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IC8_I

Magnetic minerals as markers of coastal zone evolution

State of the art
Magnetic minerals are abundant in estuarine, coastal and shelf environments. A number of environmental processes imprint specific changes onto the composition, grain size and concentration of the magnetic mineral assemblage, which can be quantified at high sensitivity. Sedimentary magnetic minerals thus have an excellent potential as markers of sediment provenance, dynamics, diagenesis and pollution in coastal and shelf environments. “Environmental Magnetism” has been used in coastal studies worldwide (Oldfield et al., 1985; Foster et al., 1991; Lees and Pethick, 1995; Zhang et al., 2001), e.g. to map the spatial distribution and composition of magnetic minerals in coastal zones, to assess their degree of sorting, and to identify the areas of sediment accumulation and erosion (Cioppa et al., 2010; Hatfield et al., 2010).
Magnetite as the most common magnetic iron-oxide mineral is generally associated with other heavy minerals of higher economic value in placer deposits. Gravitational separation of light and heavy minerals by various sorting mechanisms tends to concentrate the latter as lag deposits (Slingerland and Smith, 1986) by selective entrainment processes (Slingerland, 1977; 1986; Komar, 1987; Li and Komar, 1992). Gallaway et al. (2012) highlighted the additional role of burial mechanism in formation of magnetite enrichment in swash zones. Magnetite enrichments and placers are frequently observed at New Zealand’s west coast high-energy beaches (Bryan et al., 2007) and littoral drift belts.

Project aims
The project set out to develop and test the potential of Environmental Magnetism in a complex coastal setting with relatively well-constrained and well-known environmental conditions using enviromagnetic and sedimentological analytics in combination with numerical models. The estuarine lagoon of Tauranga Harbour and inner Bay of Plenty, an area with relatively low natural concentrations of magnetic minerals sourced from the proximal Taupo Volcanic Zone (TVZ), was chosen as a promising study site to study:
  1. the role of river input, tidal exchange, near-shore hydrodynamics and sea level change for heavy mineral enrichment patterns in estuarine lagoons and inner shelves
  2. the transport dynamics of heavy/magnetic minerals mechanisms including gravitational settling, entrainment and burial in various coastal depositional environments
Results
Surficial sediment samples were collected during two separate field campaigns (February 2010 and June 2011) within and around Tauranga Harbour, covering the downstream sections of tributary rivers, the major tidal flats and channels in the western and eastern parts of the lagoon, and 5 cross-shore profiles (0-20 m water depth) off Waihi, western, central and eastern Matakana Island and Omanu beaches.
Within the estuary, ebb-dominated tidal channels such as Western and Cutter channel act as high-energy depositional sites for magnetic minerals. As most striking large-scale feature, the magnetic susceptibility measurements revealed the presence of two coast-parallel magnetite-enriched belts along the inner Bay of Plenty shoreline. Their formative mechanism was derived from cross- and along-shore trends of magnetic enrichment and grain-size with the aid of hydrodynamic modelling: The inner belt (6-10 m depth) departs from Tauranga Entrance and is actively formed by coast-parallel drift and wave motion, while the outer belt (10-20 m depth) is composed of relict sands, which were reworked during post-glacial sea level transgression (Badesab et al., 2012). The strong positive correlation of magnetic susceptibility and magnetic grain size indicates a linkage of magnetite enrichment and selective entrainment identifying magnetite placers as (permanent or at least temporary) lag deposits.
In order to gain deeper insights into the transport dynamics of heavy mineral grains, we determined bulk and magnetite grain-size distribution curve for a collection of samples from different depositional environments in the Bay of Plenty region. Shapes and relative offsets of these distribution curves reflect the specific transport and enrichment conditions in each environment. Numerical model results showed that the rate of light mineral erosion increases with heavy mineral concentration enhancing heavy mineral enrichment. In beach environments (e.g., Matakana Beach) with equivalent grain sizes of light and heavy minerals, the bed is more stable with negligible amount of erosion compared to other bed compositions.
To investigate the factors controlling the sediment distribution and transport within the lagoonal area, the representative (riverine and estuarine) sediments within the northern and southern basins of Tauranga Harbour were characterized using magnetic, sedimentological and petrological measurements. Data showed that the variability in terrigenous sediment inputs, distinct local hydrodynamics in both basins and regional topography are the major factors controlling the sediment distribution and transport in the study area.

Members

Proponents:Prof. Dr. Tilo von DobeneckUniversity of Bremen
Dr. Hendrik Müller
Dr. Thomas Frederichs
:Prof. Dr. Karin BryanUniversity of Waikato
Prof. Dr. Roger Briggs
PhD Candidate:Firoz BadesabUniversity of Bremen

Publications

Badesab, F.K., T. von Dobeneck, K.R. Bryan, H. Müller, R. Briggs, T. Frederichs, and E. Kwoll (2012), 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, Geochem. Geophys. Geosyst., v 13, no 6, doi:10.1029/2012GC004125.

Miscellaneous

Research stays:

10.2011 - 02.2012 University of Waikato, Hamilton
02. - 10.10.2011 University of Lancaster, UK
02.2010 - 04.2010 University of Waikato, Hamilton