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

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 ©2012. American Geophysical Union. All Rights Reserved.1 of 20 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.

Magnetic fingerprinting of hydrodynamic variations and channel erosion across the turbidity maximum zone of the Yangtze Estuary, China

“…7b). Similar selective sorting of magnetic minerals has been observed in coastal beach deposits elsewhere (Hatfield et al, 2010;Holden et al, 2011;Badesab et al, 2012;Gallaway et al, 2012). (Kruiver et al, 2001).…”

“…Due to the shallower water depth of site P8 at mouth bar crest, stronger wave's effects result in frequent resuspension of surface sediments. Other factors being equal, heavy mineral grains tend to settle quickly onto the bed whereas lighter ones are easily entrained and removed (Badesab et al, 2012). Therefore, the sites with shallower water depth are not only marked by coarser particle size but also higher χ and SIRM values.…”

“…This reflects the fact that minerals of similar size may have different specific density. It is found that energetic sorting processes lead to selective entrainment and erosion of lighter quartz minerals and concentration of high density heavy minerals including coarse magnetite, which results in the enhancement of χ and SIRM values in the sediment (Hatfield et al, 2010;Badesab et al, 2012). Optical observation suggests that the N63 μm fraction from site P8, which was sampled from the shallowest water depth in the North Channel, contains more dark minerals and less light minerals (e.g., quartz) than those of P13 from the deepest water depth (Fig.…”

“…In the study of coastal deposits of the Point Pelee National Park, energetic waves and winds lead to the concentration of coarse ferrimagnetic minerals in the beach placer deposits (Hatfield et al, 2010). The selective concentration of magnetic minerals, due to tide-induced residual currents, was also observed in tidal channels of New Zealand (Badesab et al, 2012).…”

“…As a result, the dependence of magnetic properties on particle size has been used to infer sediment sources and hydrodynamics (e.g., waves, currents) in coastal and estuarine environments (Hatfield et al, 2010;Holden et al, 2011;Badesab et al, 2012;Gallaway et al, 2012). In the study of coastal deposits of the Point Pelee National Park, energetic waves and winds lead to the concentration of coarse ferrimagnetic minerals in the beach placer deposits (Hatfield et al, 2010).…”