"Biogenic magnetite from surface holocene carbonate sediments,Great Bahama Bank""

Abstract

Magnetic separates and rock magnetic tests from Holocene carbonate surface sediments of the Bahamas indicate that fine‐grained magnetite/maghemite is common at or near the sediment/water interface and forms a major portion of the magnetic fraction. Magnetite crystals range from 400 to 1000 Å in diameter as observed under the transmission electron microscope (TEM) and often occur as elongate chains or multi‐grain clusters. Crystals are very similar in dimension and grain habit to those of known biogenic origin, mainly bacteria. All measured grains are within the single‐domain stability field for magnetite based on crystal dimensions. Coercivity distributions are fairly broad (5–100 mT or greater), likely the result of partial oxidation of the magnetic minerals. No large multidomain grains were observed in the TEM study. Individual grains occur in several shapes in the microscope image, mainly hexagonal and cuboidal/prismatic. Widespread occurrence of single‐domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization. A contribution of magnetite from authigenic precipitation during burial has not been documented in these near‐surface environments.