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Ice on high mountains in the tropics.

Ice cores are used to reconstruct past
climate and environmental change in the Quaternary period by the observation of
the annual layers of snow rings in the cores. These clearly defined layers provide
a high-resolution chronology for environmental change. Annual layering has been
observed in both the Arctic and Antarctic ice sheets along with smaller ice
masses such as those found on high mountains in the tropics. Measurements taken
from these annual rings provide information on temperature, snowfall,
atmospheric composition (gases, dust, volcanic aerosols) and sunspots.

 

Along with the visible changes (largely
caused by the amount of dust in an annual ring) within the layers of the ice
cores demonstrating environmental change, seasonal layering also can also show
climate change through measurements of the physical and chemical properties of
the ice. These include measurements of stable isotope levels (?D and ?18O), variations in the number of
the heavy isotopes relative to the most common isotopes can be measured these
have been found to reflect the temperature variations through the year (Maries,
J. University of Copenhagen). Electrical conductivity of the ice,
radiocarbon dating, dust content, micro particle content and chemical element
composition of the air bubbles trapped within the ice are other ways of dating
ice cores and reconstructing climate and environmental change during the
Quaternary. More recently the development of digital scanners with powerful
computers have allowed the visual stratigraphy of ice cores to be identified
with extraordinary clarity. However, in deeper ice the annual layers are more
closely spaced and they become increasingly diffused making it more difficult
to distinguish the annual variations. The age of older ice therefore has to be
estimated through theoretical ice-flow models based on a knowledge of ice
dynamics. Alternatively, isotopic events identified in the ice cores can be
linked to other dated events that have been found in marine cores or speleothem
records, ice core time scales can then be worked out through this correlation of
data. Ice core chronologies can however, contain errors; arising from two
sources imperfections in the nature of the record or human/technical errors
that can occur; this includes incomplete core recovery, as well as the
incorrect counting of the annual horizons within the ice core (Lowe, J and
Walker, M. 2015).

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The Greenland ice sheet, has consistently
acquired snow rapidly, this has led to chronologies that span the last
glacial-interglacial cycle; making Greenland ice cores instrumental in in investigating past abrupt climate change. These
include the Greenland Ice Core Project (GICP) core which layers counted back to
14.5 ka and the Greenland Ice-Sheet Project 2 (GISP2), where multiparameter
continuous counting has been possible down to a depth of 2,800m where the ice
is dated at 110 ka (Meese, DA et al. 1997). The oldest core so far investigated
in Greenland is the 2,450m long NEEM core from the northern part of the ice
sheet, this contains the onset of the Eemian interglacial. Through linking the
CH4 and ?18O profiles from the bottom of the core with other
records from Greenland and Antarctica the NEEM ice core can be placed on the EDML
timeline. The most highly resolved chronology from Greenland is the
Greenland Ice Core Chronology (GICC05) extending back to 60 ka. This is a climatic
timescale and is based on multiparameter counting using ?D and ?18O istopic variations,
electrical conductivity measurements (ECM) and continuous flow analysis (CFA)
of water-soluble ions in three separate Greenland ice cores. However, the age
of the lower part of the ice-core record have to be estimated using ice-flow
models based on observed physical relationships between ice thickness, heat
flow, ice melt and integral ice-flow dynamics (Lowe, J and Walker, M. 2015).
The newest core from the North Greenland Ice Core Project (NGRIP) has been measured
at a very high resolution for water isotope
ratios, dust, and impurity concentrations. This is allowing researchers for the
first time to follow Greenland’s past temperature, snow accumulation, moisture
origin, and aerosol deposition at a subannual resolution. Over the very abrupt
climate changes that in the period from 15.5 to 11.0 ka. This is what
allowed for the construction of the Greenland Ice Core Chronology 2005 (GICC05).
The ?18O isotope is a proxy
for past air temperatures at the ice core sites. The ?18O levels at 14.7 ka indicate a rapid warming period
that only lasted 3 years, in comparison the warming transition at 11.7 ka
lasted 60 years. This 14.7 ka event is thought to be a Dansgaard-Oeschger
(D-O) event, each of these 25 events consisted of an abrupt warming to near
interglacial conditions followed by a gradual cooling. Related to the coldest
intervals between 6 of the D-O events are the Heinrich events, these are events
of rapid cooling. The 14.7-ka event was one of the 6 D-O events that was followed
by a Heinrich event (H1) (Stefensen, JP. Et al. 2008).

 

Ice cores can also be used to identify climatic changes in
the tropics, from select mountainous areas. Understanding how the sensitivity
of the tropics to global climate change is vital to the study of the earth’s
climate system during glacial periods (Thompson, LG. 1995). These ice core
records from high, mid and low latitude ice caps aim to create high-resolution climatic
and environmental history on a global scale, and to understand whether climatic
events were at a local or global scale. Records from a range of latitudes have shown
how widespread the effects of climatic extremes are such as The Little Age. The
comparison of two sets of ice core records, the Guliya Ice Cap in China and the
Huascarán in Peru, reveal that there was significant cooling during the Last
Glacial Cycle Maximum (LGM ?20,000 yr BP). Cores in Sajama, Bolivia and the Dasuopu,
Himalaya cores support this. Lower ?18O
values (equivalent to cooling of -8°C)
contribute to the growing body of evidence that the tropical climate was much cooler
in the past and more variable. Only during the last glacial cycle have the
tropics renewed their current role as a wet tropical climate. This has
raised questions as to the role of the tropics in global climate. In the
tropical ice cores the annual ?18O
oscillations show an inverse relationship with temperature due to
temperature and precipitation competing as controlling factors on isotopic composition
in tropical snow. However, the long term decadal-to century oscillations remain
positively correlated with temperature. This longer-term correlation is
confirmed by the presence of both major and minor large-scale climate events
within the tropical ice cores, such as the Little Ice Age on Quelccaya and the
Younger Dryas and the Last Glacial Stage in Sajama. The actual level of
snow presents in the ice cores in tropical regions is also evidence to environmental
change due to the impact caused by albedo. The level of snow in the high
Qinghai-Tibetan Plateau had an impact on the intensity of the Asian Monsoon. With
more extensive snow cover reducing the monsoons intensity caused by the high
albedo of the ice (Thompsom, LG. 2000). Cores taken from the top of Mount
Kilimanjaro record three abrupt climate changes
in this region at 8.3, 5.2, and 4 ka. The
concertation of aerosols in these cores are sporadically high. These high
levels correlate with abrupt reduction in methane at 8.2 ka in the
European Greenland Ice Core Program (GRIP) core and with the greatest Holocene
depletion of 18O in
both the GRIP and the Greenland Ice Sheet Project 2 records. The aerosol
levels in Kilimanjaro cores are dependent on lake levels with low lake levels
concentrating sodium and fluoride in the lakes. This paired with the global
reduction in CH4 inferred
from the GRIP ice core indicates that the 8.2 ka event was driven by
abrupt and sudden changes to the hydrological cycle in the tropics and
especially Africa. These changes created a much drier climate (Thompson, LG. Et
al. 2002). From evidence gathered from tropical ice cores across the globe we
can infer that the past environment of the tropics was colder and much drier
during glacial periods.

 

In conclusion ice cores are extremely useful for
reconstructing climate and environmental change during the Quaternary. There
are a number of proxies held within the ice that give evidence to the changing
of past climates both at a scopic and molecular level. Furthermore, this ice
can then be dated and a timeline of past climatic events can begin to be drawn
up. When the evidence from ice cores both in the poles and the tropics are
pieced together they form a global perspective of how the Earth’s climate has
changed. Unfortunately,
as a result of recent warming, all known tropical glaciers and ice caps are
retreating and soon will no longer continue to preserve viable paleoclimatic
records.

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