Paul B. Henderson

Shelf-derived iron inputs drive biological productivity in the southern Drake Passage

water entrainment while Fe/ 228 Ra ratios were used to calculate the Fe flux. In the summer of 2006 we found rapid mixing and significant lateral iron export, namely, a dissolved iron flux of 1.1 � 10 5 mol d � 1 and total acid leachable iron flux of 1.1 � 10 6 mol d � 1 all of which is transported in the mixed layer from the shelf region offshore. This dissolved iron flux is significant, especially considering that the bloom observed in the offshore region (0.5–2 mg chl a m � 3 ) had an iron demand of 1.1 to 4 � 10 5 mol Fe. Net vertical export fluxes of particulate Fe derived from 234 Th/ 238 U disequilibrium and Fe/ 234 Th ratios accounted for only about 25% of the dissolved iron flux. On the other hand, vertical upward mixing of iron rich deeper waters provided only 7% of the lateral dissolved iron flux. We found that similarly to other studies in iron-fertilized regions of the Southern Ocean, lateral fluxes overwhelm vertical inputs and vertical export from the water column and support significant phytoplankton blooms in the offshore regions of the Drake Passage.

Coupled radon, methane and nitrate sensors for large-scale assessment of groundwater discharge and non-point source pollution to coastal waters

We constructed a survey system of radon/methane/nitrate/salinity to find sites of submarine groundwater discharge (SGD) and groundwater nitrate input. We deployed the system in Waquoit Bay and Boston Harbor, MA where we derived SGD rates using a mass balance of radon with methane serving as a fine resolution qualitative indicator of groundwater. In Waquoit Bay we identified several locations of enhanced groundwater discharge, out of which two (Childs and Quashnet Rivers) were studied in more detail. The Childs River was characterized by high nitrate input via groundwater discharge, while the Quashnet River SGD was notable but not a significant source of nitrate. Our radon survey of Boston Harbor revealed several sites with significant SGD, out of these Inner Harbor and parts of Dorchester Bay and Quincy Bay had groundwater fluxes accompanied by significant water column nitrogen concentrations. The survey system has proven effective in revealing areas of SGD and non-point source pollution.

Seasonal evolution of water contributions to discharge from a Greenland outlet glacier: insight from a new isotope-mixing model

The Greenland ice sheet (GrIS) subglacial hydrological system may undergo a seasonal evolution, with significant geophysical and biogeochemical implications. We present results from a new isotope-mixing model to quantify the relative contributions of surface snow, glacial ice and delayed flow to the bulk meltwater discharge from a small (� 5k m 2 ) land-terminating GrIS outlet glacier during melt onset (May) and at peak melt (July). We use radioactive ( 222 Rn) and stable isotopes ( 18 O, deuterium) to differentiate the water source contributions. Atmospherically derived 7 Be further constrains meltwater transit time from the glacier surface to the ice margin. We show that (1) 222 Rn is a promising tracer for glacial waters stored at the bed and (2) a quantitative chemical mixing model can be constructed by combining 222 Rn and the stable water isotopes. Applying this model to the bulk subglacial outflow from our study area, we find a constant delayed-flow (stored) component from melt onset through peak melt. This component is diluted first by snowmelt and then by increasing glacial ice melt as the season progresses. Results from this pilot study are consistent with the hypothesis that subglacial drainage beneath land-terminating sections of the GrIS undergoes a seasonal evolution from a distributed to a channelized system.

Current and historical rates of input of mercury to the Penobscot River, Maine, from a chlor-alkali plant

Mercury inputs by surface and ground water sources to Penobscot River from a defunct Hg-cell chlor-alkali plant were measured in 2009-10 and estimated for the entire period of operation of this facility. Over the measured interval (422 days) approximately 2.3 kg (5.4 g day-1) of mercury was discharged to the Penobscot River by the two surface streams that drain the site, with most of the combined loading (1.8 kg Hg, 78%) associated with a single storm with rainfall in excess of 100 mm. Groundwater seepage rates from the site, as estimated from both a radon tracer and seepage meter methods were in the range of 3 to 4 cm day-1 and, when combined with a best estimate of the area of groundwater discharge (11,000 m2) and average seepage/porewater mercury concentration (242 ng L-1, UCL95), yielded a loading of 0.11 g day-1 for site groundwater. None of the municipal or other industrial point sources of mercury to the river between Veazie and Bucksport, Maine exceeded 1 g day-1 individually, nor was the aggregate loading of all such sources >3 g day-1 (based on State of Maine data). Mercury loadings for the three largest tributaries downstream of Veazie Dam were estimated to contribute 4.2, 3.7 and 2.5 g day-1, respectively, to the Penobscot River. Based on sampling (total Hg ~ 2 to 4 ng L-1) and historical mean discharge data (340-460 m3 s-1), the Penobscot River upstream of the plant site contributes as much as 160 g day-1 to the downstream reach depending on river discharge. Estimates of historical (1967-2012) mercury loading using both generic emission factors and measured releases ranged from 2.6 to 27 MT while the mass of mercury found in downstream sediments amounted to 9 MT.

Increased fluxes of shelf-derived materials to the central Arctic Ocean

Shelf inputs in the Arctic Ocean appear to be increasing, which could change the nutrient balance of the central basin. Rising temperatures in the Arctic Ocean region are responsible for changes such as reduced ice cover, permafrost thawing, and increased river discharge, which, together, alter nutrient and carbon cycles over the vast Arctic continental shelf. We show that the concentration of radium-228, sourced to seawater through sediment-water exchange processes, has increased substantially in surface waters of the central Arctic Ocean over the past decade. A mass balance model for 228Ra suggests that this increase is due to an intensification of shelf-derived material inputs to the central basin, a source that would also carry elevated concentrations of dissolved organic carbon and nutrients. Therefore, we suggest that significant changes in the nutrient, carbon, and trace metal balances of the Arctic Ocean are underway, with the potential to affect biological productivity and species assemblages in Arctic surface waters.

Lingering radioactivity at the Bikini and Enewetak Atolls

We made an assessment of the levels of radionuclides in the ocean waters, seafloor and groundwater at Bikini and Enewetak Atolls where the US conducted nuclear weapons tests in the 1940s and 50s. This included the first estimates of submarine groundwater discharge (SGD) derived from radium isotopes that can be used here to calculate radionuclide fluxes in to the lagoon waters. While there is significant variability between sites and sample types, levels of plutonium (239,240Pu) remain several orders of magnitude higher in lagoon seawater and sediments than what is found in rest of the worlds oceans. In contrast, levels of cesium-137 (137Cs) while relatively elevated in brackish groundwater are only slightly higher in the lagoon water relative to North Pacific surface waters. Of special interest was the Runit dome, a nuclear waste repository created in the 1970s within the Enewetak Atoll. Low seawater ratios of 240Pu/239Pu suggest that this area is the source of about half of the Pu in the Enewetak lagoon water column, yet radium isotopes suggest that SGD from below the dome is not a significant Pu source. SGD fluxes of Pu and Cs at Bikini were also relatively low. Thus radioactivity associated with seafloor sediments remains the largest source and long term repository for radioactive contamination. Overall, Bikini and Enewetak Atolls are an ongoing source of Pu and Cs to the North Pacific, but at annual rates that are orders of magnitude smaller than delivered via close-in fallout to the same area.