Sea ice seasonality during the Holocene, Adélie Land, East Antarctica

Thin sections of laminated cores from different Antarctic coastal areas have demonstrated the potential of diatom species to document climate change at the seasonal scale. Here we present the relative abundances of four diatom species and species groups (Fragilariopsis curta group as a proxy for yearly sea ice cover, F. kerguelensis as a proxy for summer sea-surface temperature, Chaetoceros Hyalochaete resting spores as a proxy for spring sea ice melting and the Thalassiosira antarctica group as a proxy for autumn sea ice formation) in core MD03-2601 retrieved off Adélie Land on the Antarctic continental shelf. These abundances were compared to surface temperatures and sea ice cover modelled over the last 9000 years. Both the marine records and the simulated climate demonstrated a cooler Early Holocene (9000–7700 years BP), a warmer Mid-Holocene (7700–4000 years BP) and a colder Late Holocene (4000–1000 years BP). Yearly sea ice cover followed an inverse pattern to temperatures with less sea ice during the Mid-Holocene Hypsithermal than during the Late Holocene Neoglacial. However, diatom census counts and model output indicate that sea ice spring melting happened earlier in the season, as expected, but that autumn sea ice formation also occurred earlier in the season during the Hypsithermal than during the colder Neoglacial, thereby following seasonal changes in local insolation.     

Observations on the relationship between the Antarctic coastal diatoms Thalassiosira antarctica Comber and Porosira glacialis (Grunow) Jørgensen and sea ice concentrations during the late Quaternary

The available ecological and palaeoecological information for two sea ice-related marine diatoms (Bacillariophyceae), Thalassiosira antarctica Comber and Porosira glacialis (Grunow) Jørgensen, suggests that these two species have similar sea surface temperature (SST), sea surface salinity (SSS) and sea ice proximity preferences. From phytoplankton observations, both are described as summer or autumn bloom species, commonly found in low SST waters associated with sea ice, although rarely within the ice. Both species form resting spores (RS) as irradiance decreases, SST falls and SSS increases in response to freezing ice in autumn. Recent work analysing late Quaternary seasonally laminated diatom ooze from coastal Antarctic sites has revealed that sub-laminae dominated either by T. antarctica RS, or by P. glacialis RS, are nearly always deposited as the last sediment increment of the year, interpreted as representing autumn flux. In this study, we focus on sites from the East Antarctic margin and show that there is a spatial and temporal separation in whether T. antarctica RS or P. glacialis RS form the autumnal sub-laminae. For instance, in deglacial sediments from the Mertz Ninnis Trough (George V Coast) P. glacialis RS form the sub-laminae whereas in similar age sediments from Iceberg Alley (Mac.Robertson Shelf) T. antarctica RS dominate the autumn sub-lamina. In the Dumont d'Urville Trough (Adélie Land), mid-Holocene (Hypsithermal warm period) autumnal sub-laminae are dominated by T. antarctica RS whereas late Holocene (Neoglacial cool period) sub-laminae are dominated by P. glacialis RS. These observations from late Quaternary seasonally laminated sediments would appear to indicate that P. glacialis prefers slightly cooler ocean–climate conditions than T. antarctica. We test this relationship against two down-core Holocene quantitative diatom abundance records from Dumont d'Urville Trough and Svenner Channel (Princess Elizabeth Land) and compare the results with SST and sea ice concentration results of an Antarctic and Southern Ocean Holocene climate simulation that used a coupled atmosphere–sea ice–vegation model forced with orbital parameters and greenhouse gas concentrations. We find that abundance of P. glacialis RS is favoured by higher winter and spring sea ice concentrations and that a climatically-sensitive threshold exists between the abundance of P. glacialis RS and T. antarctica RS in the sediments. An increase to > 0.1 for the ratio of P. glacialis RS:T. antarctica RS indicates a change to increased winter sea ice concentration (to >80% concentration), cooler spring seasons with increased sea ice, slightly warmer autumn seasons with less sea ice and a change from ~ 7.5 months annual sea ice cover at a site to much greater than 7.5 months. In the East Antarctic sediment record, an increase in the ratio from <0.1 to above 0.1 occurs at the transition from the warmer Hypsithermal climate into the cooler Neoglacial climate (~ 4 cal kyr) indicating that the ratio between these two diatoms has the potential to be used as a semi-quantitative climate proxy.     

EFFECT OF HYPEROXIA ON THE GROWTH AND PHOTOSYNTHESIS OF POLAR SEA ICE MICROALGAE1

Sea ice algal communities are naturally exposed to very high concentrations of dissolved oxygen, which are likely to lead to increasing stress levels and declines in productivity. To test this hypothesis, cultures of Fragilariopsis cylindrus (Grun?) Hasle, Pseudo‐nitzschia sp., Fragilariopsis curta (Van Heurch), Porosira glacialis (Grunow), and Entomoneis kjellmannii (Cleve) from Antarctic sea ice and Nitzschia frigida from Arctic sea ice were exposed to elevated dissolved oxygen levels, and their growth, maximum quantum yield, relative maximum electron transport rate, and photosynthetic efficiency were measured. At oxygen concentrations equivalent to approximately four times air saturation (89% oxygen), the growth rate and maximum quantum yield were significantly reduced in all taxa. When the oxygen concentration was regularly allowed to drop, the effect on growth and quantum yield was reduced. At lower dissolved oxygen concentrations (52%), the declines in growth and quantum yield were reduced but were still mostly significantly different from the controls (21% oxygen). It is likely that the generation of excess active oxygen radicals in the presence of free oxygen is responsible for most of the decline in growth, maximum quantum yield, relative maximum electron transport rate, and photosynthetic efficiency in all species.     

Protists in the marine ice of the Amery Ice Shelf, East Antarctica

Samples of marine ice were collected from the Amery Ice Shelf, a large embayed ice shelf in East Antarctica, during the Austral summer of 2001–2002. The samples came from a site ∼90 km from the iceberg calving front of the shelf, where the ice is 479 m thick and the lower 203 m is composed of accreted marine ice. Protists identified within the marine ice layer of the Amery Ice Shelf include diatoms, chrysophytes, silicoflagellates and dinoflagellates. The numerical dominance of sea ice indicator diatoms such as Fragilariopsis curta, Fragilariopsis cylindrus, Fragilariopsis rhombica and Chaetoceros resting spores, and the presence of cold open water diatoms such as Fragilariopsis kerguelensis and species of Thalassiosira suggest the protist composition of the Amery marine ice is attributable to seeding from melting pack and/or fast ice protist communities in the highly productive waters of Prydz Bay to the north.     

Diversity of the diatom genus Fragilariopsis in the Argentine Sea and Antarctic waters: morphology, distribution and abundance

Fragilariopsis species composition and abundance from the Argentine Sea and Antarctic waters were analyzed using light and electron microscopy. Twelve species (F. curta, F. cylindrus, F. kerguelensis, F. nana, F. obliquecostata, F. peragallii, F. pseudonana, F. rhombica, F. ritscheri, F. separanda, F. sublinearis and F. vanheurckii) are described and compared with samples from the Frenguelli Collection, Museo de La Plata, Argentina. F. peragallii was examined for the first time using electron microscopy, and F. pseudonana was recorded for the first time in Argentinean shelf waters. New information on the girdle view is included, except for the species F. curta, F. cylindrus and F. nana, for which information already existed. In the Argentine Sea, F. pseudonana was the most abundant Fragilariopsis species, and in Antarctic waters, F. curta was most abundant. Of the twelve species of Fragilariopsis documented, four occurred in the Argentine Sea, nine in the Drake Passage and twelve in the Weddell Sea. F. curta, F. kerguelensis, F. pseudonana and F. rhombica were present everywhere.     

Preliminary investigation of the contribution of fast-ice algae to the spring phytoplankton bloom in Ellis Fjord,eastern Antarctica

 Algae released from fast-ice in Ellis Fjord, eastern Antarctica, made little contribution to subsequent phytoplankton growth. Dominant taxa in the interior ice community included Nitzschia cylindrus (Grun) Hasle, Navicula glaciei V.H. and a dinoflagellate cyst. Diatom mortality within the ice was high. The algal contribution to the phytoplankton from the fast ice was estimated by calculating the difference between algal biomass in ice cores taken on 14 November with those taken on 18 December 1992. The biomass of sedimenting phytoplankton was estimated using sediment traps; weekly cell counts of water were used to monitor net phytoplankton growth. The low contribution from the fast-ice of Ellis Fjord to the phytoplankton is similar to results from other Antarctic fast-ice communities but is not necessarily reflective of processes occurring within either Antarctic or Arctic pack ice communities. An algal mat growing on the base of the fast-ice had a carbon standing crop of between 0.231 gC m^-2 and 0.022 gC m^-2. Much of this was delivered to the water column as the ice melted while the remainder was exported. Received: 15 March 1995/Accepted: 4 September 1995     

DEVELOPMENT OF IMMUNOASSAYS FOR THE IRON‐REGULATED PROTEINS FERREDOXIN AND FLAVODOXIN IN POLAR MICROALGAE1

While the growth of Southern Ocean phytoplankton is often limited by iron availability, there are no comparable experiments on sea‐ice algae. Here we assess the use of ferredoxin and flavodoxin to investigate the iron nutritional status of sea‐ice algae and describe the development of a quantitative immunoassay for both proteins in marine diatoms. High‐affinity monoclonal antibodies toward both proteins were produced from Cylindrotheca closterium (Ehrenb.) J. M. Lewin et Reimann, and these were used to develop Western blots. Western blots run on whole protein extracts detected both proteins with little cross‐reactivity toward other proteins. The two proteins could be successfully quantitated when applied to gels at between 5 and 50 ng in a volume of 25 μL (0.2–2 μg · mL^?1). Flavodoxin and ferrodoxin expression was examined in the Antarctic diatoms Entomoneis kjellmannii (Cleve) Poulin et Cardinal, Navicula directa (W. Sm.) Ralfs, Fragilariopsis curta (Van Heurck) Hust., Pseudo‐nitzschia sp., Porosira glacialis (Grunow) E. G. Jørg., Fragilariopsis cylindrus (Grunow) Willi Krieg., Fragilariopsis sublinearis (Van Heurck) Heiden et Kolbe, C. closterium, Nitzschia lecointei Van Heurck, and the dinoflagellate Polarella glacialis Montresor, Procaccini et Stoecker. Two Arctic isolates were also examined, Nitzschia frigida (Grunow) and Fragilariopsis oceanica (Cleve) Hasle. Significant heterogeneity of protein expression was observed despite all cultures being grown in iron‐replete f/2 medium. Only one species, F. cylindrus, displayed the expected expression of ferredoxin only in iron‐replete medium. Four were observed to produce both proteins under iron‐replete conditions. Ferredoxin was not detected at all in F. curta and Pseudo‐nitzschia sp., but distinct flavodoxin bands were observed in both of these organisms. All species examined were observed to express either flavodoxin or ferredoxin or both of the proteins as determined by Western immunoblotting.     

Summer phytoplankton distributions in the Weddell Sea

Summary The quantitative composition of phytoplankton was studied along a transect of 14 hydrographic stations, between the southern coast of the Weddell Sea and the Antarctic Peninsula, during the austral summer of 1984–1985. The most apparent feature of the phytoplankton distribution was the presence of a bloom of Phaeocystis at a frontal zone over the shelf break, and the marked contrast between a southern region, with high phytoplankton biomass, and a poorer region north of the shelf break. The most widely distributed diatom genus was Nitzschia (Fragilariopsis section). The phytoplankton assemblage of the southern region included the silicoflagellate Distephanus speculum, the diatom Rhizosolenia alata and several heterotrophic dinoflagellates such as Protoperidinium antarcticum and P. applanatum. The northern assemblage could be characterized by the abundance of flagellates and small dinoflagellates, and by diatoms such as Chaetoceros criophilum, Corethron criophilum Nitzschia kerguelensis and other Nitzschia species of the Fragilariopsis section.     

Sedimentation of biogenic siliceous particles in Antarctic waters from the Atlantic sector

Sediment traps were deployed in the Drake Passage, Bransfield Strait and west of South Orkney Islands (Powell Basin) during December 1980/ January 1981, December 1983, and between March and December 1983, respectively.Most of the trapped material is biogenic opal except in the lower half of the water column in Bransfield Strait where large amounts of resuspended aluminosilicates and quartz grains were present. Frustules and skeletons of siliceous microorganisms (diatoms, silicoflagellates, radiolarians, chrysophycean cysts), fragments and moults of crustaceans and tests of foraminifera were found. Quantitatively diatoms are the dominant constituent of the trapped biogenic material.Alteration of diatom assemblages in the water column is due to mechanical breakdown by grazing zooplankton. It mainly affects large frustules (e.g. Corethron criophilum Castracane) in the uppermost part of the water column. Dissolution of frustules occurs mostly at the sediment/water interface and leads to the enrichment of strongly silicified valves [e.g. Nitzschia kerguelensis (O'Meara) Hasle, Thalassiosira antarctica Comber (resting spores)].At the Bransfield Strait site a large part of biogenic opal was incorporated into fecal pellets of krill and copepods. The bulk of pellet content consists of fragmented diatom frustules (1–10 μm in size). Most intact valves found in the sediments have settled through the water column by means other than fecal pellet transport: e.g. settling as solitary particles or incorporated into or attached to “Marine Snow” or “Large Amorphous Aggregates”.     

Some Nitzschia and related diatom species from fast ice samples in the Arctic and Antarctic

Summary Some Nitzschia and closely related species have been examined in the light and electron microscopes from fast ice samples in the Arctic and Antarctic. Nitzschia neofrigida, forming arborescent colonies, and Nitzschia promare, forming loose ribbon colonies, are described as new species, both probably included in the distribution of other similar species. A new combination, Auricula compacta, represents the first report of this genus from ice samples. Colony formation is reported for the first time in Nitzschia arctica and Nitzschia taeniiformis. No biopolar species were found and several reports of Arctic species in Antarctic ice samples have been refuted.     

Physical parameters influencing diatom community structure in eastern Antarctic sea ice

Diatom assemblages obtained from fast ice around Prydz Bay, Antarctica, are distinctly different from those obtained from pack ice in the same area. The dominant species in all ice cores were Fragilariopsis curta, F. cylindrus, Nitzschia stellata and Pseudonitzschia turgiduloides. Entomoneis kjellmanii and Cocconeis spp. were more characteristic of fast ice samples and F. curta of pack ice samples. Ice crystal type (i.e. whether frazil or congelation crystal) is an important factor determining the algal composition of the ice. Other significant influences include the time of year the ice forms and the salinity of the ice.     

Abundance,horizontal and vertical distribution of fish in eastern Weddell Sea micronekton

Summary The spatial distribution and species composition of high-Antarctic ichthyonekton was investigated during the EPOS 3 cruise by RV Polarstern in the eastern Weddell Sea during January–February 1989. A multiple rectangular midwater trawl was used to collect samples from the surface to near the sea floor at 11 stations along a 245 kra transect off Halley Bay. Early larval stages of 18 species, representing about 24% of the known Weddell Sea ichthyofauna, were present in the water column. The Antarctic silver-fish, Pleuragramma antarcticum, over-whelmingly dominated the catches comprising 84.5% of the 5022 specimens caught. The abundance of this species markedly increased towards the offshore end of the transect with the highest numbers occurring near the shelf-break front associated with the westerly current of the southern limb of the Weddell Gyre. The increased abundance of P. antarcticum in continental slope waters was attributed to deflection of the East Weddell Coastal Current beyond the shelf/slope break by fringing ice shelves. Most larval and juvenile fish were found in the seasonally warmed upper 0–70 m layer of the Antarctic Surface Water where conditions occurred that appeared to be favourable to both feeding and growth. Cluster analysis indicated that inner-, central-and outer-shelf assemblages were represented and that the species composition was most effectively described by reference to water mass and depth.     

Relationship between the distribution of euphausiids and baleen whales in the Antarctic (35°E – 145°W)

Simultaneous whale sighting and hydroacoustic surveys were conducted from a research vessel in the Antarctic to examine the relationship between the distribution of euphausiids and baleen whales. High densities of minke whales and large aggregations of euphausiids were observed along the ice edge over the continental slope in the southeast region of area IV and in the southwest region of area V. The results suggest that the continental slope zone that coincides with the ice edge would be an important minke whale feeding area. Minke whales were rarely sighted in the offshore region even if euphausiids were abundant. Distributions of humpback whales were correlated with high euphausiid density zones, regardless of the bottom topographic features. Several groups of blue whales were sighted in the small area along the ice edge where euphausiids were abundant, but sightings were too few to draw any general conclusion about the relationship between blue whales and euphausiids. Both baleen whales and euphausiids were scarce in the area east of 170°W where sea ice covered the continental shelf and slope zone.     

Examination of diatom type material: Nitzschia delicatissima Cleve,Thalassiosira minuscula Krasske,and Cyclotella nana Hustedt

Type material of Thalassiosira minuscula Krasske and material identified by the respective authors as Nitzschia delicatissima Cleve and Cyclotella nana Hustedt (= Thalassiosira pseudonana Hasle et Heimdal) have been examined in light and electron microscopes. It is suggested that the slide labelled “Nitzschia delicatissima Helder 7/5–97” is accepted as neotype of the species since the original material must be regarded as lost, and that Thalassiosira pseudonana is identical to the diatom isolated by Guillard as clone 3-H. Nitzschia actydrophila Hasle and Thalassiosira monoporocyclus Hasle are rejected as separate species, the first being conspecific with N. delicatissima and the second with T. minuscula. The question is raised whether more emphasis should be laid on iconotypes of small diatom species identifiable only in the electron microscope than on real individuals.     

Bryozoans of the Weddell Sea continental shelf,slope and abyss: did marine life colonize the Antarctic shelf from deep water,outlying islands or in situ refugia following glaciations?

Aim At the height of glaciations such as the Last Glacial Maximum (LGM), benthic life on polar continental shelves was bulldozed off nearly all of the Antarctic shelf by grounded ice sheets. The origins of the current shelf benthos have become a subject of considerable debate. There are several possible sources for the current Antarctic shelf fauna, the first of which is the continental slope and deep sea of the Southern Ocean. The high levels of reported eurybathy for many Antarctic species are taken as evidence supporting this. A second possible source for colonists is the southern margins of other continents. Finally, shelves could have been recolonized from refugia on the continental shelves or slopes around Antarctica. The current study investigates whether the patchily rich and abundant biota that now occurs on the Antarctic continental shelf recolonized from refugia in situ or elsewhere. Location Weddell Sea, Antarctica. Methods We examined bryozoan samples of the BENDEX, ANDEEP III and SYSTCO expeditions, as well as the literature. Using similarity matrices (Sørensen coefficient), we assessed similarities of benthos sampled from around Antarctica. By assessing numbers of species shared between differing depths and adjacent shelf areas, we evaluated the origins of cheilostome bryozoan communities. Results Bryozoans decreased from 28, 6.5 and 0.3 colonies per trawl, and 0.16, 0.046 and 0.0026 colonies per cm² of hard surface from shelf to slope to abyssal depths. We found little and no support for recolonization of the Weddell Sea shelf by bryozoans from the adjacent slope and abyss, in the scenario of LGM faunal wipe‐out. The Weddell Sea shelf bryozoan fauna was considerably more similar to those on other Antarctic shelves than to that of the adjacent (Weddell Sea) continental slope. The known bryozoan fauna of the Weddell Sea shelf is not a subset of the Weddell Sea slope or abyssal faunas. Main conclusions We consider that the composition of the current Weddell Sea bryozoan fauna is most easily explained by in situ survival. Thus we consider that at least some of the Weddell Sea fauna persisted throughout the LGM, although not necessarily at the same locations throughout, to recolonize the large area currently occupied.     

CONFIRMATION OF DOMOIC ACID PRODUCTION BY PSEUDONITZSCHIA AUSTRALIS (BACILLARIOPHYCEAE) CULTURES1

Single done isolates of Pseudonitzschia australis Frenguelli (= Nitzschia pseudoseriata Hasle) isolated from a toxic bloom in Monterey Bay, California produced domoic acid in culture. Although long-term historical records do not indicate previous blooms of this species on the Pacific coast, this is probably because it has been often misidentified as Nitzschia seriata Hasle; previous evidence for toxicity is lacking. Hydrographic data suggest that areas such as Monterey Bay might be “hot spots” for domoic acid-producing blooms.     

OCCURRENCE OF AN ISOPRENOID C25 DIUNASATURATED ALKENE AND HIGH NEUTRAL LIPID CONTENT IN ANTRACTIC SEA-ICE DIATOM COMMUNITIES1

The lipid and hydrocarbon composition of natural populations of diatom communities collected during the austral spring bloom of 1985 in the sea-ice at McMurdo Sound, Antartica was analyzed by TLC-FID, GC and GC-MS. Sea-ice diatom communities were dominated by Amphiprora sp., Nitzschia stellata Manguin and Berkeleya sp. at Cape Armitage; N. stellata, Amphiprora, Pleurosigma, N. kerguelensis (O'Meara) Hasle and some small centric diatoms adjacent to the Erebus Ice Tongue; and Porosira pseudodenticulata (Hustedt) Jouse at Wohlschlag Bay. Lipid distributions of the sea ice diatom communities from the Cape Armitage and Ereus sites were characterized by high concentrations of tracylaglecycerol (triacylglycerolplar lipid = 1.0 to 1.5). The hydrocarbon n-C_21:6, common in temperate diatoms, and an isoprenoid C₂₅ diunsaturated alkene were the dominant hydrocarons detected at these two sites. Hydrogenation of the C₂₅ diene produced the known alkane 2, 6, 10, 14-tetramethyl-7- (3-methylpentyl)-pentadecane. The C₂₅ diene is one of several structurally related hydrocarbons reported in many estuarine, coastal and ocean ic sediments. We propose that certain species of diatoms are a likely source of these alkenes in sediments. The first reported biological occurrence of the C₂₅ diene in the green seaweed Enteromorpha prolifera may have been due to the presence of epiphytic microalgae in the field sample analysed.     

Allocation of photoassimilated carbon into major algal metabolite fractions: Variation between two diatom species isolated from the Weddell Sea (Antarctica)

Distribution of photoassimilated carbon into major metabolite classes differed between two Antarctic diatom species, Nitzschia curta and a small unicellular Chaetoceros sp.. Time course uptake studies (over 54 h) revealed that ¹⁴C allocation appeared to be equilibrated after approximately 8 h at light saturated photosynthesis. During short term dark periods (6 h), polysaccharides as well as low-molecular-weight compounds were catabolised to sustain protein synthesis in the dark, whilst lipid reserves were not mobilised for this process. Experiments with these two species were conducted at 0 and -1.5°C, although no difference in the distribution of radiolabel was measured between the two temperatures. It is hypothesised that under near-optimal conditions fast growing species are characterised by a high carbon turnover associated with a rapid flow of newly assimilated carbon into polymeric compound classes. On the other hand, slower growing species (such as N. curta) may store a significant amount of surplus carbon in the low-molecular-weight metabolite fraction. Species specific preferences were observed when comparing the accumulation of radiolabel into the lipid pools.     

Persistent genetic signatures of historic climatic events in an Antarctic octopus

Repeated cycles of glaciation have had major impacts on the distribution of genetic diversity of the Antarctic marine fauna. During glacial periods, ice cover limited the amount of benthic habitat on the continental shelf. Conversely, more habitat and possibly altered seaways were available during interglacials when the ice receded and the sea level was higher. We used microsatellites and partial sequences of the mitochondrial cytochrome oxidase 1 gene to examine genetic structure in the direct‐developing, endemic Southern Ocean octopod Pareledone turqueti sampled from a broad range of areas that circumvent Antarctica. We find that, unusually for a species with poor dispersal potential, P. turqueti has a circumpolar distribution and is also found off the islands of South Georgia and Shag Rocks. The overriding pattern of spatial genetic structure can be explained by hydrographic (with ocean currents both facilitating and hindering gene flow) and bathymetric features. The Antarctic Peninsula region displays a complex population structure, consistent with its varied topographic and oceanographic influences. Genetic similarities between the Ross and Weddell Seas, however, are interpreted as a persistent historic genetic signature of connectivity during the hypothesized Pleistocene West Antarctic Ice Sheet collapses. A calibrated molecular clock indicates two major lineages within P. turqueti, a continental lineage and a sub‐Antarctic lineage, that diverged in the mid‐Pliocene with no subsequent gene flow. Both lineages survived subsequent major glacial cycles. Our data are indicative of potential refugia at Shag Rocks and South Georgia and also around the Antarctic continent within the Ross Sea, Weddell Sea and off Adélie Land. The mean age of mtDNA diversity within these main continental lineages coincides with Pleistocene glacial cycles.     

Productivity events of the marine diatom Thalassiosira tumida (Janisch) Hasle recorded in deglacial varves from the East Antarctic Margin

During the last deglaciation calving bay reentrants formed over several deep inner shelf basins around Antarctica as the ice sheets retreated. Marine diatoms flourished in spring and summer within the bays, providing exceptionally high silica flux rates to the basins on an annual basis. As a result, several hundred years of the late deglaciation are archived in many of these deep basins as continuously laminated diatom-rich marine sediments. The laminae are excellently preserved and offer a unique opportunity to study deglacial processes over the Antarctic shelf at very high resolution (subseasonal to seasonal at best). Annual palaeoenvironments from these sediments have been recently reconstructed. However, finer sedimentary detail is also apparent by the preservation of sublaminae of single diatom species, which represent discrete productivity events (blooms). In this paper, we focus on Thalassiosira tumida (Janisch) Hasle sublaminae preserved in the  5 m thick deglacial laminated sequence of NBP01-01 Jumbo Piston Core (JPC) 43B from Iceberg Alley ( 67°S, 63°E) on the Mac.Robertson Shelf, East Antarctic Margin. These T. tumida sublaminae are distinctive, occur several times throughout the deglacial sequence and are always preserved within summer laminae. Importantly they carry ecological and paleoceanographic information for deglacial Iceberg Alley and possibly the entire East Antarctic Margin. We describe and illustrate the T. tumida sublaminae using scanning electron microscope backscattered electron imagery of highly polished thin sections and secondary electron imagery of sublamina fracture surfaces. T. tumida sublaminae range in thickness from 0.57 to 21.07 mm (mean = 5.12; σ = 4.49) and are defined as discrete sedimentary intervals within a summer lamination where T. tumida is the dominant diatom species (abundance > 50%; but frequently > 80%) or is very abundant (abundance 40% to 49%). Based on the stratigraphic position of these sublaminae in the annual/seasonal succession and on published morphological, ecological and distributional data of living specimens in culture and in the field, we surmise an open-water, late summer bloom for T. tumida in deglacial Iceberg Alley, at sea-surface temperatures of 0 to − 1.69 °C, under low or fading light levels just prior to significant sea-ice formation. We suggest that the sublaminae formed by a combination of episodic T. tumida blooms (surface concentration), followed by rapid deposition (high flux) and optimal frustule preservation on the sea floor. Increased sublaminae frequency upcore suggests a lengthening of the summer season, while their complete cessation 43 years prior to the end of lamination deposition in outer Iceberg Alley (JPC43B) may be due to a coastward population shift.