appearance of the pacific diatom neodenticula seminae in the northern nordic seas — an indication...
TRANSCRIPT
Marine Micropaleontology 99 (2013) 2–7
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Marine Micropaleontology
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Appearance of the Pacific diatom Neodenticula seminae in thenorthern Nordic Seas — An indication of changes in Arctic sea ice andocean circulation
Arto Miettinen a,⁎, Nalan Koç a,b, Katrine Husum b
a Norwegian Polar Institute, Fram Centre, N-9296 Tromsø, Norwayb Department of Geology, University of Tromsø, Dramsveien 201, N-9037 Tromsø, Norway
⁎ Corresponding author. Tel.: +358 9 19150817; fax:E-mail address: [email protected] (A. Mietti
0377-8398/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.marmicro.2012.06.002
a b s t r a c t
a r t i c l e i n f oArticle history:Received 30 April 2011Received in revised form 30 May 2012Accepted 4 June 2012
Keywords:Neodenticula seminaeDiatomsPacific waterArcticSea iceOcean circulationThe Nordic Seas
The marine diatom Neodenticula seminae belongs to the present day planktonic assemblage of the subarcticNorth Pacific and its high-latitude marginal seas. In the middle and high-latitude North Atlantic, N. seminaeoccurred from the middle Pleistocene to the early–middle Pleistocene transition when it became locally ex-tinct. After a long absence of 0.84 Ma, it was found again in the North Atlantic in the late 1990s (Reid et al.2007).Here we show from sediment samples taken in 2006, 2007 and 2008 that N. seminae now has appeared in theNordic Seas for the first time in geologic history, and it already has a widespread modern distribution in thenorthern Nordic Seas. The appearance of N. seminae in the Nordic Seas coincides with an increased influenceof Pacific water via the Arctic Ocean due to diminished sea ice and/or changed ocean circulation in the ArcticOcean. Our results show that trans-Arctic exchanges, which were first observed in the North Atlantic in thelate 1990's are still in motion, or possibly even accelerated during recent years. The appearance of N. seminaein the Nordic Seas might even suggest initiation of a unique climatic transition of the scale seen during themid-Pleistocene transition. More trans-Arctic exchanges can be expected in the near future if the modernwarming trend and reduction of sea ice continues in the Arctic.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
The ongoing episode of climate warming has drastic consequencesfor marine conditions especially in the northern oceans (Fig. 1). TheArctic Ocean responds more rapidly to global warming than mostareas of the planet; observations document accelerating retreat andthinning of the Arctic sea ice cover over recent decades (e.g. Comisoet al., 2008). These changes in sea ice cover substantially affect atmo-spheric, hydrographic and ecological conditions at high latitudes(Spreen et al., 2009; Leu et al., 2010; Overland and Wang, 2010;Polyak et al., 2010), e.g. freshwater export from the Arctic may modifyglobal- to basin-scale ocean circulation patterns and climate (Greeneet al., 2008; de Steur et al., 2009), or earlier phytoplankton blooms inthe Arctic Ocean may have consequences to the Arctic food chain andcarbon cycling (Kahru et al., 2011).
Neodenticula seminae (Simonsen & Kanaya) Akiba & Yanagisawa isa pennate marine planktonic diatomwhich belongs to the modern as-semblage of the subarctic North Pacific and its high-latitude marginalseas, where it commonly accounts for >40% of the diatom assem-blage (Kanaya and Koizumi, 1966; Sancetta, 1982; Sancetta andSilvestri, 1986; Takahashi, 1986; Shimada et al., 2006). The species
+358 9 19150826.nen).
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appeared in the fossil record around 2.4 Ma in the North Pacific(Akiba and Yanagisawa, 1986: Yanagisawa and Akiba, 1990, 1998).During the Holocene, N. seminae was the dominant diatom assem-blage in the Bering Sea from 6.5 cal ka BP to the present (Caissie etal., 2010). Primary productivity in the subarctic North Pacific isamong the highest in the world's oceans with the phytoplanktonserving as an efficient biological pump driving elemental cycles in-cluding the carbon content of the ocean (Honjo, 1997). The dominantdiatoms (including N. seminae) in this highly productive region playan important role in ocean-scale ecosystem and biochemical dynam-ics (Shimada et al., 2006).
In the middle and high-latitude North Atlantic, N. seminaeoccurred from the Middle Pleistocene at 1.26 Ma to the early–middlePleistocene transition at 0.84 Ma ago (Baldauf, 1986; Koç and Flower,1998; Koç et al., 1999). Baldauf (1986) interpreted that the occur-rence of N. seminae in the North Atlantic indicates the presence ofcool and low-salinity surface waters originated from the ArcticOcean in the early Pleistocene, and that this suggests that the ArcticOcean was partially ice free allowing the transportation of the speciesbetween the Bering Sea and the North Atlantic. The time intervalwhen N. seminaewas present in the North Atlantic straddles the tran-sition from the dominance of 41 ka cycles in the climate record to thedominance of 100 ka cycles. It is possible that the first occurrence ofN. seminae in the North Atlantic is an indicator of the cooling, whichstarted at 1.26 Ma, leading to the establishment of the 100 ka cycles
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and intensified Northern hemisphere glaciations. Conditions in theNorth Atlantic were too warm for subarctic N. seminae before1.26 Ma, after which conditions cooled to subarctic environment fa-vorable for the species. N. seminae thrived in the North Atlantic thenext ca. 0.4 Ma until conditions turned too severe with perennialsea ice cover leading to the disappearance of the species at 0.84 Ma.A closure of the connection between the Pacific and the Bering landbridge due to the growth of ice and falling sea level (Head andGibbard, 2005) could be another reason for the disappearance atthis time, but more likely the colder conditions in the North Atlanticled to the extinction of N. seminae in this area (e.g. Reid et al.,2007). The presence of N. seminae in North Atlantic sediments maytherefore be attributable to the unique conditions related to themid-Pleistocene transition (Koç et al., 1999).
After a long absence, re-occurrence of N. seminae was reported inthe Labrador Sea (Fig. 1) during the late 1990s as the first record inthe North Atlantic for more than 0.8 Ma (Reid et al., 2007). Theysuggested that N. seminae was carried in a pulse of Pacific water in1998/early 1999 via the Canadian Arctic Archipelago and/or FramStrait, and that the event coincided with modifications in Arctic hy-drography and circulation, the increased flows of Pacific water intothe Northwest Atlantic and the exceptional occurrence of extensiveice-free water to the North of Canada in the previous year 1998. It ap-pears therefore that the appearance of N. seminae is an indicator ofthe speed of the change that is taking place in the Arctic and North At-lantic Oceans as a consequence of regional climate warming andmarks a change in the circulation between the North Pacific andNorth Atlantic Oceans via the Arctic (Reid et al., 2007, 2008).
Although N. seminaewas found at one site north of Iceland in 2002(Reid et al., 2007), this species has never been found elsewhere in theNordic Seas (the Greenland, Iceland and Norwegian Seas). In thispaper, we report the appearance of the Pacific diatom N. seminae insurface sediment samples recovered in 2006–2008 from the northern
Fig. 1. The northern oceans. FS=Fram Strait, BS=Bering Strait, CAA=Canadian
Nordic Seas. This discovery represents a considerable expansion of itssubarctic range since its appearance in the plankton of the northernNorth Atlantic in 1999. We also discuss the causes of this appearanceand the significance of the findings as evidence of the trans-Arctic mi-gration of planktonic organisms from the North Pacific to the NorthAtlantic.
2. Material and methods
During the International Polar Year 2007–2008 (IPY), 36 stationswere sampled for surface sediments from the East Greenland andWest Spitsbergen margins, Greenland Basin and Fram Strait duringcruises by the R/V Jan Mayen in October 2006, September 2007, andAugust 2008.
Undisturbed surface sediment samples were taken by multi corerand/or box corer. As an original aim, a total of 25 surface sedimentsamples were studied for diatoms in order to extend the existingtraining data set of Andersen et al. (2004b) for diatom transfer func-tions for quantitative sea surface temperature (SST) reconstructions(Miettinen and Koç, in preparation).
The diatom samples were prepared using the method describedby Koç et al. (1993), which consists of HCl and H2O2 treatment to re-move calcium carbonate and organic matter, clay separation andpreparation of quantitative slides. A Leica Orthoplan microscopewith 1000× magnification was used to identify and count the dia-toms. The counting procedure described by Schrader and Gersonde(1978) was followed. A minimum of 300 diatom frustules (in additionto Chaetoceros resting spores) were identified for each sample.
3. Results
N. seminae was found in most of the surface sediment samples(21/25=84%) taken from the northern Nordic Seas in 2006, 2007
Arctic Archipelago, GST=Gulf of St. Lawrence. The study area is outlined.
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and 2008 (Table 1 and Fig. 2), but only as a small proportion, mostlybetween 1 and 2% of the diatom counts. The highest percentage oc-currence (5.7% at site JM-07-WP-168-MC, station no. 10) in theGreenland Sea was recorded in 2007 (Table 1 and Fig. 2). Note thatthe samples were taken from different areas in 2006–2008 and it isthus not possible to comment on any changes in the annual distribu-tion of the species, but it is clear that this Pacific species now has awide distribution across the northern Nordic Seas. While there waslittle differences in the concentration of N. seminae in the analyzedsamples, there is a tendency towards higher concentrations in thenorthern and northeastern sector of the Nordic Seas, i.e. the northernFram Strait and West Spitsbergen slope (Fig. 2).
4. Discussion
4.1. Stratigraphic and biogeographical distribution of N. seminae in theNorth Atlantic and Nordic Seas
N. seminae is found in the North Atlantic from 1.26 to 0.84 Ma(Baldauf, 1986; Koç and Flower, 1998; Koç et al., 1999). However, itis not present in the Nordic Seas, not even during the middle Pleisto-cene when it is common in the North Atlantic (e.g. Koç and Scherer,1996). N. seminae was not found in a set of surface samples fromthe Nordic Seas collected for the training data set of Andersen et al.(2004b) for quantitative sea surface temperature (SST) reconstruc-tions. It is not observed either in any Holocene paleoceanographic re-cords based on diatoms from the North Atlantic (e.g. Andersen et al.,2004a,b; Berner et al., 2008). N. seminae has first been reported in theplankton of the North Atlantic subpolar gyre (Labrador Sea) in 1999.Its geographic distribution has been extended to the Irminger Seaone year later (Reid et al., 2007), and to the Gulf of St. Lawrence in2001 and 2002 (Starr et al., 2002, 2003). In addition, it has beenreported at one station to the north of Iceland in 2002, and at two sta-tions south of Iceland in 2002 and 2003 (Reid et al., 2007). Despite thedominance of this species in phytoplankton assemblages and its highabundance in the Gulf of St. Lawrence in 2001 (Starr et al., 2002), it isnot widely distributed, e.g. it is only observed in 0.2% of the samplesof the Continuous Plankton Recorder survey (Reid et al., 2007). Ex-cept at one site north of Iceland in 2002 (Reid et al., 2007), N. seminae
Table 1Surface sediment samples analyzed for diatoms and percentage occurrence of N. seminae. S
Site no. Station number Area
1 JM06-WP-07-BC West Spitsbergen Slope2 JM06-WP-10-BC Fram Strait3 JM06-WP-12-BC Fram Strait4 JM06-WP-14-BC Western Fram Strait, sea-ice edge5 JM06-WP-16-MC Western Fram Strait, sea-ice edge6 JM06-WP-19-MC Western Fram Strait, sea-ice edge7 JM06-WP-21-MC Western Fram Strait, sea-ice edge8 JM06-WP-24-MC Western Fram Strait, sea-ice edge9 JM06-WP-26-MC Western Fram Strait, sea-ice edge10 JM07-WP-168-MC Greenland Sea11 JM07-WP-170-MC East Greenland Margin12 JM07-WP-171-MC East Greenland Margin13 JM07-WP-172-MC East Greenland Margin14 JM07-WP-176-MC Scoresby Sound Shelf15 JM07-WP-180-MC Greenland Basin, western side16 JM07-WP-182-MC Greenland Basin17 JM08-WP-337-MC West Spitsbergen Slope18 JM08-WP-338-MC West Spitsbergen Slope19 JM08-WP-339-MC Fram Strait20 JM08-WP-341-MC Fram Strait21 JM08-WP-342-MC Fram Strait22 JM08-WP-344-MC Fram Strait23 JM08-WP-345-MC Fram Strait24 JM08-WP-348-MC Storfjord Through25 JM08-WP-350-MC Hornsund Deep
has never been observed in the Nordic Seas prior to the current studyof samples from 2006 to 2008. The species is not recorded in the core-top sample of a high-resolution sediment core from the Iceland Basinof the subpolar North Atlantic which represents the year 2004(Miettinen et al., 2011, 2012). Neither is N. seminae observed in ahigh‐resolution diatom study of core MD99-2322 from the EastGreenland shelf, covering the last 2800 years (the core-top sampleis from the 19th century), even though the core site is directlyinfluenced by the East Greenland Current which carries water out-flowing from the Arctic Ocean (Miettinen and Koç, in preparation).Together these observations indicate that the recent appearance ofN. seminae in the Nordic Seas is the first time in geological history,and likely to be linked to recent changes in the Arctic Ocean.
4.2. Causes of the expansion of N. seminae into the Nordic Seas
Trans-Arctic exchanges of boreal plankton between the Pacific andAtlantic are affected by changes in sea level, (sea) ice cover, and oceancirculation (Greene et al., 2008). The Bering Strait is a gatewaybetween the Pacific and the Arctic Ocean. During the periods ofcontinental ice sheets during the Pleistocene, and thus lower globalsea level, the Bering Strait can be completely blocked. Once Pacificorganisms have reached the Arctic Ocean, sea ice cover and circula-tion patterns determine whether or not they can encroach on theNorth Atlantic through the Canadian Archipelago and/or Fram Strait(Greene et al., 2008). As the journey of organisms across the ArcticOcean takes several months, their survival is also dependent ontheir physiological ability to survive low temperatures and lightlevels.
The observed frequency of N. seminae in surface sediment samplesacross the northern Nordic Seas indicates a widespread modern dis-tribution of the species in this region. This suggests a higher influenceof the Pacific waters arriving through the Fram Strait during the lastfew years. One cause for the stronger freshwater fluxes and introduc-tion of a Pacific component into the diatom assemblages of the NordicSeas in 2006–2008 may be the recorded reductions in the extent ofsea-ice cover in 2005 and 2007 (Fig. 3).
Our results suggest that during the sea ice minima of the last fewyears, there has been a single pulse or several pulses of Pacific surface
tations numbered as JM06 were taken in 2006, JM07 in 2007, and JM08 in 2008.
Latitude Longitude N. seminae %
78 52.620 N 007 20.459 E 2.678 56.176 N 005 24.075 E –
78 54.461 N 002 24.919 E –
78 55.888 N 001 06.460 E 1.078 53.767 N 016.916 E 1.378 00.776 N 002 30.173 W 1.477 00.204 N 003 23.662 W 1.974 38.00 N 011 11.21 W 0.574 53.49 N 010 46.10 W 1.070 09.000 N 014 31.000 W 5.773 44.873 N 024 11.956 W 1.573 46.138 N 013 0.603 W 0.773 47.080 N 012 21.640 W –
69 51.585 N 022 07.906 W 0.377 28.610 N 002 02.540 W 1.377 30.540 N 000 02.270 W 1.378 08.130 N 008 30.411 E 1.978 10.029 N 006 32.731 E 1.777 59.731 N 005 58.354 E 1.077 59.720 N 004 07.442 E 0.375 59.752 N 004 19.589 E 1.375 58.938 N 008 19.384 E 2.375 59.682 N 011 00.103 E 2.375 59.737 N 013 17.892 E 0.676 47.888 N 014 58.809 E –
Fig. 2. Percentage occurrence of N. seminae in the surface sediment samples. Site numbers in Table 1. Arrows show a modern surface circulation pattern in the Nordic Seas (solidlines=cold Arctic waters, dashed lines=warm Atlantic waters, EGC=East Greenland Current) (modified from Blindheim and Østerhus, 2005).
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waters which has transported N. seminae into the Nordic Seas. Thefluxes of Pacific water through the Arctic Ocean have occurred atleast since the 1970s (e.g. Belkin, 2004) but they have not beenfound able to transport N. seminae into the Nordic Seas previously.We point out that the recent appearance of N. seminae could suggestthe occurrence of larger ice free areas in the Arctic and conditions
Fig. 3. Mean September sea ice extent in millions of km2 for the northern hemispheresince 1978 (data from National Snow and Ice Data Center). This is a measure compara-ble to annual minimum sea ice extent. The blue (gray) line indicates the decreasingtrend in September sea ice extent. The years 2006, 2007 and 2008 are highlighted.
(e.g. enough light for the photosynthesis of diatoms and maybe alsowarmer sea surface) enabling N. seminae to survive the long journeyacross the Arctic Ocean. In earlier pulses, since the 1970s, Pacificwater has probably flowed under ice with too severe conditions fordiatoms.
After arriving at the Nordic Seas, N. seminae has maintained viablestanding stocks in the area, i.e. most probably, the species belongsnow to the recent diatom assemblage of the Nordic Seas. Thus, the re-cent occurrence of N. seminae does not necessarily indicate a persis-tent input of Pacific water although this was the original source.
The observed distribution suggests that Fram Strait is the maintransport route for these Pacific diatoms into the Nordic Seas. Howev-er, a somewhat higher occurrence of N. seminae in the NE sector of theNordic Seas is difficult to explain by the ocean current system, sincethe area influenced by the East Greenland Current (which originallycarried the species into the Nordic Seas) is not so rich in N. seminae.We think that the cause for the recent distribution of the species isthe stimulating effect of the warm Atlantic waters on the growth ofN. seminae in this area. N. seminae is a subarctic species (not an arcticspecies, which are common in the area controlled by the East Green-land Current) and therefore the NE sector offers more favorable con-ditions for its growth and blooming. In principle, N. seminae in theNordic Seas could have originated from the Canadian Archipelago,which was probably the source for the species in the Labrador Sea(Reid et al., 2007). Later, it could have migrated via the North Atlanticmain stream into the Nordic Seas. However, N. seminae in the NordicSeas is heavily silicified, corresponding to the silicification level of thisspecies in the North Pacific, thereby differing clearly from less
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silicified species from the Gulf of St. Lawrence in the NWNorth Atlantic(cf. Poulin et al., 2010). This supports the interpretation of FramStrait asthe source for N. seminae in the Nordic Seas.
Earlier results from the Labrador Sea and the North Atlantic indi-cate a major reorganization of circulation patterns in the Arctic andAtlantic Oceans during the last three decades (Reid et al., 2007;Greene et al., 2008; Overland and Wang, 2010), possibly in relationwith diminished Arctic sea ice. Sea level pressure dropped precipi-tously in the central Arctic and led to the emergence of a stronglycyclonic atmospheric circulation in 1989 (Dickson, 1999). This cy-clonic atmospheric circulation resulted in the increased transport ofrelatively warm, high-salinity Atlantic water into the Arctic Oceanprimarily through the Barents Sea (Dickson, 1999; McLaughlin et al.,2002). Associated with the cyclonic atmospheric circulation and en-hanced inflow of Atlantic water, an extensive reorganization of upper-ocean circulation patterns occurred in the Arctic Ocean (Dickson et al.,2000). The Pacific water export through the Canadian Archipelago ismainly affected by large-scale atmospheric forcing resembling the Arc-tic Oscillation (AO) or the North Atlantic Oscillation (NAO). In contrast,the variability of the Pacific water inflow through Fram Strait is con-trolled by changes in Pacific water storage in the Beaufort Gyre, withan increased export during years with a cyclonic circulation anomalyover the central Arctic Ocean (Jahn et al., 2010). The reorganization ofcirculation patterns caused several changes in the central ArcticOcean: a) a shift in the front separating Atlantic and Pacific watermasses from the Lomonosov Ridge to theMendeleyev Ridge, b) aweak-ening and deflection of the Transpolar Drift, c) a weakening and shrink-ing of the Beaufort Gyre, d) an intensification and thickening of theArctic Ocean Boundary Current, and e) a significant redirection of theshallow Arctic Ocean outflow entering the North Atlantic (McLaughlinet al., 2002; Steele et al., 2004).
The appearance of N. seminae in the Nordic Seas could suggest aunique climatic transition of the scale seen during the mid-Pleistocenetransition (Koç et al., 1999), which finally led to the disappearance ofthe species from the North Atlantic at 0.84 Ma due to the severe condi-tionswith perennial sea ice cover. The present appearance ofN. seminaesuggests that the trend is reversing, i.e. it is getting warmer again andthe sea ice cover is becoming seasonal. Therefore it is possible forN. seminae to be transported from the Pacific again, and to sur-vive and bloom in the Nordic Seas as a subarctic species.
N. seminae is an example of trans-Arctic invasion which could bedue to the climate warming. In the future, the warming mayincreasingly lead to poleward expansion of temperate and tropicalspecies and interoceanic migrations, e.g. North Pacific lineages canspread into the warmer Arctic Ocean and further into the temperateNorth Atlantic affecting the overall ecological patterns of these areas(e.g. Vermeij and Roopnarine, 2008). Consequences of these possiblechanges are difficult to predict.
5. Conclusions
The Pacific diatom N. seminae was found in modern sediments inthe northern Nordic Seas in 2006, 2007 and 2008. Previous studiesof both stratigraphy dating back to the Pleistocene and modern sedi-ments up to 2004 show no presence of N. seminae in the Nordic Seas,except for a single plankton sample found in 2002 off northern Ice-land. However, our knowledge of diatom distribution in recent Arcticsediments is only based on a limited geographical data set. The obser-vations in bottom sediments indicate that the species now has awidespread distribution across the northern Nordic Seas providingevidence for a biogeographic range expansion of major importance.The appearance of N. seminae can be associated with an increased in-fluence of the Pacific waters via the Arctic, probably due to dimin-ished Arctic sea ice and/or changed ocean circulation in the ArcticOcean. Therefore it is possible for N. seminae to be transported fromthe Pacific, and to survive and bloom in the Nordic Seas as a subarctic
species. The appearance of N. seminae in the Nordic Seas could sug-gest a unique climatic transition of the scale seen during the mid-Pleistocene transition, where its occurrence in the North Atlanticindicated a transition to cold conditions. Today, however, it wouldrather indicate a transition to warmer conditions.
Our results show that trans-Arctic migrations, which were first ob-served from the North Atlantic in late the 1990's are still in motionand may have accelerated in recent years. It is likely that furthertrans-Arctic exchanges of Pacific species can be expected in the nearfuture if the warming trends and sea ice reduction continue in theArctic. Changes of this nature may pose high risks for the ecology ofthe Nordic Seas and the North Atlantic.
Acknowledgments
Weare grateful to the editor and two anonymous reviewers for theirconstructive comments and suggestions. This paper is a contribution tothe two International Polar Year (IPY) projects “Arctic Ocean warmingin the Past” (WARMPAST, IPY 786) and “Arctic Natural Climate and En-vironmental Changes and Human Adaption: From Science to PublicAwareness” (SciencePub, IPY 39) funded by the Research Council ofNorway, the University of Tromsø and the Norwegian Polar Institute.
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