Constance Sancetta

The annual cycle of sedimentation in Saanich inlet, British Columbia: implications for the interpretation of diatom fossil assemblages

Abstract Data derived from monthly sediment traps at three depths near the head and mouth of Saanich Inlet yield a detailed record of the seasonal cycle of production and vertical flux to the sediments. The material is primarily composed of diatom frustules and silt-to-clay-sized lithic fragments, with dinoflagellates and naked algae present in summer samples. Diatoms dominate from April to September while clastics dominate from October to March. Particles occur primarily in loose flocs; pellets are common in early winter and in summer material, and consist of an indiscriminant mixture of the same particles seen in co-occurring flocs. The seasonal succession of taxa is similar at both sites and is transported to the sediment with little modification by dissolution. Carbon flux is a poor indicator or productivity, due to the strong effect of recycling in the surface waters during spring and uncertainty as to the source of the carbon accumulating in the traps. Although diatom flux is an adequate indicator of primary production, transforming each taxon to equivalent cell volume yields a more accurate picture of the seasonal cycle of production. Relative abundance (taxon percentage) data can be as useful as, and in some cases more accurate than, absolute flux (number per unit time), while number-per-gram may be highly misleading as an estimator of production. Lateral advection and benthic resuspension affect monthly data and annual valve-flux data, but appear to have no net effect upon annual percentage data.

WISCONSIN--HOLOCENE PALEOENVIRONMENT OF THE BERING SEA: EVIDENCE FROM DIATOMS, POLLEN, OXYGEN ISOTOPES AND CLAY MINERALS

Abstract A core from the southeastern Bering Sea contains a depositional record of the Wisconsin glacial interval and the Holocene. Four lithologic units show distinctive changes in diatom species, palynomorphs, oxygen isotopes in diatom silica, and clay minerals. Unit 4, the oldest, probably represents the glacial advance of the early Wisconsin and records extensive sea ice cover and a soggy, herbaceous tundra on the exposed continental shelf. Unit 3 represents the stades and interstades of the middle Wisconsin, including an early period of high seasonal variability and a later period with expansion of winter sea ice in the basin and of tundra onshore. Unit 2 represents the late Wisconsin glacial maximum and subsequent deglaciation with a tundra-covered shelf. Sea ice cover increased to a maximum, during which the surface waters were probably covered by sea ice throughout the year. Subsequent to the sea ice maximum the isotope ratios record a major meltwater event. Unit 1 represents the Holocene, with expansion of shrub tundra or scrub forest in the coastal upland and waters of increased temperature and salinity in the Bering Sea.

Climatic record of the past 130,000 years in North Atlantic deep-sea core V23-82: Correlation with the terrestrial record

Abstract Quantitative paleo-environmental analyses of planktonic foraminifera in 182 samples covering the past 130,000 years in North Atlantic deep-sea core V23-82 yield time series interpreted in terms of changing surface-water conditions. An absolute chronology is estimated by linear interpolation between levels dated by 14 C or by stratigraphic correlation with other radiometrically dated climatic records. Significant events include: (1) rapid warming at 127,000 YBP, marking the start of the penultimate North Atlantic and European interglacial; (2) 124,000 YBP temperature maximum (Eemian); (3) 109,000 YBP cooling, correlated with the beginning of the last European glaciation (Wurm), and representing a temporary cooling of the North Atlantic; (4) severe cooling 73,000 YBP, marking the start of the last full glacial regime in the North Atlantic; (5) short warm intervals within the last glacial regime dated at 59,000 YBP, 48,000 YBP, and 31,000 YBP; (6) rapid termination of the last glacial interval at 11,000 YBP; (7) a 6000 YBP hypsi-saline, followed by lowering salinity values presumably associated with decreasing flux of Gulf Stream waters over the core site.

Late-glacial to holocene changes in winds, upwelling, and seasonal production of the northern California current system

Abstract A core 120 km off the coast of southern Oregon was examined for changes in lithology, diatoms, and pollen over the past 30,000 yr. Primary production during the late Pleistocene was about half that of the Holocene. Evidence from diatoms and pollen indicates that summer upwelling was much weaker, implying an absence of strong northerly winds. Early Pliocene diatoms found throughout the late Pleistocene section were probably derived from diatomites east of the Cascades and provide evidence for strong easterly winds over a dry continental interior. The findings verify predictions of a climate model based on glacial maximum conditions. There is no compelling evidence for a climatic reversal corresponding to the European Younger Dryas chron. During the early Holocene (9000−7000 yr B.P.) there may have been years when winds were insufficiently strong to support upwelling, so that warm stratified waters lay closer to the coast.

Effect of Pleistocene glaciation upon oceanographic characteristics of the North Pacific Ocean and Bering Sea

Abstract During intervals of Pleistocene glaciation, insolation of the high-latitude northern hemisphere was lower than today, particularly during summer. Growth of continental ice sheets resulted in a lowering of sea level by more than 100 m in the Bering Sea. As a result, the Bering Strait was closed and most of the Bering continental shelf exposed. A proposed model predicts that (1) sea-ice formation would occur along the (modern) outer continental shelf, (2) advection would transport the sea ice over the deep basin, and (3) brine would flow into the basin at some intermediate depth to enhance the halocline. The result would be a low-salinity surface layer with a cold, thick halocline and reduced vertical mixing. Diatom microfossils and lithologic changes in sediment cores from the North Pacific Ocean and Bering Sea support the model and suggest that the proposed oceanographic conditions extended into the North Pacific, where the cold low-salinity layer was enhanced by meltwater from continentally derived icebergs.

Seasonal associations of phytoplankton and planktic foraminifera in an upwelling region and their contribution to the seafloor

Abstract Seasonal occurrences of both phytoplankton and zooplankton are closely associated with upwelling and stabilization in waters overlying the San Pedro Basin. This region is situated within the Southern California Bight and is affected by variations in the general southward flowing California Current and variable intrusion of oceanic waters. Increased northerly wind stress in spring and early summer results in upwelling of deeper, nutrient-rich waters into the photic zone. A seven month time-series sediment trap experiment revealed clear patterns in seasonal succession of both phytoplankton and planktic foraminifera. Several distinct high and low flux periods occurred over the study period, some of which can be related to rapid changes in the hydrographic environment caused by upwelling and subsequent surface water stratification. Factor analysis of relative abundances of combined phytoplankton and foraminiferal taxa defines seven assemblages reflecting seven stages of production which range in duration from two to five weeks. Factor scores describe phytoplankton and foraminiferal taxa indicative of specific production stages. Periods of high flux of the more abundant diatoms and planktic foraminifera also illustrate preferred production stages and compare well with percentage-based results. A comparison of the sediment trap assemblage with local sediment from an oxygenated portion of the basin suggests that long-term phytoplankton accumulation in this region is dominated by taxa associated with winter bloom and post-upwelling bloom production, implying that brief but massive episodic events overwhelm and dominate the annually averaged diatom species assemblage found in the sediments. Foraminifera, however, are modified by dissolution to the extent that the high production periods of late spring and during upwelling are poorly represented. Rather, species indicative of early spring and post-upwelling conditions dominate the planktic foraminiferal sedimentary assemblage.

Comparison of phytoplankton in sediment trap time series and surface sediments along a productivity gradient

Three years of sediment trap data collected by the Multitracers project are compared with surface sediments for sites off the coast of southern Oregon, using phytoplankton microfossils. Sites were located to monitor a region frequently affected by plumes from coastal upwelling (Nearshore), one occasionally affected by the largest plumes (Midway) and an area outside the zone of coastal influence (Gyre). Diatoms overwhelmingly dominate the microfossils, with silicoflagellates an order of magnitude fewer and calcareous nanofossils absent. This absence suggests that the rarity of the group in the sediments may be a function of low production, rather than of deepwater dissolution. The Midway and Nearshore sites were generally similar in taxonomic composition and timing of flux maxima, while the Gyre site differed in both regards. Interannual variability in taxon composition appears to correlate with productivity; the most productive site (Nearshore) has the most consistent interannual trend and the least productive (Gyre) exhibits the greatest variability. The sediment assemblage at the Nearshore site is most similar to trap samples from the late fall and winter seasons, rather than the more productive spring and summer. This is partly a function of differential dissolution, but the relationship persists even when only resistant taxa are used. Two possible explanations are discussed: addition of material from the continental shelf, and inadequate representation of long-term trends by the trapping experiment. At the Midway site the data are few and scattered, but once the dissolution effect is removed it appears that the sediments contain material from several seasons of production. At the Gyre site preservation bias results in a sediment assemblage most similar to late spring and summer, only partly corresponding to maximum production. Comparison between sites indicates that the Nearshore and Gyre sediments have characteristic assemblages which are similar to the corresponding trap collections. The Midway sediments are similar to trap samples from all 3 sites, implying that the assemblage lies on a gradient between the other two and lacks a unique character of its own.

Diatom evidence on Wisconsin and Holocene events in the Bering Sea

Abstract Previous work on surface (modern) sediments has defined diatom species which appear to be good indicators of various oceanographic/ecologic conditions in the North Pacific Ocean and marginal seas. Three long cores from the eastern and northern sides of the Aleutian Basin show changes in species assemblage which can be interpreted in terms of changes in the ocean environment during the last glaciation (Wisconsin) and the Holocene. The early and late Wisconsin maxima were times of prolonged annual sea-ice cover and a short cool period of phytoplankton productivity during the ice-free season. The middle Wisconsin interstade, at least in the southern Bering Sea, had greater seasonal contrast than today, with some winter sea-ice cover, an intensified temperature minimum, and high spring productivity. Variations in clastic and reworked fossil material imply varying degrees of transport to the basin by Alaskan rivers. The results of Jouse from the central Bering Sea generally correspond with those presented here, although there are problems with direct comparison.

DIATOM STRATIGRAPHY OF THE LATE PLEISTOCENE (BRUNHES) SUBARCTIC PACIFIC

Sancetta, C. and Silvestri, S.M., 1984. Diatom stratigraphy of the late Pleistocene (Brunhes) subarctic Pacific. Mar. Micropaleontol., 9: 263--274. Three piston cores in the subarctic region of the North Pacific show consistent changes in relative abundance of diatom species throughout the Brunhes magnetic epoch. These events can be used both as stratigraphic markers to subdivide the interval and as indicators of oceanographic conditions. The stratigraphic record shows an acme for A. ochotensis from about 625--350 Kyr (Isotope Stages 15--10); a marked increase in amplitude of abundance fluctuations for R. hebetata 300--0 Kyr (Stages 8--1), the extinction of R. curvirostris at 276 Kyr (Stage 8), and abundance fluctuations of D. seminae roughly in phase with the global oxygen isotope record from 450--0 Kyr (Stages 12--1). In the subarctic Pacific, an abrupt change in species abundance is not a reliable indicator of hiatus occurrence. Although details are not clear, there appears to have been a fundamental change of the subarctic gyre during the middle Brunhes (Stages 10--8 time), with more intense glacial intervals and stronger glacial--interglacial contrast occurring after that time. Coarse clastic detritus is not restricted to glacial intervals, suggesting that ice-rafting has occurred throughout the Brunhes interval; peak levels of ice-rafting may occur during ice-growth and decay, as well as during short intervals within a stage.

Late Pliocene climate in the Southeast Atlantic: Preliminary results from a multi-disciplinary study of DSDP Site 532

Abstract Site 532 on the Walvis Ridge was sampled at 4000- to 800-year intervals from 2.24 to 2.60 Ma, spanning the three large glacial advances of the late Pliocene. An age model was created by correlating the oxygen isotope record to Site 607 with linear interpolations between tie-lines. The resultant age model differs from that in the site reports by more than 800,000 years, due to misidentification of a magnetic boundary. Sedimentation rates varied by an order of magnitude at this site, with minimum accumulation during glacial events. Interglacial intervals were charactrized by high marine production and high summer precipitation on land, while glacials had very low production and arid continental climate. During the large glacial events (Stages 96–100) conditions of low production and continental aridity reached their greatest intensity, but there is no evidence of a permanent mode shift in either marine or terrestrial records. Calcite concentration has a strong variation at obliquity frequencies, with maxima during interglacials, but occasionally shows a large amplitude at precessional frequencies as well, so that high concentrations occur in a few glacial intervals. As a result, color variation is not a reliable guide to glacial-scale cycles at this site. Composition of the phytoplankton assemblage is diverse and highly variable, and we have not been able to distinguish a clear indicator of upwelling-related production. Spectral analysis reveals obliquity and precessional signals in the pollen data, while several diatom records contain combination tones, indicating that these data represent a complicated response to both local and high-latitude forcing.