Twenty-five years ago, an important and unexpected discovery was made in the waters off Bermuda. The
deep ocean was not, as was then widely believed, a dark and ever constant environment. Rather, there was a seasonal cycle in the deep that was closely tied to the seasonal cycle at the surface via a
continuous rain of sinking particles. This revelation, one of the first results of the now 28-year-old Oceanic Flux Program
(OFP) time series, has been followed by many subsequent discoveries that have helped to shape our understanding of how the ocean operates.Elucidating the processes that control the rain, or "flux," of
particles to the deep ocean has guided my research since I assumed the leadership of the OFP from my predecessor, Werner Deuser, in the mid-1990s. What physical and biological processes control the quantity
and composition of material exported from the overlying surface waters? What processes operating within the ocean interior alter the flux of material through the water column? How do fluxes vary over time in
response to changes in these processes? The answers to these questions are important because the flux of materials through the water column regulates many aspects of ocean biogeochemistry. For example,
the flux of organic matter provides the major food source for all life below approximately 100 meters, where photosynthesis becomes light limited. The flux of nutrient elements incorporated into organic
matter, and the depths at which these are re-dissolved, regulates ocean productivity patterns. The fluxes and relative ratios of organic matter and carbonate shells of microscopic marine organisms controls,
in part, the ocean's ability to absorb excess carbon dioxide from the atmosphere. Particle flux also efficiently cleanses ocean waters of pollutants and non-biogenic minerals as they are incorporated into
sinking particles and transferred to the seafloor. At the heart of the OFP time series is a four-kilometer-long mooring with three large, funnel-shaped sediment traps. This oceanographic apparatus is
designed to capture particles as they settle through the water column. A microprocessor controls the rotation of a new collection cup under the funnel at preprogrammed intervals, enabling us to obtain a
continuous flux record. Every four months, I set sail on the Weatherbird II to retrieve the mooring, located
approximately 75 kilometers southeast of Bermuda in 4,500 meters of water. After removing the trap samples and servicing mooring components, we redeploy the mooring with a new set of cups to resume sampling. Each mooring turnaround usually consumes two to three grueling days.
Particles in the greater-than-125-micrometer size
fraction of a sediment trap are magnified 1,000 times. This sample, taken at 3,200 meters, contains zooplankton fecal pellets, shells of pelagic gastropods,
foraminifera and radiolarians, and amorphous biological aggregates. |
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Back in the lab, we process the samples using ultra-clean, carefully standardized methods to ensure sample integrity and
maintain data continuity. After removing two samples for detailed analyses of the organic and inorganic flux components, the remainder of the sample is sieved and then photographed under a
stereomicroscope to produce a digital record of the contents. We make a visual count of the different types of flux components, then freeze dry and weigh the sample to determine the mass flux.
The dried flux material, weighing in total a fraction of a gram, is chemically analyzed to determine the concentrations of organic carbon, nitrogen, carbonate, opal and lithogenic materials – the major flux
components. The remaining material is archived for future study by scientists from around the world. At any one time, there are usually several investigators
conducting a variety of research studies in collaboration with the OFP.The record of deep-ocean flux off Bermuda is now 28 years long with a temporal coverage
of more than 95 percent, making it the longest-running oceanographic time series of its kind. In the last decade or so, OFP research has benefited greatly from the establishment in 1988 of BBSR's Bermuda Atlantic Time-series Study
(BATS) near the OFP site, and the deployment of the Bermuda Testbed Mooring (BTM) in 1994. The co-location of the OFP time series and these programs, which focus on upper-ocean physics and
biogeochemistry, has provided an unparalleled opportunity to assess in detail how processes operating at the sea's surface affect fluxes to the deep ocean.
My research uses information gleaned from detailed chemical analyses of the trap samples to elucidate the processes that control flux and its temporal variability. An
intriguing finding is the importance of large, episodic, pulsed fluxes of fresh organic material to the deep ocean. Collaborating with BTM and BATS researchers, we discovered
that these flux pulses, which occur throughout the record, can often be linked to transient, physically forced productivity events at the sea surface. We are also discovering new
information on the role of mid-water ecosystems in modulating particle fluxes throughout the water column. A recent highlight of the OFP has been the rare opportunity to witness the offshore
transport of a massive sediment plume created by the passage of Hurricane Fabian over Bermuda in September 2003. The study of this event is a fine example of the synergy
among Bermuda-based observational programs. The passage of Fabian created large internal waves and rotational currents which scoured the southern slopes of the Bermuda
pedestal and carried large plumes of reef sediments – and pollutants – offshore. One such plume was intercepted by the OFP traps, 75 kilometers offshore. This event, which
generated by far the largest fluxes observed during the 28-year span of the time series, contributed as much carbonate to the deep sea as would normally accumulate in an entire year.
We now know from the OFP studies that the deep ocean is extremely sensitive to changes in the surface ocean environment on time scales ranging from days to decades.
Turbulent weather patterns and mesoscale physical variability can have a dramatic effect on the flux of particulate matter to the deep sea and, in turn, on life in the relative calm of
the deep. Although it is not yet clear how climate-induced changes will affect the processes that regulate flux to the deep sea, what is clear is that any large-scale
disturbance in the particle flux cycle due to man's activities would have enormous global repercussions. |
