Antarctica: from the beginning of the Holocene

In order to understand the changes and impacts of them on the Antarctic, we need to study its history. Since we're going through a warming period, it's important that we put in more effotrt in understanding past warming periods. First of all, in order to study the history of the Antarctic, we need to find out what happened in the past. Once again, we'll be focussing on the Antarctic Peninsula (AP) 

The Holocene (A super brief summary)

The Holocene began 12 000 years ago and is an inter-glacial stage after the last Ice Age. However, there have been observed climate shifts within the Holocene, each lasting thousands of years. The changes were marked by glacial advances and retreats. 

AP in the last 12000 years

Since the beginning of the Holocene, AP has been experiencing glacial retreats and advances. Cold events occurred inbetween warm events. For example, the George VI Ice Shelf collapsed 9.6 ka (thousand) years BP after 2000 years of relatively high early-Holocene SSTs (sea surface temperatures). It reformed and advanced again at 7.9 ka years BP as SSTs decreased. 

Early Holocene (11-9.5cal. ka)

The early Holocene was a period of warming in the AP. Warming of the AP occurred slightly before the beginning of the Holocene and carried on till it hit a climate optimum from 11 - 9.5 cal ka BP. The warming of the AP was reflected in the deglaciation of it. Most areas in the AP were under retreat by 14-13 cal. ka BP and this continued through the early Holocene. Records from the Palmer Deep record showed an increase in primary production and iceberg rafting at 11-10 cal. ka BP. 

Even during this period of time, regional differences in the effect that the warming had on the rates of ice retreat are evident. On the west side of AP, the lack of sequential retreat morianes on the continental shelf in Marguerite Bay suggests that the rate of ice retreat was quite rapid. However, ice sheet retreat was more gradual with periods of inactiveness on the east AP. 

Moreover, while many Antarctic records from ice cores, marine cores and geomorphological studies indicate that the early Holocene was a period of warming, the Palmer Deep record implies relatively cold conditions during the same period of time. the Palmer Deep record has an 'apparante cold proxy record' where there are lower diatom abundance and an assemblage characteristic of more sea ice amongst others. 

After the optimum (9.5 - 4.5 cal.ka BP)

This period after the optimum is a period of contrasting changes experienced in the AP. On the west side, the Palmer Deep record indicates that there was a southward intrusion of warmer, more subpolar waters from 9000-6700 cal. yr BP that led to the collapse of the George VI Ice Shelf that was reformed partially or completely by 7945cal yr BP. 

On the east side, the Larsen Ice Shelf-B remained intact throughout this period although the reason for that may be as simple as that it was too thick to be affected by the meltwater fracture mechanism. 

Thus this period of time for Antarctica was characterised by differential rates of deglaciation on the west and east sides of the AP. The west side had a higher rate of deglaciation and the ice retreated until at least 7-8 cal.ka BP. This was evidenced by the process of sedimentation (where sediments were carried to the bottom of lakes by melting glaciers) found in the lake basins there that were exposed then when the ice retreated. The east side however, did not become free of ice till much later in the Holocene. Some parts, like the Byers Peninsula, were ice covered till 5-3 cal. ka BP. 

Mid Holocene warm period (4.5 - 2.8 cal.ka BP) 

This was the next period considered to be when there was significant warming in the AP. Marine cores showed that this period of time was when there was reduced sea-ice coverage, greater primary production and an increase in meltwater-derived sedimentation. Lake sediments and multi proxy studies showed evidence of a warmer climate as well.

However, the Palmer Deep record (a marine record), showed that this warming wasn't out of the ordinary as suggested by other data. It indicated that the relatively warm current, the Upper Circumpolar Deep Water (UCDW) was present on the AP for a period of about 5000 years. More importantly, there was no obvious cooling between the onset of the early-Holocene warmth and this period. This relatively warm water disappears suddenly at 3600 cal. years BP compared to the gradual tailing off in lake records during this period of time. This shows that not only is there a regional difference of deglaciation rates, it strongly suggests that there are other factors affecting the extent of ice cover over the AP other than changes in SSTs.

Neoglacial interval (2.5-1.2 ka)

The end of the mid Holocene warm period was marked by a pronounced shift to colder climate conditions recorded in both the Vostok and Komsmolskaya ice cores after 2500 cal. yr BP. This is supported by findings that many biological proxy records in lakes and other sites show a positive correlation to a decline in temperature and a decline in primary production. It is still unsure whether there was glacier advances during this period of time due to poor dating.

Medieval Warm Period (1.2 - 0.6 cal.ka BP)

There may not have been a MWP in the Antarctic. Studies of various proxies and cores have not been able to allow for a conclusive statement. The only data that do show us clearly that there was a warm event occurring in the Ap at approximately the same time as the MWP are marine cores. 

And that was 12000 years of history of the Antarctic compressed into this under a thousand word blog entry. I'll be discussing the influencing factors in the extent of ice cover in the Antarctic next. The previous entry didn't do that good a job since I had limited it to only the last 50 years. the paper by J. Bentley et. al. 'Mechanisms of Holocene palaeoenvironmental change in the Antarctic Peninsula region)' (2009) does a great job explaining in detail the changes in the AP during the Holocene and the data used to understand the influencing factors of ice extent. A.E. Shevenell et. al. 'Holocene Southern Ocean surface temperature variability west of the Antarctic Peninsula' (2011) is a more detailed study of the WAP but read Bentley's one first. I found that that helped me understand the Antarctic better and allowed better understanding of Shevenell's paper.

Antarctic Peninsula: processes causing its warming climate

As the debate on climate change progresses, the focus on the poles has increased. The ice covered poles can be seen as a sort of thermometer for Earth since they respond almost immediately to changes in the climate. With technology as advanced as it is, videos showing geomorphological changes at the poles are a dime a dozen (I felt obligated to put in an amateur video because there are really cute penguins running from the waves here even though there are other videos showing part of the ice sheet collapsing clearer)

 

It is not unusual to see parts of the ice sheets break off, every summer the glaciers retreat and advance during winter. However, over the last 50 years, it has been observed that glaciers at both poles have been in retreat and despite re-growth during the winter months, there are an increasing number of glaciers that have not regained their original mass. While global warming does cause the loss of ice mass at the poles, the loss of ice mass at the poles is also contributed by ‘increasing precipitation, changing ocean temperatures’ (Cook et al, 2005). Most of the ice loss has been observed on the marginal parts of the ice sheet since they are the most susceptible to such environmental changes.

Below is map of Antarctica and the Antarctic Peninsula is indicated by the red box.

The Antarctic Peninsula is a Northwest extension of the main continent and observations have shown that it is showing much higher vulnerability to climate changes compared to the continent itself. Since the 1950s, the climate has warmed by 2 degrees (Cook et al, 2005). This regional warming of the Peninsula has led to a 10 fold increase in glacier flow and rapid ice sheet retreat (Rignort, 2006).  In the paper 'Retreating Glacier Fronts on the Antarctic Peninsula Over the Past Half-Century' by Cook et al in 2005, there are satellite images of the glacier retreat of Sheldon Glacier, Adelaide Island showing its retreat in 2001 compared to 1986. The break-up of the ice shelf is observed with the smaller ice extent and the broken, drifting ice shelf. From the earliest known position of ice shelves in 1953, 87% of glaciers in the Peninsula have shown overall retreat. The image below shows how much the Sheldon Glacier has retreated from 1947 to 2001. 

(BBC, 2005)

One of the reasons for the observed overall retreat of ice shelves is due to climate. The Antarctic Peninsula is warming at a rate of 3.4 degrees Celsius (give or take 1 degree Celsius) per century (Shevenell et al, 2011). This warming coincides with the positive rates of glacier retreat as shown by the figure below. 

However, in the figure above, while the general trend throughout the whole of the Antarctic Peninsula is a glacier retreat corresponding with a general increase in temperature there were advances in glaciers from 1945 to 1964. This suggests that there are other factors that influence the extent of the ice shelves in the Peninsula.

The reason for this discrepancy is due to the ocean-atmosphere heat exchange over the Antarctic Peninsula. It was found that from the 1950s, the Southern Hemisphere Westerlies that bring warmer air from the lower latitudes to the higher latitudes have experienced either a southerly migration (thus bring warmer air to higher latitudes) or has intensified (Shevenell et al, 2011). This then affects the Antarctic Circumpolar Current (ACC), which is an ocean current that circles the Antarctic, causing it to migrate southwards and/or intensify as well. This allows warm Circumpolar Deep Water to upwell along the Peninsula partly due to the ocean bathymetry and the ACC’s proximity and to mix with surface waters.

As a result, the advancement of glaciers prior to 1950 were due to the Westerlies being nearer to the Equator and not having that significant of an effect on the ACC, allowing for cooling of the climate. However, after that warming occurred due to the above described process and in combination with the anthropogenic global warming, this is resulted in the Antarctic Peninsula (and the rest of the Antarctic) to warm and thus experience a lesser extent of ice cover. 

In the next post, I'll go into the ocean-atmosphere heat exchange occurring over the Antarctic by taking you further back in time. Like the beginning of the Holocene 12000 years ago. 

Journals:

Cook, A.J., Fox, A.J., Vaughan, D.C., Ferrigno, J.G. (2005) 'Retreating Glacier Fronts on the Antarctic Peninsula over the Past Half-Century', Science, 308, 5721, 541-544.

Rignot, E. (2006) 'Changes in ice dynamics and mass balance of the Antarctic ice sheet', Philosophical Transactions: Mathematical, Physical and Engineering Sciences, 364, 1844, 1637-1655.


Shevenell, A.E., Ingalls, A.E., Domack, E.W and Kelly, C. (2011) ' Holocene Southern Ocean surface temperature variability west of the Antarctic Peninsula', Nature, 470, 250-255.

Antarctic glaciers show retreat 2005