We plan to collect Soutar box cores at all of the U.S. JGOFS EqPac benthic lander stations. These corers will recover about 20 KY of sedimentation at the Equator and about 50 KY of sedimentation at the ends of the transect due to decreases in sedimentation rate.
We have been funded to carry out an ancillary aerosol collection program. Aerosol collections will be made during the steam to the first station, during the transit between each station, and during the steam to port. Such sampling will provide long-duration samples within each of the trade wind systems, as well as several short duration samples.
Pumped Aerosol Samples
Two types of samples will be taken. The primary aerosol sampler will be a polypropylene-sheltered high volume pumped sample. The sample is collected on preweighed filter paper supported in plastic mesh frames. The shelter is mounted on a 30 ft aluminum tower which is secured at the bow. The tower feet are mounted in swivels so that the tower can easily be lowered to the bow deck to change samples. The shelter can be wrapped in plastic or removed during station work to prevent contamination from stack gas and particulates. The aerosol is collected by high volume pumping. The pumping times and flow volumes are monitored to provide a quantitative aerosol inventory. The filters are removed from the sampler in a small portable laminar flow hood. All post-cruise sample handling and preparation is done in a clean room.Kites
The second type of sample collected will be a 2 m
nylon
mesh kite sample of the type used by J. Prospero. The sampler is deployed
on a boom from the bow. Sea spray makes the surface tacky and traps dust
particles on the collector. Although the sample is not
quantitative and can easily be polluted with trace elements, it
is an excellent high volume sampler for mineral aerosol and provides
enough material for x-ray diffraction analysis.
We propose to analyze C
and CaCO
by coulometry and/or by
Carlo-Erba analyzer (depending on the outcome of intercalibrations still
being completed) and biogenic SiO by spectrophotometry [see Methods
section] in at least one box, spade, and gravity [trigger] core collected
at each coring site by the program [an estimated total of about 20 box
cores, 20 spade cores and 12 gravity cores]. We have designed a sampling
and analysis program for our own studies as well as to meet the needs of
PI's proposing benthic flux and process studies. We have settled on a
sampling interval of 1 mm in the upper 3 cm [for cores at lander sites
only], 1 cm for the next 5 cm, every 2 cm thereafter to a depth of 15 cm,
and every 5 cm thereafter to the total depth of the cores. We estimate a
total of about 2,000 analyses for each component from the benthic cruise.
We can provide analyses of CaCO, C and biogenic SiO in
lander sediment samples if requested.
Because of the time delays of curation and sampling of the cores and the
length of time necessary for the measurements, we cannot provide all of
the measurements by six months post-cruise. Our first priority is to
provide analyses of CaCO, C and stratigraphic information on
box cores and spade cores being used for analysis of benthic processes and
fluxes by 6 months post-cruise. Our second priority for sediment
composition will be analyses of biogenic SiO and other sediment
components [aluminosilicates, elements]. We expect to provide completed
analyses by one year post-cruise, or the time frame of the post-cruise
science meeting.
Aerosol Analyses
All aerosol samples will be analyzed using the methods described below.Aerosol Samples
The sample is analyzed by short instrumental neutron activation runs for Al, Mn, V, Ba, and other elements, and by long runs for Sc, Zn, Cr, Fe, REE and other constituents. There is generally insufficient sample to make atomic absorption analysis reasonable.The mineralogy of the sample is determined by low temperature ashing of the filter followed by analysis on zero-background quartz mounts. The samples are also inspected by SEM/EDAX for grain size, mineral content, and identification of other particle types.
Sediments
Our general sediment handling methods and specific analytical techniques have been developed over several years of experience with Pacific deep sea sediments and over two years of analyzing equatorial Pacific sediments from the WEC8803-B cruise. All sediments are sampled to avoid obvious burrows or mottles. The dry bulk density of the sediments is determined by removing the sample from the core using a constant volume sampling syringe [or a constant volume ring sampler] and by weighing this sample before and after freeze-drying. All samples used for sediment component analysis are ground before analysis to avoid errors due to the common inhomogeneities of the sediment. We have found that these errors can be considerable for C and for CaCO in the case of low
CaCO concentrations.
CaCO and C Concentration
CaCO and C contents of surface and downcore
sediments will be determined on an UIC Coulometrics Coulometer and/or on a
Carlo-Erba carbon analyzer. The basis of the coulometric technique is the
titration of CO evolved from CaCO by acid dissolution and from
C by oxidation. CaCO is measured on the inorganic carbon
attachment of the coulometer as inorganic carbon released by acidification
of the sample with either phorphoric or hydrochloric acid. Total carbon
is analyzed by combusting the sample in an oxygen atmosphere at 990°
C to convert the inorganic and C to CaCO. The evolved gas
passes through a barium chromate catalyst/scrubber to ensure that all the
carbon has been oxidized. C is calculated from the total carbon
and the inorganic carbon by difference.
The C content of equatorial Pacific sediments is small and
averages less than 1--2 wt %. Small errors in the measured value can,
therefore, have a large impact on calculated accumulation rates and
benthic fluxes. The simple technique for determining C described
above works well when the CaCO content of the samples is low. When
CaCO exceeds about 50 % of the sediment, however, the small error on
replicate inorganic carbon analyses can exceed the total concentration of
C in the sample. Therefore, we will determine C using a
sample preparation method developed by Curry and modified in our
laboratory. We determine C on a separate split of sample. The
sample is weighed and CaCO is removed by treating the sample with
50 ml of phorphoric acid. The solution is filtered through a
pre-combusted Whatman GF/F fine filter and the remaining sediment is
washed on the filter with 50 ml of deionized distilled water. The filter
is dried and folded into a precombusted aluminum foil square for insertion
into the coulometer. The precision of the method based on the average %
difference of replicate measurements of over 400 samples is ±5 % of
measured value or about ±5µgC.
Several investigators have recently found that the loss of C
during acid dissolution of CaCO in sediment trap samples and margin
sediments can be large. We tested our technique for dissolution of
C by acid predigestion: the filtrate was analyzed using the
coulometer attachment for TOC in water. We found no C in the
solution. The filtered solutions from sediment samples at 5° N and
the Equator [also therefore high CaCO and low CaCO sediments]
were also analyzed for dissolved C by E. Peltzer at Woods Hole
Oceanographic Institution using the technique of Suzuki as modified by
Peltzer. Insignificant amounts of C were present in the solution.
We believe that this is because the C in sediments from the
central equatorial Pacific has undergone considerable degradation and is
fairly refractory. This would not be the case in sediment trap material
and may not be the case in margin sediments.
Biogenic Silica
We propose to determine the biogenic silica content of surface and downcore samples by a modification of the NaCO
dissolution technique. Boucher found that grinding of deep sea sediments
before sodium carbonate dissolution was particularly important for
sediments such as those from the equatorial Pacific which contain large
diatoms and heavily silicified radiolarians.
Other Sedimentary Constituents
We propose to determine the concentration of several other elements of interest in the surface sediment and in samples from box cores. These elements include Al, Si, Fe, Mn, Cu, Ni, Zn, Co and Ba. These analyses will be conducted by XRF analysis using the URI Department of Geology XRF facility.The mineralogy of the clay and silt-sized fractions will be determined by x-ray diffraction using preparation, standardization, and data analysis methods outlined in Leinen, 1989.
I plan to conduct mineralogical and XRF analyses of samples from the spade cores. These analyses were not proposed by DeMaster et al. I plan to conduct analyses of biogenic constituents as requested by the lander group.