Ichnological analysis of pleistocene sediments from the IODP site U1385 “Shackleton Site” on the Iberian margin: approaching paleoenvironmental conditions

Abstract

Ichnological analysis, focused on trace fossil assemblages and an ichnofabric approach, with special attention to cross-cutting relationships, tiering, relative abundances and bioturbation degrees, has been used to assess environmental parameters affecting the Pleistocene macrobenthic tracemaker community at the IODP Expedition 339, Site U1385 “Shackleton Site” on the Iberian margin. The trace fossil assemblage consists of abundant Planolites, frequent yet sparsely distributed Palaeophycus, Thalassinoides (and Thalassinoides-like structures), and Taenidium, and localized Zoophycos and Chondrites. Other ichnotaxa, such as Phycosiphon and ?Scolicia, are rare. This assemblage is typical of the Zoophycos ichnofacies, though the distal expression of the Cruziana ichnofacies has a similar composition. Ichnofabrics reveal variable substrate colonization, with well defined cross-cutting relationships and tiering distribution. Differentiated ichnofabrics are: green mottled ichnofabric, Planolites ichnofabric, Taenidium and Planolites ichnofabric, Thalassinoides-like and Palaeophycus ichnofabric, Planolites and Thalassinoides/Thalassinoides-like ichnofabric, Zoophycos ichnofabric, and Chondrites ichnofabric, usually showing gradual transitions. A multi-tiered assemblage can be envisaged, with differentiation of the shallowest (biodeformational structures), shallow (Planolites, Palaeophycus and even Taenidium), middle (Thalassinoides/Thalassinoides-like structures), and lower (Zoophycos and Chondrites) tiers. According to the ichnological data, a general context of good bottom and pore-water oxygen conditions and organic matter availability can be interpreted, but localized dysaerobic intervals might be related with the record of Zoophycos and Chondrites. A constant rate of sedimentation shows only minor variations as revealed by ichnofabric succession. Soupy, soft, and stiffgrounds are interpreted as inducing changes in ichnological features. Salinity and temperature have a minor incidence on the macrobenthic tracemaker community, causing only small changes in the trace fossils.