Use of Naturally Occurring Radionuclides as Tracers for Particle and Carbon Fluxes and Cycling Rates

Michael P. Bacon and J. Kirk Cochran

Goal:

To use naturally occurring radionuclides as tracers for particle (and carbon) fluxes and cycling rates during the EqPac Time Series cruises. The in situ pumps will be used to sample suspended and sinking particles and to measure dissolved radionuclides. Sediment trap samples from moored trap arrays will also be analyzed.

Participants:

     Time Series No. 1:  T. Hammar (WHOI),
                         D. Hirschberg (SUNY)
     Time Series No. 2:  M. P. Bacon (WHOI),
                         T. Hammar (WHOI),
                         D. Hirschberg (SUNY)

Measurement and Sampling information:

Station       Depths            Sampling     Method       Analyses 
9 N      Deepwater column:      pump         trap       Th,Th,Th
          1,200 m, 2,200 m,                              Th,Pb,Po
        700 mab + 7 other depths
5 N         "                    "            "           "
5 S         "                    "            "           "
12 S        "                    "            "           "
2 N      As above, with          "            "           "
          addition of 8
           samples in
          upper 1,000 m
0           "                    "            "           "
2 S         "                    "            "           "

The Bacon pumps have been modified to use a 142-mm adaptation of the Bishop filter sequence: 53 µm Nitex, 1 µm microquartz, 0.7-µm GF/F. Thus the results should be comparable to the groups using the Bishop pumps to collect samples (Ku et al., Murray). The pumps also collect dissolved Th isotopes by retention on Mn0

absorbers, so each pump sample also will have a dissolved Th analysis.

Analytical Methods and QA/QC:

Pump samples --- Dissolved

Th will be measured aboard ship by non-destructive gamma spectrometry. The procedure will follow that of Buesseler et al. (1992a,b). Particulate

Th in the three different size fractions will be analyzed aboard ship by low-level beta counting.

For the sediment trap samples and all other isotopes on the pump samples, the sample will be totally dissolved, and Th will be purified by ion exchange chromatography and then electrodeposited onto stainless steel for alpha and beta counting (Buesseler et al., 1992b).

Pb and Po will be measured on water samples collected in a Niskin bottle mounted on the pump frame tripped at the end of the pump deployment. Analyses will be by Co-APDC precipitation on board the ship, followed by plating onto silver disks and alpha counting (Cochran et al., 1983).

Samples to be distributed to others: R. F. Anderson will analyze some of the pump prefilters for Pa. He will also measure Be on Be fractions from the pump prefilters. We will share sediment trap samples with him, but will make sure that an annual composite is measured in all cases. M. Altabet and R. Francois will measure C and N isotopes on the prefilters.

Intercalibration:

Yield tracers will be exchanged with other radiochemists. Intercalibration of

Th,

Pb and

Po analyses will be achieved by analysis of deep water samples from a mutually agreed on depth. Cochran will distribute a deep-sea sediment standard for intercalibration of sediment

Pb.

Literature Cited

Buesseler, Ken. O., J. Kirk Cochran, Michael P. Bacon, Hugh D. Livingston, Susan A. Casso, David Hirschberg, Mary C. Hartman and Alan P. Fleer (1992a).: Determination of thorium isotopes in seawater by nondestructive and radiochemical procedures. Deep-Sea Research, 39(7/8): 1103--1114.
Buesseler, Ken O., Michael P. Bacon, J. Kirk Cochran and Hugh D. Livingston (1992b).: Carbon and nitrogen export during the JGOFS North Atlantic Bloom Experiment estimated from Th:U disequilibria. Deep-Sea Research, 39(7/8): 1115--1137.
Cochran, J. K., M. P. Bacon, S. Krishnaswami and K. K. Turekian (1983).: Po and Pb distributions in the central and eastern Indian Ocean. Earth and Planetary Science Letters, 65: 433--452.