Bioturbation in an active deep-sea area: Implications for models of trace fossil tiering

The Nova Scotian continental rise below 4750 m water depth is subject to in termittently strong currents (>30 cm/s) a few times a year in deep-sea stor ms. These erode and rapidly deposit sediment on the bed, with deep erosion (>5 cm) occurring in an annual to decadal time scale. X-radiographs of subs amples from box cores taken from this area exhibit a repetitive organizatio n of primary and biogenic structures. Four tiers are recognized: 1, 2, and 3a and b. Tiers I and 2 are produced by ichnoguilds dominated by "Mycellia" B and Protopalaeodictyon, and resemble the archetypical Nereites ichnofaci es. Tier 3 is dominated by guilds comprising members of the "Scolicia group " (especially Bichordites) with an overprint of Planolites and Chondrites. However Tier 3 exhibits a vertical gradient of potential trace fossil prese rvation from 3a to 35. At depth (35), the tier becomes thoroughly bioturbat ed and finally forms the 'historical layer' of other authors. At the base o f this layer some split box cores revealed Thalassinoides 20-25 cm down. Ho wever no vertical shafts were seen linking these occurrences to the modern sea floor, and they are thought to be part of a fossil ichnocoenosis belong ing to an early post-glacial colonization of the sea bed. The extreme short -term rates of erosion and deposition in this area, the latter being up to 2.7 cm per year, result in a very low probability of preservation of Tiers 1 and 2. The material providing the long-term accumulation rate of 5 cm/ky is dominated by Tier 3. Thus only one tier would be preserved in lithified deposits. The record of intense biological and sedimentary activity seen in x-radiographs of Tiers 1 and 2 is not preserved. The accumulating sediment is ichnologically biased. This slowly accumulating sediment column is stra tigraphically "incomplete" at the I-year level required to preserve a recor d of benthic storm events, but is probably "complete" at the 100-year scale .