Fingerprinting of sediment transport processes in coastal lagoon: An environmental magnetic approach
Abstract
Sediment transport in a coastal lagoon is highly complex and is controlled by various processes (mixing, sorting, erosion, transport, deposition) that govern the distribution of sediments between sources and sinks. In this study, we explore the potential of environmental magnetism in combination with sedimentological methods to magnetically fingerprint sediment transport processes in New Zealand's largest barrier enclosed mesotidal estuarine lagoon. Measurements of bulk magnetic susceptibility and grain sizes of surficial samples collected from various parts of Tauranga Harbour including rivers, the estuary, and nearshore zone helped to identify and differentiate the sedimentary processes in and off the lagoon. The sediments were mainly dominated by variable proportion of titanomagnetite and yield different grain sizes. A general trend (NW - SE) of increasing magnetite concentration and decreasing physical grain sizes indicates the variability in sediment inputs and transport energy of flow. Higher values of SIRM / χ indicate the dominance of fine grained magnetite within riverine sediments. The low enriched fine-grained riverine sediments entering the basin are mostly flushed out to the open sea, while medium-coarse grained magnetite rich sediments gets trapped into the southern lagoonal basin forming enriched zones as inferred from the magnetic data. Within the lagoon, the intense mixing and sorting causes fractionation of heavy (magnetic) minerals which further leads to magnetic enhancement and coarsening of magnetic grain sizes within the tidal channel network of the southern basin. We observed two different patterns in sediment grain sizes. The northern lagoonal basin sediments are dominated by fine sand (~ 200 μm), while the southern basin sediments are composed of mixed grain sizes (300-500 μm). This suggests much calmer hydrodynamics conditions in the northern basin favoured accumulation of fine grained sediment while as outlined above transport is more dynamic in the southern basin. Off Tauranga Harbour, active coast- parallel transport of fine sediments results in formation of nearshore magnetite-rich belt. The fining in magnetic grain size followed by decrease in volume of fine sand mirrors NW directed alongshore sediment transport.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMGP53A1114B
- Keywords:
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- 1512 GEOMAGNETISM AND PALEOMAGNETISM Environmental magnetism;
- 1540 GEOMAGNETISM AND PALEOMAGNETISM Rock and mineral magnetism