Seasonal occurrence of silicoflagellate morphologies in different environments of the eastern Pacific Ocean

Data on weekly to bimonthly particle flux collected by sediment traps in three environments show that forms of silicoflagellates do not have a simple and consistent relationship to either temperature or productivity of the overlying water column. Sites were located in the fjords of British Columbia, along a transect off the coast of southern Oregon, and in San Pedro Basin of the Southern California Bight. Either the “cool”Distephanus speculum or the “warm”Dictyocha messanensis may dominate at temperatures over a range from 10–19°C.D. speculum dominated high-productivity periods off Oregon, whileD. messanensis dominated during coastal upwelling events in the San Pedro Basin, regardless of surface temperature.Octactis pulchra showed no distinctive trend in the San Pedro Basin, the only locality where it was observed. The isolated presence ofD. speculum in the fjords, where mixed-zone salinity may be as low as 11‰, indicates a tolerance for hyposaline waters, but no other trends could be defined with respect to surface salinity.     

Neogene Pacific microfossils and paleoceanography

Data on temporal and spacial distribution of all species of foraminifera, calcareous nannofossils, radiolaria and diatoms have been compiled from the Initial Reports of the Deep Sea Drilling Project, for the Neogene Pacific Ocean. Problematic species and sites were removed by a series of procedures. Age-depth curves for each site, based on all available biostratigraphic information, were used to choose narrow time-slices for analysis. Factor analysis was applied to the resulting data body to derive paleo-oceanographic and -ecologic information. For this data set, radiolaria were found to be the single most useful group, and calcareous nannofossils the least. Use of all groups combined delineates major provinces better than the use of any one group alone.From the Early Miocene to the present the number of plankton provinces has increased while the geographic extent of each has decreased, coincident with steepening of surface isotherms. The Tropical Province shows relatively little geographic change, but high diversification of microfossil groups. The Transitional Provinces have been remarkably stable both geographically and biologically since their appearance in the Middle Miocene, reflecting the highly adaptive nature of the component species. Subarctic and Subarctic Provinces have existed throughout the Neogene, representing the positions of the Subarctic Front and the Antarctic Convergence. The former has been generally of low diversity throughout time, while the latter has increased in diversity of siliceous organisms, and become progressively more restricted. An Antarctic Province appears in the Middle Miocene, and also shows change in species composition and geographic restriction through time. A Bering Province has appeared since the Miocene, probably in response to refrigeration of the Arctic in the Pliocene.     

Plankton response to physical forcing in the Gulf of California

Distinct increases in plankton productivity occur annually inthe Gulf of California and are related, at least in part, towind-driven changes in upper-ocean conditions. In particular,a rapid increase in plankton shell fluxes occurs in late fall(November), and is associated with a shift to northerly windsand cooling of surface temperatures that induce mixing of theupper ocean. The observed succession in the phytoplankton isattributed to this destabilization of surface waters and mayreflect the ability of different groups to rapond to varyingsurface water nutrient levels. The unraveling of such relationshipsbetween plankton production and hydrographic conditions is criticalto improving our ability to reconstruct quantitatively pastclimates.     

Spatial and temporal trends of diatom flux in British Columbian fjords

The monthly flux of diatom taxa at four sites in two fjordsof British Columbia, Canada, was determined over a 3-year periodby particle interceptor traps. Saanich Inlet is a small basinwith little horizontal density gradient while Jervis Inlet experiencesseasonal runoff and strong vertical stratification. The basicpattern of seasonal succession is the same in both fjords, butthe timing and volume of production differ. Production is spreadover a longer part of the year in Saanich Inlet, resulting ina more diverse assemblage, while in Jervis Inlet there is asingle production maximum in May, dominated by Skeletonema costatum.Most taxa occur simultaneously at all sites, but many show apreference for one fjord, or one area within a fjord. Severalspecies of Chaetoceros recur consistently in inner Saanich Inleteach fall, suggesting re-seeding by benthic spores. In contrast,Rhizosolenia setigera is more common in the outer reaches ofboth fjords, and is probably introduced from outside. In SaanichInlet interannual variability is relatively low, except forthe fall season, which may differ from year to year in bothvolume and taxonomic composition. Jervis Inlet is both spatiallyand temporally more variable: spring production may differ fromyear to year by an order of magnitude. In this fjord the degreeof stratification, controlled by freshwater influx, appearsto determine the timing and composition of the phytoplanktonassemblage. Large volumes of runoff may suppress the springbloom, and prolonged stratification through the summer producesan unusual assemblage which may be light limited. El Niñoof 1986–1987 had an opposite effect in the two fjords.In Saanich Inlet the unusual degree of sunshine during fallresulted in the largest bloom of the entire interval. Unusuallywarm air temperatures prevented the build-up of winter snowaround Jervis Inlet. As a result, the normal late-spring stabilizationof the water column from snowmelt did not occur, and the springbloom was eliminated.     

Diatom stratigraphy of the late Pleistocene (Brunhes) subarctic Pacific

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 forA. ochotensis from about 625-350 Kyr (Isotope Stages 15-10); a marked increase in amplitude of abundance fluctuations forR. hebetata 300-0 Kyr (Stages 8-1), the extinction ofR. curvirostris at 276 Kyr (Stage 8), and abundance fluctuations ofD. 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.