Abstract:

This dataset contains measurements of radium isotopes (224Ra and 226Ra) in water column samples collected aboard R/V Point Sur cruise PS19_05 in the Gulf of Mexico from 2018-09-01 to 2018-09-04. The dataset also includes the location, date, time, and depth of the sample collection. Water samples were collected from Niskin bottles during CTD casts both at the Megaplume site and in a transect extending both up current and down current of the hydrocarbon seep. Samples were collected at discrete depths through the water column at locations atop the hydrocarbon seep (‘At plume’), 1 km upcurrent of the plume, and 1,2,3 km downcurrent of the plume. Sample volumes varied between 10.7 and 121.9 L. Sample volumes were passed slowly over acrylic fibers impregnated with MnO2 to quantitatively sorb dissolved radium isotopes, then counted immediately on a Radium Delayed Coincidence Counter for total 224Ra (produced by Scientific Computer Instruments) and subsequently three weeks later for supported 224Ra. The difference between these measurements represents the excess 224Ra, serving as our tracer of hydrocarbons in the marine environment. The samples were later analyzed for 226Ra on a radon emanation line. The cruise documentation was provided for the R/V Point Sur cruise PS19_05, led by chief scientist Dr. Richard Peterson.

Suggested Citation:

Peterson, Richard. 2019. Measurements of radium isotopes (224Ra and 226Ra) in water column samples collected aboard R/V Point Sur cruise PS19_05 in the Gulf of Mexico from 2018-09-01 to 2018-09-04. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/n7-fz60-kk13

Purpose:

The purpose of this dataset is to examine the vertical and lateral distribution of a short- (224Ra) and long-lived (226Ra) radium isotope in and around an active hydrocarbon seeps in the Gulf of Mexico. Our project hypothesis is that radium is released to the surrounding water as a function of oil presence, so this dataset is exploring field evidence in support of that hypothesis.

Data Parameters and Units:

Sample Location Description (no units) -- description of sampling location with respect to the hydrocarbon seep Latitude (degrees, decimal minutes) -- sampling location Longitude (degrees, decimal minutes) -- sampling longitude Coll. Date (EST, MM/DD/YY) -- the date of sample collection in eastern standard time Coll. Time (EST, HH:MM) -- the time of sample collection in eastern standard time Depth (m) -- sampling depth in m below the sea surface Volume (L) -- sample volume in L Excess Ra-224 Activity (dpm/100L) -- measured activity of excess Ra-224 with units of dpm/100L Excess Ra-224 Activity Uncertainty (dpm/100L) -- analytical uncertainty of the measurement of excess Ra-224 activity with units of dpm/100L Ra-226 Activity (dpm/100L) -- measured activity of Ra-226 with units of dpm/100L Ra-226 Activity Uncertainty (dpm/100L) -- analytical uncertainty of the measurement of Ra-226 activity with units of dpm/100L

Methods:

Sample volumes were passed slowly (< 1 L/min) through acrylic fibers impregnated with MnO2 (which quantitatively sorbed radium isotopes; Moore, 1976). These fibers were analyzed on a Radium Delayed Coincidence Counter (RaDeCC) immediately for total Ra-224, then again after 3 weeks for supported Ra-224 (Moore and Arnold, 1996). The difference between these activity measurements is the reported excess Ra-224 activity. The fibers were then sealed in air-tight cartridges to allow Rn-222 to grow in toward equilibrium with Ra-226 and measured on a radon emanation line for sorbed Ra-226 activities (Peterson et al., 2009).

Instruments:

Radium Delayed Coincidence Counter (RaDeCC), produced by Scientific Computer Instruments. Instrument was calibrated with NIST-traceable Th-232 standards (with daughters in equilibrium) and checked periodically with this standard for QA/QC.

Error Analysis:

Analytical uncertainties are reported as 1-sigma errors based on counting statistics. Errors in count rate (i.e., counts per minute) are computed by the square root of the total counts. This error is then propagated throughout the activity calculations.

Provenance and Historical References:

Moore, W. S. (1976). Sampling 228Ra in the deep ocean. Deep Sea Research and Oceanographic Abstracts, 23(7), 647–651. doi:10.1016/0011-7471(76)90007-3 Moore, W. S., & Arnold, R. (1996). Measurement of 223Ra and224Ra in coastal waters using a delayed coincidence counter. Journal of Geophysical Research: Oceans, 101(C1), 1321–1329. doi:10.1029/95jc03139 Peterson, R. N., Burnett, W. C., Dimova, N., & Santos, I. R. (2009). Comparison of measurement methods for radium-226 on manganese-fiber. Limnology and Oceanography: Methods, 7(2), 196–205. doi:10.4319/lom.2009.7.196

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