QA_Vol 24_No 1_July 2007 - Australasian Quaternary Association

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Late Pleistocene environments in the Flinders Ranges CONTINUEDThe carbon and oxygen isotopic profiles of Section 1(Figures 4 and 5) are based on the average values of eachsample interval. These profiles show that overall δ 13 Cdecreases upwards through the section from –7.5‰ atthe base to –9‰ at the top, although there is aprominent but short-lived (∼1 ka) positive excursionwithin the upper half of the lower unit. In contrast, δ 18 Ooscillates between –4 and –2‰ through the lower unitbefore climbing from –3 to –2‰ through the upper unit.The apparent regular oscillation of the oxygen isotopedata may well be an artefact of the sample interval andshould be treated with caution until more highresolution sampling can be done. The profiles highlighta weak inverse relationship between the two parameters.The difference in overall slope of the profiles,and their contrasting fine structure, suggest subtlechanges in the freshwater setting in which thesesediments accumulated. Secular changes in the ratio ofprecipitation to evaporation (P/E) will be reflected inδ 18 O, whereas the δ 13 C values are more likely to havebeen influenced by variations in productivity, as demonstratedby, for example, the record of Late Quaternaryenvironmental change in Lake Pamvotis, Greece(Frogley et al., 2001). The significance of the trends andpatterns evident in Figure 4, particularly when coupledwith the microfaunal data, are discussed below.Palaeoenvironmental ImplicationsThe lower unit, from about 60 cm to 230 cm above thebase, contains diverse and abundant microfauna. Theostracods include free swimmers such as Cypretta sp.and benthic species that normally do not swim butcrawl on the substrate or on vegetation growing beneaththe water (Candonocypris novaezelandiae, Gomphodellamaia, Lymnocythere mowbrayensis). All these taxa live infreshwater environments, although some can tolerateslight levels of salinity (L. mowbrayensis). Some of themrequire permanent water for breeding (Darwinula sp., L.mowbrayensis). Gomphadella maia is thought to inhabitpermanent pools but De Deckker (1982b) suggests thatit may burrow into sediment when the ponds dry out.Also present in the deposits are freshwater phreaticgastropods from the Family Hydrobiidae that are foundin streams and springs, as well as land gastropods, fromthe Family Charopidae, which frequent dampvegetation. This indicates a stable, humid regime with adiversity of wetland habitats including streams, pondsand swamps. The stable isotope data (Figures 4 and 5)suggest a somewhat different setting at Brachina Gorgeprior to the LGM. Rather than the climate beinguniformly humid, the oxygen isotope profile indicates aregular oscillation between wetter and drier conditions.The amplitude of this apparent fluctuation in P/Eratio is relatively constant throughout the interval inquestion, although its periodicity shortens up thesection (see calibrated ages in Figure 3). The carbonisotope profile shows that this change of climate wasaccompanied by a steady decline in productivity of thewetland. This preliminary interpretation awaitsconfirmation by higher resolution sampling andanalysis.The upper part of the lower unit, from about 230 cmto 360 cm above the base, shows a general decline inmicrofossil diversity and abundance. Darwinula sp., afairly cosmopolitan species but requiring permanentwater, disappears while small numbers of Lymnocytheremowbrayensis persist along with Candonocypris novaezelandiae.Both these species are benthic and indicatethat the wetland was possibly contracting into smallephemeral pools that partially dry out and are fed byseepages from ground water. The free-swimmingconditions favoured by Cypretta sp. seems to no longerhave been available. A drying trend is certainly evidentin the oxygen isotope profile between 230 cm and 275cm, although a return to wetter conditions is indicatedthrough the remainder of the upper unit (Figure 4).These changes overlap the short-lived positive excursionin δ 13 C that, in lacustrine settings, would normally beattributed to enhanced primary productivity of aquaticalgae.In this same interval there is an increase in the numbersof Austropyrgus that is not as well represented inthe lower part of the section, while other Hydrobiidgastropods (including juveniles), which are not yetspecifically identified, are more abundant. More willneed to be found out about this gastropod group tounderstand the meaning of this difference. The regionaltaxonomy and ecology of this group is currently beingstudied (Clark et al., 2003).A sharp change in the physical character of the depositsoccurs at the palaeosol horizon (Williams et al., 2001)(Figures 2 and 3). The microfossil population changesacross this boundary in three ways. First, the totalnumber of ostracod and mollusc fragments increasesdramatically (Figure 4). Second, the diversity of theostracods drops to about three varieties (juvenileCandanopsid varieties, which are difficult to identify,and Darwinula sp.). Interestingly, the juvenile forms ofC. novaezelandiae, unlike the adult forms, are goodswimmers. This is an indication that more permanentwater was available again. Lymnocythere numbers dropat this stage and those of Darwinula sp. increase.Between 450 cm and 520 cm above base of the sectionthere is a drop in Darwinula sp. numbers and anincrease in the numbers of both Gomphadella maia andLymnocythere mowbrayensis (Figure 4). Both these latterspecies have been reported in slightly saline conditions,as discussed above, and this may indicate that therewas an increase in salinity in the waters at this time.Third, the large increase in total ostracod remains ismade up of non-identifiable fragments, suggesting that26 | Quaternary AUSTRALASIA 24 (2)
high-energy events such as floods prevailed duringwhich fragile shells were broken up.Following this increase in ostracod abundance, thereis a gradual decline to almost zero at 520 cm above thebase of the section (Figure 4), indicating a period ofapparent desiccation. Both the ostracod and molluscfaunas then recover towards the top of the section.Although constrained by data from just three stratigraphiclevels, the microfossil profile of the upper unitsuggests a slight increase in the wetland’s salinityfollowing the LGM, and a continuation of the slowdecline in its organic productivity. Interpretation of theresults (both isotopic and micropalaeontological) is inpart limited by the use of bulk samples. There are about30 discrete laminae between the palaeosol horizon at360–370 cm, and the top of the section at ∼580 cmabove the base of the exposed section. Only six sampleswere taken over this interval, which is hardly adequateto document the apparently increasing frequency ofturnover between wetter and drier periods. Againhigher-resolution sampling of the section is requiredto confirm this turnover.ConclusionsThis study has shown that there were two main phasesof late Pleistocene wetland sedimentation at theSlippery Dip locality in the Brachina valley of the centralFlinders Ranges. The first, from about 34,000 yr BP tothe LGM, occurred during a time of generally highprecipitation such that freshwater microfossils, includingboth free-swimming and benthic ostracods,were preserved in relatively large numbers. The carbonand oxygen isotopic signatures of the ostracods andcoeval molluscs provide an additional perspective,indicating that the prevailing moist conditions werepunctuated by drier periods. The palaeosol, about 370cm above the base of the section, marks the end of theinitial stage of deposition. This horizon is relatively freeof microfossils and coincides with a drying phase.Following the hiatus, around the time of the LGM,the second phase of sedimentation probably records aprolonged series of high-energy events (possibly floods),given the fragmented nature of the shells. A decline inmicrofossil diversity suggests a slight increase insalinity. The limited isotopic data on this interval areconsistent with the overall drying out and decline inproductivity of the wetland.This preliminary investigation highlights the need forhigher-resolution sampling of the sediments above thepalaeosol, with a view to better documenting changes intheir microfossil diversity and stable isotopic composition.Additional ages will also be necessary to establishthe nature and frequency of the events leading to theprominent lamination of this part of the Slippery Dipsection. Such information will comprise a local signatureof global events that occurred between the LGMand the dawn of the Holocene.AcknowledgementsWe thank the National Parks and Wildlife Service of SouthAustralia for their support in carrying out this research. Wethank Patrick De Deckker for his generous help in ostracodidentifi-cation, hospitality and helpful advice. Dr WinstonPonder is thanked for his help in mollusc identification.Rosemarie Glasby did invaluable hours of work on themicroscope.References• Bard, E., 1999. Ice age temperatures and geochemistry. Science,284, 1133–1134.• Chivas, A. R., De Deckker, P., Wang, S. X., Cali, J. A., 2002.Oxygen-isotope Systematics of the Nektic OstracodAustralocypris robusta. In Holmes, J. A., Chivas, A. R. (Eds.), TheOstracoda: Applications in Quaternary Research; GeophysicalMonograph published by the American Geophysical Union,Washington, DC., 301–313.• Cock, B.J., Williams, M.A.J. and Adamson, D.A.,1999.Pleistocene Lake Brachina: a preliminary stratigraphy andchronology of lacustrine sediments from the central FlindersRanges, South Australia. Australian Journal of Earth Sciences,46, 61–69.• Clark, S.A, Miller A.C. and Ponder W.F., 2003. Revision of thesnail genus Austropyrgus (Gastropoda: Hydrobiidae). Amorphostatic radiation of freshwater gastropods insoutheastern Australia. Records of the Australian MuseumSupplement, 28, 1–109.• Dalgarno, C.R., Johnson, J.E., 1965. Oraparrina 1:63,000 mapsheet. Department of Mines, South Australia.• De Deckker, P., Geddes, M.C., 1980. Seasonal fauna ofephemeral saline lakes near Coorong Lagoon, South Australia.Transactions of the Royal Society of South Australia, 103,155–168.• De Deckker, P., 1981a. Taxonomic notes on some Australianostracods with description of new species. Zoologica Scripta,10, 37–55.• De Deckker, P., 1981b. Ostracods from Australian inland waters– notes on ecology and taxonomy. Proceedings of the RoyalSociety of Victoria, 93, 43–85.• De Deckker, P., 1981c. Taxonomy and ecological notes for someostracods from Australian inland waters. Transactions of theRoyal Society of South Australia, 105, 91–138.• De Deckker, P., 1982a. Holocene Ostracods, OtherInvertebrates and Fish Remains from Cores of Four Maar LakesIn Southeastern Australia. Proceedings of the Royal Society ofVictoria, 94, 183–220.• De Deckker, P., 1982b. Non-marine ostracods from twoQuaternary profiles at Pulbeena and Mowbray Swamps,Tasmania. Alcheringa, 6, 249–274.• De Deckker, P., 1982c. Late Quaternary ostracods from LakeGeorge, New South Wales. Alcheringa, 6, 305–318.• De Deckker, P., 1983. Notes on the ecology and distribution ofnon-marine ostracods in Australia. Hydrobiologia, 106,223–234.• Fairbanks, R.G., Mortlock, R. A., Chiu T.-C., Cao, L., Kaplan, A.Guilderson, T.P., Fairbanks, T.W., Bloom, A.L., Grootes, P.M.,Nadeau, M.-J., 2005. Marine Radiocarbon Calibration CurveSpanning 0 to 50,000 Years B.P. Based on Paired 230Th/234U/238U and 14C Dates on Pristine Corals. QuaternaryScience Reviews, 24, 1781–1796.27 | Quaternary AUSTRALASIA 24 (2)
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Page 3 and 4: EditorialPresident’s PenDear Fell
Page 5 and 6: Interview withProfessor Liu Tungshe
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