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Late Holocene paleoceanography in the Chukchi and Beaufort Seas, Arctic Ocean, based on benthic foraminifera and ostracodes

  • Julia L. Seidenstein
  • Thomas M. Cronin
  • Laura Gemery
  • Lloyd D. Keigwin
  • Christof Pearce
  • Martin Jakobsson
  • Helen K. Coxall
  • Emily A. Wei
  • Neal W. Driscoll
Original Article
Part of the following topical collections:
  1. PAST Gateways

Abstract

Calcareous microfossil assemblages in late Holocene sediments from the western Arctic continental shelf provide an important baseline for evaluating the impacts of today’s changing Arctic oceanography. This study compares 14C-dated late Holocene microfaunal assemblages of sediment cores SWERUS-L2-2-PC1, 2-MC4 and 2-KL1 (57 mwd), which record the last 4200 years in the Herald Canyon (Chukchi Sea shelf), and HLY1302-JPC-32, GGC-30, MC-29 (60 mwd), which record the last 3000 years in the Beaufort Sea shelf off the coast of Canada. Foraminiferal and ostracode assemblages are typical of Arctic continental shelf environments with annual sea-ice cover and show relatively small changes in terms of variability of dominant species. Important microfaunal changes in the Beaufort site include a spike in Spiroplectammina biformis coinciding with a decrease in Cassidulina reniforme in the last few centuries suggesting an increase of Pacific Water influence and decreased sea-ice. There is low-amplitude centennial-scale variability in proportions of benthic foraminiferal species, such as C. reniforme. In addition to these species, Cassidulina teretis s.l., Elphidium excavatum clavatum and Stainforthia feylingi are also common at this site. At the Herald Canyon site in the last few centuries, C. reniforme peaks around 150 years BP and then decreases while Spiroplectammina earlandi spikes and Acanthocythereis dunelmensis decreases also suggesting an increase in Pacific Water influence and decreased sea-ice at this site. This site also includes Buccella spp. and Elphidium excavatum clavatum. Differences in benthic foraminifera and ostracode species dominance between the two sites may be due to a greater influence of Pacific Water in the Chukchi shelf, compared to the more distal Beaufort shelf, which is also affected by the Beaufort Gyre and the Mackenzie River.

Keywords

Paleoceanography Holocene Microfossils Arctic Pacific Water 

Introduction

The Arctic region is sensitive to ongoing climate change, which has led to dramatic sea-ice losses [2, 50, 53, 64] and ecological impacts [16] in the western Arctic Ocean. The Chukchi Sea is a highly productive marine ecosystem and an important carbon sink. This productivity is driven by spring sea-ice melt and break-up, which also allows stratification of the water column [16] and inflow of nutrient-rich Pacific-origin water (PW). Likewise, the Beaufort Sea ecosystem is influenced by PW as well as Mackenzie River outflow and Atlantic-origin water [3]. These areas will continue to experience sea-ice loss and ecosystem changes, thus it is necessary to understand pre-anthropogenic environmental changes during the Holocene interglacial period [the last 11.7 kiloanum (ka)] as a baseline for interpreting modern changes.

In the western Arctic Ocean, the shallow continental shelves of the Chukchi and Beaufort Seas are influenced by a number of climatologically and oceanographically sensitive features. One of these is the influx of water originating from the Pacific Ocean that flows through the shallow (~ 50 mwd) Bering Strait (Fig. 1a) and underneath the Polar Surface Layer (− 2 to 0 °C). Inflowing PW includes the cold, saline, nutrient-rich Anadyr Water on the western side, the warm, less saline and nutrient-poor Alaskan Coastal Water on the eastern side and between them is Bering Shelf Water, with intermediate salinity [8, 15, 57, 58]. Waters originating on the Bering Shelf combine with Anadyr Water to form Bering Sea Water, which flows north into the Herald Canyon and Central Canyon. The Alaskan Coastal Water hugs the coastline of Alaska and flows to the shelf region of the Beaufort Sea by way of Barrow Canyon. The mean sea level in the Chukchi Sea is less than the sea level of the Bering Sea, and this difference is the main cause of PW flowing north into the Arctic [6, 32] despite mean winds blowing from the north [62]. In general, PW is nutrient-rich, warmer and less saline than Arctic water masses [58, 61].

Fig. 1

a Map showing the locations of the SWERUS-L2-2-PC1 at 72 m water depth in the Herald Canyon of the Chukchi Sea and HLY1302-JPC-32, GGC-30 and MC-29 at 60 m water depth from the Beaufort Sea shelf off the coast of Canada. Basemap from the International Bathymetric Chart of the Arctic Ocean (IBCAO) [21]. Ocean currents adapted from Grebmeier et al. [15] (Fig. 1). Sea-ice minimum line (pink dashed line) indicates the September mean ice edge based on AMSR-E satellite data from 2003 to 2011 from Frey et al. [11], b SWERUS-L2-2-PC1 chirp profile. c HLY1302 cores projected on chirp profiles

The flow of warm, nutrient-rich PW through the Bering Strait, which influences sea-ice melting in the Chukchi Sea [19, 50, 63, 64] and productivity of the Bering–Chukchi ecosystem [14, 57] is affected by broader atmosphere–ocean patterns related to the strength of the Aleutian Low [9, 41]. A study by Stein et al. [51] using biomarkers as proxies for sea-ice extent in the Arctic suggests that in the Chukchi Sea during the late Holocene, sea-ice extent increased during the last 2 kyears BP possibly due to the decreased inflow of PW combined with decreased primary production. Furthermore, Polyak et al. [38], using biomarkers, also identifies late Holocene changes in Bering Strait inflow as well as a stronger Beaufort Gyre (BG) in the early Holocene.

The Beaufort Sea shelf is influenced by the Alaskan Current flowing near the Alaskan coastline and by inputs from the BG and the Mackenzie River, including annual variations in sea-ice cover and river inflow [3]. Pacific Water also periodically affects the western Beaufort Sea shelf [15]. The BG primarily flows clockwise and influences the movement of sea-ice and surface water [7]. Polyak et al. [38] suggests that the BG has gradually weakened throughout the Holocene because of increasing sea-ice extent and stability caused by decreasing insolation.

In this study, we examine the hypothesis that PW variability during the late Holocene might be closely linked to sea-ice extent and thickness. SWERUS-L2-2-PC1/MC4/KL1 (site 2-PC1) is located in the Herald Canyon, which is one of the main pathways for PW entering the Arctic Ocean [36, 62]. The 2-PC1 site lies near the present day seasonal sea-ice minimum edge (1981–2010 NSIDC) so it is an ideal location for the reconstruction of past sea-ice variability. HLY1302-JPC32/GGC30/MC29 cores (site JPC32) record changes in the BG in part due to influence from the Mackenzie River. In addition, both core sites are significant because they record the Late Holocene at a high-resolution and were taken in shallow regions of the Chukchi and Beaufort Seas.

Benthic foraminifera and ostracodes are useful proxies for reconstructing environmental conditions on Arctic continental shelves. Previous studies have contributed to our knowledge of modern Arctic benthic foraminiferal species distributions [20, 29, 37, 43, 45, 60]. Benthic foraminifera can be used to infer paleo sea-ice margins because they respond to the surplus of food at sea-ice margins and can tolerate low-oxygen bottom-water conditions associated with sea-ice margins [48]. For example, C. reniforme is a key species in reconstructing seasonal sea-ice coverage [37]. Ostracode data is helpful for paleoceanographic reconstructions because ostracodes have a wide distribution in sediments and biogeographical and ecological limits controlled by environmental conditions such as sea-ice, temperature, salinity, and nutrient availability [13, 52]. Previous studies on Arctic ostracodes have contributed to this study [12, 13, 30, 52].

Materials and methods

Sediment cores

Site 2-PC1 includes piston core SWERUS-L2-2-PC1, multicore 2-MC4 and kasten core 2-KL1, which were collected in 57 m water depth from the northeastern end of the Herald Canyon during Leg 2 of the 2014 SWERUS-C3 (Swedish–Russian–US Investigation of Climate, Cryosphere and Carbon interaction) Expedition on Swedish icebreaker (IB) Oden (Table 1; Fig. 1a). A chirp sonar profile (Fig. 1b) across the 2-PC1 coring site reveals an approximately 20 ms (∼ 15 m) thick acoustically stratified upper unit with no apparent signs of disturbances [22]. The 8.2 m long 2-PC1 recovers < 55% of this upper unit. The sediment consists of dark gray clay–mud with some fine sand (2%) in places.

Table 1

Core numbers with date collected, locations, water depth and core lengths

Core number

Date collected

Latitude (N)

Longitude (W)

Water depth (m)

Recovered length (cm)

SWERUS-L2-2-PC1

August 2014

72.52°

175.32°

57

820

SWERUS-L2-2-MC4

August 2014

72.43°

175.343°

56

24

SWERUS-L2-2-KL1

August 2014

72.34°

176.44°

73

18

HLY1302-JPC-32

July 2013

69.974°

137.239°

60

1415

HLY1302-GGC-30

July 2013

69.974°

137.242°

60

290

HLY1302-MC-29

July 2013

69.975°

137.242°

60

45

Site JPC32 includes piston core HLY1302-JPC-32, gravity core GGC-30 and multicore MC-29 which were collected in 60 mwd during the USCGC Healy 1302 expedition in 2013. A chirp profile (Fig. 1c) shows that the 14 m JPC-32 is well laminated from 0 to 7 m, but penetrates an area with possible disturbances from 7 to 14 m. The sediment is primarily dark gray clay-mud with some fine sand. Site JPC32 is located in the moat of a pingo-like feature where rapid sedimentation occurred during the last few 1000 years is preserved on the Beaufort shelf. Pingo-like features, as described by Paull et al. [34] on the Beaufort Sea shelf, are caused by methane gas hydrate decomposition. These shallow shelf areas were once uncovered permafrost. During a past sea-level rise, the permafrost was flooded which caused the subsurface to warm and the gas hydrate stability zone moved downward. The gas hydrate zone released methane gas into the sediment causing pressure so that the Pleistocene-aged sediment was pushed up into a mound and the gas vented. The area around this mound that experienced this loss of the Pleistocene-aged sediment and degassing then slumped to create a ring-like depression or moat which was filled in by younger sediment [34]. It is this younger sediment that was cored. The JPC32 site pingo-like feature shows up the chirp profile with a 15-m diameter mound (Fig. 1c).

Microfossil sample preparation and assemblage analysis

JPC32 site cores were sampled in September of 2016 at Woods Hole Oceanographic Institution (WHOI). Core samples were taken at 1-cm intervals for MC-29, every 5 cm for GGC-30 and every 10 cm for JPC-32. The 2-PC1 core was sampled at 5-cm intervals. Sediment samples were washed through a 63-µm sieve and dried on filter paper overnight in a 50 °C oven. Foraminifera, including calcareous and agglutinated, and ostracodes were picked from the > 63-µm size fraction to ensure that juveniles would be included. Each sample was picked by spreading sieved sediment evenly onto a small tray and picking specimens with a fine brush. Because the foraminifera were so abundant, each sample was picked up to 200 specimens. Ostracodes were not as abundant as benthic foraminifera, so all specimens were picked and counted. Planktic foraminifera were very rare and were not studied. Taxa were identified to a species level using the taxonomy of various papers listed in Table 2.

Table 2

Foraminifera ecology of major species

Species

Author and year

Picture sources

Kara Sea environment [37]

Notes

Cassidulina teretis s.l.

Cassidulina teretis Tappan, 1951

Scott et al. [43], p. 246, pl. 6, fig. 1–14

 

Deep water [60], Atlantic water [23, 55]

Cassidulina reniforme

Norvan, 1945

Rodrigues et al. [66], p. 53, pl. 3, figs. 5, 6, 9, 11, 12

River-intermediate

Common in the Arctic. Prefers cold water areas (temps below ca. 2 °C) with seasonal sea-ice coverage and muddy sediments and is typically not found in decreased salinities (below about 30 psu) [37]

Elphidium excavatum forma clavatum

Cushman, 1930

Loeblich and Tappan [29], p. 98, pl. 19, fig. 4–10; Feyling-Hanssen [67] pls. 1, 2

Widespread on Arctic shelves

Widespread on the Arctic shelves and adapted to harsh environments (Polyak 2002); tolerates lower salinity conditions, high abundance indicates cold and unstable environments [5]; cold, low-productivity waters [33]

Buccella spp.

Buccella frigida Cushman, 1922; Buccella hannai arctica Voloshinova, 1960

Polyak et al. [37], p. 261, pl. 2, Figs. 14–17

River-proximal

Shallow shelf areas down to 50 m [33]

Spiroplectammina spp.

Spiroplectammina biformis Parker and Jones, 1864; Spiroplectammina earlandi Parker, 1952

Wollenburg and Mackensen [60], p. 178, pl. 2, figs. 15–18

 

Shallow water, estuary species [43]; S. earlandi and S. biformis typical of glaciomarine habitats [26]

Stainforthia feylingi

Knudsen & Seidenkrantz, 1994

Knudsen and Seidenkrantz [24]

 

Reaching up to 20–30% gives evidence for unstable environment with periodically low-oxygen conditions at the sea floor. May be found in connection to high productivity in early phases of break-up of sea-ice [24, 25]

Stainforthia loeblichi

Feyling-Hanssen, 1954

Polyak et al., p. 261, pl. 2, fig, 19; Ovsepyan et al. [33], p. 651, pl. 1, fig. 12

River-intermediate

Infaunal, opportunistic, high seasonal productivity

Chronology

Eighteen radiocarbon samples based on mollusk and foraminifera calcite material from the JPC32 site cores were processed at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) of WHOI (Table 3; Fig. 2). The material chosen was well-preserved and samples were cleaned of any sediment using a wet brush. Ages are reported using the BP 1950 time scale and were calibrated using Calib 7.10 [54] and the Marine13.14c calibration curve [40]. A marine reservoir correction of R = 477 ± 60 years was applied based on Holocene sediments independently dated by volcanic ash [35] and verified by reservoir estimates based on cores from nearby locations in the Beaufort Sea and Chukchi Sea [31].

Table 3

Radiocarbon dates and calculations for HLY1302-MC-29/GGC-30/JPC-32

Depth (cm)

Depth cumulative (cm)

Material

14C age (years BP)

Age Err

ΔR

Calibrated age range (2σ)

Median (cal years BP)

From (cal age BP)

To (cal age BP)

30(MC)

30

Foraminifera: mixed benthic species

615

20

 

144

309

268

154(GGC)

175

Mollusc

1760

15

477 ± 60

690

942

826

290(GGC)

311

Foraminifera: Islandiella and Cassidulina

2340

20

477 ± 60

1283

1543

1412

70(JPC)

375

Mollusc: Thyasira flexuosa

2530

20

477 ± 60

1801

1478

1626

250(JPC)

552

Mollusc fragments

2890

20

477 ± 60

2248

1886

2050

260(JPC)

562

Foraminifera: mixed benthic species

3180

20

477 ± 60

2659

2281

2419

380(JPC)

689

Mollusc: Nuculidae and fragments

3380

20

477 ± 60

2809

2465

2672

450(JPC)

752

Mollusc: Portlandia arctica and fragments

3250

15

477 ± 60

2684

2341

2513

620(JPC)

922

Mollusc: Portlandia arctica and fragments

3570

15

477 ± 60

3031

2737

2869

710(JPC)

1012

Mollusc fragments

3700

15

477 ± 60

3205

2856

3034

880(JPC)

1182

Foraminifera: mixed benthic species

4920

20

477 ± 60

4797

4442

4623

920(JPC)

1222

Mollusc: Portlandia arctica

4630

20

477 ± 60

4410

4045

4226

1050(JPC)

1352

Mollusc fragments

5120

20

477 ± 60

5048

4687

4874

1150(JPC)

1452

Mollusc: Portlandia arctica and fragments

3490

15

477 ± 60

2939

2683

2791

1250(JPC)

1552

Mollusc: Portlandia arctica

3810

20

477 ± 60

3348

2998

3183

1250(JPC)

1552

Foraminifera: mixed benthic species

3800

20

477 ± 60

3340

2985

3169

1380(JPC)

1682

Mollusc fragments

3210

20

477 ± 60

2665

2311

2458

1415(JPC)

1717

Foraminifera: Elphidium and Cassidulina

3710

25

477 ± 60

3226

2858

3047

Depth (cm) refers to the depth in each individual core at the HLY1302 site and depth cumulative (cm) is the cumulative depth of the composite core

BP before present 1950 CE

Fig. 2

Age model from radiocarbon dates for HLY1302-JPC-32/GGC-30/MC-29. Ages are reported as years before present (yrs BP), referring to years before 1950 and were calibrated using Calib and the marine13.14c calibration curve [40]. A marine reservoir correction of R = 477 ± 60 years was used based on data from Holocene sediments independently dated by the 3.6 ka Aniakchak tephra [35]. The radiocarbon dates are shown with 2σ error bars

The JPC32 site age models are based on radiocarbon dates from the three different cores, each of which had its own age model. The graph of the age model (Fig. 2) is based on cumulative depths, which are reported here in parentheses after the depths in the individual cores. The JPC-32 age model was constructed linearly from the date of 1626 years BP at 70 cm (372 cm cumulative) to 3034 years BP at 710 cm (1012 cm cum.). Below a core depth of 710 cm, there is a cluster of three dates around 4500 years BP followed by a cluster of five dates dated around 3000 years BP. This age reversal suggests a reworking of sediments so samples from the core below 710 cm (1012 cm cum.) were not included in the age model. This discrepancy may be due to a structural disturbance in the sediment caused by the pingo-like feature. The age model for MC-29 was fit linearly based on a bottom core sample of 268 years BP at 30 cm and the assumption that the 0–1 cm interval represents the calendar year 2013 (− 63 years BP), the year the core was taken. By matching the faunal assemblages of GGC-30 with MC-29, it was determined that 20 cm was missing from the top of GGC-30 and that the top of GGC-30 has a date of 106 years BP. Using this approach, the age model for GGC-30 was fit linearly based on three samples throughout the core: 113 years BP at 0 cm (21 cm cumulative), 826 years BP at 154 cm (175 cm cum.) and 1412 years BP at 290 cm (311 cm cum.). There remains some uncertainty about the ages of the breaks between JPC-32, GGC-30 and MC-29 at this site so interpretation of changes in faunal assemblages in these intervals merits caution.

The sedimentation rate for the JPC32 composite was relatively high. From 1012 to 372 cm cum., the sedimentation rate was 450 cm/kyear, 220 cm/kyear from 372 to 113 cm cum. (GGC-30) and 120 cm/kyear from 318 to − 55 cm cum. (MC-29).

An age model for 2-PC1 was established by Pearce et al. [35] based on radiocarbon dates and tephrochronology. The 2-PC1 core had high sedimentation rates ranging from 200 cm/kyear for the older section of the core, 100 cm/kyear for the middle section and 300 cm/kyear for the last 700 years. The 2MC-4 age model was created by matching up the benthic foraminifera data and using the sedimentation rate of 300 cm/kyear, which is the same as the top of 2-PC1.

Microfossil taxonomy and ecology

We followed Yasuhara et al. [65], Gemery et al. [13] and Cronin et al. [4] for taxonomy and ecology of Arctic ostracodes. We applied the foraminiferal taxonomy and ecology of Loeblich and Tappan [29], Wollenburg and Mackensen [60], Polyak et al. [37], Schell et al. [42], Scott et al. [43, 44, 45], and Taldenkova et al. [55], with more details in Table 2. One taxonomic note is needed about the important species Cassidulina teretis Tappan [56], first described from the Alaskan Coastal Plain and which has consistently been used as an indicator of Atlantic Water in the mid-depth Arctic Ocean. We use the name Cassidulina teretis s.l. where s.l. refers to sensu lato, which according to the International Code of Zoological Nomenclature means “in the wide sense”. This classification reflects the varying views on the taxonomy of C. teretis and related species from the Nordic Seas and eastern Arctic Ocean (such as Cassidulina neoteretis Seidenkrantz [47]). Because our material is from the Western Arctic Ocean near the type locality of C. teretis Tappan 1951, we follow early workers [27, 29, 56] and more recent workers [28, 43] who retained the name C. teretis.

Microfossil data calculations and plotting

Benthic foraminifers and ostracodes were present in all cores from both the 2-PC1 and JPC32 sites and were very well preserved throughout the cores (see Plates 1, 2). In the JPC32 site, up to 200 foraminifera specimens were picked. Fatela and Taborda [10] concluded that 100 specimens is the minimum for assemblage assessments in less detailed studies. For a few samples, all foraminifera were picked, but for most samples around 200 specimens were picked. Samples with fewer than 100 individual foraminifera were omitted. At 2-PC1, all benthic foraminifers and ostracodes were picked from each sample while 2-MC4 was picked to a maximum of about 200 specimens. Samples yielding less than 100 foraminifera were omitted from analyses. Because of low total ostracode abundances in both cores, species counts from adjacent samples were binned to provide a semi-quantitative measure of assemblage composition. The grouping was done by combining data from adjacent samples into one binned sample comprising a larger depth interval. At JPC32, the GGC-30 data grouped into 10-cm bins containing two adjacent samples and the JPC-32 data grouped two adjacent samples into 20-cm bins. Ostracodes in MC-29 were rare, so this core was not included in the ostracode plots. From the 2-PC1 site, ostracode data from 5-cm intervals was grouped into 20-cm bins (four adjacent samples grouped together). Bins with less than four specimens were omitted from analyses.

Plate 1

The following specimens are from HLY1302-JPC32 220–220 cm: 1: Elphidium excavatum forma clavatum; 2: Cassidulina teretis s.l.; 3: Cassidulina reniforme; 4: Stainforthia feylingi; 5: Spiroplectammina biformis; 6: Spiroplectammina earlandi; 7: Buccella frigida; 8: Buccella frigida, The following specimens are from SWERUS-L2-2-PC1 15–17 cm: 9: Elphidium excavatum forma clavatum; 10: Buccella frigida; 11: Buccella frigida; 12: Cassidulina reniforme; 13: Spiroplectammina biformis; 14: Spiroplectammina earlandi

Plate 2

1: Acanthocythereis dunelmensis, adult, left, GGC30 115–117 cm; 2: Acanthocythereis dunelmensis, juvenile, left, GGC30 100–102 cm; 3: Elofsonella concinna, adult, left, SWERUS 2-PC1 60–62 cm; 4: Elofsonella concinna, adult, right SWERUS 2-PC1 541–543 (interval view); 5: Kotoracythere arctoborealis, adult, right, GGC30 55–57 cm; 6: Kotoracythere arctoborealis, adult, left SWERUS 2-PC1 414–416 cm (interval view); 7: Normanicythere leioderma, adult, left, SWERUS 2-PC1 298–300 cm; 8: Paracyprideis pseudopunctillata, adult, left, GGC30 0–2 cm; 9: Sarsicytheridae bradii, adult, right, GGC30 150–152; 10: Semicytherura complanata, adult, right, GGC30 0–2 cm

Relative frequencies (RF, percent abundance of total assemblage, also called proportions) were calculated for each species of benthic foraminifers and ostracodes. Dominant species for each group were plotted. For foraminifers, a three-point smoothing was applied to the data by averaging each data point’s RF with the RFs before and after, shown on the graphs as a black line (Figs. 3, 4), and 95% confidence limits were calculated using the algorithm for binomial probability [10, 39] to identify significant changes in assemblages.

Fig. 3

Composite down-core record of Late Holocene benthic foraminifers from the Beaufort Sea shelf (site JPC32). Benthic foraminiferal species relative frequencies from the Beaufort Sea shelf (site JPC32) are plotted against age (reported as calibrated years before present, see Fig. 2). C. teretis s.l., C. reniforme, E. clavata, S. biformis and S. feylingi are the most abundant species. MC-29 samples are plotted in green, GGC-30 samples in red, JPC-32 samples in blue and the 3-point running mean in black. Confidence limits were calculated using the algorithm for binomial probability [39]. Red arrows mark important faunal changes including the Assemblage Zones are delineated as AZ and represent changes in the dominant species

Fig. 4

Late Holocene Herald Canyon benthic foraminifers (site 2-PC1). Benthic foraminifera relative frequency from the Herald Canyon (site 2-PC1) are plotted against age (reported as cal. years BP) on 4a and 4b. Included here are the dominant species, E. e. clavatum, Buccella spp., C. reniforme and S. earlandi. A 3-point running mean is plotted as a black line and confidence limits are also plotted. Important faunal changes are marked with a red arrow. Major changes occur in the last 200 years as shown in 4b. Assemblage Zones are delineated as AZ and represent changes in the dominant species

Results

Beaufort Sea shelf benthic foraminifera

In the Beaufort Sea shelf core site (Fig. 3), the dominant foraminifers are Cassidulina reniforme with an average RF of 34% per sample. Other important taxa downcore include Elphidium excavatum forma clavatum with an average RF of 21% and Cassidulina teretis s.l. with a RF of 13%. Spiroplectammina biformis, the dominant agglutinated species, and Stainforthia feylingi are rare from 3000 to 1500 years BP, but comprise 14 and 8%, respectively, of the average RF per sample from 1500 years BP to present.

Several foraminiferal Assemblage Zones (AZ) were identified.

  • AZ 4 occurs from 3000 to 1400 years BP. During this period, C. teretis s.l., C. reniforme and E. e. clavatum were the dominant species and experience RF fluctuations. C. teretis s.l. fluctuates between 10 and 30% RF. C. reniforme exhibits a large fluctuation and reaches a maximum RF of around 50% from 2800 to 2550 years BP. S. feylingi becomes more abundant at around 10% in the middle of this zone. Towards the end of this period, C. reniforme increases from around 30–50%.

  • AZ 3 occurs from 1400 to 450 years BP. The start of this period is defined by S. biformis becoming abundant with a RF around 20% (red arrow in Fig. 3) and S. feylingi becoming more abundant with an RF around 10%. These species along with C. reniforme and E. e. clavatum dominate while C. teretis s.l. becomes less common. C. reniforme has a high of around 40% from 800 to 600 years BP. At the end of this period, S. biformis decreases from about 30–10%.

  • AZ 2 occurs from 450 to 100 years BP. This period is defined by an increase in C. reniforme from around 25–55% at the end of this period (red arrow in Fig. 3). C. reniforme along with E. e. clavatum dominate during this time period.

  • AZ 1 occurs from 100 to − 56 years BP. This time period is defined by a spike in S. biformis to 60% RF and a sharp decrease in C. reniforme from 55 to 0% followed by a slight increase to 12%. C. teretis s.l. E. e. clavatum and S. feylingi decrease during this period.

Herald Canyon benthic foraminifera

In the Herald Canyon 2-PC1 site (Fig. 4), the dominant foraminifer is E. e. clavatum with an average RF of 55% per sample. Buccella spp. (including the species Buccella frigida, Buccella hannai arctica and Buccella tenerrima, as grouped by Polyak et al. [37]) averages 13%, while C. reniforme is 12% and S. earlandi is 3%. Distinct Assemblage Zones (AZ) were identified as follows.

  • AZ 4 covers 4200–1400 years BP. In this period, E. e. clavatum dominates with Buccella spp. and C. reniforme is also abundant. E. e. clavatum fluctuates between 35–70% through this time period. C. reniforme is less common during this time period except for an increase from 4000 to 3500 years BP from about 10% to a peak of 40%.

  • AZ 3 covers 1400 to 450 years BP. This period is defined by a decrease in Buccella spp. of 20–5% and an increase in C. reniforme of 5–25% RF. From around 1150 years BP, E. e. clavatum RF decreases from 70 to 50%.

  • AZ 2 covers 450–200 years BP. This period is defined by a decrease in Buccella spp. from 20 to 15%. C. reniforme increases to a peak of around 30% RF at 200 years BP (red arrow in Fig. 4). E. e. clavatum decreases down to about 40% at the end of this period.

  • AZ 1 covers 200 to − 63 years BP. This period is defined by a peak of C. reniforme of 30% followed by a decline down to near 0%. During the last 30 years, E. e. clavatum and Buccella spp. decrease down to near 0% in the modern while S. biformis increases from a RF of around 20–80% (see Fig. 4b).

Beaufort Sea shelf ostracodes

On the Beaufort Sea shelf, Paracyprideis pseudopunctillata is the most abundant species with an average RF of 69% followed by Semicytherura complanata with an average RF of 8%. Kotoracythere arctoborealis has an average RF of 6% (Fig. 5). A few other species such as Acanthocythereis dunelmensis, Cluthia cluthae and Cytheropteron elaeni were present in low abundance with average RFs of less than 7%. These species are not shown on the figure due to their low abundances. The total number of ostracodes in the binned samples varies from 10 to 120 specimens throughout the core and notably increases after 1500 years BP (red arrow in Fig. 5). Assemblage zones were not created for ostracodes because of low abundances. Faunal transitions of note include:

Fig. 5

Late Holocene Beaufort Sea shelf ostracodes (site JPC32). Ostracode species’ relative frequencies were calculated by dividing the number of specimens for each species in a sample by the total number of ostracodes in that sample. The percent abundances are plotted against age (cal. years BP). Samples from GGC-30 are in red and JPC-32 samples are in blue. P. pseudopunctillata and S. complanata are the dominant species with K. arctoborealis also present. Total ostracode numbers are also plotted

  • P. pseudopunctillata RF gradually declines from the bottom of the core with an RF of around 80% to the top of the core with an RF of around 25% (gray arrow in Fig. 5). Within that time, there is one zone of especially low RF (0–20%) between 625 and 444 years BP.

  • S. complanata varies from 0 to 20% during 3000–950 years BP. Its abundance abruptly increases to 63% around 650 years BP (red arrow in Fig. 5) and decreases to 0–20% from 500 to 100 years BP.

  • K. arctoborealis is relatively constant through the core with RFs between 0 and 20%

Herald Canyon ostracodes

Ostracode samples in the Herald Canyon 2-PC1 site (Fig. 6) usually average less than 20 specimens per binned sample, with the exception of samples from the top of the core (from 42 to − 32 years BP) where a few intervals contained 60–78 specimens. Acanthocythereis dunelmensis and Kotoracythere arctoborealis have the highest average RFs of 33 and 20%, respectively. Other less common ostracodes are Elofsonella concinna (RF of 8%), Normanicythere leioderma (8%), Semicytherura complanata (5%), Cytheropteron elaeni (5%), Sarsicytheridae bradii (3%) (Fig. 6). The following faunal changes are of note:

Fig. 6

Late Holocene Herald Canyon ostracodes (site 2-PC1). Ostracode relative frequencies are plotted against age for the major ostracodes at site 2-PC1. The most abundance species, A. dunelmensis, K. arctoborealis, E. concinna, N. leioderma, S. bradii and S. complanata are plotted here along with the total ostracode numbers. A. dunelmensis shows a general trend of increasing in relative abundance towards the present (gray trend line). K. arctoborealis oscillates from 0 to nearly 60% during the last few 1000 years

  • During 4250–2340 years BP, A. dunelmensis has low RFs and then increases in abundance (red arrow in Fig. 6) to 35–100% from 2000 to 50 years BP. However, at the very top of the core, A. dunelmensis decreases slightly from 80% at 50 years BP to 64% at − 30 years BP (red arrow in Fig. 6).

  • K arctoborealis is abundant but variable from 3950 to 1950 years BP with RF that vary from 0 to 60%.

  • N. leioderma peaks in RF to 45% at 2000 years BP while RFs range from 0 to 25% in the rest of the core.

Discussion

Benthic foraminifers

Foraminiferal assemblage zones at the JPC32 (Beaufort Sea) and 2-PC1 (Chukchi Sea) sites cover roughly the same period of late Holocene deposition, but are defined by transitions of different species assemblages. A total of 30 species were identified in both cores, however, the same five species for JPC32 and four species for 2-PC1 are predominant in the cores. All of the common species are typical of Arctic shallow water, however, there are several significant faunal changes worth further discussion.

AZ 4 in both cores is dominated by C. reniforme and E. e. clavatum although in 2-PC1, Buccella spp. is also prevalent and in JPC32, C. teretis s.l. is common. Through AZ 4, the RFs of dominant species in both cores fluctuate moderately, indicating possible low-amplitude centennial-scale changes through the core. For example, the RF of E. e. clavatum in 2-PC1 fluctuates between 35–70% while in the JPC32 site, C. reniforme fluctuates between 30 and 50%. E. e. clavatum is widespread on modern Arctic shelves and uses both epifaunal and infaunal strategies to adapt to cold and variable environments especially at ice-margin interfaces [5, 37]. Some authors report that E. e. clavatum tolerates a wide range of habitats and can survive in nutrient-poor conditions [37, 60]. The infaunal species Cassidulina reniforme is an important indicator of glaciomarine ice-proximal environments and generally suggests the influence of cold water masses [17, 49]. In the Arctic, C. reniforme is a common calcareous species living on the shelf typically in water below ca. 2 °C with seasonal sea-ice coverage and salinities above 30 psu [37]. The presence of C. teretis s.l. in JPC32 could be interpreted as evidence for Atlantic-origin water incursions on the shelf since C. teretis s.l. is widely considered an indicator of Atlantic water [1, 23, 48, 55, 60]. The absence of C. teretis s.l. in 2-PC1 could show that there is little or no Atlantic Water reaching this part of the Chukchi Sea.

In AZ 3, several faunal changes occur in both cores. At JPC32, S. biformis appears in high proportions. S. biformis is a shallow water and estuary species [43], is typical of glaciomarine estuarine habitats [26] and was found in Arctic Surface Water by Ishman and Foley [20]. The abundance of agglutinated foraminiferal species like S. biformis indicates increased meltwater output and reduced carbonate preservation according to some authors [46]. This suggests a possible increase in the influence of Mackenzie River discharge at JPC32 during AZ 3, starting around 1400 years BP. Stainforthia feylingi also increases in RF at JPC32 which might suggest unstable environmental conditions including periods of low dissolved oxygen and periods of high productivity during break-up of sea-ice [24, 25]. Seidenkrantz [48] found that abundant S. feylingi might be related to algal blooms during sea-ice retreat at the sea-ice cover margin.

At the 2-PC1 site, Buccella spp. decreases and then increases towards the end of this period. Buccella spp. occurs in deeper shelf sites, but shows a preference for shelf areas affected by rivers [37], shallow shelf areas down to 50 m [33] and areas influenced by seasonal sea-ice [18]. C. reniforme at 2-PC1 gradually increases through this period possibly indicating the increasing influence of cold water masses. In the latter half of AZ 3 (800–600 years BP) in JPC32, C. reniforme RF peaks while S. biformis and S. feylingi both decrease which could indicate less sea-ice break-up. C. teretis s.l. becomes less common during this time period, perhaps due to changes in the influence of Atlantic Water beneath the Polar Surface Layer.

The age of AZ 2 is slightly different in each core. 2-PC1 AZ2 occurs from 450 to 200 years BP and JPC32 AZ2 occurs from 450 to 100 years BP. During this time period, C. reniforme increases in both cores indicating a possible increase in colder water masses reaching both sites. This might reflect Little Ice Age cooling in the Chukchi and Beaufort Seas, a trend that began in AZ 3 at 2-PC1. These changes could be showing the decreased influence of PW in the Chukchi starting 1.5 kyears BP. This is consistent with the findings of Stein et al. [51] who found that increasing sea-ice extent in the last 2 kyears BP might be due to decreased PW flow. S. biformis decreases to 0–15% in JPC32 during this time period which might indicate more sea-ice cover during the Little Ice Age.

The age of AZ 1 is ~ 100 to − 53 years BP in JPC32 and ~ 200 to − 63 years BP in 2-PC1 when there is a dominance of agglutinated species. In 2-PC1, E. e. clavatum, Buccella spp. and C. reniforme decrease towards 0% RF while S. earlandi becomes the dominate species. Spiroplectammina earlandi and S. biformis may be morphotypes of a single species and probably inhabit similar environmental conditions [26]. In 2-PC1, S. earlandi increases around the same time as S. biformis does suggesting large environmental changes at both sites such as reduced sea-ice cover and warmer near-surface condition [46], perhaps due to inflow of warm, nutrient-rich Pacific water. The agglutinated spike is unlikely to be a product of dissolution of calcareous species because calcareous foraminifera species still exist in this part of the core and do not show signs of dissolution. In addition, the decrease of C. reniforme in both cores indicates a decrease in cold water which supports the Spiroplectammina spp. data. The AZ 1 Spiroplectammina invasion in 2-PC1 is exceptional in the new 4000 kyears long Holocene record, implying a mode shift in Herald Canyon benthic regime in the most recent 60 years. The 2-PC1 site in the Chukchi Sea is located directly in the path of the warm PW coming through the Herald Canyon from the Bering Strait. The JPC32 site is further from the Bering Strait so although it is also affected by PW, this region is also affected by the Mackenzie River and the Beaufort Gyre.

Ostracodes

Due to low specimen counts in 2-PC1 and JPC32, the interpretation of results must be considered cautiously. Nevertheless, these results provide qualitative information on changing oceanographic conditions. As typical modern Arctic inhabitants, A. dunelmensis and K. arctoborealis are common in varying frequencies down 2-PC1. Acanthocythereis dunelmensis is the dominant species during the last 1000 years while K. arctoborealis is the dominant before 1000 years BP. Acanthocythereis dunelmensis inhabits frigid, marine, shallow depths (< 100 m) of the Arctic’s marginal seas. Kotoracythere arctoborealis, a species with Pacific Ocean affinities, decreases from 4000 to 950 years BP while A. dunelmensis increases, which indicates colder waters on the shelf in the last 1000 years [12, 13]. The decrease in A. dunelmensis in the last century could indicate a decrease in cold water perhaps due to an in increase in warm PW influencing the Chukchi Shelf.

Like K. arctoborealis, N. leioderma provides evidence for oceanographic changes. In 2-PC1, this species was more common from ~ 4450 to ~ 1150 years BP with abundances up to ~ 20% (Fig. 6). Today, N. leioderma occurs in subarctic and Arctic regions, but is a dominant species in subarctic waters of the Bering Sea [12]. S. bradii and S. complanata are common in both subarctic to Arctic environments. In 2-PC1, these species occur in sediments dated at around 4450–3950 and 3950–1950 years BP.

In the JPC32 site, the euryhaline species P. pseudopunctillata dominates the assemblages. Typically, P. pseudopunctillata inhabits shallow (< 50 mwd), nearshore environments often near river mouths such as Norton Sound [12]. It is tolerant of seasonally fluctuating salinities and water temperatures [30, 52]. Today, P. pseudopunctillata is common in the Chukchi, Beaufort Seas and Laptev Sea assemblages. It is found in very high proportions in modern shallow (< 20 m) surface sediments of the E. Siberian Sea.

Conclusions

High-resolution sediment core foraminiferal and ostracode records from the Chukchi and Beaufort Seas provide important information on benthic ecosystem changes during the late Holocene. However, when compared to major microfaunal changes during the last deglaciation in the Laptev Sea [55] and Barents Sea [59], the Chukchi and Beaufort shelf faunal changes of the past ~ 4000 years are relatively small in terms of the variability in dominant species. Nonetheless, these cores do provide evidence for changes in the influence of warmer, high nutrient Pacific Water inflow in the western Arctic Ocean. The abundance of C. reniforme from 1400 to 200 years BP at the Chukchi Sea 2-PC1 site suggests colder ocean temperatures possibly due to a decrease in PW influence. The decline in abundance of C. reniforme and increase in S. earlandi from 200 to − 63 years BP suggests a greater influence of nutrient-rich, warmer PW and a decline in seasonal sea-ice cover. Acanthocythereis dunelmensis also declines in 2-PC1 in the last few centuries supporting the possible increasing influence of PW. A similar pattern is observed at the Beaufort Sea shelf JPC32 site where C. reniforme declines and S. biformis increases ~ 100 to − 56 years BP. In addition, quasi-cyclic changes of relative abundance of species at both sites may signify subtle, but distinct centennial-scale variability. Further studies of high-resolution cores from the Beaufort shelf and use of additional proxies will aid our understanding of Holocene ocean variability on western Arctic Ocean shelves.

Notes

Acknowledgements

Thanks to the scientists and crew on the icebreaker Oden for the Swedish-Russian-US Investigation of Climate, Cryosphere and Carbon interaction (SWERUS-C3) expedition in 2014 and USCGC Healy 1302 expedition in 2013. The SWERUS-C3 program was funded by a grant from the Knut and Alice Wallenberg Foundation and by the Swedish Polar Research Secretariat. A National Science Foundation grant (ARC-1204045) funded the Healy 1302 expedition. Thanks to A. Ruefer, A. Xu and S. Fisher for sample processing and M. Toomey for help with the age model. The U.S. Geological Survey Climate and Land Use R&D Program funded this study. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Compliance with ethical standards

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Supplementary material

41063_2018_58_MOESM1_ESM.xlsx (168 kb)
Supplementary material 1 (XLSX 168 KB)

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Copyright information

© US Government (outside the USA) 2018

Authors and Affiliations

  • Julia L. Seidenstein
    • 1
    • 6
  • Thomas M. Cronin
    • 1
  • Laura Gemery
    • 1
  • Lloyd D. Keigwin
    • 2
  • Christof Pearce
    • 3
    • 4
  • Martin Jakobsson
    • 3
  • Helen K. Coxall
    • 3
  • Emily A. Wei
    • 5
  • Neal W. Driscoll
    • 5
  1. 1.Natural Systems Analysts, IncU.S. Geological SurveyRestonUSA
  2. 2.Department of Geology and GeophysicsWoods Hole Oceanographic InstitutionWoods HoleUSA
  3. 3.Department of Geological Sciences and Bolin Centre for Climate ResearchStockholm UniversityStockholmSweden
  4. 4.Department of Geoscience, Arctic Research Centre and iClimateAarhus UniversityAarhusDenmark
  5. 5.Scripps Institution of OceanographyUniversity of California San DiegoLa JollaUSA
  6. 6.Department of GeosciencesUniversity of Massachusetts-AmherstAmherstUSA

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