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Australia: The Land Where Time Began |
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Climate Controls of the Present in the Southwest Pacific The current climate of the southwest Pacific region
is subject to several major oceanographic and atmospheric controls.
Variations in the position of the Inter-Tropical Convergence Zone (ITCZ)
influences the northern part of this region, and during the Southern
Hemisphere summer the ITCZ migrates southwards into the extreme northern
part of Australia, and is a major influence controlling the supply of
moisture as far south as 30oS. The Indo-Pacific Warm Pool
(IPWP), the region with mean temperatures of above 28oC,
which directly influences the northern part of Australia is a major
source of global latent heat. The size of the IPWP has been found to be
sensitive to ENSO changes, and therefore the strength of the atmospheric
circulation across the Pacific. The IPWP contracts towards the equator
during El Niño (warm ENSO) events, the South Pacific Convergence Zone
(SPCZ) migrates into the ITCZ, and a high-pressure anomaly develops in
the mid-latitudes over southern Australia (Hooker & Fitzharris, 1999).
The Interdecadal Pacific Oscillation appears to modulate the degree of
strengthening across the region of the teleconnection.
The dry, sinking air of the subtropical
high-pressure belt that migrates across the continent through the year
dominates the climate of Australia. Westerly airflow in the
mid-latitudes centred over 40oS roughly tracks the flow of
subantarctic waters. During the Southern Hemisphere winter when the
high-pressure systems move north, westerly winds and rain-bearing cold
fronts influence southern Australia (Sturman & Tapper, 1996). Ocean circulation plays a potentially important
role in the transmission of ENSO signals to high southern latitudes via
the Antarctic Circumpolar Wave (ACW) (White and Peterson, 1996) through
the transmission of sea surface temperature (SST) anomalies. Rossby
Waves transmit SST anomalies, where they are subsequently propagated to
the east into the South Atlantic and Indian Ocean. However, at the
present the long-term stability of the ACW and the IPO are not known.
The oceans transport heat to the south via the East Australian Current
(EAC) independently of the ENSO-generated anomalies, and the EAC has a
significant influence on the coastal regions of central Australia, as
approximately half of the EAC moves to the east at the Tasman Front
(about 34oS), the remainder continuing to the south (Roughan
and Middleton, 2004). The tropical Leeuwin Current (LC), which is warm
and of low salinity, flows polewards then to the east along the coasts
of western and southern Australia (Okada & Wells, 1997). Climate
Quantification Palaeoclimatic change in the Australasian area has
generally been based on proxy data, pollen in particular, that has been
described on a qualitative basis. Quantitative approaches, that Turney
et al. describe as promising, have begun to be developed more recently.
Results have been generated for Coleoptera (Porch & Elias, 2000),
Chironomidae (Dimitriadis & Cranston, 2001), pollen (Kershaw, 1998) and
plant δ13C Turney et
al.,
1999) though these have not yet been applied systematically to the
period of interest here. In the marine realm where changes in planktonic
assemblages of foraminifera have generated robust temperature
reconstructions for key periods is where the greatest success has been
achieved in developing quantified climate changes (Barrows & Juggins,
2005). Turney et
al.
say that periglacial, glacial and dune deposits can provide quantified
measures, in geomorphological contexts, of the climate at the time of
their formation (Galloway, 1965; Nanson et
al., 1995; Barrows et
al., 2001, 2004), though they
are inherently single estimates representing broad time periods. Also,
there is a preservation bias in such deposits. It was shown by a review
of desert climates in Australia of the Late Pleistocene (Hesse et
al., 2004) that between 100 ka
and the present there is an exponential rise in the number of dated
samples for dune material being deposited, which is partly due to the
older dunes being progressively reworked or their ‘younging’. Another
complication is that records of maximum advance or rapid collapse in the
chronologies of glacial moraines, while the bias of dunes is to the
episodes of waning activity and stabilisation of the dunes. Landscape
modifications often resulting from combinations of climatic variables
are often reflected by both, only rarely relating to a single
temperature or change in precipitation.
Geochronology Radiocarbon dating has principally been used in
determining the timing of events from the close of the glacial period to
the Early Holocene within marine and terrestrial records. In terms of
precision and accuracy, the robustness of these chronologies depends on
a number of considerations that include the number of radiocarbon ages
that are available for each sequence and the nature of samples that are
dated (bulk sediment, selected fossils or chemical fractions (Turney et
al., 2000; Lowe et
al., 2001). Within the
Australasian region there are only a small number of radiocarbon ages
that have been obtained from sequences from the LGM and termination
(obtained largely from bulk sediment samples) and the standard error on
the ages, are typically large (>100 yr at 1σ). As a result of this the
assessment of the accuracy of these ages, isolation of aberrant results
(which result e.g. from
in situ
taphonomic or biogeochemical processes, contamination in either the
field or lab), and the obtaining of realistic calibrated estimates of
age is often difficult for all these reasons. It is ideal if terrestrial
plant macrofossils can be utilised due to their reflection directly of
the atmospheric content of 14C; though this is possible only
rarely (if ever) within marine contexts. The radiocarbon ages have been
previously reported by the original workers for the sites discussed in
this study. In this study Turney et
al., calibrated the
radiocarbon ages <20 ka 14C BP against INTCAL04 (Reimer et
al., 2004), and those >20 ka
14C BP have been converted by the use of the data sets that
were obtained from the Cariaco Basin (Hughen et
al., 2004) or tropical corals
(Fairbanks et
al., 2005).
A method that offers considerable promise is the
use of ‘wiggle-matching’ of radiocarbon datasets to the global
radiocarbon calibration curve, though single age estimates that fall
within plateaus may calibrate to span several centuries. The methodology
is based on the principle that inflections in the calibration curve,
such as plateaus and transitions that are steep sided, must be reflected
in time-depth functions in radiocarbon ages that are obtained from
sequences that are stratified, and the matching of the latter to the
former can be used to derive calendar ages for the horizons that are
radiocarbon dated. In contrast to studies that have been carried out in
the Northern Hemisphere (e.g. Gulliksen et
al., 1998), there are no
lacustrine or marine sequences that have been dated to sufficient
resolution to undertake such an exercise in Australia. It has been
demonstrated that Huon pine from Tasmania has the greatest potential for
such an approach (Barbetti et
al.,
2004; Fig. 2). Here, the combined sequences of tree rings extends over
680 yr and the interhemispheric offset in radiocarbon years was small,
between 10.3 and 10.1 cal. ka BP, a period of time during which
14C
was rising, though increased to 50 years or more between 10.1 and 10.0
cal. yr. BP, a period during which atmospheric
14C was
falling, differences comparable to changes over the past millennium
(Hogg et
al., 2002). Turney et
al., say future work will
allow the precise comparison of climate proxies that had been derived
from the Huon pine with other datasets from the Northern Hemisphere on
the same absolute timescale. Thermoluminescence (TL) has been used to date
events (Nanson et
al., 1992a,
1995, 2003) and the dating of exposure
in situ (10Be,
26Al,
36Cl) (Barrows et
al., 2001, 2002, 2004; Fink et
al., 2000, and unpublished data; Kiernan et
al., 2004), while a precise
age-depth profile of ice-core ages from the Law Dome (Antarctica) has
been achieved by synchronisation to the GRIP ice core through changes in
gas (methane) content (Morgan et
al., 2002; van Ommen et
al.,
2004). All of these methods are independent of fluctuations in
atmospheric content of 14C.
Climatic and
environmental changes – the last glacial period to the Early Holocene,
30-8 cal. ka BP The Australian region differs from the North
Atlantic region in that for most of this period it cannot be subdivided
into a succession of accepted chronostratigraphic units on which useful
discussions can be based (Mangerud et
al., 1974; Lowe et
al., 2001). In this paper
Turney et
al. use general
descriptions of major periods of change in order to compare different
datasets. They do not imply that the definitions and timing of these
periods are fixed, given the problems associated with precise and
accurate dating of these events and the inherent danger that some may
record change that is time-transgressive. In some cases the events in
individual sites have been shown to straddle the boundaries of periods
described elsewhere in this paper. Also, they anticipate that more
events will be identified and future research will allow the precise
definition of the age of their boundaries.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |