This paper presents an overview of the ways that natural radionuclides have been used as tracers of biogeochemical processes occurring in the coastal ocean. The radionuclides involved include those in the uranium and thorium decay series and those produced in the atmosphere by the interaction of cosmic rays with atmospheric gases (i.e., cosmogenic radionuclides). The property of radioactivity provides a chronometer with which to measure the rates of oceanic processes, and the fundamental biogeochemical behavior of the radionuclide determines which process(es) it may be used to trace. Examples from recent research will be presented in the talk.

As part of the first investigation of the North Water region of Baffin Bay to specifically examine carbon cycling in this unique and highly productive area, we found that the distributions of carbon within these waters were controlled by a complex system of transport and biological processes. We systematically collected samples throughout the North Water during April–July 1998 and August–September 1999 and analyzed them for total dissolved inorganic carbon (DIC), alkalinity, dissolved organic carbon, and total suspended particulate carbon. Consistent with biogenic drawdown, surface DIC concentrations dropped by as much as 250μmolkg−1 during the summer and began to increase again by the end of September. Although the surface waters were supersaturated with carbon dioxide in early spring, extensive ice cover limited CO2 outgassing at that time. As the ice cleared, decreasing surface DIC concentrations supported significant fluxes of CO2 into the ocean. In late September and early October, when ice again was beginning to cover the area, the surface waters were still undersaturated in CO2, implying that the North Water could be a net sink of atmospheric carbon, if winter air–sea fluxes are minimal. There is strong evidence that horizontal advection plays an important role in controlling DIC distributions, although we were unable to independently quantify the advective fluxes. Based on the observed changes in total carbon concentrations and estimates of air–sea fluxes, we found that carbon was lost from the surface waters between April 1998 and October 1999, probably due to both biological and advective export.

The North Water, a large polynya in northern Baffin Bay, has an extensive period of continuous primary production lasting up to 6 months. The well-known richness of this polynya in higher trophic levels, including numerous marine mammals and birds, must be based on the magnitude and duration of that production. As part of the International North Water Polynya Study, we observed that two groups of phytoplankton were responsible for most of the primary production in 1997–1999: the large centric diatoms, Thalassiosira spp., and the smaller, colonial centric diatom, Chaetoceros socialis Lauder. We studied the phytoplankton community assemblage in the North Water in August 1997, April–July 1998, and August–September 1999 using epifluorescence microscopy on samples fixed and filtered at sea, with unpreserved cells observed in July 1998. Blooms of C. socialis occurred at southern stations in June and throughout the polynya from July to September. Maximum concentrations reached 30,100 cellsml−1 (496μg Cl−1) in the western area in early July. Blooms of C. socialis directly followed those of Thalassiosira spp., which were more intense but also more transient. C. socialis appeared to maintain its population in the euphotic layer for up to 3 months by surviving at low nutrient levels, sinking to depths where dynamic physical regimes brought nutrient replenishment, and producing resting spores (up to 233μg Cl−1). The resting spores may survive low-nutrient periods and be introduced, some from fecal pellets, to waters of more favorable growth conditions; in July, 86% of fecal pellets and 76% of aggregates observed in shallow, floating sediment traps contained resting spores of C. socialis. Colonies of C. socialis were a potential mid-water food source and major contributor to the benthos: C. socialis contributed up to 91% of total phytoplankton cells (49% phytoplankton carbon) identified in moored sediment traps. Mats of polymer gels (mucous), likely formed from the observed C. socialis colonies, some containing frustule and setae fragments as well as resting spores, contributed massively to the material in traps in the southern and western polynya. C. socialis appears to be a very important primary producer throughout the productive season of the North Water.

Water-column deficits of 234Th (half-life=24.1d) relative to its parent 238U were used to estimate fluxes of 234Th and particulate organic carbon (POC) through the euphotic zone of the North Water, a polynya in northern Baffin Bay. Samples collected in May 1998, early in the seasonal development of the polynya, showed small deficits of 234Th in the upper 100m. Deficits were greater in July 1998 and August–September 1999. Sinking fluxes of POC were calculated from the water-column 234Th deficits by multiplying the latter by the POC/234Th ratio on sinking particles (assumed to be the >70-μm filterable fraction). POC fluxes were greatest in July 1998 (mean 27.4±13.4mmolCm−2d−1) and declined in August 1999 (mean 8.7±1.7mmolCm−2d−1), largely due to a decrease in the POC/Th ratio in the 1999 samples. Regionally, POC fluxes were greatest in the western and northern portions of the North Water in July 1998 and markedly lower in the west in August 1999. This pattern resembles spatial and temporal changes in primary production in the polynya determined independently. POC fluxes determined from water-column 234Th profiles agreed within factors of 2–5 with values determined from floating sediment traps deployed in the upper 100m. Thorium-based indices of new production (ThE ratios) obtained within the polynya encompassed a wide range (0.1–0.9), with western areas showing substantially higher transfer efficiency of carbon fixed by phytoplankton at the surface. Thorium-derived POC fluxes at 100m for both July 1998 and August 1999 correlated positively with biomass of the dominant diatom in the North Water, Chaetoceros socialis, suggesting that regional variations in carbon export can be further understood by factors controlling the distribution of this species.