Executive Summary Project title: A Pilot Study to Investigate the Possible Linkages Between Submarine Groundwater Fluxes and Amphitheater-Shaped Scarps on the Northern Insular Margin of Puerto Rico Date: May 20, 2008 Project Number: #R-122-1-04 Investigators and affiliation: Principal Investigator: Nancy R. Grindlay, Center for Marine Science, University of North Carolina Wilmington Co-Principal Investigator: Willard S. Moore, Department of Geological Sciences, University of South Carolina Dates Covered: March 1, 2004 - February 28, 2008 Summary of Impacts and Contributions including: Objectives: This was an interdisciplinary pilot study to investigate the geological and chemical aspects of a unique deep-sea habitat on the northern margin of the island of Puerto Rico. The objectives of our investigation were two-fold: 1) To determine the composition and measure the flux of fluids along the northern insular margin of Puerto Rico. This objective has been met. A 2005 sampling program revealed excess activity of four naturally occurring radium isotopes 223Ra and 224Ra (half lives of 11 and 3.6 days, respectively), and 226Ra and 228Ra (half lives of 5.7 and 1600 yrs, respectively) in deep and shallow offshore waters, and rivers and springs of northern Puerto Rico. Water samples from depths of 3117 and 3469 m at one station within the Arecibo Amphitheater, a giant (55 km-wide) curved-shaped scarp carved out of a northerly dipping carbonate platform on the northern Puerto Rico margin, had clear excess 223Ra and 228Ra. The 228Ra activity at 3117 m was about an order of magnitude higher than samples collected at similar depths a few 100 kms north of Puerto Rico. The 226Ra in this sample was about 20% higher than expected. The combination of strong signals of both long and short-lived isotopes made this our best case for recent radium additions due to deep fluid flow. Additional deep samples from other stations also had clear excess signals of some or all radium isotopes. Surface water samples collected nearest the shore have excess activities of both these isotopes. The surface samples are, with one exception, enriched in 226 Ra and 228Ra compared to tropical Atlantic water. This indicates radium additions by SGD are widespread offshore the northern coast of Puerto Rico. However, analyses of coastal springs, seeps, and rivers along the north coast show relatively low activities of 223Ra and 224Ra that are similar to low salinity samples we have measured elsewhere. 2) To conduct high-resolution geological mapping of the amphitheater-shaped scarp, upper platform and debris slopes using a towed camera system. This objective was met. In September 2006 we conducted a five-day cruise that included the use of a deep-towed camera system (TOWCAM) to visually document seep sites on the Puerto Rico margin. The TOWCAM was equipped with a CTD to measure bottom temperature, salinity and turbidity during the tows, as well as three 10-liter Niskin bottles for water collection. Target areas of TOWCAM deployments were chosen on the basis of areas of excess radium activity identified in the 2005 cruise. These areas included a 20-30 km-long crescentic-shaped fault observed in the carbonate platform on the eastern flank of the Mona Rift, the headwall and crown of the Arecibo Amphitheater, and a crescentic-shaped fault about 10 km landward of the crown of the amphitheater. A total of seven camera tows and eight CTD casts and water sampling stations at depths ranging from 1800 m to 3500 m where done during the cruise. Advancement of the Field: We have measured activities of radium isotopes offshore northern Puerto Rico that exceed background oceanic values and indicate local additions of radium. Such additions are likely due to fluids passing through rocks or sediments. The source of these fluids may be submarine groundwater discharge (SGD) through shallow permeable sediments or discharge through deep formations (cold seeps). Excess activity of the short-lived isotopes 223Ra and 224Ra indicate recent additions of radium to the water. Surface water samples collected nearest the shore have excess activities of both these isotopes. The surface samples are, with one exception, enriched in 226Ra and 228Ra compared to tropical Atlantic water. This indicates radium additions by SGD are widespread offshore the northern
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