Intermediate water ventilation in the Nordic seas during MIS 2

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /grl.50325, 2013 Intermediate water ventilation in the Nordic seas during MIS 2 Kari-Lise Rørvik, 1,2 Tine L. Rasmussen, 1 Morten Hald, 1 and Katrine Husum 1 Received 14 January 2013; revised 3 March 2013; accepted 5 March 2013; published 12 May [1] A high-resolution marine record from the northern Norwegian continental margin off Lofoten is used to reconstruct changes in the oceanography of the Nordic seas during marine isotope stage (MIS) 2 including the Last Glacial Maximum and early deglaciation circa 25 to 16 ka. The period was characterized by generally warm subsurface water temperatures >2 C and inflow of Atlantic surface water. Several events were characterized by decrease in ventilation of the intermediate water and low subsurface temperature and salinity. They correlate with colder atmospheric temperatures as seen in ice cores. The events terminated with gradual strengthening of the intermediate water ventilation and increase in subsurface temperature. The generation of intermediate water was unstable and experienced climate and ventilation changes on millennial and centennial timescales. The changes appear consistent with modeling experiments that predict short-lasting circulation stops during MIS 2 due to release of meltwater in the Nordic seas. Citation: Rørvik, K.-L., T. L. Rasmussen, M. Hald, and K. Husum (2013), Intermediate water ventilation in the Nordic seas during MIS 2, Geophys. Res. Lett., 40, , doi: / grl Introduction [2] The Atlantic Meridional Overturning Circulation (AMOC) is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic Ocean and a southward flow of cold water generated by deep convection in the Nordic seas and North Atlantic Ocean. This ocean circulation system transports large amounts of heat from the southern hemisphere to the northern hemisphere. Today, in the Nordic seas, inflow of warm and saline Atlantic Water constitutes the northernmost surface component of the AMOC (Figure 1). Previous investigations have shown that during the marine isotope stage (MIS) 2, when ice sheets covered the northern European and American continents, Atlantic Water reached to the Fram Strait in the polar North Atlantic [e.g., Hebbeln et al., 1994]. MIS2 also comprises the Last Glacial Maximum (LGM), when ice volume reached its maximum circa ka [Martinson et al., 1987]. However, despite numerous published records, a detailed reconstruction of the marine paleoenvironment in the Nordic seas is lacking, and the paleoceanographic and climatic development of the MIS 2 interval is still poorly understood [e.g., CLIMAP Project Members, 1981; Legrande and Wunsch, 1995;Pflaumann et al., 2003]. [3] Here we present a multiproxy study of the highresolution core MD from the northern Norwegian continental margin. The record offers the opportunity to reconstruct the properties of the surface and intermediate water masses in the Nordic seas during MIS 2 on multidecadal time resolution. The study is primarily based on the distribution pattern of planktonic foraminifera, absolute summer subsurface temperature calculated by transfer functions using planktonic foraminifera census data, planktonic and benthic stable isotope values, absolute salinity derived from planktonic d 18 Ovalues and ice-rafted debris (IRD). The results are compared to records from the northeastern Atlantic Ocean (Figure 1). 2. Material and Methods [4] The giant piston core MD ( N/ E) was taken in 1999 at 1122 m water depth off Lofoten in northern Norway. The slope is steep, and the site of MD is located close to the former ice margin of MIS 2 and the LGM (Figure 1) [Laberg and Vorren, 2004]. The lithology of the record is described in detail by Laberg and Vorren [2004], and the IRD record and age model are described in detail by Rørvik et al. [2010]. For this study, oxygen and carbon stable isotope measurements were performed using the planktonic foraminifera species Neogloboquadrina pachyderma sinistral (s) and the shallow-infaunal benthic foraminifera species Cassidulina neoteretis. Planktonic foraminifera ( mm size fraction) were counted and identified to species level (>300 specimens/sample). Some samples from intervals with low IRD contained too few planktonic specimens for quantification. Absolute summer temperatures were calculated by transfer functions based on planktonic foraminifera in the size fraction >100 mm [Husum and Hald, 2012] using the C2 program [Juggins, 2010]. We calculated the subsurface temperatures at water depth 100 m (sea surface temperature; SST-100 m), because N. pachyderma scalcifies at m below the sea surface [e.g., Bauch et al., 2001], The Weighted Average Partial Least-Squares method with three components were used according to the recommendations of Birks [1998]. The Root Mean-Squared Error of Prediction is 0.56 C. The planktonic d 18 Ovaluesmeasured on N. pachyderma (s) were corrected for ice volume changes and converted to salinity values using the equation published in Lubinski et al. [2001] and the mixing line published in stlund et al. [1987]. The calculated salinity represents subsurface waters at 100 m water depth below the surface. 1 Department of Geology, University of Tromsø, Tromsø, Norway. 2 Ross Offshore AS, Sandefjord, Norway. Corresponding author: K.-L. Rørvik, Ross Offshore AS, Leif Weldings Vei 14, NO-3208 Sandefjord, Norway. (kari.lise.rorvik@rossoffshore.no) American Geophysical Union. All Rights Reserved /13/ /grl Results and Interpretation [5] The studied time interval covers most part of MIS 2 including the LGM and early deglaciation and dates circa ka (Figure 2). The planktonic d 18 O values in core MD are roughly parallel to the North Atlantic record MD published by Peck et al. [2006] and with 1805

2 grained, with low content of IRD, which indicates fewer melting icebergs (Figure 2i). Parts of the layers are laminated [Laberg and Vorren, 2004; Rørvik et al., 2010]. [8] The events terminate with a near-surface warming seen as a reduction in the percentage of N. pachyderma s and relatively rapid increase in SST-100 m (Figures 2a and 3a), which indicates a warming of the surface water. In summary, all proxies show synchronous biological, geochemical, and sedimentological anomalies during the events. The onsets are abrupt and occur within circa 100 years, whereas the recoveries to average values are more gradual and occur within circa 500 years (Figure 2). Figure 1. Location of studied core MD (closed circle) and other cores (open circles). The modern circulation of the North Atlantic region is indicated. NAC, North Atlantic Current; NC, Norwegian Current; NCC, Norwegian Coastal Current; WSC, West Spitsbergen Current; EGC, East Greenland Current. similar timing of the well-known cold Heinrich events H2 and H1 and the warm interstadial 2 (IS2) (Figures 2a, 2b, and 2c) [Bond et al., 1993]. At site MD , the planktonic foraminiferal fauna is dominated by the polar species N. pachyderma s (Figure 2a). The absolute SST-100 m show that subsurface temperatures varied between 1.7 and 4 C (Figure 3d). [6] The percentage of N. pachyderma s, the summer SST-100 m, salinity and the d 18 O records (Figures 2a 2d, 3d, and 3e) show several rapid subcentennial scale oscillations indicating rapid fluctuations in temperature and salinity, probably as a result of the proximity to the former ice sheet margin. The planktonic and the benthic oxygen isotopes records mirror each other almost completely with the same magnitude and number of excursions. Also the planktonic and benthic d 13 C records parallel each other (Figures 2e, 2f, and 2g). This indicates an overall well-mixed water column over the core site for most part of the studied time interval. [7] A notable feature in the three d 13 C records are the three events of depletions in both planktonic and benthic 13 C (numbered 1 3) (Figures 2e, 2f, and 2g). Event 3 is uncertain as it is situated at the end of the studied core sections and has limited number of data points. However, in all three events in core MD , the benthic d 13 C values are generally lower by about 0.7 1%. The depletion in 13 C could indicate reduced ventilation of both the intermediate and surface waters of the Nordic seas similar to the events in the northeastern North Atlantic Ocean (Figure 2f) [Peck et al., 2006]. The 13 C events are marked by minima in the concentration of both planktonic and benthic foraminifera (Figure 2h). In contrast to H1 and H2, the sediments of events 1 3 in MD are fine 4. Discussion 4.1. Intermediate Water Ventilation During MIS 2 [9] Our proxy records show relatively warm subsurface conditions with temperatures mostly >2 C indicative of relatively strong advection of Atlantic Water to the Nordic seas (Figures 2a and 3d). The generally high benthic d 13 C values indicate good ventilation of the intermediate water (Figures 2f and 2g). The water mass probably represents the Glacial North Atlantic Intermediate Water, which is characterized by its very high d 13 C values [Oppo and Lehman, 1993; Millo et al., 2006; Rasmussen and Thomsen, 2009]. Our results thus point to an overall active intermediate water circulation in the Nordic seas during most part of MIS 2. The period is interrupted by events with lower subsurface temperatures and reduced influence of the Atlantic surface water and strong decrease in benthic d 13 C values (Figure 2). We suggest that the decrease in d 13 C values is linked to reductions in the circulation and formation of intermediate water in the Nordic seas. This is in agreement with the sedimentological evidence. According to Laberg and Vorren [2004], the laminations of the sediment layers originated from unstable bottom water conditions with fluctuations in bottom current activities that interrupted the intervals of more stable bottom currents. The latter periods correlate with lower abundances of N. pachyderma s and higher subsurface temperatures and probably stronger inflow of Atlantic Water (Figures 2a and 3d). [10] The generally close correlation in time of the events of low d 13 C of core MD with core MD from the Ireland margin [Peck et al., 2006] (Figures 2f and 2g) supports the interpretation of d 13 C in terms of variation in formation of intermediate water. The two records are very similar in outline and suggest a direct link between changes in the Nordic seas and in the Northeast Atlantic Ocean Stability of Intermediate Water Circulation During MIS 2 [11] The surface ocean conditions during parts of MIS 2 is considered to have been relatively stable with low variability in the oxygen and carbon isotope values [e.g., Duplessy et al., 1988; Oppo and Lehman, 1993; Sarnthein et al., 1994; Weinelt et al., 2003; Lekens et al., 2006; Knutz et al., 2007; Rasmussen and Thomsen, 2008]. The interval represents a period with a steady state ocean circulation, the glacial mode [Ganopolski and Rahmstorf, 2001]. The results from this study and Peck et al. [2006] show at least two events (and possibly three; Figures 2g, 3b, and 3f) of severe reductions in the flow of intermediate water. [12] To better compare the 13 C-depletion events to other cores in the pathway of the Atlantic Water inflow to the 1806

3 Figure 2. Records from core MD (this study, unbroken lines) and core MD off Ireland [Peck et al., 2006] (broken lines) plotted against calibrated kilo years (ka). The two records are presented with each their independent age model. (a) Relative abundance of the planktonic foraminifera species Neogloboquadrina pachyderma s, (b) Planktonic d 18 O values measured in N. pachyderma s from core MD , (c) Planktonic d 18 O values measured in N. pachyderma s, (d) Benthic d 18 O values measured in Cassidulina neoteretis, (e) Planktonic d 13 C values measured in N. pachyderma s, (f) Benthic d 13 C values measured in Cibicidoides wuellerstorfi from core MD , (g) Benthic d 13 C values measured in C. neoteretis, (h) Concentration of total planktonic and benthic foraminifera per gram dry weight sediment (note logarithmic scale), (i) Ice-rafted debris (IRD/cm) from Rørvik et al. [2010]. Light grey vertical bars highlight benthic d 13 Cminima 1 3 discussed in the text. Dark grey bars mark Heinrich events H2 and H1. IS2, interstadial 2. Nordic seas, we have tied the record of MD to the timescale of the GISP2 ice core record and core DAPC-02 from Rosemary Bank, which has been tuned to the GISP timescale by Knutz et al. [2007] (Figure 3). We used the H1, H2, IS2, and the warming after event 2 as tie points assuming these events were synchronous. Between these correlation points, linear sedimentation rates were assumed. Although the records may not be entirely synchronous, it appears that the events 1 3 correlates with cold climate as seen in the GISP2 record and with high percentages of N. pachyderma s in the marine record of DAPC-02 indicating cold sea surface temperatures (Figures 3a and 3b). The rapid warm-cold oscillations during MIS 2 are accompanied by fairly large changes in salinity, which can be interpreted as a sign of high instability of the sea surface conditions (Figure 3e). H2 and H1 apparently show the lowest subsurface salinity. However, due to the low number of foraminifera in the events of low benthic d 13 C values, we were not able to calculate the salinity or the salinity has only few data points (Figures 3d and 3e). The lack of planktic foraminifera may suggest that the surface salinity was even lower than during the Heinrich events as planktonic foraminifera tends to be almost absent in such conditions [Johannessen, 1986]. We note that hardly any ventilation change occurred during the course of H2, while some reduction occurred during H1 although based on few data points (Figure 3f). In the North Atlantic Ocean, only short-lasting transient reductions in benthic d 13 C values are seen during the Heinrich events (Figure 2f) [Peck et al., 2006]. 1807

4 Figure 3. Ice core and marine core records. (a) Oxygen isotope values from the GISP2 ice core [Grootes and Stuiver, 1997], (b) Relative abundance of N. pachyderma s in core DAPC-02 from the northeastern North Atlantic Ocean [Knutz et al., 2007], (c f) Records from core MD (this study), (c) Relative abundance of N. pachyderma s, (d) Absolute subsurface temperatures calculated by transfer functions on planktic foraminifera census data, (e) Calculated subsurface salinity (see text for explanation), (f) Benthic d 13 C values measured in C. neoteretis. Locations of cores are shown in Figure 1. Light grey vertical bars highlight benthic d 13 C minima events 1 3. Dark grey bars mark Heinrich events H2 and H1. Note that the timing of events in MD-2294 has changed very little from the previous age model based on the calibrated 14 C dates (Figure 2). IS2, interstadial 2. [13] The reason for the d 13 C reductions during the events 1 3, which are smaller and of shorter duration than the Heinrich events is difficult to explain. However, a modeling study may point to that meltwater injection to the Nordic seas of this apparently very sensitive area of the steady state glacial ocean may have had the ability to severely perturb the generation of deep and intermediate water in the Nordic seas [Levine and Bigg, 2008]. The model estimates a complete shutdown of the AMOC for freshwater fluxes of 0.4 Sv (sverdrup). The shutdown is followed by a complete recovery (Figure 4). Smaller freshwater fluxes result in a partial reduction of the AMOC. The main impact time is 100 years, and the recovery back to the normal range of AMOC strength occurs within 500 years. The freshwater is slowly dispersed by the AMOC [Levine and Bigg, 2008]. Thus, the model compares well with the proxy records of core MD , which show reduced ventilation of the intermediate and subsurface waters attributed to reduced flow of the intermediate water in the Nordic seas (Figure 4). Following the rapid decreases in d 13 C, the signals of both the planktonic and benthic records in core MD indicate a gradual acceleration of the intermediate water circulation as also described by the modeling experiment (Figure 4). The final onset of intermediate water formation was apparently rapid and consistent with the increase in subsurface temperature and salinity reflecting strengthening of Atlantic Water flow (Figure 3). [14] The model experiments of Levine and Bigg [2008] demonstrate that a northerly localized salinity anomaly in the Nordic seas leads to decrease in the strength of the North Atlantic Current, an increase in sea ice cover and a more northerly extent of cooling. The model indicates that the longest recovery time results from meltwater released directly into the Nordic seas compared to experiments with release of meltwater from the Hudson Strait and Gulf of Saint Lawrence. For Heinrich event, H2 meltwater flows in 1808

5 therefore could have been more severe than for Heinrich event H2, where meltwater predominantly reached the North Atlantic Ocean south of the Nordic seas. [16] Acknowledgments. The project was funded by the University of Tromsø and the Research Council of Norway through the SPONCOM project. The core was taken during the international IMAGES cruise in 1999 with RV Marion Dufresne. We warmly thank V. L. Peck, P. C. Knutz, and G. R. Bigg for kindly sharing their data with us. Figure 4. Selected records from core MD across event 2 together with results of a modeling experiment from Levine and Bigg [2008] showing Atlantic overturning (Sv) in connection with a 0.4 Sv meltwater pulse. the Nordic seas were limited according to reconstructions by Lekens et al. [2006] and the circulation of intermediate water although diminished did not stop [Rasmussen and Thomsen, 2004, 2009]. Therefore, we suggest that the reduced intermediate water circulation during the events 1 3 and the low benthic d 13 C values (e.g., compared to Heinrich event H2) can be caused by meltwater flows in the Nordic seas. The numerous other coolings and reductions in subsurface salinity seen in the North Atlantic and Nordic seas records were short-lasting and apparently without any significant effect on the flow of intermediate water (Figures 2 and 3) also in accordance with the model experiments. 5. Conclusions [15] Our results show that the flow of Atlantic surface water into the Nordic seas was interrupted several times, which probably caused extensive cooling in the Nordic seas. The subsurface water during MIS 2 circa ka experienced fairly large variations in salinity. The results suggest that in a glacial steady state, the ocean circulation was not stable and was characterized by repeated intrusions of Atlantic water interrupted by meltwater supply. 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