Western Ross Sea Research Articles

Temporal and spatial patterns in the Ross Sea: Phytoplankton biomass, elemental composition, productivity and growth rates

The temporal and spatial patterns of phytoplankton biomass, productivity, and particulate matter composition in the Ross Sea were assessed during cruises in January 1990 and February 1992. Biomass and primary productivity in the southern Ross Sea were greatest during mid‐January, with surface chlorophyll concentrations, particulate organic carbon levels, and integrated primary productivity averaging 4.9 μg L−1, 0.54 mg L−1 and 2.63 g C m−2 d−1, respectively. Comparable mean concentrations and rates for February were 1.1 μg L−1, 0.29 mg L−1, and 0.78 g C m−2 d−1 (decreases of 76, 46, and 70%, respectively), indicative of the scale of temporal changes. A distinct south‐north transition also was observed both in productivity and phytoplankton biomass, with the lowest values occurring in the northern Ross Sea. East‐west gradients in phytoplankton biomass and composition occurred within the southern Ross Sea. The areal productivity of the Ross Sea ranged from 0.15 to 2.85 g C m−2 d−1 and is among the highest found in the entire Antarctic. Carbon:chlorophyll ratios were uniformly high but were highest (150) in 1990 in the diatom‐dominated western Ross Sea. Surface growth rates were modest, averaging less than 0.2 day−1 during both seasons. We hypothesize that the marked seasonality in the region provides an environment in which net growth rates, although slow, are maximized through low loss rates and which allows biomass to accumulate in the surface layer. Furthermore, the temporal variations are quantitatively similar to the observed spatial variations. Therefore the dominant determinant of phytoplankton biomass and productivity at any one point on the Ross Sea continental shelf is the stage of the seasonal growth cycle.

Spatial patterns in phytoplankton biomass and pigment distributions in the Ross Sea

The distribution of phytoplankton biomass and pigments was determined in the Ross Sea during late austral summer 1992. Large gradients in biomass were noted both in the east‐west and north‐south direction, with maximum particulate matter concentrations occurring in the southwest portion of the Ross Sea. Two xanthophyll pigments dominated the profiles, fucoxanthin (indicative of diatoms) and 19′ hexanoyloxyfucoxanthin (hex) (indicative of the prymnesiophyte Phaeocystis antarctica). Fucoxanthin concentrations were greatest near the coast of Victoria Land and within the northern transect, whereas hex concentrations were highest offshore in the SE Ross Sea. Particulate carbon:chlorophyll a (POC:chl) ratios in diatom‐dominated waters of the western Ross Sea were relatively high (210). The POC:chl ratio for Phaeocystis antarctica populations from the SE Ross Sea was substantially less (92) than the ratios observed in diatom‐dominated waters. Pigment 14C labeling indicated that phytoplankton carbon in the diatom‐dominated northern Ross Sea accounted for approximately 30% of the POC, with a phytoplankton C:chl ratio of 130. Short‐term (24 hour) vertical fluxes of pigments ranged from 3 to 40 μg chl a m−2 d−1. Diatom‐dominated regions had greater fluxes of phaeophorbides, suggesting that metazoans were the most important grazers at these locations. In contrast, the phaeophytin/total phaeopigment ratio was highest in waters dominated by Phaeocystis antarctica. The distribution of phytoplankton biomass and pigments revealed a spatially variable distribution of taxa, one which clearly has important consequences to food‐web dynamics, biogeochemical cycles, and vertical flux patterns in the Ross Sea.

Late Pleistocene interactions of East and West Antarctic Ice-flow Regimes: evidence from the McMurdo Ice Shelf

AbstractWe present new interpretations of deglaciation in McMurdo Sound and the western Ross Sea, with observationally based reconstructions of interactions between East and West Antarctic ice at the last glacial maximum (LGM), 16000, 12000, 8000 and 4000 BP. At the LGM, East Antarctic ice from Mulock Glacier split; one branch turned westward south of Ross Island but the other branch rounded Ross Island before flowing southwest into McMurdo Sound. This flow regime, constrained by an ice saddle north of Ross Island, is consistent with the reconstruction of Stuiver and others (1981a). After the LGM, grounding-line retreat was most rapid in areas with greatest water depth, especially along the Victoria Land coast. By 12000 BP, the ice-now regime in McMurdo Sound changed to through-flowing Mulock Glacier ice, with lesser contributions from Koettlitz, Blue and Ferrar Glaciers, because the former ice saddle north of Ross Island was replaced by a dome. The modern flew regime was established ∼4000 BP. Ice derived from high elevations on the Polar Plateau but now stranded on the McMurdo Ice Shelf, and the pattern of the Transantarctic Mountains erratics support our reconstructions of Mulock Glacier ice rounding Minna Bluff but with all ice from Skelton Glacier ablating south of the bluff. They are inconsistent with Drewry’s (1979) LGM reconstruction that includes Skelton Glacier ice in the McMurdo-Sound through-flow. Drewry’s (1979) model closely approximates our results for 12000-4000 BP. Ice-sheet modeling holds promise for determining whether deglaciation proceeded by grounding-line retreat of an ice sheet that was largely stagnant, because it never approached equilibrium flowline profiles after the Ross Ice Shelf grounded, or of a dynamic ice sheet with flowline profiles kept low by active ice streams that extended northward from present-day outlet glaciers after the Ross Ice Shelf grounded.

Chronology of late Wisconsin ice retreat from the western Ross Sea, Antarctica

Lithologic data from marine sedimentary cores and accelerator mass spectrometer (AMS) radiocarbon dates indicate that grounded ice did not advance to the western Ross Sea continental shelf edge during the last glacial maximum (LGM). A chronology of the timing of ice retreat was provided by 26 AMS dates, obtained from 12 cores. Dates ranging from 20 to 29 ka suggest that the outer continental shelf (beyond ca. lat 74°S) was not covered by grounded glacial ice prior to and during the early stages of the LGM. 14C dates just above transitions from subglacial diamictons to marine muds indicate that the area around the Drygalski ice tongue was deglaciated by at least 11.5 ka. The Ross Ice Shelf reached its present-day position near Ross Island by about 7 ka. A hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf. The hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf.

Late Pleistocene interactions of East and West Antarctic Ice-flow Regimes: evidence from the McMurdo Ice Shelf

AbstractWe present new interpretations of deglaciation in McMurdo Sound and the western Ross Sea, with observationally based reconstructions of interactions between East and West Antarctic ice at the last glacial maximum (LGM), 16000, 12000, 8000 and 4000 BP. At the LGM, East Antarctic ice from Mulock Glacier split; one branch turned westward south of Ross Island but the other branch rounded Ross Island before flowing southwest into McMurdo Sound. This flow regime, constrained by an ice saddle north of Ross Island, is consistent with the reconstruction of Stuiver and others (1981a). After the LGM, grounding-line retreat was most rapid in areas with greatest water depth, especially along the Victoria Land coast. By 12000 BP, the ice-now regime in McMurdo Sound changed to through-flowing Mulock Glacier ice, with lesser contributions from Koettlitz, Blue and Ferrar Glaciers, because the former ice saddle north of Ross Island was replaced by a dome. The modern flew regime was established ∼4000 BP. Ice derived from high elevations on the Polar Plateau but now stranded on the McMurdo Ice Shelf, and the pattern of the Transantarctic Mountains erratics support our reconstructions of Mulock Glacier ice rounding Minna Bluff but with all ice from Skelton Glacier ablating south of the bluff. They are inconsistent with Drewry’s (1979) LGM reconstruction that includes Skelton Glacier ice in the McMurdo-Sound through-flow. Drewry’s (1979) model closely approximates our results for 12000-4000 BP. Ice-sheet modeling holds promise for determining whether deglaciation proceeded by grounding-line retreat of an ice sheet that was largely stagnant, because it never approached equilibrium flowline profiles after the Ross Ice Shelf grounded, or of a dynamic ice sheet with flowline profiles kept low by active ice streams that extended northward from present-day outlet glaciers after the Ross Ice Shelf grounded.

Spring Phytoplankton Production in the Western Ross Sea

Coastal zone color scanner (CZCS) imagery of the western Ross Sea revealed the Presence of an intense phytoplankton bloom covering >106,000 square kilometers in early December 1978. This bloom developed inside the Ross Sea polynya, within 2 weeks of initial polynya formation in late November. Primary productivity calculated from December imagery (3.9 grams of carbon per square meter per day) was up to four times the values measured during in situ studies in mid-January to February 1979. Inclusion of this early season production yields a spring-to-summer estimate of 141 to 171 grams of carbon per square meter, three to four times the values previously reported for the western Ross Sea.

Transfer faults in the western Ross Sea: new evidence from the McMurdo Sound/Ross Ice Shelf aeromagnetic survey (GANOVEX VI)

Aeromagnetic data collected on the GANOVEX IV and GANOVEX VI expeditions are combined in this report to give a synoptic view of the western Ross Sea, Antarctica. The addition of the new GANOVEX VI data allows the identification of the southern boundary of the “Ross Sea Unit” — a magnetic unit containing rift-fabric anomalies of the West Antarctic rift system in the Victoria Land basin. Although this boundary has a similar WSW–ENE orientation to the northern boundary, as identified in the GANOVEX IV survey, the newly identified southern magnetic unit (called the “Ross Island and Ice Shelf Edge Unit”) includes evidence of the S–N rift-fabric that is not found in the north, i.e. the rift-fabric continues farther south. The linear boundaries themselves are interpreted as transfer faults as proposed by previous workers for the tectonic development of the Ross Sea area.

Antarctic climate and tectonic history: The Cape Roberts Project

Scientists and logistics experts from Germany, Italy, New Zealand, the United Kingdom, and the United States met from September 21 to 23, 1993, near Washington, D.C., to discuss management, logistics, and science objectives of a proposed international stratigraphic drilling project.The project plans to core 1500 m of strata beneath the western Ross Sea off Cape Roberts, where shipboard geophysical surveys indicate that dipping sedimentary strata occur close to the seafloor beneath a thin blanket of much younger sediments (Figure 1). These sediments include the oldest sedimentary sequences in the Ross Sea and are interpreted to be from 30‐ to more than 100‐m.y. old. No rocks of this age are known to crop out in the Ross Sea‐Transantarctic Mountains region or other parts of east Antarctica.

Cyclonically Forced Barrier Winds along the Transantarctic Mountains near Ross Island

Abstract The effect of the Transantarctic Mountains on cyclonically forced boundary-layer winds in the vicinity of Ross Island, Antarctica (77.5°S, 167°E), is discussed. When cyclones are present over the western Ross Ice Shelf and Ross Sea, the low-level easterly airflow is toward the mountains. A barrier wind regime is set up as the flow is turned northward and becomes parallel to the mountain range. IT is found that cyclonically forced barrier winds occurred around 5% and 8% of the time during 1984 and 1985, respectively. The case histories of two well-defined barrier wind events lasting for 24 h are discussed in detail, with regional analyses based on satellite photographs and automatic weather station data. One case is for a katabatic wind-forced mesoscale cyclone forming to the north of Ross Island, and the other is for a synoptic-scale cyclone moving through the western Ross Ice Shelf-Ross Sea region. A numerical model for the vertically integrated boundary-layer flow that calculates two horizontal...

Tectonic controls on the geochemical composition of Cenozoic, mafic alkaline volcanic rocks from West Antarctica

Cenozoic, mafic alkaline volcanic rocks throughout West Antarctica (WA) occupy diverse tectonic environments. On the Antarctic Peninsula (AP), late Miocene-Pleistocene (7 to <1 Ma) alkaline basaltic rocks were erupted <1 to 45 million years after subduction ceased along the Pacific margin of the AP. In Marie Byrd Land (MBL), by contrast, alkaline basaltic volcanism has been semi-continuous from 25–30 Ma to the present, and occurs in the West Antarctic rift system. Together, these Antarctic tectono-magmatic associations are analogous to the Basin and Range, Sierran, and Coast Range batholith provinces. Unlike the western US, however, basaltic rocks throughout WA have uniform geochemical characteristics, with especially narrow ranges in initial87Sr/86Sr (0.7026–0.7035),143Nd/144Nd (0.51286–0.51299), and La/Nb (0.6–1.4) ratios, suggesting very limited liput from “old” subcontinental lithosphere or crustal sources during magma genesis. However, there are significant differences in the relative and absolute abundances of the LILE (large-ionlithophile elements), and these divide WA into two provinces. Basalts from the AP region have unusually high K/Ba and K/Rb ratios (50–140 and 500–1500 respectively) and marked Ba depletion (Ba/Nb=2.5–8.0; Ba ppm 66–320) relative to MBL basalts, which have LILE distributions within the range for OIB (ocean-island basalt) (K/Ba <50, Ba/Nb 5–20). This geochemical contrast is accompanied by a three-fold increase in the age range of volcanic activity and a three orders of magnitude increase in the volume of eruptive products, within MBL. The regional differences in geochemistry, and in the volume and duration of volcanic activity, are best explained by a plume-related origin for MBL basalts, whereas alkaline magmatism in the AP is causally related to slab window formation following the cessation of subduction. Plume activity has alreadybeen proposed to explain tectonic doming and associated spatial patterns of volcanism in MBL. Most MBL geochemical traits are shared by the volcanic rocks of the western Ross Sea, suggesting that a large plume head underlies the West Antarctic rift system. The uniformity of basalt compositions throughout WA and the entire rift system suggest uniformly minimal extension throughout this region during late Cenozoic time. Differences in crustal thicknesses can be explained by early Cenozoic or pre-Cenozoic extension, but restraint on extension is suggested by the size of the region and the implied size of the plume. The c. 95% encirclement of the Antarctic plate by mid-ocean ridges and transforms restrains extension on a regional scale, leading to nonadiabatic plume rise and correspondingly little decompression melting.

Breeding habitats of emperor penguins in the western Ross Sea

Within the Western Ross Sea, there are six emperor penguin colonies of widely different size that occur exclusively on sea ice. In 1990 a survey of all six sites, two by close overflights and four from the ground, showed that the breeding habitats were highly variable. The most important physical characteristics of these habitats appear to be stable fast ice, nearby open water, access to fresh snow, and shelter from the wind.

Structural characteristics and development of sea ice in the western Ross Sea

The internal structure of ice cores from western Ross Sea pack ice floes showed considerable diversity. Snow-ice formation made a small, but significant contribution to ice growth. Frazil ice was common and its growth clearly occurred during both the pancake cycle and deformation events. Congelation ice was also common, in both its crystallographically aligned and non-aligned varieties. Platelet ice was found in only one core next to the Drygalski Ice Tongue, an observation adding to the increasing evidence that this unusual ice type occurs primarily in coastal pack ice near ice tongues and ice shelves. The diverse internal structure of the floes indicates that sea ice development in the Ross Sea is as complex as that in the Weddell Sea and more complex than in the Arctic. The mean ice thickness at the ice core sites varied between 0.71 m and 1.52 m. The thinnest ice generally occurred in the outer pack ice zone. Regardless of latitude, the ice thickness data are further evidence that Antarctic sea ice is thinner than Arctic sea ice.

Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone

During the austral summer of 1990 an intense, diatom-dominated, ice-edge phytoplankton bloom in the southwestern Ross Sea resulted in depletion of silicic acid, nitrate and phosphate to concentrations much lower than is typical for Antarctic surface waters. Silicic acid was depleted to <6 FM within the core of the meltwater field, where biogenic particulate silica concentrations exceeded 20 pm01 1-' Three Si uptake kinetic experiments were conducted on natural phytoplankton assemblages from the nutrient-depleted zone; 30Si-labeled Si(OH), was used to measure the uptake rate at concentrations ranging from 1 to 20 PM above ambient. Dependence of the spec~fic uptake rate ( V ) on silicic acid concentration conformed well to a Michaelis-Menten saturation model; maximum uptake rates (V,,,,,) ranged from 0.0022 to 0.0028 h' which corresponds to maximum growth rates of 0.08 to 0.10 doublings d-l. Half-saturation constants (km) ranged from 1.1 to 4.6 PM, a range similar to values found in other areas of the ocean and considerably lower than those previously reported for several Antarctic d~atom species in culture studies. Results indicate detectable, but weak, substrate limitation of s~llcic acid uptake rate by the naturally occurring diatom assemblage in the western Ross Sea. Significant Si limitation in other subsystems of the Southern Ocean would be possible only if their resident diatom assemblages had much lower affinity for silicic acid than we observed.

Cycling of biogenic silica within the upper water column of the Ross Sea

We determined the distributions of biogenic particulate silica and dissolved silicic acid in the upper 80 m over the continental shelf of the southern Ross Sea, Antarctica, during two occupations of an east-west transect at 76°30′S in mid-January and early February 1990. There was a persistent (at least 3½ weeks in duration) diatom bloom within a surface meltwater lens extending 100–150 km seaward from the edge of the receding pack ice, with biogenic silica concentrations frequently exceeding 20 μmol l −1 in the upper 15 m. There were also other significant maxima in biogenic silica, with concentrations greater than 7 μmol l −1 at distances of 250–500 km seaward of the ice edge. These maxima were apparently unrelated to meltwater effects. 30Si tracer experiments to measure the production and dissolution rates of biogenic silica as a function of depth within the upper 50 m at three stations within the ice-edge diatom bloom indicate that the specific production rate (i.e. the rate per unit of biogenic silica present) ranged from 0.05 to 0.12 day −1 ( 0.07–0.17 doublings day −1). The resulting vertically integrated silica production rates ranged from 27 to 50 mmol m −2 day −1, with a mean of 34. Vertically integrated rates of biogenic silica dissolution ranged from 16 to 28 mmol m −2 day −1, with a mean of 22, or 64% of the mean silica production rate. The mean resulting net rate of biogenic silica production within the bloom, 12 mmol m −2 day −1, leads to an estimated net annual silica production rate of 1.0 mol m −2 in the upper 50 m of the southwestern Ross Sea. A revised silica budget for the western Ross Sea, incorporating the most recent estimates of production, redissolution, accumulation in the sediments and efflux from the seabed, indicates that virtually all silica exported from the upper 50 m must reach the sea-floor unless advective transport from the east is significant.

Chlorofluorocarbon evidence for rapid ventilation of the Ross Sea

Chlorofluorocarbon (CFC)-12 and -11 (CF 2Cl 2 and CFCl 3) measurements were made in seawater on the Ross Sea continental shelf and adjacent slope region in 1984. Concentrations of CFC-12 and CFC-11 in Ross Sea continental shelf water averaged nearly half that of saturated surface water. Circumpolar Deep Water within 50 km of the Ross Sea continental shelf also contained measureable CFC-12 and CFC-11, but an order of magnitude less than shelf waters. CFC-12 and CFC-11 concentrations in the deep water overlying the continental slope increased with depth, indicating recent ventilation and bottom water formation near the continental shelf. Several water masses on the continental shelf that are commonly distinguished on the basis of temperature and salinity characteristics also varied in CFC content, and thus in their modes and rates of ventilation and renewal. A time-dependent model reproducing the 1984 sub-surface shelf water CFC concentrations demonstrates the relative importance of mixed layer entrainment, gas exchange through leads in the winter sea ice field, and mixing with source waters derived from the Circumpolar Deep Water. Model results show that mixed layer entrainment at the beginning of winter is the dominant process ventilating sub-surface shelf water in the eastern Ross Sea, while western shelf waters also require significant gas exchange during winter. Shelf water residence time can only be constrained by the model and available data to 10 years, but most probable values are ∼2.5 years in the eastern Ross Sea and ∼4 years in the western Ross Sea. CFC concentrations in water which has circulated beneath and has been modified by the Ross Ice Shelf indicate that the transformation time from High Salinity Shelf Water to Ice Shelf Water can be as short as 3.5 years. With reasonable oxygen consumption and nutrient regeneration rates, the balance of ventilation and mixing processes that reproduced observed shelf water CFC-12 concentrations also can account for observed oxygen and nitrate levels, if the western surface waters are undersaturated by ∼10% at entrainment.

Geochemical characteristics of Western Ross Sea (Antarctica) sediments

A geochemical study of bottom sediments from 50 stations in various Ross Sea sedimentary environments, ranging from the coastal strip to the external platform, situated off Southern Victoria Land between the David and Mariner glaciers has demonstrated the silicate nature of these sediments and significant variations in the major element concentrations. The presence and concentrations of the trace elements studied (Cr, Ni, Co, V, Cu, Pb, Zn and Cd) differ from one geographic area to another, and within a given basin. A statistical analysis of the geochemical variables using the cluster method shows that Si is independent of the other elements, which group into associations. A multivariate statistical analysis based on geochemical, granulometric and environmental parameters identifies five distinct sediment groups located in different areas of sedimentation and with peculiar geochemical characteristics. Group 1, on the external platform, which comprises very fine sediments, is characterised by the highest contents of Mg, LOI, Cr, Ni and V, and of Cu and Zn. Group 2, from the banks formed by pelitic sands, is distinguished on the basis of lower Cu and Zn and higher Cr and Si. Group 3, on the internal platform consisting of sandy sediments, comprises high values of Na, Al, Ca and Pb and low values of Zn and V. Group 4, in the coarse coastal sands, is enriched in Co and Ca, Na, K and Al. Group 5, formed by materials with different grain-size distribution located south of Coulman Island, is distinguished by a very high content of Zn, Fe, Ti and Na, with Cr, Co, Pb and Si being found at lower levels. The materials are glacial-detrital, are derived from the rocky hinterland formations, and are reworked by transport and deposition processes depending on the energy level of the marine sedimentary environments. Given the scarcity of comparison data from the hinterland rocks it is difficult to establish the parent rocks of these glaciomarine sediments on the basis of geochemical data alone. Only in the case of the sediments south of Coulman Island can one be certain of a contribution from the McMurdo volcanics. For the external platform sediments, the highest contents of Si and L.O.I. confirm the input of material from biogenic neoformation.

High-resolution seismic-reflection interpretations of some sediment deposits, Antarctic continental margin: Focus on the western Ross Sea

High-resolution seismic-reflection data have been used to a varying degree by geoscientists to interpret the history of marine sediment accumulations around Antarctica. Reconnaissance analysis of 1-, 3.5-, and 12-kHz data collected by the U.S. Geological Survey in the western Ross Sea has led to the identification of eight echo-character facies and six microtopographic facies in the sediment deposits that overlie the Ross Sea unconformity. Three depositional facies regions, each characterized by a particular assemblage of echo-character type and microtopographic facies, have been identified on the continental shelf. These suites of acoustic facies are the result of specific depositional processes that control type and accumulation of sediment in a region. Evidence of glacial processes and products is uncommon in regions 1 and 2, but is abundant in region 3. McMurdo Sound, region 1, is characterized by a monospecific set of acoustic facies. This unique assemblage probably represents turbidity current deposition in the western part of the basin. Most of the seafloor in region 2, from about latitude 77°S to 75°S, is deeper than 600 m below sealevel. The microtopographic facies and echo-character facies observed on the lower slopes and basin floor there reflect the thin deposits of pelagic sediments that have accumulated in the low-energy conditions that are typical of deep-water environments. In shallower water near the boundary with region 3, the signature of the acoustic facies is different from that in deeper water and probably indicates higher energy conditions or, perhaps, ice-related processes. Thick deposits of tills emplaced by lodgement during the most recent advance of the West Antarctic Ice Sheet are common from latitude 75°S to the northern boundary of the study area just south of Coulman Island (region 3). The signature of microtopographic facies in this region reflects the relief of the base of the grounded ice sheet prior to decoupling from the seafloor. Current winnowing and scour of shallow parts of the seafloor inhibits sediment deposition and maintains the irregular, hummocky relief that characterizes much of the region. Seafloor relief of this type in other polar areas could indicate the former presence of grounded ice.

Multiple sources of alkanes in Quaternary oceanic sediment of Antarctica

Normal alkanes ( n-C 13 n-C 36), isoprenoid hydrocarbons ( i-C 15, i-C 16, i-C 18, i-C 19, and i-C 20) triterpanes (C 27 C 32), and (C 27 C 29) are present in low concentrations offshore Antarctica in near-surface, Quaternary sediment of the Wilkes Land continental margin and of the western Ross Sea. The distributions of these hydrocarbons are interpreted relative to possible sources and processes. The hydrocarbons appear to be mixtures of primary and recycled material from marine and terrigenous sources. The n-alkanes are most abundant and are characterized by two distinct populations, one of probable marine origin and the other likely from terrigenous, vascular plant sources. Because the continent of Antarctica today is devoid of higher plants, the plant-derived hydrocarbons in these offshore sediments probably came from wind-blown material and recycled Antarctic sediment that contains land-plant remains from an earlier period of time. Isoprenoid hydrocarbons are partially recycled and mainly of marine origin; the dominance of pristane over phytane suggests oxic paleoenvironmental conditions. Both modern and ancient triterpanes and steranes are present, and the distribution of these indicates a mixture of primary and recycled bacterial, algal, and possible higher-plant materials. Although the sampled sediments were deposited during the Quaternary, they apparently contain a significant component of hydrocarbons of pre-Quaternary age.

Phytoplankton bloom dynamics of the western Ross Sea ice edge—II. Mesoscale cycling of nitrogen and silicon

In the austral summer of 1983 we examined the spatial distributions of nutrients and phytoplankton biomass, nutrient assimilation rates and phytoplankton growth rates in an intense diatom bloom associated with a receding ice edge in the western Ross Sea. There was significant near-surface depletion of nitrate, phosphate and silicic acid within the bloom, but at almost all stations their concentrations remained higher than those generally considered limiting to phytoplankton growth. Ammonium concentrations within the bloom were frequently below detection limits (>0.1 μM in this study), and nearly always >0.25 μM. Rates of nitrate uptake were high enough to satisfy approximately 65% of the phytoplankton nitrogen demand estimated from 14C-primary productivity and particulate organic carbon: nitrogen ratios. Growth rates estimated for the overall phytoplankton assemblage based on 15N uptake, and the diatom assemblage based on 30Si uptake, were generally between 0.07 and 0.4 divisions per day. The mean rate of biogenic silica production within the bloom was 38 mmol Si m −2 day −1, which is an order of magnitude higher than previous estimates of silica production in the Southern Ocean, and comparable to rates observed in coastal upwelling systems at low latitudes. Accumulation of biogenic silica within the surface layer very nearly matched the depletion of dissolved silicic acid (i.e. the sum of these two quantities was very nearly conservative). The observed high rates of nitrate uptake and biogenic silica production imply that ice-edge phytoplankton blooms are a quantitatively important net source of organic and siliceous biogenic material within the Ross Sea.

Phytoplankton bloom dynamics of the western Ross Sea ice edge—I. Primary productivity and species-specific production

During the austral summer in 1983, studies of primary productivity within a phytoplankton bloom associated with a receding ice edge in the western Ross Sea, Antarctica were conducted. Primary production was elevated within the ice-edge bloom, with integrated euphotic zone production averaging 40.1 mg C m −2h −1. The production was elevated in regions where the ice melt had created a strong vertical stability, and was decreased outside the zone of melt-water influence. Microautoradiographic analyses and species abundance data showed that a single species of diatom, Nitzschia curta, a species generally considered to be epontic, contributed up to 93% of the total productivity and 70% of the number of phytoplankton cells. Growth rates determined from autoradiography and carbon estimates based on cell volume as well as those determined from bulk production data agreed well and ranged from 0.1 to 0.5 divisions per day. No difference in growth rates between populations within and from outside the bloom was noted, an observation which we believe reflects the recent mixing of the phytoplankton as vertical stability weakened. We conclude that the bloom was seeded by algae released from melting ice, and that an ice-edge bloom can have substantial impact on the biogeochemical cycles and foodweb dynamics of the Southern Ocean.