Australia: The Land Where Time Began

A biography of the Australian continent 

Last Glacial Period Termination – Climatic and Environmental Changes 20-11.5 Cal. ka BP

At 16.8 cal. ka BP a limited suite of rainforest angiosperm, gymnosperm taxa and sclerophyll taxa first appear near the site in Lake Euramoo, which Turney et al. suggest were possibly as patches of incipient rainforest forming mosaics with wet sclerophyll woodland. Similar changes were noted almost 4,000 years later in Lynch’s Crater (Walker & Chen, 1987; Hiscock & Kershaw, 1992) and may reflect closer proximity of Lake Euramoo to glacial rainforest refugia as precipitation and temperature increased through the termination. Associated with this phase are organic accumulation and burning of biomass (Haberle, 2005), which suggests there was a rise in the biomass of woody plants that may have been a factor in the increasing rate of accumulation of charcoal particles in the sediments.

At Tower Hill there are 2 periods of significant climate change that can be identified (D’Costa et al. 1989; Turney et al., 2006) within this period. The first sustained increase in Eucalyptus pollen at 17 cal. ka BP with the steppe grasslands taxa of the LGM being replaced, indicate there was an increase in temperature. Following this time, conditions appear to have been relatively dry, with an increase in Chenopodiaceae suggesting salt marsh invasion of an ephemeral saline lake (Turney et al., 2006). There is the beginning of a substantial rise of Casuarinaceae, the tree family, about 14 cal. ka BP, as well as a decrease in Asteraceae (steppe), which suggests there was a further increase of temperature at this time. About 13.7 cal. ka BP it appears wetter conditions developed, with Botryococcus, a fresh to brackish water alga, replacing Chenopodiaceae Turney et al., 2006).

A speleothem from Chillagoe in north Queensland produced oxygen isotope data that the growth of the stalagmite commenced with a period that was moderately mild, 15.4-14 ka, which was followed by a reversal between 14 and 10 ka that was relatively dry, which was characterised by ¹⁸O values that were relatively enriched. It appears the trend of carbon isotope followed that of the δ¹⁸O, probably because of corresponding changes in vegetation. It is not easy to reconcile these inferred changes with the records of the Wet Tropics pollen. It is unclear if this indicates that at this time environmental changes were decoupled from climatic changes across Queensland.

Immediately to the north of Finke River source-bordering sand dunes, that are mostly younger, 17-0 ka, indicate that there was a very substantial wind shift, by a about 25^o, from due south to southeasterly, over this period (Nanson et al., 1995). A post-LGM windshift, from due south to southeast, has also been identified (Hollands et al., 2006). The wind field that controls alignments of linear dunes in the deserts of Australia shifted southwards about 160 km, or 1.5^o, following the LGM, though before the alignment of the dunes of the Holocene, that Turney et al. suggest was most probably associated with the migration of the high-pressure system over Australia at this time.

Similar precipitation and flow-regime changes to the much larger rivers in eastern NSW were experienced by small catchments in New South Wales, such as those of the Nambucca River and Bellinger River. However, because of their confined nature they have been flushed of most of their older deposits from the Pleistocene, though they have retained a sensitive record of the flow regime changes that were less pronounced (Nanson et al., 2003). The enhanced fluvial activity of the Yanco phase spanned between 20-30 ka and was preserved widely as a period during which there was a major flow and reworking of alluvium.  As well as this increased availability of moisture the frequency of fire around this time appears to have been relatively low. It is indicated by the concentration of charcoal that in New South Wales the sites of Gooches Swamp (Black & Mooney, in press) and Redhead Lagoon (Williams, 2005) experienced little fire throughout much of this time

Exposure dating of recessional moraine sequences at a number of sites in Tasmania show that at about 20-19 ka glacial retreat began, continuing unabated to about 15-14 ka, against the backdrop of temperatures that were rising on the mainland of Australia and offshore (Barrows et al., 2001, 2002; Fink et al., 2002, and unpublished data); supporting the interpretation of a warming trend through this transitional period. However, it has been reported (Barrows et al., 2001, 2002) that there was an advance that was centred on 16.8 ± 1.4 cal. ka (Mt Twynam Advance) in both the Snowy Mountains and the highlands of Tasmania. Coupled to this it has been suggested there may have been a reactivation of periglacial activity dated to 16.6 ± 0.7 cal. ka, though it is not clear that the latter was widespread in alpine areas (Barrows et al., 2004). It at first appears to be contradictory that temporary growth of glaciers could occur at a time when it is implied by vegetation that temperatures were increasing. A potential scenario that has been suggested is that in this period there was increased seasonality (Kershaw, 1995), with climatic conditions being such that summers were dry and winters were wet, though this remains uncertain.

The ocean record SO14-08-05 indicates there was a substantial increase in dust flux from about 16 cal. ka, that was associated with these changes, which is consistent with the strengthening of the Australian monsoon that occurred at this time, though it pre-dates the onset at 14 cal. ka that has been reported elsewhere (Wyrwoll & Miller, 2001). Contrasting with this, the dust flux from E26.1 decreased significantly after the LGM, which according to Turney et al. suggests westerly airflow over southeastern Australia moved to the south, becoming more humid after the LGM (Hesse, 1994; Hesse & McTainsh, 1999).

High-resolution analyses of skeletal Sr/Ca  and δ¹⁸O obtained from giant fossil Diploastrea heliopora coral at Vanuatu indicates that average sea surface temperatures were cooler by 4.5 ± 1.3^oC than at present (Corrège et al., 2004). U-series dating was used to date this coral, the results indicating the record spans the period 12.4-11.7 ka, which provides a unique record at high resolution of changing sea surface temperature in the tropical Pacific. It is indicated by the amplified annual cycle of sea surface temperature, relative to that of the present, that cooling was caused by compression of tropical waters towards the equator. It was found that there was a positive correlation in the record between the oxygen isotope ratios of sea water and sea surface temperature which suggests that the South Pacific Convergence Zone (SPCZ) which brings ¹⁸O-depleted precipitation to the area at the present, was not active from the LGM (Barrows & Juggins, 2005) to the time when the coral at Vanuatu was forming. At that time conditions were comparable to those of contemporary El Niño events.

It is suggested by similar conditions that persistent El Niño events are recorded throughout the Pacific region. A marine record in the Great Australian Bight indicates enriched ¹⁸O in surface waters that spans 12.3-11.1 cal. ka BP (Andres et al., 2003), which appeared to coincide with changes in circulation. These results, which are also consistent with clear skies under anticyclonic pressure systems and increased evaporation, were originally interpreted to represent cold oceanic conditions. Further afield, it is indicated by speleothem growth in the southwestern USA that a sustained period of wet conditions between 12.4 and 11 cal. ka BP, which is consistent with El Niño disruption of the California Current (Polyak et al., 2004). Turney et al., suggest it is possible that an important role in regional climate change during the end of the last Termination and the beginning of the Holocene may have been played by long-term ENSO activity changes, though there needs to be a significant improvement in dating and quantification.

In the coastal Antarctic record of Law Dome this period is one of pronounced warming that commenced about 17.7 ka (Morgan et al., 2002). Except for a brief interruption between 15 and 13 ka, this warming trend continued through to the Holocene. The latter event that has been described as the Antarctic Cold Reversal (ACR) is chronologically distinct from the Younger Dryas Stadial in the Northern Hemisphere, and it has been identified in numerous ice cores across Antarctica (Blunier et al., 1998). Linking of gas records between Greenland and the Law Dome high-resolution core has made possible the precise dating to demonstrate that the cooling that occurred at the start of the Antarctic Cold Reversal led to the abrupt warming that occurred during the Bølling Interstadial in the Northern Hemisphere (GL-1) about 14.7 ka (Morgan et al., 2002). It is implied by this that in the Southern Ocean changes in temperature are not a direct response to abrupt changes in thermohaline circulation in the North Atlantic (cf. Broecker, 1998). The implications of this have still not been fully realised.

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.