Earth and Planetary Science Letters 210 (2003) 249^258
www.elsevier.com/locate/epsl
Tracing the source of sediment and phosphorus into
the Great Barrier Reef lagoon
Malcolm McCulloch a; , Christine Pailles b;1 , Philip Moody b ,
Candace E. Martin a;2
b
a
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
Department of Natural Resources and Mines, Natural Resource Sciences, 80 Meiers Road, Indooroopilly, QLD 4068, Australia
Received 18 July 2002; received in revised form 24 January 2003; accepted 14 March 2003
Abstract
Neodymium and strontium isotopic systematics show that terrestrial phosphorus (P) entering the inner Great
Barrier Reef (GBR) is dominated by the transport and dispersal of fine-grained basaltic soils. Soils derived from alkali
basalts have high total P (3000^4000 mg/kg) and distinctive 143 Nd/144 Nd isotopic signatures (ONd V+3 to +5), while
the more common Palaeozoic granitic/metamorphic soils have much lower total P (300^600 mg/kg) and 143 Nd isotopic
signatures (ONd V38). The nearshore environment ( 6 5 km from the coast) is dominated by coarse-grained, graniticderived fluvial detritus, while s 20 km from the coast, carbonate-rich sediments with increasing contributions from
basaltic components become more important. In the offshore sites adjacent to coral reefs, it is shown that basaltderived sediments can account for s 90% of the terrestrial P, although making up less than half of the total
terrigenous detritus. Equilibrium phosphorus concentration measurements on the marine sediments indicate that P
enters the GBR lagoon via a two-stage process. Firstly, during episodic flood events, P is transported into the GBR
lagoon on P-retentive fine-grained suspended sediments, with only minor desorption of P occurring in the low-salinity
flood plumes. Desorption of P mainly occurs over longer timescales, predominantly in regions of sediment anoxia,
with release of PO33
4 directly into marine pore waters probably via reduction of ferric phosphates, and subsequent
release into the water column by re-suspension. This process causes P depletion of the re-deposited sediments.
= 2003 Elsevier Science B.V. All rights reserved.
Keywords: sediment phosphorus tracing; Great Barrier Reef; degradation; neodymium^strontium isotopes
1. Introduction
* Corresponding author.
E-mail address: malcolm.mcculloch@anu.edu.au
(M. McCulloch).
1
Present address: C.E.R.E.G.E., Europole Mediterraneen
de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex, France.
2
Present address: Geology Department, University of
Otago, Dunedin, New Zealand.
Enhanced levels of nutrients and sediment are
one of the major causes of coral reef degradation.
The reasons are complex and incompletely understood, but it is clear that enhanced nutrient loads
from erosion of agricultural land [1^4], acting together with climatic stresses, such as cyclones and
unusually high ocean temperatures [3,5], are prov-
0012-821X / 03 / $ ^ see front matter = 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0012-821X(03)00145-6
EPSL 6623 22-4-03 Cyaan Magenta Geel Zwart
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M. McCulloch et al. / Earth and Planetary Science Letters 210 (2003) 249^258
ing to be a lethal combination. Furthermore, the
regeneration of coral reefs following extreme climatic events, such as the 1997^98 coral mass
bleaching [3,5^7], is often being inhibited due to
algal blooms and eutrophication from high nutrient loads in the water column. Determining
the source and pathways through which anthropogenically derived sediments and nutrients enter
coral reefs is thus an essential prerequisite for
ensuring their long-term sustainability.
The Great Barrier Reef (GBR) is a complex
assemblage of barrier, patch, platform and fringing reefs, extending for over 2000 km along the
northeastern coastline of Australia. Although large
sections are distal from direct terrestrial in£uences, inshore parts, especially in the central and
northern GBR, are impacted by runo¡ from large
rivers. The river £ows are generally highly episodic, being associated with cyclones or occasionally intense monsoonal depressions. During these
high-intensity events there can be massive discharges of freshwater and suspended sediments
into the nearshore GBR lagoon. For example,
during the Cyclone Sadie £ood event of 1994,
both the volume of water discharged and the suspended loads of the Johnstone River increased by
approximately two orders of magnitude [8]. The
river £ow increased from an average rate of V60
m3 /s to 6000 m3 /s with the suspended load increasing from V6 mg/l to 220^550 mg/l [9,10].
As a consequence, large, low-salinity, high-turbidity £ood plumes were discharged into the inner
GBR lagoon. Depending on hydrodynamic conditions [11,12] such plumes generally persist for at
least several weeks and occasionally for several
months as they are advected northwards along
the coast.
In order to constrain the provenance of phosphorus £uxes that are entering the inshore region
of the GBR lagoon, we have determined P concentrations and Nd^Sr isotopic compositions of
terrestrially derived sediments from the Johnstone
River £ood plume and from bottom sediments in
a transect to the reef. The Johnstone River drains
a medium-sized catchment of 1634 km2 . Catchment geology comprises Palaeozoic granitic and
metamorphic rocks, overlain by the Tertiary,
Atherton basalts [13] (Fig. 1). Most of the agri-
culture (sugar, banana plantations and tropical
fruits) is restricted to the basaltic provinces where
fertile ferrosol soils have developed. These soils
have excellent drainage characteristics, are well
structured and have high retention capacities for
the important nutrient element phosphorus [14].
Leaching of P in soluble form is thus usually minimal, but soil loss into rivers may occur from
stream-banks and cultivated slopes during highintensity rainfall events[14^17]. Basaltic soils typically have high indigenous contents of P (3000^
4000 mg/kg), which together with their large proportion of ¢ne clay-size particles makes them a
potentially important source of P for the nearshore GBR.
2. Methods
Systematic sampling of bottom sediments [16^
18] was undertaken along both the Johnstone and
South Johnstone rivers, the estuary and from two
VE^W marine transects from the river mouth to
the Gibson and Howie reefs in the central GBR
(Fig. 1). Marine samples located at a depth of
6 25 m were collected by SCUBA divers, with
all other samples (river sediments and o¡shore
samples at s 25 m depth) being collected using
a Van Veen grab sampler capable of recovering
an undisturbed sample of the upper 10 cm of
bottom sediment. Plume sediment samples were
collected in February 1994 in the Johnstone River
estuary following Cyclone Sadie. Grab and plume
samples were immediately frozen at 320‡C until
analysis. Phosphorus was separated into organic
and inorganic P using 20% HCl with total P being
estimated by the sum of these two fractions using
procedures described in [19]. Surface soil samples
were also obtained from the Berner Creek, a
headwater catchment of the North Johnstone
River. This catchment was chosen as it is dominated by basaltic soils and has regions where
phosphate fertilisers have been applied.
For Nd^Sr isotopic analyses 50^100 mg samples were dissolved in Te£on bombs using a mixture of nitric and HF acids and separated using
cation exchange columns. Prior to this, marine
samples were subjected to a leaching by acetic
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M. McCulloch et al. / Earth and Planetary Science Letters 210 (2003) 249^258
251
Fig. 1. Map showing the distribution of the main geologic units [13] in the Johnstone River catchment and location of coral
reefs. Soils developed on Tertiary basalts are an important constituent of both the North and South Johnstone River sub-catchments. Solid symbols show locations of marine sediment samples.
acid to remove any carbonate component. Nd
and Sr isotopic compositions were determined
on a Finnigan MAT 261 thermal ionisation
mass spectrometer. Usual within-run precisions
of better than R 0.00002 were obtained on 87 Sr/
86
Sr and 143 Nd/144 Nd ratios. Sr ratios are normalised to 86 Sr/88 Sr = 0.1194 and Nd ratios to 146 Nd/
144
Nd = 0.7219. The NBS 987 standard gave an
average of 87 Sr/86 Sr = 0.71020 R 2 (2c, n = 8) and
the nNd-1 inhouse standard 143 Nd/144 Nd =
0.512188 R 7 (2c, n = 7). 143 Nd/144 Nd ratios are
reported as ONd values with ONd = [(143 Nd/
144
Ndsample )/(0.51265)31]U104 .
3. Results
3.1. Phosphorus and Nd elemental abundances
Within the Johnstone River, sediments have total P ranging from 400 to 800 mg/kg (Fig. 2) with
the inorganic fraction generally making up more
than 75% of the Ptot [16]. The organic component
of P (Table 1) was most important in the estuarine sediments, where it contributed up to 50% of
the Ptot [16]. Tributaries (not shown in Fig. 1)
have even higher Ptot , approaching those of the
basalt-derived soils (Ptot V2000^4000 mg/kg). No
EPSL 6623 22-4-03 Cyaan Magenta Geel Zwart
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M. McCulloch et al. / Earth and Planetary Science Letters 210 (2003) 249^258
di¡erences were observed in the Ptot levels for
basalt-derived soils that had been subjected to
phosphate fertilisers compared to unfertilised soils
(Table 1). This is consistent with the high indigenous contents of P in basaltic soils.
On the GBR inner shelf, within 5 km of the
coast, the Ptot ranged from 110 to 200 mg/kg
with the sediments being dominated by coarse
£uvial sands ( 6 10% silt+clay) [18]. From 5 to
12 km, the sediments consisted of anoxic mud
(80% silt+clay) with Ptot ranging from V250 to
330 mg/kg (Fig. 2). From 12 to 30 km the amount
of CaCO3 (mainly skeletal debris) increased signi¢cantly with a corresponding decrease in the
silt+clay fraction ( 6 10% at s 20 km). Despite
dramatic changes in the sedimentology, especially
decreasing amounts of silt and clay, Ptot remained
almost constant at V250^300 mg/kg. These levels
of Ptot are similar to those previously obtained for
GBR reef sediments [20,21] and much lower than
the values in the suspended loads carried in £ood
plumes (Ptot V1900 mg/kg). The relatively uni-
form levels of P within the sediments of the inner
GBR lagoon may result from mixing by acrossreef dispersal processes [22,23] and/or approximately equal proportions of P coming from marine versus terrestrial sources.
The Nd elemental abundances are shown in
Fig. 2. The lowest concentrations (Nd 6 10 ppm)
are present in the quartz/feldspar £uvial sands
close to the river mouth and highest concentrations (NdV40 ppm) in the clay^silt-dominated
anoxic zone 5^12 km o¡shore. An unexpected observation is that in the more distal carbonate-rich
sediments (20^32 km o¡shore) the Nd concentrations remain nearly constant at moderate levels
(8^14 ppm), that is 1/3 to 1/2 of typical continental abundances [24,25], despite increasing percentages of CaCO3 . Since Nd is almost exclusively
derived from terrestrial sources (abundances in
marine carbonates and seawater are U1033 ^
U1036 lower than average continental crust)
this indicates the presence of a widely distributed
terrestrially derived clay component that makes
up 20^30% of the total carbonate-rich sediments.
3.2. Nd^Sr isotopic constraints on sediment
provenances
Fig. 2. Total P (inorganic+organic) in sediments from the
Johnstone River, estuary [18] and o¡shore transects, plotted
versus distance from the Johnstone River mouth. Suspended
river sediments (open circles) have the highest Ptot (800^2000
mg/kg) re£ecting the importance of ¢ne-grained high-P basaltic soils. The o¡shore bottom sediments (solid circles) have
signi¢cantly lower Ptot (V300 mg/kg) than either the riverbottom or river-bank sediments (Ptot V800 mg/kg) or suspended £ood-plume sediments (Ptot V1900 mg/kg), indicating
large-scale ( s 80%) P desorption in the marine bottom sediments. Nd concentrations in the marine sediments show that
the highest Nd concentrations occur in the nearshore anoxic
muds. Nd concentrations are also relatively high in the carbonate-rich reefal sediments, indicating the presence (V30%)
of widely distributed ¢ne-grained terrestrial clays.
To further constrain the provenance of the sediments and hence particulate-bound phosphorus
that is entering the GBR lagoon, Nd^Sr isotopic
analyses have been undertaken (Fig. 3) on the
marine sediments, suspended sediments from a
£ood plume and the range of soil types within
the Johnstone River catchment. Neodymium is a
particularly powerful isotopic tracer [24,25] as distinctive isotopic terrestrial signatures are preserved through the recent geochemical cycle of
soil production, sediment transport and redeposition in the coastal environment. The Johnstone
River catchment consists of isotopically contrasting rock types (Table 1), Tertiary basalts with
relatively high 143 Nd/144 Nd (ONd V+3 to +5) [26]
compared to more evolved Palaeozoic granites
and metamorphics (ONd V37 to 39). Along the
VE^W transect into the GBR, the ONd values
increase progressively (i.e. to less negative values)
from ONd = 38 to 34.5. The most rapid change is
within the ¢rst 5 km of the coast, which is dom-
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253
Table 1
Isotopic and elemental data for the Johnstone River and Great Barrier Reef sediments
Sample
GBR sediments [18]
GG 9984 W
G (acetic)
FF 9983 W
F
EE
CC 9980 W
C
AA 9978 W
J
BB 9979 W
II 9985 W
B
KK 9987 W
LL
Cyclone Sadie [17]
Sadie SJ
Sadie NJ
Estuary sediments [16]
Bay 10010 W
Coccoo 10008 W
Rocky
Soils
Metamorphic
Tully Met 0^10 cm
Tully Met 20^30 cm
Granitic
Tully Gr 0^10 cm
Tully Gr 20^30 cm
Basaltic
Innisfail Bas 0^10 cm
Innisfail.Bas 20^30 cm
Berner Creek soils
IL950011/1
IL950012/1
IL95030 fert
IL95007/1 fert
IL95007/7 fert
IL95015 unfert
IL95021 unfert
IL95023 unfert
Basalts [26]
ath12
rr225
bk112
bk114
Distance
(km)
ONd
87
Sr/86 Sr
Nd
(ppm)
8.9
32
32
29
29
24
18
18
13
11
10
6
5
3
2
34.6 R 0.5
35.1 R 0.2
35.2 R 0.2
34.9 R 0.3
35.3 R 0.2
35.0 R 0.5
36.0 R 0.2
36.1 R 0.2
36.3 R 0.2
36.3 R 0.4
36.5 R 0.2
36.9 R 0.2
38.1 R 0.4
37.9 R 0.3
0.71447 R 1
0.71129 R 5
0.71806 R 2
0.70973 R 1
0.71127 R 1
0.71810 R 1
0.71653 R 2
0.71504 R 2
0.71527 R 1
0.71549 R 1
0.71900 R 6
0.71665 R 1
0.72077 R 1
0.72020 R 1
31.0
35.6 R 0.4
33.4 R 0.3
0.71816 R 1
0.71496 R 9
30.6
31.4
32.5
37.1 R 0.3
37.2 R 0.4
36.5 R 0.3
0.71922 R 2
0.72067 R 3
0.71544 R 1
36.4 R 0.3
0.71378 R 2
0.71705 R 1
14.2
14.1
40.4
32.0
Porganic
(ppm)
Ptotal
(ppm)
67.5
79
61
55
39
34
39
14
24.5
17.5
2.5
14
0.3
0.3
18
328
49
9.5
298
260
31
235
35
287
77
334
1.5
0.5
8.3
1.2
0.98
37.7 R 0.3
38.0 R 0.4
0.82493 R 3
0.83519 R 2
18.9
14.0
0.2 R 0.4
0.0 R 0.2
0.70738 R 3
0.70779 R 5
12.9
2.4 R 0.3
3.9 R 0.3
4.7 R 0.3
3.6 R 0.3
3.3 R 0.3
2.8 R 0.3
4.1 R 0.3
3.5 R 0.3
0.70630 R 2
0.70455 R 2
0.70549 R 2
0.70598 R 2
0.70690 R 2
0.70513 R 2
0.70556 R 2
0.70552 R 2
5.2 R 0.3
3.4 R 0.3
3.0 R 0.3
5.8 R 0.3
0.70391 R 2
0.70461 R 2
0.70472 R 2
0.70367 R 2
inated by coarser £uvial sediments, presumably
derived mainly from the granitic soils. The ONd
values then increase steadily with distance o¡shore, from ONd = 36.5 to 34.5 in the carbon-
CaCO3
(%)
440
290
1200
760
13.5
4080
2100
3893
4759
4080
2343
1979
1615
4241
4248
22.3
25.2
25.4
22
2760
1970
2260
4080
ate-dominated reef sediments. The trend towards
less negative values with increasing distance from
shore implies larger proportions of sediment derived from basaltic soils. This is consistent with
EPSL 6623 22-4-03 Cyaan Magenta Geel Zwart
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M. McCulloch et al. / Earth and Planetary Science Letters 210 (2003) 249^258
Fig. 3. Plot of the Nd isotopic composition of marine bottom sediments (solid), estuarine sediments (open), and suspended sediments collected from the North (NJ) and South
(SJ) Johnstone rivers during the 1994 Cyclone Sadie. The
systematic increase in ONd values with distance o¡shore indicates an increasing proportion of ¢ne-grained basaltic-derived
soils. The proportion of basaltic relative to granitic soils is
calculated assuming linear mixing with 40% by mass of basalt soil (ONd V3 R 1) contributing s 80% of the terrigenous
derived P near the coral reefs (30^35 km). Within V5 km of
the river mouth, sediments consist mainly of coarse-grained
£uvial sediments of granitic origin and hence have more negative ONd values similar to their granitic parents (ONd V
38 R 1).
(87 Sr/86 SrV0.705 R 0.001) compared to the parent
basalt (87 Sr/86 SrV0.7040 R 0.0005), possibly indicating a small marine cyclic salt and/or aeolian
contribution. Soils derived from the Palaeozoic
granites, however, have very distinctive isotopic
values, with ONd = 38 R 1 and 87 Sr/86 SrV0.82.
The Sr isotopic composition of the granitic soil
is likely to be highly variable, as high Rb/Sr
phases such as mica have much higher 87 Sr/86 Sr
ratios than the low Rb/Sr Ca-feldspars. For this
reason the granitic soils would be expected to
have a broad range of grain size-dependent 87 Sr/
86
Sr ratios. An additional complication is that, in
contrast to Nd, appreciable amounts of Sr are
present in marine CaCO3 sediments with seawater
87
Sr/86 SrV0.7092. Although the carbonate component was largely removed from the sediments
by leaching with acetic acid, small residues would
shift the Sr isotopic composition to seawater ratios (Fig. 4).
Assuming either linear or curvilinear mixing
(Fig. 4) gives mass fractions of basaltic relative
to granitic soils of V10 R 5% for sediments in
the estuary and nearshore £uvial zone, increasing
to V40 R 10% for the marine sediments nearest to
observations that the basalt-derived soils are ¢ner-grained, having high clay contents (60^80%),
and that once discharged into the inshore region
of the GBR, prevailing hydrodynamic conditions
allow only the ¢ner particles to be transported
o¡shore. Deposition of coarser £uvial sands is
generally restricted to the estuary and nearshore
areas [22,23].
In order to better quantify the proportions of
basalt- versus granite-derived soils entering the
GBR, mixing relationships have been examined
using combined Nd^Sr isotopic systematics. Basaltic soils from Berner Creek, a headwater catchment, have ONd = +2 to +5, a range which is very
similar to their parent Tertiary basalts (Fig. 4).
The fertilised soils do not show any shifts in ONd
indicative of fertiliser addition [25]. Two size fractions from a basalt soil collected from the Johnstone River bank near Innisfail have ONd = +0.0
and 0.2. Strontium isotopic compositions of the
soils tend to be shifted to slightly higher values
Fig. 4. Mixing relationships for Nd^Sr isotopic systematics
in sediments and end-member components. The marine sediments are modelled as representing mixtures of soils derived
from Tertiary basalts [26] (ONd V3 R 1 and 87 Sr/86 SrV0.7055)
and Palaeozoic granites and metamorphics (ONd V38 R 1 and
87
Sr/86 SrV0.72^0.82). Two bulk mixing curves are shown
with (Sr/Nd)basalt soil /(Sr/Nd)granitic soil ranging from 1 (linear)
to 20 (hyperbola), which give similar mass fractions of basalt. The near-reef sediments (25^35 km) have lower than anticipated 87 Sr/86 Sr, which may re£ect small amounts of residual marine carbonate having seawater 87 Sr/86 Sr (0.7092).
EPSL 6623 22-4-03 Cyaan Magenta Geel Zwart
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255
Fig. 5. Schematic diagram showing the transport of suspended sediments and P predominantly as particulate form into the GBR
lagoon. The release of P probably occurs as a consequence of iron reduction in anaerobic zones in the sea-£oor environment
[27^29]. In the carbonate-rich sediments adjacent to the coral reefs, the Nd isotopic constraints indicate that V40% of the terrigenous sediment is derived from basalt soils. This implies that s 80% of terrestrial P entering the GBR is possibly derived from
erosion of basaltic soils.
the Gibson and Howie coral reefs (Fig. 1). Uncertainties in the mass fractions re£ect those in the
Nd and Sr concentrations and isotopic compositions of the respective end-member components.
Thus granitic detritus is the dominant sedimentary component, consistent with the observations
that the main river channels are cut into granitic
bedrock, and where present, the Tertiary basalts
form only thin cappings [13,26].
Suspended sediments collected during the 1994
Cyclone Sadie [17] (Figs. 3 and 4) have Nd^Sr
compositions that are either similar to the nearshore marine sediments (i.e. South Johnstone
ONd = 35.6) or have slightly higher values (i.e.
North Johnstone River ONd = 33.6), implying a
greater proportion of basalt soil component
(V50 R 10%) in the latter. Using the measured
Ptot in the North Johnstone Sadie-suspended sediment (1900 mg/kg), together with P in basaltic
(V3500 mg/kg) and granitic soils (600 mg/kg),
also indicates approximately equal proportions
of basalt versus granitic soils of 50 R 10% in the
North Johnstone Sadie sample. This compares
with V25% of basaltic soil in the South Johnstone suspended sediment. Interestingly, the average of the ONd values for the North and South
Johnstone Cyclone Sadie soils is indistinguishable
from the ONd composition of the most distal o¡shore sediments. This provides further con¢rmation that it is the ¢ne-grained suspended sediments that are transported more e⁄ciently
o¡shore and northwards with the prevailing currents and winds. The presence of additional small
contributions of terrigenous components from either more southerly river plumes such as from the
Tully, Herbert and Burdekin rivers or possibly
minor contributions from distal volcanic sources
such as the Indonesian island-arc cannot be excluded, but in this study are not apparent.
4. Discussion and conclusions
From the combined P^Nd^Sr systematics and
EPSL 6623 22-4-03 Cyaan Magenta Geel Zwart
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M. McCulloch et al. / Earth and Planetary Science Letters 210 (2003) 249^258
sedimentological constraints, a relatively coherent
model can now be developed for the transport of
terrestrial P into the GBR lagoon (Fig. 5). Basaltic soils make up 10^40% of the terrigenous sediment component deposited in the GBR from the
Johnstone River catchment, with the proportion
increasing with distance o¡shore (Fig. 3). However, in terms of overall P budgets, the basaltic
soil is by far the most important component, accounting for up to 80^90% of the total terrestrialderived P reaching the coral reefs of the GBR.
Assuming approximately equal contributions of
P from terrestrial and marine sources this indicates that basalt-derived P may account for up
to V40^50% of the total P incorporated in the
coral reef sediments. The dominant mechanism
for transport of terrestrial P into the nearshore
environment of the GBR is via particulates, with
£uvial-suspended sediments typically accounting
for s 80% of total P (Ptot ) budget in £uvial systems [8]. There is no evidence for large-scale desorption of P from suspended sediments during
transport into the marine environment. For example, equilibrium P concentration experiments [15,
17] conducted on suspended sediments ( s 0.45
Wm) collected in the Johnstone River estuary
5 days after £ooding associated with the 1994 Cyclone Sadie have been shown to have desorbed
little or no P ( 6 1% of total P) when suspended
in P-free water. Suspension of the same sediment
in 3.5% NaCl (similar conductivity to seawater)
indicated that salinity did not promote further P
desorption. This suggests that as the £ood plume
moves o¡shore, P is likely to undergo only limited
desorption from suspended particulates, despite
encountering increasing salinity [8,15^17]. Direct
measurements of dissolved inorganic and organic
P from the GBR £ood plumes [31] generally show
the most elevated concentrations in the lowest-salinity portions ( 6 V2.5% NaCl) of £ood plumes.
This suggests that, although the P budget in £ood
plumes is generally dominated by particulate P, on
shorter timescales direct input by rivers of nutrients (P and N) in dissolved form is still important [8,31]. These dissolved nutrients often result in
enhanced productivity in the frontal convergence
zone of £ood plumes, with rapid biological uptake
and recycling of nutrients by phytoplankton [31].
Close to the estuary, £uvial sediments dominate
[22,23], but in a zone 5^12 km o¡shore, ¢negrained anoxic muds (clay contents of 60^80%)
predominate, with P an order of magnitude lower
than that measured in the suspended plume sediments. This indicates that desorption of P occurs
in the bottom sediments (Fig. 4), probably via the
reduction of ferric phosphates [27^29]. On longer
timescales phosphorus is thus mainly solubilised
through the diagenesis of bottom sediments,
which in this region appear to have released
V80% of their P into the water column. Solubilisation of P is likely to involve the release of Prich pore waters from bottom sediments during
wind- or cyclone-driven sediment re-suspension
events [29]. The timescales for these processes
are unknown but are likely to occur over months
to many years. The nearshore anoxic zone therefore has the potential to be a long-term source of
dissolved P to the marine environment.
Although the amount of terrigenous sediment
declines with increasing distance o¡shore, the relatively constant Nd concentration at V1/3 of
crustal levels in the carbonate-rich reef sediments
implies the presence of widely distributed ¢negrained terrestrial components in the reef sediments. This contrasts with the budget for organic
carbon derived from carbon isotopic analyses,
which show [32] that the terrestrial plant detritus
is mainly deposited within V2 km of the river
mouth with no terrestrial 13 C signatures being
present in the o¡shore reefs. This is consistent
with much more rapid recycling of organic carbon
compared to the rare earth element Nd, the latter
essentially remaining ¢xed in the sedimentary column. Furthermore, since Nd is derived almost
exclusively from the land, its isotopic signature
is una¡ected by biogeochemical processes.
The Nd isotopic results also show that the relative proportion of basaltic-derived soils in the
terrigenous sedimentary component increases
with proximity to the coral reef. As the ¢negrained basaltic soils contain high P, they are
likely to have a disproportionately larger e¡ect
on the ecology of GBR than hitherto realised.
Whilst direct measurements of recent deposition
rates of terrigenous components into this region
of the GBR are limited [23], trace element studies
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M. McCulloch et al. / Earth and Planetary Science Letters 210 (2003) 249^258
of corals have shown an increase in terrestrially
derived sediments by U4^8 since European settlement [30]. It is thus likely that more intense land
use, particularly on the basaltic soils, has enhanced the P £uxes. This study therefore provides
further support for the need for general adoption
of minimum-impact soil surface management
practices (e.g. minimum tillage, green cane trash
blanketing) combined with measures such as the
re-establishment of riparian vegetation zones to
reduce transport especially of P-rich basaltic soils
into coral reefs. Unfortunately, the e¡ects of the
past 100 years of inappropriate land-use practices
cannot be rapidly reversed, as drainage lines still
contain accumulated ¢ne-grained sediments and
in the nearshore environment sediment desorption
of P will be an ongoing process.
Acknowledgements
[6]
[7]
[8]
[9]
[10]
[11]
This project was partially supported by an Australian Research Council grant to M.M. on the
quanti¢cation of anthropogenic £uxes to the
Great Barrier Reef. We appreciate the helpful
comments by Elbaz-Poulichet and anonymous reviewers. We thank E. Bard for pointing out an
oversight in our cartography of the Australian
coastline.[BARD]
[12]
[13]
[14]
[15]
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