Stratigraphic variation in ichnofabrics at the “Shackleton Site” (IODP Site U1385) on the Iberian Margin: Paleoenvironmental implications

Abstract

Ichnofabric analysis was conducted on cores from Site U1385 (IODP Expedition 339) to interpret paleoenvironmental conditions at the southwest margin of the Iberian Peninsula during the Pleistocene. Site U1385 provides an important record for the study of orbital and suborbital-scale climate variability, changes related to ocean/atmosphere dynamics, and affected environmental parameters. A detailed study of the ichnofabric characterization is presented, focusing on the types of ichnofabrics, relative abundance, amount of bioturbation, grouping, ichnofabric succession/transitions and vertical distribution. Seven ichnofabrics were differentiated; green mottled ichnofabric, Planolites ichnofabric, Taenidium &Planolites ichnofabric, Thalassinoides-like &Palaeophycus ichnofabric, Planolites &Thalassinoides/Thalassinoides-like ichnofabric, Zoophycos ichnofabric, and Chondrites ichnofabric. They exhibit significant differences in terms of ichnofabric features as well as in their stratigraphic distribution. The most abundant ichnofabrics are Planolites &Thalassinoides/Thalassinoides-like ichnofabric and green mottled ichnofabrics, with a dominance of the middle tier ichnofabrics. The degree of bioturbation is moderate, with a mean BI around 3, but showing a clear bimodal distribution: one group of intervals is characterized by high bioturbation (BI=6), while another displays low-moderate BI values (BI=1–2). In general, ichnofabric features confirm generally good environmental conditions (oxic environment and high food availability) for the macrobenthic tracemaker community, especially favorable in the uppermost part of the sediment. A complete ichnofabric succession, from deep to shallow tier ichnofabrics, is commonly registered (i.e., Zoophycos ichnofabric or Chondrites ichnofabric to Planolites &Thalassinoides/Thalassinoides-like ichnofabric, and then either to green mottled ichnofabric, or to Planolites ichnofabric and then green mottled ichnofabric). However, there are some intervals where an incomplete succession between deep and shallow ichnofabrics is observed (i.e., Chondrites ichnofabric to green mottled ichnofabric or Zoophycos ichnofabric to green mottled ichnofabric), indicating relevant modifications of environmental parameters such as oxygen or food supply. Types of ichnofabrics and the Bioturbation Index show significant short-term changes through the studied core. Such variations are probably correlated to millennial-scale climatic perturbations, such as glacial terminations and related phenomena (Heinrich events, ice-rafting events, etc.), or long-term cyclic patterns related to orbital climate variability. The patterns could be tied to a significant change in the climate system, such as the one associated with the middle Pleistocene transition.