Journal of Paleolimnology

, Volume 45, Issue 2, pp 273–285

Late Holocene Adélie penguin population dynamics at Zolotov Island, Vestfold Hills, Antarctica

Authors

  • Tao Huang
    • Institute of Polar EnvironmentUniversity of Science and Technology of China
    • Institute of Polar EnvironmentUniversity of Science and Technology of China
  • Yuhong Wang
    • Institute of Polar EnvironmentUniversity of Science and Technology of China
    • National Institutes of Health
  • Deming Kong
    • Institute of Polar EnvironmentUniversity of Science and Technology of China
Original paper

DOI: 10.1007/s10933-011-9497-x

Cite this article as:
Huang, T., Sun, L., Wang, Y. et al. J Paleolimnol (2011) 45: 273. doi:10.1007/s10933-011-9497-x
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Abstract

We inferred late Holocene Adélie penguin occupation history and population dynamics on Zolotov Island, Vestfold Hills, Antarctica, using geochemical data from a dated ornithogenic sediment core (ZOL4). Radiocarbon dates on fossil penguin bones in the core indicate that Adélie penguins occupied the island as early as 1,800 years before present (yr BP), following the retreat of the Sørsdal glacier. This occupation began ~1,200 years later than that observed at Ardley Island and King George Island, in the South Shetland Islands. Phosphorus was identified as the most indicative bio-element for penguin guano in core ZOL4, and was used to infer past penguin population dynamics. Around 1,800 years ago, the Adélie penguin populations at both Zolotov Island and Ardley Island increased rapidly and reached their highest levels ~1,000 yr BP. For the past ~900 years, the penguin populations at Zolotov Island have shown a general rising trend, with fluctuations, while those at Ardley Island have shown a moderate decreasing trend. The Adélie penguin populations at both Ardley Island and Zolotov Island showed a clear decline ~300 years ago, which we interpret as a response to the Little Ice Age, or a neoglacial cooling event.

Keywords

Adélie penguinAntarctic climatesIce coreOrnithogenic sedimentsWestern Antarctic PeninsulaLittle Ice Age

Introduction

Polar seabirds provide important linkages between marine ecosystems and terrestrial environments. They transport marine-derived nutrients and contaminants onto land via their guano and physical remains (Sun and Xie 2001a; Blais et al. 2005, 2007; Xie and Sun 2008; Yin et al. 2008; Brimble et al. 2009; Keatley et al. 2009; Michelutti et al. 2010). Lake sediments in areas visited by seabirds can therefore contain materials of both marine and lacustrine origin (Sun and Xie 2001b). In Antarctica, chemical signatures from penguin droppings and physical signatures such as bones, feathers and hairs in lake sediments have been used to infer the past population dynamics of penguins and seals, as well as their responses to changing climate and human activities (Hodgson and Johnston 1997; Sun et al. 2000, 2004a, b, 2005; Wang et al. 2007; Huang et al. 2009a; Yang et al. 2010). For example, the penguin populations at Ardley Island and King George Island, South Shetland Islands, showed a dramatic decline in the neoglacial period, 2,300–1,800 yr BP (Sun et al. 2000), indicating the negative impacts of cooling climate on penguin populations.

Adélie penguins (Pygoscelis adeliae) are the most abundant seabirds in Antarctica and the bellwether of Antarctic climate change (Ainley 2002). Their population dynamics are influenced by climatic and environmental factors such as sea ice extent and duration, sea surface temperature, air temperature and snow cover (Fraser et al. 1992; Wilson et al. 2001; Jenouvrier et al. 2006; Bricher et al. 2008), and thus they provide an integrated response to ecological and climate changes (Croxall et al. 2002). In the past few decades, observational records of changing Adélie penguin populations have shown strikingly different trends in the Antarctic Peninsula and East Antarctica regions. The Adélie penguin populations in East Antarctica have shown a sustained increase, while those in the Antarctic Peninsula region have decreased (Woehler et al. 2001). These opposite population trends are likely associated with differences in regional climate and environment, such as sea ice extent and related changes in prey abundance (Fraser and Hofmann 2003; Forcada et al. 2006). Records of penguin population changes over larger spatial and temporal scales are required to provide a long-term record of natural variability and to understand the population responses of Adelie penguins to changes in climate and marine ecosystems.

The Vestfold Hills is one of the larger East Antarctic oases (Fig. 1), located east of Prydz Bay. Many Adélie penguin colonies are present on its western coastal islands. During the 1981/1982 seasons, it was estimated that there were 196,592 pairs of breeding Adélie penguins on these islands, of which 17,496 were on Zolotov Island (Whitehead and Johnstone 1990). In a previous study, we reconstructed an 8,500-year record of Adélie penguin population dynamics at Gardner Island, Vestfold Hills and examined associations with climate and environmental changes (Huang et al. 2009a). Temporal resolution of the inferred penguin population shifts at Gardner Island, however, was relatively low, and detailed penguin population changes over the past 2,000 years were not resolved. In the present study, we explore geochemical and chronological data from sediment core ZOL4 taken in a lake on Zolotov Island. We extracted the bio-elements and inferred late Holocene Adélie penguin occupation and population dynamics. We also compared the penguin population changes at Zolotov Island with records from Ardley Island in the South Shetland Islands over the past 1,800 years, and examined associations with regional climate changes.
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Fig. 1

Map of the Vestfold Hills and South Shetland Islands, including the sampling sites on Zolotov Island (ZOL4) and Ardley Island (Y2)

Materials and methods

Study site and sample collection

Zolotov Island is located about 10 km southwest of the Australian Antarctic Davis Station in Vestfold Hills, East Antarctica (68°39′S, 77°52′E; Fig. 1). The island is about 2 km long and 1.5 km wide, and has a maximum altitude of 28 m above sea level. It possesses a large number of breeding Adélie penguins. Sediment core ZOL4 was retrieved from a lake near a large Adélie penguin colony. The catchment is located in a low-lying basin at the center of this island, and is about 120 m long and 55 m wide. It lies at an altitute of ~7 m. During field investigations, the lake was very shallow. A 12-cm-diameter PVC pipe was pushed vertically into the deepest part of the lake to collect the sediment core. After the PVC pipe was retrieved, its bottom and top were sealed. In the laboratory, the 40-cm core was opened and sectioned at 1-cm intervals. Penguin remains such as bones, feathers and eggshell fragments were found in the upper 17 cm, and hand picked. The 40 subsamples were stored frozen prior to analysis. Before chemical analyses, each subsample was air-dried in a clean laboratory and homogenized.

Radiocarbon dating and geochemical analyses

We dated four fossil penguin bones (collagen) and two bulk sediment samples (organic carbon) by Accelerator Mass Spectrometry (AMS) 14C to establish the depth/age profile for core ZOL4. Dates on fossil bone were corrected for the marine carbon reservoir effect using the dataset of Marine04 (Hughen et al. 2004) to give a ΔR 880 ± 15 years, the age of local modern penguin bone (Huang et al. 2009b), and calibrated using the CALIB 5.1.0 program (Stuiver et al. 2005). In this study, the calibrated 14C dates were reported in calendar years before present (cal yr BP).

All air-dried subsamples were measured to determine the concentrations of 16 major and trace elements (P (P2O5), S, Cu, Zn, Ni, Cd, Pb, K (K2O), Na (Na2O), Ca (CaO), Mg (MgO), Fe (Fe2O3), Al (Al2O3), Mn, Cr and Ti), total carbon (TC) and total nitrogen (TN). For chemical element analyses, subsamples were sieved though a 70-μm mesh, and then ground to powder after removal of large rock fragments. About 0.25 g of each powder sample was taken, weighed, and digested (HNO3-HF-HClO4) in a Teflon crucible with electric heating. The digested samples were analyzed for P, Cu, Zn, Ni, Cd, Pb, K, Na, Ca, Mg, Fe, Mn, Cr, Al and Ti using an ICP-OES DV2100 (PerkinElmer, USA). Standard sediment reference materials were included with every batch of samples. The analytical values for major elements and trace elements are within ±0.5% and ±5% of the certified standards, respectively. TC, TN and S were measured by vario EL III (Elementar, Germany) with a relative error of 0.1%.

We ran R-mode clustering analysis, Principal Component Analysis (PCA) and Pearson correlation analysis on these data using SPSS16.0, to examine the relationship between element concentrations in the sediments and their controlling factors. The concentration data of fresh penguin guano and bedrock at nearby Gardner Island were also included in this study to help discern the bedrock signature from that of penguin guano.

Results

Sedimentology and chronology

Sediment core ZOL4 is 40 cm long. Two sedimentary units were identified from macroscopic description, color, smell and sedimentology (Fig. 2). Unit 1 extends from the base to 17 cm, and is characterized by dominance of greyish deposits consisting of mainly sand, a moderate amount of silt, and some small gravels. Unit 2 spans from 17 cm to the surface and consists of olive to dark olive grey sediment, which compared with Unit 1, has more silt and clay. Unit 2 also contains many physical penguin remains such as bones, feathers and eggshells, and has a strong smell of penguin guano, and is identified here as a penguin ornithogenic sediment layer, i.e. sediments that contain penguin guano and body remains. Unit 1 has a distinct sedimentology compared with Unit 2, and it is unlikely amended by penguin guano. TC and TN are very low in Unit 1, but very high in Unit 2 (Fig. 2; Table 1), indicating a substantial increase in organic inputs to Unit 2.
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Fig. 2

The sedimentology and elemental concentration profiles in sediment core ZOL4

Table 1

Element mean concentrations in the sediments of ZOL4, together with reference data from penguin guano and bedrock

Elements

Range

CV (%)

ZOL4 (0–40 cm)

ZOL4 (0–17 cm)

ZOL4 (17–40 cm)

*Guano (n = 3)

*Bedrock (n = 3)

TC (%)

1.16–3.74

37.2

2.16

3.05

1.50

10.25

0.23

TN (%)

0.14–2.05

97.0

0.61

1.17

0.19

2.84

0.03

P (%)

0.19–5.72

125.4

1.96

4.17

0.33

6.31

0.33

S (%)

0.26–0.41

76.0

0.31

0.35

0.29

2.11

0.46

Mg (%)

4.82–9.63

25.6

6.50

8.08

5.34

5.57

4.62

Cu (μg/g)

66–315

52.5

164

252

99

310

144

Zn (μg/g)

65–126

21.0

90

106

78

130

209

Ni (μg/g)

88–198

24.3

127

155

106

22

52

Cd (μg/g)

0.69–2.29

32.5

1.30

1.66

1.03

0.95

0.40

Fe (%)

4.91–11.66

23.4

8.33

5.83

10.19

3.52

17.7

Ti (mg/g)

2.02–6.44

32.8

4.38

2.54

5.75

2.42

11.59

Al (%)

4.24–7.47

15.5

6.23

5.01

7.12

7.04

7.51

Ca (%)

3.79–6.83

14.4

5.61

4.79

6.21

4.21

9.64

Pb (μg/g)

4.91–9.48

15.3

7.43

7.82

7.14

5.83

9.52

K (%)

1.34–2.23

11.1

1.67

1.62

1.70

0.78

0.21

Na (%)

2.59–3.72

8.3

3.18

3.02

3.31

3.11

1.42

Mn (%)

0.85–1.55

13.1

1.15

1.13

1.16

0.03

0.19

Cr (μg/g)

109–217

13.6

163

146

175

CV is coefficient of variation; * Collected from Gardner Island, which is adjacent to Zolotov Island in Vestfold Hills; – not determined

Radiocarbon results are shown in Table 2. The mean calibrated ages of penguin bones at 1, 8, 14, and 17 cm are about 80, 930, 1,280, and 1,765 cal yr BP, respectively, and they show a fairly linear trend with depth (r = 0.99, n = 4). Ages of the two bulk sediment samples at 30 and 39 cm depth are 14,110 ± 50 yr BP and 17,080 ± 60 yr BP (14C dates), respectively, very different from the ages determined on bones. The “old carbon” reservoir effect on bulk sediments is difficult to estimate precisely because there are multiple sources of carbon to the sediment. Nevertheless, these dates suggest that Unit 1 includes material of late glacial age, and predates Unit 2. The chronology of the upper unit (17–0 cm) was established using linear interpolation between the four calibrated AMS 14C dates on macrofossils (Fig. 3).
Table 2

AMS 14C dates and calibrated ages using CALIB 5.1.0 and the Marine04 datasets (∆R 880 ± 15)

UCIAMS number

Sample number

Sample material

Depth (cm)

Conventional 14C age (yr BP)

Calibrated age (cal yr BP)

Mean

Range (2σ)

*39438

DG-34

Bone

Modern

880 ± 15

Modern

Modern

55716

ZOL4–1

Bone

1

1,340 ± 15

83

0–139

55717

ZOL4–8

Bone

8

2,250 ± 15

928

904–963

55736

ZOL4–14

Bone

14

2,595 ± 15

1,281

1,252–1,309

55718

ZOL4–17

Bone

17

3,030 ± 15

1,765

1,702–1,817

55794

ZOL4–30

Bulk sediments

30

14,110 ± 50

55795

ZOL4–39

Bulk sediments

39

17,080 ± 60

The dates were measured at the W.M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory, University of California Irvine (KCCAMS UCI); * 39438 was presented in Huang et al. (2009b)

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Fig. 3

The age profile of ZOL4 based on linear interpolation between the calibrated 14C dates (Note that the dates below 20 cm are not plotted on the same scale)

Element concentrations

Mean concentrations of elements in the sediments of ZOL4 are listed in Table 1. Among the major elements, Fe has the highest mean concentration of 8.33%, followed by Mg (6.50%), Al (6.23%), Ca (5.61%), Na (3.18%), K (1.67%), Ti (4.38‰) and Mn (1.15‰). The mean concentration of P is 1.96% and as high as 4.17% in Unit 2. The elemental concentration profiles of ZOL4 are plotted in Fig. 2. They show substantial fluctuations, consistent with the sedimentology. Similar to TC and TN, the concentrations of most elements show an abrupt shift at 17 cm depth. The elements P, Mg, Cu, Zn, Ni, S, Cd and Pb have similar concentration profiles in that their concentrations are high in the upper, 17–0 cm (Unit 2), but very low and stable in the lower 40–17 cm (Unit 1). The elements Al, Ca, Fe and Ti show an opposite trend. Their concentrations are high in sediments from 40–17 cm and low in sediments from 17–0 cm. The large changes in elemental concentrations at 17 cm indicate a change in the source of material delivered to the sediment. The elements K, Na, Cr and Mn do not show clear trends, and thus appear to have been less impacted by the changes in the sedimentary source materials.

Statistical analysis

Assemblages of bio-elements in lake sediments have been used successfully to infer their material sources (Sun et al. 2000, 2004a; Liu et al. 2006; Huang et al. 2009a). We performed R-clustering, Pearson correlation analyses and PCA to obtain an assemblage of bio-elements for the penguin ornithogenic sediments of ZOL4.

The R-clustering results for the elemental concentrations in ZOL4 are shown in Fig. 4 and show that P, TN and S belong to the first group, TC, Cu, Ni, Zn, Mg and Cd the second, and Al, Ca, Na, K, Cr, Mn, Pb, Fe and Ti the third. Pearson correlation analyses were performed to confirm the clustering results. The coefficients are listed in Table 3 and are consistent with the R-clustering results. The concentrations P, Mg, TN, TC, Cu, Zn, S, Ni and Cd are significantly correlated with each other and they are negatively correlated with lithologic elements Al, Ca, Fe, Ti, Cr and Na.
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Fig. 4

R-mode clustering results for the elements measured in sediment core ZOL4

Table 3

Correlation coefficients between the elements in the ZOL4 sediment core

 

P

Mg

TC

TN

Cu

Zn

S

Ni

Cd

Al

Ca

Fe

Ti

Cr

Na

Pb

Mn

K

P

1

                 

Mg

.98*

1

                

TC

.89*

.84*

1

               

TN

.95*

.93*

.89*

1

              

Cu

.89*

.86*

.91*

.80*

1

             

Zn

.80*

.76*

.77*

.65*

.90*

1

            

S

.72*

.70*

.70*

.68*

.79*

.71*

1

           

Ni

.72*

.74*

.79*

.60*

.93*

.88*

.71*

1

          

Cd

.69*

.62*

.67*

.57*

.75*

.88*

.58*

.69*

1

         

Al

−.95*

−.91*

−.91*

−.91*

−.90*

−.79*

−.60*

−.76*

−.68*

1

        

Ca

−.88*

−.85*

−.76*

−.90*

−.72*

−.56*

−.54*

−.53*

−.44*

.89*

1

       

Fe

−.95*

−.89*

−.91*

−.92*

−.87*

−.81*

−.73*

−.69*

−.76*

.93*

.85*

1

      

Ti

−.94*

−.89*

−.90*

−.90*

−.92*

−.82*

−.81*

−.78*

−.72*

.92*

.87*

.96*

1

     

Cr

−.65*

−.57*

−.63*

−.68*

−.47*

−.44*

−.19

−.27

−.53*

.69*

.67*

.72*

.61*

1

    

Na

−.56*

−.49*

−.44*

−.51*

−.48*

−.54*

−.07

−.33*

−.57*

.70*

.63*

.61*

.54*

.71*

1

   

Pb

.29

.28

.27

.22

.43*

.37*

.72*

.46*

.20

−.21

−.23

−.29

−.45*

.26

.10

1

  

Mn

−.09

−.02

−.08

−.23

−.01

.09

.12

.26

.01

.24

.34*

.19

.18

.33*

.46*

.09

1

 

K

−.22

−.13

−.23

−.29

.01

−.07

.32*

.18

−.21

.30

.28

.31*

.10

.70*

.52*

.64*

.35*

1

Significant at the level of 0.01 (2-tailed)

PCA is an effective statistical method for separating elemental assemblages and elucidating the main controlling factors on chemical components in lake sediments (Hodgson et al. 2001; Liu et al. 2006). We therefore performed PCA on the elemental concentrations in sediments of core ZOL4. Two factors accounting for approximately 80.97% of the variance in the data were obtained, and their loadings are plotted in Fig. 5. Component 1 accounts for 64.13% of the total variance, and is the controlling factor. The variables P, Mg, TN, TC, Cu, Zn, S, Ni and Cd have very high positive loadings on Component 1. These nine elements are significantly inter-correlated and show similar concentration profiles (Table 3; Fig. 2). Their mean concentrations in the penguin ornithogenic layer (Unit 2) are higher than those in Unit 1 (Table 1), and therefore Component 1 is likely linked with the input of penguin guano or guano-derived materials. Al, Ca, Fe and Ti have negative loadings on Component 1, likely caused by the input of a large amount of penguin guano and the consequent dilution and reduction in their relative concentrations. Component 2, which accounts for 16.84% of the total variance, is characterized by high positive loadings of K, Cr, Pb, Na and Mn and the negative loadings of P, Mg, TN, TC and Cd. S, Ni, Cu, Zn, Al, Ca, Fe and Ti have positive loadings on Component 2, therefore Component 2 probably represents the geochemical contributions from other sources such as the local bedrock.
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Fig. 5

Distributions of the factor loadings from the PCA analysis of the elements and oxides in sediment core ZOL4

To confirm the results of PCA on separating the element concentrations in ZOL4, we included the concentrations of elements in fresh penguin guano and bedrock from nearby Gardner Island (Table 1). The concentrations of P, TN, TC, S, Cu, Cd and Mg in penguin guano are much higher than those in bedrock, suggesting that these elements are derived mainly from penguin guano on Gardner Island and Zolotov Island. In contrast, Zn, Ni, Pb, Mn, Fe, Ti, Ca and Al show geochemical characteristics different from penguin guano, indicating that guano is not their primary source.

As indicated above, P, Mg, Cu, S and Cd in the ornithogenic sediment layer of ZOL4 are derived mainly from penguin guano and are thus used here as geochemical markers of penguin populations. These elements, however, also exist in the weathered bedrock, especially Mg, Cd and Cu (Table 1). Therefore it was necessary to identify those elements that have very low concentrations in local weathered bedrock, but high concentrations in penguin guano. We define these as optimal bio-elements. The sediments in the upper unit, 17–0 cm, are affected by penguin guano input, but those in the lower unit, 40–17 cm, are not. Thus elemental concentrations between 40 and 17 cm mainly reflect the background values of local bedrock. Here we use elemental enrichment ratios to evaluate the impacts of penguin guano on the elemental concentrations in the ZOL4 sediments. We define the enrichment ratios as ERi = Cmean 17–0/Cmean 40–17, where Cmean 17–0 is the mean elemental concentration in the upper 17 cm and Cmean 40–17 is the mean concentration in the lower unit, 40–17 cm. The elemental enrichment ratios are plotted in Fig. 6. Phosphorus has the maximum enrichment ratio, 12.64, and was identified as the optimal bio-element to represent Adélie penguin guano in this study.
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Fig. 6

Enrichment ratios of elements in sediment core ZOL4

Discussion

Bio-elements as geochemical markers of penguin guano

Relative to typical Antarctic soils and bedrock, many elements are enriched in penguin guano, ornithogenic soils and ornithogenic sediments. Sun et al. (2000) reported that nine elements including F, P, S, Se, Cu, Zn, Ca, Sr and Ba were enriched significantly in penguin ornithogenic sediments in core Y2 relative to the surrounding Antarctic soils on Ardley Island and King George Island, South Shetland Islands. Hofstee et al. (2006) reported that P, S, Mg, Ca, As, Cu, Zn, and Cd were concentrated in penguin guano from Cape Hallett in northern Victoria Land. Similar enrichment of certain elements in fresh penguin guano has also been reported at Edmonson Point, Terra Nova Bay and Admiralty Bay, King George Island, South Shetland Islands (Ancora et al. 2002; Zdanowski et al. 2005). Previously, we reported very high concentrations of F, P, S, Se, Cu, Sr and As in Adelie penguin guano compared to bedrock at Gardner Island, Vestfold Hills, East Antarctica (Huang et al. 2009a). In the present study, Se, Sr and As in ZOL4 were not determined. Elements P, Mg, Cu, S and Cd were present in high concentrations in the ornithogenic layer of ZOL4, due to their high levels in guano. However, Zn and Ni were present in higher concentrations in local bedrock (Table 1).

Elements Zn and Ni are easily enriched by organic matter in the earth’s surface (Goldschmidt 1954), and their high levels in the ornithogenic layer of ZOL4 may be due to a combination of guano and local bedrock inputs. Similar high levels of Zn and Ni in ornithogenic sediment core DG4 from nearby Gardner Island were also observed (Huang et al. 2009a). High levels of P, S and Cu in guano mainly originate from Antarctic krill (Euphausia superba), a main dietary component of the Adélie penguins (Tatur and Keck 1990), and high levels of Cd in guano are likely from upwelling Antarctic deep water (Ancora et al. 2002). Mg and P in the ornithogenic layer of ZOL4 show significant correlations (r = 0.98, p < 0.001), indicating likely deposition of struvite (Mg(NH4)PO4·6H2O), a typical component of fresh penguin guano and ornithogenic soils (Tatur and Keck 1990).

Elements P, Mg, Cu, S and Cd in ZOL4 are therefore mainly derived from penguin guano, and these elements are considered immobile in Antarctic lake sediments (Sun et al. 2000; Liu et al. 2005). Consequently, concentrations of these elements in ZOL4 are considered to be good geochemical markers for changes in penguin guano inputs and thus penguin numbers. Of these elements, P is most enriched and can therefore be used to infer past Adélie penguin population dynamics around the lake catchment.

Late Holocene Adélie penguin occupation and population dynamics at Zolotov Island

Adélie penguins are sensitive to Antarctic climate and environmental changes. The establishment of their breeding colonies is well correlated with the deglaciation and formation of ice-free areas in Antarctica (Baroni and Orombelli 1994; Emslie and McDaniel 2002; Emslie et al. 2003; Huang et al. 2009b). In the Vestfold Hills, there was a large increase in sea ice extent between 2,500 and 2,000 yr BP (corrected 14C dates). This event, termed the Chelnock Glaciation by Adamson and Pickard (1986), has been identified from terrestrial evidence (Adamson and Pickard 1986), lake sediments (Roberts and McMinn 1999), and marine sediments (McMinn et al. 2001; Taylor and McMinn 2002). After 2,000 yr BP, the sea-ice extent was reduced, but it was still substantially greater than that prior to 2,500 yr BP (McMinn et al. 2001). The sediments of Abel and Platcha Bays in the southern Vestfold Hills recorded a possible ice cap retreat at approximately 1,750 yr BP (corrected 14C dates) (McMinn 2000), coinciding with a warm period (2,000–1,750 yr BP) recorded in the sediments from nearby Lake Nicholson (Bronge 1992). In the present study, the AMS 14C dates on the fossil penguin bones from sediment core ZOL4 indicate that Adélie penguins occupied Zolotov Island only in the late Holocene, since ~1,765 cal yr BP, corresponding to this period of local warm climate and the retreat of nearby Sørsdal glacier, as recorded in lake sediments (Bronge 1992; McMinn 2000).

Bulk sedimentation rates in the upper 17 cm of ZOL4 show only minor fluctuations, therefore the P concentration in the upper 17 cm is considered to be a reliable indicator of the relative abundance of penguin guano and therefore penguin numbers. Overall P-inferred Adélie penguin population dynamics at Zolotov Island show a rising trend for the past 1,800 years (Fig. 7). The P-inferred Adélie penguin populations at Zolotov Island show high levels during the periods ~1,160–990 cal yr BP, ~690–450 cal yr BP and the last ~200 years, and low levels during ~930–810 cal yr BP and ~320 cal yr BP, although some of the latter periods are indicated by single data points and are therefore less reliable.
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Fig. 7

Adélie penguin population dynamics and wider regional climate records. a Penguin population changes at Zolotov Island as indicated by the P concentrations in ZOL4; b Climate changes over the past 2,000 years from δ18O in the Taylor Dome ice core (5-points smoothed); the original data were reported by Steig et al. (2000), and in the Law Dome ice core (figure from Morgan 1985); c Penguin population changes at Ardley Island as indicated by the P concentrations in sediment core Y2 (Sun et al. 2000)

The inferred Adélie penguin population dynamics at Zolotov Island can be compared with the climate changes over the past 1,800 years inferred by Morgan (1985) from the oxygen isotope data in an ice core from Law Dome, East Antarctica. During the isotope-inferred warm period from 1,700 to 1,000 yr BP, the penguin population displayed a rising trend and peaked between ~1,160 and 990 cal yr BP (Fig. 7a, b). The dramatic reduction of the penguin population between ~930 and 810 cal yr BP coincided with rapid cooling between 1,000 and 700 yr BP. During the relatively mild climate from 650 to 400 yr BP, the penguin population maintained high levels between 690 and 450 cal yr BP. The oxygen isotope data in the ice core from Law Dome recorded a cooling from ~400 yr BP to a cold period between 250 and 200 yr BP, followed by rapid warming, and this is consistent with the decline of the penguin population at ~320 cal yr BP and the increase in the last ~200 years. Similar associations between penguin populations and climate were also observed on adjacent Gardner Island. There, the inferred Adélie penguin populations reached high levels ~4,700–2,400 cal yr BP during the warmer mid-Holocene, and then the populations showed a significant decline, corresponding to the onset of local neoglaciation (Huang et al. 2009a).

Such associations can be explained by the impacts of climate-related changes on penguin nesting and foraging. During warm climates, the island is exposed adequately and thus provides ample nesting and breeding sites for penguins. Warm climate may also increase marine productivity, thereby providing sufficient food supply to support the growth of penguin populations.

Comparison of late Holocene penguin populations in Antarctica

Sun et al. (2000) reported a 3,000-year record of penguin population changes at Ardley Island, King George Island, South Shetland Islands, and the results indicated a rapid increase in local penguin populations from ~1,800 cal yr BP (Fig. 7c). Adélie penguins occupied Zolotov Island, Vestfold Hills from ~1,765 cal yr BP (Fig. 7a), and their populations experienced a similar and rapid increase. In the Ross Sea area, after the known ‘penguin optimum’ between about 4,000 and 2,000 cal yr BP (Emslie et al. 2003, 2007), there was a disappearance of Adélie penguins between 2,000 and 1,100 cal yr BP. Emslie et al. (2007) attribute the disappearance of the penguin to semi-permanent sea ice blocking shorelines. Around 1,000 years ago, Adélie penguin populations at Zolotov Island and Ardley Island reached high levels (Fig. 7a, c), corresponding to a warm period recorded in East Antarctic ice cores (Morgan 1985; Masson et al. 2000; Steig et al. 2000) and in marine sediments from the Antarctic Peninsula (Domack et al. 2003). Between 1,200 and 600 cal yr BP, Adélie penguins reoccupied Prior Island along the Victoria Land coast (Baroni and Orombelli 1994), indicating favorable climate conditions, similar to the northern hemisphere Medieval Warm Period. The penguin populations at Zolotov Island decreased dramatically to a very low level ~930 cal yr BP, whilst those at Ardley Island only showed a moderate reduction (Fig. 7a, c). However, since ~900 cal yr BP, the penguin populations at Zolotov Island show an overall rising trend with fluctuations, but those at Ardley Island show a continued, moderately decreasing trend, except for a single dramatic decline ~300 cal yr BP that coincides with a clear decline in penguin populations between 450 and 200 cal yr BP inferred from another ornithogenic sediment core (Y4) on the same island (Liu et al. 2005). These opposing trends are similar to observational data for the past few decades, during which Adélie penguin populations have shown an increasing trend in East Antarctica, while decreasing in King George Island and the Palmer station area, Western Antarctic Peninsula (Woehler et al. 2001). These opposite population trends over the past decades are likely caused by differences in regional sea ice extent and prey availability, driven by local climate (Forcada et al. 2006). Opposite penguin population trends inferred over the past ~900 years at these two islands may be a consequence of internal biological processes such as predation pressure. Physical processes such as icebergs blocking access (Arrigo et al. 2002) may also play a role, in addition to external climatic factors.

The penguin populations at both Zolotov Island and Ardley Island experienced an abrupt decline around ~300 cal yr BP (Fig. 7a, c), corresponding to a cold climate anomaly recorded in ice cores from East Antarctica (Morgan 1985; Masson et al. 2000; Li et al. 2009) and West Antarctica (Mayewski et al. 2004), and in marine sediments from the South Shetland Islands (Yoo et al. 2009; Milliken et al. 2009) and Antarctic Peninsula area (Domack et al. 1995; Leventer et al. 1996; Smith et al. 1999). These recorded cold climates in Antarctica coincide approximately with the northern hemisphere Little Ice Age (Lamb 1977; Grove 1988; Mann et al. 2009), but the timing of the Little Ice Age in Antarctica is different (Masson et al. 2000; Bentley et al. 2009; Verleyen et al. in press).

Studies have shown that both extreme warm and cold climates could reduce Adélie penguin populations by impairing their access to nesting and foraging habitats (Fraser et al. 1992; Sun et al. 2000). There seems to be an optimal combination of climatic and marine ecosystem conditions for Adélie penguins, and the marked ‘penguin optimum’ around 4,000–2,000 cal yr BP in Antarctica (Baroni and Orombelli 1994; Emslie et al. 2003, 2007; Huang et al. 2009a) is a period when such conditions likely occurred.

Conclusions

AMS 14C dates on fossil penguin bones from the ZOL4 core show that Adélie penguins occupied Zolotov Island, Vestfold Hills from at least 1,765 cal yr BP, corresponding to the local retreat of the Sørsdal Glacier. Phosphorus was identified as the optimal bio-element indicative of penguin guano input to the sediments in ZOL4, and was used to infer penguin population changes for the past 1,800 years. Adélie penguin population dynamics at Zolotov Island correlated well with regional climate changes. Adélie penguin populations at Ardley Island and Zolotov Island both reached high levels around 1,000 cal yr BP, and experienced a decline around ~300 cal yr BP, suggesting that cold climates are detrimental to Adélie penguin populations.

Acknowledgments

This study was funded by the National Natural Science Foundation of China (No. 40730107), the National Science and Technology Supporting Program (2006BAB18B07), the Major State Basic Research and Development Program of China (973 program, No. 2010CB428902) and the Australian Antarctic Division Science Project (AAD2873). We thank the Chinese Arctic and Antarctic Administration, Polar Research Institute of China, Australian Antarctic Division for logistical support in field, Dr. Renbin Zhu for collection of samples and Dr. William Cooper for assistance with radiocarbon dating. We especially thank the reviewers for their critical comments and Dr. Dominic Hodgson and Dr. Mark Brenner for their help in improving this manuscript.

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© Springer Science+Business Media B.V. 2011