Source of trace element variability in Great Barrier Reef corals affected by the Burdekin flood plumes

Geochimica et Cosmochimica Acta, Vol. 67, No. 2, pp. 231–246, 2003 Copyright © 2003 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/03 $22.00 ⫹ .00

Pergamon

PII S0016-7037(00)01055-4

Source of trace element variability in Great Barrier Reef corals affected by the Burdekin flood plumes CHANTAL ALIBERT,1,* LES KINSLEY,1 STEWART J. FALLON,1 MALCOLM T. MCCULLOCH,1 RAY BERKELMANS,2 and FELICITY MCALLISTER3 1

Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia 2 Great Barrier Reef Marine Park Authority, Townsville, Queensland 4810, Australia 3 Australian Institute of Marine Science, Townsville, Queensland 4810, Australia (Received November 28, 2001; accepted in revised form July 5, 2002)

Abstract—Massive corals in the Great Barrier Reef, analyzed at high-resolution for Sr/Ca (thermal ionization mass spectrometry) and trace elements such as Ba and Mn (laser ablation inductively coupled plasma mass spectrometry), can provide continuous proxy records of dissolved seawater concentrations, as well as sea surface temperature (SST). A 10-yr record (1989 to 1998) from Pandora Reef, an inshore reef regularly impacted by the freshwater plumes of the Burdekin River, is compared with an overlapping record from a midshelf reef, away from runoff influences. Surface seawater samples, taken away from river plumes, show little variability for Sr/Ca (8484 ⫾ 10 ␮mol/mol) and Ba (33.7 ⫾ 0.7 nmol/kg). Discrete Ba/Ca peaks in the inshore coral coincide with flood events. The magnitude of this Ba/Ca enrichment is most likely controlled by the amount of suspended sediments delivered to the estuary, which remains difficult to monitor. The maximum flow rate at peak river discharge is used here as a proxy for the sediment load and is shown to be strongly correlated with coral Ba/Ca (r ⫽ 0.97). After the wet summer of 1991, the coral Ba/Ca flood peak is followed by a plateau that lingers for several months after dissipation of plume waters, signifying an additional flux of Ba that may originate from submarine groundwater seeps and/or mangrove reservoirs. Both Mn and Y are enriched by a factor of ⬃5 in inshore relative to midshelf corals. Mn/Ca ratios show a seasonal cycle that follows SST (r ⫽ 0.7), not river discharge, with an additional high variability in summer suggesting a link with biological activity. P and Cd show no significant seasonal variation and are at a low level at both inshore and midreef locations. However, leaching experiments suggest that part of the coral P is not lattice bound. Copyright © 2003 Elsevier Science Ltd long-term trend back to the early 1900s, as illustrated in Figure 1A (Nicholls and Kariko, 1993; Lough, 1993; Suppiah and Hennessy, 1995; Power et al., 1998; Hennessy et al., 1999). This study is aimed at quantifying relationships between riverine inputs to the GBR waters and trace element records in massive corals. For this purpose, a coral from an inshore reef affected by flood plumes was compared with a coral from a midshelf reef, away from runoff influences. Sr/Ca and Ba were also analyzed in seawater samples collected across the GBR Lagoon, away from river plumes. To monitor riverine inputs, the trace elements Ba, Mn, P, Y, and Cd, found at low concentration in the open ocean, have been measured in the coral skeleton by laser ablation inductively coupled plasma mass spectrometry (ICPMS). High-precision Sr/Ca measurements, fitted to instrumental sea surface temperature (SST) variations, have been used to determine the timing for the Ba or Mn enrichments observed in the coral in an effort to better understand processes at work in coastal waters. We also examined the relationship between coral Ba/Ca, river discharge, and rainfall variability in North Queensland.

1. INTRODUCTION

The impact of increasing agricultural and urban development, use of fertilizers and pesticides, and loss of coastal wetlands on river water quality in North Queensland, Australia, and the subsequent delivery of nutrients, sediments, and pollutants into the Great Barrier Reef (GBR) lagoon, are now recognized as a threat to GBR ecosystems (Brodie, 1998; Furnas et al., 1998; Brodie et al., 2001; Furnas and Mitchell, 2001; special issue of Marine Pollution Bulletin, 2001). Sediment and nutrient export fluxes to the ocean have been assessed (Moss et al., 1992; Neil and Yu, 1995; Furnas et al., 1998), but there are still few data about the major cation and trace element composition of groundwater and river waters, particularly along salinity gradients that extend far from estuarine mixing zones during major floods. Such data are needed to quantify current exports to coastal waters and the open ocean. Models of erosion and sediment transport have suggested a fourfold increase in the sediment export since European settlement ca. 1850 as a result of exacerbated erosion in grazing lands. Recent hydrological models by Prosser et al. (2002) have produced a comparable estimate of a sixfold increase. Proxy data are necessary to document this major change and an anthropogenic signal should be easily identifiable against the rainfall record for northeast Australia, which does not show any significant

* Author to whom correspondence ([email protected]).

should

be

2. REGIONAL SETTINGS

2.1. Burdekin Catchment The Burdekin River basin drains a large area of 130,000 km2 between the latitudes 18°S and 24°S and the longitudes 144.5°E and 148.5°E. A poor vegetation cover, along with intense summer tropical storms, favor runoff as the main source

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Fig. 1. (A) The average summer rainfall (standardized anomalies) for six stations in the Burdekin catchment between 1885 to 1998, using monthly data from the Australian Bureau of Meteorology, is compared with (B) the Burdekin discharge (December to April) near the river mouth at Home Hill and Clare, back to 1922. Both records are characterized by strong interannual and decadal variability, the latter outlined by 5-yr running mean trends (bold line). There is no significant long-term upward or downward trend (dotted line). (C) The histogram of the Burdekin MFR, using 5-d smoothed data, shows that 54% of the data fall in the range 1600 to 4700 m3/s. (D) The relationship between average rainfall and the summer discharge anomalies, back to 1922, is best fitted by a logarithmic regression (solid line, r ⫽ 0.91, significant above the 99.9% confidence level calculated for 12 effective degrees of freedom, after Davis, 1976) or a power regression (dashed line, r ⫽ 0.87).

of freshwater to the sea. Groundwater from the delta is likely to contribute to this flux only during the dry season (Bristow et al., 2001). The upper catchment has a long history of grazing back to the 1850s, and the lower catchment is a major region of sugarcane production. An assessment of the impact of agricultural practices on the coastal environment (Moss et al., 1992) has established that areas where grazing is the dominant land use represent ⬃90% of the total Burdekin-Haughton catchment, with less than 2% of remaining pristine areas, mainly mangrove, rainforest, and woodlands. According to Barson et al. (2000), most of the clearing occurred during the 1950s and the 1970s, but extensive clearing is still continuing. The completion of the Burdekin Falls Dam in the mid-1980s has reduced flooding in the lower reaches, except when associated with heavy coastal rainfall, and retains a large part of the coarse sediment load. The Burdekin River is the largest contributor of sediments to the central GBR, with estimates between 3 to 8.5 ⫻ 106 tonnes/yr (Moss et al., 1992; Neil and Yu, 1995; Furnas, personal communication), based on limited measurements of

suspended sediments. Recent hydrological models (Prosser et al., 2001a,b, 2002) predict a mean annual sediment load for the Burdekin of 2.4 ⫻ 106 tonnes/yr and indicate a relatively high sediment delivery ratio of ⬃13% that results from the enhanced sediment transport on hill slopes during summer tropical storms. The latter authors also show that most of the sediment load is delivered to the coast from grazing lands in the lower part of the catchment. 2.2. Relation between Regional Precipitation and the Burdekin Flow Annual rainfall in North Queensland, dominated by heavy summer rainfall events associated with monsoon depressions or tropical cyclones (e.g., Lough, 1994), decreases rapidly inland from the coastal region. Townsville receives an average of 1039 mm of rain between November to April, which is about twice as much as inland. Summer rainfall is characterized by its high interannual variability of 43% relative standard deviation

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(rsd), mainly related to El Nin˜ o-Southern Oscillation (ENSO) (e.g., McBride and Nicholls, 1983), and interdecadal variability, with wet periods occurring in clusters such as during the mid-1950s, mid-1970s, 1988 to 1991, and currently since 1998 (Fig. 1A). An average record for the Burdekin catchment region was estimated from six stations, part of the high-quality historical precipitation data set of Lavery et al. (1997). Two stations were selected near the coast (station 32040 Townsville, 19.25°S, 146.76°E, 1894 to 1998, and station 33035 Home Hill), with the other stations in the upper catchment (station 32018 Glen Eagle, station 32044 Valley of Lagoons, station 33082 Strathmore, and station 34000 Balfes Creek P.O.). The good coherence between these records ensures that they capture a pattern of rainfall variability representative of the vast catchment. Precipitation anomalies blended for the six stations are compared in Figure 1 to the Burdekin discharge calculated from daily flow gauge data from the Queensland Department of Natural Resources (DNR) (available at http://www.dnr.qld.gov.au) from Home Hill (1921 to 1953) and Clare since 1951. Summer flow and maximum flow rate (MFR) vary by an order of magnitude from year to year (Fig. 1C). A majority of the flood events produce 1 to 9 millions of megaliters, with a MFR between 1600 to 4700 m3/s (Fig. 1B). During February 1991, the MFR reached 14,000 m3/s during a few days. By comparison, the Fly and Sepik Rivers (Papua New Guinea) have average discharges of 4470 m3/s and 3800 m3/s, respectively (Meybeck and Ragu, 1997). Figure 1D shows the relationship between rainfall and river discharge.

2.3. Corals and River Plumes The flood plumes of the Burdekin stretch several hundreds of kilometers northward, regularly impacting inshore corals between 19.6°S and 17°S, and also some midshelf corals, principally north of Palm Islands where the isobaths, and hence reefs, come closer to the coast. Pandora Reef (18.8°S, 146.4°E) is a small reef near Palm Island, 17 km from the shore and 175 km to the north of the Burdekin River mouth (Fig. 2). Flood plumes from the Herbert River are deflected to the north and rarely affect this region except when high discharge is combined with northwesterly winds. Modeling of the Burdekin River plumes back to 1966 (King et al., 2001; also available at http://www.aims.gov.au) developed from observations of river flow and wind and physical oceanographic parameters shows that the brackish waters usually reach the Palm Isles after 1 to 2 weeks and have an average residence time from a few weeks up to 2 months, depending mainly on the magnitude of the discharge and also on the wind speed and direction (dominant southeasterlies). The mixing zone, corresponding to the 0 to 10 salinity range, extends northward toward Cape Bowling Green (Fig. 2), and a steep decrease of fine suspended sediments has been reported in that region (Wolanski and Jones, 1981; Orpin et al., 1999) . Davies Reef (18.8°S, 147.7°E), ⬃70 km east-northeast of Townsville, is out of reach of river plumes (Fig. 2), except during the 1974 major floods, when salinity may have fallen to 33 (King et al., 2001).

Fig. 2. Salinity map for the 1990 moderate flood of the Burdekin River, after King et al. (2001), with Pandora and Davies reefs outlined (star). Seawater samples were collected at Myrmidon, Davies and Yongala Wreck reefs, and along a transect off Cairns, to the north of this map. The two major rivers are the Burdekin to the south of Townsville and the Herbert to the north near Hinchinbrook Island.

3. METHODS 3.1. Water Collection Water samples I to VIII were collected along a transect offshore from Cairns (17°S) in July 1996 on board the R.V. Lady Basten Australian Institute of Marine Science (AIMS). Temperature and salinity were obtained by CTD casts. Stations I and II, on the shelf slope off Norman Reef and Euston Reef, respectively, were ⬃200 m above the seafloor, and waters were taken at depths of 1, 100, and 150 m. Stations V to VIII were along the coast, 12 to 20 m above the seafloor, between Cape Tribulation and the Low Isles, with station VI in front of the Daintree River mouth. Samples of seawater (1 L) were immediately drawn from an allocated PVC Niskin bottle and half of it filtered (0.45 ␮m; Durapore) on board. The filtering unit was rinsed between samples with Milli-Q water. Both filtered and unfiltered waters were stored in acid cleaned polyethylene bottles and immediately acidified with distilled nitric acid. Additional surface water samples were manually collected in January 1999 at Myrmidon Reef near the shelf edge, Davies Reef ⬃70 km off shore of Townsville, and near the coast at the Yongala Wreck site, ⬃10 km off Cape Bowling Green and 35 km from the Burdekin estuary. These samples were unfiltered but immediately

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acidified. Surface water from the Yongala site was obtained again in January 2002, during a minor flood of the Burdekin, filtered less than 48 h later at the laboratory, and analyzed for Ba. A low salinity around 32, measured during this recent sampling by conductivity probe, is likely to reflect the heavy rainfall of the previous days, rather than detached patches from the river plume, because clear waters were observed at this site. 3.2. Coral Collection Pandora Reef was chosen because previous work by Sinclair (1999) had documented the strong relationship between the major flood events of the early 1970s and the presence of fluorescent lines and high Ba/Ca in corals from this reef. Several 40-cm-long cores were collected in October 1998 to document the response of inshore Porites corals to the moderate 1998 flood, also followed by a severe bleaching event (Berkelmans and Oliver, 1999). The sample PAN-98 was analyzed, using two adjacent slices B3 (pieces 1 and 2) and B1 (pieces 3 and 4) to optimize sampling along growth vectors. The sample Davies-2 (side) from Davies Reef was analyzed by laser ablation ICPMS along a track parallel to a previously determined Sr/Ca record (Alibert and McCulloch, 1997), and the time series was derived after appropriate scaling of the distance axis for the period 1989 to 1993. 3.3. ICPMS, Mass Spectrometry, and Multicollector ICPMS Methods The laser ablation ICPMS analysis of coral is similar to that reported by Sinclair (1999) and Fallon et al. (1999). During the period of this study, two quadrupole ICPMS, Fisons PQII and Agilent 7500s, were used. The sample stage holds 45-mm-long coral samples. An ArF excimer laser beam is focused on the coral surface to a 500- by 50-␮m rectangle, and is scanned along the coral growth axis at 2 mm/mn. Before analysis, a surface-cleaning scan was routinely carried out and followed by preconditioning scans, monitoring the level of trace elements such as Pb, Ce, and Cd, which are sensitive to surface contamination. An analytical scan, measuring isotopes of the abundant elements 43Ca, 84Sr, 25Mg, 11B, 238U, and 138Ba, at 5 Hz laser pulse rate was followed by a second scan at 20 Hz pulse rate for the trace element isotopes Ba137, P31, Mn55, Y89, Cd111, U238, and Ca46. Under these conditions, a layer of a few microns of aragonite was removed at each passage. Integrated counts were averaged to correspond to a spatial resolution of ⬃100 ␮m, before normalizing to Ca. Coral analyses were bracketed by standard analyses using the glass standard NIST 614 (National Institute of Standards and Technology) and a pressed-powder coral disk for which Ba/Ca and U/Ca ratios had been determined by isotope dilution ICPMS (Fallon et al., 1999), and Sr/Ca ratios by thermal ionization mass spectrometry (TIMS) (this study). Matrix effects associated with the low level NIST 614 glass were found to be minimal for Ba/Ca ratios. The NIST 614 glass was calibrated against NIST 612. Standard concentrations are reported in Table 1 and compared with accepted/certified values (Horn et al., 1997). Trace elements such as Cd and Y in Davies 2 and PAN 98-2 (piece 4) were measured with the Agilent 7500s instrument, which had higher sensitivity. Seawater Sr and Ca were collected together after passage of ⬃10 ␮L through a long cation exchange column, with 2.5 N HCl used as eluant. The dry residue was taken up with a drop of perchloric acid and evaporated at high temperature to eliminate resin organics that could interfere with Ca during mass spectrometer analysis (after Bacon and Edmond, 1972). High-precision Sr/Ca ratios reported in Table 2 have been obtained by ID-TIMS (Finnigan MAT 261) with a mixed 43Ca84 Sr spike and measuring the 42,43,44Ca isotopes. Reproducibility of Sr/Ca ratios was better than 0.15%. Ba concentrations in seawater were determined by isotope dilution with the Finnigan multicollector ICPMS “Neptune,” measuring 135,137,138 Ba isotopes after spiking with enriched 135Ba (purchased from Oak Ridge, diluted to a concentration of 1 nmol 135Ba per gram). To avoid the direct introduction of seawater into the plasma, Ba was extracted from ⬃2 ml seawater through ion exchange columns (the same as those used for Sr), and Ba was recovered in 5 mL of 6 N HCl. Solutions of normal Ba and 135Ba spike, at 1 ppm concentration, were measured for isotopic composition on the “Neptune” instrument. The measured 137Ba/138Ba ratio in the normal was 0.15450 ⫾ 0.00005

Table 1. ICPMS standards measurements.

Sr/Ca (TIMS)

Ba/Ca (IDICPMS)a

U/Ca (ID-ICPMS)a

0.0887 0.0002

3.81E-06

1.08E-06

Coral standard (mol/mol) SD SRM NIST 614

P

Mn

Sr

Cd

Ba

U

(ppm) 2␴ SE Certified values 2␴ SE Accepted values (LA-ICPMS)b 1␴ SD (SIMS)b 1␴ SD

11.8 0.4

1.37 0.01

45.7 0.1 45.8 0.1

0.52 0.02

3.12 0.04

0.837 0.004 0.823 0.002

11.78 0.06

1.37 0.08

45.5 0.3

3.29 0.04 3.02 0.05

0.83 0.01

a b

Fallon et al. (1999). Horn et al. (1997).

(SD), which corresponds to a mass bias of 1.3 to 1.4% per mass unit, determined by correcting 137Ba/138Ba to 0.156655 (as measured by TIMS by Eugster et al., 1969). A 10 ppb Merck standard solution, measured at regular intervals between spiked samples, confirmed the stability of the mass bias, and the exact mass bias was determined for each spiked sample after removing the spike contribution. Instrument sensitivity was ⬃30 mV 138Ba per ppb Ba (1011 gain on the Faraday cup amplifiers); 140Ce and 139La were monitored, and a correction on the 138Ba isotope was introduced when the Ce or La signal was ⬎ 0.1 mV. Results for filtered surface seawaters I to VIII from the transect, the Yongala Wreck site, and the open ocean standard NASS-4 are presented in Table 3, together with the river water SLRS-2 for which a value within 2% of the certified value was obtained (direct measurement). Analytical errors, including the approximation for the mass fractionation law, are less than 0.2%, and the accuracy of ⬃2% on Ba concentrations, by comparison with SLRS-2, mainly reflects weighing errors and their effect on spike and normal concentrations. 3.4. SSTs and Tidal Data Sr/Ca time series were compared with weekly averaged and blended SSTs from in situ measurements at three sites around Orpheus and Pelorus islands, part of a monitoring program by Great Barrier Reef Marine Park Authority and at the Davies Reef weather station maintained by AIMS. Satellite-derived weekly data from Integrated Global Ocean Services System (IGOSS) (at http://www.ingrid.columbia.edu) at the 1° ⫻ 1° grid centered at 18.5°S, 147.5°E, were also used. Daily and monthly values for mean sea level at Cape Ferguson (Townsville), used in the discussion, were obtained from The National Tidal Facility, Flinders University, South Australia, back to September 1991 and School of Ocean and Earth Science and Technology (SOEST) (University of Hawaii) for historical data back to 1984 (ftp://ilikai.soest.hawaii.edu). 4. RESULTS

4.1. Chronological Control: Sr/Ca Surface seawater samples measured for Sr/Ca (Table 2) give an average of 0.00848 mol/mol, and the 100- to 150-m-deep waters from station II are enriched up to 0.00853 mol/mol, in line with high nutrients typical for these subthermocline waters (19.6 to 24°C, salinity of 35.4 to 35.7), which are part of the Subtropical Lower Water reservoir. The homogeneity of Sr/Ca in surface waters across the GBR lagoon supports the hypoth-

Trace element variability in corals from the Great Barrier Reef Table 2. Seawater TIMS Sr/Ca data.a

Station

Reef location

Water Salinity temp (CTD) (°C) (psu)

Davies 3-Sur Mid reef Yongala 4-Sur Inshore Myrmidon 1-Sur Outer reef Myrmidon 2-Sur Outer reef Myrmidon 2-Sur d Outer reef I-Sur-F Outer reef II-Sur-F Outer reef 25.7 II-Sur-F d Outer reef II-Sur-F d Outer reef II-100 m-F Outer reef 24.1 II-150 m-F Outer reef 19.6 VII-5 m-F Inshore 23.5 Average surface waters

35.03 35.44 35.65 34.67

235

Table 3. Seawater Ba concentrations.a

Sr/Ca (mol/ mol)

2␴ error (␮mol/mol)

0.008487 0.008491 0.008477 0.008477 0.008485 0.008480 0.008479 0.008482 0.008510 0.008519 0.008534 0.008485 0.008484

0.7 0.8 0.6 0.9 0.5 0.8 0.6 0.6 0.7 0.5 0.5 0.7 10 (SD)

Sur ⫽ surface water (⬃1 m below surface); F ⫽ filtered; d ⫽ duplicate analysis. a

esis that the variation of Sr/Ca ratios in the GBR corals reflects the ambient water temperature. TIMS Sr/Ca measurements in the two corals PAN 98-2 and Davies 2 (Figs. 3A, B) have been transformed into time series by direct comparison with in situ water temperatures averaged at a similar weekly resolution. The two least-squares linear regressions between SST and Sr/Ca (Fig. 3C) are largely within errors of each other with Sr/Ca ⫻ 103 ⫽ 10.29 (⫾0.02) ⫺ 0.0537 (⫾0.0006) ⫻ SST, for PAN 98-2, and Sr/Ca ⫻ 103 ⫽ 10.46 (⫾0.02) ⫺ 0.060 (⫾0.001) ⫻ SST, for Davies 2. Because laser ablation ICPMS produces less precise Sr/Ca ratios, as seen in Figures 3D and E, the chronology of PAN 98-2, slice B1, was derived by matching only a small number of data points to the SST record (mainly satellite-derived data) between 1988 to 1993. The calculated extension rate is faster and more variable for the inshore coral (60 ␮m/d ⫾ 37% rsd), with a significant slowdown occurring during the two coldest winter months, than for the Davies Reef coral (30 ␮m/d ⫾ 19% rsd). For PAN 98-2, the top three samples (Fig. 3A) were excluded from the time fit because their Sr/Ca ratios are much higher than expected for a reasonable extension rate of 20 to 40 ␮m/d and appear to reflect stress on the coral colony. X-ray photographs revealed very low skeleton density at the top 2 to 3 mm, above the summer high-density band. It is inferred that after the 31°C maximum reached in mid-February 1998, which is accurately recorded by the coral, polyps did not calcify normally after early April. This is also in good agreement with field observations (Berkelmans and Oliver, 1999) reporting that decline or death of the total coral cover occurred among the coral communities around Orpheus Island 9 weeks after the peak temperatures of midFebruary, which marked the onset of widespread bleaching. 4.2. Trace Elements in Coral Skeleton: Barium, Manganese, Phosphorus, Cadmium, and Yttrium Barium in coral skeleton reflects seawater-dissolved Ba, with a partition coefficient DBa ⬇ 1 (Lea et al., 1989). Ba measured here in nine surface waters, away from river plumes, is fairly

Station I-Sur-F II-Sur-F III-5 m-F IV-5 m-F V-5 m-F VI-5 m-F VII-5 m-F VIII-5 m-F Yongala-Sur-F Jan 02 Average SD NASS-4 (open ocean) SLRS-2 (river water) certified value

Salinity (psu)

SS (mg/L)

Ba (nmol/kg)

— 35.0 35.0 35.0 35.0 34.5 34.7 34.7 32.0

— 0.4 0.9 0.9 1.8 2.3 2.4 3 16

31.3 –

– –

33.17 35.18 32.94 33.20 33.12 34.00 34.01 33.98 33.72 33.70 0.70 46.7 102.6 100.5

a Waters collected near surface (⬃1 m) or at 5 m depth (III to VIII), filtered (0.45 ␮m) and acidified. SS ⫽ suspended solids estimated from filters.

constant (Table 3) at 33.7 ⫾ 0.7 nmol/kg or 4.63 ⫾ 0.1 ppb, close to the value of 4.65 ppb reported for near-surface waters from the western Pacific (GEOSECS III) by Bacon and Edmond (1972). Waters from station II, on the outer reef, give values for Ba (35.2 nmol/kg) and Sr/Ca (0.00851 mol/mol) that are slightly above average and reflect a weak upwelling of nutrient-rich waters. Ba/Ca ratios between 3.5 to 4 ␮mol/mol measured in the Davies 2 and PAN 98-2 corals (Fig. 4A, B) give DBa ⬇ 1.2. Discrete Ba/Ca peaks in PAN 98-2 are closely associated with flood events carrying freshwater to the reef site. In spite of different hydrodynamic conditions associated with each flood plume (Fig. 5, bottom), a good agreement is found between coral Ba/Ca and salinity calculated near Pandora Reef (Fig. 5, top), with a small Ba peak in 1996 corresponding to a local salinity of 34 (5% freshwater), and the major Ba peak for 1991 to a salinity of ⬃27 (25% freshwater). Without presuming any value for river water Ba or for the suspended sediment load, this relation (r ⫽ 0.86 for the period 1988 to 1998) shows that coral, and hence seawater Ba, can be roughly predicted from water salinity around the Palm Isles because the latter is mainly controlled by the Burdekin River discharge. In the Davies coral, minor Ba/Ca peaks occur in early summer with no relation to river runoff. The concentration of Mn in the Davies coral is lower (0.075 ppm, or Mn/Ca ⫽ 0.136 ␮mol/mol) than in the Pandora coral (0.36 ppm, or Mn/Ca ⫽ 0.66 ␮mol/mol). It has been previously shown, however, that Mn is discriminated against by coralline aragonite relative to Ca (DMn ⫽ 0.1 to 0.5; Shen and Boyle, 1988; Shen et al., 1991), in line with its preference for rhombohedral carbonates, so that the ratio of ⬃5 between inshore and midshelf corals may not necessarily represent the actual enrichment of coastal waters. A major difference with Ba is that Mn shows a seasonal signal (Fig. 4A, B), with a summer/winter ratio of ⬃2. Transient spikes are a robust feature of the Mn pattern, defined by several data points and reproducible on distinct laser tracks. It would be very difficult to produce such continuous and detailed Mn records from seawater samples, unless shipboard real-time measurements can be developed, as described recently for Ba (ICPMS) by Volpe et al. (2001).

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Fig. 3. Time series for Sr/Ca (TIMS) in the corals (A) PAN 98-2, slice B3 (1993 to 1998) and (B) Davies 2 side (1988 to 1993). and water temperature are adjusted by least-square fitting (in situ SSTs in dark blue, satellite-derived SSTs in black). A relatively large number of Sr/Ca data points has been matched to salient detail features of the SST. (C) Least-squares linear regressions (implemented using the Williamson method) for Davies 2 side (blue) and PAN 98-2 (red) show that the two calibrations are within errors of each other (95% error envelope shown as dashed line, n ⫽ 192 data points out of 199 for PAN 98-2, n ⫽ 213 out of 215 data points for Davies 2, errors as 2␴). (D, E) Laser ablation ICPMS Sr/Ca ratios measured for the PAN 98-2 coral along one laser track for slice B1 and two adjacent laser tracks B and C for slice B3. The scaling of the water temperature (blue line) is the same as in (A) and (B).

Yttrium (Figs. 4C, D) is fairly constant at a concentration six times higher for the Pandora coral ( 0.07 ppm) than the Davies coral (0.011 ppm), with a small peak in PAN 98-2 associated to the February 1991 flood. The Mn/Y ratio (mol/mol) is similar at both sites (11.0 at Davies vs. 8.3 at Pandora) and both concentrations, and Mn/Y ratios are close to reported for other Porites corals (Mn/Y ⫽ 7.6; Fallon et al., 2002). Phosphorus (⬃20 ppm or P/Ca ⫽ 65.4 ␮mol/mol) and cadmium (0.012 to 0.014 ppm or Cd/Ca ⫽ 9 to 11 nmol/mol) are low at both reef sites. This Cd/Ca ratio is within ranges (2 to 7 nmol/mol and 12 to 25 nmol/mol) reported for Pavona clavus corals from the Galapagos Islands by Shen et al. (1987) and Delaney et al. (1993), respectively. The elevated Ba and P between May 1992 and October 1993, at the top 7 mm of Davies 2 (Fig. 4C), correspond to remnants of coral tissue. We observed a non reproducible behavior for P and Cd (Fig. 4E) depending on sample preparation. Before analysis, slice B3 of PAN 98-2 was strongly leached with concentrated H2O2 in an ultrasonic bath, to remove organics such as coral tissue and endolithic algae often found at its base, before being extensively rinsed in Milli-Q water under an ultrasonic probe. This procedure does not affect Sr/Ca or Ba/Ca ratios. The second slice B1 was only slightly leached. When successive analyses were run along the same laser track on slice B3, P concentrations were offset downward at each new analysis from several hundred ppm down to ⬃100 to 150 ppm. Furthermore, some

seasonal variations were consistently observed in this top slice, which were not observed in B1, the latter giving a more reproducible lower concentration of ⬃30 ppm. Surface contamination is not a plausible explanation at these high concentration levels. A similar behavior was observed for other corals also leached in concentrated hydrogen peroxide. Coral slices that were only slightly leached in H2O2 gave reproducible low P and Cd concentrations, with a hint of slight enrichment in summer (Figs. 4C, D, E, left). The surface of coral PAN 98-2, slice B3 was examined under a scanning electron microscope (SEM). The laser track (top half of the SEM image in Fig. 6) is bordered by condensation droplets from the laser-generated plasma and is conspicuous by a higher porosity at the micron scale, compared with the adjacent surface. The latter shows the characteristic smooth-surface hexagonal aragonite crystals. SEM examination of laser tracks on coral surfaces only slightly treated with H2O2, show a distinctive edge and surrounding ejecta blanket, but lack the pervasive porosity shown in Figure 6. 5. DISCUSSION

5.1. Sr/Ca Temperature Relationship A similar Sr/Ca-temperature calibration is obtained here for both an inshore coral, where water turbidity and nutrients are

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Fig. 4. Laser ablation ICPMS measurements of Ba/Ca and Mn for Davies 2 (A) and PAN 98-2 (B). Note the different scale for Mn. The small difference between average winter Ba/Ca values between the two corals is not thought to be significant but rather to represent natural variability between coral colonies. Three Ba analyses are overlaid between 1989 to 1991, two of them representing the same record measured with the Agilent ICPMS but calibrated against the coral standard (dark brown line slightly shifted above the two others) and the glass standard NIST 614, whereas the third record was measured together with Sr/Ca by means of the coral standard. The two Mn records overlaid were also obtained with the two instruments and show perfect agreement. P, Cd, and Y (Agilent ICPMS) for Davies 2 (C) and PAN 98-2, B1, piece 4 (D), show a higher concentration of Y inshore, but similar P and Cd, whereas (E) the two slices B1 and B3 of PAN 98-2 measured for P and Cd (Fison PQII instrument) show anomalous high values for slice B3 (right) strongly leached with H2O2 prior analysis, compared with slice B1 (left).

high, and for a midshelf coral living in clear oligotrophic waters. This is a strong indication that environmental factors alone cannot account for the large discrepancies reported in the literature for this thermometer (Beck et al., 1992; de Villiers et al., 1994; Shen et al., 1996; Alibert and McCulloch, 1997; Boiseau et al., 1997; Heiss et al., 1997; Crowley et al., 1999; Evans et al., 1999). There is, however, mounting evidence that the slope of the linear regression is the only robust parameter, with the intercept varying from one coral to the next. The origin of this variability remains elusive, in spite of multiple attempts, at Research School of Earth Sciences (Fallon, 2000; Marshall, 2000; E. Hendy, personal communication), to relate it to environmental or coral growth factors. It is a possibility that the range of intercept values found for different Porites corals is related to variations in the photosynthetic activity of the symbiotic Zooxanthellae, but this has not yet been rigorously demonstrated (Cohen et al., 2002). The role of “vital effects” needs to be better evaluated because trace element compositions in coral skeleton are strongly out of equilibrium, in a thermodynamic sense, as are marine inorganic aragonites with Sr2⫹/ Ca2⫹activity ratios an order of magnitude smaller than expected from equilibrium with seawater at 25°C (Plummer and Busenberg, 1987).

5.2. P, Cd, and Y in Porites Corals The export of phosphorus to the GBR lagoon is a major environmental concern. Low phosphate concentrations have been reported by Furnas and Mitchell (2001), who found that the majority of P was exported off North Queensland rivers in particulate form. Two important sinks for phosphate in the GBR lagoon are likely to be biologic uptake and adsorption on carbonate sediments (Millero et al., 2001). The latter process has been suggested (Entsh et al., 1983; Koch et al., 2001) to explain relatively low exchangeable and porewater P at Davies Reef and Florida Bay, respectively. Figure 7 showing Ba vs. P (ppm) for the Davies 2 time series reveals two distinct linear relationships, one defined by Ba and P in the tissue layer with a slope of 0.4, and a second one for the remaining skeleton with a lesser enrichment and a slope of 0.2. The latter comprises the minor Ba and P peaks observed in the coral in early summer (arrows in Fig. 7A) and these are believed to reflect minute amounts of organic matter incorporated into the coral skeleton. This interpretation is similar to that of Tudhope et al. (1996) who proposed that organically bound Ba could explain the seasonal enrichment observed in corals from the Oman Sea, away from river influence.

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Fig. 5. Coral Ba/Ca for PAN 98-2 as a time series between 1988 to 1998 is compared with daily model salinity near Pandora Reef (thick gray line) (King et al., 2001) by use of 5-d smoothed data to match the coral sampling resolution. The amplitude of the Ba/Ca peaks appears proportional to the freshening of waters during most flood events. The two bottom graphs illustrate the freshening near Pandora Reef (salinity as gray line) in response to discharge at the Burdekin River mouth during the single-pulse flood event of 1998 and the multipulse event of 1990 –1991.

The coral leaching experiments described in section 4.2 suggest that a significant proportion of P may be adsorbed on surfaces rather than substituting for major cations in the aragonite crystal. H2O2 leaching has a strong etching effect, exposing deep intracrystalline spaces. As a result, the extraction of material along the surfaces of these cavities is likely to be greatly enhanced under the high-energy laser beam and may contribute to the anomalous P enrichment observed in the first scans of the laser over the same track. Because it has been observed that H2O2 oxidation does not destroy completely organics in coral skeleton, an organically bound origin for the anomalous high P and Cd cannot be excluded. Entsh et al. (1983) suggested that mineralization of tissue P by microbial

activity could also contribute to the adsorption of P to the skeleton. These authors reported soluble reactive P concentrations around 0.1 ␮mol in the water column at Davies Reef. Taking ⬃20 ppm P in the coral skeleton, a partition coefficient of 0.15 is obtained, showing that even at this relatively low concentration, P appears discriminated against by coralline aragonite, which is expected from the different structure of phosphate. In contrast, the partitioning of cadmium, which substitutes for Ca in aragonite (similar 2⫹ effective ionic radius), between either coral/seawater (Shen, 1986) or benthic foraminifera/ seawater (Boyle et al., 1995) is close to 1, so that Cd is a reliable tracer of nutrients and pollutants (Shen et al., 1987). In

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Fig. 6. SEM images showing the track left after ablation with the excimer laser (top) on the surface of the PAN 98-2 coral (slice B3, strongly leached in H2O2 before analysis). (A) The ablated surface has a higher porosity at the micron scale, better seen in the enlargement below. (B) A sharp step of a few microns and presence of condensation droplets characterize the laser track. Comparison with pristine coral slices (not shown) indicates that the etching effect associated with H2O2 increases the microporosity, which in turn may favor the extraction by the laser beam of material within holes. The material preferentially removed by the leaching is likely to be remains of organic material but also some secondary aragonitic material.

that respect, the low and similar Cd level of 12 to 14 ppb measured here in the Pandora and Davies corals indicates the absence of significant contamination of ambient waters and no

noticeable change since the late 1980s. Although soluble Cd data are still not available for GBR waters, a seawater Cd of 0.096 nmol/L is predicted, which is three to five times higher

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Fig. 7. (A) The time series featuring Sr/Ca (TIMS), P, and Ba for the Davies 2 coral collected in October 1993 show that combined small peaks of Ba and P occur in early summer (arrows). Note also the enrichment in the tissue layer. (B) A corresponding scatterplot for Ba and P suggests the presence of two different types of organic material enriched in Ba and P.

than measured in New South Wales coastal waters (Apte et al., 1998). More data are necessary to trace, for example, Cd from fertilizers, expected to be washed out into coastal waters by heavy coastal rainfall from the sugarcane fields of the lower catchment. Finally, yttrium appears to follows Mn in terms of enrichment in coastal waters relative to the midshelf and is less sensitive than Ba to monitor terrestrial inputs associated with the summer floods. 5.3. Coral Barium and the Burdekin River Floods Terrestrial Ba is carried out to the ocean as a dissolved river pool that is mainly controlled by the geology and weathering

conditions in the watershed and by river discharge. Runoff over heavily weathered tropical soils is known, however, to provide low Ba (Edmond et al., 1978), unless chemical weathering can be assisted by biologic activity (Markewitz et al., 2001). Although the relative proportions of particulate/dissolved Ba are not known for the Burdekin, the very high suspended sediment load associated with weathered soils in the catchment and protracted drought periods suggest that Ba desorbed from riverborne sediments in the mixing zone is dominant relative to river-dissolved Ba. Estuarine release has been shown to double the river-dissolved Ba pool. Resuspension of bottom sediments in the estuary zone is a rather unlikely source (Hanor and Chan, 1977; Coffey et al., 1997; Shiller, 1997; Horowitz et al., 2001).

Trace element variability in corals from the Great Barrier Reef

For the 1991 flood event, an effective river end-member concentration (after Boyle et al., 1974) of 340 nmol/kg has been estimated by extrapolating to zero salinity the plume water Ba value of 100 nmol/kg calculated from coral Ba/Ca enrichment at Pandora Reef (DBa ⫽ 1, error of ⬃15% representing the reproducibility of the Ba/Ca flood peak measured along different laser tracks, several millimeters apart) and by using seawater Ba ⬇ 33.7 nmol/kg (this study). This figure is similar to values reported for the Delaware, Zaire, or Amazon Rivers but likely to change for each flood event, depending on which part of the catchment is affected by rainfall. We now turn to the relationship between coral Ba and hydrology. In the absence of data for the suspended sediment load associated with each flood, the river MFR has been taken as proxy, in agreement with the strong relation between flow rate and particulate nutrient concentrations found by Mitchell and Furnas (1997). For each flood event, a maximum Ba/Ca value (⌬Ba/Ca) was calculated by averaging the two consecutive highest values (corresponding to ⬃0.5 mm of coral growth) and subtracting an average winter value. Flow rate at peak discharge was calculated for each event as the maximum value for five consecutive days (from smoothed data). The best least-squares fit between ⌬Ba/Ca and MFR for the seven flood events between 1989 to 1998 (Fig. 8) is logarithmic: ⌬Ba/Ca ⫽ 2.48 ⫻ 10⫺6 Ln(MFR) ⫺ 1.67 ⫻ 10⫺5 (r ⫽ 0.97), rather than linear (r ⫽ 0.80). A slightly better fit, obtained by adding a linear term can be explained by the double origin of Ba. The dissolved Ba river pool is involved in the logarithmic part, as some degree of dilution is expected during prolonged wet seasons (limited reservoir), whereas Ba desorbed from clayrich sediments is expected to linearly follow the MFR. This relationship can be further complicated when short but intense rainfall events occur after several years of drought—for example, in 1927, 1946, or 1968 — because floods then have a high erosive effect and are likely to promote the release of extra Ba. This has indeed been observed in a long coral record from a nearby reef (McCulloch, unpublished data). Summer river discharge (Q) is another parameter of general interest for climatic studies because it reflects precipitation at the large scale of the catchment (Fig. 1). Its relation to the MFR between 1921 to 1998 is best-fitted by a power law: Q ⫽ 6.87 ⫻ 103 MFR0.814 (r ⫽ 0.94), which is reminiscent of rating curves, as reported in the literature, between discharge and suspended sediment load. The good correlation between ⌬Ba/Ca and Q between 1989 to 1998 (r ⫽ 0.96) suggests that coral Ba/Ca may carry a valuable precipitation signal. Another feature of coral Ba time series is the occasional departure of peak shape from the simple rapid fall reflecting discharge pattern and plume dissipation. Similar tailing was observed by Sinclair (1999) for several of the Ba flood-related peaks of the 1970s. In 1991, the first flood occurred in early January in the wake of tropical cyclone “Joy,” which brought heavy rains mainly along the coast in late December 1990. This event did not produce a noticeable Ba enrichment in the coral. A second event in mid-February was associated with a very active monsoon (Bannister and Smith, 1993) and widespread flooding in the Burdekin catchment produced two peak discharges (Fig. 5, bottom), with the second one pushed rapidly toward the Palm Isles by steady southeasterlies, as explained by King et al. (2001). This last pulse is well recorded in the coral,

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but the Ba/Ca peak is followed by a plateau that lingers into early October (Fig. 9). According to river discharge and salinity patterns, brackish waters were well mixed with coastal seawater by May. Areas under the Ba/Ca curve vs. time (Fig. 9) have been estimated to derive the relative proportions of Ba delivered by the river plume and the second source, the limit between the two domains being taken at the end of April. A ratio of 0.8 between the two areas indicates that nearly as much Ba was discharged to coastal waters from the second source. Mean sea level in the Townsville region has a sinusoidal seasonal pattern that peaks between January and April, closely following the SST cycle. In addition to the buoyancy of the freshwater plume that contributes to high sea level along the coast (Wolanski, 1994), the highest sea level in early 1991 corresponded precisely with the occurrence of the Burdekin plume, and the coral Ba/Ca peak (Fig. 9). These high sea level conditions are favorable to the deposition of river-borne sediments in the freshwater part of the estuaries and in fringing mangroves. During the subsequent low discharge season, the low sea level from May onward coincides with a drop of the water table. Wolanski et al. (2001) observed that during the dry season, mangrove creeks become hypersaline and groundwater level rises/drops significantly between spring/ebb tides. It is proposed here that sediment-borne Ba locked in the coastal mangroves could provide a source of soluble Ba by ion-exchange reaction with more saline waters encroaching inland at the highest tides during the dry season. This is similar to the mechanism proposed by Carroll et al. (1994) to account for high Ba and 226Ra in coastal waters during low river discharge for the Ganges-Brahmaputra system. An alternative Ba source may be found in the freshwater seeps near Palm Island, locally known as “Wonky Holes,” which have been recently identified by Stieglitz and Ridd (2001) as ancient riverine channels. These could provide an efficient pathway to transport groundwaters enriched in Ba to the GBR Lagoon. The importance of aquifer reservoirs as a source for coastal water Ba has previously been demonstrated for the inner shelf waters of the South Atlantic Bight (Moore, 1996; Shaw et al., 1998), the Ganges-Brahmaputra mixing zone (Moore, 1997) and along the coast of Florida (Swarzenski et al., 2001). The analysis of waters from tidal channels of coastal mangroves and from the inshore freshwater seeps will be necessary to test these two hypotheses. 5.4. Coral Mn as a Tracer for Primary Productivity and Benthic Fluxes Riverine particulate matter is likely to be the major source of Mn oxy-hydroxides along the coast of central GBR. The short coral records presented here have shown that, firstly, floodrelated Ba peaks do not have systematic Mn counterparts, except in 1991 and 1998. The Mn spike observed in February 1994 precedes the Ba flood peak of March 1994, and could instead be related indirectly (see discussion below) to the moderate flood of the Herbert River caused by tropical cyclone “Sadie,” inland of North Queensland. The second major feature of coral Mn is the seasonal signal associated with high variability during summer. This seasonal signal rises in October and decreases sharply during March of the next year. The annual maximum insolation in November– December coincides with a relaxation of the southeasterly

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Fig. 8. Coral Ba enrichment (⌬Ba/Ca) as a function of the river MFR. The best fit is obtained with a logarithmic law (r ⫽ 0.97). The dotted line is obtained by iteratively adding a small linear term until the fit includes the highest discharge of 1991.

winds and sharp increase in SSTs. Together, these conditions are likely to boost the primary productivity. The close link with temperature is shown by the good correlation obtained between Mn and SST (Fig. 10) (r ⫽ 0.72 or 0.74, for a linear or exponential best fit, respectively). Two possible origins for the seasonal variability, both linked to the availability of organic matter, are briefly discussed below. The first hypothesis calls for benthic fluxes, either sustained or transient. Sediments around Pandora Reef include sandy muds that could provide low oxygen conditions required for the degradation of organic material. The average high winter value observed for the inshore relative to midshelf corals, calls for a steady-state benthic input. At the midshelf reef site, the seasonal Mn signal recorded in the Davies 2 coral may also be related to weak benthic fluxes, as low redox potential values were reported by Entsch et al. (1983) in Davies Reef sediments, at a few cm below the surface. On the other hand, in dynamic coastal environments, nutrients (including Ba) and micronutrients such as Mn are known to vary on a short timescale and transient benthic fluxes could be triggered by the deposition of decaying plankton in spring– summer. This dual dependency of Mn fluxes on temperature and primary productivity levels has been reproduced experimentally (Hunt, 1983) as well as observed (e.g., Schoemann et al., 1998). Furnas and Mitchell (1997) and Brodie et al. (1997) have shown that the inshore waters of central GBR are rich in chlorophyll, especially during blooms triggered by flood plumes (up to 20 ␮g/L chlorophyll reported for the 1991 flood). Decayed plankton and bacteria can flocculate easily with ambient fine suspended sediments to form a “marine snow” as documented by Wolanski (1995) and Wolanski et al. (2001).

These aggregates can also scavenge Mn oxides, thus providing suitable suboxic conditions needed for rapid microbial degradation on the seafloor. Another source of soluble Mn is required to explain the Mn spikes observed during the dry summers of 1992, 1994, or 1995 (Fig. 10). The photo-assisted reduction of particulate Mn(IV) by organic substances in the water column, such as fulvic and humic acids, has been shown to be an efficient process (Sunda et al., 1983; Stone and Morgan, 1984; Waite et al., 1988; Sunda and Kieber, 1994; Matsunaga et al., 1995) and the absence of Mn-oxidizing bacteria in the tropical Pacific has been noted by several authors (e.g., Moffett, 1997). As the level of nutrients and organic matter is greatly enhanced during flood plume conditions, it is expected that remineralization at the seaward edge of plumes may produce large amounts of soluble Mn. Competition by phytoplankton uptake may limit the availability of this short-lived stock but in the case of the major flood events, such as in 1991 or 1998, the enrichment of ambient seawater may exceed demand, causing the sharp peaks recorded in coral skeleton. 6. CONCLUSIONS

Porites corals can successfully monitor past environmental changes that affect the surface water concentrations of Ba, Mn, Y, and Cd. Riverine inputs have been identified as the major source of Ba into GBR waters, but an additional source is found after major wet summers, which could be related to submarine groundwater seeps and/or a reservoir associated with mangroves. On the basis of these short coral records, Ba/Ca time series from corals growing in the path of river plumes of major

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Fig. 9. Detail of the coral Ba/Ca time series for 1991 (two black lines representing the same data calibrated either with the coral standard or the NIST 614 glass). The Ba/Ca peak after the major February 1991 flood is tailing off toward the end of the dry season. Sea level variations (daily data for Cape Ferguson obtained from NTF/SOEST) feature a maximum between January and April, coincident with the occurrence of the river plume, which may augment the deposition of suspended sediments in fringing mangroves along the coast and in the freshwater part of the estuary. The two sea level signals (gray dotted lines) correspond to mean weekly sea level and maximum sea level at weekly resolution, taking the highest daily sea level value for each week.

rivers such as the Burdekin, are expected to monitor long-term increases in sediment export resulting from changes in land use in the last 150 yr, and may also capture decadal-scale changes in the North Queensland rainfall pattern. In this tropical catchment, where the intense runoff events are expected to cause a rapid transport of pollutants and nutrients into the GBR Lagoon, it has been surprising to find that coral phosphorus is not

enriched after flood events and that a significant proportion of P is probably adsorbed on coral surfaces and/or organically bound. Coral Cd concentrations were low at both reef sites, indicating that ambient waters are enriched only three to five times compared with pristine coastal waters around New South Wales. Finally, the complexity of the Mn record reflects dynamic

Fig. 10. Time series for SSTs and coral Mn for PAN 98-2, both smoothed with a 3-week running average to enhance the seasonal signal. The linear regression (r ⫽ 0.72) between Mn and water temperature confirms that the two signals are largely in phase, but Mn carries an additional variability during summer.

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coastal processes, showing seasonality and transient enrichments during summer that do not always coincide with flood events. A joint approach involving water and sediment analyses for both the river network and the coastal mixing zone, together with hydrological models, would be warranted to assess exports of solute elements and sediments to the GBR Lagoon. Acknowledgments—The collection of coral cores at Pandora Reef benefited from the support and assistance of Michele Devlin and David Haynes (Great Barrier Reef Marine Park Authority) and Dan Sinclair and Erica Hendy (Research School of Earth Sciences). We also thank John Marshall and Tim Wyndhan for collecting seawater samples at Davies, Myrmidon and Yongala Wreck reefs in January 1999 and 2002. We thank the crew and scientific team of the R.V. Lady Basten (AIMS) during the July 1996 cruise, particularly Alan Mitchell and Michelle Skuza, who processed the CTD casts and analyzed nutrients and chlorophyll in waters, and Miles Furnas, who kindly gave permission to use unpublished data. Rainfall data were provided by the Australian Bureau of Meteorology, mean sea level data by The National Tidal Facility at Flinders University, and river flow data by the Queensland DNR. Discussions with Erica Hendy were appreciated, and she also helped us improve an early version of the article in manuscript. Associate editor: D. W. Lea REFERENCES Alibert C. and McCulloch M. T. (1997) Strontium/calcium ratios in modern Porites corals from the Great Barrier Reef as a proxy for sea surface temperature: Calibration of the thermometer and monitoring of ENSO. Paleoceanography 12, 345–363. Apte S. C., Barley G. E., Szymczak R., Rendell P. S., Randall L., and Waite T. D. (1998) Baseline trace metal concentrations in New South Wales coastal waters. Mar. Freshwater Res. 49, 203–214. Bacon M. P. and Edmond J. M. (1972) Barium at GEOSECS III in the southwestern Pacific. Earth Planet. Sci. Lett. 16, 66 –74. Bannister A. J. and Smith K. J. (1993) The South Pacific and southeast Indian Ocean tropical cyclone season 1990 –91. Aus. Met. Mag. 42, 175–182. Barson M., Randall L., and Bordas V. (2000) Land Cover Change in Australia. Bureau of Rural Sciences, Canberra. Beck J. W., Edwards R. L., Ito E., Taylor F. W., Recy J., Rougerie P., Joannot P., and Henin C. (1992) Sea-surface temperature from coral skeletal strontium/calcium ratios. Science 257, 644 – 647. Berkelmans R. and Oliver J. K. (1999) Large-scale bleaching of corals on the Great Barrier Reef. Coral Reefs 18, 55– 60. Boiseau M., Cornu H., Turpin L., and Juillet-Leclerc A. (1997) Sr/Ca and ␦18O ratios measured from Acropora nobilis and Porites lutea: Is Sr/Ca paleothermometry always reliable? C. R. Acad. Sci. Paris 325, 747–752. Boyle E. A., Collier R., Dengler A. T., Edmond J. M., Ng A. C., and Stallard R. F. (1974) Chemical mass balance in estuaries. Geochim. Cosmochim. Acta 38, 1719 –1728. Boyle E. A., Labeyrie L., and Duplessy J.-C. (1995) Calcitic foraminiferal data confirmed by cadmium in aragonitic Hoeglundina: Application to the last glacial maximum in the northern Indian Ocean. Paleoceanography 10, 881–900. Bristow K. L., Charlesworth P. B., Lowis B., Laidlow G., and Gilbey P. (2001) The Lower Burdekin Initiative: An industry/science partnership to facilitate improved water management. In Proceedings of the Australian National Committee on Irrigation and Drainage (ANCID 2001) Conference. Bunburry, Australia, 11 pp, CSIRO Land and Water. Brodie J. (1998) The water quality status of the Great Barrier Reef World Heritage Area. In State of the Great Barrier Reef World Heritage Area Workshop (eds. D. R. Wachenfeld, J. K. Oliver, and J. I. Morissey). Great Barrier Reef Marine Park Authority. Brodie J. E., Furnas M. J., Steven A. D. L., Trott L. A., Pantus F., and Wright M. (1997) Monitoring chlorophyll in the Great Barrier Reef lagoon: Trends and variability. In Proceedings of the 8th Interna-

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