SeamountFlux: Pore water geochemistry and sediment characteristics (Guatemala Basin, East Pacific)
Abstract
The scientific goal of the project "SeamountFlux" is to study a process, which can possibly contribute significantly to large scale cooling of the oceanic crust. The focus of the study is to investigate the exchange of matter and energy between the ocean and the upper young oceanic crust in the vicinity of unexplained circular depressions in the sedimentary cover, which are widely spread in the equatorial Pacific. A possible model for the formation of these so-called "hydrothermal pits" is the dissolution of calcium carbonate (CaCO3) minerals by upward flowing warm fluids, which are under-saturated in CaCO3. Seamounts are suspected to act as an entrance point for cold seawater, which then equilibrates with the basaltic oceanic crust prior to exiting through the pits. Such a process should affect the thermal structure in and around the pits as well as the geochemistry of the pore water. During cruise SO207 (June/July 2010 on the German RV Sonne) in the vicinity of ODP/IODP Site 1256 and ODP site 844 (Guatemala Basin, eastern equatorial Pacific) we collected 24 gravity cores varying in length between 3.3 m and 10.6 m from 3 distinct working areas. The cores were generally taken along a transect, starting in the center of a pit, across the pit slope to the surrounding seafloor. From these cores a total of 451 pore water samples were collected and analyzed onboard (pH, Eh, Fe, NH4 and alkalinity), as well as in the laboratory (Cl, SO4, Na, Mg, K, Ca, Sr and Mn). First results do not show any obvious evidence for the "hydrothermal" hypotheses. Pore water profiles were more or less identical, whether collected from within or from outside a pit. Chloride, SO4, Na, Mg, K, Ca, and Sr concentrations were identical to those in seawater. Particularly higher Ca and Sr concentrations were to be expected if calcium carbonate dissolution took place within the sediments. With respect to sediment composition and physico-chemical conditions in the pore waters, we observed larger differences between the 3 working areas than between the inside and outside samples. However, there is indirect evidence that the sediments inside the pits show a higher degree of compaction. With one exception we found that those gravity cores collected from inside the pits were shorter, which would indicate a harder, more compacted sediment. A simple mass balance calculation, using the maximum degree of calcium carbonate under saturation, in conjunction with measured pore water composition and observed calcium carbonate content in the sediments was carried out. Results from this calculation indicate that a minimum 6 Ma of constant fluid circulation would have been necessary to cause the removal of sufficient calcium carbonate to account for the average pit depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T31A2315P
- Keywords:
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- 1033 GEOCHEMISTRY / Intra-plate processes;
- 1050 GEOCHEMISTRY / Marine geochemistry;
- 1051 GEOCHEMISTRY / Sedimentary geochemistry;
- 8130 TECTONOPHYSICS / Heat generation and transport