Radium isotopes as tracers of coastal circulation pathways in the Mid-Atlantic Bight

Rasmussen, Linda L.

doi:10.1575/1912/2444

Cited by 7 publications

(5 citation statements)

References 66 publications

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“…The end of the transect (>250 km) had elevated 224 Ra and 228 Ra values, which would require a very fast offshore transport of coastal waters without significant dilution with low‐radium offshore waters. A recent formation of a large eddy entraining shelf water or shelf streamers similar to those observed by Rasmussen [2003] could be responsible for quick lateral transport preserving coastal 224 Ra/ 228 Ra ratios. Alternatively, high particulate 228 Th, which we did not measure and therefore cannot account for in calculating excess 224 Ra, may be responsible for false apparent excess 224 Ra.…”

Section: Discussionmentioning

confidence: 59%

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

Dulai

Ardelan

Henderson

et al. 2009

Global Biogeochemical Cycles

122

108

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In the Southern Ocean near the Antarctic Peninsula, Antarctic Circumpolar Current (ACC) fronts interact with shelf waters facilitating lateral transport of shelf‐derived components such as iron into high‐nutrient offshore regions. To trace these shelf‐derived components and estimate lateral mixing rates of shelf water, we used naturally occurring radium isotopes. Short‐lived radium isotopes were used to quantify the rates of shelf water entrainment while Fe/228Ra 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 × 105 mol d−1 and total acid leachable iron flux of 1.1 × 106 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 × 105 mol Fe. Net vertical export fluxes of particulate Fe derived from 234Th/238U disequilibrium and Fe/234Th 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.

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Section: Discussionmentioning

confidence: 59%

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

Dulai

Ardelan

Henderson

et al. 2009

Global Biogeochemical Cycles

122

108

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“…Tidally driven input from the coastal ocean is another possibility, however; 228 Ra/ 226 Ra ARs on the Mid-Atlantic Bight shelf are always Ͻ2 and typically Ͻ1 (Rasmussen 2003). Therefore, we infer that the groundwater we sampled was not the only groundwater source to the estuary.…”

Section: Discussionmentioning

confidence: 87%

“…The last endmember is the surficial aquifer (marsh), which we assume is the Columbia aquifer driving groundwater flow through the salt marsh sediments. The following three equations, station near the mouth of Chesapeake Bay in Rasmussen (2003). The Yorktown aquifer Ra endmember was chosen as the sample with both the highest 226 Ra activity and lowest 228 Ra/ 226 Ra AR (0.5).…”

Section: Discussionmentioning

confidence: 99%

Submarine groundwater discharge of nutrients and copper to an urban subestuary of Chesapeake Bay (Elizabeth River)

Charette¹,

Buesseler²

2004

Limnology & Oceanography

162

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We investigated the role submarine groundwater discharge (SGD) plays in the delivery of nutrients and copper to the Elizabeth River (Virginia) estuary, a major subestuary of lower Chesapeake Bay. Using an approach based on radium isotopes, we concluded that two distinct sources of groundwater were equally impacting the estuary: a surface (marsh) aquifer and deep aquifer source each with a unique 228 Ra/ 226 Ra activity ratio. Considering each of these sources, we calculated an SGD flux of 1 ϫ 10 6 m 3 d Ϫ1 (Ϯ10%), which represented ϳ6% of the SGD flux for the entire Chesapeake Bay and ϳ5% of the James River, a major source of freshwater to lower Chesapeake Bay. SGD-derived dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) fluxes averaged 4.5 (Ϯ4.6) and 0.16 (Ϯ0.17) mmol m Ϫ2 d Ϫ1 , respectively, and compared well with area-normalized fluxes to Chesapeake Bay. In contrast, SGD-derived Cu input of 730 (Ϯ390) kg yr Ϫ1 was a relatively small source of Cu (ϳ3%) to the Elizabeth estuary given that surface water inputs, such as antifouling paints associated with naval operations, are a major component of the Cu budget for this system. These findings were in general agreement with prior studies of SGD for this region.Submarine groundwater discharge (SGD) is often ignored when constructing geochemical budgets for elements in nearshore environments, mainly because the volume flux is difficult to estimate. However, many studies indicate that SGD may carry significant quantities of nutrients and trace metals to the ocean Moore 1996;Krest et al. 2000;Montlucon and Sanudo-Wilhelmy 2001). In the case of nutrients, SGD has been the principal mechanism for eutrophication in many coastal embayments throughout the world (e.g., Valiela et al. 1990).Various approaches for quantifying SGD include water budgets, seepage meters, and natural tracers (see review by Burnett et al. 2001). The water budget approach, which attempts to balance aquifer inputs (precipitation) with losses (evapotranspiration), often results in a large uncertainty, since SGD is the difference between two very large numbers (e.g., Cambareri and Eichner 1998). Seepage meters, though useful for understanding local-scale patterns of SGD, are labor intensive and therefore require significant effort for quantifying SGD on large spatial scales (e.g., Michael et al.

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“…It is a key variable in understanding chemical and biological processes of riverine plumes and nearshore upwelling and also necessary to determine the fluxes of SGD and associated terrestrial materials. This can be estimated via physical methods (Pilson, 1985;Sanford et al, 1992), numerical models (Das et al, 2000), or isotopic water mass balances (Kelly and Moran, 2002;Delhez et al, 2003;Rasmussen, 2003). Radium isotopes have been used to estimate the time elapsed since a water parcel was last in contact with a boundary (Charette et al, 2008;Peterson et al, 2008b).…”

Section: Water Ages Derived From 223 Ra/ 228 Ramentioning

confidence: 99%

How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?

et al. 2012

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In order to assess the role of submarine groundwater discharge (SGD) and its impact on the carbonate system on the northern South China Sea (NSCS) shelf, we measured seawater concentrations of four radium isotopes 223,224,226,228Ra along with carbonate system parameters in June–July, 2008. Complementary groundwater sampling was conducted in coastal areas in December 2008 and October 2010 to constrain the groundwater end-members. The distribution of Ra isotopes in the NSCS was largely controlled by the Pearl River plume and coastal upwelling. Long-lived Ra isotopes (228Ra and 226Ra) were enriched in the river plume but low in the offshore surface water and subsurface water/upwelling zone. In contrast, short-lived Ra isotopes (224Ra and 223Ra) were elevated in the subsurface water/upwelling zone as well as in the river plume but depleted in the offshore surface water. In order to quantify SGD, we adopted two independent mathematical approaches. Using a three end-member mixing model with total alkalinity (TAlk) and Ra isotopes, we derived a SGD flux into the NSCS shelf of 2.3–3.7 × 108 m3 day−1. Our second approach involved a simple mass balance of 228Ra and 226Ra and resulted in a first order but consistent SGD flux estimate of 2.2–3.7 × 108 m3 day−1. These fluxes were equivalent to 12–21 % of the Pearl River discharge, but the source of the SGD was mostly recirculated seawater. Despite the relatively small SGD volume flow compared to the river, the associated material fluxes were substantial given their elevated concentrations of dissolved inorganic solutes. In this case, dissolved inorganic carbon (DIC) flux through SGD was 153–347 × 109 mol yr−1, or ~23–53 % of the riverine DIC export flux. Our estimates of the groundwater-derived phosphate flux ranged 3–68 × 107 mol yr−1, which may be responsible for new production on the shelf up to 0.3–6.3 mmol C m−2 d−1. This rate of new production would at most consume 11 % of the DIC contribution delivered by SGD. Hence, SGD may play an important role in the carbon balance over the NSCS shelf

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Radium isotopes as tracers of coastal circulation pathways in the Mid-Atlantic Bight

Cited by 7 publications

References 66 publications

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

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

Submarine groundwater discharge of nutrients and copper to an urban subestuary of Chesapeake Bay (Elizabeth River)

How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?

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