Micropalaeontological evidence for deglacial marine flooding of the ancient courses of the River Murray across the Lacepede Shelf, southern Australia

Accepted Manuscript Micropalaeontological evidence for deglacial marine flooding of the ancient courses of the River Murray across the Lacepede Shelf, southern Australia

Graham J. Nash, Patrick De Deckker, Carol Mitchell, Colin V. Murray-Wallace, Quan Hua PII: DOI: Reference:

S0377-8398(18)30014-8 doi:10.1016/j.marmicro.2018.04.002 MARMIC 1692

To appear in:

Marine Micropaleontology

Received date: Revised date: Accepted date:

13 February 2018 25 April 2018 25 April 2018

Please cite this article as: Graham J. Nash, Patrick De Deckker, Carol Mitchell, Colin V. Murray-Wallace, Quan Hua , Micropalaeontological evidence for deglacial marine flooding of the ancient courses of the River Murray across the Lacepede Shelf, southern Australia. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Marmic(2018), doi:10.1016/j.marmicro.2018.04.002

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Micropalaeontological evidence for deglacial marine flooding of the ancient courses of the River Murray across the Lacepede Shelf, southern Australia Graham J. Nash1, Patrick De Deckker1*, Carol Mitchell 2, Colin V. Murray-Wallace2, Quan Hua3 1

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Research School of Earth Sciences, Australian National University, Canberra, ACT

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2600, Australia. 2

School of Earth and Environmental Sciences, University of Wollongong, Wollongong,

NSW 2522, Australia. 3

Australian Nuclear Science and Technology Organisation. Locked Bag 2001, Kirrawee

DC, NSW 2232, Australia.

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 deceased

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* Corresponding author:

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E-mail address: [email protected]

ABSTRACT

the

course

of

the

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Previous sub-bottom profiling of the Lacepede Shelf, southern Australia, had inferred palaeo-River

Murray

during

periods

of

low

sea

level.

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Micropalaeontological analysis of two cores taken during the SST-02-07 cruise, confirm these earlier observations, and reveal the past existence of permanently open,

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estuarine environments substantially larger than currently prevailing at the mouth of the terminal lakes of the River Murray, Australia’s largest exorheic river. The characteristics of the sediment and contained microfossils provide substantial evidence for the repeated development of large, oxygen depleting algal blooms (sapropels). Detailed analyses of the ostracod and foraminifer microfaunas, together with other fossil remains such as pteropods, bryozoan, sponge spicules and echinoid spines, combined with the dating of selected ostracods and bivalve molluscs (by AMS radiocarbon) and bivalve molluscs (by amino acid racemization) reveal rapid sediment accumulation within the ancient channels of the River Murray under estuarine conditions. The oldest

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infill relates to the end of the rapid sea-level rise following Meltwater Pulse IA ~13,600 cal. years BP and the second one that coincides with the Meltwater Pulse IB at 11,50011,000 cal. years BP; in both cases, marine microfauna confirm the presence of marine incursions. The palaeoenvironmental reconstructions based on microfossils presented here rely on the study of six southeastern Australian contemporaneous estuaries where foraminifers

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and ostracods were collected, together with some ecological data such as salinity,

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dissolved oxygen and presence/absence of aquatic vegetation.

Keywords AAR, AMS

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C, sapropel, Ostracoda, Foraminifera, Pteropoda, rapid sea level rise,

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MWP IA, MWP IB, pyrite oxidation, estuarine faunas. 1. Introduction

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Given the generally high level of tectonic stability of the southern Australian passive continental margin, the Late Quaternary sea level record of the region (past 132 ka) is highly fragmentary in nature (Murray-Wallace, 2002). The most detailed

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knowledge of palaeosea-level during this period relates to the sea level highstands of the last interglacial maximum (Marine Isotope Substage 5e centred on 125 ka ago;

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Murray-Wallace and Belperio, 1991; Murray-Wallace et al., 2016; Pan et al., 2018) and the current, Holocene interglacial, the latter sea level highstand attained some 7 ka ago

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at the culmination of post-glacial sea-level rise following the Last Glacial Maximum (Belperio et al., 2002; Lewis et al., 2013). The inferred palaeosea-level records for

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these two interglacials have been derived from extensive sectors of the South Australian mainland coastline, including the marine incursions, Gulf St Vincent and Spencer Gulf (fig. 1) (Belperio et al., 1983; Belperio et al., 2002; Murray-Wallace and Belperio, 1991).

The palaeosea-level records of this vast sector of the southern

Australian coastline reveal that during the last interglacial (MIS 5e), sea level ranged between 2.1±0.5 and 4.8±1 m APSL (above present sea level) (Murray-Wallace et al., 2016; Pan et al., 2018). The Holocene sea level record in this far-field region reveals the attainment of the highstand by 7 ka ago, followed by a spatially variable fall in relative sea level to present level, relating predominantly to hydro-isostasy. The highest

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magnitude of relative sea-level fall during the Holocene highstand is for regions farthest from the continental shelf edge, such as northern Spencer Gulf where a fall of up to 4.5 m is registered (Belperio, 1995). Along the open ocean coastline, a lower fall of relative sea level of approximately 0.5 m is evident (Murray-Wallace and Woodroffe, 2014). Knowledge of the history of relative sea-level changes during the last glacial cycle is, however, more incomplete for this margin, in part due to the logistical

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difficulties of sampling in submarine contexts, particularly the continental shelf. A major

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vibracoring program in northern Spencer Gulf, revealed the presence of interstadial marine strata between last interglacial and Holocene successions, and were inferred to correlate with the Late Pleistocene warm interstadials MIS 5c and 5a; 105 ka and 82 ka with relative sea levels of -8 and -14 m, respectively. However, the ages of these sedimentary successions were not established by geochronological methods (Hails et

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al., 1984a,b). In southern Gulf St Vincent and northern Spencer Gulf, the presence of interstadial marine strata of MIS 3 age (64-32 ka), relating to sea levels of –30 to -27 m

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have been identified in several submarine vibracores (Cann et al., 1988; Cann et al., 1993; Cann et al., 2000; Murray-Wallace et al., 1993). Hitherto, the only investigation examining the early portion of post-glacial sea-level rise in earliest Holocene time was

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from southern Gulf St Vincent from vibracores collected from a present water depth of approximately 40 m (Cann et al., 2006). Radiocarbon dating of shallow subtidal to

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intertidal shelly faunas from three vibracores revealed evidence for environmental changes in southern Gulf St Vincent accompanying post-glacial sea-level rise for the

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period from 10 ka ago. In particular, the development of estuarine-lagoonal environments in the early evolutionary history of Gulf St Vincent was noted (Cann et al.,

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2006). The

present paper documents

the

first comprehensive

investigation of

sedimentary palaeoenvironment on the central Lacepede Shelf of Southern Australia based on the analysis of two vibracores. The work provides the opportunity to examine land-sea correlations of Late Quaternary environmental change based on the preserved marine faunas.

The geographic location of the vibracores (Fig. 1) and their greater

water depth provides the opportunity to extend the temporal coverage of Late Quaternary environmental changes within the broader region. The Lacepede Shelf is a large (30,000 km2) relatively flat, submerged extension

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of the Murray Darling Basin. Modelling of sea levels along the southern Australian continental margin by Lambeck and Chappell (2001) indicates substantial episodic exposure of the Lacepede Shelf since glacial Termination 2 (T2). The area offshore from the mouth of the modern River Murray was extensively surveyed from RV Southern Surveyor during the SS02-06 and SST02-07 research cruises. Sub-bottom profiling revealed an anastomosing series of depositional facies that extends some 200

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km beyond the modern shoreline (Fig. 1). These are morphologically consistent with

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fluvial and possibly lacustrine environments, and have been attributed to past extension of the ancient River Murray during periods of low sea-level (Hill et al., 2009). This paper is primarily concerned with the identification of palaeoenvironments of the ancestral River Murray that prevailed on the exposed Lacepede Shelf during late Quaternary low (glacial) stands of sea level. Sediments deposited in those

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environments now lie submerged by water that flooded the Lacepede Shelf during the post-glacial marine transgression; i.e. the present sea-level high stand. Cores taken by

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a vibracorer, deployed by the Southern Surveyor recovered successions of sediments together with their preserved fossils, principally foraminifers, ostracods and bivalve molluscs. Vibracoring during the SST02-07 cruise recovered nine cores >200 cm in

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length, providing almost continuous sampling of the sea floor between 39 and 71 m below present sea level. Correlation of this vertical distribution with the sea level curve

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of Lambeck and Chappell (2001) indicates a potential palaeoenvironmental coverage extending back to marine isotope stage 5d (Fig. 2). This period encompasses the

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“Mungo wet phase” of early human habitation at the Willandra Lakes, higher in the Murray-Darling catchment (Bowler et al., 2012). These, and many other inland

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Australian lake systems, have subsequently undergone extreme aridification, and now exist as ephemeral, wind-deflated depressions. Accordingly, the possible existence of undisturbed lacustrine/fluvial facies on the Lacepede Shelf, contemporaneous with the Mungo wet phase is of considerable interest in understanding the dynamics of the largest river system in Australia during the last glacial cycle. This study reports on invertebrate fossils, principally foraminifers and ostracods, identified in cored sediment recovered from the Lacepede Shelf during the SST-02/07 cruise. The preserved microfossils are used as proxies to infer palaeoenvironments. Complementary radiocarbon and amino acid racemization (AAR) dating of extracted

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fossil material constrains the timeframe represented and the rate of sedimentation for those periods. Of key interest is verification of the buried channel system inferred from sub-bottom profiling from RV Southern Surveyor (Hill et al., 2009). Identified marine, estuarine and potential lacustrine phases, provide a lithological and biostratigraphic framework to compare with the Late Quaternary record of relative sea-level changes based on coral terraces (Bard et al., 2010; Camoin et al., 2012; Camoin and Webster,

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2015). 2. Regional Setting

The Lacepede Shelf exists as a discrete and slightly depressed portion of the passive south-eastern continental margin of southern Australia. It is bounded to the north-east by the extensive and relatively flat arcuate coastal beach-dune barrier, the

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Coorong lagoon and the Younghusband Peninsula, and to the north-west by the higher relief landscape of Kangaroo Island and the mainland Fleurieu Peninsula. The most

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northern extent of the Lacepede Shelf is closely aligned with the present mouth of the River Murray, at the terminus of the 1.1x106 km2 Murray-Darling Basin (MDB) (Fig. 1). The Lacepede Shelf, and the MDB as a whole, have their origins in the westerly

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dipping, Paleocene subsidence of a large section of the eastern Australian continental interior. Subsequent episodes of intraplate folding and glacioeustatic sea-level changes

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caused an oscillation between long erosional and depositional phases (Hill et al., 2009). The retreat of the last major continental transgression, from about 6 Ma, is marked by a

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series of arcuate residual shoreline features that extend northwards from the Coorong, into far western New South Wales (Murray-Wallace, 2002; Bowler et al., 2006). These

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facies form an effective “palaeo-contour map” due to the primacy of eustatic over tectonic sea level control during the period of their formation (Bowler et al., 2006). Throughout the Pleistocene, significant global glacioeustatic sea level change is thought to have occurred on ~103 occasions (Lieskicki and Raymo, 2005). Based on the models constructed by Lambeck and Chappell (2001), these oscillations fall mostly within a spatial range defined by the submerged edge of the Lacepede Shelf and the present southern Australian coastline. Relative sea level has also been influenced by the gradual neotectonic east-west subsidence of the landward margin of the Lacepede Shelf (Murray-Wallace, 2002).

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Discharge from the MDB to the ocean is presently ~1.26% of average recorded annual precipitation over the entire catchment, and an estimated 3.24% immediately before European settlement (MDBC, 2006), while the average annual sediment discharge is estimated at 1.16 million tons (Department of Environment Water Heritage and the Arts, 2002). These figures are very low by global standards (Li et al., 1996a, 1996b). The relatively modest influence exerted by the modern River Murray ensures a

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tenuous connection with the sea at the fluvial/marine interface. Complete closure and

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several virtual closures of the Murray mouth have occurred during modern times (Harvey, 2002; Walker, 2002; Webster, 2005). Wave domination during prehistoric times is evident in the landward extension of the Coorong barrier complex (Frick et al., 1996). Submerged strandline facies similar to those associated with the Coorong are known to exist in relatively shallow water (<40 m deep) on the Lacepede Shelf (James

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et al., 1992). However, the majority of the shelf has only subtle surface relief features, with ~60% lying within a depth range of 40-70 m (Hill et al., 2009). The shelf edge lies

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at a depth of 140-250 m, and has been deeply incised by the submerged Murray Canyons ~100 km south, and south-east, of Kangaroo Island. Clay mineral analysis demonstrates a strong link between sediments from these canyons and those of the

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terrestrial River Murray catchment (Gingele and De Deckker, 2004). Sub-bottom profiling of the Lacepede Shelf has subsequently revealed facies consistent with an

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ancient system of anastomosing channels that connect the Murray Canyons system and current mouth of the River Murray (Hill et al., 2009).

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The modern Lacepede Shelf is storm-dominated and characterised by big seas, especially in winter. Waves generally approach from the south-west with modal deep-

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water wave heights often in excess of 2.5 m and swell periods frequently greater than 12 secs (James et al., 2001; James and Bone, 2011). This provides for a deep oscillatory motion, able to move sediments at depths exceeding 100 m. Surface circulation is dominated by the warm, easterly moving Flinders Current and is supplemented by vertical movements of cold water at the shelf edge (Li and McGowran, 1998; O’Hara et al., 2000; James et al., 2001; James and Bone, 2011). Today, conditions on the Lacepede shelf are conducive to prodigious heterozoan carbonate production, even at the global scale (Hill et al., 2005). Spiculitic and delicate branching bryozoans thrive, particularly along the shelf edge where there is hard, stable substrate

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and regular upwelling of nutrients (James et al., 2001; James and Bone, 2011). Extensive reworking and winnowing of the skeletal carbonate sediment occurs where water depths fall within the range of the powerful wave base. This reduction in sediment size is synchronous with increasing numbers of microscopic, generally benthic, heterozoans such as foraminifers. These tiny organisms constitute the dominant in situ living assemblage over much of the shelf (James et al., 2001; James and Bone, 2011).

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By comparison, the spatial, if not ecological distribution of planktic foraminifers is

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heavily constrained by the highly energetic SE movement of the water over the Lacepede Shelf (Li et al., 1996a; Bourman et al., 2016). In addition, a large portion of the upper sedimentary cover of the Lacepede Shelf consists of red brown quartz sand that was originally deposited as dunes when the Shelf was subaerially exposed during periods of very low sea levels such as during the Last Glacial Maximum. In fact, the

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nature of this quartz-rich compact sand precluded penetration below a few centimeters by a gravity corer, but subsequently a vibracorer was used successfully to penetrate

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this compact sandy layer. Two of the cores are the focus of this study. 3. Materials and Methods

Two

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3.1. Vibracores

vibracores from the SST-02/07 cruise were selected for detailed analysis

sea-level changes:

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because of their substantial length and in the hope of targetting significant periods of

292 cm.

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• VC-9: location 36o14.470’S - 138o48.408’E, present water depth 51.7 m, length

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• VC-17: location: 36o24.695’S - 138o02.257’E, present water depth 67.5 m, length 342 cm.

The cores were cut into manageable lengths upon retrieval from the sea floor. All subsequent processing was conducted at the Micropalaeontology Laboratory, Research School of Earth Sciences [The Australian National University (ANU)]. In the Laboratory, core sections were split longitudinally, photographed and logged. Samples comprising ~2 cm3 of sediment were taken at 10 cm intervals along the central axis of the core and/or adjacent to stratigraphically significant features. Fossil molluscs were sampled for amino acid racemization (AAR) analyses [University of Wollongong (UW)], and for

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radiocarbon

dating

[Australian

Nuclear

Science

and

Technology Organisation

(ANSTO))]. The individual core sections are retained in refrigerated storage (4 oC) at ANU. Following extraction, the sediment samples were disaggregated by soaking for several days in 3% H2O2. The resultant material was then washed through a 130 m sieve to remove the fine fraction and the retained portions (>130 m) were oven dried

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at <60oC. Initial wet sediment, and oven-dried masses were recorded to provide an

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approximate yield. The washed and dried sediment was further reduced into unbiased subsample batches by using a micro-splitter. The sub-sample batches were picked for individual fossil foraminifers and ostracods, and other key palaeontological indicators such as pteropod and bryozoan fragments. Identifications relied on the previous works of various authors, principally the illustrated publications of Albani (1968), Quilty (1977),

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Hartmann-Schroder and Hartmann (1978), Hartmann (1979, 1980, 1981), Apthorpe (1980), Yassini & Jones (1995), and Cann et al. (2000, 2002, 2006a, 2006b). Actual dry

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and wet sediment yields were then derived based upon the micro-splitter iterations employed for each picked subsample batch. Abundance and diversity ratios for the two principal fossil groups (foraminifers and ostracods) were determined to recognise

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marked species biases, aberrations relating to very low counts and key biofacies relationships (following the methods set out in Nash et al. (2010)). All processed

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materials are stored at the ANU Micropalaeontology Laboratory. Scanning Electron Microscopy (SEM) was used to illustrate species/morphological

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variations and evidence of inferred taphonomic processes in the sample material. Specimens were gold coated using an Emitech(TM) low vacuum sputter coater, and

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imaged at 200-400 x magnification using the Cambridge S360 Scanning Electron Microscope in the Electron Microscopy Unit at the ANU.

3.2 Microfaunal analysis A total of 69 samples were taken from cores VC9 and VC17 and 4149 mixed individual specimens >130 m were recovered. These comprised 1361 foraminifers (60 species), 1764 ostracod valves (882 inferred individuals from 44 species), and 1024 other specimens of mixed origin (principally whole or parts of diatoms, pteropods, echinoids, bryozoans and sponge spicules).

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3.3. Geochronology Eight radiocarbon ages were determined, four from each core. Depending on availability, the fossils analysed comprised either individual mollusc valves or multiple ostracods drawn from life assemblages [= juveniles and adults, thus indicating minimal post-mortem transport (De Deckker, 2002). In both cases, radiocarbon ages were

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derived from samples of multiple ostracods due to the paucity of molluscs in the fine

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sediments of the lower portions of both cores. The small mass of these batch samples required that the  13C values be assumed as zero per mil,the mean value of marine carbonates (Stuiver and Polach, 1977) (Table 1).

AAR analyses were undertaken exclusively on fossil molluscs. One hundred and ten specimens (including replicates from the same individuals), predominantly from the

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upper, coarse-grained sediment occupying the top 100 cm of both cores were analysed. Here, the determined ages range from 6,080±900 to 13,500±2000 cal. years BP. The

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oldest age determined by AAR was 15,500±2300 years BP (VC9-193.5 cm) (see Table 2).

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3.4 Sampling of modern estuaries

Six modern estuaries in southeastern Australia provided a basis for interpretation

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of fossils preserved in the cores (Fig. 1). Fauna that might provide insight as environmental proxies were identified and environmental parameters such as dissolved

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oxygen and salinity were measured. Refer to Supplementary Section 1.

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3.5 Taxonomic notes of the microfossils preserved and identified in the cores Description of the important microfossil taxa and relevant ecological notes based from published accounts from southeastern Australian estuaries are provided in supplementary section 2.

3.6 Determination of biofacies based on the microbiota and associated material All the relevant information used to determine biofacies based on the microbiota and associated material for use with the microfossil remains extracted in the cores is available in supplementary section 1.

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4. Radiocarbon and AAR dating Several fossil specimens were extracted from cores VC9 and VC17, and their ages determined using either accelerator mass spectrometry (AMS) radiocarbon (Table 1) and/or amino acid racemization (AAR) dating. The fossils selected for analysis comprised several finely preserved specimens of commonly occurring molluscs, and ostracod

valves

that showed

signs of attrition due to

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several disarticulated

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reworking/transport (Table 2). In addition, a sample consisting of a life assemblage of the ostracod Osticythere baragwanathi and another of the same species of ostracod combined with well-preserved benthic foraminifers were used for AMS radiocarbon dating (Table 1).

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4.1 AMS radiocarbon dating

Sample preparation for radiocarbon dating varied depending upon the type and

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volume of sediment available. Bivalve shells were physically cleaned using a dental drill to remove possible carbonate contamination, and then acid-etched using 0.25M HCl for 30 minutes (resulting in a ~50-70% reduction of the original sample mass). Ostracod

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cleaning was limited by the small available mass of material (<0.5 mg/valve), and involved rinsing in dilute HCl (0.001 M) for 3 minutes to remove surface contaminants.

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All specimens (shells and ostracods) were then rinsed with deionised water in an ultrasonic bath 3 times for 5 minutes each, and oven dried over 12 hours at 60 oC before

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hydrolysis.

The cleaned (pre-treated) samples were hydrolysed to CO2 using 85%

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phosphoric acid, then converted to graphite using the H2/Fe method. The technical methods are described in Hua et al. (2001, 2004). A small portion of graphite from the prepared samples was employed for the determination of  13C using the Micromass IsoPrime Elemental Analyser/Isotope Ratio Mass Spectrometer (EA/IRMS) at ANSTO. AMS

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C measurements were performed using the STAR and ANTARES facility at

ANSTO (Fink et al., 2004) for shells and ostracods, respectively. The results are reported as conventional radiocarbon ages after correction for  13C (Table 1). Calibrated

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C calendar ages at 2σ (95.4% confidence level) were

calculated using the Marine13 data set (Reimer et al., 2013) with a marine radiocarbon

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C yrs and the calibration

program for southern Australia is the weighted mean value of 7 data points for Gulf of St. Vincent, northern Spencer Gulf and Pondalowie Bay listed in the Online Marine Reservoir Correction Database (http://calib.org/marine/). All the radiocarbon ages mentioned in the text are in calibrated years BP.

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4.2 Amino acid racemization dating

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AAR determines fossil age based on the increasing ratio of D- to L-amino acids. The extent of racemization of the amino acids glutamic acid and valine in the extracted samples was determined by reverse phase, high performance liquid chromatography (RP-HPLC). The analytical protocol followed that of Kaufman and Manley (1998). Mollusc specimens were first acid etched in 2 M HCl to remove diagenetically modified

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outer portions of shells and any surface-adhering sedimentary particles. These samples were then digested in 8 M HCl and spiked with the internal standard L-homoargenine Analyses were undertaken on the total hydrolysable amino acids after

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(0.01 M).

hydrolysis for 22 h at 110 oC in 7 M HCl. The analytical procedure involved the precolumn derivatization of DL-amino acids with o-pthaldialehyde (OPA) together with the thiol,

N-isobutyryl-L-cysteine

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chiral

(IBLC)

to

yield

fluorescent

diastereomeric

derivatives of the chiral primary amino acids. Amino acid D/L value determinations

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were undertaken using an Agilent 1100 HPLC with a C-18 column and auto-injector. Sample replicates represent analyses on more than one portion of the same individual

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and the number of replicate subsamples (n) is given in Table 2. To assist in the calibration of the extent of AAR with time and enable the

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determination of AAR numeric ages from D/L values, five molluscan specimens (Nuculana crassa) were also analysed by AMS-radiocarbon and linear regressions of the extent of valine racemization was plotted following the methods set out in Clarke and Murray-Wallace (2006; Table 2). This permitted a direct comparison of AAR D/L value and radiocarbon age based on the analysis of the same fossils.

Given that

racemization rate varies with fossil genus, AAR numeric ages were only determined for the species Nuculana crassa. 5. Results and discussion

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5.1 Core descriptions Both cores are characterised by a lower section of clay-rich sediment, overlain by coarse mixed-quartz and skeletal carbonate sands, containing disarticulated bivalves. Sedimentological descriptions of the cores are provided below and logs are presented in Figs. 2 and 3. Core VC9:

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85-0 cm: basal layer of unbroken, but disarticulated bivalve shells; the entire interval

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comprises carbonate-quartz sand with mica and many fragmented shells; sediment colour lighter above 50 cm (Munsell 2.5Y5/4).

155-85 cm: relatively homogenous sandy clay; abundant fine black particulates, framboidal pyrite and numerous blackened bivalve shells.

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215-155 cm: homogenous grey clay matrix and strongly reduced black particulate material; several lenses of very fine quartz-carbonate sediment; bioturbated; abundant molluscan shell fragments at 189 and 163 cm.

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270-215 cm: generally homogeneous grey clay (Munsell 7.5Y4/1), with discrete laminar inclusions of black framboidal pyrite; a bioturbation void infilled with dark particulate material and shell fragments at 256 cm.

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290-270 cm: mottled grey/light olive clay; no evidence of hard contact at the maximum

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penetrated depth.

Core VC17:

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93-0 cm: yellow colour (Munsell YR5/8) contrasts markedly with the lower part of the core; well sorted, quartz-carbonate sand; numerous shell fragments, more and

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abundant

larger

with

depth;

a

pronounced

layer

of

blackened

disarticulated/broken bivalve shells (i.e. shell hash) at 93-89 cm. 168-93 cm: inter-bedded clay and extremely fine quartz-clay sand; the clay layers exhibit discontinuous laminae due to the inclusion of thin surfaces of blackened particulate matter and fine framboidal pyrite; this particulate matter forms some discrete darkened patches with a clayey sand texture; unidentifiable shell fragments in a sub-horizontal bed at 164 cm and at 142.5 cm. 200-168 cm: much lighter in colour (Munsell 5Y4/1); outwardly homogenous but with laminar inclusions of very fine clayey quartz sand; no recognisable shell material.

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238-200 cm: predominantly grey clay (Munsell 7.5Y4/1); numerous darker, slightly coarser laminae; no observable shell; hard pyritised concretions at 263.5 and 260 cm; fibrous vegetal material at 290 cm and at the base of the core; infilled bioturbation voids perpendicular to the bedding at 245-242 cm and at 229-227 cm

5.2 Fossils and related zonations in the cores

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The greatest inferred yield per gram of unprocessed sediment was 1216

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individual microfossils (538 foraminifers and 678 ostracods) at 151-150 cm in core VC9. While the bottom 50 cm of the same core returned the lowest, virtually non-existent yields. ‘Filtering’ the results from each sample to present only the three most numerous ostracods and foraminifers preserved in good condition (and thus, likely in situ specimens) returned a total of 51 species, 23 of which were common to both cores. The

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‘filtered’ results facilitated the recognition of biofacies specific clusters, and thus four facies were recognised: lagoon, estuary, tidal, and open marine. Key species: biofacies

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relationships based on the incidence of well-preserved species within a processed sediment are shown below (Table 3); these were derived from ranked, stratigraphicallykeyed, distribution matrices (see supplement section 1).

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The down-profile distribution of foraminifers and ostracods in cores VC9 and VC17 show a strong correlation between discrete biofacies groups and the

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lithostratigraphy. Much of the biofacies variation in the two cores is presumed to reflect changes in the water salinity of the inferred environment as well as post-depositional

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processes. The inferences derived from both cores are explained and subsequently related to the wider palaeoenvironmental context (See supplement section 1 for

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additional information on inferred biofacies). The preservation state of the microfossils in VC9 and VC17 and their inferred biofacies reveal a series of changes in accord with the main stratigraphical divisions (Figures. 4-5, Table 4). In summary, the upper section of both cores, is characterised by a marine biofacies, with evidence of abundant mechanical abrasion and sorting of the sediment and associated fauna, and a strong numerical bias of foraminifers over ostracods. Below that section, the sediment in both cores is characterised by a progressively changing mix of non-marine biofacies, a greater proportion of ostracods and evidence of chemical rather than physical degradation of the fossil taxa. Biofacies divisions are

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more apparent in VC9, and also more tightly constrained to the stratigraphic divisions in the core.

5.2.1 Core VC9 290-270 cm (basal unit, Fig. 2): This section of the core is characterised by fine mottled clay and is numerically dominated by the ostracod Xestolebris cedunaensis and the

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large diatom Campylodiscus sp. The ostracods are stained grey while the diatoms are

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commonly coated with pyrite (Plate 3 -1,2). The presence of aquatic vegetation can be inferred from the presence of X. cedunaensis (Yassinni & Jones, 1987, 1995; Reeves et al., 2007). The greying of the preserved ostracod valves and the partial pyritisation of the fossils indicates oxygen deprivation following deposition (De Deckker, 1988; De Deckker & Forester, 1988). These faunal/diagenetic patterns indicatie a shallow,

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quiescent lagoonal or restricted estuarine environment with periodic development of large algal blooms (i.e. sapropels). In the presence of adequate supplies of iron

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particles, such conditions are conducive to the formation of pyrite via the anaerobic bacterial digestion of accumulated algal remains (Berner, 1984). The general paucity of other microfossils and the poorly preserved state of those present indicates a

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subsequent oxic phase and the resultant chemical weathering of fossils by pyrite alteration (as described by Merrits et al., 1998).

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270-220 cm: The majority of this section comprises very fine sediment with laminae (Fig. 2). However, the upper 5 cm are marked by inclusions of fine sandy clays and very

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small shell fragments. These features imply a quiescent to transient energy setting. Variable oxygen saturation and redox conditions are indicated by infaunal bioturbation

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traces (oxygenated sediments), and in the general distribution of oxidised/pyritized organic particulates and iron-oxide rich clay laminae (alternate reducing and oxidising conditions). The wide proliferation of pyritized diatoms (see Plate 3-1,2) and the general paucity of other well-preserved microfossils (despite the evidence of bioturbation) are again indicative of episodic algal blooms and the subsequent pyrite-derived chemical corrosion of the carbonate fossil material. (Plate 3- 3-5). This is obviously a post depositional phenomenon. Small numbers of the ostracods Loxoconcha, Pectocythere, and planktonic foraminifers is evidence for a degree of tidal/marine influence (Yokoyama et al., 2000, 2001; Clarke et al., 2001), particularly towards the top of the

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section where there is a dramatic increase in both diversity and quartz sediment. This unit is considered to be representative of a protected estuarine setting adjacent to a tidal channel. 220-155 cm: (note that at 210 cm, a years BP age (± 2

14

C date on ostracods returned a 12,750±279 cal.

The fine grey sediment dominating this section is indicative of

relatively low dissolved oxygen levels and a low energy, depositional environment (Fig.

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2). This is consistent with the observed large numbers of the ostracod Osticythere

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baragwanathi (Warne, 2006), which are both well preserved and accompanied by a large range of instar stages. As such, these specimens are considered to represent an in situ life assemblage. Outwardly, this suggests a lagoonal environment. However, this possibility is discounted by the presence of Pectocythere, Loxoconcha, Xestolebris, Ammonia sp. and Elphidium excavatum (also in life assemblages). Collectively, these

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species indicate a calm, brackish environment with aquatic vegetation, a degree of tidal exposure and oxygen depletion at the sediment/water interface. Water stratification due

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to low mixing is likely and supported by that the decline in the number of Osticythere at 200 cm and 160 cm where there are marked increases in quartz and reworking of the faunas,

respectively

(i.e.

increased

energy/mixing).

Overall,

the

environmental

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conditions represented by this section of the core are inferred to be analogous to the submerged mudflats adjacent to the tidal channel of the modern Hopkins estuary in

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south-western Victoria as described by Warne (2006). 155-85 cm: Several ages (3 radiocarbon and 3 AAR) were obtained for the upper half

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of this section (Fig. 4). The sediment is characterised by olive-grey sandy clay (Munsell 7.5Y4/1) and dispersed, worn molluscan shell fragments (Fig. 2). Brackish water is

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indicated by the presence of Ammonia sp. (refer to taxonomic notes in Supplement 2) and E. excavatum, which dominate as a pair in a range of environments including saline lakes and coastal lagoons such as the Coorong. Elphidium excavatum is a great survivor as salinity increases; Ammonia sp. survives in large numbers in moderately hyposaline conditions (unsuitable for shelf-dwelling faunas that could migrate into an estuary). Accordingly, the ratio of numbers of these two foraminifers can be a useful indicator of salinity. These are typified by the ostracod Pectocythere sp. which constitutes the dominant microfaunal life assemblages throughout. Open estuarine conditions are inferred from the presence of several distinctly marine species including

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pteropods, which occur at 140-120 cm. Fluctuating salinity is shown in the oscillating ratio of foraminifers:ostracods (the F:O ratio which can at best provide an estimate of truly marine and/or non-marine conditions. The assumption is that when this ratio is high, foraminifers abound and thus truly marine conditions prevail. The opposite is interpreted when ostracod numbers are highest). These oscillations are consistent with environmental seasonality (e.g. the passage of flood pulses, possibly caused a sea

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level rise/surge). Evidence supporting this view is the contemporaneous clustered

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preservation of articulated ostracods, which are indicative of rapid sedimentation (see De Deckker, 1988). Noteworthy is the presence of abundant (but tiny) remains of siliceous sponge spicules, echinoid spines and bryozoan fragments at 140 cm (see Plate 2). These are coincident with a dramatic alteration to the F:O ratio to favour foraminifers (suggesting affinity with marine conditions), but no change in the proportion

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of quartz sand (which would increase in the event of an incursion). Given the open estuarine conditions, such an event would have been capable of thoroughly mixing the

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contents of the estuary.

85-50 cm: (note that AAR ages range from 12,500±1900 to 6,900±1000 years BP): This section of the core is characterised by a sudden and marked increase in all of the

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indicators associated with a marine environment (abundant marine microfossil taxa and pteropods) and a consistent numerical dominance of foraminifers over ostracods. The

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sediment within this unit is characterised by its dark colour (Fig. 2), an obvious clay component, randomly-oriented bivalve fragments and several estuarine microfossils,

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particularly ostracods of the genera Pectocythere and Callistocythere, and the milliolid foraminifer of the genera Quinqueloculina and Triloculina. Several AAR ages <10 ka

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were determined for this section of the core using valve fragments and disarticulated individuals of the mollusc species Nuculana crassa (Table 2). Sediment reworking is indicated by the inclusion of much older shells, and by the non-monotonic distribution of the dated material. This mixed age pattern is consistent with the higher-energy conditions on the Lacepede Shelf at the time of deposition and the physical intermixing of fossil shells of differing ages. Quartz, where present, is clear and mostly angular (see Plate 2-1). The characteristics described here suggest a submerged delta front deposit. Sedimentation in such an environment would reflect the balance between riverine and marine dynamics, permitting periods of sediment reworking, and also of gentle

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accumulation (as required to preserve the numerous fragile pteropods). The sudden change represented by this deltaic sediment is unrelated to those in the lower part of the core (indeed it shares more characteristics with the material that overlies it). It can therefore be assumed that it represents the basal component of the Holocene transgression. 50-0 cm: (note an AAR age at 20 cm of 6,080±900 years BP). Clean quartz-dominated

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sand, stained and reworked microfossils, coarse skeletal carbonate sand, reworked

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shell fragments (Fig. 2), and the absolute dominance of the biofacies by holomarine species marks this final transition as an open marine or shelfal environment (i.e. the fully marine conditions of the Holocene high stand).

5.2.2 Core VC17 14

C age of 13,410±300 years BP (± 2

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338-200 cm: (basal unit, and at 330 cm a

The extensive dark micaceous clays, and laminar features of the basal unit (Fig. 3) are

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characteristic of continuous sediment accumulation in relatively quiescent conditions. Abundant pyritization of organic material and the incidence of blackened ostracod valves are suggestive of a generally low oxygen saturation state during the submerged

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phase. However, fluctuating incidences of benthic species such as Ammonia sp.and E. excavatum, and the bioturbatred sediments, reveal that the oxygen level at the were

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sediment-water interface

subject to significant variation. The intermittent

proliferation of large diatoms, particularly between 338 and 260 cm, is coincident with

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marked declines in the number of calcareous microfossils, and with large amounts of fibrous, but unidentifiable vegetal remains. A substantial nutrient inflow, followed by

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algal blooms and the oxygen depletion of the bottom water (during bloom decay) can be inferred. Such a pattern is consistent with the development of sapropel conditions (Edwards et al., 2006). These phenomena were clearly common and, combined with the general decay of other accumulated organic matter, would have contributed markedly to fluctuations in oxygen levels at the sediment-water interface and also to the abundance of biologically-formed pyrite. Widespread oxidation of particulate matter and the chemical degradation of microfossils are indicative of dissolution during pyrite oxidation at some later stage. In general, the environment represented here was particularly conducive to the

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ostracod O. baragwanathi (supplement section 2). This species appeared in wellpreserved assemblages throughout the entire section. Curiously, many of these assemblages lack adults, but do include pristine examples of multiple instar stages, including very large juvenile forms. Plausible explanations for these observations include: firstly, the separation of the adults and the juveniles or, secondly, that something prevented the juveniles from reaching maturity, It is probable that the small

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juveniles were hydrodynamically suspended and redeposited, and that high summer

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water temperatures, or more likely, decreased oxygen levels caused them to die before reaching adulthood. We favour the latter option otherwise life assemblages could not occur due to sorting while in transport. The presence of open marine and tidal conditions indicates an opening to the sea, and provides a means by which the Osticythere could be preferentially transported (i.e. via tidal currents). The intermittent

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presence of aquatic vegetation can be inferred from the presence of X. cedunaensis. Gaps in the recorded incidence of Xestolebris are coincident with the sapropels. This

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could demonstrate a loss of Xestolebris due to either salinity changes, or a loss of the vegetation, or any combination of the two.

200-168 cm: Relative to the basal unit, this section of the core is marked by a

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noticeable change in colour and homogeneity of the sediment (Fig. 3). This comprises massive, light grey clays (Munsell 5Y4/1), occasionally very fine quartz and mica-rich

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sandy lenses and isolated inclusions of framboidal pyrite. Above 182 cm, increasing marine influence is reflected in the decrease of the F:O diversity ratio from 0.66 to

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0.83:1 (implying more saline/marine conditions) and also in the increased proportion of open estuarine and shelfal species observed (refer to Supplement 3). Both chemical

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and physical alteration are implicated in the low preservational quality of many of the observed microfossils, and it is probable that a number of the previously-degraded specimens were reworked during the deposition of a sandy layer at 190 cm. The environment for this section is still estuarine and very similar to the lower part of the core. However, the observed features relate to an increase in water depth, salinity and environmental energy (refer to Supplement 3). The migration of the tidal channel to encompass the core location is a scenario that accounts for the various traits observed in this section of the core. 168-93 cm: (note at 160 cm a 14C age gave a 13,450±340 cal. years BP age (± 2

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This section is also indicative of a continuous phase of organic-rich sediment accumulation (Fig. 3) and redox variability. Brackish, commonly vegetated conditions are inferred from the high incidence of E. excavatum and X. cedunaensis respectively. The numerical domination of Pectocythere sp. in mixed life assemblages with slightly lower numbers of Loxoconcha australis and O. baragwanathi indicates a protected open estuarine environment. Notably this entire section of core VC17 was characterised

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by microfossils that are stained, partially dissolved or poorly calcified (Plate 3-3 to 5).

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Poor calcification is indicative of a low environmental pH and/or low calcite saturation levels during the life phase of many of the observed specimens, while dissolution again relates to post-burial taphonomic processes acting upon the sediment containing the fossil material. Based on the patterns observed by Yassini & Jones (1987), the causal agent would most likely be algal blooms. This view is unsupported by any marked

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incidence of diatoms in the sample material. However, it is possible that many diatoms were not detected because they were less than 130 µm in size and thus, passed

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through the sieve in the early stages of sample processing. It is also possible that chemical degradation of the calcareous fossil material in this section related to younger sapropel events, and the contribution acid leachate from a part of the deposit that was and/or eroded. The

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subsequently reworked

high incidence

of Osticythere is

demonstrative of both low O2 and low pH conditions at the sediment-water interface

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(Warne, 2006). 93-0 cm: (note at 84 cm a

14

C date gave an age of 13,550±240 cal. years BP (± 2

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There is a pronounced sediment and biofacies transition at 93 cm depth. This contact is marked, at its base, by abundant disarticulated, blackened bivalves and numerous

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blackened shell fragments (Fig. 3). This implies the deflation and reworking of the underlying, commonly anoxic, estuarine sediments. The remainder of the core, above 82 cm, is dominated by coarse skeletal carbonate sand, randomly oriented shell valves and fragments, and holomarine microfossil biofacies. Notwithstanding slight differences in racemization rate between mollusc genera, the amino acid D/L values in this portion of the core (0-93 cm) reflect localized reworking under moderate energy conditions. Holocene, open marine-shelfal conditions are inferred.

5.2.3 Correlation of cores VC9 and VC17

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Cores VC9 and VC17 are characterised by facies that imply relatively quiescent but consistently open estuarine conditions, followed by an unconformable transition to open marine conditions (Figs. 4, 5) Indicator species recognised in both cores are presented in Table 3. In addition, dated fossils from the two cores indicates a relationship limited to the period of post-glacial sea-level rise on the Lacepede Shelf. Demonstrated changes in salinity are attributed to altered flow regimes and the impact

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of algal blooms on oxygen and pH levels at the sediment-water interface. However,

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neither core contained microfossils consistent with the development of extremely saline, athalassic [=non-marine] conditions.

As both cores are located at different depths on the Lacepede Shelf and returned different ages based on their fossil assemblages, we cannot directly correlate them. Nevertheless, important remarks can be made: (1) both cores register sea-level rises as

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a result of the global deglaciation; (2) when both sites were transgressed by the ocean, rapid sedimentation rates occurred. This is confirmed by the fact that well over 1 m of

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sediment recovered in each core (in VC9: from 210 to 95cm (Fig. 4); in VC17: from 330 to 90cm (Fig.5)), ages are statistically similar; and (3) the top section of both cores comprise reworked Holocene marine sands (Figs. 4, 5). Much of the sand consists of

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reworked aeolian quartz dunes that originally capped the Lacepede Shelf during the glacial regression leaving the Shelf completely sub-aerially exposed.

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Seasonal interruptions to salt wedge conditions would have resulted from large summer melts associated with the deglaciation of the alpine areas of south-eastern

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Australia (see Gingele et al. 2007). Indeed, the pre-transgression radiocarbon and AAR ages (~13,500 cal. yr BP) determined for the lower section of cores VC9 (Fig. 4) and

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VC17 (Fig. 5) coincide with the “Yanco” high-discharge phase of the Murrumbidgee River (a significant tributary of the River Murray) (Page et al. (1996); Barrows et al. (2001)). These major seasonal discharges caused extensive mixing and changes to water salinity in the estuarine environments represented by cores VC9 and VC17 demonstrated in the fossil samples by alterations to the F:O ratio and in the rapid death and subsequent articulated preservation of various ostracods (refer to Supplement 3). 5.3 Consequences of rapid sea-level rises and the ‘Meltwater Pulses 1A and 1-B Table 4 and figure 6 present a summary of the biofacies recognised in both cores

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and associated chronologies. It is clear upon examination of the different facies that sedimentation rates were times quite extensive (see notes in section 5.2.3). Camoin and Webster (2016), in their summary of post-glacial sea-level changes discussed the nature of those very rapid rises associated with the now well-documented meltwater pulses MWP-1A that occurred between 14.6 and 14.35 ka and MWP-1B that occurred between 11.4 and 11.2 ka. Camoin and Webster (2016) summarised that

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during MWP-1A an accelerated sea-level rise of ~46 ± 6 mm yr-1 occurred, heralding an

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overall sea-level rise of 16 ± 2m in amplitude in less than 500 years. The same authors also indicated that in the Papeete cores, no significant acceleration of sea-level rise during MWP-1B.

The rapid sea-level rise during MWP-1A would have had a significant impact on the Lacepede Shelf. Unfortunately, core VC-17 is located too high (67.5 m bsl) on the

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Shelf to detect the sudden sea-level rise that occurred during MWP-1A. Nevertheless, when sea-level was approaching ~70 m below present sea level and still rising, a

core for the first time; a

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transition above 200 cm in core VC17 is noticeable with marine taxa appearing in the 14

C age on a life assemblage of the ostracod O. baragwanathi

(implying an in situ estuarine sample at 160 cm in the core confirms an age of 13,440 ±

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320 cal. yr BP (Fig. 6). This dated horizon gives an age that coincides with a level of close to 70 m in the Tahiti core (Fig. 7). When truly marine conditions are finally 14

C ages on the mollusc N. crassa as

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registered in core VC17 above 90 cm (Fig. 6), two

well as the AAR age on the same taxon all indicate deposition at 13.5 ka corresponding

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with the post-MWP-1A phase.

In core VC9, the length of the core encompasses the depth on the Lacepede Shelf

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coincident with meltwater pulse MWP-1B when sea level increased from ~-56 m to ~51m as recognised in the Tahiti cores (Bard et al., 2010; Camoin et al., 2012). Two

14

C

ages on the mollusc N. crassa correlate well with the ages and relative depths of meltwater pulse MWP-1B; these 2 molluscan samples are found in what we interpreted as a marine-estuarine facies (Figs. 4, 6) and the third N. crassa dated by AAR, although with a large uncertainty, also covers MWP-1B (Fig. 6 and Table 2).

5.4 Taphonomic processes and the problem of sample selection for geochronological analyses and palaeoenvironmental interpretation

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Several issues arose when interpreting the ages obtained from microfossils as well as from molluscs. The robust nature of the fossil molluscs means that they can be transported and selectively reworked, sometimes through several sedimentary cycles without showing much evidence of mechanical abrasion and/or dissolution. Several fossil mollusc shells showed signs of reworking based on their fragmented nature and the occurrence of shells with higher D/L values reflecting older ages, superposed on

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younger shells with lower D/L values. Accordingly, the ages listed in bold in Table 2

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reflect the most likely timing of deposition as they represent the lowest D/L values and hence youngest ages. Nevertheless, the material that was analysed by AAR needs to be selected so as to ascertain confidence in the obtained ages. In our case, we only considered the ages that are listed in bold in Table 2.

Secondly, in environmental settings such as shallow estuaries and lagoons,

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winds can stir the bottom of those water bodies and microfossils and molluscan bivalves can be transported and mixed repeatedly. Hence, a total ostracod fauna needs

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to be examined and mixed assemblages that originally belong to different environments need to be assessed. The advantage with ostracods is that by examining both instars and adult valves it may be determined if the faunal assemblage has been transported

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and/or reworked. A sample representing a ‘life assemblage of a specific ostracod’ would therefore ensure that its age obtained by any dating technique will relate to an ‘in situ’

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material. Unfortunately, in the study reported here, ‘in situ’ assemblages of ostracods were rarely identified; we have attributed the mixed and transported assemblages to the

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shallow nature of the continental shelf environment in which the organisms grew. Thirdly, the nature of estuaries is such that there are diurnal changes in salinities

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and currents that are driven by tides. In some circumstances, tidal regimes can at times be extremely vigorous and, as a consequence and once again, cause microfaunas and the larger bivalve molluscs to be transported and selectively reworked. Hence, caution is required when identifying the nature of the environments in which the fossils finally preserve. In addition, a substantial discharge of freshwater into the estuary may cause organisms to die, and this perhaps would explain, for example, the absence of adult ostracods ion what is considered a (unreworked) life assemblage. Finally, the shallow nature of the Lacepede Shelf is an environment, especially in winter, which is strongly affected by strong winds that originate from the Southern

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Ocean with consequently high waves with amplitudes that will affect the sea floor (see section 2 on regional setting). For further details, refer to the important compendium volume on Australian southern shallow seas by James and Bone (2011). As a result, much reworking of the sediments, including the faunas will occur. This has been a feature of the Shelf since the post-glacial transgression even if sea level has not changed may eventually become mixed with much younger material. Consequently,

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caution is required when examining faunas and their host sediments. 6. Conclusions

Microfaunal analysis of cores VC9 and VC17 unambiguously demonstrate that both core sites are associated with the passage of the palaeo-River Murray across the Lacepede Shelf. and

micropalaeontological

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Sedimentological

analysis

indicates

that

the

Pleistocene/Holocene boundary is represented by the unconformable changes from

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quartz-rich sand to clay. The top section of both cores comprise sediment that has been frequently reworked during the late stages of post-glacial sea-level rise and the subsequent sea-level highstand. The importance of taphonomic processes involved in

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estuarine and lagoonal environments is also stressed. Close examination of the preservation state of preservation of fossiliferous material is required to ensure that

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dated material and environmental reconstructions are derived from ‘in situ’ contexts. The Holocene open marine phase in each core is unambiguous and easily identified by

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the omnipresence of pteropods and reworked microfossils. The lower (Pleistocene) part of each core appear more complex, but can be explained in terms of the rapid sea-level

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rises as recognised in Barbados (see Fairbanks et al. (2005), although with too few dates] and better constrained through the recent drilling at Tahiti (Bard et al. (2010); Camoin et al. (2012) and Camoin and Webster (2015)). Overall, the microfossil diversity, combined with seismic surveys carried out during the SST02/07 research voyage, show that the older estuaries associated with the River Murray (now drown were maintained in an open state throughout the entire period documented by the cores. In addition, the ocean was proximal enough to ensure the coexistence of mixed life assemblages not commonly observed in the modern southern Australian setting. The tidal action was also not sufficiently strong to affect good mixing

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of the estuarine water, but this was not always the case. This resulted in development of salt wedge conditions, and a hypoxic sediment water interface (i.e.conditions readily exploited by the ostracod O. baragwanathi). Finally, nutrient and dissolved iron-rich water from terrestrial sources fuelled the episodic sapropel phases identified in both cores, and facilitated biologically mediated pyrite formation during the oxygen depleting decomposition phase of these events. This is best evidenced in the lower portion of

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disruptions at the sediment-water interface.

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cores VC9 and VC17 where critically low oxygen levels also caused major ecological

This study found no evidence for a truly lacustrine facies in the cores. As discussed above, the microfossils and molluscan remains of both cores are only indicative of either very open estuarine conditions from the period following the Last Glacial Maximum, or marine conditions associated with the final post-glacial

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transgression. Neither core shows evidence of hard contact at their base, so it is possible that the in situ sediment record is more extensive than that which was

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available for analysis, this is expected to be the case upon the examination of the seismic profiles presented in Hill et al. (2009).

The study of cores VC9 and VC17 demonstrated that, in order to draw viable

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inferences, numerous micropalaeontological clues and environmental factors need to be simultaneously considered. In particular, an understanding of likely environmental

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functioning under a range of scenarios, and the identification of key indicator species, can provide relevant information. The ostracods X. cedunaensis and O. baragwanathi

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are of critical importance to document bottom water salinity and the status (presence and condition) of the aquatic vegetation when considering the palaeoenvironments

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represented by the cores.

Finally, we clearly document that Meltwater Pulse 1-B is detected in core VC9 because its depth of the sea floor is such that recognised and dated marine taxa are coincident with an age of ~11,500 to 11,000 cal. years BP. The marine transgression that followed Meltwater Pulse MWP-1A ~13,600 cal. years BP is also recognised in core VC17 based on the fossil taxa and acquired ages.

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Acknowledgements We are extremely grateful to Professor J. H. Cann who reviewed the manuscript and provided very detailed comments which considerably helped improve the manuscript. We also benefited from the comments of an anonymous reviewer and the editorial comments of Dr Xavier Crosta. Many thanks to all. Dr Mark Warne for ostracod taxonomy and advice on current sedimentation in Victorian estuaries. We are very

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grateful to the entire crew and scientific staff of the RV Southern Surveyor (research

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cruises SS02-06 and SST02-07). Geoscience Australia provided the vibracorer operated by Nigel Craddy of ANU. An ARC-DP grant awarded to PDD funded part of the cruises expenses.

We gratefully acknowledge an AINSE Grant (08/051) for radiocarbon dating of fossil molluscs and ostracods from the Lacepede Shelf.

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We would like to dedicate this paper to the memory of Carol Mitchell who carried

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out the AAR analyses with great enthusiasm and interest. She is sadly missed.

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Figure captions

Fig. 1 Schematic map of the southern Australian continental margin, showing the

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Lacepede Shelf, the present River Murray and Coorong Lagoon barrier complex, the inferred courses of the ancient River Murray across the Lacepede Shelf (based on sub-

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bottom profiling and interpreted by Peter Hill) and the location of cores VC -9 and VC-

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17. (Image modified after Hill et al., 2009).

Fig. 2. Detailed log of core SST02/07 VC9. Refer to table 1 for more information and material dated and the nature of preservation.

Fig. 3. Detailed log of core SST02/07 VC17Refer to table 2 for more information and material dated and the nature of preservation.

Fig. 4. Diagram showing stratigraphically-keyed biofacies and special indicators for core

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VC9. Biofacies groups are based on the species shown in the incidence matrices (see supplement section 3). The horizontal broken red lines denote major stratigraphic divisions. “diatoms*” imply the likely presence of a sapropel; “veg*” is indicative of a vegetal substrate; “eury*” is indicative of euryhaline or potentially hypersaline conditions. Ages in black represent radiocarbon dates and in red AAR ages (for further

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details refer to Tables 1 and 2).

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Fig. 5. Diagram showing stratigraphically-keyed biofacies and special indicators for core VC17. Same comments as for fig. 5

Fig. 6. Plots of depth versus age for the two Lacepede cores (VC9 and VC17) showing along the depth axis the different recognised facies and the results of dates obtained for

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the varied material analysed. The age extent of the two meltwater pulses (MWP-1B and MWP-1A) are also plotted on these diagrams to show that several of the dates obtained

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in the cores pertain to those pulses.

Fig. 7. Combined plots of sea-level changes in metres over the period of 18 to 4 ka

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obtained from various sources (Bard et al., 2010; Camoin et al., 2012; Camoin and Webster, 2015). The timing of the two meltwater pulses (MWP-1A and 1B) are also

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indicated as well as the intervals encompassed by the two cores (VC9 and 17) are also indicated. Abbreviations for the coral assemblages recognised at the Papeete and branching Acropora and Pocillopora; PM: branching

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Tiarei drill sites are: AP:

Pocillopora and massive Montipora; PP: branching Porites and Pocillopora; PPM:

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brancing Porites and encrusting Porites and Montipora; mP: massive Porites; AFM: encrusting agaricids and faviids. For further information see references listed above.

Plate captions

Plate

1. Scanning electron microphotographs of various ostracod taxa

encountered in modern esutuaries of southeatern Australian and the 2 cores VC9

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and VC17. Osticythere baragwanathi 1: Left view of carapace of adult female; 2: Left view of carapace of adult male; 3: left valve of instar A-1; 4: left valve of instar A-2; 5: left valve of instar A-3; 6: left valve of instar A-4; 7: ventral view of adult carapace. Leptocythere sp. 8: lateral view of right valve of carapace; 9: right valve of instar A-1; 10: right valve of instar A-2; Paracypria sp. 11: ventral view of adult carapace showing appendages; 12: tilted side view of carapace showing mostly left valve; 13: left view of

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adult carapace; 14-15: left valves of instars. Xestoleberis chilensis austrocontinentalis

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16: left valve of adult. Xestoleberis cedunaensis 17-18: instars. Parakrithella australis 19: adult right valve; 20-21: left valve of instars. Microcytherura sp. 22: right valve of female?; 23: right valve of male? Hiltermannicythere sp. 24: left valve of female; 25: left valve of male. Pectocythere sp. 26: left valve of instar A-2; 27: left valve of instar A-1; 28: left valve of male?; 29: left valve of female? Loxoconcha gillii 30: left valve of adult;

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31: left valve of instar A-1. Loxoconcha australis 32: left valve of adult female?; 33: left valve of male? Cytherella sp. 34: left valve of adult; 35: left valve of instar A-1?

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Callistocythere dorsotuberculata 36: right valve of juvenile instar; 37: right valve of partiallly dissolved adult valve. Hemicytherura sp. 38-41: left valves of adults showing

43-44: left valve of adults.

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various forms of reticulation. Bairdopillata sp. 42: right valve of adult? Paranesidea sp.

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Plate 2. Scanning electron microphotographs of selected detrital grains, remains of microfossils including numerous foraminifers. 1-3: Quartz grains displaying

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different shapes: 1: angular; 2: subrounded; 3: rounded. 4-5: Broken siliceous sponge spicules. 6-8: Fragments of bryozoans showing evidence of transport. 9: Juvenile

AC

bivalve mollusc: 10-12: Fragments of echinoid spines. 13-14: Well preserved pteropod conchs which are usually very fragile. 15: Elphidium excavatum indicative of estuarine, euryhaline/hypersaline conditions. 16: Elphidium macelliforme indicative of marine conditions. 17: unidentified elphidiid (possibly E. chalottense of Hayward et al., (1997) J. Cann pers. comm.). 18: Elphidium crispum juvenile indicative of a vareity of biofacies but is found on sea grass. 19-20: Elphidium advenum advenum (or possibly poorly preserved E. crispum (J. Cann pers. comm.) indicative of estuarine conditions. 21-22: Ammonia sp. (dorsal and umbillical views respectively) indicative of estuarine, euryhaline and hypersaline settings. 23: juvenile globigerinid foraminifer, normally open

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marine but likely transported ininto shallow water and possibly estuary. 24: bolivinid foraminifer indicative of shelf and open marine. 25-27: agglutinated foraminifers all indicative of lagoonal conditions with potentially elevated salinities and low pH conditions: 25: Miliammina fusca; 26: Trochamina inflata; 27: Ammobaculites sp. 28-31: Quinqueloculina tasmanica and Q. poeyana showing aberrant form in 31, all indicative of estuarine conditions. 32-34: discorbid foraminifers including co-joined Glabratella sp.,

T

all common in tidal and open marine settings. 35: Triloculina oblonga. 36-38: Triloculina

SC RI P

inflata. 39: Quinqueloculina seminulum. Note 35-39 represent species that occupy a variety of shelfal and estuarine environments (including mangrove fields, but which show a high tolerance to elevated salinities. 40: Sigmamiliolinella australis, a true marine species.

NU

Plate 3. Examples of the effects of pyritisation and its alteration in specimens from cores VC9 and VC17. 1: Two fragments of the large diatom Campylodiscus sp. ’girdles’

MA

separated by pyrite crystals (VC9-289.5cm); 2: Pyrite cast from a totally dissolved planispirally coiled foraminifer (VC17-140cm); 3: Unidentifiable rotaliid foraminifera in the advanced stages of dissolution (VC17-130cm). Examples of post depositional

ED

chemical dissolution (indicated by the arrows) caused by subaerial exposure of the sediment and development of acidic fluids (likely H2SO4) as a result of pyrite dissolution

PT

by oxidation. 4: Osticythere baragwanathi; 5: Elphidium excavatum (both from VC17-

CE

130cm).

AC

Tables captions

Table 1. Radiocarbon ages obtained from fossil molluscan Nuculana crassa and ostracod Osticythere baragwanathi specimens extracted from cores VC9 and VC17 from the Lacepede Shelf.

Table 2. Results of amino acid racemization (total hydrolysable amino acids) and calculated ages of fossil molluscs extracted from cores VC9 and VC17 from Lacepede

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Shelf. Samples shown in bold represent the youngest well-preserved examples from the dated horizon. Table 3. List of the top indicator species recognised in cores VC9 and VC17, as identified by species incidence matrices (refer to supplements 1 and 3). Species most indicative of the nominated biofaces groups as demonstrated by the incidence matrices.

T

“Times in top 3” = incidence within the matrix (high = strong relationship), “Occurrences” assemblage, “size variation”

SC RI P

= total number of samples in which the species occurred, “adult + juvenile” = mixed = maximum variation observed (and sample horizon).

Eury = euryhaline, veg = presence of aquatic vegetation.

Table 4. Principal biostratigraphic divisions and environmental conditions recognized in

NU

cores VC9 and VC17.

Marine Micropaleontology.

MA

Supplementary information to be made available at the Elsevier web site for

ED

Supplement 1. Information on the sampled modern estuaries and collections.

PT

Supplement 2. List of species list encountered during the sampling of estuaries and also in cores VC9 and 17. Taxonomic descriptions and ecological note are provided.

AC

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Supplement 3. Incidence matrices using microfossils calculated for the two cores VC9 and VC17.

References

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ACCEPTED MANUSCRIPT Table 1: Radiocarbon ages obtained from fossil molluscan Nuculana crassa and ostracod Osticythere baragwanathi specimens extracted from cores VC9 and VC17 from the Lacepede Shelf. Depth in core (cm)

Sample Type

AAR-lab code

AMS-lab Code

 13 C

Conventiona l 14 C age ± 1σ (‰) ± 1σ (BP)

Calibrated age at 2 (cal BP)** Age range Age ± 2

UWGA7548 UWGA7550 UWGA7553

OZL200

0.0*

10,260±100

11,280±400

OZL201

0.9±0.2

10,410±80

OZL202

-0.1±0.1

9,900±80

OZN432

0.0*

11,280±120

10,88011,680 11,15011,830 10,53011,070 12,48013,020

OZL204

0.1±0.1

13,31013,790 13,28013,760 13,11013,790 13,11013,710

13,550±240

110 120 210

Nuculana crassa Nuculana crassa Nuculana crassa Osticythere baragwanathi

160 330

UWGA7171 UWGA7517

OZL205 OZN433

Osticythere baragwanathi Osticythere baragwanathi + forams

OZN434

12,140±90

-0.5±0.2

12,110±90

0.0*

12,020±150

MA

90

Nuculana crassa Nuculana crassa

ED

84

NU

Core VC17

SC RI P

105

T

Core VC9

0.0*

11,990±130

11,490±340 10,800±270 12,750±270

13,520±240 13,450±340 13,410±300

AC

CE

PT

Note : * - Assumed δ 13 C value due to sample material is not enough for a reliable δ 13 C measurement ** - Age calibration was carried out using Marine09 Marine13 (see Reimer et al. ,2013) data set with an ΔR value of 62 ± 61 yrs and the calibration program CALIB version 7.0.2.

ACCEPTED MANUSCRIPT Table 2. Results of amino acid racemization (total hydrolysable amino acids) and calculated ages of fossil molluscs extracted from cores VC9 and VC17 from Lacepede Shelf Samples shown in bold represent the youngest well preserved examples from the dated horizon

Lab Code (UWGA)

Species

n

Preservation D/L VAL state*

Nuculana crassa Shell fragment (UAF) Polinices pyriformis Placamen placidium Knobbly shell fragment Polinices pyriformis Polinices pyriformis Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Mactra sp. Polinices pyriformis Nuculana crassa knobbly shell piece Nuculana crassa

1

Fr(OGE)

1

Fr(UAF)

2

GE

1

Fr(OGE)

1

Fr(UAF)

2

GE

Core VC9

7522

50

7523

60

7526

61

7527

61

7528

70

7531

70

7532

77

7534

77

7535

77

7536

98

7540

98 98

7541 7542

98 98

7543 7544

100

7545

100

7546

105

7547

SC RI P

46

6080±900

0.031±0.005

0.082±0.003

0.012±0.005

0.074

0.213

0.206

0.096±0.004

0.147±0.005

GE

0.097±0.003

0.148±0.009

Dis(WP)

0.066±0.001 0.120±0.004 6900±1000

2

Fr(OGE)

0.084±0.004

0.133±0.001

9400±1400

2

Fr(OGE)

0.106±0.006

0.146±0.001

12,500±1900

2

Dis(WP)

0.066±0.007 0.118±0.001 6900±1000

2

Dis(WP)

0.094±0.006

0.143±0.006

10,800±1600

2

Dis(WP)

0.086±0.017

0.130±0.016

9700±1500

1

Dis(WP)

0.107

0.142

12,600±1900

2

Dis(WP)

0.110±0.011

0.147±0.008

13,000±2000

1

Dis(WP)

0.119

0.142

14,300±1600

2 1

Fr GE

0.100±0.006 0.143

0.177±0.001 0.189

2

Fr(OGE)

0.092±0.013

0.136±0.009

1

Fr(UAF)

0.426

0.352

2

Dis(WP)

0.080±0.008 0.128±0

2 2

NU

7521

0.053 0.174

MA

46

0.06

APK age (VAL) and cal. a BP

0.148

ED

7520

PT

30

CE

7519

AC

20

D/L GLU

T

Depth (cm)

10,500±1600

8900±1300

ACCEPTED MANUSCRIPT

140

7556

140

7558

170 170

7564 7565

180

7566

190

7567

190

7568

193.5

7569

193.5

7570

210

7571

211.5

7572

216

7573 7501

31.5

7502

31.5

7503

45

AC

31.5

45

7199

50

7196

55

7190

55

7191

55

7192

31.5

7504 7198

2

Dis(WP)

0.094±0.007 0.185±0.077 10800±1600

1 2

GE Dis(WP)

0.109 0.106±0.015

0.142 0.144±0.012

12500±1900

1

Dis(WP)

0.128±0.031

0.149

15,500±2300

1

Fr(UAF)

0.388

0.379

2 2

GE Fr(UAF)

1

Fr(UAF)

2

Fr(OGE)

2 2

Solen vaginoides Katelysia scalarina Solen vaginoides Katelysia scalarina Granulina nympha Gibberula subbulbosa Solen vaginoides Venericardia bimaculata Gibberula subbulbosa Myadora

CE

Core VC17

0.381±0.016

T

7554 7555

0.421±0.025

SC RI P

120 140

Fr(UAF)

0.162 0.428±0.007

0.176 0.387±0.001

0.498

0.419

0.112±0.013

0.159±0.016

13300±2000

Fr(OGE)

0.114±0.014

0.141±0.004

13600±2000

Fr(H)

0.109±0.016

0.141±0.001

12900±1900

Fr(H)

0.124±0.003

0.149±0.001

15,000±2300

Fr(OGE)

0.126±0.017

0.166±0.016

2

Fr(H)

0.127±0

0.161±0.011

1

GE

0.114

0.203

1

Fr(OGE)

0.077

0.117

2

Fr(OGE)

0.050±0.004

0.107±0.008

1

Fr

0.103

0.140

2

Fr(OGE)

0.027±0.004

0.069±0.007

1

GE

0.092

0.122

1

GE

0.083

0.106

1

Fr

0.042

0.117

Fr

0.112

0.177

1

Fr

0.048

0.115

2

Dis

0.092

0.179±0.008

2 2

NU

7551

2

MA

110

knobbly shell fragment Nuculana crassa Diala sp. Nuculana crassa Nuculana crassa knobbly shell fragment Diala sp. knobbly shell fragment knobbly shell fragment Nuculana crassa Nuculana crassa Nuculana crassa Nuculana crassa Katelysia sp. (?) Mactra sp.(?) Batillaria australis

ED

7552

PT

110

ACCEPTED MANUSCRIPT

7186

57

7187

60 60 60

7180 7181 7182

64

7176

64

7179

70 84

7518 7167

84

7169

84

7171

84

7172

84

7173

90

7514

90

7515

90

7516

90

7517

92

7512

Nuculana crassa Katelysia scalarina Gibberula subbulbosa Glycymeris radians Irus crenatus Nuculana crassa

2

Dis

0.067±0.004

0.138±0.005

2

Dis

0.047±0.004

0.102±0.002

Dis

0.105±0.009

0.174

2

Dis

0.142±0.022

0.229±0.018

2 2 2

Fr Dis Dis

0.094±0.013 0.062±0.008 0.125±0.008

0.119±0.011 0.122±0.004 0.182±0.004

2

Dis

0.065±0.010

0.123±0.018

2

Fr

0.053±0.016

0.145±0.077

1 2

Fr(H) GE

0.152 0.103±0.045

0.174 0.161±0.010

2

Fr(OGE)

0.084±0.004

0.150±0.006

2

Dis(Ab)

0.132±0.003

0.196±0.009

Dis

0.132±0.014

0.167±0.005

2

Fr(OGE)

0.095±0.008

0.178±0

1

GE(Ab)

0.150

0.191

2

Dis

0.110±0.009

0.189±0.007

2

Dis(Ab)

0.168±0.009

0.244±0.013

2

Art(WP)

0.115±0

0.168±0.004

3

T

57

0.181±0.011

SC RI P

7185

0.093±0.005

NU

57

Dis

MA

7194

2

ED

55

rotunda Myadora rotunda Glycymeris radians Katelysia peronii Venericardia bimaculata Myadora rotunda Katelysia sp. Lucina sp. Myadora rotunda Sunetta vaginalis Gibberula subbulbosa Lucina sp. Friginatica beddomei Katelysia scalarina Nuculana crassa

PT

7193

CE

55

13,530±220 cal a BP (C14) 13,500±2000

13,500±220 cal a BP (C14)

AC

Irus 2 Dis(Ab) 0.234±0.007 0.320±0.004 crenatus 92 7513 Irus 2 Dis(Ab) 0.173±0.009 0.253±0.009 crenatus 92 7507 Gibberula 2 Ab&Pt 0.241 0.297 subbulbosa Last Interglacial, Glanville Formation, Mundoo Island 100 Katelysia 3 Art(WP) 0.419±0.010 0.437±0.009 125 ka (MIS scalarina 5e) Late Pleistocene interstadial marine sediments, Core VC20 Lacepede Shelf (Hill et al., 2009) 130 6337 Irus 2 0.183±0.004 0.223±0.002 39.4±5.9 ka crenatus (AAR) 130 6338 Nuculana 2 0.182±0.007 0.212±0.009 35.6±5.3 ka illepida (AAR)

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI P

T

*Preservation state : Art – Articulated; Dis – Disarticulated; Fr – fragment; WP – well preserved; H – hinge; OGE – outer growth edge; EV – entire valve; Ab – Abraded; Pt – pitted surface; UAF – unrecognisable affinity; GE – gastropod complete specimen.

ACCEPTED MANUSCRIPT

Table 3. Top indicator species in cores VC9 and VC17, as identified by species incidence matrices (refer to supplementary section 1).

Biofacies

Top indicator species

Tim es in top 3

Occurrences

Grow th stage

# size variations present (core & depth in cm )

Juvenile

Osticythere baragwanathi

28

36

x

x

7 (VC17-331)

Estuary

Pectocythere sp.

38

44

16

12

x x

9 (VC9-160)

Loxoconcha australis Cibicides lobatulus

14

2

x x x

Elphidium macelliforme

10

14

Triloculina inflata

14

Elphidium crispum

5

x x x

x x x

Special * (eury)

Elphidium excavatum

14

Special ** (veg)

Xestoleberis cedunaensis

9

x x

x x

Marine

19 18 23 15

8 (VC9-150) 1 (several) 6 (VC9-60) 6 (VC9-40) 10 (VC9-83) 4 (VC9-90) 6 (VC9-210)

NU

Varied/mixed

SC RI P

Tidal

T

Adult

Lagoon

Species most indicative of the nominated biofaces groups as demonstrated by the incidence matrices. “Times in top 3” = incidence within the matrix (high = strong

MA

relationship), “Occurrences” = total number of samples in which the species occurred, “adult + juvenile” = mixed assemblage, “size variation” = maximum variation observed

AC

CE

PT

ED

(and sample horizon). Eury = euryhaline, veg = presence of aquatic vegetation.

ACCEPTED MANUSCRIPT Table 4 Environmental affinities of common species and species groups identified ope n oce an

F o r x x

sh elf

inn er sh elf x

int er tid al x

tidal chan nels

shelt ered bays

x x

x x

coas tal lago ons x x

x

x

x

x x

x

x x

x x

x

x

hypers aline lake x

x

x

AC

CE

PT

ED

MA

NU

SC RI P

Taxon name Triloculina inflata x x Triloculina oblonga Quinqueloculina x seminulum x x x x Quinqueloculina x tasmanica x Quinqueloculina x poeyana x Elphidium x excavatum x x x Elphidium crispum x x x Elphidium x macelliforme x x x x Globigerina sp. x x x Planoglabratella x nimai x x x Agilloecia sp. x x x Ammonia sp. x x Callistocythere dorsotuberculata x x x Cibicides lobatulus x x x Leptocythere spp. x x Loxoconcha australis x x x Loxoconcha judithaea Microcytherura sp. x x x Osticythere baragwanathi Paracypria sp. Pectocythere spp. x x x x x Trochammina inflata x x Xestoleberis cedunaensis x x x x “For” column denotes oraminifers, all others are ostracods

open estua ries

T

in cores SST02/07 VC9 and VC17

x x

x

x

x

x x

x

x

x

x x

x x

x

ACCEPTED MANUSCRIPT

Table 5 Principal biostratigraphic divisions and environmental conditions recognized in cores VC9 and VC17

Depth (cm ) & Age

VC9

85-155 ~11 ka

- m arine biofacies - high levels of quartz sediment, coarse marine material and rew orked microfossils - abundant fragmented material -

predom inance of estuary and tidal biofacies euryhaline and aquatic vegetation indicators very high levels of preservation in good condition highest incidence of articulated ostracod valves in core

T

0-85 Holocene

Com m ents

SC RI P

Core

- lagoon biofacies 155-220 - intermittent euryhaline and aquatic vegetation indicators ~11.5 -~12.7ka - extensive dissolution of foraminiferal tests

0-93 Holocene

- absence of essentially all biofacies groups , coincident w ith abundant diatoms - high proportional representation of stained and pyritised microfossils - m arine biofacies - high levels of quartz sediment, coarse marine material and rew orked microfossils - abundant fragmented material

NU

220-290 >12.7 ka

-

168-200 ~13.5 ka

- m arked decrease in all biofacies, m arine biofacies at 190cm - quartz sand and shell grit at 190cm - extensive physical/chemical degradation of fossil material

200-338 ~13.5 ka

- m ixed biofacies, diatom-rich below 250cm - intermittent euryhaline and aquatic vegetation indicators - high levels of fossil preservation

ED

AC

CE

PT

VC17

lagoon and estuarine biofacies abundant euryhaline indicators and intermittent aquatic vegetation indicators preservation levels variable, extensive damage to fossil material at 160cm blackened sandy clay betw een 161-155cm

MA

93-168 ~13.5 ka

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA

NU

SC RI P

T

Highlights:  We use the ecological information of Foraminifera and Ostracoda collected from various estuaries in southern Australia to interpret environmental changes recorded in 2 vibracores obtained from the Lacepede Shelf, offshore South Australia  Detailed studies of the microfauna and sediments from the 2 cores inform us on salinity and sea-level changes that postdate the Last Glacial Maximum  The microfauna in the cores record the 2 significant and rapid sea level changes, and in particular Meltwater Pulse IB at 11,500-11,000 cal. years BP.  We illustrate the effect of micropyrite dissolution that affects calcareous microfossil ‘corrosion’

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7