Formation of magnetite-enriched zones in and offshore of a mesotidal estuarine lagoon: An environmental magnetic study of Tauranga Harbour and Bay of Plenty, New Zealand

Authors

  • Firoz Badesab,

    1. MARUM-Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Klagenfurter Strasse, DE-28359 Bremen, Germany
    2. Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand
    Search for more papers by this author
  • Tilo von Dobeneck,

    1. MARUM-Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Klagenfurter Strasse, DE-28359 Bremen, Germany
    Search for more papers by this author
  • Karin R. Bryan,

    1. Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand
    Search for more papers by this author
  • Hendrik Müller,

    1. MARUM-Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Klagenfurter Strasse, DE-28359 Bremen, Germany
    Search for more papers by this author
  • Roger M. Briggs,

    1. Department of Earth and Ocean Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand
    Search for more papers by this author
  • Thomas Frederichs,

    1. MARUM-Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Klagenfurter Strasse, DE-28359 Bremen, Germany
    Search for more papers by this author
  • Eva Kwoll

    1. MARUM-Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Klagenfurter Strasse, DE-28359 Bremen, Germany
    Search for more papers by this author

Abstract

[1] Magnetic iron minerals are widespread and indicative sediment constituents in estuarine, coastal and shelf systems. We combine environmental magnetic, sedimentological and numerical methods to identify magnetite-enriched placer-like zones in a complex coastal system and delineate their formation mechanisms. Magnetic susceptibility and remanence measurements on 245 surficial sediment samples collected in and around Tauranga Harbour, the largest barrier-enclosed tidal estuary of New Zealand, reveal several discrete enrichment zones controlled by local hydrodynamic conditions. Active magnetite enrichment takes place in tidal channels, which feed into two coast-parallel nearshore magnetite-enriched belts centered at water depths of 6–10 m and 10–20 m. A close correlation between magnetite content and magnetic grain size was found, where higher susceptibility values are associated within coarser magnetic crystal sizes. Two key mechanisms for magnetite enrichment are identified. First, tide-induced residual currents primarily enable magnetite enrichment within the estuarine channel network. A coast-parallel, fine sand magnetite enrichment belt in water depths of less than 10 m along the barrier island has a strong decrease in magnetite content away from the southern tidal inlet and is apparently related to active coast-parallel transport combined with mobilizing surf zone processes. A second, less pronounced, but more uniform magnetite enrichment belt at 10–20 m water depth is composed of non-mobile, medium-coarse-grained relict sands, which have been reworked during post-glacial sea level transgression. We demonstrate the potential of magnetic methods to reveal and differentiate coastal magnetite enrichment patterns and investigate their formative mechanisms.

Ancillary