Late Quaternary paleoceanography and paleo-sea ice conditions in the Mackenzie Trough and Canyon, Beaufort Sea 1

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1 Late Quaternary paleoceanography and paleo-sea ice conditions in the Mackenzie Trough and Canyon, Beaufort Sea Trecia M. Schell, David B. Scott, André Rochon, and Steve Blasco Abstract: The Mackenzie Trough provides a high resolution signal for paleoceanography as a result of high sedimentation rates at the mouth of the Mackenzie River. Three cores were collected along a transect covering a depth range of m and the time period of the last cal BP. Prior to the last * cal BP, the distal core is characterized by laminated sediment and a foraminiferal fauna of Arctic Bottom Water calcareous species and abundant planktic foraminifera suggesting little freshwater runoff and (or) perennial sea-ice cover. This occurs at a similar time as laminated sediments from the west of this site, which have been suggested to be part of the Lake Agassiz flood outburst and (or) cold period. If this outburst occurred, the very positive oxygen isotope values from our core (PC3; >+3.0 ppm) indicate that it did not flow through the Mackenzie Trough. After 9000 cal BP, the faunas in the three cores differ because of timing and different water depths. However, it is possible to see a progression of cold saline water prior to cal BP, with a freshening of surface water after cal BP where tintinnids (brackish water ciliates) occur with incursions of deep water Arctic calcareous species to *3000 years BP. A sequence of mixed faunas appears as sea ice returns, at least periodically in the last 3000 cal BP; but (in core PC2 only) a return to more sea ice is recorded by both foraminifera and dinocysts in the last few hundred years. Résumé : La fosse du Mackenzie fournit un signal à haute résolution pour la paléocéanographie en raison des taux élevés de sédimentation à l embouchure de la rivière Mackenzie. Trois carottes ont été prélevées le long d un transect couvrant une plage de profondeurs de 58 à 671 m; la période de temps couvre les derniers années calendaires avant le présent (années cal. BP). Avant les derniers * années cal. BP, la carotte distale est caractérisée par des sédiments laminés et une faune de foraminifères d espèce calcaire des eaux du fond de la mer Arctique et les nombreux planctons foraminifères suggèrent peu d écoulement d eau douce et (ou) un couvert permanent de glace de mer. Cela se produit en même temps qu une déposition de sédiments laminésàl ouest de ce site; ces sédiments feraient partie du débordement de l inondation et (ou) de la période froide du lac Agassiz. Si ce débordement a eu lieu, les valeurs très positives de l isotope de l oxygène de notre carotte (PC3; >+3,0 ppm) indiquent qu il ne s est pas écoulé à travers la fosse du Mackenzie. Après 9000 années cal. BP, les faunes dans les trois carottes diffèrent en raison des moments de déposition et des profondeurs d eau différentes. Il est toutefois possible de voir une progression de l eau froide saline avant années cal BP. Après années cal. BP, on note un adoucissement de l eau de surface; en effet, des tintinnides (un cilié planctonique d eau saumâtre) se retrouvent mélangésàdes espèces calcaires d eau profonde de l Arctique jusqu à *3000 années BP. Une séquence mixte de faunes apparaît au retour de la glace de mer, du moins périodiquement, au cours des dernières 3000 années cal. BP. Cependant, un retour à plus de glace de mer est enregistré dans les foraminifères et les dinokystes au cours des quelques dernières centaines d années (seulement dans la carotte PC2). [Traduit par la Rédaction] Introduction This study sought to determine temporal changes in various elements (sea-ice cover, sediment flux, freshwater flux) between the Beaufort Shelf and the Arctic Ocean in the late Quaternary in relation to the largest flux of freshwater into the Canadian Arctic Ocean sector over the last years. The primary goal was the determination of paleo-sea-ice cover with relation to present conditions, looking at changes of various fossil proxies through time. To achieve these Received 13 March Accepted 29 September Published on the NRC Research Press Web site at cjes.nrc.ca on 23 January Paper handled by Associate Editor P. Hollings. T.M. Schell and D.B. Scott. 2 Centre for Environmental and Marine Geology, Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 4J1, Canada. A. Rochon. Institut des Sciences de la Mer de Rimouski (ISMER), Université du Québec à Rimouski, 310, allée des Ursulines, Rimouski, QC G5L 3A1, Canada. S. Blasco. Natural Resources Canada, Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, NS B2Y 4A2, Canada. 1 This article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network. 2 Corresponding author ( david.scott@dal.ca). Can. J. Earth Sci. 45: (2008) doi: /e08-054

2 1400 Can. J. Earth Sci. Vol. 45, 2008 goals, an area with a high resolution temporal record is required and the Mackenzie Trough is uniquely suited for this. A major part of the sediment coming into the Beaufort Shelf area is transported by the Mackenzie River, providing a high sedimentation rate in the trough where the cores were collected, which in turn produces a high resolution paleorecord, especially in the Holocene. Distribution and extent of sea-ice cover influences wind-driven currents as well as surface productivity, and consequently the redistribution of particles. Hence, proxies for reliable reconstruction of paleo-sea-ice cover are important, and foraminifera and related organisms offer some of the best proxies. Of major importance globally is that less sea ice in the Arctic means much more warming in the summer as the sunlight penetrates the water instead of being radiated back into the atmosphere by the ice. Recent records from satellites suggest up to a 30% decrease in sea-ice cover over the last 30 years during the summer months; the question is whether this has occurred repeatedly during the Holocene or this is a new phenomenon resulting from the industrial age of the last few hundred years. The cores in this study and other nearby sites (Scott et al. 2007; Schell et al. 2008) range in age from the end of the last glaciation (* cal BP) to the present, providing a long-term source of indications for the paleoceanography and paleo-ice cover in the Mackenzie Trough using the previously mentioned proxies. This study, combined with others in the area using assemblages of dinoflagellates in relation to ice cover (Rochon 2006; Rochon et al. 2006; Richerol et al. 2008a, 2008b), seeks to provide multi-proxy data. Schell et al. (2008) discuss some box core data from across the Amundsen Gulf, which is quite different from Mackenzie Trough data; Scott et al. (2008a, 2008b) discussed the foraminiferal assemblages on the Beaufort Shelf, Slope, and in Amundsen Gulf. The foraminiferal data together with the other proxies are the basis for interpreting the core data both here and in other studies. Scott et al. (2007) discussed the glacial deglacial record in two slope locations using data subsequently published in Scott et al. (2008a, 2008b). These studies combined provide a more comprehensive picture of paleoceanography since the last glaciation for the Beaufort Shelf Amundsen Gulf region. Environmental setting The Mackenzie Trough is a major asymmetric, linear, northwest-trending paleo-valley that separates the eastern and western Beaufort shelves. This submarine feature is 80 km wide and 150 km long and has incised >400 m into the broad, flat continental shelf, extending into the Mackenzie Canyon farther offshore, finally reaching into the Beaufort Sea (Fig. 1). The Mackenzie Trough is partially filled in by >300 m of Quaternary sediments and has a bathymetric relief of 100 m with seaward dipping gradients of 1.58 (Blasco et al. 1990). Sea-level rise in the late Quaternary averaged 2.5 mm/year ( cal BP) in the Mackenzie Trough area, followed by a rapid Holocene sea-level rise (7.14 mm/year) to 3000 BP, which brought the Mackenzie Trough to its present configuration (Hill et al. 1985). The Mackenzie River is one of several large Arctic rivers that discharge freshwater into the Arctic Ocean. The Mackenzie River has a lower discharge rate than the Siberian rivers (Lena, Ob, and Yensei), but the suspended particulate matter (SPM) load of the Mackenzie is three to four times larger than the three Siberian rivers combined (Matthiessen et al. 2000). The river typically discharges a volume of m 3 of freshwater per year into the Beaufort Sea (Hirst et al. 1987; Solomon et al. 2000). In contrast, the supply of total organic carbon (TOC) by the Eurasian rivers is much larger (>30% of SPM) than by the North American rivers (<5% of SPM) because of the differences in the geology of the respective catchment areas (Pocklington 1987). The Mackenzie River erodes steeply sloping alluvial strata, and its TOC consists mainly of particulate organic carbon (POC; Macdonald et al. 1998). Most of the sediment that reaches the Beaufort Sea is in the form of suspended silt and clay (Solomon et al. 2000). The total sediment load delivered to the head of the delta ranges up to 128 Mt/year (Carson et al. 1998). The Arctic Ocean is characterized by a stratified water column of three water masses: the Arctic Surface Water (ASW; top 200 m including the uppermost seasonally mixed m) is a cold and diluted surface layer, often containing Mackenzie River freshwater runoff in the ice-free months; the Atlantic Intermediate Water (AIW; m water depth) is a warmer and more saline water mass; while the deepest body of water is the Arctic Bottom Water (ABW; > 400 m) that is cold and saline (Scott and Vilks 1991). The seismics for the three cores (Fig. 2) were obtained along a transect in water depths of m with three cores collected (671 m, MR2002-K05 PC1; 223 m, MR2002-K05 PC2; 58 m, MR2002-K05 PC3). Hence, they have the potential to provide records of movements or changes of all the major water masses that cover the present Mackenzie Trough. Little river influence extends beyond the deepest core site except in the upper few metres. Previous work Vilks (1989) provided the first comprehensive investigation of Beaufort shelf benthic foraminifera resulting from the circum Americas cruise of the CCGS Hudson in ; at that time the multiyear ice margin was almost 100 km farther offshore than it was when sampled by the Canadian Arctic Shelf Exchange Study (CASES) expedition in 2004 (Scott et al. 2008a, 2008b). Vilks (1989) was the only previous study to examine distributions of foraminifera on the Beaufort Shelf, but processing techniques at the time precluded rigorous comparisons to the 2004 data. Other previous studies served to establish the taxonomy but little on the distributions of species (see review in Scott et al. 2008a, 2008b). Lagoe (1977, 1979) established much of what we know about the distribution of deep water Arctic species from data collected from the T-3 ice island occupied by an international group of scientists for several years (Clark et al. 1980). These studies were added to by examination of material from the ice stations in the central Arctic (LOREX, FRAM, CESAR; Markussen et al. 1985; Scott and Vilks 1991). Scott et al. (1989a) provided the first detailed paleoclimatic and stratigraphic record from the central Arctic from the collection of cores from the Alpha Ridge, but these cores only provide a broad framework with a low resolution of years/cm. Although many of the deep water species (e.g., Stetsonia arctica Green 1960 and Buliminella hensoni Lagoe 1977) find their way into the Mackenzie Trough, they are not dominant species there. More

3 Schell et al Fig. 1. Location map, with an inset of 2002 RV Mirai core sites MR2002-K05 PC1, PC2, and PC3 (adapted from Rochon et al. 2003). recent work done by Schröder-Adams et al. (1990), Poore et al. (1994), Bergsten (1994), Wollenburg and Kuhnt (2000), and Polyak et al. (2002, 2004); Wollenburg et al. (2001, 2004) will be integrated into the discussion with results from this paper. Andrews and Dunhill (2004) examined a 5 m core just west of the Mackenzie Trough in 405 m of water from the US Beaufort Shelf, but the record just covers the cal BP time frame and that area was not under the direct influence of the Mackenzie River. However, they suggest that there was a Lake Agassiz outburst event at * years BP with isotopic evidence from Cassidulina (= Islandiella in this paper) and planktic foraminifera. However, there was insufficient chronological control in their core data to bracket the event because the youngest date determined by their study after cal BP was 9680 cal BP, much younger than the end of the event in their core P189ARP45. Distinct foraminiferal faunal zonations in the Beaufort Shelf region result from the influence of local as well as regional Arctic oceanography (Scott et al. 2008a, 2008b and references therein). Results from these studies and several others discussed in Scott et al. (2008a, 2008b) indicate that distinct faunas characterize deep, cold, saline Arctic Bottom Water (ABW; below 400 m), warmer Intermediate Atlantic Water (IAW; 400 m upwards to 200 m), and Arctic Surface Water (ASW; m) along the Arctic margin. However, the present Mackenzie Trough and Canyon contain slightly less saline and highly variable water masses at varying depths; these water masses have changed in character and position (both vertically and laterally during the Holocene; Stein and Macdonald 2004). The foraminiferal studies aforementioned provide a solid basis for interpretation of core faunas in this study. In terms of techniques for detecting presence or absence of sea ice, several recent papers have used various microfossil proxies for paleo-ice conditions: chlorophycean algae from river runoff (Matthiessen et al. 2000); multi-proxy data from a thermokarst lake on Richards Island in the Beaufort Sea (pollen, dinoflagellates, foraminifera, thecamoebians, and geochemistry; Solomon et al. 2000); and papers examining paleo-ice conditions in the eastern versus western Arctic (Mudie et al. 2005, 2006; Rochon 2006, Rochon et al. 2006; Fisher et al. 2006); see Discussion section. All of these studies use proxies as sea-ice indicators in various ways: the surface dwellers (diatoms, dinoflagellates, pollen), foraminifera, thecamoebians, and geochemistry all have different responses and leave different signals in the sediments depending on sea-ice coverage. Most relevant are two papers by Richerol et al. (2008a, 2008b), in which dinoflagellates are examined in the surface sediments of the

4 1402 Can. J. Earth Sci. Vol. 45, 2008 Fig. 2. Subbottom profiles for each site location, with location insets (adapted from Rochon et al. 2003). Mackenzie Trough from the same stations used in this study (Richerol et al. 2008a) and from box cores (Richerol et al. 2008b). In the present paper, we use the relative presence of several indicative calcareous and agglutinated species of foraminifera, the presence of planktic foraminifera, and the abundance of tintinnids for ice-cover proxies. As shown in cited papers (Scott and Vilks 1991; Bergsten 1994; Wollenburg and Kuhnt 2000; Wollenburg et al. 2001, 2004), the presence of calcareous species in the bottom sediments in polar regions often suggests more permanent sea ice because with less sea ice the surface productivity increases, increasing the flux of organic matter to the sea floor, which in turn causes oxygen depletion in the sediments leading to reducing conditions at the sediment surface; even though there may be many more planktic foraminifera living in seasonally open water they are not well preserved on the seafloor. Tintinnids are ciliates that are often found in brackish environments that have high suspended loads; they occur at present in most parts of the Beaufort Shelf region, and in these cores indicate the presence of both river runoff and high suspended loads, both of which suggest limited permanent sea-ice cover (Scott et al. 1995, 2008a, 2008b, Schell et al. 2008). Materials and methods Fieldwork In September 2002, a sample suite of piston, trigger weight, and box cores was collected at three core sites (Fig. 1; Table 1), with the RV Mirai, in association with Joint Western Arctic Climate Studies Japan Marine Science and Technology Center (JWACS JAMSTEC), Natural Resources Canada (NRCan Geological Survey of Canada (GSC) Atlantic), and the CASES project (Rochon et al. 2003). Upon collection, all materials were sealed and refrigerated at 4 8C

5 Schell et al Table 1. Core numbers with locations, date collected, water depth (m) and recovered core lengths (cm). Core number Date collected Latitude (N) Longitude (W) Water depth (m) Recovered length (cm) MR2002-K05 PC MR2002-K05 PC MR2002-K05 PC bc Fig. 3. Lithologic summary for each core (MR2002-K05 PC1, PC2, and PC3) (adapted from Rochon et al. 2003; and K. Jenner, personal communication). ybp, years before present. until further sampling. The cores were analyzed by multisensor core logger, split, described, photographed, and subsampled for various proxies by the JAMSTEC coring crew and A. Rochon. For the foraminiferal samples, 5 10 cm 3 of sediment were collected at 10 cm intervals in the piston and trigger weight cores. The cores were resampled in 2004 at the JAMSTEC core storage facility in Mutsu, Japan. The sediment cores have been stored in the permanent collection of JAMSTEC since 2006 in Yokohama. Core lithologic summaries are shown in Fig. 3. Foraminifera sample processing and analyses The core samples were stored in a cold room (4 8C) at Dalhousie University until they could be processed. Wet sieving was done with distilled water through 63 and 45 mm mesh sieves and separated size fractions were placed in containers with buffered formalin added to prevent bacterial growth. To maintain statistical validity in the analyses, at least 300 specimens of foraminifera were counted (where possible) per sample for each size fraction. For samples with large numbers (>1000/10 cm 3 ), the samples were wet split following the method of Scott and Hermelin (1993) to obtain a manageable sample. The sample counts were converted to a percentage distribution for comparison and for simplicity of graphing purposes; only the species with >5% representation were selected. The smaller size fraction of mm has not commonly been examined in most other works from the Arctic and elsewhere, but in this paper and Scott et al. (2008a, 2008b) it has become recognized as a very important part of the faunal assemblage in some cases, >80% of the sample is represented by the smaller size fraction. In Figs. 4 6, the percentage abundances for the most prominent species represent the combined size fractions, although part of the graph in Figs. 4 6 shows the

6 1404 Can. J. Earth Sci. Vol. 45, 2008 Fig. 4. Core lithology, ages, total numbers of foraminifera and foraminiferal occurrences in % for selected species, and total numbers for other indicators per 10 cm 3 sample for PC3. Vertical numbers 1 and 2 refer to assemblage numbers in Table 4. ybp, years before present. percentage of the mm fraction for the total sample. Species are discussed individually as the percentage of mm is concentrated in some individual species. The calcareous foraminiferal tests were generally preserved in good condition, although there were some intervals containing tests that were slightly dissolved or etched. Once counted, the samples were preserved in small glass vials, in solution in equal amounts of distilled water and 10% ethanol (for preservation). For longer term archiving of the counted samples, 1 2 ml of buffered formalin was added because alcohol will sometimes evaporate away even in sealed vials, which exposes the samples to bacterial action that might destroy not only the calcareous material but also the inner linings of agglutinated species. Chronological methods Several sample intervals were picked for calcareous foraminifera in PC1 to obtain material for accelerator mass spectrometer (AMS) 14 C dating. Shell dates were conventional radiocarbon dates (Table 2), and the AMS sample weight varied from 30 to 50 mg carbonate. These samples were combined planktic and benthic species, since these are not stable isotope measurements but 14 C dates, which measure in percentages, not parts per million (ppm) as is the case for stable isotope measurements. Since these samples are from relatively shallow water, a reservoir correction of only 400 years was applied to all AMS dates and the difference between surface and bottom ages would be insignificant, since we know from the foraminiferal assemblages there was little deep sea water at any of these sites. The AMS samples were sent to J. Southon, University of California, Irvine, California, for AMS dating analysis for the smallest samples (lower part of core PC1), while the remainder were obtained as conventional 14 C dates by Beta Analytic Inc., Miami, Florida, from mollusc shells using a 250 year reservoir correction. All values and corrections are shown in Table 2. Stable isotopic work was carried out at Université du Quebéc á Montréal in the laboratories of Geochemistry and Geodynamics Research Centre (GEOTOP). The samples were processed on a Micromass MultiCarb online with a triple collector dual inlet Micromass Isoprime IRMS. The technique involves delivering three drops of 100% H 3 PO 4 acid on mg of sample under vacuum at 90 8C. The evolved gases were dried on a cool trap ( 80 8C) and trapped in a double cold finger system (liquid N 2 at * 180 8C). The reference gas used is CO 2 evolved from an in-house carbonate standard (UQ6). Data were corrected by analyzing the in-house reference material at the beginning (four times)

7 Schell et al Fig. 5. Core lithology, ages, total numbers of foraminifera and foraminiferal occurrences in % for selected species, and total numbers for other indicators per 10 cm 3 sample for PC2. Vertical numbers 3, 4 and 5 refer to assemblage numbers in Table 4. ybp, years before present. and the end (four times) of the analytical sequence. Replicate measurements of the in-house standard (standard deviation of the eight measurements) are better than ±0.05% for both d 13 C and d 18 O. Results are given with respect to Vienna Pee Dee Belemnite (VPDB). Geochemical reference material standard UQ6 d 13 C = +2.25% versus VPDB and d 18 O = 1.40% versus VPDB. Standard UQ6 has been calibrated using the usual international reference material many times over the past years. Results Each core site has a unique record, as they were obtained from different water depths, different distances from the Mackenzie River mouth, and covering differing chronologies. This influences both the sedimentation rates and surface salinities overlying the core sites (Rochon et al. 2003). Not surprisingly the sedimentation rates are highest closest to the river mouth where the sediment laden river water first comes in contact with the saline ocean water, which causes clay flocculation and massive fallout of sediment (Kranck 1973; Table 3). The cores are discussed with respect to water depth starting with the shallowest core (faunal assemblage zones for each core are displayed in Table 4). Clearly, these cores cannot be correlated directly because the faunas change with depth in core as well as water depth along the transect; hence, faunas change according to environment as well as through time. This is compounded by the fact that no dates were determined between and 8293 cal BP; however, these cores have many more dates than most other cores collected on the Beaufort Shelf and other Arctic shelf areas. The agglutinated or non-calcareous species are discussed in the following sections as a single group, not as individual species. The major species in this group are Trochammina spp., Spiroplectammina biformis (Parker and Jones, 1865), Textularia earlandi Parker 1952, Reophax scottii Chaster 1892, and Saccammina atlantica (Cushman, 1944) together with several other less common species. Also, although the percentage of mm and >63 mm fractions are not shown separately for each species in Figs. 4 6, the species that have the largest percentages of the small fraction are the deep water species (Buliminella hensoni, Bolivina arctica Herman 1973) and the tintinnids. Taxonomically the Islandiella teretis (Tappan, 1951) species includes several species of other workers using the genus Cassidulina (C. teretis, C. neoteretis to name a few), and all the planktics are Neogloboquadrina pachyderma (sinistral) (Ehrenberg, 1861) or left coiling. The planktic percentages (P) are included with the benthic percentages (B), since the B/P ratio is one of the more important parameters

8 1406 Can. J. Earth Sci. Vol. 45, 2008 Fig. 6. Core lithology, ages, total numbers of foraminifera and foraminiferal occurrences in % for selected species, and total numbers for other indicators per 10 cm 3 sample for PC1. Vertical numbers 6, 7, 8, and 9 refer to assemblage numbers in Table 4. ybp, years before present. in determining paleo-ice cover. All these data are contained in supplementary Tables S1 S3. 3 Chronology of the three cores The dates for these cores were obtained from mollusc shells in the two shallower cores (PC2, PC3) and from combined benthic and planktic foraminifera in the deepest core (PC1). The foraminifera in the two shallower cores had very thin and fragile tests and therefore could not provide sufficient material even for AMS dates. However, in the laminated section of core PC1 foraminiferal tests were sufficiently thick to provide the weight required for AMS dates (see Table 2). Core PC3 contains abundant shell material, particularly in the cm interval. Six of these shells were submitted for 14 C dating; 14 C dates are listed versus core depth in Figs. 4 7 and Table 2. MR2002-piston core 3 (PC3) This core was recovered at the shallowest (57.5 m) and most river proximal site (Fig. 2); it is also the shortest core (642 cm) with the shortest chronological record (7000 years) but the highest resolution (Figs. 3, 4). Core PC3 consists of olive-grey, bioturbated clay with intense bioturbation from 100 to 200 cm. There are also several small intervals of laminations at 150 and cm (Fig. 3). At the time of sampling, the thermocline was not well defined; there were several peaks suggesting strong mixing at this time (July). However, the pycnocline and transparency data had a strong gradient at *10 m water depth. These results reflect the strong input of the river during the summer the conditions in winter presumably would be much more stabilized; however, it is almost impossible to get these measurements in the winter. Details of data for all three stations can be located at the CASES web site ( cases/fieldwork2003.asp). There are two assemblage zones in PC3. Assemblage zone 1 Assemblage zone 1 (late Holocene) extends from 0 to 225 cm and averages individuals per 10 cm 3 3 Supplementary data for this article are available on the journal Web site (cjes.nrc.ca) or may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD For more information on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/cms/unpub_e.shtml.

9 Schell et al Table 2. Carbon-14 dates from University of California, Irvine (AMS dates), California, and Beta Analytic Inc., Miami, Florida (conventional radiocarbon dates) including core number, sample depth, material dated, laboratory number, uncorrected 14 C date, calibrated ages (1s and 2s), and DR value. Core number Composite depth (cm) Material dated Laboratory number 14 C age BP uncorrected Calibrated age BP 1s Calibrated age BP 2s DR value MR2002-K05-PC1 705 Foram UCIAMS ± Foram UCIAMS ± MR2002-K05-PC Shell Beta ± Shell Beta ± Shell Beta ± MR2002-K05-PC Shell Beta ± Shell Beta ± Shell Beta ± Shell Beta ± Shell Beta ± Shell Beta ± Note: Sidereal dates (for calibration) determined using methods of Stuiver et al. (1998). Foram refers to bulk calcareous foraminifera (benthic and planktic combined) and shell refers to assorted mollusc shells. Table 3. Sedimentation rates for each core determined by extrapolating from 14 C dates versus depths in Table 2; accuracy of these rates is dependent on the vertical core distance between the various dates. With so few dates it is impossible to put error bars on the rates. Core name Sample depth (cm) Sample age (years BP*) Sedimentation rate (cm/1000 years) Time interval for sedimentation, rates shown in left column (years BP) PC * *11540 PC * * *8293 PC * * * * * *6998 Table 4. Foraminiferal assemblages for the three cores. Assemblages for core PC3 Assemblage zone 1 (0 225 cm) F. fusiformis, E. exc. clavatum, Islandiella spp, (10% 25%), C. reniforme and planktics (<5%) Assemblage zone 2 ( cm) Agglutinated species (30% 40%), E. clavatum, Islandiella spp. (10% each), and other small species <5% Assemblages for core PC2 Assemblage zone 3 (0 50 cm) E. clavatum, C. reniforme, Islandiella, planktics all common together with agglutinated forms Assemblage zone 4 ( cm) More abundant agglutinated forms with the same set of calcareous spp. and an increased presence of Tintinnids Assemblage zone 5 ( cm) Deep water Arctic species B. hensoni dominates with agglutinated forms; tintinnids, other species from zone 4 also present Assemblages for core PC 1 Assemblage zone 6 (0 100 cm) Very low numbers but mostly Trochammina spp. in the small size fraction with a few peaks of planktics and other benthic calcareous spp. Assemblage zone 7 ( cm) Co-dominated B. hensoni, Islandiella spp., E. clavatum and planktics with a significant number of Tintinnids Assemblage zone 8 ( cm) B. hensoni, and Islandiella spp. with decreasing planktics. Tintinnids abrupty disappear Assemblage zone 9 ( cm) Almost 100% calcareous with abundant Islandiella spp. and planktics

10 1408 Can. J. Earth Sci. Vol. 45, 2008 Fig. 7. A compilation of the paleoenvironmental interpretations for all cores by sedimentology, ages, and foraminifera assemblages. Carbon ( 12 C/ 13 C) and oxygen ( 16 O/ 18 O) ratios are shown at far left for core PC1 in the Agassiz flood period. ybp, years before present. (Fig. 4; Table S1). The foraminifera are larger in size (more individuals >63 mm), more diverse, and the fauna contains slightly more calcareous taxa. Tintinnids are most numerous in this interval and are accompanied by some partially dissolved tests and organic linings. This assemblage zone is dominated by Fursenkoina fusiformis (Williamson, 1858) (25%) and agglutinated species, while Elphidium excavatum (Terquem, 1876) forma clavatum (Cushman 1930) and Islandiella spp. are each 10%. Small individuals of Cassidulina reniforme Norvang 1945 and N. pachyderma are present in proportions <5%. It was not possible to sample the upper 30 cm of this core; however, there was a surface sample from the same site, box core 912 (collected in July 2004; Scott et al. 2008a, 2008b). This surface sample indicates a significant change in the fauna from the 30 cm level in PC3 with planktic foraminifera present, more C. reniforme, and even more significant, no tintinnids and many fewer F. fusiformis. Assemblage zone 2 Assemblage zone 2 (mid- to late Holocene) extends from 225 to 656 cm and ranges from 500 to 1000 individuals per 10 cm 3 of larger foraminifera. The diversity in this assemblage zone ranges from 5 to 20 species and is dominated by agglutinated species (30% 40%) and some E. excavatum f. clavatum and Islandiella spp. (10% each). C. reniforme (except at 590 cm), F. fusiformis, and N. pachyderma are present in minor proportions (<5%). There fewer tintinnids, and more numerous partially dissolved calcareous tests and organic inner linings. MR2002-piston core 2 (PC2) This is the mid-water core (223 m), collected at a middistance between the river mouth and shelf edge (Figs. 2, 3). The core is cm long and spans just over the last 8000 cal BP. The sediments consist of an olive-grey, bioturbated clay with a decreasing degree of bioturbation in the uppermost 200 cm (Fig. 3). At the time of sampling, this site had a very strong pycnocline, transparency gradient, and thermocline that were coincident, unlike PC1, at a depth of 20 m. As with PC3 these gradients would be completely different in the winter. There are 3 assemblage zones in PC2. Assemblage zone 3 Assemblage zone 3 (late Holocene to present) extends from 0 to 50 cm and averages individuals per 10 cm 3. The foraminiferal taxa are larger (with the exception of B. hensoni) and more diverse (Fig. 5; Table S2). There is a 60% peak of the large calcareous foraminifera at

11 Schell et al cm of E. excavatum f. clavatum, Islandiella spp., and N. pachyderma. Just above the peak of calcareous foraminifera the agglutinated species again dominate until just below the sediment surface, with minor amounts of Islandiella spp., N. pachyderma, organic linings, and tintinnids. Box core 906 from this site contains the true surface and has a slightly different fauna, with Cassidulina reniforme as the dominant species (25%); this fauna doesn t occur as a dominant in any other assemblages below this level, suggesting the loss of the true surface in PC2. Assemblage zone 4 Assemblage zone 4 (mid- to late Holocene) extends from 50 to 350 cm and averages 1000 individuals per 10 cm 3 of small agglutinated foraminifera. This assemblage zone has a stable diversity of species. It is dominated by agglutinated species with E. excavatum f. clavatum, C. reniforme, Islandiella spp., and N. pachyderma present in proportions <5%. In this assemblage zone, tintinnids are the most numerous, and there are some organic inner linings. Assemblage zone 5 Assemblage zone 5 (early Holocene to mid-holocene) extends from 350 to cm and is characterized by small calcareous foraminifera ranging from 500 to 3500 individuals per 10 cm 3. Foraminiferal diversity increases upwards in this assemblage zone. This zone is dominated by the ABW calcareous species B. hensoni (80%), but lower in this interval contains 40% 60% agglutinated species with some E. excavatum f. clavatum, Islandiella spp., and N. pachyderma, also present in proportions <15%. In this interval, tintinnids are found and partially dissolved calcareous tests of C. reniforme and Islandiella spp.; and remnant organic inner linings of completely dissolved calcareous tests are relatively abundant. In general, there is a large percentage of the smaller size fraction throughout all the foraminiferal zones with B. hensoni comprising most of that fraction in assemblage zone 2 and agglutinated species in all zones above that. MR2002-piston core 1 (PC1) This core is from the deepest water (671 m) and most river distal site at the mouth of the Mackenzie Trough (Fig. 1). It contains the longest record both chronologically (* years) and core lengthwise (Figs. 2, 3). There is a significant change in the core from an olive-grey, finegrained, laminated silty clay below 500 cm, overlain by an olive-grey homogenous clay between 400 and 500 cm, followed by an olive-grey, bioturbated clay above 400 cm. Based on the two 14 C dates the sedimentation rates decreased significantly after BP. Above the laminated sequence (0.5 m in core PC1) there is a slower sedimentation rate (*0.45 m/1000 years) after the uppermost date of BP. The sedimentation rate in the laminated sediments would be *10 m/1000 years (over a 200 year interval; Fig. 3), *20 times greater than in the Holocene (Table 3). There are four foraminiferal assemblage zones in PC1. Assemblage zone 6 Assemblage zone 6 (late Holocene to present?) extends from 0 to 100 cm with 20 to 1800 individuals per 10 cm 3. The foraminifera are generally small (50% 90% <63 mm) and agglutinated (Fig. 6; Table S3), dominated at >80% by the genus Trochammina spp., except at cm where there is a 50% peak of E. excavatum f. clavatum. with lesser percentages of N. pachyderma, Islandiella spp., B. hensoni, agglutinates, and F. fusiformis. In general, the smaller size fraction dominates what is thought to be the Holocene (i.e., nonlaminated sediments). Assemblage zone 7 Assemblage zone 7 (?mid- to late Holocene) extends from 100 to 400 cm ranging from 500 to 2000 individuals per 10 cm 3 of smaller (45 63 mm) with a diverse foraminiferal assemblage. It is dominated in places by B. hensoni, with some alternation with agglutinated species N. pachyderma, Islandiella spp., and E. excavatum f. clavatum also present in proportions of 20% 40%; F. fusiformis is sporadic in this zone. In addition, tintinnids, partially dissolved calcareous tests of C. reniforme and Islandiella spp., and remnant organic inner linings are abundant (Fig. 6); this may correspond to assemblage zone 5 in PC2. Assemblage zone 8 (early postglacial?) is a transitional zone between 400 and 500 cm to smaller (45 63 mm) and more diverse foraminiferal fauna ranging from 1000 to 2000 individuals per 10 cm 3. It is dominated 80% 100% by the ABW calcareous species B. hensoni (generally <63 mm) or the ASW agglutinated species group with F. fusiformis also present in proportions <10%. In this interval, Tintinnopsis rioplatensis Souto 1973 (a tintinnid ciliate) disappears while the partially dissolved calcareous tests of C. reniforme, Islandiella spp., and remnant organic inner linings of completely dissolved calcareous tests begin to appear. Assemblage zone 9 Assemblage zone 9 (late postglacial; cal BP) extends from 500 to 1023 cm with significantly lower numbers ranging from 100 to 1000 individuals per 10 cm 3 but usually <200 per 10 cm 3. It is dominated by low diversity, larger sized (>63 mm) calcareous foraminifera, such as N. pachyderma and Islandiella spp., with some agglutinated spp. (usually <10%) and F. fusiformis present in proportions <10%; the smaller size fraction is <50% in most cases. Taxonomy and plates for all major species discussed here are published in Scott et al. (2008b). Discussion Comparison with previous paleoceanographic studies Most of the Arctic Ocean waters are characterized by a reverse thermocline, with uppermost surface waters being less saline but cooler than the mid-atlantic waters found just below. This is particularly acute in the Mackenzie Trough with the very large freshwater discharge from the Mackenzie River. The strength of this stratification is extremely influential in the type and amount of sea ice that is present (de Vernal et al. 2005a). Times of weaker stratification lead to stronger heat fluxes across water masses, and thus less sea-ice formation (i.e., the warmer mid waters are

12 1410 Can. J. Earth Sci. Vol. 45, 2008 able to discharge heat upwards, and more vertical mixing can occur). In the western Arctic, previous sea-surface reconstructions indicate that from 6000 to *9000 cal BP there was continuous sea-ice coverage (de Vernal et al. 2005a, 2005b; Rochon et al. 2005, 2006). However, from 2000 to 6000 cal BP, conditions in the western Arctic were warming and there was less sea-ice cover, with the Holocene Thermal Optimum occurring between 4000 and 6000 cal BP. But from 2000 cal BP until present, sea-ice conditions inferred by dinoflagellate assemblages indicate conditions of persistent pack-ice in the western Arctic (de Vernal et al. 2005a, 2005b; Rochon et al. 2005; Schell et al. 2008). It has also been documented that the western Arctic s response to climate change differs from the eastern Arctic (Hillaire-Marcel et al. 2004; de Vernal et al. 2005a, 2005b; Mudie et al. 2005, 2006; Slubowska et al. 2005; Fisher et al. 2006; Rochon 2006; Rochon et al. 2006). However, Richerol et al. (2008a, 2008b), using dinocysts, have refined this scenario with much finer detail. The data from the dinocysts suggest that warming has slowed in the last 200 years at some of the same core locations as the present study (their box core 909 = PC2; their box core 912 = PC3). They suggest a slowing in warming from 1850 AD to the present (* C warming) from the previous period of AD when there was *2 5 8C warming. The dinocyst foraminiferal record of cores 909 and PC2 match well but the uppermost section of PC3 was missing so no comparison could be made. This may explain the inconsistent uppermost parts of PC2 (assemblage zone 3) where there is a sudden decrease in tintinnids and more planktics and (or) calcareous species at this time. One of our piston cores was in the same location as the box core of Richerol et al. (2008a, 2008b), it appears that the foraminiferal tintinnid faunas are detecting the same signal from at least one site except that the foraminifera do not provide as precise a temperature range as the dinoflagellates. This also agrees with a record from a box core in 1000 m of water just to the east of the Mackenzie Trough (core 750, Scott et al. 2007), which indicates a return to more sea ice in the last 100 years with the return of calcareous species to a site that had no calcareous fauna after the end of the last glaciation. This type of comparison clearly illustrates the utility of multi-proxy work with climate studies. Comparison of cores from this study The shallowest water core, PC3, spans *7000 cal BP (Figs. 4, 7). This core site is the most proximal to the Mackenzie River mouth with little sea-ice influence apparent in the paleo-record with trace amounts of N. pachyderma, together with the common calcareous Arctic species E. excavatum f. clavatum and Islandiella spp. At this site, the ratios of calcareous to agglutinated foraminifera vary from 0 to 1:1 through assemblage zones (AZs) 1 and 2, and Islandiella spp. to E. excavatum f. clavatum are approximately equal throughout the core, which indicates the equal influence of shelf and river waters. What separates AZs 1 and 2 is high abundance of F. fusiformis in AZ 1. This benthic foraminiferid is an opportunistic species that thrives in organic-rich, low-oxygen habitats (Scott et al. 1980; Alve 2003). Also, during the same intervals brackish water tintinnids appear, indicating an increase in suspended organic material in the water column, which results in more organic material on the seafloor. The combination of these species suggests a significant increase in the delivery of organic material to this site in the last 2000 years. It is not evident whether the increase in SPM is a result of more organic production in the estuary or increased organic carbon coming from the river itself and into the trough in the late Holocene. This pattern is not repeated in the two deeper water cores, suggesting that this site is responding more to very local river input as opposed to oceanic influence and also the freshwater marine interface, which causes massive flocculation of fine particulates. PC2, the intermediate site, is located at the boundary depths between AIW and ASW and spans a paleoceanographic record of *9000 cal BP (Figs. 5, 7). The site is more proximal to the Mackenzie River and receives more organic material and sediments than the distal PC1. Agglutinated faunas are very well represented, as well as tintinnids, indicating significant amounts of river and shelf sediment transport, and open water with less sea-ice coverage at this site over time. Those trends are coupled with decreasing amounts of calcareous foraminifera (fewer N. pachyderma, B. hensoni, and Islandiella spp.) upwards in the core (AZ 4). However, in AZ 3 (0 50 cm) there is a large increase of calcareous species common to the shelf waters (E. excavatum f. clavatum, C. reniforme, Islandiella spp., and N. pachyderma), which might suggest a late increase in sea-ice cover, which is also reflected in dinocyst data from the same site. From 400 cm to the base of the core (*7000 cal BP), there are significant amounts of ABW calcareous species present (foraminiferal assemblage zone 5) mixed with varying amounts of tintinnids, agglutinated species, and other calcareous species indicating fluctuating sea-ice conditions between 4000 and 7000 cal BP at this site. The foraminiferal proxy data are similar to the results of the marine palynological data. PC2 is the only piston core with published dinoflagellate data so far (Rochon et al. 2005, 2006; Rochon 2006). The palynological analyses reveal relatively diverse dinoflagellate cyst assemblages throughout the PC2 core. Rochon (2006) found that the bottom 5 m of the core (between 6340 and *9000 cal BP) were characterized by the Arctic cold-water suite of species, indicating low sea-surface temperature and significant sea-ice cover. They also found a change in the assemblages between *2 and 4 m (3575 *5422 cal BP) where the coldwater species reached a minimum, while the relative abundance of Operculodinium centrocarpum (a dominant of modern northern Pacific Ocean assemblages) reached a maximum (50%). The maximum abundance of these species during that interval suggests a more important input of North Pacific waters in the Beaufort Sea area and corresponds to the Holocene thermal optimum. Thereafter, the abundance of the cold-water suite of species gradually increases toward modern values, indicating a return to the modern temperature and sea-ice regime (Rochon et al. 2005). PC1 contains the longest record, spanning * cal BP of late glacial to Holocene climate history (Figs. 6, 7). It is the only core from this suite with pre-holocene sediments containing laminated silty clays with an almost 100% calcareous foraminiferal fauna in the lower 5 m of core, dated be-

13 Schell et al tween and cal BP. This fauna has very high numbers of planktic species (*1:1 benthic/planktic (B/P) ratio), which suggests permanent ice cover (e.g., Scott and Vilks 1991). It is known that even seasonally open water in the Arctic promotes high productivity that feeds large amounts of organic material to the seafloor (especially at a relatively shallow site such as that at core PC1), which in turn promotes reducing conditions at the seafloor resulting from organic breakdown and causing dissolution of carbonate (Scott and Vilks 1991; Wollenburg and Kuhnt 2000; Wollenburg et al. 2001, 2004; Mudie et al. 2006). However, the absolute numbers of foraminifera (usually <1000/10 cm 3 of combined B/P) indicate the sedimentation rate was high but productivity was relatively low with a 1:5 B/P ratio in the laminated interval (5 10 m). PC1 contained few shells or fragments and little CaCO 3, except for the larger calcareous foraminifera in the older section. The two dates in the laminated interval (covering only 200 years in *4 m of core section) indicate a very high rate of sedimentation (1427 cm/ 1000 years) at the end of isotope stage 2. At this time, sea level would have been m lower (Hill et al. 1985; Blasco et al. 1990), which means the PC3 site was not under the sea and there was a reduced record at PC2, which may explain why these two cores have a shorter chronology. The Agassiz flood sediments are they really coming into the Beaufort Sea? Laminations, such as those observed in the lower part of core PC1, are also reported from the Alaskan shelf (Andrews and Dunhill 2004), just west of the Mackenzie Trough, at a slightly later time ( cal BP). Andrews and Dunhill observed an indication of a meltwater spike at cal BP that they suggest is associated with the glacial Lake Agassiz flood, which is suggested to have come from the Mackenzie River both by Andrews and Dunhill and by Poore et al. (1999) and Nørgaard-Pedersen et al. (2007). The foraminiferal fauna and time frame for the Andrews and Dunhill (2004) data are almost identical to the fauna found in the lower part of PC1. Their core P189AR- P45 was collected in 405 m water depth, similar to water depth of core PC1. Stable isotopic values of the calcareous planktic foraminifera (N. pachyderma, sinistral) from the laminated sediments in PC1 indicated relatively heavy d 18 O values (2.7% 3.5%; Fig. 7; Table 5), which are even heavier than the 2.5% values reported by Andrews and Dunhill (2004) from farther west of the Mackenzie Trough. These values are also heavier than those recorded by Poore et al. (1999) and Nørgaard-Pedersen et al. (2007) for the same planktic species at approximately the same time. These sites are all to the west of the Mackenzie Trough, and the stable isotope values are lighter to the west, suggesting that if there was meltwater it would have been coming from the Siberian Shelf, not the Mackenzie River. It is difficult to imagine significant meltwater in surface Arctic water near the Mackenzie Trough at this time with isotopic measurements from planktic values we have measured both in this paper and values obtained from modern planktic foraminifera close to the Mackenzie Trough (Scott et al. 2007). These are values similar to what was measured from the Alpha Ridge cores at the same time (Scott et al. 1989a) and it is unlikely there was much meltwater in the central Arctic at any time in the Table 5. Stable isotopes (C, O) for levels 517, 558, 725, 765, 805, 838, 878, 937, and 1006 cm in PC1. Npl, N. pachyderma (left coiling). Core No. (depth in cm) d 13 C/ PDB d 18 O/ PDB 2002 MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > MIRAI PC , Npl > Note: See Tables S1 S3. last years. However, there is an obvious event at this time both near the Mackenzie Trough and on the Alaskan Shelf with strongly laminated sediments at the same time in both areas. Core PC1 appears to capture the lower part of the event, while core P189AR-P45 captures the last part of it, although error bars for these dates overlap so it is most likely the same event. This provides a partial time frame for a duration of at least years for this event; the question is just what type of paleoenvironment this represents. If the meltwater were substantial, the stable isotopic values measured by others, in addition to our measurements in the Mackenzie Trough, would have been much lower or even negative, assuming there would have been any calcareous material under conditions of lowered salinity combined with glacial temperatures. It is clear that there was an unusual depositional event both in the Alaskan core and core PC1 with large sedimentation rates, but it could have occurred under sea ice, which would explain the distinct laminations; however, it is problematic to explain the rapid sedimentation rates for the laminated units under sea ice. Possible oceanographic conditions at the Agassiz time The actual oceanographic conditions in Agassiz time period, as indicated by the foraminifera, suggest oceanographic conditions similar to those from the Amundsen Gulf, i.e., high salinities and cold temperatures. The low B/P ratio of foraminiferal assemblage zone 9 also suggests the presence of significant sea-ice cover or floating glacial ice (Schröder-Adams et al. 1990; Scott and Vilks 1991; Schell et al. 2008). Before cal BP (* cal BP) in the Amundsen Gulf there is evidence of icebergs calving off the glacier grounded in that area which deposited the ice-rafted debris (IRD) observed in a core from directly in front of the former ice margin (Scott et al. 2007); this is also true for the core in 1000 m of water just to the east of the Mackenzie Trough (Scott et al. 2007). It is suggested that there was a significant influx of meltwater into the western Arctic Ocean as recorded in box cores from the Mendeleyev Ridge during the early part of marine oxygen isotope stage 1 (OIS 1) * cal BP (Poore et al. 1999). Oxygen isotope records from several box cores raised from the Mendeleyev Ridge (Poore et al. 1999) have negative values in the lower part of OIS 1 and are interpreted to represent an influx of meltwater during the last de-