The Deep Waters of the Eurasian Basin, Arctic Ocean: Geothermal Heat Flow, Mixing and Renewal
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ARTICLE IN PRESS Deep-Sea Research I 53 (2006) 1253–1271 www.elsevier.com/locate/dsr The deep waters of the Eurasian Basin, Arctic Ocean: Geothermal heat flow, mixing and renewal$ Go¨ran Bjo¨rka,Ã, Peter Winsorb aDepartment of Oceanography, Earth Sciences Centre, Go¨teborg University, P.O. Box 460, 405 30 Go¨teborg, Sweden bPhysical Oceanography Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Available online 12 July 2006 Abstract Hydrographic observations from four separate expeditions to the Eurasian Basin of the Arctic Ocean between 1991 and 2001 show a 300–700 m thick homogenous bottom layer. The layer is characterized by slightly warmer temperature compared to ambient, overlying water masses, with a mean layer thickness of 5007100 m and a temperature surplus of 7.072 Â 10À3 1C. The layer is present in the deep central parts of the Nansen and Amundsen Basins away from continental slopes and ocean ridges and is spatially coherent across the interior parts of the deep basins. Here we show that the layer is most likely formed by convection induced by geothermal heat supplied from Earth’s interior. Data from 1991 to 1996 indicate that the layer was in a quasi steady state where the geothermal heat supply was balanced by heat exchange with a colder boundary. After 1996 there is evidence of a reformation of the layer in the Amundsen Basin after a water exchange. Simple numerical calculations show that it is possible to generate a layer similar to the one observed in 2001 in 4–5 years, starting from initial profiles with no warm homogeneous bottom layer. Limited hydrographic observations from 2001 indicate that the entire deep-water column in the Amundsen Basin is warmer compared to earlier years. We argue that this is due to a major deep-water renewal that occurred between 1996 and 2001. r 2006 Elsevier Ltd. All rights reserved. Keywords: Deep water; Convection; Mixing; Geothermal heat; Heat flow; Nansen Basin; Amundsen Basin; Lomonosov Ridge; Arctic Ocean 1. Introduction tions, quite different from the uniform mixing rate assumed in models. While we understand much Diapycnal mixing in the oceans is a prerequisite about the processes responsible for mixing, and for the renewal of the deep waters and is essential have a rough qualitative sense of their distribution, for upper ocean heat transfer and storage. Under- quantitative information on mixing is lacking for standing this critical process has become more most of the world oceans. This is especially true for urgent, as it is now realized that mixing processes the ice-covered and poorly sampled Arctic Ocean. have heterogeneous and complex spatial distribu- Observations of mixing in the upper ocean of the Arctic show that energy levels associated with $Woods Hole Oceanographic Institution contribution 11081. internal waves are roughly 1/3 of those found in ÃCorresponding author. Tel.: +46 31 7732958. mid- and low-latitude regions of the oceans E-mail address: [email protected] (G. Bjo¨rk). (D’Asaro and Morehead, 1991; Plueddemann, 0967-0637/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2006.05.006 ARTICLE IN PRESS 1254 G. Bjo¨rk, P. Winsor / Deep-Sea Research I 53 (2006) 1253–1271 1992). It has been argued that more subtle, double- respectively, divided by the Gakkel Ridge having a diffusive processes may control much of the interior broken structure and generally low altitudes. The mixing (e.g. Padman, 1994). However, processes only direct deep-water communication is with the responsible of abyssal mixing and ventilation in the Greenland-Iceland-Norwegian (GIN) seas through Arctic are still relatively unknown. Studies suggest Fram Strait with a sill depth of about 2500 m (see that the deep waters may be ventilated slowly by Fig. 1 for geographical information). Thus, the renewal of shelf water created by freezing and brine Arctic deep waters below 2500 m are isolated from rejection on the shelves (e.g. Aagaard et al., 1981; direct influence of the surrounding oceans. The Rudels et al., 2000; Winsor and Bjo¨rk, 2000)or estimated isolation ages for the Eurasian Basin and from influxes from the adjacent Norwegian Sea Canadian Basin bottom waters, based on 14C (Aagaard et al., 1985; Jones et al., 1995). measurements, are 250 and 450 years, respec- There are also topographic constraints that tively (Schlosser et al., 1997). determine the type of processes that may affect the The isolated nature of the deep waters suggests deep-water mixing. The Arctic Ocean is divided into that relatively weak fluxes may affect the deep-water two main basins: the Eurasian Basin and the properties over time. One possible flux is heat Canadian Basin, separated by the central Lomono- supplied to the deep ocean from the Earth’s interior, sov Ridge. The Eurasian Basin consists of two sub- the geothermal heat flow. Most observations of basins: the Nansen and Amundsen Basins with geothermal heat flux have focused on extreme fluxes typical maximum depth of about 4000 and 4500 m, at small point sources, such as hydrothermal vents, Fig. 1. Map showing the positions of the CTD stations used in the paper. Squares (black) are from Oden’91 (1991), triangles (green) AOS94, circles (red) ARKXII (1996), and stars (blue) from AO-01 (2001). Filled markers indicate where a homogeneous bottom layer was found. Bathymetry is from the International Bathymetric Atlas of the Arctic Ocean (IBCAO). ARTICLE IN PRESS G. Bjo¨rk, P. Winsor / Deep-Sea Research I 53 (2006) 1253–1271 1255 but there is also a background heat flux associated Arctic: the Oden’91 cruise on I/B Oden (1991), the with the warm interior of the Earth. Stein and Stein AOS94 cruise on CCGS Louis S. St-Laurent (1994), (1992) estimated a global mean geothermal heat flux the ARK XII expedition of 1996 with Polarstern, of 87 mW mÀ2 (1 mW ¼ 10À3 W), with a back- and the AO-01 expedition in 2001 with I/B Oden. ground level of about 50 mW mÀ2 over deep abyssal Potential temperature Y (referenced to the surface plains. Mid-ocean ridges generally have larger unless otherwise noted) and potential density s were values 4200 mW mÀ2 (Murton et al., 1999). computed from standard algorithms (UNESCO, Although the geothermal heat flux is relatively 1983). Fig. 1 shows a map of the Eurasian part of small compared to other fluxes, it has been shown in the Arctic Ocean with bathymetry and positions of several studies that it may have a significant effect the different hydrographic stations used here. on the large-scale circulation. Gustafsson (2002) used the mean global geothermal heat flux to estimate the production of potential energy and 2.2. Geothermal heat flow compared this with estimates of the total amount of energy available from tides and winds for deep Observations of geothermal heat flow are based water mixing. She found that the heating of the deep on measurements of the temperature gradient in the waters of the world’s oceans by geothermal heat sediment. The heat flow Q is determined from flow represents about 4% of the heating from Q ¼ kdT/dz, where k is the thermal conductivity, vertical diffusion, but that the actual work against and dT/dz is the temperature gradient in the buoyancy forces done by geothermal heating can be sediment, usually measured by a probe inserted 10% of the total work by turbulent diffusion. The into the bottom, e.g. a gravity corer. Most published geothermal heat flux can therefore be an important data from the Arctic region comes from the component of the global thermohaline circulation. Canadian Basin, and is available through the This was also shown in modeling studies by Adcroft Global Heat Flow Data Set stored at the US et al. (2001) and Scott et al. (2002). They used a National Geophysical Data Center (NGDC). A spatially uniform heat flux (50 mW mÀ2) through general presentation of this extensive data set is the ocean floor to investigate its effect on the given in Pollack et al. (1993). meridional overturning circulation and found that Langseth et al. (1990) presented an overview of the imposed heat flux caused a perturbation of the geothermal heat flow measurements in the Arctic, meridional overturning cell on the order of several with most observations originating from the Cana- Sverdrups, connecting with an upper level circula- dian Basin. They found typical heat fluxes ranging tion at high latitudes where the imposed heat flow between 30 and 105 mW mÀ2, with an overall mean altered the deep-water circulation in the oceans of 46 mW mÀ2. Majorowicz and Embry (1998) significantly. estimated heat flows in the 35–90 mW mÀ2 range, In this paper we use hydrographic measurements with a mean of 53712 mW mÀ2 from 156 sites in the from icebreaker expeditions to the Arctic from 1991 Sverdrup Basin, Canadian Arctic. Earlier observa- to 2001, focusing on the deep-water properties of tions over the Alpha Ridge during the Cesar the Eurasian Basin, and in particular, the vertically experiment found an average heat flow of homogeneous bottom layer presumably caused by 60 mW mÀ2 (Taylor et al., 1986). Recent measure- geothermal heat flow. Since many of the processes ments on the continental slope of the Laptev Sea that affect the observed layer in the Eurasian Basin revealed relatively high heat flow values between 85 are unknown, we investigate the dynamics of this and 117 mW mÀ2 (Drachev et al., 2003). layer in an exploratory fashion as a starting point Observations from the central Eurasian Basin are for future investigations. relatively sparse. The data used in this paper come from yet unpublished material collected during the 2.