Sediment Trap and ²³⁴Th methods for Particulate Organic Carbon Export in the upper ocean: Current Status

Annual report- 2001

Year 1 activities

SCOR WG 116 met for the first time in Amsterdam on July 8-10, 2001 just prior to the IGBP co-sponsored conference: "Challenges of a Changing Earth". Meetings were held at the Dutch Royal Academy Building, and 9 of 10 WG members were in attendance for the entire meeting, in addition to Dr. S. Tsunogai, our WG reporter, and Ed Urban from SCOR, attended parts of this meeting. In general, the meeting was broken into 3 parts: Day 1- Individual presentations & discussions; Day 2- discussion on what we know and don't know regarding upper ocean fluxes; Day 3 (afternoon only)- future WG plans (full agenda can be found at the end of this report).

During our meeting, we discussed the current Terms of Reference, and there was some feeling that the TOR are a bit narrowly focused. For example, during our WG deliberations we looked beyond simply "upper ocean export" to consider deeper trap fluxes, and we went beyond "organic C" to consider other elemental or radionuclide fluxes. However, we did not feel the need to alter the TOR in order to include this broader discussion. In the end, our report will remained focused on the TOR as approved by SCOR and accepted by the WG.

During our meeting, it was noted that WG 116 can take considerable advantage of a recent report edited by W. Gardner (Sediment Trap Technology and Sampling in Surface

Waters, report from the JGOFS Symposium in Villefranche sur Mer in May 1995). The goal of WG 116 is to go beyond this report, and review more recent data, including results from a wider range of international studies of upper ocean flux that would be relevant to our TOR. Specifically, with regards to upper ocean sediment traps, we spent some time discussing new data on: 1) hydrodynamics and particle sorting; 2) swimmers; 3) solubilization of particles in traps; and 4) particle source regions. It was noted, for example, that new neutrally buoyant sediment traps have both improved hydrodynamic characteristics over drifting trap designs and reduce the overall swimmer flux as well. Regarding swimmers, it was noted that no detailed protocols or intercomparisons between swimmer picking procedures have been published, though some WG members had experiences to share. One outcome of this discussion was a proposal to include a better grounded data base and protocol discussion in our WG report on swimmer removal methods. Overall, the biggest potential contribution of this WG would be in the compilation of detailed methods and protocols that might be applied to future upper ocean trap studies.

Analytical methods for ²³⁴Th have been improved considerably in recent years, allowing for wider application of the tracer. Easier methods also allow for the measurement of 3-D and time-series changes in the ²³⁴Th activity balance, which is needed to tightly constrain ²³⁴Th fluxes. Some examples of these highly resolved ²³⁴Th activity balances were shown, and it was noted that the data base for ²³⁴Th generally has increased considerably over the past 5 years. Overall, however, the biggest limitation in applying ²³⁴Th as a tracer of particulate organic carbon (POC) or other elemental fluxes, is in the collection and analyses of samples for determination of the POC/²³⁴Th ratio on sinking particles. Examples were shown in which filters and shallow traps provide a different estimate of this ratio, and arguments for and against each method were made. The WG is optimistic that considerable advances are possible with the application of new sampling techniques (SPLITT; Lagrangian traps), and that field intercomparisons would prove worthwhile (see #5, below). In addition, lab studies of the adsorption/desorption characteristics of ²³⁴Th with specific marine particles were thought to be important for better understanding changes and controls on the POC/²³⁴Th ratio.

WG Products

The meeting discussed a number of potential WG products that could result from our activities, including:

1) continued use of a Web site to disseminate WG relevant reports, papers, etc.

2) publication of a short EOS article (late 2001/early 2002) telling the broader community about our plans (Buesseler to lead; drafts to all WG participants in fall)

3) creation of a WG-relevant bibliography that would be on-line on our Web site. The goal would be to compile all the references related to upper ocean sediment traps and ²³⁴Th studies, including methods, lab work, field studies, and protocols. The focus will be on papers relevant to observational approaches used to estimate particle export, rather than modeling only of particle dynamics. A master list of key words would allow efficient searching and a common software format (EndNote) would be used. The bibliography will be started at WHOI, and include all of the papers provided to Buesseler at our WG meeting. A draft bibliography would be sent to all WG participants, and they will add appropriate references to fill out and complete this list. Since many relevant papers are not readily available in all countries, this reference list was thought to be quite useful to both the expert and broader community.

4) individual projects by many WG members are very relevant to the theme, and continued reporting on progress was likely to lead to a new understanding of upper ocean particle fluxes and methods that would be incorporated into our final report.

5) there was considerable enthusiasm for a collaborative intercomparison of methods to collect particles and measure flux in the upper ocean. In particular, it was noted that the WG members possess collectively a unique set of skills and sampling methods that have never been applied in a coordinated fashion at any single field site. One proposal was to hold an upcoming WG meeting at a site where other funds could be used to sponsor a short field experiment and intercomparision of methods, similar to the activities of SCOR/LOICZ WG 112 on Magnitude of Submarine Groundwater Discharge and its Influence on Coastal Oceanographic Processes. Possible sites discussed included off Catalina Island, California (USC Marine lab site); off France (DYFAMED Program site offshore from Villefranche/Monaco); or the ESTOC time series site off the Canary Islands. Some effort was spent on the experimental design features of this type of intercomparison. Without additional financial support, it was realized that his effort would not be possible, and options for seeking IOC or US NSF support were briefly discussed. Individual WG members were to return to their own labs and seek out opportunities for this plan.

WG 2002 plans

It was noted that two other groups—G. Jackson, US JGOFS SMP "Twilight Zone" Working Group and R. Schlitzer, International JGOFS Global Synthesis Working Group—were planning meetings in 2002 that might overlap in interest with WG 116. If possible, some coordination between groups should be encouraged and/or direct co-sponsorship of a single meeting sought. As of August 2001, it does not seem like the timing (early 2002 for both meetings) nor topics (mid-water zooplankton and modeling for Jackson; and global modeling of fluxes for Schlitzer) are sufficiently relevant that WG 116 would plan to hold a meeting at the same time/place. We are seeking finds in 2002 from SCOR for sending one WG 116 member to each of these spring 2002 meetings to report back to our group at our next meeting.

For planning purposes, we propose to hold our next WG 116 meeting during the summer/fall of 2002 in Southern France. Three events drive the timing and location of the meeting. First, proposals are under review to expand work at the DYFAMED site off France, to include a wider range of particle studies. If so, tying our WG meeting into possible cruise and lab activities associated with these cruises would be pursued. In addition, discussions are underway post-Amsterdam to submit proposals for field work in 2003 as outlined above and at our Amsterdam WG meeting. By next summer, we will know if we are successful at securing additional funds or not (possible sources include US NSF, IOC). Hence, we would use the 2002 WG meeting for either (1) planning details of our final report for our SCOR WG (summary article(s) in peer reviewed journals) or (2) planning for participation of WG members, if successful in our fund raising efforts, in an international study of upper ocean particle flux and composition. Finally, on 2-5 September, 2002, the IAEA lab in Monaco (Dr. Fowler's home Institution) is co-hosting an International Conference on Radioactivity in the Environment. Some of the WG 116 participants are likely to be attending this conference, and we might be able take advantage of this opportunity to make presentations on the WG theme plus ease some of the travel costs and logistics by holding our WG meeting either before or after this conference (more information on this conference can be found at www.elsevier.nl/locate/jer2002)

Final agenda SCOR WG #116

Sunday, July 8, 2001

9:00-10:00

introductions & logistics & overall plans (Buesseler)

initial discussion of Terms of Reference & possible products

10:00 - 12:00 Individual Presentations

(30 minutes each, generally 20 min. presentation w/discussion)

K. Buesseler

S. Fowler

M.M. Sarin

A. Antia

12:00-1:30 lunch break

1:30- 4:15

(30 minutes each with one 15 min. break)

M. Rutgers van der Loeff

D. Steinberg

K. Harada

C. Ming

T. Trull

4:15 - 5:00 Wrap up with science items from absent SCOR WG members- Gustafsson/Michaels

Monday, July 9

9:00- 12:00 (w/one 15 min. break)

General Science Discussion related to Upper ocean fluxes

1. Issues related to trapping (S. Folwer leads discussion)

Swimmers

Hydrodynamics

Solubilization/preservation

Particle source region

Particle sorting

12:00 - 13:30 lunch break

13:30 - 16:00 (w/one 15 min. break)

2. Issues related to Thorium methods (M. Rutgers van der Loeff leads discussion)

Activity balance- non-steady state & physical transport

POC/234Th ratios

residence time approaches

other elemental fluxes (PON, bSi, PAH, PCB's)

16:00 - 17:30 start discussion of

3. Other methods . (T. Trull leads discussion)

Approaches which depend on particle characteristics for yields and interpretations :-

Neutrally buoyant traps

SPLITT

particle counting/optical methods

mass balance approaches & modeling

other isotopes

Tuesday, July 10 (afternoon only)

13:30 - 17:00 (w/one 15 min break)

Finish discussion of: 3. Other methods

Finalize Terms of Reference (Buesseler lead)

-include deep traps? other elements beyond POC? etc.

What comes next (Buesseler)

Proposals for next meeting

Activities beyond Terms of Reference?

Tasks between meetings & assignments

New associate members?