BENTHIC FORAMINIFERAL FAUNAS REVEAL TRANSPORT DYNAMICS AND NO-ANALOG ENVIRONMENTS ON A GLACIATED MARGIN (GULF OF ALASKA)

Glaciated margins have complex sedimentation histories, which can confound reconstruction of their paleoenvironmental histories. Benthic foraminifera are commonly used in facies analysis and have played a role in reconstructing transport and mixing in sedimentary records. Here we combine modern analog techniques based on 225 modern benthic foraminiferal samples from the Gulf of Alaska with taphonomic and sedimentological analyses to (a) differentiate autochthonous sedimentation from transported sediments (b) evaluate the method of transport and (c) determine if no-analog fossil assemblages are due to no-analog environments or to taphonomic alteration.

IODP Expedition 341 Site U1419 in the Gulf of Alaska is located on the continental slope and experienced periods of sedimentation up to 100’s cm/kyr over the ~60,000-year record. Because it is located in a tectonically active region with episodes of glacial advance and retreat, U1419 may be especially prone to receiving downslope sediment. Intervals of transport must be recognized before paleoecologial and paleoenvironmental interpretations can proceed.

Similarity between fossil and modern assemblages are evaluated using the correlation of species-level rank-abundances. Some downcore benthic foraminiferal assemblages are analogous to faunas at the modern U1419 site, but others are analogous to modern faunas on the upper slope and continental shelf, indicating times of sediment transport. The high fidelity of these transported assemblages to upslope faunas, the lack of evidence for faunal mixing, and the taphonomic similarity of transported and autochthonous assemblages suggest transport as coherent slump blocks.

Assemblages with no-modern analogs are interpreted as autochthonous or allochthonous by their similarity in sedimentary context and faunal composition to assemblages with modern analogs. Preservation quality and lack of size sorting suggest the no-analog faunas represent ecological rather than taphonomic processes and thus likely indicate no-analog environmental conditions. The dominant taxa in these no-analog assemblages suggest episodes of lower-oxygenation and episodes of lower productivity than experienced in the modern Gulf of Alaska. Geochemical proxy data will be necessary to fully interpret these faunas.