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2008-2010 Research Projects
A Pilot Study to Investigate the Possible Linkages Between Submarine Groundwater Fluxes and Amphitheater-shaped Scarps on the Northern Insular Margin of Puerto Rico
Dr. Nancy Grindlay is the PI for R-122-1-04. The project was extended due to problems early on during the course of her project. The initial marine field research was originally scheduled to be conducted aboard the R/V Chapman, owned and operated by the University of Puerto Mayagüez, in late January 2005. However, in mid-December of 2004, Grindlay was informed that the R/V Chapman did not have the necessary length of CTD cable for deep-water sampling and would not be available for her field program. She was able to reschedule the cruises aboard the UNOLS vessel R/V Pelican operated by Louisiana University Marine Consortium.
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 this investigation were two-fold: 1) To determine the composition and measure the flux of fluids along the northern insular margin of Puerto Rico and 2) To conduct high-resolution geological mapping of the amphitheater-shaped scarp, upper platform and debris slopes using a towed camera system.
The first, objective was 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 226Ra 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 the investigators have measured elsewhere.
The second, objective was also met. In September 2006, the PI 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 crescent-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 crescent-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 1800m to 3500m were done during the cruise.
The Study made advances in the field as it provided first time, in situ, measurement of the activities of radium isotopes offshore northern Puerto Rico and exposed that they exceed background oceanic values, indicating 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 coast of Puerto Rico. A deep-towed camera system was used to visually document seep sites on the margin. Digital still images documented white halos, presumably carbonate deposits associated with seeps, around a string of E-W trending fissures at the crown of an amphitheater-shaped scarp where clear radium excess was measured. The TOWCAM system measured near-bottom turbidity and temperature anomalies associated with these features.
This study is a necessary first step toward evaluating the groundwater contribution to the coastal waters off northern Puerto Rico. This is an important and practical issue that needs to be addressed in order to quantify the amount of potential pollutants (e.g. excess nutrients) that contaminate coastal waters. If on-land springs and rivers are the sources of excess radium offshore northern Puerto Rico, very large volumes of SGD are required to balance the coastal signal. Therefore effective management of runoff and ground water quality onshore is imperative given the potentially significant impact on the coast ocean environment.
Two MS students were supported by this study. Both students graduated and are now working;
Meghan Hearne – University of North Carolina Wilmington Geology MS, Investigation of submarine landslides along the northern Puerto Rico-Virgin Islands margin, p.87, 2004., graduated August, 2004, now at Mindo and Moore, San Diego, CA, meggiehearne@yahoo.com
Steve Mondziel – University of North Carolina Wilmington Geology MS, Morphology, structure and tectonic evolution of Mona Canyon, Puerto Rico, p. 93, 2007, graduated Dec. 2007, now at Hess Corporation, Houston, TX, smondziel@yahoo.com
Publications
Grindlay, N.R., M. Hearne, and P. Mann, A High Risk of Tsunami in the Northern Caribbean, Eos Transactions. American Geophysical Union, 86(12), 121, 2005.
Planned publications
Hearne, M., N. R. Grindlay, and P. Mann, Recurrent submarine landsliding offshore northern Puerto Rico, Marine Geology, in revision.
Hornbach, M., Mondziel, S., Grindlay, N.R., P. Mann, and Frolich, C., Did a submarine slide trigger the great 1918 Puerto Rico tsunami? Science of Tsunami Hazards, in review.
Grindlay, N.R. and Moore, W.S. ,Evidence for deep advective fluid flow on the northern Puerto Rico margin, in prep. Geochemistry, Geophysics, Geosystems, Anticipated submission July 2008
Grindlay, N.R., Mondziel, S., Mann, P., Escolona, A and Abrams, L., Cretaceous-recent extension of the northern Puerto Rico Margin: Tectonic evolution of the Mona Canyon, in prep. Marine Geology, Anticipated submission August 2008.
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