Origin of Brines in Neogene Sediments of the Ross Sea, Antarctica: And-2a Core, Andrill Southern Mcmurdo Sound Project

Gui, Zi ¹; Frank, Tracy D.^1
1 Geosciences, University of Nebraska-Lincoln, Lincoln, NE.

Brines with salinities that approach six times sea water salinity were encountered in Neogene sediments of the Ross Sea recovered during the ANDRILL Southern McMurdo Sound Project. This study integrates geochemical and isotopic data from pore waters with petrographic, C-O isotopic, and clay mineralogical data from sediment samples to investigate the origin of these unusual pore waters. During drilling, 35 pore water samples were extracted from whole-round sections of core collected from 9.67 to 545.01 meters below sea floor (mbsf) and subjected to the following analyses: alkalinity, pH, δ¹⁸O and δ¹³C_DIC values, and major ion concentrations (Cl, Br, Sulfate, Na, K, Mg, and Ca). Sediment samples were collected every 10 to 20m for petrographic examination, XRD determination of clay mineralogy, and stable isotopic analysis of carbonate phases. Alkalinity, salinity and the concentrations of most major ions increase from near-seawater values near the top of the core to values that are several fold higher near the base. With the exception of sulfate, all ions that behave conservatively in seawater maintain a constant relationship with Cl. These patterns suggest that brines evolved from seawater via a process that removes H₂O while leaving salts behind. Petrographic observations suggest that the most plausible explanation involves chemical exchange between pore fluids and reactive volcanogenic particles, which make up a significant proportion of the sediment. The major processes include uptake of H₂O during hydration of volcanic glass and the formation of hydrous secondary minerals such as zeolite, layered silicates, and chlorite. A relatively high geothermal gradient (~50°C/km) may be enhancing reaction rates in the AND-2A core. Petrographic observations and XRD analyses confirm that these secondary phases are present throughout much of the section. Chemical exchange between pore water and silicate minerals is reflected in δ¹⁸O values of pore waters, which decrease with increasing depth. δ¹³C_DIC values as low as -24‰ suggest that organic matter degradation is also occurring. Stable isotopic analyses of carbonate cements will provide additional insight into diagenetic processes. Results indicate that seawater-rock exchange in volcanic-rich marine sediments can produce brines at relatively low temperatures and over geologically short time intervals.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009