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U.S. Geological Survey Open-File Report 2010-1155

Terrigenous Sediment Provenance from Geochemical Tracers, South Molokai Reef Flat, Hawaii

By Renee K. Takesue

ABSTRACT

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Land-derived runoff is one of the greatest threats to coral-reef health. Identification of runoff sources is an important step in erosion mitigation efforts. A geochemical sediment provenance study was done in uplands and across the adjacent fringing reef on the southeast shore of Molokai, Hawaii, to determine whether sediment runoff originated from hillsides or gulches. Source-region identification was based on geochemical differences between alkalic basalt, which outcrops on hillsides, and tholeiitic basalt, which outcrops in gulches. In Kawela watershed, copper to iron ratios (Cu/Fe) were distinct in hillside soil versus gulch sediment and suggest that hillside erosion is the predominant mechanism of sediment delivery to the nearshore. This suggests that runoff-mitigation efforts should take steps to reduce hillside erosion. Cadmium to thorium ratios (Cd/Th) in nearshore sediment suggest that there is a high-Cd source of runoff east of Kamalo Gulch. This compositional difference is consistent with the predominance of tholeiitic basalt on the eastern end of Molokai.

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For additional information:
Contact Information, Pacific Coastal and Marine Science Center
U.S. Geological Survey
Pacific Science Center
400 Natural Bridges Drive
Santa Cruz, California 95060
http://walrus.wr.usgs.gov/

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Suggested citation:

Takesue, R.K., 2010, Terrigenous sediment provenance from geochemical tracers, south Molokai reef flat, Hawaii: U.S. Geological Survey Open-File Report 2010-1155, 17 p.



Contents

Abstract

Introduction

Study Goals

Molokai Geology and Climatology

Methods

Results and Discussion

Summary

Acknowledgments

References

two appendixes


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