Investigation of Quaternary diatoms in sediment core CRP-1 from the Ross Sea, Antarctica

In the first season of drilling, the Cape Roberts Project (CRP) recovered one drillcore (CRP-l) from Roberts Ridge in western McMurdo Sound, Ross Sea, Antarctica Diatom biostratigraphy places the upper six lithostratigraphic units (Units 1.1, 2.1, 2.2, 2.3, 3.1, and 4.1) of CRP-l (0.0 to 43.15 mbsf) within the Quaternary. Both non-marine and marine Quaternary diatoms occur in variable abundance in the Quaternary interval of CRP- 1 Biostratigraphic data resolve two Quaternary time slices or events within CRP-1. Marine diatom assemblages in Units 4.1 and 3.1 represent sedimentation within the diatom Actinocyclus ingens Zone (1.35 to 0.66 Ma). Further refinement of the age of Unit 3.l places deposition in the interval 1.15 to 0.75 Ma based on the common occurrence of Thalassiosira elliptipora and correlation to the Southern Ocean acme of this taxon The absence of ActiActinocyclus ingens and the presence ot Thalassiosira antarctica in Unit 2.2 require a younger zonal assignment for this interval, within the diatom Thalassiosira lentiginosa Zone (0.66 to 0.0 Ma). A new diatom species. Rouxia leventerae, is described from marine assemblages of Units 2.2, 2.3, 3.1, and 4.l. Lithostratigraphic Unit 3.1 (33.82 to 31.89 mbsf) is a bryozoan-dominated skeletal-carbonate facies. Low abundance of Fragilariopsis curta and Fragilariopsis cylindrus within this unit combined with the relatively high abundance of species associated with open water indicates deposition in waters that remained ice free for much or all of the year Diatom assemblages suggest carbonate deposition in Unit 3.1 is linked to a significant early Pleistocene event in McMurdo Sound, when elevated surface-water temperatures inhibited the formation of sea ice.

Planktonic and benthic stables Isotopes, age model and carbon analysis of HUD91/039 from the northernmost Baffin Bay

A multiproxy study of palaeoceanographic and climatic changes in northernmost Baffin Bay shows that major environmental changes have occurred since the deglaciation of the area at about 12 500 cal. yr BP. The interpretation is based on sedimentology, benthic and planktonic foraminifera and their isotopic composition, as well as diatom assemblages in the sedimentary records at two core sites, one located in the deeper central part of northernmost Baffin Bay and one in a separate trough closer to the Greenland coast. A revised chronology for the two records is established on the basis of 15 previously published AMS 14C age determinations. A basal diamicton is overlain by laminated, fossil-free sediments. Our data from the early part of the fossiliferous record (12 300 - 11 300 cal. yr BP), which is also initially laminated, indicate extensive seasonal sea-ice cover and brine release. There is indication of a cooling event between 11 300 and 10 900 cal. yr BP, and maximum Atlantic Water influence occurred between 10 900 and 8200 cal. yr BP (no sediment recovery between 8200 and 7300 cal. yr BP). A gradual, but fluctuating, increase in sea-ice cover is seen after 7300 cal. yr BP. Sea-ice diatoms were particularly abundant in the central part of northernmost Baffin Bay, presumably due to the inflow of Polar waters from the Arctic Ocean, and less sea ice occurred at the near-coastal site, which was under continuous influence of the West Greenland Current. Our data from the deep, central part show a fluctuating degree of upwelling after c. 7300 cal. yr BP, culminating between 4000 and 3050 cal. yr BP. There was a gradual increase in the influence of cold bottom waters from the Arctic Ocean after about 3050 cal. yr BP, when agglutinated foraminifera became abundant. A superimposed short-term change in the sea-surface proxies is correlated with the Little Ice Age cooling.

Radiocarbon ages, diatom abundance and laminae description of sediment core MD03-2597

Laminated sediments are unique archives of palaeoenvironmental and palaeoceanographic conditions, recording changes on seasonal and interannual timescales. Diatom-rich laminated marine sediments are examined from Dumont d'Urville Trough, East Antarctic Margin, to determine changes in environmental conditions on the continental shelf from 1136 to 3122 cal. yr BP. Scanning electron microscope backscattered electron imagery (BSEI) and secondary electron imagery are used to analyse diatom assemblages from laminations and to determine interlamina relationships. Diatom observations are quantified with conventional assemblage counts. Laminae are primarily classified according to visually dominant species identified in BSEI and, secondarily, by terrigenous content. Nine lamina types are identified and are characterized by: Hyalochaete Chaetoceros spp. resting spores (CRS); CRS and Fragilariopsis spp.; Fragilariopsis spp.; Corethron pennatum and Rhizosolenia spp.; C. pennatum; Rhizosolenia spp.; mixed diatom assemblage; Stellarima microtrias resting spores (RS), Porosira glacialis RS and Coscinodiscus bouvet; and P. glacialis RS. Formation of each lamina type is controlled by seasonal changes in sea ice cover, nutrient levels and water column stability. Quantitative diatom assemblage analysis revealed that each lamina type is dominated by CRS and Fragilariopsis sea ice taxa, indicating that sea ice cover was extensive and persistent in the late Holocene. However the lamina types indicate that the sea ice regime was not consistent throughout this period, notably that a relatively warmer period, ~3100 to 2500 cal. yr BP, was followed by cooling which resulted in an increase in year round sea ice by ~1100 cal. yr BP.

Mid-Pliocene diatom and palynological assemblages north-northeast of Cape Adare, Antarctica

Microfossil assemblages in Pliocene sediments from DSDP Site 274 (68°59.81'S, 173°2564'E) provide data on the age of the sediments and suggest the presence of Nothofagus (southern beach) in Antarctica during the Pliocene. A suite of 17 samples was collected in an interval from Samples 28-274-6R-1, 83-87 cm to 28-274-11R-4, 73-77 cm (48.33-100.29 mbsf). Biostratigraphic study of the abundant diatom assemblages combined with published radiolarian data indicates that the sample interval ranges in age from 5.0 to 2.2 Ma, with an apparent unconformity between about 3.8 and 3.2 Ma. Nothofagidites (the genus for fossil pollen referable to Nothofagus) occurs throughout the interval, as well as pollen and spores with known stratigraphic ranges that unequivocally indicate reworking from older rocks. Species of Nothofagidites recovered include N. asperus, N. brachyspinulosus, N. flemingii, N. senectus, and N. sp. cf. N. lachlaniae; the latter form is previously known from the Sirius Group in the Transantarctic Mountains. Abundant palynomorphs were recovered in only three of the samples from Site 274 (Samples 28-274-9R-2,15-19 cm; 28-274-9R-2,48-52 cm; and 28-274-9R-2,65-69 cm). Based on the diatom and radiolarian biostratigraphic data, the ages of these samples range from 3.00 to 3.01 Ma. The relative abundance of N. sp. cf. N. lachlaniae in the three samples is an order of magnitude higher than relative abundances for the other species of Nothofagidites in the same samples. The signiticantly higher relative abundance of N. sp. cf. N. luchlaniae suggests that this pollen was derived from trees of Nothofugus that were living in Antarctica during the mid Pliocene. Diatom assemblages from these three samples indicate that sediments in this interval were rapidly deposited as biogenic oozes in an open-ocean setting relatively free of sea ice, thus decreasing the possibility of reworking from a single source bed rich in N. sp. cf. N. lachlaniae. Clearly, more detailed work in additional well-dated cores from around Antarctica is needed before a clear picture of the Neogene history of Antarctic terrestrial vegetation emerges.

Trace metals in sediments of the Var Canyon turbidite system at the DYFAMED station in the Ligurian Sea

The magnitude and the chronology of anthropogenic impregnation by Hg and other trace metals of environmental concern (V, Cr, Ni, Cu, Zn, Ag, Cd and Pb, including its stable isotopes) in the sediments are determined at the DYFAMED station, a site in the Ligurian Sea (Northwestern Mediterranean) chosen for its supposed open-sea characteristics. The DYFAMED site (VD) is located on the right levee of the Var Canyon turbidite system, at the end of the Middle Valley. In order to trace the influence of the gravity current coming from the canyon on trace metal distribution in the sediment, we studied an additional sediment core (VA) from a terrace of the Var Canyon, and material collected in sediment traps at the both sites at 20 m above sea bottom. The patterns of Hg and other trace element distribution profiles are interpreted using stable Pb isotope ratios as proxies for its sources, taking into account the sedimentary context (turbidites, redox conditions, and sedimentation rates). Major element distributions, coupled with the stratigraphic examination of the sediment cores point out the high heterogeneity of the deposits at VA, and major turbiditic events at both sites. At the DYFAMED site, we observed direct anthropogenic influence in the upper sediment layer (<2 cm), while on the Var Canyon site (VA), the anthropization concerns the whole sedimentary column sampled (19 cm). Turbiditic events superimpose their specific signature on trace metal distributions. According to the 210Pbxs-derived sedimentation rate at the DYFAMED site (0.4 mm yr-1), the Hg-enriched layer of the top core corresponds to the sediment accumulation of the last 50 years, which is the period of the highest increase in Hg deposition on a global scale. With the hypothesis of the absence of significant post-depositional redistribution of Hg, the Hg/C-org ratio changes between the surface and below are used to estimate the anthropogenic contribution to the Hg flux accumulated in the sediment. The Hg enrichment, from pre-industrial to the present time is calculated to be around 60%, consistent with estimations of global Hg models. However, based on the chemical composition of the trapped material collected in sediment traps, we calculated that epibenthic mobilization of Hg would reach 73%. Conversely, the Cd/C-org ratio decreases in the upper 5 cm, which may reflect the recent decrease of atmospheric Cd inputs or losses due to diagenetic processes.

Diatom biostratigraphy and sea ice concentration reconstruction from core DA06-139G, Vaigat Strait in Disko Bugt, West Greenland

A diatom-based sea-ice concentration (SIC) transfer function is developed using 72 surface samples from west of Greenland and around Iceland, and through comparison with the associated modern SIC. Canonical correspondence analysis on surface sediment diatoms and monthly average of SIC reveals that April SIC is the most important environmental factor controlling the distribution of diatoms in the area, and permits the development of a diatom-based SIC transfer function. The consistency between reconstructed SIC based on diatoms from West Greenland and the instrumental and documentary data during the last ~75 years demonstrates that the diatom-based SIC reconstruction is reliable for studying the palaeoceanography off West Greenland. Relatively warm conditions with strong influence of the Irminger Current (IC) are indicated for the early part of the record (~5000-3860 cal. yr BP), corresponding in time to the latest part of the Holocene Thermal Maximum. The April SIC oscillated around the mean value between 3860 and 1510 cal. yr BP and was above mean afterwards, particularly during the time interval 1510-1120 cal. yr BP and after 650 cal. yr BP, indicating more extensive sea-ice cover in Disko Bugt. A high degree of consistency between the reconstructed April SIC and changes in the diatom species suggests that the sea-ice condition in Disko Bugt is strongly influenced by variations in the relative strength of two components of the West Greenland Current, i.e. the cold East Greenland Current and the relatively warm IC.