English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Diatoms occur sporadically in lower Miocene to Holocene sediments recovered at ODP Site 645 and in upper Plio­ cene to Holocene sediments at ODP Site 646. The diatom assemblage at Site 645 contains rare stratigraphic indicators. Fragmentation of frustules and the occurrence of species characteristic of high-latitude shelf, upper-slope environments suggest current transportation from the shelf. The diatom abundance and preservation at Site 646 probably reflect cli­ matic changes and are also affected by dissolution, but it is not possible to detect the dominant factor. Therefore, the stratigraphic ranges of the primary and secondary biostratigraphic indicators are often unreliable. INTRODUCTION Leg 105 is the first Deep Sea Drilling Project-Ocean Drilling Program (DSDP-ODP) cruise to drill in Baffin Bay and to core Neogene sediments in the Labrador Sea continuously (Sites 645 and 646, Fig. 1). Some of the objectives of Leg 105 included the recovery of continuous stratigraphic sequences in the Labrador Sea and Baffin Bay, the completion of a high-resolution stratig­ raphy, and the correlation of this stratigraphy with those previ­ ously completed for the North Atlantic and the NorwegianGreenland Sea. Investigating the responses of oceanographic circulation to changing climatic conditions was another interest of the Leg 105 Shipboard Scientific Party. Diatom biostratigraphic and paleoceanographic studies have previously been conducted for Neogene and Quaternary sedi­ ments from the high-latitude Atlantic (north of 50°N; Baldauf, 1984, 1987; Schrader, 1979) and from the Norwegian-Greenland Sea (Schrader and Fenner, 1976; Dzinoridze et al., 1978; Thiede et al., in press). We recovered Pliocene and Pleistocene diatoms at Sites 645 and 646. At Site 645, diatoms occur sporadically within Plio­ cene and Pleistocene sediments. The diatom assemblage is com­ posed of species typical of shelf and upper-slope deposits, sug­ gesting downslope redeposition of the sediments. The diatom flora at Site 646 consists of three primary components: (1) a high-latitude assemblage composed of nearshore species simi­ lar to those observed at Site 645, in the Norwegian-Greenland Sea (Schrader and Fenner, 1976), in the Bering Sea (Baldauf, 1982; Sancetta, 1982), and elsewhere in Baffin Bay (Williams, 1986); (2) an assemblage similar to that defined for the warmtemperate North Atlantic (Schrader, 1979; Baldauf, 1984, 1986, 1987); and (3) an assemblage composed of pelagic species com­ mon to the Norwegian-Greenland Sea (Schrader and Fenner, 1976, Dzinoridze et al., 1979), the North Pacific (Barron, 1980, 1985a; Koizumi, 1973, Koizumi and Tanimura, 1985; Akiba, 1985), and the Bering Sea (Sancetta, 1982). The occurrence of these three distinct diatom assemblages within the sediments re­ covered at Site 646 reflects changes in surface-water conditions. Primary biostratigraphic markers do occur at Site 646, but their stratigraphic range is generally unreliable because of the inSrivastava, S. P., Arthur, M., Clement, B., et al., 1989. Proc. ODP, Sci. Re­ sults, 105: College Station, TX (Ocean Drilling Program). 2 Universite de Bretagne Occidentale, GIS "Oceanologie et Geodynamique," 6, avenue Victor Le Gorgeu, 29287 Brest Cedex, France. 3 Ocean Drilling Program, Texas A&M University Research Park, 1000 Dis­ covery Drive, College Station, TX 77840. consistent occurrence and poor to moderate preservation. There­ fore, the diatom biostratigraphy at Site 646 relies in part on sec­ ondary stratigraphic markers. METHODS Approximately 1.5 cm of the sample was placed into a 400 mL beaker and disaggregated by the addition of 10 mL of 30% hydrogen peroxide. We then added 30 mL of 37% HC1 and gently heated the sam­ ple until carbonates and organic carbons were removed. We diluted the sample by adding approximately 300 mL distilled water and decanted it after 2 hr settling time. We repeated the decanting process until a pH of 6 was reached. Strewn slides were prepared on 22 x 30 mm cover glasses and mounted in Hyrax on 22 x 75 mm glass slides. For Site 646 material, we reexamined strewn slides of acid-clean ma­ terial prepared aboard JOIDES Resolution as well as additional samples processed at onshore facilities. For material from Holes 645B to 645F, most of the samples we studied consisted of core-catcher samples ob­ served on board JOIDES Resolution and reexamined onshore. Table 1 lists the abundance and preservation of samples that contain diatoms from Site 645. We examined one strewn slide of each sample in its en­ tirety at x 500 with a light microscope; we confirmed species identifica­ tions at x 1250. The range chart for Site 646 is presented in Table 2. Species were recorded as abundant (A) if two or more specimens were present in one field of view at x 500, common (C) if one specimen oc­ curred in two fields of view, few (F) if one specimen was observed in each horizontal transect, and rare (R) if specimens were encountered less frequently than that. The quality of fossil preservation in the samples—poor (P), moder­ ate (M), and good (G)—is based on the number of finely silicified spe­ cies such as Pseudoeunotia doliolus and the Nitzschia group, compared with the number of heavily silicified ones such as Stephanopyxis horridus, Thalassiosira zabelinae, and Coscinodiscus marginatus. The geo­ chronology used in this report follows that of Berggren et al. (1985). Correlation of the diatom events to the magnetostratigraphic record fol­ lows that of Barron (1985b), Barron et al. (1985), Baldauf (1984, 1987), Morley et al. (1982), Koizumi and Tanirama (1985), and Sancetta and Silvestri (1984, 1986) (Fig. 2). RESULTS Site 645 Site 645 (70°27.43'N, 64°39.26'W) is situated on the lower continental slope east of Baffin Island in water depths of about 2006 m (Fig. 1). The sediments recovered have a pronounced ter­ rigenous character with surprisingly few microfossils (with the exception of dinocysts), which could be the consequence of low surface-water productivity within this region of Baffin Bay. Dia­ toms occur sporadically in Miocene to Holocene sediments. The diatom assemblages at this site are characterized by species typi­ cal of high-latitude shelf, upper-slope environments.

Miocene To Holocene Diatom Biostratigraphy from Baffin Bay and Labrador Sea, Ocean Drilling Program Sites 645 and 646