OVERCOMING PITFALLS IN INTERPRETING PALEOENVIRONMENTS FROM GLACIATED MARGINS: EXAMPLES FROM RECORDS OBTAINED BY IODP EXPEDITION 341 IN THE GULF OF ALASKA

Integrated Ocean Drilling Program Expedition 341 drilled a transect from the Bering Trough to the distal Surveyor Fan to record Neogene tectonic and climate events on the southern Alaska margin. This record captures distal and proximal advances and retreats of Bering Ice Stream (BIS), which is an extension of the northwestern Cordilleran Ice Sheet. While these sites have yielded a better understanding of glacial history, several obstacles to interpreting paleoenvironments must be overcome.

First, limited core recovery, especially at the shelf (U1420 – 14%) and slope (U1421 – 23%) sites, occurred because of the drilled lithology – washed clasts, drilled rocks, gravels, and clast-rich diamicts. To overcome this, we integrated the sedimentological, paleontological, and seismic record of intervals with good recovery, which correspond to mud and muddy diamicts and parallel, high-amplitude, continuous reflections. Well-recovered intervals are muds indicative of glacial distal or open ocean conditions and muddy diamicts have varying amounts of ice-rafted debris (IRD) that reflect retreat facies. Poor recovery intervals consist of chaotic and muted seismic reflections on the shelf and discontinuous reflections on the slope, representing reworked advance/retreat sediments and sediment gravity flow deposits during advance, respectively.

Second, caution must be taken when using paleoenvironmental models where there are no modern analogs (i.e. temperate glaciers cutting across a shelf into an open ocean basin). Sedimentary models developed in places that are not influenced by wet-based glaciers (parts of Antarctica and Greenland) should not be applied to coastal Alaskan glaciers. While there are a few studies of living foraminifera near the BIS, none of the sites is under the influence of the high volume of sediment and meltwater delivery produced by BIS is abundant in muddy diamicts. Interpretation based on modern models alone of muddy diamicts containing Elphidium excavatum, a common high-latitude, inner-shelf taxon, would suggest downslope transport; however, no sedimentologic evidence exists to support gravity flows in many of the muddy diamicts. We interpret E. excavatum in intervals with high IRD as being indicative of high meltwater production and a proximal glacier, irrespective of bathymetry.