Fish otolith microchemistry: Snapshots of lake ...

Quaternary International 463 (2018) 29e43

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Fish otolith microchemistry: Snapshots of lake conditions during early human occupation of Lake Mungo, Australia Kelsie Long a, *, Rachel Wood a, Ian S. Williams a, John Kalish b, Wilfred Shawcross c, Nicola Stern d, Rainer Grün a, e a

Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia Australian Safeguards and Non-proliferation Office, Department of Foreign Affairs and Trade, R G Casey Building, John McEwan Crescent, Barton, ACT 0221, Australia c 25 Fairfax Street, O'Connor, ACT 2602, Australia d Department of Archaeology, La Trobe University, Melbourne, VIC 3086, Australia e Research Centre of Human Evolution, Environmental Futures Research Institute, Griffith University, Nathan QLD 4111, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 March 2016 Received in revised form 17 October 2016 Accepted 21 October 2016 Available online 21 February 2017

The d18O, Strontium/Calcium and Barium/Calcium values recorded in golden perch otoliths collected from two evaporative lakes, modern Lake Hope and ancient Lake Mungo, have been used to reconstruct changes in water composition and environmental conditions during the life of the fish. Lake Hope was filled by floodwaters in 1989 and 1990, then a period of lake drying was followed by a natural fish death event in 1994. Otoliths from these fish have d18O profiles reflecting the earlier floods, and the progressive evaporation of the lake. Sr/Ca ratios start to follow the d18O trend only after evaporation is well advanced, probably after the fish became stressed. Otoliths from a period of early human occupation at Lake Mungo, 14 C age range ca. 37e42 cal kBP, record a different history. Most otoliths show a relatively stable d18O profile throughout the life of each fish, with no evidence of significant lake flooding or drying. Sr/Ca ratios are similarly stable, indicating that over a period of ca. 5 ka evaporation and inflow remained in relative balance. All the otoliths have high Ba/Ca ratios during the early years of the fish, likely a juvenile biological effect in common. The Mungo otoliths differ, in also showing a rise in Ba/Ca ratios in the outermost layers, as yet unexplained. One Mungo otolith, 14C dated at ca. 19.3 cal kBP, does show evaporation and stress trends in d18O and Sr/Ca ratios respectively, consistent with other evidence that Lake Mungo was subject to frequent drying at that time. © 2016 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Inland archaeological sites in the Australian arid zone contain few palaeoclimate archives. For those archives that do exist, such as sedimentary records, it is difficult to associate the environmental conditions they record directly with the time scales of human occupation. At the world heritage site of Lake Mungo, in north western New South Wales, lake shore lunettes preserve a record of human occupation, and of alternating phases of wet and dry conditions in the adjacent lake (Bowler et al., 2003, 2012; Stern et al., 2013; Stern, 2015). Dunes often form over a much longer time period than human lives, however, making it difficult to identify in

* Corresponding author. E-mail address: [email protected] (K. Long). http://dx.doi.org/10.1016/j.quaint.2016.10.026 1040-6182/© 2016 Elsevier Ltd and INQUA. All rights reserved.

the stratigraphy specific events that would have had a direct impact on past peoples within their lifetimes. Golden perch are fish that can live up to 40 years (Pritchard, 2004), a period comparable to a human life span. As the fish grow so do their ear bones (otoliths), preserving a record of the changing chemical and isotopic composition of the water in which the fish lived. Otoliths are abundant within the Lake Mungo lunettes, preserving an untapped record of past changes in lake levels and environmental conditions that can be associated directly with human occupation. Here, we report the results of a study that explored the use of fish otolith chemical and isotopic signatures to track changes in lake level during the earliest period of human occupation at Lake Mungo. The first part of the study focused on modern otoliths obtained from a mass fish death event at Lake Hope (Pando Penunie), an evaporative lake in South Australia, to test the effect of evaporation and salinity change on golden perch

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K. Long et al. / Quaternary International 463 (2018) 29e43

otolith Sr/Ca, Ba/Ca and Oxygen isotope ratios. The second part used the results of this test to interpret analyses of otoliths excavated from the Lake Mungo shoreline. Those findings were integrated with site stratigraphy and archaeology to determine the relationships between lake level change and evidence for human occupation. The results demonstrate the value of chemical records from excavated otoliths in providing insights into palaeoenvironmental conditions at inland archaeological sites. 1.1. Otoliths Fish otoliths are paired structures that form within the endolymph sac of the inner ears of bony fish. They develop by the incremental deposition of calcium carbonate onto an organic matrix, forming annual growth rings. There are three types of otolith: sagittae, lappili and asteriscii. As in this study, sagittae are most commonly used in research because they are the largest and are composed of dense aragonite (Panfili et al., 2002). Otoliths are often well preserved in archaeological records. They provide information about the size, age and species of fish that were consumed at the site (Casteel, 1976; Nolf, 1985; Colley, 1990; Dubois and Scartascini, 2012). As otoliths grow they take up and preserve a record of the trace element and isotopic compositions of the ambient water (Campana, 1999). Some of these chemical markers are affected by changes in water level and temperature. When otoliths are found in close association with archaeological materials, these records can be related directly to past human occupation (eg. Walker and Surge, 2006; Wang et al., 2013; Amekawa et al., 2016). 1.2. Golden perch (Macquaria ambigua) All otoliths included in this study are from golden perch (Macquaria ambigua) or a closely related subspecies (Lake Eyre golden perch Macquaria ambigua sp.). The golden perch is a long-lived species that is found predominately in lowland, warmer, turbid, slow flowing rivers (Lintermans, 2007). The species can move >1000 km prior to spawning, which in most cases coincides with a rise in water levels (Reynolds, 1983; Ye et al., 2008; King et al., 2009; Faulks et al., 2010). This upstream migration appears to be a method of compensating for the downstream flow of pelagic eggs and larvae (Rowland, 1996). Outside the breeding season, however, adults can occupy areas of 100 m2 for weeks or months before relocating (Lintermans, 2007). Golden perch can grow up to 740 mm in length and 23 kg in weight, but commonly weigh less than 5 kg. They are opportunistic carnivores. Juvenile fish feed on aquatic insect larvae and microcrustaceans whilst adults eat mainly shrimp, yabbies, small fish and benthic aquatic insect larvae (Lintermans, 2007). Typically, thin sections of otoliths are used to age fish, a technique validated for golden perch up to 26 years of age (MallenCooper and Stuart, 2003). Otoliths recovered from golden perch preserved in the Lake Mungo lunette mostly come from relatively young fish, up to about 12 years old (Long et al., 2014; Boljkovac, 2009), but a few otoliths from Garnpung, higher in the lake system, have been aged beyond 40 years (Pritchard, 2004). Golden perch can tolerate a wide range of salinities (0e33‰) and temperatures (4e37 C) and both juveniles and sub-adults can survive in seawater (Langdon, 1987). 1.3. Oxygen isotopes (O isotopes) The O isotope ratios in water change or fractionate due to physical processes such as evaporation and condensation. During evaporation the lighter isotope 16O enters vapour preferentially,

leaving behind a body of water enriched in the heavier isotope, 18O. The change in 18O/16O is expressed as a change in parts per thousand, d18O. About 20% evaporation increases water d18O by about 5‰ (Gonfiantini, 1986). Otolith calcium carbonate is deposited in near O isotopic equilibrium with the surrounding water (Patterson et al., 1993; Thorrold et al., 1997; Campana, 1999), however, species specific effects cannot be ruled out (Høie et al., 2004; Radtke, 1984; Matta et al., 2013). This means that the otolith d18O reflects both the d18O and temperature of the water at the time the otolith was forming. The influence of temperature is about a 1‰ change for every 4.6 C (Kim et al., 2007); higher temperatures lead to a decrease in otolith d18O. Changes in salinity are sometimes used as a proxy for changes in water d18O (Gonfiantini, 1986). Bastow et al. (2002), for example, found a 1‰ increase in otolith d18O for a 10‰ increase in water salinity at sampling sites within the hypersaline Shark Bay. However, Bastow et al. (2002) did not measure ambient water d18O values and it is likely that as salinity increased so too did the d18O of the water due to evaporation. Elsdon and Gillanders, 2002 did identify a slight increase in otolith d18O values from ca. 31.2‰ SMOW to ca. 32.1‰ SMOW with a change of 5e30‰ salinity. There is the possibility that both salinity and ambient water composition interact to influence otolith d18O values but either way otoliths from fish living in an evaporating lake are expected to show a progressive increase in d18O values. Long et al. (2014) used O isotope compositions measured across the age rings of golden perch (Macquaria ambigua) otoliths from Lake Mungo hearth sites and surface finds (Boljkovac, 2009) to identify evaporative lake conditions. This built on and included the pioneering oxygen analyses of ancient golden perch otoliths conducted by Boljkovac (2009). From the otolith O isotope values and radiocarbon dating, Boljkovac (2009) identified two periods of different lake conditions separated by 1000 years, the earliest representing a time of increasing evaporation of the lake (BMLM 007, BMLM 211) and the later indicating lake full conditions (BMLM 158). Possible temperature fluctuations of 1e2‰ were also noted but without knowing the ambient water d18O, temperature values could not be calculated. An increase of 4‰ in otolith d18O leading up to fish death was interpreted as an evaporation effect, and an increase in Sr/Ca ratios was hypothesised to indicate fish entry into the highly saline water body. These interpretations were based on published tank and wild studies of salinity and temperature effects on otolith d18O and elemental Sr/Ca ratios (Kalish, 1991; Elsdon and Gillanders, 2002; Wells et al., 2003; Gillanders, 2005). Little is known, however, about the effects of evaporation on O isotopes and Sr/Ca ratios in golden perch otoliths from Australian ephemeral lakes. The samples from modern Lake Hope (Pando Penunie) analysed here tested the use of O isotopes in golden perch otoliths as indicators of lake evaporation and whether Sr/Ca and Ba/Ca ratios showed similar or different trends. 1.4. Ba/Ca and Sr/Ca ratios in otoliths 1.4.1. Salinity, ambient water composition, temperature and physiology Salinity is often cited as the main influence on Sr/Ca and Ba/Ca ratios in otoliths due to observed differences between marine and freshwater occupation; otolith Sr/Ca ratios increase in marine settings (Brown and Severin, 2009; Gillanders, 2005) whilst Ba/Ca decreases (Elsdon and Gillanders, 2005a). These changes are actually due to differences in the ambient composition of the water, however. Kraus and Secor, 2004 showed that when Sr levels were manipulated under constant salinity and Ca levels there was a significant increase in otolith Sr/Ca between low and high Sr treatments whilst the effect of salinity was insignificant. Chang


et al. (2004) in a similar test of the relationships between water Sr/ Ca, salinity and otolith Sr/Ca found that a salinity change from 5‰ to 35‰ had no effect on otolith Sr/Ca ratios. Given that the Sr/Ca ratio in otoliths is most strongly influenced by changes in water Sr/Ca ratio, as has been shown in experimental studies (Bath et al., 2000; Kraus and Secor, 2004; Collingsworth et al., 2010) and in studies mapping fish migration (Elsdon and Gillanders, 2005b; Arai, 2010; Pangle et al., 2010), then fish living in an evaporating lake would be expected to show no change in Sr/ Ca ratio unless the water Sr/Ca ratio changed. This is unlikely unless water levels were so low and saline as to cause mineral precipitation. The evaporation-related increase in otolith Sr/Ca ratios observed by Long et al. (2014) is probably physiologicaldsomething must affect the ability of the fish to discriminate between Ca and Sr under highly saline conditions and the associated suite of chemical and biological changes in the lake environment. Sr incorporation in fish otoliths is at least partially dependent on temperature and/or physiology. The relationships observed between temperature and otolith Sr/Ca have been inconsistent, ranging from positive (Kalish, 1989; Bath et al., 2000; Collingsworth et al., 2010) to negative (Radtke, 1989; Townsend et al., 1992) to non-existent (Kalish, 1991; Arai, 2010). Townsend et al. (1992) found that otolith Sr/Ca ratios decreased with increasing water temperature (6 mmol mol1 at 2 C, 1 mmol mol1 at 18 C) and observed that at temperatures of 54,400

± ± ± ± ± ± ± ± ±

438 361 377 377 410 1163 429 607 678

Calibrated (cal kBP 95.4%)

Modelled (cal kBP 95.4%)

from

to

from

to

19.5 42.2

19.0 40.7

39.9

37.7

38.2 40.0

35.0 37.7

38.1 38.4 42.0 46.9 39.8 42.1 42.8

36.4 36.4 40.4 42.6 37.4 40.0 40.6

43.4 41.9 43.9

41.1 39.7 41.4

38.4 38.6 41.8 42.2 39.8 41.8 42.1 42.3 42.7 43.0 43.0 43.1 44.3

36.4 36.5 40.2 37.0 37.4 38.8 40.0 40.7 41.1 41.5 41.2 41.5 41.5


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Fig. 9. Radiocarbon dates on otoliths calibrated against ShCal13 (Hogg et al., 2013) and modelled and plotted in Oxcal v4.2.4 (Bronk Ramsey and Lee, 2013).

the otoliths blew in from older sediment exposures further along the lunettes, or were reworked from older sediments through, for example, the burrowing activity of wombats at the excavation site (Shawcross, 1998). The earliest artefacts excavated from Trench B in spit 15, are likely to be older than 42.7e41.1 cal kBP (95.4% probability, Boundary Unit B End). However, this should be regarded as a tentative constraining age as there is no functional association between the dated otoliths and the oldest lithic artefacts.

4.2.2. Analytical results The analyses of 11 ancient otoliths from Trench B are plotted in stratigraphic order in Figs. 7 and 8. A consistent feature of all the O isotopic records is relatively low d18O (0 ± 2‰) early in life, followed by a rise over time to no more than 10‰ at the time of death. Several of the records are virtually flat. Except in A4/3/1, B5/13/4 and B4/14/2, there is minimal evidence for progressive evaporation of the lake, and only in B8/11/1 is there a mid-life drop in d18O that might be associated with fresh water recharge. Sr/Ca ratios in most otoliths stay at or below 5 mmol mol1 for the life of the fish. The most marked exception is otolith A4/3/1, in which the Sr/Ca ratios rise rapidly from ca. 3e12 mmol mol1, coinciding with a rise in d18O, over the 2 years before fish death, a likely sign of increased stress as the lake began to evaporate. A similar but more muted signal is seen in B7/9/1, with coincident rises in d18O and Sr/Ca ratios in the year before the fish died. In all other cases, and particularly B8/11/1, the shifts in d18O and Sr/Ca ratios appear to be independent. All the Mungo otoliths have peaks in the Ba/Ca ratios in the first

one or two years of life, just like the modern otoliths from Lake Hope. They differ from the modern, however, in most also having a peak in the Ba/Ca ratios just before death. Most of the peaks in early life are large, > 0.15 and many >0.2 mmol mol1. Several of the endof-life peaks are also >0.2 mmol mol1. A feature of several trace element patterns not seen in the modern otoliths is a partial correlation between Sr/Ca and Ba/Ca ratios, particularly in mid-life. In A4/3/1, B7/9/1, B8/11/1, B8/12/4 and B5/13/4 in particular, mid-life rises and falls in Sr/Ca and Ba/Ca ratios track very closely. This contrasts with the trends at end of life seen in several otoliths, e.g. A7/8/2, B7/9/1 and B8/12/4, when the Sr/Ca and Ba/Ca ratios diverge. The largest disparity between the d18O and trace element trends is seen in B8/11/1, which was from a layer of beach gravels and quartz sands indicative of full freshwater lake conditions. The rise in d18O between 2 and 4 years of age is not reflected in the Sr/Ca ratios; neither is the plunge in d18O between 5 and 6.5 years. In fact, in the latter case both Sr/Ca and Ba/Ca ratios rise.

5. Discussion The results obtained from modern otoliths provide a basis on which to interpret those from archaeological sites. Only d18O is directly related to ambient water composition during evaporation, a process which depletes the water in 16O, and fresh water recharge, which replaces it. Although Sr/Ca ratios in otoliths commonly rise at the same time as d18O, it is not in itself a reflection of ambient water Sr/Ca ratios or evaporation because Sr/Ca is bioregulated. Only when evaporation raises salinity to a level at which

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bioregulation begins to fail do Sr/Ca ratios in otoliths begin to rise. Ba/Ca ratios show no such trend and appear to be determined entirely by biological processes, particularly when the fish are young. 5.1. Evaporation and flood events 5.1.1. Golden perch from Lake Hope Based on the earlier work by Long et al. (2014) it was expected that the otoliths from fish trapped in evaporating Lake Hope would show a progressive increase in d18O and possibly Sr/Ca ratios leading up to the time of their death, and that was indeed the case in the four otoliths analysed. The Lake Hope otoliths also revealed additional detail, regarding the relationship between Sr/Ca and d18O. Otoliths LH1 and LH130 (Fig. 5) recorded a strong, rapid evaporation between the ages of 1 and 2, four years before Lake Hope dried out. It is known that during periods of flooding fish are transported into Lake Hope from deep waterholes upstream (Turner, 1994). It is also known that golden perch most commonly spawn during periods of increased water level (Reynolds, 1983; Ye et al., 2008; King et al., 2009; Faulks et al., 2010). The birth of these fish coincided with an initial flooding event that brought water to Lake Hope in 1989 (Turner, 1994; Kingsford et al., 1999), but the fish evidently did not enter the lake at that time. The rise in d18O that they recorded subsequently is too rapid for the evaporation of a large body of water. It is more consistent with an evaporating waterhole. The following flood event in 1991 was recorded in the otolith composition as a sudden 11e15‰ drop in d18O. The slow rise in d18O and Sr/Ca ratios that followed is consistent with both fish having been advected into Lake Hope by that flood, eventually to be subjected to environmental and physiological stress and die as that larger body of water evaporated over a period of ca. 3 years. The 3-year rise in d18O was matched by a progressive increase in Sr/Ca ratios, but the rise in Sr/Ca ratios during the first, rapid evaporation event was much smaller and began only midway through the event. Previous tank and wild studies of both freshwater and marine fish have shown that the strongest influence on otolith Sr/Ca ratios is the ambient concentration of Sr and Ca in the water (Elsdon and Gillanders, 2005b; Arai, 2010; Kraus and Secor, 2004). If fish move between different lakes and rivers the Sr/Ca ratios in their otoliths will change if the ambient Sr and Ca concentrations in the water change. The response in such cases is expected to be prompt, not progressive over several years. If the Sr concentration in water always increases with evaporation, then the otoliths should have shown the same increase in Sr/Ca ratios during both evaporation events, but they did not. Some other factor affected otolith Sr/Ca ratios during the second evaporation event, and it was not a factor during the first. It is known that stressful conditions such as very low temperatures (Townsend et al., 1992), disease and extreme evaporation (Kalish, 1992) can influence Sr uptake and deposition in otoliths. Kalish (1989) suggested that the substitution of Sr for Ca in the matrix of otoliths depends on a variety of factors including stress, which possibly influences the mobilisation and availability of Sr and Ca in the blood plasma. In a study of juvenile marine fish trapped in an isolated stream Kalish (1992) found an increase in Sr/ Ca ratios in the outer layers of all fish otoliths that could not be attributed to salinity or temperature effects. This change was interpreted to be a result of stress on the fish as they experienced atypical environmental conditions. This might explain why Sr/Ca ratios began to rise in the Lake Hope otoliths only after evaporation (as recorded by rising d18O) and hence salinity crossed a critical threshold. Otoliths LH2 and LH129 (Fig. 6) did not record the 1991 flood

event, except possibly for the small drop in LH2 d18O at two years of age. Instead d18O increased steadily from birth to death, but as in LH1 and LH130, Sr/Ca ratios began to increase only in the 3rd year, after evaporation had pushed d18OVPDB up to about 5‰. This is consistent with stress being a cause of increasing otolith Sr/Ca ratios. Only when evaporation raised the salinity of the water above a threshold did the fish start to lose their ability to discriminate against Sr. 5.1.2. Golden perch from Lake Mungo The analyses of Lake Hope otoliths show that, in freshwater fish, d18O and Sr/Ca ratios record short-term changes in the ambient environment (changes in water composition due to evaporation and flooding) and the response of the fish to those changes (stress). A similar record has been found previously in the otoliths of fish harvested from Lake Mungo ca. 19 ka ago by indigenous peoples (Long et al., 2014) and in otoliths from the northern lunettes analysed by Boljkovac (2009) also dated to around the LGM (21,000e19,000 BP). The present study examined fish that lived in Lake Mungo at an earlier time, ca. 40 ka ago. A consistent feature of the 11 otoliths analysed was that the fish hatched in relatively fresh water, resulting in otolith d18O of 0 ± 2‰, but soon moved to water with higher values resulting in otolith d18O > 5‰, in which they lived the remainder of their lives. The implication is that the fish hatched in a river during a flood, but soon afterwards moved to the lake, which, because there was no outflow, had elevated d18O due to long-term evaporation. In most cases the d18O of the lake water, and presumably the salinity, remained relatively stable during the whole life of the fish, implying a large body of water steadily recharged by fresh water. The one notable exception is otolith A4/3/ 1 (Fig. 7). Otolith A4/3/1 was recovered from the very top layers of the excavation, the red soil layer of Unit C. Its radiocarbon age places it around the time of the LGM (ca. 19e20 ka) a period of fluctuating lake levels and drying. The trend of increasing d18O and Sr/Ca ratios, similar to those in the Lake Hope otoliths, is consistent with the fish having lived in evaporative conditions in the years leading up to its death. The changes in Ba/Ca ratios are different from those observed in the modern Lake Hope otoliths, however. In contrast to the Lake Hope otoliths, its Ba/Ca ratios do not stay low but rise strongly during the final evaporation event. The same fish species are involved in both cases, so the effect is not species related. More work is needed on golden perch raised under controlled conditions to determine which biological and environmental factors are the main controls of Ba/Ca ratios. The range of otolith Sr/Ca ratios found in the present study is much larger than that previously reported for freshwater fish. It is common to use Sr/Ca and sometimes Ba/Ca profiles in otoliths to determine the timing of fish migration between fresh and marine water. Brown and Severin, 2009, for example, analysed Sr/Ca ratios across the annual increments of 28 freshwater, 21 diadromous and 32 marine fish otoliths. The Sr/Ca ratios in freshwater fish were low and relatively constant (1 mmol mol1) in anadromous fish the values fluctuated between 3 and 4 mmol mol1, and exclusively marine fish had Sr/Ca up to ca. 10e15 mmol mol1. Both modern and ancient otoliths studied here have Sr/Ca ratios well above 1 mmol mol1, reaching ca. 12 mmol mol1 in B7/9/1 and ca. 13e15 mmol mol1 in LH1, even though none of these fish ever entered marine waters. The controls on the incorporation of Sr into fish otoliths are complex, and failing to consider factors such as stress induced by temperature extremes, overcrowding, and rising salinity due to evaporation could lead to erroneous interpretations when reconstructing fish migration histories based on otolith trace elements. None of the ancient otoliths show O isotopic evidence for large


flood events. Only in A4/3/1 was there a distinct (ca. 5‰) drop in d18O that might record a flood (Fig. 7). The spacing of the analyses was too wide to determine the rate of the drop, but given that the otolith was dated to around the LGM when water levels were fluctuating, it is probably a flood signal. It could be argued that the wider spacing of the ancient otolith analyses resulted in evidence of flood events being missed. This is unlikely, however, as the d18O in each otolith changed so little over many years of life. Lake conditions over at least 4 ka remained relatively stable, the effects of evaporation on d18O being balanced by a steady inflow of freshwater due to more effective precipitation in the water catchment (Stern, 2015). The exception is B8/11/1, in which there is d18O evidence that the fish entered Lake Mungo just before a 3 year period during which the inflow of freshwater was high enough to slowly counteract a previous drying trend and drive the lake water back down to a freshwater composition (Fig. 8). 5.2. Seasonality, diagenesis and growth effects With the exception of A4/3/1 the ancient otoliths showed no consistent change in Sr/Ca ratios associated with fish age, so Sr/Ca ratio effects related to somatic growth or penetrative diagenesis can be ruled out. Ba/Ca ratios, however, increased markedly in the outermost layers of all the ancient otoliths, an effect not present in the modern otoliths, so the possibility of some near-surface alteration of Ba/Ca ratios in the ancient otoliths cannot be ruled out. The O isotopic analyses of the ancient otoliths were not spaced closely enough to pick up seasonal fluctuations in every year of the life of the fish, but in some regions of denser sampling (e.g., B8/12/4 years 8e9, B5/13/4 years 6e7, B4/14/2 years 7e9) fluctuations of 1e2‰ were evident. These might relate to seasonal temperature change. B4/15/4 (Fig. 8) was the only otolith sampled with more than 2 spots per age increment throughout. It showed a consistent rise and fall in d18O of 1e2‰, equivalent to ca. 9 C, that might be seasonal variation. Without knowing the d18O of the water at the time we cannot determine specifically what the temperatures were. We also cannot distinguish movement of the fish, to waters of different d18O, from seasonal temperature effects. 5.2.1. Ba/Cadbiology, ambient water composition or diagenesis? Both the modern and ancient otoliths showed relatively high, and sometimes sharply fluctuating, Ba/Ca ratios within the first 1e2 years of life (Figs. 5e8). Although this could be due to high Ba/Ca ratios in the ambient water in which the fish hatched, it is more likely to be a juvenile biological signal. Similar peaks in Ba/Ca have been reported in the juvenile portion of black bream otoliths recovered from archaeological contexts by Disspain et al. (2011). Those peaks, which were not restricted to the nucleus, were interpreted to record periods of freshwater occupation. Martin et al. (2013) studied temporal and spatial variations in Sr/ Ca and Ba/Ca ratios in eight natal rivers and streams in south-west France, and then investigated the relationship between those variations and measurements of Sr/Ca and Ba/Ca ratios in the otoliths of juvenile Atlantic salmon collected from those waterways. They also found a large peak in the otolith Ba/Ca ratios in the early stages of life, which was followed by a progressive decrease to a stable value until death. The changes in otolith Ba/Ca ratios could not be explained by changes in the ambient water Ba/Ca ratios and was therefore interpreted as an ontogenetic signal. The Ba content of river water can vary greatly in space and time (Hanor and Chan, 1977). Durum and Haffty, 1961 found that the concentration of dissolved Ba in the Mississippi River at Baton Rouge varied from 72 to 127 mg/l over the period of a year. Ba/Ca ratios and Ba concentrations are rarely measured in Australian lakes and rivers, and there are no relevant records for Cooper Creek,

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source of the water for Lake Hope, or the Lachlan River, source of the water for the Willandra Lakes. Without such data the fluctuations in otolith Ba/Ca ratios once the Lake Hope and Lake Mungo fish had matured remain difficult to explain. 5.3. Trench B sedimentary record, archaeology and otolith compositions The otoliths from Trench B showed no evidence of burning and were not directly associated with evidence for human occupation in the form of hearths. More than 75% of the artefacts found were from spits 3e6, a period from which few to no otoliths were recovered. Although otoliths were found in most spits, most were found concentrated in spit 10. All otoliths analysed here, except for A4/3/1, were from below spit 7. The sedimentology for those spits suggests that high lake levels and freshwater dominated that period of Lake Mungo's history. Spits 11 to 13 (Fig. 4) are characterised by beach gravels (spit 11) and lacustrine sand and clay (spits 12e13) indicating high freshwater lake levels. The analyses of the otolith from spit 11 (B8/11/1) suggest that the ambient water d18O increased both early and in the final two years of the fish's life, but only by about 6‰, not the >20‰ recorded by fish that died following evaporation of Lake Hope. Past analyses of Lake Mungo surface otoliths (Boljkovac, 2009) and hearth otoliths (Long et al., 2014; Long, 2012) dated to around the LGM (ca 21,000e19,000 years ago) have found similar increases in O isotope values of 4e5‰.The otoliths from Mungo presented here have trace element ratios as low and stable (Sr/Ca 5 mmol mol1) as those previously found in otoliths from Mulurulu, a lake higher in the system and closer to river recharge (Long et al., 2014). Otolith A5/10/1 (Fig. 8) from the large accumulation in spit 10 showed no increase in d18O or Sr/Ca ratio in the years leading up to fish death. There is no evidence that the fish died because Lake Mungo evaporated, consistent with the spit 10 sediments having been deposited when the lake was at high/overflow levels (Walshe, 1987; Shawcross, 1998; Bowler, 1998). The large accumulation of otoliths in spit 10 must have another explanation, e.g. deoxygenation by an algal bloom, a physical concentration by sedimentary processes or a human harvested death assemblage. Whatever the cause of fish death, there is no evidence that the fish was stressed before it died. The d18O in all the Mungo otoliths is consistently high relative to that from fish in modern river systems (Nelson et al., 1989; Dufour et al., 2005) indicating that for the period ca. 38e42 ka Lake Mungo had minimal outflow, allowing 18O to build up through evaporation. This period falls between peaks in glacial activity (Barrows et al., 2001) and following this was a period of high run-off conditions in the Lachlan at ca. 34 ka (Kemp and Spooner, 2007). Steady recharge of the lake from periodic but not substantial floodwaters would keep the evaporation effect low but the overall salinity of the lake high. Observed fluctuations of 1e2‰ in otolith d18O could be related to temperature changes or small flood events. There is no O isotopic evidence for a cycle of flooding and drying like that seen in the otoliths from late in the lake's history (Long et al., 2014). 6. Conclusions The main finding of this study is that chemical and isotopic composition of otoliths from inland archaeological sites is a valuable tool for reconstructing past evaporation and flooding events but temperature information remains elusive. In summary: The only direct evidence for flooding and drying is d18O, although once fish become stressed by the effects of

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evaporation, the rise in otolith d18O is accompanied by a rise in Sr/Ca ratios. Ba/Ca ratios do not behave similarly, being high in the otoliths of juvenile fish and fluctuating apparently independently of environmental conditions thereafter. There is also an increase in Ba/ Ca ratios at the edges of the ancient otoliths, possibly indicating a diagenetic effect. The low d18O in the juvenile stages of otolith growth shows that the golden perch from the modern and ancient Lachlan and Cooper river systems breed during periods of high fresh water flow. Without knowing the ambient water O isotope values, we cannot currently recover temperature data from otolith chemistry alone. Radiocarbon dates on the excavated otoliths support the original OSL chronology for the site. The contents of the excavation, aside from one LGM otolith at the very top and one beyond the limits of radiocarbon dating near the base, represents a period of ca. 35e45 cal kBP. The sediments at the site suggest lake full conditions for most of this period and this is supported by the O isotope and Sr/Ca values of the majority of the otoliths. Only A4/3/1 which was dated to the LGM shows similar trends to those found in LGM hearth otolith (Long et al., 2014) and surface otolith (Boljkovac, 2009).

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