Australia: The Land Where Time Began

A biography of the Australian continent 

Penguin Colonisation of the Vestfold Hills, East Antarctica, Following the Last Glacial-Interglacial transition

The population history of the Adélie penguin, Pygocelis adeliae, as well as well as its response to climatic and environmental changes, have been studied widely in maritime Antarctica and several glacial refugia of Adélie penguins have been dated back to the  Last Glacial Maximum (LGM). The process of colony expansion during the subsequent glacial-interglacial transition was, however, less well-documented. In this paper, Gao et al. investigated an ornithogenic core, from the Vestfold Hills, East Antarctica, in order to improve knowledge of postglacial penguin colonisation. Around 15.6 ka local deglaciation occurred, based on a detrital layer that was coarse-grained, this date being earlier than dates reported from other studies from the same region. It is suggested by geochemical analysis of the core that penguins began colonising the Vestfold Hills about 14.6 ka, which is the earliest geological record in East Antarctica on the occupation by penguins, after which their population exhibited a trend that was broadly increasing. In the mid-Holocene, at about 6,740 BP, the population expanded, at which time the climate corresponded to a local climatic optimum. Population changes of local sub colonies was the basis for this inference, which corresponded well with the reconstruction of local climate change, as well as the penguin population changes that were based on genetic studies. Gao et al. proposed that shortly after the retreat of the glaciers the emerging ice-free areas were colonised by sea birds, based on a consideration of the results of previous studies of the history of occupation following deglaciation in the Artic, as well as other parts of Antarctica. The availability of breeding habitat over geological timescales is a key factor that controls the size of the population and the activity range of Antarctic Adélie penguins.

One of the most important ice free areas in East Antarctica is the Vestfold Hills, located at 68^o33ʹS, 78^o15ʹE, Prydz Bay (Macintosh et al., 2014), and it’s climate history during the Holocene has been studied widely. Regions of the Vestfold Hills and Larsemann Hills, located at 69^o24ʹS, 76^o13ʹE, experienced the ‘Early Holocene optimum’ during the Early Holocene from 11.5-9.5 ka, at which time the climate was warm and humid, as was inferred from  lake sediment records (Verleyen et al., 2011). A cold, dry period from 9.5-7.4 ka followed the early-Holocene optimum, a time of low water levels in lakes of the Larsemann Hills (Verleyen et al., 2004).  It is suggested by studies of marine sediments from Prydz Bay that between 7.4-5.2 ka ocean conditions were warmer (Verleyen et al., 2004). A period of intense hydrodynamics between 7.0 and 6.7 ka is also indicated by penguin guano sediments from Gardner Island located at 68^o34ʹS, 78^o52ʹE, in the Vestfold Hills (Huang et al., 2009). Also, a mid-late Holocene thermal optimum was reconstructed in the Vestfold hills. It was proposed (Pickard et al., 1986) that there was an optimum from 4.7-3.0 ka based on the sediment record from Watts Lake, 68^o35ʹS, 78^o00ʹE; and it is indicated by studies of lacustrine diatom assemblages that there was a warm, wet period from 4.2-2.2 ka (Roberts & McMinn, 1996, 1999). A neoglaciation was experienced by the Vestfold Hills at around 2.5 ka, concurrent with many other locations in east Antarctica (Ingόlfsson, 2004; McMinn et al., 2001).

This process of ice retreat during the last glacial-interglacial transition in the Vestfold Hills remains unclear, in spite of the foregoing (Mackintosh et al., 2014). Throughout the Last Glacial Maximum (LGM) some parts of the Vestfold Hills may have been free of ice (Colhoun et al., 2010; Gibson et al., 2009); 11.4 ka some lakes became open seasonally, when diatoms began to appear in the sediments (Roberts & McMinn, 1999; Cromer et al., 2005). Adjacent areas produced additional records. It is indicated by offshore sediments from the central Rauer Islands (68^o51ʹS, 77^o50ʹE) that the ice shelf receded from this location at around 13.3-12.7 ka (Barbara et al., 2010). Also, in the Larsemann Hills many lakes were deglaciated from 14.5-11.5 ka at the beginning of sedimentation (Hodgson et al., 2005; Verleyen et al., 2004, 2005). On the Stoknes Peninsula, located at 69^o26ʹS, 76^o05ʹE), however, several lakes in the Larsemann Hills, were not permanently covered by ice from 40 ka, at least (Hodgson et al., 2001). Further evidence of the timing of deglaciation is provided by glacial deposits. At about 12.5-12.4 ka deposition of glacial sediments in Terrasovoje Lake and Beaver Lake at Amery Oasis, located at 70^o40ʹS, 68^o00ʹE, terminated (Wagner et al., 2004, 2007), and the areas below the current Amery ice shelf, 69^o45ʹS, 71^o00ʹE, were transformed to an open water environment by 1.1 ky (Barbara et al., 2010; Leventer et al., 2006); Mackintosh et al., 2014). At Burton Basin, 67^o35ʹS, 68^o07ʹE, the western margin of Prydz Bay, was deglaciated at 12.8 ka (Borchers et al., 2016). And in the East Prydz Channel region (67^o30ʹS, 72^o00ʹE) ended at about 13.0 ka (Domack et al., 1998; Mackintosh et al., 2014).

Currently there are more than 250 breeding sites of Adélie penguins (Pygocelis adeliae) in East Antarctica, and 30% of breeding pairs globally (Southwell et al., 2017; Lynch & LaRue, 2014). A distinctive trend of population change in East Antarctica over a timescale of a decade, compared to other parts of Antarctica has been revealed by studies of the modern ecology of Adélie penguins. The population of Adélie penguins in East Antarctica exhibits an increasing trend, though the population of the penguins in the Arctic Peninsula has decreased over the last decades (Southwell et al., 2015).

In East Antarctica, however, the long-term history of population dynamics of Adélie penguins is not as well known as in the Antarctica Peninsula and the Ross Sea region. Reconstructions of the Early Holocene and earlier populations of penguins have been performed only by genetic researchers. A slight positive trend in the Adélie penguin population since 1 Ma was discerned (e.g., Li et al., 2014); the trend becoming very significant at 150 ka, then after the cooling of 60 ka it started to reverse. These genetic reconstructions focused, however, the historical change of a specific lineage and they do not provide information on where the change in population occurred. The earliest fossil evidence of colonisation by Adélie penguins was a bone from the Ross Sea region dating to 45 ka (Emslie et al., 2007); no residuals were, however, discovered that dated to the LGM. Extensive grounds that were free of ice were produced during the last deglaciation, which would have provided suitable breeding grounds for penguins, and, based on genetic studies, Adélie penguin populations in the Antarctic Peninsula began to expand from 16 ka (Clucas et al., 2014). Reconstruction changes in the population of Adélie penguins in East Antarctica from 20 ka and an increase was noted from ~14 ka (Younger et al., 2015); In the ice core from Dome C this pattern corresponded to changes in the temperature record in many lakes in the Vestfold and Larsemann Hills (Cromer et al., 2005; Hodgson et al., 2001; Roberts & McMinn, 1999; Verleyen et al., 2004, 2005). This reconstruction has a large degree of temporal uncertainty, however, with a 95% confidence of 19-11 ka (Younger et al., 2015); also it does not provide information on reconstructed population.

The oldest current geological evidence of penguin colonisation of East Antarctica is from 9 ka and 8.5 ka in the records from the Windmill Islands (66^o20ʹS, 110^o28ʹE) (Emslie & Woehler, 2005) and Gardner Island, in the Vestfold Hills, respectively (Huang et al., 2009). These records began much later then that date of local ice retreat, however. Following the last glacial-interglacial transition geological records of colonisation by penguins and habitat expansion are generally absent in East Antarctica.

A 15.6 kyr record was obtained by Gao et al. from a sediment core (RN-01) from a small catchment on the Long Peninsula in the northern Vestfold Hills in this study. A phase of deglaciation that was characterised by rapid accumulation of sediment and a high coarse-grained lithogenic material content was identified by analyses of ¹⁴C lithology, grain size and geochemical proxies. Gao et al. subsequently used multivariate statistical analysis of the geochemical proxies in order to estimate the time of initial colonisation by penguins and subsequent changes in the population over the last 14 kyr. Gao et al. also compared their results with the results of other reconstructions of processes of colonisation of seabirds in the Arctic, as well as other areas of Antarctica, and they discussed the primary drivers of the expansion of penguin colonies following the last deglaciation.

Conclusion

A near-instantly deposited layer that was 19 cm thick and dated the time of the local deglaciation to ~15.6 ka, which is 2,000 years earlier than most of the other records from the region, was revealed by study of an ornithological sediment core from the western Long Peninsula in the Vestfold Hills of East Antarctica. At about 14.6 ka penguins began to colonise the area, which is 6,000 years earlier than previous records obtained from the Vestfold Hills. Also, it is likely the sampling site was the location of one of the earliest Adélie penguin breeding colonies in East Antarctica. Gao et al. also proposed that growth of the population and the expansion during the last glacial-interglacial transition were driven mainly by the availability of extensive summer grounds that were free of ice that had resulted from retreat of the ice.

Sources & Further reading

Gao, Y., et al. (2018). "Penguin colonization following the last glacial-interglacial transition in the Vestfold Hills, East Antarctica." Palaeogeography, Palaeoclimatology, Palaeoecology 490: 629-639.