This study focuses on understanding diagenesis within the sediments from the Grenada Basin, located within the southern Lesser Antilles Volcanic Arc. Sediment in this region include tephra-rich volcanic sands, hemipelagic mud sequences, and carbonate-rich sequences, and vary widely in their proportions over short depth (cm) intervals.
A variety of dissolved constituents and sedimentary solid phases, including dithionite extractable iron and manganese are used here to constrain the diagenetic reactions occurring within the sediment package in the Grenada Basin. Core material was obtained during IODP Expedition 340, and for this study I focus primarily on five sites, U1394, U1395 and U1396 located in the northern region off the coast of Montserrat and U1399 and U1400 in the south, located off the island of Martinique.
Pore fluid chemical compositions reflect heterogeneity in the dominance of reactions occurring within the sediment. Some sites clearly show evidence for alteration of volcanic material as an important diagenetic process while other sites appear to be dominated by carbonate precipitation or dissolution reactions. For example, at sites U1395, U1396, and U1400 increases in Ca mirror decreases in Mg, suggesting that alteration of volcanic matter is the dominant diagenetic reaction occurring. Dissolved minor element distributions (Si, Li, B, Rb) support this notion with Si and Li generally increasing and B and Rb generally decreasing. In contrast, at sites U1394 (upper 100 meters) and U1399, decreases in both Ca and Mg with depth suggest that carbonate precipitation is an important diagenetic process.
Regardless of the main sediment type, organic carbon content is uniformly low with average values of 0.19 ± 0.11% for U1394, 0.13 ± 0.07% for U1395, 0.13 ± 0.06% for U1396, 0.28 ± 0.08% for U1399, and 0.23 ± 0.15% for U1400. Carbonate contents in the north are more variable, ranging between 1 and 80%, than cores in the south, 1 to 40%. These large variations in CaCO3 likely reflects variable dilution with volcanogenic sediment. Reactive Fe ranges from 0.18 to 0.75% in cores in the north and 0.4 to 1.4% in the south whereas, reactive Mn ranges from 0 to 0.09% in the north and 0 to 0.3% in the south. Relationships among reactive manganese, organic carbon, and calcium carbonate suggest that organic material may be associated with reactive Mn and carbonate. Notably, it appears that reactive iron does not appear to be closely associated with organic matter, although the cycling of this metal may play a secondary role in carbon preservation.