The development of the Western Boundary Undercurrent (WBUC) in a changing climate since the beginning of the Miocene


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Antje.Mueller-Michaelis [ at ] awi.de

Abstract

The present deep Western Boundary Undercurrent (WBUC) at the Eirik Drift off the southern tip of Greenland is mainly fed by the overflows over the Greenland-Scotland ridge (GSR), the Denmark Strait Overflow Water (DSOW) and Iceland-Scotland Overflow Water (ISOW), respectively (Fig. 1). Interactions of the atmosphere with the warm and saline Atlantic surface inflow to the Norwegian-Greenland Sea (NGS) yield in convective mixing and the production of these dense overflow water masses (Van Aken, 2007). Therefore, the WBUC at the Eirik Drift is sensitive to changes in the deep-water formation and hence, to climate changes. Changes in the WBUC will, in turn, affect the world's climate. The WBUC is the main contributor to the lower branch of the North Atlantic Thermohaline Circulation (THC). The upper counterpart of the THC is responsible for the global redistribution of heat and freshwater via the surface ocean. The Eirik Drift has been shaped by the WBUC since the Miocene. It therefore archives information of changes in depositional processes and hence, changes in the pathways and strength of the WBUC. The high-resolution multichannel seismic reflection data network collected during RV Maria S. Merian cruise MSM 12/2 crossed ODP Leg 105 Site 646 and IODP Expedition 303 Sites U1305, U1306, and U1307 (Fig. 1). Thus, the seismic reflection data could be incorporated with geological information from the ODP and IODP sites (Channell et al., 2010; Srivastava et al., 1989) to deduce information on the development of the WBUC and a much clearer understanding of the evolution of the climate in the northern North Atlantic during the Neogene. Synthetic seismograms based on density and P-wave velocity data from ODP Leg 105 Site 646 and IODP Expedition 303 Sites U1305, U1306, and U1307 were correlated with the processed seismic reflection data. We identified four seismic units and the reflectors defined by Arthur et al. (1989) and refined the stratigraphy by horizons A1 (0.8 Ma), A2 (1.4 Ma) and A3 (19-17 Ma) (Table 1). We tracked the reflectors and seismic units from ODP Leg 105 Site 646 over our complete study area. The erosional and depositional centers of each (sub)unit were interpreted with respect to the prevailing deep current activity. The shapes and locations of the depocenters of the seismic (sub)units as well as the internal structure and reflection characteristics are used to decipher the changes in the WBUC's pathways and intensity in the study area. In sediment drifts deposited under the influence of along-slope processes, unit thinning, converging internal reflections and reflector truncations indicate the erosional centers of deep current cores. In the Northern Hemisphere, the depocenters with diverging internal reflections are found to the right of the deep current flow (Faugères et al., 2008; Faugères et al., 1999; Nielsen et al., 2008). The location and orientation of the depocenters as well as the morphology of the basal horizons help to connect the erosional centers to pathways over the study area as contour currents follow certain depth ranges. We observe shifts in the location and orientation of the depocenters and crest migrations among the seismic subunits. This indicates a highly variable deep current activity at the Eirik Drift. The interpretation of our data leads to a model of the paleocirculation at the Eirik Drift with pathways and intensity of the WBUC. Here, the observations in every (sub)unit and their interpretations are discussed with respect to the evolution of deep currents, shifts in deep water formation regions and climate changes in the northern North Atlantic. The various depocenters of subunit SUIV-c (~40-19 Ma; Table 1) are interpreted as an infill and drape of the hummocky basement (~60-40 Ma) in a tranquil environment at the Eirik Drift (Mueller-Michaelis et al., 2012 (submitted)). Not surprisingly, no WBUC is observed at the Eirik Drift as the GSR has not reached the critical level to allow NCW overflow until early Miocene (Vogt, 1972). But still there is an indication for existing NCW deep circulation in the NE North Atlantic within this time interval observed with a major increase in drift accumulation at the Feni Drift (Wold, 1994) (~35-33 Ma; Fig. 1). This NCW flow is found southward directed from the Rockall Trough to the North Atlantic (Fig. 2a). Two possible deep-water formation regions have been hypothesized to be responsible for that: South of the GSR in the Rockall Trough (Stoker, 1998) or north of the sill in the SE Norwegian Sea with an open gateway at the distal eastern end of the GSR via the Faroe-Shetland basin (Davies et al., 2001). Indications for the latter are also documented in the Faroe Drift NE of the GSR (Davies et al., 2001) and therefore seems more likely than the Rockall Trough theory. We observe the onset of WBUC activity at ~19 Ma at the Eirik Drift. A widespread erosional event in the North Atlantic in the early Miocene accounts also for the onset of drift building at horizon A3 (~19-17 Ma, Table 1) (Mueller-Michaelis et al., 2012 (submitted)). This event is also observed in Bjorn, Gardar and Feni Drifts (Fig. 1) (Miller and Tucholke, 1983). We suggest the deep-water formation region in the NGS and the onset of NCW overflows following the subsidence of the GSR in early Miocene (Vogt, 1972). The NCW flow remained intense at the Eirik Drift until ~15 Ma, but was depleted between 15-12.5 Ma (Mueller-Michaelis et al., 2012 (submitted)). This is in accordance to the general observation of low NCW flux between ~16 to 12.5 Ma (Woodruff and Savin, 1989; Wright et al., 1992). We suggest that the onset of the NCW overflows and intensification of the deep-water formation reinforced the Miocene warming phase with an intensified northward transport of warm and saline Atlantic surface water. Increased freshwater input to the Nordic Seas towards the mid Miocene Climatic Optimum may then have yielded in a strong, stable ocean stratification with a freshwater-barrier-layer, which inhibited deep convection in the Nordic Seas. The decrease in NCW formation, in turn, accounted for a decrease in northward heat transport to high latitudes and assisted the cooling of the mid Miocene Climate Transition (~14 Ma). We expect this climate cooling to have resulted in increasing instability of the surface ocean stratification due to heat loss and salt input at the oceans surface. This yields a renewed NCW production and thus, a renewed onset of WBUC has been observed at ~12.5 Ma at the Eirik Drift with the formation of horizon R5 (~12-10 Ma; Table 1). The renewed NCW production is also documented in increased accumulation observed at Snorri, Bjorn, Gardar and Hatton Drift (Fig. 1) between 13 and 10 Ma (Wold, 1994). Between ~10 and 8.1 Ma we observe a shallower WBUC than between 19 and 15 Ma at the Eirik Drift. The shallower WBUC is interpreted as the result of less dense and thus, weaker NCW flow. With our knowledge of the present WBUC at the Eirik Drift, we would expect a strong WBUC along with higher NCW production and vice versa. Higher NCW production, in turn, is expected in a cool regime, as surface cooling and salt input due to formation of sea-ice increase the ocean surface density and thus, the deep-water production (Van Aken, 2007). However, we observe a decrease in WBUC intensity along with the ongoing Miocene cooling phase. The WBUC transports lower (DSOW) and upper (ISOW) NCW in different core depths. It is possible that we only observe one of the two different cores at the Eirik Drift region. As we observe a shallowing and thus weakening during a cold phase, we interpret the observed NCW flow as the upper NCW (ISOW), which more likely weakened and shallowed in a cooling regime. We suggest that the lower NCW (DSOW) flowed at larger depths outside of our study area (Fig. 2b). At 8.1 and 7.5 Ma the reflector doublet R3/R4 was formed by NCW pulses with inhibited NCW flow at 9 Ma and 7 Ma (Mueller-Michaelis et al., 2012 (submitted)). We suggest short-term temperature fluctuations to result in strong NCW pulses with similar pathways like between 12 and 8 Ma (Fig. 2b). During the following deposition of subunit SUIII-b (7.5-5.6 Ma) the observed WBUC influence at the Erik Drift further weakened. The WBUC here was presumably too weak to (re)deposit sediments on top of the piled drift axis. We address this weakening to the ongoing climate cooling, resulting in an intensification of a deeper, denser lower NCW (DSOW) flow outside of our study area, and a weakening of the observed upper NCW (ISOW) flow. Thus, the WBUC at the Eirik Drift stayed weak until the climate started to warm again at ~6 Ma with an increase in WBUC activity in the study area observed at reflector R2 (5.6 Ma). Between 5.6-4.5 Ma we observe an increasing WBUC at the Eirik Drift with its maximum intensity leading to the formation of the erosional unconformity at ~4.5 Ma (Table 1) (Kaminski et al., 1989) along with climate warming. For the first time, we observe two branches of NCW flow at the Eirik Drift. The lower branch flows over ODP Leg 105 Site 646 and was identified as rather DSOW (lower NCW) then ISOW (upper NCW) (Kaminski et al., 1989). We therefore conclude that the upper branch transported ISOW (upper NCW). The two branches of WBUC at the Eirik Drift are of maximum intensity observed here. Nevertheless, we interpret this observation as a shallowing and weakening of the NCW flow system in the northern North Atlantic during warming climate. During the preceding cool phases the one observed branch is assumed to transport ISOW (upper NCW) and the flow of DSOW (lower NCW) is suggested to proceed at greater depth and/or different pathways outside of our study area. Along with the climate warming the WBUC system shallows and hence, the intense deep branch of NCW flow lies within our study area. The DSOW (lower NCW) flow during this warming period is, even though it is assumed to have shallowed and weakened, still of higher intensity than the upper NCW (ISOW) flow during the cold phases. With the end of mid Pliocene warmth cooling started again at ~3.6 Ma, and the WBUC intensity of DSOW and ISOW flow remained strong and the pathways almost the same until ~2.5 Ma. The onset of drift building at the Gloria Drift (Fig. 1) at ~4 Ma (Wold, 1994) indicates the onset of deep-water formation in the Labrador Sea along with the Pliocene cooling (Fig. 2c). Entrainment of less dense LSW to the dense overflows may explain why we do not observe intensification and deepening of the WBUC system during Pliocene cooling. At 2.5 Ma the intensification of Northern Hemisphere Glaciation (NHG) is documented in the sedimentary record of ODP Leg 105 Site 646 (Arthur et al., 1989). We observe a gradual shallowing and deepening of 2 WBUC branches after 2.5 Ma at the Eirik Drift NHG. The expanded Nordic ice shields isolate the surface ocean from exchanges in heat, freshwater and momentum with the overlying atmosphere and might have shifted the deep water formation region to the south. This southward shift possibly account for less dense overflow waters even in a cold regime. Also the changes of NCW composition due to the entrainment of LSW should be considered. Based on our data and the geological records at the ODP/IODP sites at the Eirik Drift, we deciphered parts of the complex interplay of changes in the NCW flow and climate in the northern North Atlantic. References: Arthur, M.A., Srivastava, S.P., Kaminski, M., Jarrard, R.D., Osler, J., 1989. Seismic Stratigraphy and History of Deep Circulation and Sediment Drift Development in Baffin Bay and the Labrador Sea, in: Srivastava, S.P., Arthur, M., Clement, B., et_al. (Eds.), Scientific Results. Ocean Drilling Program, College Station, pp. 957-988. Channell, J.E.T., Sato, T., KANAMATSU, T., STEIN, R., Alvarez Zarikian, C.A., 2010. Expedition 303/306 synthesis: North Atlantic climate, in: Channell, J.E.T., KANAMATSU, T., Sato, T., STEIN, R., Alvarez Zarikian, C.A., Malone, M.J., Scientists, a.t.E. (Eds.), Proceedings of the Integrated Ocean Drilling Program. Integrated Ocean Drilling Program, College Station, TX. Davies, R., Cartright, J., Pike, J., Line, C., 2001. Early Oligocene initiation of North Atlantic Deep Water formation. Nature 410, 917-920. Faugères, J.C., Stow, D.A.V., Camerlenghi, M.R.a.A., 2008. Chapter 14 Contourite Drifts: Nature, Evolution and Controls, Developments in Sedimentology. Elsevier, pp. 257, 259-288. Faugères, J.C., Stow, D.A.V., Imbert, P., Viana, A.R., 1999. Seismic Features Diagnostic of Contourite Drifts. Marine Geology 162, 1-38. Kaminski, M.A., Gradstein, F.M., Scott, D.B., Mackinnon, K.D., 1989. NEOGENE BENTHIC FORAMINIFER BIOSTRATIGRAPHY AND DEEP-WATER HISTORY OF SITES 645, 646, AND 647, BAFFIN BAY AND LABRADOR SEA, in: Srivastava, S.P., Arthur, M., Clement, B., et_al. (Eds.), Scientific Results. Ocean Drilling Program, College Station, pp. 731-756. Miller, K.G., Tucholke, B.E., 1983. Development of Cenozoic Abyssal Circulation south of the Greenland-Scotland Ridge, in: Bott, M.H.P., Saxov, S., Talwani, M., Thiede, J. (Eds.), Structures and Development of the Greenland-Scotland Ridge - New Methods and Concepts. Plenum Press, New York and London, pp. 549-589. Mueller-Michaelis, A., Uenzelmann-Neben, G., Stein, R., 2012 (submitted). Onset of drift building at the Eirik Drift during the Miocene: Evidence from high-resolution seismic reflection data. Marine Geology. Nielsen, T., Knutz, P.C., Kuijpers, A., Camerlenghi, M.R.a.A., 2008. Chapter 16 Seismic Expression of Contourite Depositional Systems, Developments in Sedimentology. Elsevier, pp. 301-321. Srivastava, S.P., Arthur, M.A., Clement, B., et_al_(eds.), 1989. Proceedings ODP, Scientific Results, 105. ODP, College Station, TX (Ocean Drilling Program). Stoker, M.S., 1998. Geological Processes on Continental Margins, in: Stoker, M.S., Evans, D., Cramp, A. (Eds.), Geological Processes on Continental Margins. Geological Society, London. Van Aken, H.M., 2007. The Oceanic Thermohaline Circulation. Springer, New York. Vogt, P.R., 1972. The Faroe-Iceland-Greenland aseismic ridge and the western boundary undercurrent. Nature 239, 79-81. Wold, C.N., 1994. Cenozoic Sediment Accumulation on Drifts in the Northern North Atlantic. Paleoceanography 9, 917-941. Woodruff, F., Savin, S., 1989. Miocene deepwater oceanography. Palaeogeography 4, 87-140. Wright, J.D., Miller, K.G., Fairbanks, R.G., 1992. Early and middle Miocene stable isotopes: implications for deepwater circulation and climate. Paleoceanography 7, 357-389.



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Conference (Invited talk)
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Not peer-reviewed
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Published
Event Details
ICDP/IODP Kolloquium Freiberg 2013, 25 Mar 2013 - 27 Mar 2013, TU Bergakademie Freiberg.
Eprint ID
32221
Cite as
Mueller-Michaelis, A. and Uenzelmann-Neben, G. (2013): The development of the Western Boundary Undercurrent (WBUC) in a changing climate since the beginning of the Miocene , ICDP/IODP Kolloquium Freiberg 2013, TU Bergakademie Freiberg, 25 March 2013 - 27 March 2013 .


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