Deep-Water Renewal in a Scottish Fjord: Temperature, Salinity and Oxygen Isotopes

Deep-water renewal in a Scottish fjord: temperature, salinity and oxygen isotopes

William E. N. Austin & Mark E. Inall

The sea lochs (fjords) of north-west Scotland are located in a region of Europe particularly well situated to monitor changes in westerly air streams. The moisture transported in these air streams has a profound effect on regional precipitation, freshwater run-off and, in turn, sea loch circulation. The gentle slope of the regional salinity:δ18O mixing-line, deÞ ned as 0.18 ‰ per salinity unit, suggests that the temperature:δ18O relationship may be readily resolved in these coastal waters. Deep-water renewal events, both observed and predicted from empirical models, in the bottom-waters of Loch Etive provide an opportunity to assess the 18 18 temperature, salinity and δ O relationship. Predicted changes in δ Ocalcite as a function of changing salinity (∆S) and changing temperature (∆T) during deep-water renewal events suggest that > 80 % fall above analytical detection limits. The theoretical likelihood of recording such renewal events in the “palaeoclimate” record appears to be promising, but temperature and salinity change during renewal events may have either sign. Scottish fjords, because of the relatively small impact which salinity 18 has on δ Owater , may provide useful study sites in palaeoclimate research, particularly where palaeotemperature is the primary record of interest.

W. E. N. Austin, School of Geography and Geosciences, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9AL, UK; M. E. Inall, Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll, Scotland PA37 1QA, UK.

The fractionation and resulting ratio changes of is also temperature dependent (e.g. Urey 1947). the stable oxygen isotopes 18O and 16O provide a The Þ rst empirical temperature:δ18O relationship powerful tool in our understanding of the global was developed by McCrea (1950) and the Þ rst hydrological cycle (e.g. Dansgaard 1964; Craig & “palaeotemperature” equation was published by Gordon 1965). In mid-latitude coastal regions, Epstein et al. (1953). Changes in both salinity and freshwater run-off and ground waters (depleted temperature therefore have the potential to be in 18O) mix with oceanic (shelf) water (enriched recorded in the palaeoclimate record of marine 18 18 18 in O) to deÞ ne salinity:δ Owater relationships organisms (δ Ocalcite). It is therefore highly (e.g. Mikalsen & Sejrup 2000). desirable to understand the factors (both spatial An understanding of the quantitative salinity: and temporal) governing δ18O within modern sea δ18O relationship provides a means of model- loch environments ling the δ18O response in these coastal/fjord Scottish fjords are generally deep, semi- waters. If salinity changes, then so too will enclosed basins with a shallow sill at the mouth. A the δ18O of that water. The incorporation of tidal variation in sea level is forced at the mouth, 18 the δ Owater signal into the calcium carbonate and fresh water is input into the interior. In general 18 skeletal structure of marine organisms (δ Ocalcite) there may also be variation in baroclinic forcing

Austin & Inall 2002: Polar Research 21(2), 251–258 251

Fig. 1. Location map. Inset shows locations of Loch Etive and Loch Sunart on the Scottish west coast.

from changes in density and stratiÞ cation at the either low precipitation (summer) or sub-zero seaward side of the sill, driven, for example, by catchment area temperatures (winter). Thus, upwelling or downwelling along the continental whilst deep-water renewal events generally result shelf margin. The presence of a shallow sill at the in a sharp increase in the salinity of basin deep- fjord mouth cuts off basin water, lying below a water, the event may be accompanied by a temp- certain level, from direct communication with erature change of either sign, depending on the the continental shelf and the open ocean. As surface water temperature at the time of renewal. a result the deep-water properties are altered Between renewal events, deep-water density is only through local diffusion within the basin, gradually reduced by diffusive vertical mixing convective overturning within the basin, or with fresher surface waters. This diffusive when the cross-sill inß ow is sufÞ ciently dense mixing, thought to be mediated by the action of to sink into the basin and uplift the bottom- internal wave breaking (Inall & Rippeth 2002), waters. preconditions the deep-water for the next renewal The most dramatic changes in deep-water event. properties occur during the last of these three processes, usually referred to as a deep-water Study area renewal event. In shallow-silled fjords with strong tidal currents, deep-water renewals Two fjords on the west coast of Scotland have generally occur after a period of low freshwater been selected for this study: Loch Sunart and run-off. Low freshwater run-off may result from Loch Etive (Fig. 1).

252 Deep-water renewal in a Scottish fjord Loch Sunart is 33 km long, with an average a period of less than 15 years (Appenzeller et al. width of 1.5 km, a maximum depth of 124 m, 1998). and consists nominally of six sills and basins Scottish sea lochs bridge the land–ocean (Edwards & Sharples 1986). The tidal regime interface, are highly sensitive to freshwater run- in the loch is predominantly semidiurnal, with off, and therefore provide a potential link between a range of 4 m at springs and 1 m at neaps. The the climate ß uctuations associated with westerly catchment area is approximately 300 km2, with air streams and the physical and chemical mountainous ground to the north and east, and properties of the bottom-water of fjords. The aim approximately half the freshwater enters the of this study is to establish the regional salinity: system in the upper basin. Loch Sunart has one δ18O relationship and to evaluate the potential of the smallest freshwater discharge to tidal ß ow of recording deep-water renewal events in the ratios among Scottish lochs (Edwards & Sharples sea loch environment. It is beyond the scope of 1986), probably contributing to the relatively this paper to address how such events might be frequent renewal of its bottom-waters (Gillibrand recognized and resolved in the palaeo-record. et al. 1995). Loch Etive, on the other hand, has one of the highest freshwater discharge to tidal ß ow ratios Methods among Scottish lochs (Edwards & Sharples 18 1986). The upper basin of Loch Etive is 16 km Salinity and δ Owater long, with an average width of 1.2 km, and a max- imum depth of 145 m. The sill is narrow (200 m), A total of 22 water samples collected from Loch and shallow (approximately 14 m). Tidal forcing Sunart during June 1999 are employed to derive is predominantly semi-diurnal, with a neap range a regional salinity:δ18O relationship for fjord of 1.1 m and a spring range of 1.8 m. Tidal ß ows waters. Salinity measurements were made using in excess of 1.5 ms-1 have been observed over the a Guildline salinometer, at the School of Ocean sill, resulting in rapid loss of tidal energy near Sciences, University of Wales, and are reported the sill, which may in turn reduce the efÞ cacy with a precision of +/- 0.02 salinity units. The of diffusive pre condi tioning. This, com bined stable oxygen isotope measurements were made at with the high freshwater input, results in a mean the Scottish Universities Environmental Research renewal period of 1.3 years (Edwards & Edelsten Centre. The δ18O measurements are based on 1 1977). Renewal events, when they do occur, result ml volumes, determined via CO2 equilibration in rapid deep-water property changes of typically with an automated water equilibrator attached 1 unit in salinity, and warming or cooling of 1 to to an isotope ratio mass spectrometer. Data are 2 °C. reported with standard deviations of about +/ - 0.15 ‰ relative to Vienna standard mean ocean Rationale of the study water (VSMOW).

The sea lochs (fjords) of north-west Scotland Deep mooring observations are located in a region of Europe particularly well situated to monitor changes in westerly air A standard U-shaped mooring was maintained in streams. Interannual atmospheric circulation the deep basin of upper Loch Etive from 6 April changes at this latitude are largely governed by 2000 until 10 July 2001 (position 56° 27.37’ N, the North Atlantic Oscillation (NAO), a large- 05° 11.17’ W; water depth 145 m). At a distance scale meridional oscillation of atmospheric mass of 1.7 km landward of the sill, the mooring was between the subtropical anticyclone near the located on the central axis and in the deepest Azores and the subpolar low pressure system part of the upper basin and instrumented with near Iceland (Hurrell 1995). The inß uence of a Seabird T/S sensor and an Aanderaa current the NAO on both precipitation and westerlies is meter at 15 m above the sea ß oor. Further T/S well established (e.g. Trigo et al. 2002). Long- instruments were positioned throughout the water term variability in the NAO index, reß ecting the column. Observations from the T/S sensor located dominant mode of atmospheric behaviour in the at 15 m above the sea ß oor are reported here. Two North Atlantic region, is observed over a range complete renewal events were observed during of multi-annual periodicities, most of which have the mooring deployment.

Austin & Inall 2002: Polar Research 21(2), 251–258 253 underway (Mikalsen & Austin unpubl. data). In the high latitudes, for example, the impact of sea ice formation and melting may signiÞ cantly 18 alter the seasonal evolution of salinity:δ Owater relationships (e.g. Strain & Tan 1993).

Deep-water renewal: temperature and salinity changes Prior to the Loch Etive deep-water renewal event of May 2000 (Fig. 4) a slow, gradual decline in bottom-water salinity of < 0.1 over a period δ18 Fig. 2. Salinity: O mixing line, Loch Sunart, July 1999 of weeks is observed, a signature of vertical (n = 22). δ18O = 0.18*S – 6.0 (R2 = 0.998). water diffusive mixing between deep-water and fresher intermediate water. On 9 May, between 10:08 and 11:22 on a ß ooding spring tide, the salinity dropped by 0.39 and the temperature by Results and discussion 1.63 °C. The mean current speed rose from 2.4 cm/s-1 during the 24.84 hours before the event to 18 -1 Regional salinity: δ Owater relationship 10.1 cm/s over the same period after the event. CTD casts made on 25 April and 5 June revealed 18 -1 The regional salinity:δ Owater mixing-line for an increase in dissolved oxygen from 0.9 mg/l Scottish west coast waters, ranging from fresh to 9.5 mg/l-1 at 15 m above the sea ß oor. All these water to fully marine water (Fig. 2), is deÞ ned by pieces of evidence demonstrate the effect of a the following relationship: full deep-water renewal event on the deep basin water of upper Loch Etive. The dense coastal δ18O = 0.18*S - 6.0 (1) water water, presented at the sill by the spring tide, 18 At zero salinity (S = 0), the predicted δ Owater ß oods over the sill, runs down the inner face of of freshwater entering the coastal environment the sill as a density current, and displaces upward is -6.0 ‰. Observed values, collected from the the old water previously residing in the depths of freshwater tributaries entering Loch Sunart in the basin. July 1999, range from -5.76 to -6.32 ‰. These observed and predicted values are consistent with 18 18 Predicting δ Owater and δ Ocalcite 18 the weighted annual δ Owater in precipitation from 18 this region of north-west Europe (Fig. 3), which Regional salinity:δ Owater mixing-lines provide 18 lies within the range -5 to -8 ‰ (IAEA 2001). a means to predict the δ Owater composition of The fairly broad range of the latter reß ects the coastal water from observations of salinity. The fact that data have been contoured, based on the Loch Etive deep-water renewal event of May interpolation of a fairly limited amount of Global 2000 (Fig. 4) is characterized by a sudden decline Network for Isotopes in Precipitation (GNIP) in bottom-water salinity of nearly 0.5. Applying 18 station data, spanning a wide geographical region Eq. (1) to estimate δ Owater , a corresponding and (J. Birks, pers. comm. 2002). equivalent shift of about 0.09 ‰ is obtained. The The extent to which seasonal precipitation effect of salinity associated with the renewal event 18 differences in δ Owater , dampened as they are of May 2000 is therefore less than the analytical in stream waters (e.g. Soulsby et al. 2000), precision of our measurements (+/- 0.15 ‰). inß uence the slope of the coastal mixing lines In this study, the temperature: δ18O relationship is largely unknown. Periods of spring snow adopted comes from Bemis et al. (1998): melt, for example, may release large volumes T(°C) = 16.5 -4.8(δ18O - δ18O ) (2) of isotopically depleted water into the coastal calcite water environment and may, for a short time, increase To explore the impact of temperature on “equil- 18 18 the slope of the salinity:δ Owater mixing-line. An ibrium” calcite, the δ Owater correction of extended regional st udy of nor th-west European to -0.27 ‰ (Hut 1987) is employed to convert from 18 Arctic salinity:δ Owater mixing-lines is currently the VSMOW to the Vienna Pee Dee belemnite

254 Deep-water renewal in a Scottish fjord Fig. 3. North-western European mean annual δ18O ‰ (VSMOW) in precipitation (modiÞ ed from IAEA 2001).

18 Fig. 4. Mooring data, Deep Basin, Loch Etive (May 2000): (a) temperature (°C); (b) salinity; (c) δ Owater (‰, VSMOW); (d) 18 δ Ocalcite (‰, VPDB).

Austin & Inall 2002: Polar Research 21(2), 251–258 255 Fig. 5. Temperature and salinity changes of the bottom-waters

of Loch Etive from before to after renewal events. Triangles indicate modelled renewal events of Edwards & Edelsten (1977); the circle indicates the observed event reported here. Contours are of predicted changes in 18 δ Ocalcite , produced as a result of temperature and salinity changes; the area between the broken lines is below the detec- tion limits of the measuring technique.

(VPDB) scale: renewal event recorded in May 2000 (Fig. 4). All events are described in terms of the change δ18O = δ18O - 0.27 (3) water(VPDB) water(VSMOW) in salinity and temperature experienced in the From Eqs. (1), (2) and (3), it is possible to derive bottom of the deep basin before and after renewal an estimate of the theoretical “equilibrium” (Fig. 5). Equations (1) and (2) are differentiated 18 calcite (δ Ocalcite) from any given observation with respect to S and T, respectively, and of salinity and temperature. Using the deep combined and rearranged to give an expression 18 mooring data from Loch Etive, we calculate a for the change in δ Ocalcite as a function of theoretical δ18O change of approximately 0.3 ‰ a change in salinity (∆S) and a change in associated with the deep-water renewal event temperature (∆T). 18 of May 2000 (Fig. 4). In terms of the potential Calculating changes in δ Ocalcite over a typical 18 impact on “equilibrium” δ Ocalcite , temperature range of ∆S and ∆T, and noting that measurement 18 is the most signiÞ cant feature of this renewal precision of the δ Ocalcite is +/- 0.05 ‰, allows us event. Analytical precision of the δ18O analyses to predict which of the historical overturn events of marine carbonates is often reported as +/ can be detected. From the twelve deep-water - 0.05 ‰ relative to the VPDB standard (e.g. renewal events considered, only two fall below Bemis et al. 1998) and it should, theoretically, the detection limit, while a further two lie close be possible to detect such events if they are to the limit (Fig. 5). preserved and resolved in the palaeoclimate record of Scottish sea lochs. Conclusions Historic renewal events in Loch Etive The data presented in this paper provide the Þ rst 18 Twelve renewal events for the upper basin of regional salinity:δ Owater mixing-lines for the Loch Etive have been examined to determine coastal waters of north-western Scotland. Deep- 18 the probability of event detection by δ Ocalcite water mooring data from Loch Etive provide measurement. Eleven of these events come from rare insight into the timing and magnitute of a the empirical model predictions of Edwards deep-water renewal event. As anticipated, the & Edelsten (1977), the twelfth is a deep-water slope of the mixing-line (0.18) is less than similar

256 Deep-water renewal in a Scottish fjord relationships deÞ ned for the North Sea (0.25; References Israelson & Buchardt 1991) and the Sognefjord, western Norway (0.31; Mikalsen & Sejrup Bemis, B. E., Spero, H. J., Bijma, J. & Lee, D. W. 1998:Re- 2001). Such spatial gradients in the salinity: evaluation of the oxygen isotopic composition of plank- tonic foraminifera: Experimental results and revised paleo- δ18 Owater mixing-lines, from the coastal regions temperature equations. Paleoceanography 13, 150–160. of north-western Europe to the Arctic, are to Craig, H. & Gordon, L. I. 1965: Deuterium and oxygen 18 be expected because of the regional pattern of variations in the oceans and the marine atmosphere. In E. 18 Tongiorgi (ed.): Stable isotopes in oceanographic studies δ Owater in precipitation (Fig. 3; Mikalsen & and paleotemperatures. Pp. 9–130. Pisa: Laboratory of Austin, unpubl. data). Scottish fjords, because Nuclear Geology, National Research Council. of the relatively small impact which salinity Dansgaard, W. 1964: Stable isotopes in precipitation. Tellus 18 has on δ Owater , may provide useful study sites 16, 436–468. in palaeoclimate research, particularly where Edwards, A. & Edelsten, D. J. 1977: Deep water renewal of palaeotemperature is the primary record of Loch Etive: a three basin Scottish fjord. Estuar. Coast. Mar. Sci. 5, 575–595. interest. Sclerochronological and stable isotopic Edwards, A. & Sharples, F. 1986: Scottish sea lochs: a cata- analyses of banded mollusc shells, such as the logue. Spec. Publ. 134. Oban, Scotland: Scottish Assoc- long-lived species Arctica islandica, may provide iation of Marine Science. a means of detecting such events in the recent Epstein, S., Buchsbaum, R., Lowenstam, H. A. & Urey, H. C. 1953: Revised carbonate-water isotopic temperature scale. geological record. Geol. Soc. Amer. Bull. 64, 1315–1325. Gillibrand, P. A., Turrell, W. R. & Elliott, A. J. 1995: Deep- water renewal in the upper basin of Loch Sunart, a Scottish fjord. J. Phys. Oceanogr. 25, 1488–1503. Hurrell, J. W. 1995: Decadal trends in the North Atlantic Oscil lation—regional temperature and precipitation. Sci- ence 269, 676–679. Hut, G. 1987: Consultants group meeting on stable isotope reference samples for geochemical and hydrological investigations, 16–18 September 1985, Vienna. Report to Director General, International Atomic Energy Agency. IAEA (International Atomic Energy Agency) 2001: GNIP maps and animations. Vienna: IAEA. Accessible at http: //isohis.iaea.org. Inall, M. E. & Rippeth, T. P. 2002: Dissipation of tidal energy and associated mixing in a wide fjord. Environ. Fluid Mech. 12, 219–263. Israelson, C. & Buchardt, B. 1991: The isotopic composition of oxygen and carbon in some fossil and recent bivalve shells from East Greenland. LUNDQUA Rep. 33, 117–123. Lund University. McCrea, J. M. 1950: On the isotopic chemistry of carbonates and a paeotemperature scale. J. Chem. Phys. 18, 849–857. Mikalsen, G. & Sejrup, H. P. 2000: Oxygen isotope composi- Acknowledgements.—WENA thanks Anne Hamerstein tion of fjord and river water in the Sognefjorden drain age (School of Ocean Sciences [SOS], University of Wales) for area, western Norway: implications for palaeoclimate help with salinity measurements, Hilary Kennedy (SOS), studies. Estuar. Coast. Shelf Sci. 50, 441–448. Andrew Tait and Tony Fallick (Scottish Universities Environ- Soulsby, C., Malcolm, R., Helliwell, R., Ferrier, R. C. & mental Research Centre, East Kilbride)—all for help with Jenkins, A. 2000: Isotope hydrology of the Allt a’Mharcaidh stable isotope measurements. Jack Jarvis and Richard catchment, Cairngorms, Scotland: implications for hydro- Bates provided Þ eld assistance. Financial support from logical pathways and residence times. Hydrol. Process. 14, the Royal Society, Carnegie Trust, and EU Framework V 747–762. (HOLSMEER) are gratefully acknowledged. WENA also Strain, P. M. & Tan, F. C. 1993: Seasonal evolution of oxygen thanks Gaute Mikalsen, Phil Gillibrand, Tony Fallick and isotope–salinity relationships in high-latitude surface Jean Birks for their helpful comments and advice. MEI thanks waters. J. Geophys. Res. 98(C8), 14 589–14 598. Colin GrifÞ ths (DML) for mooring design, deployment and Urey, H. C. 1947: The thermodynamic properties of isotopic recovery, instrument calibration and data processing. substances. J. Chem. Soc. 1947, 562–581.

Austin & Inall 2002: Polar Research 21(2), 251–258 257