Accepted Manuscript
Using oxygen isotopes to establish freshwater sources in Bedford Basin, Nova Scotia, a Northwestern Atlantic fjord
Elizabeth A. Kerrigan, Markus Kienast, Helmuth Thomas, Douglas W.R. Wallace
PII: S0272-7714(17)30276-7 DOI: 10.1016/j.ecss.2017.09.003 Reference: YECSS 5605
To appear in: Estuarine, Coastal and Shelf Science
Received Date: 10 March 2017
Please cite this article as: Kerrigan, E.A., Kienast, M., Thomas, H., Wallace, D.W.R., Using oxygen isotopes to establish freshwater sources in Bedford Basin, Nova Scotia, a Northwestern Atlantic fjord, Estuarine, Coastal and Shelf Science (2017), doi: 10.1016/j.ecss.2017.09.003.
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Using oxygen isotopes to establish freshwater sources in Bedford Basin, Nova Scotia, a
Northwestern Atlantic fjord
Elizabeth A. Kerrigan a, , Markus Kienast a, Helmuth Thomas a, Douglas W. R. Wallace a*
aDalhousie University, Department of Oceanography, 1355 Oxford Street, PO Box 150000, B3H
4R2, Halifax, NS, Canada
*Corresponding Author: Douglas W.R. Wallace ([email protected])
Room 2-35, Steele Ocean Sciences Building, 1355 Oxford St
PO Box 15000, Halifax, Nova Scotia, Canada B3H 4R2
Additional e-mail addresses: Elizabeth A. Kerrigan ([email protected]), Markus Kienast ([email protected]), Helmuth Thomas (helmuth.th [email protected])
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Abstract:
A weekly time-series of oxygen isotope ( δ18 O) measurements was collected over a 16-month period from near-surface (1m) and near-bottom (60m) waters of Bedford Basin, a coastal fjord adjacent to the Scotian Shelf, off eastern Canada. The time-series was complemented with δ18 O measurements of local precipitation (rain and snow), river, and wastewater runoff. The isotopic composition of precipitation displayed strong seasonality with an average (volume-weighted)
δ18 O value of -5.39‰ (± 0.96) for summer and a depleted value of -10.37‰ (± 2.96) over winter.
Winter precipitation exhibited more depleted and variable δ18 O of solid precipitation relative to rainfall. The annual, amount-weighted average δ18 O of Sackville River discharge (-6.49‰ ±
0.82) was not statistically different from precipitation (-7.24‰ ± 0.92), but exhibited less seasonal variation. Freshwater end-members (zero-salinity intercepts) estimated from annual and
δ18 seasonal regressions of O versus salinity (S) for BedfordMANUSCRIPT Basin near-surface samples were consistent with the δ18 O of summer precipitation and the annual, amount-weighted average for the Sackville River. However, the isotopically depleted signature of winter precipitation was not observed clearly in near-surface waters of Bedford Basin, which might reflect isotope enrichment during sublimation from accumulated snowfall prior to melting and discharge, or retention and mixing within the drainage basin. In near bottom waters, most of the δ18 O-S variation (average freshwater end-member: 7.47‰ ± 2.17) could be explained by vertical mixing with near-surface waters (average freshwater end-member: -6.23‰ ± 0.34) and hence with locally-derived freshwater.ACCEPTED However the near-bottom δ18 O-S variation suggested an additional contribution of a freshwater end-member with a δ18 O of -15.55‰ ± 2.3, consistent with a remotely-derived freshwater end-member identified previously for the Scotian Shelf. Residuals from a long-term regression of δ18 O-S were generally within the range expected due to analytical
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uncertainty (± 0.05); however near-surface waters exhibited seasonal variability of small amplitude, which was consistent with the timing and δ18 O variability of local freshwater inputs.
Keywords: Bedford Basin; fjord; oxygen isotopes; salinity; seasonality; freshwater inputs
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1.0 Introduction:
The freshwater balance of the northern North Atlantic Ocean is changing as a result of changing
Arctic sea-ice cover and increased melting of glacial ice (e.g., on Greenland; Bamber et al.,
2012). Much of this Arctic-derived freshwater is transported southwards along the east coast of
North America, as far as Cape Hatteras, by a 5000 km long boundary current (Chapman and
Beardsley, 1989). This long-range influence of Arctic freshwater potentially impacts alongshore nutrient transport and stratification-dependent biological production over continental shelves.
The alongshore continuity of this boundary current was first identified on the basis of an analysis of oxygen isotopes of water (Fairbanks, 1982).
The oxygen isotope composition of H 2O molecules has been used widely as a tracer of the hydrological cycle (Craig, 1961), ocean freshwater sources (e.g., Craig & Gordon, 1965; Fairbanks, 1982; Khatiwala et al., 1999), and sourc esMANUSCRIPT of deep water masses (Bauch et al., 1995). Craig and Gordon (1965) first used δ18 O as an oceanographic tracer and established that, when paired with salinity (S), it can be used to characterize mixing of water masses and to distinguish different sources of freshwater (e.g., precipitation, river discharge, meltwater, etc.).
For the North Atlantic as a whole, freshwater originating from Arctic outflow dominates, with a
δ18 O end-member of -21‰ (Khatiwala et al., 1999). Oxygen isotope analysis of the waters over the Scotian Shelf and Scotian Slope, off the east coast of Canada, revealed that the main freshwater input ACCEPTEDhas its origin at high-latitudes (Fairbanks, 1982; Khatiwala et al., 1999). This
Arctic freshwater source is complemented in spring and summer by the outflow of the Gulf of St.
Lawrence (Shadwick and Thomas, 2011). Khatiwala et al. (1999) noted that the 18 O-salinity
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signature and resulting freshwater end-member of waters over the Scotian Shelf (-15.55‰;
Fairbanks, 1982) are influenced strongly by sea-ice formation and brine rejection over the
Labrador Shelf and by the heavier freshwater end-member derived from the St. Lawrence River
(-10.3‰). The two main sources of freshwater on the Scotian Shelf, high-latitude (i.e. Arctic) runoff and St. Lawrence River water (SLRW), contribute freshwater in an approximately 2:1 ratio (Khatiwala et al., 1999).
A number of previous studies have used the δ18 O-S relationship within estuaries and fjords
(Martin & Letolle, 1979; Austin & Inall, 2002), typically focussing on distinguishing various sources and sinks (precipitation, evaporation, river discharge, groundwater input, ice-melt) (e.g.,
Azetsu-Scott & Tan, 1997; Corlis et al., 2003; MacLachlan et al., 2007; Stalker et al., 2009; Turk et al., 2016; Whitney et al., 2017). The δ18 O-S relationship allows characterization of freshwater inputs with distinct δ18 O signatures in an estuary. However, MANUSCRIPT few studies have examined the seasonality of the δ18 O-S relationship.
Here we present results from a 16-month, weekly time-series of δ18 O collected from Bedford
Basin, a coastal fjord adjacent to the Scotian Shelf, together with measurements of local river water, wastewater, and precipitation. The data provide insight into the distribution and fate of freshwater sources as well as a reference point against which to evaluate longer-term changes in freshwater inputs,ACCEPTED both local and remote, that might arise due to climate change.
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2.0 Study Site:
2.1 Scotian Shelf
The Scotian Shelf is a 700 km long region of the continental shelf off Nova Scotia, varying in width from 120 – 240 km, and covering 120,000 km 2 with an average depth of 90 m. This region is bound by the Laurentian Channel to the northeast and by the Northeast Channel and the Gulf of Maine to the southwest (Shadwick & Thomas, 2011) (Figure 1). Three major water masses contribute to Scotian Shelf waters: (1) Warm Slope Water (WSW), a mixture of Labrador Slope
Water (LSW) and Gulf Stream (GS) water, (2) Labrador Shelf Water (LShW), derived from the inner branch of the Labrador Current, and (3) St. Lawrence Estuary Water (SLEW), which is influenced by the St. Lawrence River (Khatiwala et al., 1999; Shadwick & Thomas, 2011).
2.2 Bedford Basin Bedford Basin is a fjord (volume: 5.6 x 10 8 m3, area: MANUSCRIPT 17 km 2, max. depth: 71 m) situated on the Atlantic coast of Nova Scotia, Canada, and separated from the adjacent Scotian Shelf by a 10 km long channel that is less than 400 m wide at its narrowest point, with a sill depth of ~20 m (Petrie
& Yeats, 1990; Gregory, 1993; Li & Harrison, 2008) (Figure 1; 2a).
2.2.1 Freshwater inputs & circulation
The local input of fresh water to Bedford Basin is small relative to exchanges with the adjacent
Scotian Shelf, with a tidal inflow to freshwater input ratio of 109.38 for Bedford Basin (Gregory et al., 1993). Near-surfaceACCEPTED salinity within Bedford Basin is typically in the range of 29-30.5 and hence lower than surface salinities of 30-31.5 measured at HL-2 (see Figure 1). The annual average discharge of fresh water into Halifax Harbour is 15.7 m 3/s (Buckley & Winters, 1992)
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and is supplied from a watershed with area of 281 km 2 (Li & Harrison, 2008). The largest single source is the Sackville River, which enters Bedford Basin at its northwestward end and has an average discharge of 5.41 m 3/s (Buckley & Winters, 1992), ranging from 2 m 3/s to 9 m 3/s in the summer (July – September) and spring (March and April) respectively (Fournier, 1990). The remaining freshwater discharge is via a number of small streams and sewers (Buckley &
Winters, 1992) with c. 16% of the total delivered by the Halifax wastewater treatment system, whose water originates from two lakes, Pockwock Lake and Lake Major, as well as a number of smaller reservoirs (Fader & Buckley, 1995). Direct precipitation onto Bedford Basin contributes an additional 0.8 m 3/s (Kerrigan, 2015). The contribution of submarine groundwater discharge
(SGD) to overall freshwater input is not known. However given the low permeability bedrock
(Figure 1), a groundwater recharge ratio for the catchment area of 0.12 (Gavin Kennedy, Nova
Scotia Department of Natural Resources, personal communication, July 2017), and the likelihood that much of the recharge is delivered to Bedford B asinMANUSCRIPT via rivers and streams, we consider additional input by SGD to be small for this water body.
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FIGURE 1 : General movement of water masses along the Scotian Shelf (Khatiwala et al., 1999;
Shadwick & Thomas, 2011). SLEW: St. Lawrence Estuary Water, LShW: Labrador Shelf Water,
WSW: Warm Slope Water, LSW: Labrador Slope Water, GS: Gulf Stream. Halifax Line stations
are indicated along with station numbers (1 – 7) and the grey star identifies Bedford Basin. The
bedrock geology surrounding Bedford Basin includes granite but is composed largely of very
fine- to medium-grained metasandstone and slate, with relatively low groundwater permeability
ACCEPTEDand yield (Kennedy & Drage, 2009).
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Bedford Basin is stratified by temperature seasonally, and by salinity almost year-round due to local freshwater inputs (Li & Harrison, 2008). In some years, near-bottom waters can become
-1 hypoxic ([O 2] < 20 µmol kg ) by late fall and early winter (Hargrave et al., 1976; Punshon &
Moore, 2004). However, bottom waters are replenished periodically, either as a result of (1) wind-induced vertical mixing during periods of weaker stratification in winter (Platt et al., 1972;
Punshon & Moore, 2004), or (2) lateral mixing with saltier waters from offshore, typically during spring and fall, due to strong alongshore winds and large tides or storm events (Burt et al.,
2013). These lateral intrusions can result from a build-up of coastal sea level, and bring dense offshore continental shelf water with higher O 2 levels over the sill in the Narrows (20 m) and into deep (70 m) Bedford Basin (Burt et al., 2013). Temperature and salinity as well as O 2 data tend to change abruptly with these intrusion events (AMEC Earth and Environmental, 2011; Burt et al., 2013), supporting a shallow offshore intrusion source, consistent with lateral intrusion mechanisms described by Platt (1975). MANUSCRIPT
The overall circulation within Bedford Basin is estuarine with lower-density upper water flowing out towards the Scotian Shelf, and deeper saline water flowing occasionally into the basin over the sill (Figure 2b). The mean tidal range is 1.5 m and the flushing time of near-surface waters ranges from 10 (Gregory et al., 1993) to 40 days (Shan & Sheng, 2012), while the flushing time for the entire basin was determined to be ~ 90 days.
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FIGURE 2: (a) Sample sites for this study, and their position within Halifax Harbour. The grey star denotes the “Compass Buoy” station (see text) where most sampling took place. (b) A cross-
section schematicACCEPTED of the two-layered flow in Halifax Harbour, depicting dense offshore water
inflow, and freshened surface water outflow, adapted from Fader and Miller (2008).
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3.0 Methods:
3.1 Sample collection
3.1.1 Scotian Shelf sample collection
Samples for δ18 O and salinity were collected offshore of Halifax along the “Halifax Line”
(Therriault et al., 1998; Figure 1) during cruises of the CCGS Hudson in October 2008 and April
2009 (Shadwick & Thomas, 2011). Seawater samples were collected at each site and sample depth for isotopic analyses ( δ18 O) from 20 L Niskin bottles, mounted on a General Oceanic 24- bottle rosette equipped with a CTD (SeaBird).
3.1.2 Bedford Basin sample collection
Sampling for oxygen isotopes of H 2O was supported by the Bedford Basin Monitoring Program
(BBMP), which has maintained weekly sampling in Bedford Basin (Compass Buoy Station: 44°41’37” N, 63°38’25” W; Figure 2a) at four depths MANUSCRIPT (1, 5, 10, and 60 m) since 1992 (Burt et al., 2013; Li, 2014). Oxygen isotope samples were collected at 1 and 60 m to represent surface and deep water respectively. Following collection, 4 L Niskin bottle samples were sub-sampled into
250 ml (for salinity) and 60 ml (for δ18 O) Boston Round bottles with Poly-Seal-Lined caps wrapped with electrical tape to prevent evaporation during storage. δ18 O samples were stored at
4 °C until analysis (typically between 1 – 4 weeks), while salinity samples were stored at room temperature (c. 20 °C) and analyzed every 3 to 4 months. ACCEPTED 3.1.3 Sackville River sample collection
Samples from the Sackville River were collected with a bucket lowered from a pedestrian bridge,
1.2 km from the river mouth, and the bucket contents were transferred directly into 60 ml ( δ18 O)
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Boston Round glass bottles. Samples were collected monthly, except for January and February of
2013 when the river was frozen.
3.1.4 Wastewater sample collection
Wastewater samples were collected on April 3, 2014 from the five major wastewater treatment facilities (WWTFs) in Halifax: Herring Cove, Halifax, Mill Cove, Dartmouth, and Eastern
Passage. Collected samples represent the outflowing, treated wastewater that enters Halifax
Harbour. Samples were collected in either Nalgene or glass bottles before being transferred to
Dalhousie University, where they were stored in 60 ml Boston Round glass bottles before analysis took place.
3.1.5 Precipitation sample collection Samples of precipitation were collected from June 2MANUSCRIPT012 to October 2013 from the roof of the Oceanography Department at Dalhousie University (43°38’10” N, 63°35’38” W) away from any potential sources of contamination. Rainfall samples were collected using a 500 ml Pyrex bottle fitted with a glass funnel (135 mm diameter). The bottle was typically collected between 8:00 and 10:00 Monday to Friday following a precipitation event. On occasion, samples were collected later in the day following the completion of an event, in order to collect a single sample representative of the event. During major, longer, rainfalls samples were collected throughout to avoid the risk of evaporation or overflow. No samples were collected between 17:00 Friday and
8:00 Monday. FollowingACCEPTED sample collection, a 60 ml Boston Round glass bottle was filled, capped, taped, and stored at 4 °C. The volume of the remaining sample was measured and the
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contents transferred into a large glass bottle, containing all of the precipitation samples collected during that month to provide a “monthly-integrated” sample.
Samples of solid precipitation (snow, ice, and hail) were collected using a bucket, weighed down with rocks and lined with plastic. Since snow collection is influenced strongly by wind, some samples could not be volume-weighted accurately. This occurred on three occasions in February
2013, when large snowdrifts were present and snow sample amounts were not representative of the total volume that fell.
Snow samples were collected prior to melting and transferred into 1 L Nalgene bottles. These bottles were stored at 4 °C to allow the snow to melt gradually, and covered to prevent isotopic fractionation during evaporation (Cooper, 1998). Once melted, the water volume was recorded and a 60 ml Boston Round bottle was filled for isot opicMANUSCRIPT analysis. Any remaining meltwater was added to the total monthly precipitation, or “monthly-integrated”, bottle (with the exception of samples that could not be volume-weighted).
3.2 Volume-weighting of precipitation samples
Volume-weighted averages were calculated for monthly and yearly precipitation, as both the number of precipitation events and the volume of precipitation that fell in a single event were variable (from 50 to 1200 ml). Volume-weighted averages ( ̅) of δ18 O for years, months and seasons, were calculatedACCEPTED as: