Tropical river flow and rainfall reconstructions from coral luminescence: Great Barrier Reef, Australia

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1 PALEOCEANOGRAPHY, VOL. 22,, doi: /2006pa001377, 2007 Tropical river flow and rainfall reconstructions from coral luminescence: Great Barrier Reef, Australia Janice M. Lough 1 Received 11 October 2006; revised 22 January 2007; accepted 12 February 2007; published 27 June [1] Rainfall and river flow in northeast Queensland, Australia, are highly seasonal and show high interannual and decadal variability that is modulated by El Niño Southern Oscillation (ENSO) events and the Pacific Decadal Oscillation (PDO). Reconstructions of October September freshwater input to the Great Barrier Reef lagoon and October September Queensland rainfall are developed from visual assessment of the occurrence and intensity of luminescent lines in massive Porites from up to 25 coral cores from 15 nearshore reefs regularly influenced by river flood plumes. Separate reconstructions are developed for four rivers (Herbert, Burdekin, Pioneer, and Fitzroy), and these are used to reconstruct total annual freshwater flow into the Great Barrier Reef (69 74% variance calibrated) and an index of Queensland rainfall (53 57% variance calibrated). The reconstructions extend back to 1631 but are most reliable from 1661 and capture significant decadal variability. The reconstructions provide insights into long-term tropical rainfall and river flow variability and the behavior of ENSO and the PDO over several centuries. Significant, though weak, relationships are found between these reconstructions and an independent reconstruction of ENSO. The reconstructions highlight that observations from the instrumental records of high interannual and decadal rainfall and river flow variability in northeast Australia also characterize the past few centuries. Although there appears to be no overall trend toward wetter or drier conditions, the reconstructions suggest that the variability of rainfall and river flow has increased during the twentieth century with more very wet and very dry extremes than in earlier centuries, as projected for the region as a consequence of global warming. Citation: Lough, J. M. (2007), Tropical river flow and rainfall reconstructions from coral luminescence: Great Barrier Reef, Australia, Paleoceanography, 22,, doi: /2006pa Introduction [2] High-resolution proxy climate records provide insights into the nature and causes of past climate variability and assist in detecting current climate changes associated with global warming due to the enhanced greenhouse effect [Jones and Mann, 2004]. Proxy evidence for climate changes over the past millennia is, however, biased toward temperature variables and the northern hemisphere. There is a need to extend such proxy climate information into the tropics, the southern hemisphere and to consider variables other than temperature. Reliable proxy climate reconstructions from the tropics are also of considerable importance in improving our understanding of the major sources of short-term climate variability, El Niño Southern Oscillation (ENSO) events, and decadal variability such as the Pacific Decadal Oscillation (PDO) [Deser et al., 2004; van Oldenborgh and Burgers, 2005; Gergis et al., 2006]. 1 Australian Institute of Marine Science, Townsville, Queensland, Australia. Copyright 2007 by the American Geophysical Union /07/2006PA [3] A variety of records contained in annually banded massive coral skeletons have proved to be important sources of proxy climate information for shallow water tropical ocean regions. The majority of coral proxy records published to date have focused on seawater temperature reconstructions obtained from geochemical tracers such as d 18 O and Sr/Ca ratios (see reviews of Gagan et al. [2000], Cole [2003], and Felis and Patzold [2003]). Coral records are also increasingly being incorporated into multiproxy reconstructions of regional and large-scale climate variables [e.g., Evans et al., 2001, 2002; D Arrigo et al., 2005a, 2006a, 2006b]. The number and geographic coverage of proxy climate records from corals is improving, though many of the best coral records are relatively short [Wilson et al., 2006]. There has also been a reliance on interpreting single coral samples which can affect the reliability of the records and their longterm climatic interpretation [Lough, 2004]. There is a growing recognition, however, that multiple, replicated samples are as fundamental to extracting reliable proxy climate records from corals as they are to dendroclimatology [e.g., Hendy et al., 2002; Suzuki et al., 2005; Smith et al., 2006]. [4] This paper addresses some of the needs identified above by developing robust reconstructions back to the early seventeenth century of tropical river flow and rainfall for northeast Australia from luminescent lines in multiple coral samples from the Great Barrier Reef (GBR), Australia. 1of16

2 The regional climate and environmental setting is first reviewed, followed by the application of luminescent lines in corals to hydroclimate reconstructions. Development of the river flow and rainfall reconstructions is then described, along with their characteristics and some comparisons made with independent proxy climate records. The reconstructed series are available as auxiliary material Regional Climatic and Environmental Setting [5] Average surface climate in northeast Australia and along the GBR is dominated by the southeasterly trade wind circulation in winter and the westerly monsoon circulation in summer [Sturman and Tapper, 1996]. These effectively divide the year into the warmer summer wet season (October to March) and the cooler winter dry season (April to September) and makes the water year, October to September (dated by the January), the most appropriate annual period rather than the calendar year. Tropical cyclones are an important feature of the summer monsoon circulation and can occur on the GBR between November and May with peak activity in January to March [Puotinen et al., 1997]. The summer monsoon brings the majority of annual rainfall (80%) to northeast Australia [Lough, 1991]. Summer rainfall usually occurs in several bursts of activity (typically rain falls on only 30% of days), often linked to the day Madden Julian Oscillation [Suppiah, 1992]. Rainfall also exhibits considerable interannual variability (40% cv). The highly seasonal and variable rainfall regime of northeast Australia results in the highly variable river flows, which are characteristic of Australian rivers in comparison to rivers in other regions of the world [Finlayson and McMahon, 1988]. There are 35 drainage basins in Queensland with rivers flowing eastward into the GBR and whose catchments cover 26% of the land area of the state [Queensland Water Resources Commission, 1980]. About 80% of the total river flow into the GBR occurs between S with highest flows in March, a month after the annual rainfall maximum. [6] The GBR extends for 2000 km adjacent to the coast of Queensland between 8 24 S with the nearly 3000 coral reefs located from directly off the coast up to 200 km offshore. The characteristics of coral reef communities and coral growth rates vary with latitude [Lough and Barnes, 2000]. There are also significant cross-shelf changes in coral community characteristics along a gradient from terrestrially influenced coastal waters to open ocean waters at the shelf edge. The massive Porites corals used in this study are found on all coral reefs throughout the GBR [Veron, 1986]. Modeling and observational studies [Devlin et al., 2001; King et al., 2001a] show that the episodic lowsalinity river flood plumes associated with rainfall events during the wet season annually affect nearshore reefs and occasionally affect some midshelf reefs. The river flood plumes generally move northward close to the coast, driven by the Coriolis force and prevailing winds and have been observed to reach the bottom in shallow coastal waters and still be up to m thick on reaching midshelf reefs during large flood events [King et al., 2001b]. There have 1 Auxiliary materials are available at data.html. been considerable changes in land use in the coastal catchments of the GBR since European settlement in the late nineteenth century with extensive clearing of woodlands for beef production and, to a lesser extent, cropping agriculture and urban development [Devlin et al., 2001]. These changes in land use in northeast Queensland appear to have altered the water quality of nearshore reef environments [McCulloch et al., 2003] with some indications of consequent impacts on algal, coral and fish assemblages [Fabricius, 2005; Fabricius et al., 2005]. Concern about water quality and its effects on the GBR [e.g., Brodie and Mitchell, 2005] has led to the implementation of a state and federal government initiative to improve water quality on the GBR (see gov.au/coasts/pollution/reef/). Delivery of low-salinity waters (which can be detrimental to coral reef organisms), sediment, nutrients and pollutants to reefs of the GBR is achieved during the highly variable river flood events. It is therefore of interest to know whether the chances of delivery of these materials have changed through time. One of the purposes of the present study is to provide this longer-term perspective on the amount of freshwater entering the GBR lagoon. [7] Distinct and different climate anomalies occur in northeast Australia and along the GBR during ENSO events and are partly responsible for the high rainfall and river flow variability. During typical El Niño events, the summer monsoon circulation is weaker than normal and associated with reduced rainfall, river flow and tropical cyclone activity. During typical La Niña events, the summer monsoon circulation is more vigorous than normal leading to increased rainfall, river flow and tropical cyclone activity. ENSO extremes also influence air and sea surface temperatures [Lough, 1994, 2001]. The strength of ENSO teleconnections with northeastern Australian climate is modulated by the Pacific Decadal Oscillation (PDO) [Mantua et al., 1997; Power et al., 1999]. Teleconnections between Australian rainfall and ENSO events are strong, significant and more predictable during PDO cool phases than during PDO warm phases. Additionally, during PDO cool phases there is greater spatial coherence of northeast Australian rainfall anomalies and greater interannual variability (larger extremes) than during PDO warm phases. Significant lowfrequency variability has been documented for rainfall over eastern Australia [Meinke et al., 2005] and a strong control of ENSO and the PDO on rainfall and flood risk in eastern Australia [Kiem et al., 2003; Verdon et al., 2004] with both an enhanced flood risk during La Niña events compared to El Niño events and a further enhancement of La Niña flood risk during cool PDO phases. These decadal variations will also affect river flow entering the GBR, so improved reconstructions of river flow can contribute to understanding these decadal modulations of northeastern Australian hydroclimate. [8] Also of interest is whether Australian rainfall is changing. Analysis of instrumental all-australian rainfall trends over the period identified 1974 as the wettest year and 1902 as the driest year [Smith, 2004]. Australian rainfall appears to have increased since 1901 with most changes occurring in the summer half year and since These changes are, however, dominated by changes over the extensive inland areas of western, northern 2of16

3 and central Australia rather than the higher-rainfall regions of eastern Australia. As climate continues to warm because of the enhanced greenhouse effect, the global hydrological cycle is projected to be enhanced with more extreme droughts and floods [Intergovernmental Panel on Climate Change, 2001]. Regional projections of changes in average rainfall in northeast Australia are, however, less clear. This is due, in part, to the poor ability of current climate models to correctly simulate the Australian summer monsoon [Moise et al., 2005] and the resulting considerable uncertainty among different climate models about the direction and magnitude of rainfall changes [Whetton et al., 2005; Hennessy et al., 2006]. Assessment of regional rainfall and river flow changes is also confounded by the high natural interannual variability and uncertainty as to how ENSO and PDO may change in a warming world. It is likely, however, that a given rainfall deficit in a warmer world will result in more intense droughts because of higher temperatures [Walsh et al., 2002] as already observed in a recent drought [Nicholls, 2004]. Most climate models also suggest that even where changes in average rainfall are small or unclear that the intensity of extreme rainfall events will increase [Walsh et al., 2002]. Thus the frequency of extreme wet and dry years in northeast Australia is likely to increase. Extended reconstructions of rainfall and river flow would provide perspectives on any such longer-term changes. 3. Coral Luminescence as a Proxy for River Flow and Rainfall [9] When placed under ultraviolet (UV) light, slices from certain massive corals display bright luminescent lines. These were first identified in nearshore corals of the GBR and linked to the occurrence and intensity of river flood events [Isdale, 1984; Neil et al., 1995]. The robustness of luminescent lines in massive corals as a proxy for annual river flow and seawater salinity was demonstrated in an analysis of >200 Porites coral colonies from throughout the GBR [Lough et al., 2002]. The average intensity of luminescence is inversely related to the relative distance of the reef across the shelf and the average water depth between a reef and the mainland. Reefs could be divided into three groups: those that never recorded river runoff (>36 m water depth), those that occasionally recorded runoff (20 30 m water depth) and those that recorded runoff every year (<20 m water depth). This study also demonstrated the high reproducibility of the occurrence and intensity of luminescent lines within corals, between corals and between reefs. The occurrence and intensity of luminescent lines recorded in coral skeletons is significantly linked with river flood Figure 1. Annual (October September) (a) Queensland rainfall, ; (b) Herbert River flow, ; (c) Burdekin River flow, ; (d) Pioneer River flow, ; (e) Fitzroy River flow, ; and (f) all rivers freshwater flow into the GBR, Thick line is 10-year Gaussian filtered series. 3of16

4 Table 1. Instrumental River Flow Statistics River Period Median Flow, km 3 Coefficient of Variation, % Maximum Flow Volume, km 3 Year Minimum Flow Volume, km 3 Year Herbert Burdekin Pioneer < Fitzroy All rivers Table 2b. Correlations Between October September Instrumental Rainfall, River Flow, and Niño 3.4 for PDO Warm and Cool Phases a Period Rainfall Rivers Herbert Burdekin Pioneer Fitzroy All Rivers PDO Phase warm cool warm a Bold indicates values significant at 5% level. plumes and modeled salinity variations (1 2% lowering) associated with such flood events [King et al., 2001a]. [10] Several studies have now demonstrated strong links between the occurrence and intensity of luminescent lines and river flows and rainfall in a variety of massive coral species on reefs other than the GBR [Smith et al., 1989; Scoffin et al., 1989; Ramsay and Cohen, 1997; Peng et al., 2002; Nyberg, 2002; Ayliffe et al., 2004]. In addition, the occurrence and intensity of luminescent lines is often used to assist in dating other coral records [Scoffin et al., 1992; Hudson et al., 1994; Cole et al., 2000; Smithers and Woodroffe, 2001; Suzuki et al., 2003; Hendy et al., 2003a, 2003b]. [11] The cause of luminescent lines in massive coral skeletons is still being debated. Originally, they were attributed to terrestrial humic materials transported in the river flood plumes and incorporated into the coral skeletons [Boto and Isdale, 1985; Susic et al., 1991]. A more generalized model [Barnes and Taylor, 2001] suggested that variations in skeletal architecture in response to reduced calcification during periods of low salinity were the primary cause of luminescent lines. This model resolved a number of apparent contradictions in the literature, including situations where luminescent lines were reported in corals that could not have been influenced by terrestrial humic acids (e.g., off southern Oman [Tudhope et al., 1996]). Further experiments have, however, shown that changes in skeletal architecture alone cannot account for the observed intensity of luminescent lines and that there may be substantial contributions from changed crystal size and packing, possibly associated with changes in crystal chemistry [Barnes and Taylor, 2005]. Table 2a. Correlations Between October September Instrumental Rainfall, River Flow, Niño 3.4, and PDO for a Rivers All Niño Rainfall Herbert Burdekin Pioneer Fitzroy Rivers 3.4 PDO Rainfall 1 Herbert Burdekin Pioneer Fitzroy All rivers Niño PDO a Bold indicates values significant at 5% level. [12] There are two previously published reconstructions of river flow into the GBR from coral luminescence. A significantly calibrated (83% variance explained) and verified 337-year reconstruction of Burdekin River flow [Isdale et al., 1998] was developed from luminescence measured in two long coral cores (Havannah Island and Pandora Reef used in this study). There appeared, however, to be some discrepancies between the two coral records and although the reconstruction suggested a long-term drying trend this was due primarily to the contribution of the Havannah Island coral. Visual assessments of luminescence from up to 8 long coral cores (3 from midshelf locations and 5 from nearshore reefs used in this study: Kurrimine, Brook, Havannah, Pandora and Great Palm Island) from the central GBR provided a 373-year accurately cross-dated luminescence chronology that was a good proxy for Burdekin River flow (67% variance explained) and Queensland summer rainfall (42% variance explained) [Hendy et al., 2003a]. These authors found a dating error in the Havannah coral record used by Isdale et al. [1998] and no evidence for the downward trend in river flow suggested in the earlier analysis. The Hendy et al. [2003a] master luminescence chronology (see NOAA paleoclimatic database available at is therefore the more reliable estimate of Burdekin flow and supersedes the Isdale et al. [1998] reconstruction. Variations in the strength of the relationship between the proxy Burdekin record and reconstructed Niño 3 index [Mann et al., 2000] suggest that teleconnections between northeast Australia climate and ENSO have not been stationary over the past few centuries. 4. Data 4.1. Instrumental Data [13] Interannual rainfall variations over Queensland show a high degree of coherence and a relatively small number of stations can be used to capture this variability [Lough, 1991]. An index (expressed as percentage of long-term mean) of October September (water year) rainfall, was used based on 16 stations obtained from the Australian Bureau of Meteorology [Lough, 1997] (Figure 1a). [14] Four rivers were selected based on their proximity to several nearshore coral cores contained within the collection of the Australian Institute of Marine Science [Lough et al., 1999]. Monthly flow volumes were obtained from the Queensland Department of Natural Resources, Mines and Water (available at index.html) and totaled for the October September water year for (1) the Herbert River at Ingham (18.6 S, 4of16

5 Table 3. Details of Coral Cores a Reef Core Identification Latitude S Longitude E Start End Distance From Mainland, km Herbert River Reconstruction Normanby NOR01B Kurrimine KMN02B Dunk DUN02B Coombe COO01B COO01E Brook BRO01A Burdekin River Reconstruction Great Palm GPI01A GPI02A GPI02B Pandora PAN04A PAN04B PAN07B PAN08B PAN09A PAN10B Havannah HAV01A Magnetic MAG01D Pioneer River Reconstruction Hook HKO01B N Molle NMI01B S Molle SMI01C Cid Harbour CID01A Lupton LUP01A LUP01C Fitzroy River Reconstruction Humpy HMP01A HMP01B a See Lough et al. [1999] E) with a catchment area of 8581 km 2, (Figure 1b), (2) the Burdekin River at Clare/Home Hill (19.7 S, E) with a catchment area of 129,870 km 2, (Figure 1c), (3) the Pioneer River at Pleystowe Mill (21.1 S, E) with a catchment area of 1430 km 2, (the Pleystowe Mill gauged record ended in 1988 and the series for was estimated from Mirani Weir) (Figure 1d), and (4) the Fitzroy River at Yaamba (23.1 S, E) with a catchment area of 136,398 km 2, (the Yaamba gauged record ended in 1973 and the series for was estimated from Riverslea) (Figure 1e). The two largest rivers, the Burdekin and Fitzroy, drain 64% of the total catchment area and contribute, on average, 25% of the total runoff into the GBR lagoon [Furnas and Mitchell, 2001]. An estimate of the total freshwater volume entering the GBR lagoon based on instrumental records from all rivers [Furnas, 2003] was totaled for the October September water year, Average statistics of these five series (Table 1) illustrate the extreme interannual variability of river flow in northeast Queensland. [15] ENSO activity was described by the Niño 3.4 sea surface temperature anomaly index (Climate Prediction Center, available at to 2005; extended back to 1872 using HadLSST [Rayner et al., 2003]). Monthly values were averaged for the October September water year. [16] The Pacific Decadal Oscillation (PDO) [Mantua et al., 1997] was described by the PDO index values (leading principal component of monthly SST anomalies in the Pacific Ocean north of 20 N with global average SST anomalies removed to separate any global warming signal). Data from 1900 (available at Table 4a. Details of Coral Luminescence Used to Reconstruct Herbert River Flow a Median Flow for Years 1, km 3 Median Flow for Years 3, km 3 r Value Reef Brook Coombe Kurrimine Dunk Normanby Average a Correlation (r value) between annual luminescence and instrumental river flow from 1916; median instrumental river flow for years with visual luminescence values at individual reefs of 1 and 3. Median flow of the Herbert River is 3.4 km 3. 5of16

6 Table 4b. Summary of Regressions for Herbert River Reconstruction a Number Coral R 2, % Start End a Explained variance (R 2 ) for separate regressions over the calibration period using number of available coral series for each time period of the reconstruction indicated by start and end years. pdo/pdo.latest) were averaged for the October September water year. [17] All instrumental rainfall and river flow series were significantly correlated with each other and with the Niño 3.4 and PDO indices, (Table 2a). The rainfall and river flow series had high interannual variability and none showed any significant trend toward either wetter or drier conditions (Figure 1). All series showed considerable decadal variability: notably wetter/high-flow conditions in the 1950s and 1970s and drier/low-flow conditions in the 1960s. For Queensland rainfall back to 1891, 1902 (the culmination of the Federation Drought ) was the driest year and 1974 the wettest year on record (as found for all-australia rainfall [Smith, 2004]). As noted earlier [Power et al., 1999; Kiem et al., 2003; Verdon et al., 2004], the strength of the relationship between ENSO and Queensland rainfall and river flow is, also, modulated by the PDO phase. During cool PDO periods Queensland rainfall tends to have greater interannual variability, greater spatial coherence of anomalies and strong and significant teleconnections with ENSO. In contrast during warm PDO periods, Queensland rainfall tends to have lower interannual variability, lower spatial coherence of anomalies and the teleconnections with ENSO are not significant (Table 2b) [Lough, 1991]. Figure 2. Reconstructed (solid line) and instrumental (shaded line) October September, Gaussian-filtered river flows for (a) Herbert River, (b) Burdekin River, (c) Pioneer River, and (d) Fitzroy River Reconstructed ENSO and PDO [18] The reconstructed Queensland rainfall and river flow indices were compared with reconstructions of ENSO and PDO. The reconstruction of Niño 3 sea surface temperature (SST) (MAN) index from multiple proxy data sources including tropical series directly affected by ENSO [Mann et al., 2000] and the reconstruction of Niño 3 SST from subtropical North American tree ring chronologies which relies on climate teleconnections with ENSO [D Arrigo et al., 2005b] (DAR) were used. The two series were significantly correlated over the period (r = 0.73) and over the earlier period, (r = 0.43) and are both significantly correlated with the instrumental Niño 3.4 SST index used here, (r = 0.77 (MAN) and r = 0.65 (DAR)). [19] Several reconstructions of the PDO have been developed from tree ring chronologies from western America [Biondi et al., 2001; D Arrigo et al., 2001; Gedalof and Smith, 2001; Macdonald and Case, 2005] and from Chinese documentary rainfall records [Shen et al., 2006]. Although 6of16

7 Table 5a. Details of Coral Luminescence Used to Reconstruct Burdekin River Flow a Median Flow for Years 1, km 3 Median Flow for Years 3, km 3 r Value Reef Havannah Pandora Magnetic Great Palm Average a Correlation (r value) between annual luminescence and instrumental river flow from 1922; median instrumental river flow years with visual luminescence values at individual reefs of 1 and 3. Median flow of the Burdekin River is 6.1 km 3. the different series calibrate significant PDO variance and show significant multidecadal variability, their PDO histories prior to the calibration period of the twentieth century are not necessarily in agreement (not shown; see also [D Arrigo et al., 2005a]). Verdon and Franks [2006] define common PDO phase shifts over the past 400 years from these various PDO reconstructions, which they suggest is achievable with a reasonable amount of coherency between the various proxy series. They conclude that the regime shifts of the twentieth century also characterized the past 400 years with 8 PDO cool phases ( , , , , , , and ) and 8 PDO warm phases ( , , , , , , and ) over the period Reconstructions From Coral Luminescence 5.1. Dating and Measurement of Luminescence Indices [20] Visual assessment of luminescence intensity and dating of the coral cores followed the procedures described by Lough et al. [2002]. Slices of coral were viewed under UV light in a darkened room. For each year and coral slice the appearance of the luminescence lines was ascribed to one of four categories: 0 = no visible line, 1 = faint luminescent line, 2 = moderate luminescent line, and 3 = intense luminescent line. Occasionally very intense luminescent lines were ascribed a value of 3.5. Dated luminescence indices were obtained for 25 coral core slices from 21 different corals from 15 nearshore reefs between S along the GBR (Table 3). This relatively simple approach provides robust indices of luminescence that relate Table 6a. Details of Coral Luminescence Used to Reconstruct Pioneer River Flow a Median Flow for Years 1, km 3 Median Flow for Years 3, km 3 r Value Reef Hook Cid Lupton S Molle N Molle Average a Correlation (r value) between annual luminescence and instrumental river flow from 1917; median instrumental river flow for years with visual luminescence values at individual reefs of 1 and 3. Median flow of the Pioneer River is 0.5 km 3. to proximity to river plumes, river flow and rainfall and the luminescent indices are easily cross dated between different corals ensuring high dating accuracy [Lough et al., 2002; Hendy et al., 2003a]. Dated luminescence indices from coral cores from the same coral and/or reef were averaged to form a single index for each of the 15 reefs. Linear regression used to develop the reconstructions of river flow and rainfall with separate regressions developed according to the available series which were then spliced together Reconstructing River Flows Herbert River [21] Herbert River flow was reconstructed from visual luminescence indices from 6 coral cores from 5 nearshore reefs (Table 3). All luminescence series and the 5-reef average series were significantly correlated with instrumental Herbert flow and showed a clear separation between observed flows in years with visual luminescence values 1 and 3 (Table 4a). The correlation between the 5-reef average series and Herbert river flow was stable through time (r = 0.76, and r = 0.72, ). A reconstruction of Herbert River flow was developed from linear regression of the average luminescence series of between one and five coral series for the period and was most reliable with three or more cores, , with 53 55% variance explained (Table 4b). Over the period , 69% of the instrumental decadal variability was explained (Figure 2a) Burdekin River [22] Burdekin River flow was reconstructed from visual luminescence indices from 6 coral cores from 4 nearshore reefs (Table 3). All luminescence series and the 4-reef average series were significantly correlated with instrumental Burdekin flow and showed a clear separation between observed flows in years with visual luminescence values 1 and 3 (Table 5a). The correlation between the 4-reef Table 5b. Summary of Regressions for Burdekin River Reconstruction a Number Coral R 2, % Start End a See Table 4b. Table 6b. Summary of Regressions for Pioneer River Reconstruction a Number Coral R 2, % Start End a See Table 4b. 7of16

8 Table 7a. Calibrated Variance for All Rivers Reconstruction and for Two Subperiods a Number Reconstruction Reconstruction Instrumental Reconstruction a Percent variance calibrated by instrumental river flow records also given. average series and Burdekin River flow was stable through time (r = 0.84, and r = 0.82, ). A reconstruction of Burdekin River flow was developed from linear regression of the average luminescence series of between one and four coral series for the period and was most reliable with two or more cores, , with 64 66% variance explained (Table 5b). Over the period , 81% of the instrumental decadal variability was explained (Figure 2b) Pioneer River [23] Pioneer River flow was reconstructed from visual luminescence indices from 6 coral cores from 5 nearshore reefs (Table 3). All luminescence series and the 5-reef average series were significantly correlated with instrumental Pioneer flow and showed a clear separation between observed flows in years with visual luminescence values 1 and 3 (Table 6a). The correlation between the 5-reef average series and Pioneer River flow was stable through time (r = 0.84, and r = 0.79, ). A Figure 3. Reconstructed October September (a) all-rivers index as anomaly from long-term median flow and (b) Queensland rainfall as percentage of long-term mean, Solid line is 10-year Gaussian filter. For both series the instrumental observations have been added from 1984 to 2005 and shown in light shading. 8of16

9 Table 7b. Calibrated Variance for Queensland Rainfall Reconstruction and for Two Subperiods a Number Instrumental Reconstruction Reconstruction Reconstruction a Percent variance calibrated by instrumental river flow records also given. reconstruction of Pioneer River flow was developed from linear regression of the average luminescence series of between one and five coral series for the period and is most reliable with three or more cores, , with 62 64% variance explained (Table 6b). Over the period , 74% of instrumental decadal variability was explained (Figure 2c) Fitzroy River [24] Fitzroy River annual flow was reconstructed from visual luminescence indices from 2 coral cores from the same coral (Table 3). Both luminescence series were significantly correlated with instrumental Fitzroy flow (r = 0.70, ) and showed a clear separation between observed flows for years with visual luminescence values 1 (1.0km 3 ) and 3 (14.1 km 3 ); median flow of the Fitzroy River is 3.0 km 3. The correlation between the average series and Fitzroy River flow was stable through time (r = 0.64, and r = 0.79, ). A reconstruction of Fitzroy River flow was developed from linear regression of the average luminescence series of between one and two coral series for the period , based on one core and explaining 49% variance and from based on 2-core average series and explaining 50% variance. Over the period , 53% of instrumental decadal variability was explained (Figure 2d) All Rivers Freshwater Input to Great Barrier Reef [25] All rivers, October September, freshwater input to the GBR was reconstructed from the four river reconstructions described in the preceding section: Burdekin ( ), Pioneer ( ), Fitzroy ( ) and Herbert ( ). Separate regression models were developed with 1 to 4 rivers over the calibration period, and this calibration was stable over 2 subperiods. Instrumental river flows from the same rivers would explain 82 95% of the all rivers index (Table 7a). The reconstruction (Figure 3a) covers the period with greatest reliability from 1660 with 69 74% variance explained. Over the period , 90% of instrumental decadal river flow variability was explained Queensland Rainfall [26] Queensland, October September, rainfall was reconstructed from the four individual river reconstructions. Separate regression models were developed with 1 to 4 rivers over the calibration period, and this calibration was stable over 2 subperiods. Instrumental river flows from the same rivers would explain 46 60% of the rainfall index (Table 7b). The rainfall reconstruction (Figure 3b) accounts for 15% less variance than for the river flow reconstruction and was also considered to be most reliable from 1660 with 56 58% variance explained. Over the period , 73% of instrumental decadal rainfall variability was explained. 6. River and Rainfall Reconstructions 6.1. Characteristics [27] The two reconstructed series (Figures 3a and 3b) were obviously very similar but do display some differences in terms of extremes (Table 8). Both showed the high interannual and decadal variability that characterizes the instrumental records of the twentieth century (Figures 1a and 1d) and capture, for example, the wetter 1950s and 1970s and drier 1960s. Comparable wet decades were reconstructed in the late nineteenth and mid seventeenth centuries. The wettest 30-year period was reconstructed in the twentieth century and the driest in the mid nineteenth century. Importantly, neither series shows a significant trend toward either wetter or drier conditions over the past 3 1 = 2 centuries. The wettest and driest years and 10-year periods were, however, all reconstructed during the twentieth century. [28] The highly variable nature of northeastern Queensland rainfall and river flows means that it is the extremes, droughts and floods, that are of most significance and which the luminescence indices appear to reliably capture (see comparisons of observed median flows for extremes of visual indices in Tables 4a 6a 6b). To test whether the reconstructions showed any evidence for long-term changes in extremes, the reconstructed river flow series, , was sorted into deciles and each year classified as very high, high, average, low or very low flows (Table 9). The frequency of these extremes was calculated Table 8. Annual, 10-Year and 30-Year Extremes of Reconstructed All-Rivers and Queensland Rainfall a Maximum Minimum All Rivers Rainfall All Rivers Rainfall Annual Period Year Period Year Period a Bold indicates 10- or 30-year period significantly different from longterm mean at 5% level. 9of16

10 Table 9. Reconstructed October September Freshwater Flow Into GBR From Coral Luminescence: Classification of Extremes, and Instrumental Record Flow in 1600s Flow in 1700s Flow in 1800s Flow in 1900s Flow in 2000s Year River Year River Year River Year River Year River 1631 average 1700 very high 1800 low 1900 very low 2000 high 1632 average 1701 low 1801 high 1901 average 2001 average 1633 low 1702 low 1802 average 1902 lowest 2002 low 1634 high 1703 average 1803 high 1903 average 2003 low 1635 very low 1704 average 1804 average 1904 low 2004 average 1636 low 1705 average 1805 low 1905 average 2005 low 1637 high 1706 high 1806 average 1906 very high 1638 very high 1707 average 1807 average 1907 very low 1639 high 1708 very high 1808 average 1908 high 1640 low 1709 average 1809 average 1909 very low 1641 low 1710 very low 1810 average 1910 very high 1642 very high 1711 very high 1811 high 1911 very high 1643 low 1712 average 1812 average 1912 low 1644 high 1713 average 1813 low 1913 high 1645 low 1714 low 1814 average 1914 average 1646 low 1715 low 1815 average 1915 low 1647 low 1716 average 1816 average 1916 low 1648 very low 1717 high 1817 very high 1917 very high 1649 very high 1718 average 1818 low 1918 very high 1650 low 1719 low 1819 high 1919 very low 1651 very low 1720 average 1820 average 1920 low 1652 high 1721 average 1821 low 1921 average 1653 low 1722 average 1822 average 1922 high 1654 high 1723 low 1823 low 1923 very low 1655 high 1724 average 1824 very low 1924 average 1656 low 1725 average 1825 high 1925 average 1657 average 1726 high 1826 high 1926 very low 1658 average 1727 high 1827 very low 1927 high 1659 very low 1728 very low 1828 average 1928 high 1660 average 1729 average 1829 low 1929 average 1661 average 1730 average 1830 low 1930 high 1662 average 1731 high 1831 high 1931 very low 1663 very high 1732 high 1832 low 1932 low 1664 very high 1733 high 1833 average 1933 average 1665 average 1734 average 1834 average 1934 high 1666 low 1735 average 1835 average 1935 very low 1667 very low 1736 average 1836 low 1936 high 1668 low 1737 average 1837 average 1937 average 1669 average 1738 low 1838 high 1938 low 1670 high 1739 high 1839 low 1939 low 1671 average 1740 average 1840 high 1940 very high 1672 average 1741 low 1841 average 1941 average 1673 average 1742 average 1842 very low 1942 average 1674 average 1743 average 1843 low 1943 average 1675 low 1744 average 1844 average 1944 average 1676 average 1745 average 1845 very low 1945 low 1677 high 1746 average 1846 average 1946 average 1678 average 1747 average 1847 high 1947 high 1679 average 1748 low 1848 high 1948 very low 1680 average 1749 high 1849 average 1949 very high 1681 low 1750 high 1850 average 1950 very high 1682 low 1751 low 1851 average 1951 high 1683 average 1752 average 1852 average 1952 very low 1684 average 1753 average 1853 very low 1953 average 1685 high 1754 high 1854 low 1954 very high 1686 high 1755 high 1855 average 1955 very high 1687 average 1756 high 1856 average 1956 very high 1688 average 1757 average 1857 high 1957 average 1689 average 1758 high 1858 very low 1958 very high 1690 average 1759 high 1859 high 1959 high 1691 low 1760 average 1860 high 1960 average 1692 average 1761 very high 1861 low 1961 low 1693 low 1762 average 1862 very low 1962 average 1694 very high 1763 average 1863 average 1963 high 1695 low 1764 high 1864 very high 1964 low 1696 low 1765 high 1865 low 1965 low 1697 average 1766 average 1866 average 1966 very low 1698 low 1767 low 1867 average 1967 low 10 of 16

11 Table 9. (continued) Flow in 1600s Flow in 1700s Flow in 1800s Flow in 1900s Flow in 2000s Year River Year River Year River Year River Year River 1699 average 1768 average 1868 low 1968 very high 1769 high 1869 very low 1969 very low 1770 low 1870 very high 1970 low 1771 average 1871 very low 1971 very high 1772 average 1872 average 1972 high 1773 average 1873 very high 1973 low 1774 low 1874 average 1974 highest 1775 low 1875 average 1975 average 1776 high 1876 average 1976 high 1777 average 1877 average 1977 high 1778 average 1878 high 1978 average 1779 average 1879 high 1979 very high 1780 average 1880 average 1980 very low 1781 very low 1881 average 1981 high 1782 high 1882 high 1982 very low 1783 average 1883 average 1983 high 1784 low 1884 average 1984 average 1785 average 1885 low 1985 low 1786 high 1886 very low 1986 low 1787 average 1887 high 1987 low 1788 high 1888 average 1988 average 1789 average 1889 low 1989 high 1790 high 1890 very high 1990 average 1791 average 1891 very high 1991 very high 1792 average 1892 very low 1992 very low 1793 average 1893 average 1993 low 1794 very low 1894 very high 1994 low 1795 very high 1895 very high 1995 very low 1796 average 1896 very high 1996 average 1797 average 1897 average 1997 average 1798 average 1898 average 1998 average 1799 low 1899 high 1999 average for three subperiods (Table 10). These results suggest that the most recent period has experienced more frequent very high and very low flow extremes than in earlier centuries, even though there appears to be no overall trend toward either higher or lower flows, i.e., that the variance rather than the mean of river flow in northeast Queensland has changed. (Similar results were obtained for reconstructed Queensland rainfall, not shown.) In addition, the percentage of coefficient of variation (cv) of the reconstructed river flow series for the 3 subperiods was 50.1%, 48.1%, and 64.3%, respectively, again suggesting an increase in variability in the most recent period. A similar conclusion was drawn by Hendy et al. [2003a] from their analysis of Burdekin River flow and they suggested that this could reflect increasing magnitude of ENSO events though the authors caution that this could also be due to more series being included. [29] Two additional analyses were performed to test whether this suggested recent increase in river flow/rainfall variability was real and not an artifact of the varying number of coral series through time. First, a reconstruction of river flow from the 4 longest continuous coral series (Pandora 08B, Havannah 01A, Hook 01B and Humpy 01B), , showed the same increase in variance (though similar median flows) between and with the percentage cv increasing from 56.1% to 64.3%. The anomaly of percentage of years for very high and very low flow events changed from 5%, to +7%, (see Table 10). Second, the frequency of luminescent lines at three midshelf reefs, which only occasionally experience the largest flood events [Lough et al., 2002], was examined. At Otter Reef (18 S, 60 km from land) the observed frequency of luminescent lines (i.e., large flood events reaching the reef) decreased from every 14 years ( ) to every 5 years ( ). At Rib Reef (18.5 S, 56 km from land), the frequency decreased from every 4.3 years ( ) to every 2.4 years ( ). At Wheeler Reef (19 S, 70 km from land) the frequency halved from every 20 years ( ) to every 9 years ( ). Both these Table 10. All-River Reconstruction: Percentage of Years Classified According to Flow Level Expressed as Difference From Expected Percentage for Three Subperiods Flow Class Very low 4% 2% +6% Low +6% 1% 5% Average +4% +3% 10% High 4% +7% 1% Very high 2% 7% +10% Very low and low +2% 4% +2% Very high and high 6% +1% +9% Very high and very low 6% 9% +16% 11 of 16

12 Table 11. Correlations Between Instrumental ( i ) and Reconstructed ( r ) Rainfall, River Flow, and ENSO Indices for , PDO Warm Phase, and PDO Cool Phase a Riv r Riv i RF r RF i N3 DAR N3 MAN N34 i Riv r 1 Riv i RF r RF i N3 DAR N3 MAN N34 i PDO Warm Phase Riv r 1 Riv i RF r RF i N3 DAR N3 MAN N34 i PDO Cool Phase Riv r 1 Riv i RF r RF i N3 DAR N3 MAN N34 i a Values in bold are significant at 5% level. Abbreviations are Riv, river; RF, rainfall; N3 DAR, reconstructed Niño 3 SST index [D Arrigo et al., 2005b]; N3 MAN, reconstructed Niño 3 SST index [Mann et al., 2000]; and N34 i, instrumental Niño 3.4. analyses support an increase in river flow/rainfall variability since the late nineteenth century Comparisons With Other Proxy Climate Records [30] Independent and significantly calibrated proxy climate records from a range of locations is the first step in piecing together the nature and causes of past climate variability and change and detecting current climate changes. Do the reconstructed river flow and rainfall indices for northeast Australia show any significant relationships with proxy ENSO records? This would increase the confidence in the reliability of the reconstructions developed here from GBR coral luminescence. [31] Over the common period, , the instrumental and reconstructed river flow and rainfall indices were significantly correlated with the instrumental Niño 3.4 ENSO index and the reconstructed Niño 3 indices [Mann et al., 2000; D Arrigo et al., 2005b] (Table 11). The relationships with the instrumental and reconstructed ENSO indices were not significant for the PDO warm period, (Table 11, compare Table 2b) and significant for the PDO cool period, (Table 11). [32] Over the earlier common period, , the reconstructed rainfall and river flow indices were weakly, but significantly correlated with the Mann et al. [2000] Niño 3 index but only marginally significant with the D Arrigo et al. [2005b] index (Table 12). Overlapping 21-year correlations between the reconstructed river flow and Mann Niño 3 index (Figure 4a) show that the magnitude and significance of the correlations and the variability of the reconstructed flow (Figure 4b) have varied over time as would be expected if the PDO modulation of ENSO teleconnections was also operating in earlier centuries. Periods of significant correlations, which may represent past PDO cool phases were: , , , , and Two notable episodes of extremely weak correlations occurred in the late seventeenth century and the first half of the nineteenth century (also noted by Mann et al. [2000] and D Arrigo et al. [2005b]) which may be PDO warm phases comparable to that of the middle decades of the twentieth century when ENSO teleconnections broke down. [33] The PDO phase shifts suggested from analyses of various proxy PDO indices over the past four centuries [Verdon and Franks, 2006] were used to compare the correlations between the reconstructed river flow series and the Mann et al. [2000] Niño 3 index. For years classified as PDO cool phases, the correlation was 0.35 (n = 170) and for years classified as PDO warm phases, the correlation was 0.24 (n = 149) over the period Both correlations were significant at the 5% level. For the independent period of the river flow reconstruction (i.e., prior to 1924), the correlations were also significant but did not show such a marked difference between PDO cool phases (r = 0.24, n = 137) and PDO warm phases (r = 0.28, n = 125). A similar analysis using the three PDO warm and three PDO cool phases identified from Chinese documentary drought and flood indices [Shen et al., 2006] prior to 1880 gave a significant correlation of 0.23 (n = 124) for cool phases and a nonsignificant correlation of 0.15 (n = 107) for warm phases. A similar, inconclusive result was obtained when the North Pacific Index reconstruction [D Arrigo et al., 2005a] was used to identify warm and cool phase correlations between reconstructed river flow and the Niño 3 index: r = 0.26 (n = 132) for warm phase periods and r = 0.28 (n = 140) for cool phase periods prior to [34] A final test compared the extremes of the reconstructed river flow and Mann et al. [2000] Niño 3 index. Over the independent period of the reconstruction ( ), the standardized Niño3 index was used to define El Niño extremes (index 1 sd, 47 years) and La Niña extremes (index 1sd, 34 years). Over this period the average reconstructed river flow was 22.8 ± 12.2 km 3. The average reconstructed river flow for the El Niño years was 19.7 ± 10.6 km 3 (t value = 1.88, not significantly different from the long-term mean) and for the La Niña years was 30.8 ± 14.6 km 3 (t value = 3.40, significantly different Table 12. Correlations Between Reconstructed Rainfall, River Flow, and ENSO Indices for a Riv r RF r N3 DAR N3 MAN Riv r 1 RF r N3 DAR N3 MAN a Bold indicates significant at 5% level. 12 of 16