Abiotic characteristics and diatom abundance and biomass in different sectors of the Argentinien Shelf and antartic waters

A large spatial scale study of the diatom species inhabiting waters from the subantarctic (Argentine shelf) to antarctic was made for the first time in order to understand the relationships between these two regions with regard to the fluctuations in diatom abundances in relation with environmental features, their floristic associations and the effect of the Polar Front as a biogeographic barrier. Species-specific diatom abundance, nutrient and chlorophyll-a concentration were assessed from 64 subsurface oceanographic stations carried out during the austral summer 2002, a period characterized by an anomalous sea-ice coverage corresponding to a ''warm year". Significant relationships of both diatom density and biomass with chlorophyll-a (positive) and water temperature (negative) were found for the study area as a whole. Within the Subantarctic region, diatom density and biomass values were more uniform and significantly (in average: 35 and 11 times) lower than those of the Antarctic region, and did not correlate with chlorophyll-a. In antarctic waters, instead, biomass was directly related with chlorophyll-a, thus confirming the important contribution of diatoms to the Antarctic phytoplanktonic stock. A total of 167 taxa were recorded for the entire study area, with Chaetoceros and Thalassiosira being the best represented genera. Species richness was maximum in subantarctic waters (46; Argentine shelf) and minimum in the Antarctic region (21; Antarctic Peninsula), and showed a significant decrease with latitude. Floristic associations were examined both qualitatively (Jaccard Index) and quantitatively (correlation) by cluster analyses and results allowed differentiating a similar number of associations (12 vs. 13, respectively) and two main groups of stations. In the Drake Passage, the former revealed that the main floristic change was found at the Polar Front, while the latter reflected the Southern ACC Front as a main boundary, and yielded a higher number of isolated sites, most of them located next to different Antarctic islands. Such differences are attributed to the high relative density of Fragilariopsis kerguelensis in Argentine shelf and Drake Passage waters and of Porosira glacialis and species of Chaetoceros and Thalasiosira in the Weddell Sea and near the Antarctic Peninsula. From a total of 84 taxa recorded in antarctic waters, only 17 were found exclusively in this region, and the great majority (67) was also present in subantarctic waters but in extremely low (< 1 cell/l) concentrations, probably as a result of expatriation processes via the ACC-Malvinas Current system. The present results were compared with those of previous studies on the Antarctic region with respect to both diatom associations in regular vs. atypically warm years, and the distribution and abundance of some selected planktonic species reported for surface sediments.

Diatom biostratigraphy and datum events of ODP Hole 183-1140A

Ocean Drilling Program (ODP) Leg 183 Site 1140 provided a lower Oligocene to middle Miocene record of diatom assemblages from the northern Kerguelen Plateau. Samples were examined to improve the resolution of shipboard diatom biostratigraphy. The material is complementary to that recovered during ODP Legs 119 and 120, and the diatom zonation of Harwood and Maruyama could be readily applied. A standard succession of biostratigraphic zones from the middle Miocene and lower Oligocene was delineated, although some zones were unrecognizable because of poor core recovery. The detailed diatom biostratigraphy presented here agrees well with shipboard calcareous nannofossil biostratigraphy. Sediment accumulation rates based on diatom bioevents average 1.26 cm/k.y.

Diatom biostratigraphy of ODP Holes 191-1179B and 191-1179C

Although the objective of Ocean Drilling Program Leg 191 was to install a seismic monitoring station and to test a hard rock reentry drilling system, several good, near-continuous sedimentary core sequences were recovered during the cruise. Two holes, 1179B and 1179C, yielded an upper Miocene to Pleistocene diatom record through an expanded section with excellent recovery. Because diatom species included in both low-latitude and high-latitude biostratigraphies are present, zonations for the equatorial Pacific and northwest Pacific are applied to the sediments. The oldest zones from each zonation that are represented in the cores are the Nitzschia miocenica Zone and the Rouxia californica Zone, respectively. Only one zonal boundary is not observed within the diatom assemblage, that being the top of the Nitzschia jouseae Zone and base of the Rhizosolenia praebergonii Subzone A (equatorial Pacific). Preservation is good overall, and sample abundances vary from rare to abundant. This would be an excellent section for further biostratigraphic, paleoclimatic, and paleoceanographic study.

Palaeoceanographic data of sediment core MD01-2416

We present an SiF4 separation line, coupled to a laser fluorination system, which allows for an efficient combined silica d18O and d30Si analysis (50 min per sample). The required sample weight of 1.5-2.0 mg allows for high-resolution isotope studies on biogenic opal. Besides analytical tests, the new instrumentation set-up was used to analyse two marine diatom fractions (>63 µm, 10-20 µm) with different diatom species compositions extracted from a Bølling/Allerød-Holocene core section [MD01-2416, North-West (NW) Pacific] to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species-related isotope effects (vital and environmental effects). While d30Si offsets between the two fractions were not discernible, supporting the absence of species-related silicon isotope effects, systematic offsets occur between the d18O records. Although small, these offsets point to species-related isotope effects, as bias by contamination can be discarded. The new records strengthen the palaeoceanographic history during the last deglaciation in the NW Pacific characterized by a sequence of events with varying surface water structure and biological productivity. With such palaeoceanographic evolution it becomes unlikely that the observed systematic d18O offsets signal seasonal temperature variability. This calls for reconsideration of vital effects, generally excluded to affect d18O measurements.

Siliceous microplankton in surface sediments and the water column of the Red Sea

We studied the siliceous microplankton assemblages (mainly diatoms) from plankton tows (mesh size 20 µm) and surface sediment samples collected along a N-S transect in the northern Red Sea (28-21°N). In addition, we analyzed differences/similarities between plankton and sediment assemblages within a brine-filled basin (the southern basin) of the Shaban Deep and compared these assemblages with those from outside the brine. Plankton samples revealed the overwhelming dominance of diatoms over other siliceous groups. Diatoms accounted for ca. 97% of all biosiliceous particles at 120-20 m (vs. 2.9% silicoflagellates and 0.4% radiolarians), and ca. 94% at 200-120 m (vs. 4.5% silicoflagellates and 1.6% radiolarians). In general, a marine, warm-water (tropical/subtropical) diatom assemblage characterizes the plankton samples. Representatives of the Nitzschia bicapitata group are by far the most abundant contributors at both depth intervals (average=43%), ranging from ca. 30% in the North to ca. 60% in the South. Biogenic opal content in non-brine surface sediments is very low, (below 0.2 wt.% SiO2); and concentration of siliceous microorganisms is also low and of the order of 5*10**3-10**4 microorganisms/g dry sediment. Diatoms are the main contributors to the opal signal in the 20-40 µm fraction, while they share dominance with radiolarians in the >40 µm fraction. Total diatom concentrations average 1.2*10**4 valves/g in the 20-40 µm fraction and 4*10**3 valves/g in the >40 µm fraction. Robust taxa of warm water affinity (Alveus marinus, Azpeitia neocrenulata, Azpeitia nodulifera and Roperia tesselata) characterize the surface sediments. In contrast, biogenic opal content in brine surface sediment samples is much higher than in the non-brine samples, ranging from 2.8 to 3.8 wt.% SiO2, and concentration of siliceous microorganisms is 3-4 orders of magnitude higher. In addition here, diatoms dominate the opal signal. The taxa found in these samples are a mixture of non-brine and plankton samples, and fragile forms (e.g., N. bicapitata group, Neodelphineis indica) are well preserved in these sediments. Thus, brine sediments in this region seem to offer a great potential for palaeoenvironmental studies.

Diatom biostratigraphy, Argon ages and chronology of ODP Hole 183-1138A

A thick Neogene section was recovered in the upper ~300 m of Ocean Drilling Program Hole 1138A, drilled on the Central Kerguelen Plateau in the Indian sector of the Southern Ocean. Sediment lithologies consist primarily of mixed carbonate and biosiliceous clays and oozes, with several thin (1-3 cm) tephra horizons. The tephras are glass rich, well sorted, and dominantly trachytic to rhyolitic in composition. Volcaniclastic material in these horizons is interpreted to have originated from Heard Island, 180 km northwest of Site 1138, and was likely emplaced through both primary ash fall and turbiditic, submarine flows. A Neogene age-depth model for Hole 1138A is constructed primarily from 36 diatom biostratigraphic datums. Nannofossil and planktonic foraminifer biostratigraphy provides supporting age information. Additionally, four high-precision 40Ar-39Ar ages are derived from ash and tephra horizons, and these radiometric ages are in close agreement with the biostratigraphic ages. The integrated age-depth model reveals a reasonably complete lower Miocene to upper Pleistocene section in Hole 1138A, with the exception of a ~1-m.y. hiatus at the Miocene/Pliocene boundary. Another possible hiatus is also identified at the Oligocene/Miocene boundary. High Neogene sedimentation rates and the presence of both calcareous and siliceous microfossils, combined with datable tephra horizons, establish Site 1138 as a suitable target for future drilling legs with paleoceanographic objectives. This report also proposes two new diatom species, Fragilariopsis heardensis and Azpeitia harwoodii, from Pliocene strata of Hole 1138A.

Abundance of diatom cells in sediments from the Okhotsk and Bering Seas and the Northwest Pacific Ocean

The present investigation was targeted at diatom composition studies in the surface sediments (0-1 cm) sampled in the Sea of Okhotsk and the northwest Pacific in the depth range from 130 to 6110 m. The taxonomic analysis, as well as the quantitative (the diatom cell abundance per sediment dry weight unit) content and ecological group definition, was applied. Ten diatom taxa are the main body (80-100%) of the diatom assemblages: Bacterosira bathyomphala, Chaetoceros spp. (spores), Actinocyclus curvatulus, Thalassiosira latimarginata (group), T. antarctica (spores), Neodenticula seminae, Rhizosolenia hebetata f. hiemalis, Thalassiothrix longissima, Coscinodiscus marginatus, Coscinodiscus oculus iridis. The relative content of these species reflects the sedimentation conditions for different parts of the sea: the shelf, the continental slope, the open sea, and the ocean. The highest diatom content (45.6.3-60.0 mln per g of dry weight) was found for the surface sediments in the central part of the Sea of Okhotsk and the continental slope of western Kamchatka.

Diatom abundance in sediments from ODP Leg 167 sites

The Pliocene and Pleistocene periods are known for the onset and consequent amplification of glacial-interglacial cycles. The California margin, situated in the mid-latitudes of the northern Pacific Ocean, is expected to be one of the most interesting regions for Pliocene to Pleistocene paleoceanography because this area occupies a unique position in the ocean-atmosphere system over the region. In this study, we investigated paleoceanographic history, using fossil diatoms, since the Brunhes/Matuyama (B/M) paleomagnetic boundary in which glacial and interglacial periods began to alternate in 100-yr cycles. In Hole 1018A, to a depth corresponding to the beginning of Northern Hemisphere glaciation (late Pliocene), we investigated the responses of the ocean-atmosphere system to stepwise cooling in the California margin. Although the work is still continuing, this data report shows that fossil diatoms of Pliocene and Pleistocene sediments significantly changed both in quality and quantity and implies a possible relationship to global climatic changes.