Latitudinal (Climatic) Controls on Neoichnological Assemblages of Modern Marginal-Marine Depositional Environments
Murray Gingras, Shahin E. Dashtgard, and George Pemberton
University of Alberta, Edmonton, AB
Neoichnological trace assemblages developed at high (temperate) latitudes differ from low-latitude (tropical) assemblages as a result of significant changes in physical and chemical conditions between seasons in temperate settings. Consequently, the infauna encountered in intertidal and shallow-subtidal environments varies from tropical to temperate climatic zones. Animal indicators of temperate conditions include large, deep-burrowing bivalves, such as Panopea generosa and Tresus nutalli, which are increasingly common towards temperate latitudes. Terrebellid polychaetes – a known producer of Rosselia-like structures – are also abundant in temperate settings. Some species of Thalassinid shrimp – at least species known to produce lined domiciles akin to Ophiomorpha— are most common in semi temperate to tropical climates. Finally, Opheliid polychaetes, which produce Macaronichnus in modern settings, are common in temperate-zone foreshores. These and similar animal distributions demonstrate that a latitudinally sensitive distribution of traces (or trace fossils) is resident in the rock record.
In a majority of marginal-marine depositional environments, the intertidal zone is the most (ichnologically) sensitive to extreme climate. In semitemperate settings, tidal-flat deposits are normally highly burrowed with a range of moderately sized trace-making organisms (Tillamook Bay, OR, Cape Beal, BC, Willapa Bay, WS). However, tidal flats that are exposed to regular winter freeze (Bay of Fundy and Kouchibouguac, NB) have a much lower diversity, smaller trace makers, and are consistently in a state of animal colonization. The ichnological signature, when combined with the sedimentological signatures of ice rafting, ice-gouging and ice crystal formation produce a depositional model that is very different from warmer settings.