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 30^oS. The Indo-Pacific Warm Pool (IPWP), the region with mean temperatures of above 28^oC, 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 40^oS 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 34^oS), 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.

Sources & Further reading

1. Turney, C. S. M., S. Haberle, D. Fink, A. P. Kershaw, M. Barbetti, T. T. Barrows, M. Black, T. J. Cohen, T. Corrège, P. P. Hesse, Q. Hua, R. Johnston, V. Morgan, P. Moss, G. Nanson, T. van Ommen, S. Rule, N. J. Williams, J. X. Zhao, D. D'Costa, Y. X. Feng, M. Gagan, S. Mooney and Q. Xia (2006). "Integration of ice-core, marine and terrestrial records for the Australian Last Glacial Maximum and Termination: a contribution from the OZ INTIMATE group." Journal of Quaternary Science 21(7): 751-761.