Buried nodules from the central Indian Ocean basin

Abstract

The occurrence of buried nodules is rare compared to surface nodule abundance in the deep sea. Surface nodules are collected with freefall grab, vanveen /peterson grabs, dredges and sediment cores whereas buried nodules with exact depth are recovered only by sediment cores. Despite, the slower growth rates of nodules (few mm/Ma) compared to associated sediment (few mm/ka), the phenomena of keeping nodules at the sediment surface still remains an enigma. More than 60 sediment cores (spade and gravity) were collected from the Central Indian Ocean Basin to understand the source of elements and paleoclimatic studies. Twelve sediment cores showed presence of buried nodules at different depths and water depth in these cores varies from 4740 to 5800 m. Maximum of 15 buried nodules were recovered in one core (AAS-22/ GC-07) and maximum depth where buried nodule recovered is at 5.35 m depth below seafloor. In the present study a total of twenty buried nodules from four sediment cores were utilized. Of these, 13 buried nodules are from two sediment cores in siliceous ooze and seven from two sediment cores in a red clay area. The morphology, size, surface texture and chemical composition of buried nodules from two different sediment type have been studied here. The buried nodules are elliptical, elongated, rounded, sub rounded, irregular and polynucleated. Majority of these nodules are approx. 2 cm in diameter and have both smooth and rough surface texture. Manganese and Fe content of nodules from siliceous ooze varies from 18.7 to 29.1%; 1.73 to 14.1% whereas from the red clay they range from 17.4 to 20.9% and 8.1 to 11.1% respectively. The Mn/Fe ratio suggests that nodules from siliceous ooze are hydrogenous, diagenetic and early diagentic in nature where as nodules from red clay area are formed by only hydrogenetic process. Nickel is more enriched compared to Cu in the buried nodules. The factors responsible for the elemental variations and genesis of these buried nodules are discussed.