinking of cold water. Downcore analysis of cores lying adjacent to the Polar Front and the diatom ooze—pelagic clay boundary show evidence for past climatic variation. The low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality, make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations. The position of the Polar Front has migrated at least three times within the last 0.015 m.y. B.P. Within the last 0.3 m.y. B.P. warm maxima have occurred at approximately 0.0, 0.015, 0.125 and 0.3 m.y. B.P.

Die pelagischen Ostracoden der MARCHILE I‐Expedition (Südost‐Pazifik), I: Verbreitung, Zoogeographie und Bedeutung als Indikatoren für Wasserkörper. (Crust., Ostracoda: Myodocopida)

Abstract The pelagic Ostracods of the MARCHILE I‐Expedition were studied from samples collected during February and March 1960 off the Chilean coast between 30° S and 42° S. The study area in the Peru‐Chile Coastal Current had a length of more than 1500 km and a breadth of 250 km. The upper 200 m (Epipelagial) consisted of five water masses characterized by different values of temperature, salinity and dissolved oxygen. Two of these water masses were associated with zones of upwelling. The horizontal spatial distributions of six very abundant species are uniquely associated with specific water masses, as follows: Subtropical Surface Water and Porroecia porrecta pacifica. Antiboreal Water (=Subantarctic Surface Water) and Orthoconchoecia haddoni marchilensis. Low Salinity Water and Conchoecia lophura lissoides. Equatorial Subsurface Water (=Gunther Current) and Conchoecetta giesbrechti giesbrechti. Subantarctic Subsurface Water and Pseudoconchoecia serrulata. Upwelling areas of Antiboreal Water and Orthoco...

Surface distribution of albacore tuna,Thunnus alalungaBonnaterre, in relation to the Subtropical Convergence Zone east of New Zealand

Trolling surveys showed that albacore tuna are about equally abundant in subtropical and Subtropical Convergence Zone surface waters off the south‐east coast of New Zealand. Few albacore were located in Subantarctic Surface Water, indicating that the Subtropical Convergence Zone is the southern limit for albacore in this area.

Mesopelagic faunal transition across the Subtropical Convergence east of New Zealand

Abstract The winter distributions of mesopelagic fishes, crustaceans, and cephalopods across the Subtropical Convergence east of New Zealand are shown to reflect the hydrological changes associated with convergence of Subtropical and Subantarctic Surface Waters. Samples collected in the upper 400 m in each of these water masses and in mixed water over the Chatham Rise contained members of faunal associations apparently characteristic for each area. The area of strongest hydrological change was characterised, at least for fishes and crustaceans, by a considerable decrease in species richness. Use of cluster analysis confirmed the clear correpondence between geographic species associations and water masses.

Antarctic Polar Frontal Zone from Australia to the Drake Passage

North-south temperature sections drawn from historical XBT observations averaged over various longitude bands are compared with sections drawn from existing hydrographic data. Corresponding features in both types of sections are related to changes in upper-layer water masses by the use of temperature-salinity relationships. A series of six of these sections covering the area from south of Australia east to the Drake Passage are employed to define the positions of two discontinuities or fronts. These fronts are the northern and southern boundaries of a complex transition region, called the Antarctic Polar Frontal Zone, between the Antarctic and Subantarctic Surface Waters. This transition region varies in width both zonally and seasonally, and is the site of eddies and meanders of the boundaries.

An Antarctic oceanographic section along 170°E

The circulation pattern of Antarctic and sub-Antarctic waters is strongly influenced by the complex marine topography south of New Zealand. The distribution of water characteristics indicates that the Antarctic Circumpolar Current undergoes intense wave-like migrations, which may be transient in nature, apparently associated with the Macquarie Ridge and Campbell Plateau. The bulk of the circumpolar current transport flows eastward over the northern flank of the mid-ocean ridge, but a strong, apparently jet-like, current exists around the periphery of the Campbell Plateau. The water column of the upper 1000 m possesses much thermohaline complex structure on vertical scales of 20–200 m in the Polar Front zone along 170°E from 56 to 62°S. South of 62°S the water column is well stratified with a strong temperature minimum at 100 m, while north of 56°S is the well-mixed, thick, Sub-Antarctic Surface Water. The geostrophic volume transport relative to the deepest common depth of station pairs, is 135 × 10 6 m 3 s −1. The bottom velocities are not zero and the actual transport is quite different from the baroclinic transport. Bottom current (100 m above the sea floor) at 56°S average, for a three-day period, is 29 cm s −1 towards 065° T. Using this for geostrophic reference suggests that the current around the Campbell Plateau periphery has a surface zonal velocity of 1·5 knots.

PELAGIC PHOSPHORUS METABOLISM IN ANTARCTIC WATERS1

Eleven large quantitative samples of the particulate material in Antarctic and subantarctic surface waters were obtained with a continuous flow system. Each sample was divided into those particles which were retained by a No. 20 plankton net with a pore size of about 30 µ (class I) and those which passed through the net but were retained by a Sharples Supercentrifuge (class II). Total phosphorus and organic nitrogen were determined on aliquots of class I and II samples, and the remaining material was chemically fractionated into acid‐soluble organic phosphorus, phospholipids, orthophosphate, oligopolyanions, and ribonucleic acid‐polyphosphates.About two‐thirds of the nonparticulate phosphorus was orthophosphate. From 3 to 7.5% of total phosphorus was particulate. Over half of the particulate phosphorus was in class II; 32P‐phosphate experiments indicated that these particles incorporated phosphate at least six times as rapidly as class I particles. In class II particles orthophosphate and RNA‐polyphosphate constitute 80 to 85% of the total phosphorus; in class I the phospholipids are also a large fraction. RNA‐polyphosphate was the most rapidly labeled fraction from the class II particles.