Abstract:
Laminated sediments from Saanich Inlet, an anoxic fjord in southeastern Vancouver Island, British Columbia constitute a unique paleoenvironmental archive. Integrating core description, X-radiography, secondary-electron and backscattered-electron microanalysis, these sediments provide unique insights into sedimentary processes and paleoenvironmental evolution. In particular, the content and taphonomy of siliceous microfossil communities were assessed. Diatom blooms are conspicuous in these sediments and contributed to the formation of distinct sedimentary laminae. Our data reveal that the diatom, Skeletonema costatum was ubiquitous throughout our samples; monospecific assemblages of S. costatum also occur. Aggregation is a prerequisite to diatom sedimentation. Modern S. costatum has a high stickiness coefficient, which aids in the formation of large aggregates, facilitating rapid sinking of opal and organic carbon from the euphotic zone to the sediments. In addition, high stickiness may render a competitive advantage to Skeletonema, since aggregate formation may entrap competitor diatom species and larger zooplankton species, thus removing these obstacles to maximum growth from the photic zone (Hansen et al., 1995). Once removed from the photic zone, Skeletonema is able to dominate due to opportunistic growth dynamics. High stickiness of modern Skeletonema and consequent formation of rapidly-settling aggregates is comparable to biologically-mediated aggregation ("self-sedimentation" of Grimm et al., 1997) evidenced by the planktic diatom Chaetoceros. Monospecific assemblages of pristine Dichtyocha speculum also occur that are attributed to similar biologically-mediated flux mechanisms. We suggest that biologically-mediated aggregation constitutes an efficient flux mechanism, that bypasses trophic recycling to deliver pristine opal and organic carbon to the sediments. In addition, diatom self-sedimentation may have important ecological and evolutionary implications. The formation of S. costatum aggregates and the removal of competitor phytoplankton species ("kamikaze" event) apparently enabled S. costatum to dominate some phytoplankton assemblages. Thus, the ubiquity of Skeletonema and Dichtyocha and their prominence in some laminae throughout the Saanich ODP cores provides insight into sedimentary processes over broad geological spans, thus linking organism-scale biology to Earth's environmental evolution.