Pore Water Constraints on the Competition Between Carbonate and Tephra Diagenesis in Sediments from the Lesser Antilles Volcanic Arc: Implications for Carbon Preservation
Abstract
We present sediment pore fluid results obtained during IODP Expedition 340, from five sites located within the Grenada Basin of the southern Lesser Antilles Volcanic Arc region. In addition to reactions related to organic matter diagenesis, deep pore fluid chemistry indicates that the diagenetic reactions fall under two broad categories; (1) reactions related to material exchange with volcanogenic material and (2) reactions related to carbonate dissolution, precipitation, or recrystallization. For example, portions of the sedimentary sequences at sites U1395, U1396, and U1400 show increases in Ca with coeval decreases in Mg, suggesting that exchange reactions from the alteration of volcanic material are occurring within these sites. At Sites U1394 (upper 100 meters) and U1399, decreases in both Ca and Mg with depth imply that Ca-Mg carbonate precipitation is a dominant diagenetic process at these sites, and for selected samples, X-ray diffraction results confirm the presence of this phase. Mn also covaries with Ca and Mg at these sites suggesting the likelihood of rhodochrosite formation, although this phase, if present, is likely to be a trace component of the sediment package. 87Sr /86Sr of the carbonate fraction from Site U1396 is generally consistent with the contemporaneous seawater isotope value. The 87Sr /86Sr of the non-carbonate fraction, however, ranges from 0.7074 to 0.7052, and these values likely represent a mixture of local arc volcanic sources and trans-Atlantic eolian sources. At all sites, pore fluid 87Sr /86Sr reflects the competition between diagenesis of volcanogenic material and marine carbonates. The high carbonate background effectively buffers pore fluid 87Sr /86Sr from the much larger changes in dissolved 87Sr /86Sr that might otherwise be expected given the high abundance of tephra in these sediments. These data coupled with solid phase data suggest that, despite the abundance of tephra, organic carbon preservation is primarily tied to carbonate burial with a potential secondary contribution from a Mn-rich phase.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP14B..06M
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1051 Sedimentary geochemistry;
- GEOCHEMISTRYDE: 4914 Continental climate records;
- PALEOCEANOGRAPHYDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY