- Email: [email protected]
Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates
Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 71 (2007) 4693–4703 www.elsevier.com/locate/gca
Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates Nicola Allison a
a,*
, Adrian A. Finch a, Jody M. Webster
b,c
, David A. Clague
c
School of Geography & Geosciences, University of St. Andrews, St. Andrews, Fife KY16 9AL, UK b School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia c Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA
Received 10 September 2006; accepted in revised form 8 July 2007; available online 7 September 2007
Abstract The geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry. We used secondary ion mass spectrometry (SIMS) to measure Sr, Mg, B, U and Ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil Porites corals from submerged reefs around the Hawaiian Islands. Cement and primary coral geochemistry were significantly different in all corals. We estimate the effects of cement inclusion on climate estimates from drilled coral samples, which combine cements and primary coral aragonite. Secondary 1% calcite or 2% aragonite cement contamination significantly affects Sr/Ca SST estimates by +1 C and 0.4 to 0.9 C, respectively. Cement inclusion also significantly affects Mg/Ca, B/Ca and U/Ca SST estimates in some corals. X-ray diffraction (XRD) will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection (1%) of the technique. Thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed. To confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton. Sr/Ca, B/Ca and U/Ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces i.e. the coral structures maintain their original composition and selective SIMS analysis of these structures offers a route to the reconstruction of accurate SSTs from altered coral skeletons. However, Mg/Ca and Ba/Ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high Mg calcite cement. We hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure. 2007 Elsevier Ltd. All rights reserved.
1. INTRODUCTION The geochemistry of coral skeletons is affected by the environmental factors prevailing at the time of skeletal deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However, fossil corals can undergo post-depositional diagenesis during which parts of the original coral skeleton (the primary aragonite) may dissolve and secondary minerals (e.g. aragonite, low- and
*
Corresponding author. E-mail address: [email protected] (N. Allison).
0016-7037/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.07.026
high-Mg calcite, depending on the diagenetic environment) may be deposited. For the purposes of this work, we collectively term all secondary minerals as cements although total infilling of skeletal porosity is rarely observed in the corals examined here. For palaeoenvironmental reconstruction corals are typically sampled by microdrill and several mg of carbonate are drilled from successive positions in the skeleton to create a time series. Cements may have significantly different geochemistry compared to the primary coral aragonite (e.g. Muller et al., 2001) and their inclusion in drilled coral samples may lead to erroneous estimates of past climate. Few studies have quantified the effects of diagenesis on palaeoenvironmental estimates (Enmar et al.,
4694
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
2000; Muller et al., 2001; McGregor and Gagan, 2003; Lazar et al., 2004; Quinn and Taylor, 2006). Cement compositions have been determined by electron microprobe (e.g. MacIntyre, 1977; Bar-Matthews et al., 1993; Sherman et al., 1999; Enmar et al., 2000) but these studies are restricted to elements above the limit of detection of the technique (notably Sr and Mg are of palaeoenvironmental significance) and are hampered by poor analytical precision (e.g. typically 5% for Sr). Other authors used bulk methods to analyse drilled samples (combining primary coral and cements) and have estimated the relative composition of the cement based on estimates of cement abundance made by XRD (McGregor and Gagan, 2003) or by petrography (Enmar et al., 2000). In this paper, we assess the significance of cements on climate reconstructions using a suite of fossil Porites spp. corals collected from submerged reefs around the Hawaiian Islands. Many drowned Hawaiian reefs are well preserved offering excellent opportunities for palaeoenvironmental reconstruction in the sub-tropical Northern Pacific Ocean. We have used SIMS to determine the concentrations of a range of potential palaeoenvironment proxies (Sr, Mg, B, Ba and U) in primary coral aragonite and secondary cements. The high spatial resolution of SIMS allows the independent analysis of small areas of secondary minerals or primary skeleton. We also compare the geochemistry of primary coral aragonite in cemented and uncemented areas of the skeleton to investigate if the trace element concentrations of the primary aragonite are affected by local cement precipitation. We estimate the likely effects of cements on climate estimates from drilled coral samples. Coralline Sr/ Ca is commonly used to indicate SST (e.g. Beck et al., 1992) while Ba is a combined indicator of SST and of seawater Ba/Ca composition which reflects upwelling and freshwater runoff (Shen and Sanford, 1990). Mg, B and U have been related to SST in some corals (e.g. Min et al., 1995; Mitsuguchi et al., 1996; Sinclair et al., 1998, respectively) although some correlations are poor (e.g. Quinn and Sampson, 2002) and the robustness of these proxies has yet to be proved. U series dating in corals is an important tool in both dating palaeoenvironmental specimens and in developing models of sea levels changes from raised coral terraces (e.g. Yokoyama et al., 2001) while B isotopic composition is a potential indicator of local seawater pH (Honisch et al., 2004). A comparison of the boron and uranium isotopic compositions of the primary skeleton and cements is beyond the scope of this study, however our data on the concentrations of these elements provides some insight into the likely impact of diagenesis on isotope-based proxies. 2. MATERIALS AND METHODS 2.1. Corals Fossil Porites samples were collected using a submersible (Pisces) and a ROV (Tiburon) from a series of drowned reefs around the Big Island and Oahu, Hawaii. Porites is an annually banded reef-building coral and is the genus most commonly used for palaeoenvironmental reconstruction. We examined 20 coral specimens by scanning electron
microscopy and selected diagenetically altered sections of seven specimens for detailed analysis. Sample sites, mineralogy and specimen ages are summarised in Table 1. 14C and U/Th ages were made on apparently pristine sections of two specimens and the other corals were dated stratigraphically (Table 1). 2.2. Sample characterisation A 10 mm thick slice was cut through the centre of each sample and two thin strips were cut along the axis of maximum skeletal growth from each slice. One strip was finely ground for XRD analysis with a Philips PW1050 diffractometer using SIROQUANT software (Sietronics Pty. Ltd.) to provide quantitative estimates of modal concentrations of all phases. Analysis of standard mixtures indicated that the limit of detection of calcite in aragonite was <1%. The second strip was divided into 15 mm lengths to produce thin sections suitable for SIMS. Samples were mounted onto 2.5 cm diameter glass slides with epoxy resin (Buehler Epothin) and ground to a thickness of 50 lm. For scanning electron microscopy (SEM) additional pieces of the skeletons were fractured to expose the internal structure, cleaned in an ultrasonic bath, dried and gold coated. Samples were photographed in a JEOL JSM 5600 microscope in the School of Chemistry at the University of St Andrews, UK. 2.3. SIMS Primary coral aragonite and cements were identified in the thin sections and analysed by SIMS. To investigate if the precipitation of cements in skeletal pore spaces affects the geochemistry of the primary coral aragonite we compared the trace element composition of coral fasciculi (the bundles of acicular needle crystals) in adjacent areas of the skeleton which did and did not contain cements. Trace element/Ca ratios were determined using a Cameca ims-4f ion microprobe in the School of GeoSciences at the University of Edinburgh, UK. Thin sections were gold coated and analysed with a 16O ion beam, accelerated at 14.5 kV. Instrument conditions were primary beam current = 20 nA, energy offset = 75 eV, imaged field = 25 lm, field aperture 1 (1800 lm) and contrast aperture 3 (150 lm). The analytical diameter was 25 lm. Secondary ion counts were measured using an electron multiplier by sequential stepping of the magnetic field through a cycle of relative atomic masses. Each analysis is the sum of ten cycles. Count times and statistics are summarised in Table 2. Background counts were measured at mass 4.7 and were typically 0.02 cps. The total time per analysis (including other isotopes not reported here) was 15 min. We used a pre-analysis burn-in time of 3 min to remove surface contamination. Trace element/Ca ratios on the sample did not change significantly over the 10 cycle period. Internal reproducibility (the precision at a single point) was calculated from the standard deviation (s) of the trace element/ p Ca ratio in the ten cycles in each analysis as (t*(s/ 10)), (t = 2.26). Mass interferences for Sr (Allison and Finch, 2004) and Mg and Ba (Allison, 1996b) are insignificant.
Effects of submarine coral diagenesis on climate estimates
4695
Table 1 Details of fossil corals studied. Bold dates are 14C or U/Th age estimates from actual specimens and range dates indicate predicted age ranges based on the sample’s stratigraphic relationship to dated samples (Ludwig et al., 1991; Moore et al., 1990a; Riker-Coleman et al., 2005; Webster et al., 2004) Sample
Age (ky) a
P5-67-3 P5-78-5 T304-R3 T277-R8 T286-R8 T301-R43 T313-R23
13.48 +0.35/0.43 13.9 ± 0.3b 14–125 130–140 225–276 370–400 1000–1500
Depth of fossil reef (m)
Mineralogy
175 207 250 412 593 967 499
98% 94% 99% 77% 98% 99% 92%
aragonite, aragonite, aragonite aragonite aragonite, aragonite, aragonite,
2% Mg calcite 4% Mg calcite
2% Mg calcite 1% calcite 8% calcite
Mineralogy of specimens was determined by X-ray diffraction. All specimens were collected from reefs around the Big Island with the exception of T313-R23 which was sampled off Oahu. a 14 C age, Moore et al., 1990a, calibrated in Webster et al., 2004. b U/Th age, Webster et al., 2004.
Table 2 SIMS count times, count rates and analysis statistics on coral Isotope
Count time per cycle (s)
Total counts per analysis
Internal precision, M/Ca (%)
Total counts per analysis in epoxy resin
11
5 3 2 2 15 20
25000 80000 6.9 · 106 2.4 · 106 4000 450
4.5 3.5 — 0.5 7 10
680 2000 216 40 6 0
B Mg 44 Ca 88 Sr 138 Ba 238 U 26
Internal precision was calculated from the trace element(M)/Ca ratio, see text.
We observe no counts at mass 238 when analysing on areas of the section filled with epoxy resin, indicating that the 197 Au40Ca1H trimer does not contribute significantly to counts attributed to 238U. We anticipate no significant interference for 11B. Data were collected over a total of four analytical sessions (each of one week duration). Trace element count rates were normalised to 44Ca and compared to standards (Table 3). The relative ion yields (RIYs) of Mg, Sr and Ba to Ca were estimated daily after multiple analyses (n = 3–10) on the carbonate standard OKA carbonatite (Ca concentration = 40.04%). The standard was usually analysed at the start of each day. Occasional analyses on the standard at the end of the day confirmed that RIYs did not vary significantly within a day and we observed
Table 3 Details of standards used for SIMS analysis Element
Standard
Std concentration, ± 1s (mmol mol1)
External precision, M/Ca (%)
RIY
B Mg Sr
Apatite OKA OKA
3.3 0.7 0.5
— 0.2895 1.0497
Ba U
OKA Apatite
Unknown 2.75 ± 0.13 (n = 3) 13.66 ± 1.65 (n = 3) 0.95 ± 0.22 (n = 4) 4.84 · 103
0.9 2.1
1.0104 0.3575
no significant variations in RIYs within an analytical session. Sr and Ba to Ca RIYs did not vary significantly between analytical sessions while the Mg to Ca RIY varied by up to 4%. B and U counts in this standard are low and we analysed an apatite standard to check instrument stability for these elements. The B concentration of the standard is unknown but 11B counts are relatively high (300 cps). Analyses on the apatite were made on 1 or 2 days in each session and we have assumed no significant changes in RIY over the remainder of the session. The p external reproducibility of analysis (t*(s/ 10)) was calculated from the standard deviation (s) of 10 analyses on each standard (Table 3).We estimate a B/Ca RIY of 0.095 by making multiple (n = 58) SIMS analyses across a 1 year transect of a modern Porites lobata coral collected from Lanikai, Oahu and assuming a B/Ca concentration of 0.391 mmol mol1 (reported for a shallow water Porites compressa from a similar locality (Honisch et al., 2004)). The accuracy of our SIMS estimates is affected by uncertainty in the composition of the standards (e.g. reflecting interlaboratory differences and/or variations in the different OKA batches analysed by each laboratory, Table 3) and by potential matrix effects resulting from chemical and physical differences between the standards and sample, i.e. aragonite, calcite and apatite. We have compared estimates of trace element concentration along adjacent skeletal transects by SIMS and by bulk methods of analysis. We analysed a 10 mm transect (equivalent to 1 years growth) of a fossil Porites from Papua New Guinea (our reference H96-18), previously drilled at 1 mm resolution and precleaned and analysed for Mg/Ca, Sr/Ca and U/Ca by ICP-MS in David Lea’s laboratory, University California, Santa Barbara, USA. We also analysed a 17 mm transect (2 years) of a Pavona clavus from the Galapagos previously analysed for Ba/Ca (Shen et al., 1992). The Ba composition of this specimen (Table 4) is similar to that of modern (Allison and Finch, 2007) and fossil (Table 5) Porites. We standardised all our SIMS data to these bulk values (Table 4). We were unable to compare SIMS and bulk B/Ca estimates for the same coral and accept that data for this element may be inaccurate. However our % estimates of analytical precision of SIMS are good (Tables 2 and 3).
4696
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
Table 4 Mean trace element concentrations and 95% confidence limits determined by SIMS and bulk analyses of parallel coral transects SIMS
Bulk analysis
Standardisation factor
Porites fossil Mg/Ca mmol mol1 Sr/Ca mmol mol1 U/Ca lmol mol1
4.88 ± 0.28 (n = 34) 8.630 ± 0.053 (n = 34) 0.344 ± 0.014 (n = 29)
3.919 ± 0.090 (n = 11) 8.994 ± 0.056 (n = 11) 1.166 ± 0.018 (n = 11)
0.80 1.04 3.39
Pavona clavus Ba/Ca lmol mol1
3.97 ± 0.13 (n = 95)
4.32 ± 0.14 (n = 8)
1.09
We have considered the likely impact of contamination of some of our analyses by small volumes of epoxy resin, e.g. where the diameter of the cement may be <25 lm or where spaces between acicular secondary aragonites have filled with epoxy resin. We observed no significant differences in total Ca counts in analyses on primary coral and secondary minerals indicating that contribution of epoxy resin to the analysis was low (<10%). We made several SIMS analyses on the epoxy resin under the same sputtering conditions as used on the carbonate samples. Count rates in the epoxy resin are low for all elements (Table 2) and we estimate that introducing a 10% volume of epoxy into a coral analysis would affect the original trace element/Ca ratio by <0.3% for B/Ca and Mg/Ca and by <0.02% for Sr/Ca, Ba/Ca and U/Ca. Count rates for B, Mg and Sr may be lower on some cements if they are present in lower concentrations compared to the coral (see Section 3). We estimate that a 10% volume of epoxy into cements of low B, Mg and Sr concentration would affect the original trace element/Ca ratio by <2% for B/Ca and Mg/Ca and by <0.02% for Sr/Ca. 3. RESULTS AND DISCUSSION 3.1. Distribution of cements All of the corals selected contain cements (Figs. 1 and 2). Secondary aragonite occurs as acicular cements syntaxial to the primary coral aragonite (Figs. 1a–b and 2a and c), i.e. the secondary aragonites precipitate onto and have the same lattice orientation as the original coral aragonite. The interface between the primary and secondary aragonite is best observed in thin section in transmitted light (e.g. Fig. 1a). Primary aragonite crystals of Porites corals are densely packed with little space between adjacent crystals (Fig. 1b and 2b). Secondary aragonite crystals become interspaced with voids as the crystals grow and extend away from the primary aragonite. This is shown in Fig. 2c where individual secondary aragonite crystals at the outermost edge of the circular skeletal unit may be separated by spaces of 5 lm or more. Similar spaces can be observed in Fig. 2f. When viewed in thin section by transmitted light these voids scatter light resulting in dark patches or lines between crystals. The exact interface between the primary and secondary aragonite may not be apparent by SEM (Fig. 2c) but is usually clear in transmitted light (Fig. 1a). We note that in some cases the interface may be difficult to discern, e.g. the interface to the bottom of Fig. 1b is obvious but is less clear at the top of the image. For the purposes of this
work we have restricted SIMS analyses to crystal types which can be identified clearly. Secondary calcite occurs in a range of textures: as micrite and as steep-sided rhombs, continued growth of which produced dog tooth crystals (Fig. 1c), suitable for SIMS. Calcite completely infills some interskeletal pore spaces and appears dark (Fig. 1d). The distribution of cements was very heterogeneous and we observe large variations in the abundance and types of cement within even single coral thin sections (15 · 15 mm, e.g. Fig. 1). Zones of different cement texture and/or abundance are frequently separated by dissepiments (Fig. 1a and 2a and d) which are thin layers of primary skeleton accreted approximately 2– 3 mm from the coral surface and parallel to it. They are produced approximately monthly and seal off the bulk of the skeleton from the uppermost skeletal layer, which is still occupied by the coral tissue. We did not observe neomorphism, i.e. the replacement of the primary coral aragonite by secondary calcite (Sherman et al., 1999) and found no evidence of barite or of brucite which has been observed in modern coral (Buster and Holmes, 2006). SIMS (see next section) indicates that all calcite cements analysed are high Mg calcite. High Mg calcite and secondary aragonite are typical shallow marine reef cements (MacIntyre, 1977). Aragonite cements may be precipitated within decades of accretion of the original skeleton (Enmar et al., 2000; Muller et al., 2001). Similarly secondary calcite has been observed along microborings of living corals (MacIntyre and Towe, 1976). We observe no consistent pattern between abundance or type of cement and age/depth of coral. Cementation is most pronounced in exposed reefs (e.g. MacIntyre, 1977) and the local positions of the coral specimens in each reef will have affected their exposure to wave activity. We observe dissolution of some of the centres of calcification (COCs, primary coral aragonite structures) in the fossil corals (Fig. 2b–d) leaving voids of 20 lm diameter. COCs in unaltered modern corals are composed of granular submicron crystals (Cohen et al., 2001) forming a structure of diameter 5–10 lm (Fig. 3a and b). We observed no alteration of the primary coral fasciculi which are formed from bundles of acicular crystals (e.g. Fig. 2b and h). Dissolution and cementation may occur simultaneously in different parts of submerged modern reefs reflecting variations in the composition of interstitial waters (Tribble et al., 1990). Dissolution below the aragonite compensation depth (1000 m at present) is unlikely to have been a significant factor at the depths of most of the corals studied here (Table 1) but may also occur in the vadose environment. The
1.62 ± 0.20 1.03 ± 0.80 1.42 ± 0.12 (n = 12) 15.33 ± 1.89
3.2. Composition of primary coral aragonite and cements and effects of cement inclusion on palaeoenvironmental estimates
T313-R23
Mean concentrations and 95% confidence limits are shown. Nd means not determined.
2.74 ± 0.19 144.2 ± 9.0 2.56 ± 0.19 59.9 ± 7.7 0.55 ± 0.05 0.25 ± 0.06 (n = 3) 0.56 ± 0.06 (n = 6) 0.13 ± 0.05 7 7 13 9 Primary coral Cement Primary coral Cement
4697
reefs sampled here were probably accreted during ice sheet growth when sea level fall and island subsidence occurred at similar rates, and stable shorelines conditions prevailed (Moore and Fornari, 1984). During deglaciations, sea level rose rapidly, drowning the reefs. It is possible that eustatic sea level changes exposed the reefs to the vadose environment before subsequent drowning, e.g. Sherman et al. (1999) infer subsequent periods of marine, meteoric and post-meteoric marine diagenesis in a nearshore submarine reef terrace in Oahu. However, we observe no low Mg calcite cements in these specimens which argues against subaerial exposure and meteoric diagenesis.
9.23 ± 0.22 1.51 ± 0.50 9.42 ± 0.14 0.99 ± 0.24
2.11 ± 0.37 2.73 ± 0.94 1.54 ± 0.30 1.64 ± 0.32
T313-R23
T286-R8
T304-R3
P5-78-5
Calcite cements T277-R8
coral
coral
coral
coral
11 12 10 6 8 8 9 8 13 8 Primary Cement Primary Cement Primary Cement Primary Cement Primary Cement
coral P5-67-3
nd nd 0.60 ± 0.04 0.15 ± 0.11 0.53 ± 0.08 0.12 ± 0.04 0.56 ± 0.15 0.20 ± 0.07 (n = 3) 0.56 ± 0.06 (n = 6) 0.15 ± 0.02 (n = 6)
2.93 ± 0.25 0.71 ± 0.26 3.16 ± 0.20 0.74 ± 0.41 3.41 ± 0.29 0.78 ± 0.27 3.04 ± 0.15 0.49 ± 0.15 2.56 ± 0.19 0.54 ± 0.53
9.47 ± 0.21 11.37 ± 0.49 8.95 ± 0.12 12.96 ± 1.66 9.26 ± 0.13 11.46 ± 0.37 9.40 ± 0.17 10.90 ± 0.33 9.42 ± 0.14 11.52 ± 0.38
(n = 6)
(n = 12) (n = 7)
1.42 ± 0.12 1.67 ± 0.17 1.13 ± 0.28 1.48 ± 1.03 1.27 ± 0.26 1.96 ± 0.40 1.46 ± 0.12 2.52 ± 0.46 1.42 ± 0.12 2.18 ± 0.44 2.57 ± 0.50 2.94 ± 0.37 2.24 ± 0.42 6.28 ± 2.31 5.26 ± 1.62 (n = 6) 4.16 ± 0.59 2.12 ± 0.31 3.27 ± 0.90 1.54 ± 0.30 3.35 ± 1.46
(n = 10)
U/Ca (lmol mol1) Mg/Ca (mmol mol1) B/Ca (mmol mol1) n Coral aragonite cements
Table 5 Composition of primary coral aragonites and adjacent cements, deposited in skeletal pores
Sr/Ca (mmol mol1)
Ba/Ca (lmol mol1)
Effects of submarine coral diagenesis on climate estimates
We performed SIMS analyses on Mg calcite and aragonite cements and on adjacent coral fasciculi. We examined thin sections in transmitted light before SIMS to select suitable areas for analyses. We avoided COCs and microborings, both of which are often associated with significantly different geochemistry compared to the surrounding skeleton (Allison, 1996a; Cohen et al., 2001; Allison and Finch, 2004; Meibom et al., 2004). Primary and secondary aragonites were readily distinguished in transmitted light (Fig. 1a and b) and we examined cemented areas carefully to avoid secondary calcite crystals interspersed between secondary aragonite (as in Fig. 2f). Most cement analyses were performed on substantially infilled pore spaces (e.g. Fig. 1a and d) to reduce any effects of contamination by epoxy resin. However some analyses were also performed on small areas or thin rims (e.g. Fig. 1c) of cement to check their composition. We observe no significant differences in the composition of these small areas compared to the more infilled areas and we have grouped together all analyses of each cement type in each coral for brevity. B was introduced into the analytical programme relatively late so fewer points have been analysed. We observe significant differences between the composition of both types of cements and the primary coral aragonite (Table 5 and Fig. 4). We also observe considerable variability in the composition of the cements. The coral and cement compositions can be interpreted in the light of likely controls on their chemistry, namely the composition of the precipitating solution and the trace element/ Ca partition coefficient. The cements precipitate from reef pore waters which reflect the chemistry of local seawater potentially modified by the precipitation and/or dissolution of other minerals in the reef pores. The coral skeleton precipitates from a calcifying fluid the composition of which is determined by the relative rates of active and passive ion transport to the calcification site. Carbonate composition may also be affected by pH and/or redox changes in the precipitating fluid, which can alter trace element speciation and affect the relative abundance of species which participate in carbonate precipitation. The pH of coral porewaters is considerably lower (7.5–7.7 Tribble et al., 1990; Enmar et al., 2000) than the pH of the coral calcifying fluid (ranging from 8 in dark conditions to >9 in the light, Al-Horani et al., 2003). Finally trace element/Ca partition coefficients
4698
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
Fig. 1. Transmitted light micrographs of coral T313-R23. The types and abundance of cements was heterogeneous and primary coral aragonite (PA), secondary aragonite (SA) and secondary calcite (SC) were observed within one thin section. Dissepiments (D) are marked. The interface between the primary and secondary aragonites is marked in (b).
Fig. 2. Scanning electron micrographs of corals T286-R8 (a–c), T313-R23 (d–f) and T277-R8 (g–i). Notation as for Fig. 1. The distribution of cements is heterogeneous. (a) No cement is visible to the left of the dissepiment (shown at higher magnification in (b) while aragonite cement is abundant on the right (shown at higher magnification in (c)). (d) Mg calcite crystals are distributed across the skeletal wall to the left of the dissepiment (shown at higher magnification in (e), while to the right of the dissepiment, aragonite cements are dominant with some Mg calcite crystals deposited between the aragonite crystals (shown at higher magnification in (f)). (g and h) Coral T277-R8 is almost completely infilled with micritic Mg calcite cement (shown at higher magnification in i). Centres of calcification in b, c and d are marked COC. Scale bars are 100 lm (a, d and g), 20 lm (b, c and h) and 10 lm (e, f and i).
may be affected by temperature, the carbonate phase, crystal shape and other kinetic factors, e.g. precipitation rate. For example, the trace element geochemistry of naturally precipitated non-biogenic shallow marine carbonates can
be highly variable, e.g. see Mucci (1987) for a review of Mg concentrations in calcites. We conclude that the variable composition of the cements reflects a wide range of pore water conditions.
Effects of submarine coral diagenesis on climate estimates
4699
Fig. 3. Scanning electron micrographs of a modern coral indicating the normal appearance and dimensions of a centre of calcification (COC). Scale bars are 20 lm (a) and 10 lm (b). Microboring holes (MB) are visible in this coral.
3.2.1. Sr/Ca All the aragonite cements contain significantly more Sr than the primary coral aragonite while all calcite cements contain significantly less Sr. Our estimates of Sr/Ca in aragonite cements are similar to one report (10.8 mmol mol1, MacIntyre, 1977) but higher than 2 other reports (9.0 and 10.1 mmol mol1, Sherman et al., 1999; Enmar et al., 2000, respectively). This may reflect real variations in coral composition or a difference in the calibration of the techniques used (e.g. previous estimates were made by electron probe microanalysis). The magnitude of Sr enrichment in the aragonite cements compared to the primary coral skeleton, is similar (typically 20%) in this and other studies (MacIntyre, 1977; Enmar et al., 2000) indicating that aragonite cements increase the Sr content of the whole skeleton (primary skeleton plus cements) by a similar amount in all cases. Using the mean compositions of the primary coral aragonite and cements in Table 5 we have estimated the effects of inclusion of 1% cement (by mass) on SST estimates from drilled coral samples (Table 6). Contamination of a pristine skeleton by 1% calcite cement causes an increase in SST estimates of 1 C while 1% aragonite cement decreases SST estimates by 0.2–0.3 C (although a larger decrease was observed in one coral). Sr/Ca determinations of drilled coral samples are usually made by thermal ionisation mass spectrometry (TIMS) or inductively coupled plasma atomic
Primary skeleton
0
B/Ca mmol mol-1 0.2 0.4 0.6
emission spectrophotometry (ICP-AES) which have typical analytical precision (2s) of <0.03% and <0.4%, respectively (Beck et al., 1992; Schrag, 1999). The inclusion of <1% or 2% of aragonite cement in drilled samples will significantly affect Sr/Ca estimates by each of these techniques respectively while <1% calcite cement samples will significantly affect Sr/Ca estimates by either of these techniques. 3.2.2. Mg/Ca The aragonite cements contain significantly less Mg than the primary coral aragonite while the calcite cements contain significantly more Mg. Estimates of the temperature dependence of Mg/Ca in Porites corals range from 0.088 to 0.164 mmol mol1 C1 (Fallon et al., 1999; Wei et al., 2000). Using an estimate in the middle of this range (Mitsuguchi et al., 1996), we calculate that the contamination of a pristine skeleton by 1% aragonite cement reduces SST estimates by 0.2 C while 1% calcite cement increases SST estimates by 4 and 11 C (Table 6). The typical precision (2s) of Mg/Ca analysis of drilled samples by ICPAES is <0.07 mmol mol1 (Mitsuguchi et al., 1996) and the inclusion of <1% calcite and 3% aragonite cement in drilled samples will significantly affect Mg/Ca estimates. 3.2.3. Ba/Ca We observe significantly higher Ba concentrations in the aragonite cements of 2 corals, compared to the primary
Cement
0
Mg/Ca mmol mol-1 2 4
Sr/Ca mmol mol-1 4 8 12
0
Ba/Ca μmol mol-1 2 4 6 8
0
U/Ca μmol mol-1 1 2 3 Aragonite cements
P5-67-3
0
P5-78-5 T304-R3 T286-R8 T313-R23
15.3
144
T313-R23
60
Calcite cements
T277-R8
Fig. 4. Composition of the primary coral aragonite and calcite cements of each skeleton. Values are means and 95% confidence limits.
4700
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
Table 6 Estimated effect of 1% cement contamination on SST estimates from the Mg/Ca, Sr/Ca, U/Ca and B/Ca composition of each coral Coral sample
Mg/Ca (C)
Sr/Ca (C)
U/Ca (C)
B/Ca (C)
Aragonite cements P5-67-3 P5-78-5 T304-R3 T286-R8 T313-R23
0.17 0.19 0.20 0.20 0.16
0.24 0.47 0.27 0.19 0.26
ns ns 0.15 0.23 0.16
nd +0.22 +0.20 +0.17 +0.21
Mg calcite cement T277-R8 T313-R23
+11.0 +4.44
+0.96 +1.05
ns 2.96
+0.14 +0.20
The temperature dependences used for the calculations for Sr/Ca (in Hawaiian Porites) and Mg/Ca, U/Ca and B/Ca in coral aragonite are 0.080 mmol mol1 C1, 0.13 mmol mol1 C1, 0.047 lmol mol1 C and 0.021 mmol mol1 C1 (de Villiers et al., 1994; Mitsuguchi et al., 1996; Min et al., 1995; Sinclair et al., 1998, respectively). ns indicates that the composition of the cements and coral were not significantly different (at p = 0.05). Nd, not determined.
aragonite (P5-78-5 and T313-R23) but no significant differences in the Ba compositions of the coral aragonite and cement of the remaining corals. In one case (coral p5-785) the Ba concentrations of aragonite cements were nearly 3x the concentrations observed on the primary coral aragonite. The inclusion of 5% cement in this coral sample can increase total Ba by up to 10%. The magnitude of the annual Ba signal in pristine coral is 20% in a Pavona clavus specimen from the Galapagos (Shen et al., 1992) indicating that cement inclusion can significantly affect total Ba and seasonal Ba trends. 3.2.4. B/Ca All the cements (aragonite and Mg calcite) contain significantly less B/Ca than the primary coral. The B concentration of inorganically precipitated calcite decreases as seawater pH decreases (Sanyal et al., 2000). The pH of coral porewaters is considerably lower (7.5–7.7) than that of the coral calcifying fluid (8 to >9 in the light) and the large decrease in the B/Ca concentration of the cements is therefore consistent with a pH effect. The pH dependence of B carbonate concentration may reflect the increase in the relative abundance of borate at higher pH if borate is the species predominantly incorporated in carbonates (see Pagani et al., 2005 for a review) or may reflect other factors e.g. crystal growth rate (Hobbs and Reardon, 1999). Variations in the B/Ca composition of the aragonite and Mg calcite cements are small and inclusion of 1% of either of these cements increases SST estimates by 0.2 C (Table 6). We estimate that d11B of coral aragonite is relatively robust to the effects of cement precipitation. d11B of abiotic shallow marine aragonites (22‰, Hemming and Hanson, 1992) and inorganically precipitated calcite (19‰ at pH 7.9, Sanyal et al., 2000) are less than that of coral aragonite (24‰, Hemming and Hanson, 1992). The reproducibility of d11B analysis is typically 0.5‰ and we estimate that 25% calcite cement by mass would be required to significantly reduce coral d11B using these estimates of carbonate d11B and the B concentrations observed in Table 5. The inclusion of 25% aragonite cement by mass reduces d11B by <0.2‰.
3.2.5. U/Ca U/Ca concentrations in the aragonite cements are higher than in the adjacent primary coral (although the difference is significant in only 3 of the 5 specimens) and are similar to a previous estimate (Lazar et al., 2004) in diagenetically altered living corals. The concentrations of U/Ca in the calcite cements were considerably higher than initially expected. The partition coefficient of U in inorganically precipitated calcite (0.3) is at least an order of magnitude less than that in aragonite (2–10, Meece and Benninger, 1993) and U(VI) in calcite is incorporated in a disordered and apparently less stable co-ordination environment than in aragonite (Reeder et al., 2000). We had therefore anticipated that U/Ca concentrations would be significantly less in the calcite cements. However in one specimen the calcite cements had similar concentrations to the primary aragonite and in the other coral (T323-R23) concentrations were significantly higher (15.3 lmol mol1) than in the primary aragonite or in the secondary aragonite cements analysed in other corals. High U/Ca partition coefficients (60– 430) have been estimated in spar calcite in zinc ores and the authors concluded that U (IV) substitutes for divalent Ca to form a stable complex under reducing conditions (Sturchio et al., 1998). Reducing environments can occur in the interior of living reefs (Tribble et al., 1990) and we hypothesise that the relatively high U concentrations observed in the Mg calcites reflect precipitation under these conditions. Inclusion of 1% aragonite cement decreased U/Ca based SST estimates by 0.2 C while inclusion of 1% of the Mg calcite cement, which had a significantly different U/Ca compared to the coral, decreased the SST estimate by 3 C. The typical precision (2s) of U/Ca analysis of drilled samples by TIMS is 2‰ (Min et al., 1995) and contamination of a pristine skeleton by <1% calcite or aragonite cement in drilled samples will significantly affect U/Ca estimates. Both high Mg calcite and aragonite cements can significantly enrich coral d234U (Henderson et al., 1993) so that true coral ages are underestimated. The magnitude of this effect is dependent on the timing and duration of diagenesis and will be insignificant if diagenesis occurs during or shortly after the life of the coral (Lazar et al.,
Effects of submarine coral diagenesis on climate estimates
geochemistry. We hypothesise that some alteration of the primary aragonite has occurred in this coral. Coral skeletons contain micropores at a range of scales from the micron sized borings produced by micro-organisms, e.g. algae and fungi (Fig. 3) to the sub micron spaces which can occur between skeletal fibres (Perrin, 2003). Infilling of these intraskeletal spaces by calcite could significantly affect the geochemistry of the primary skeleton. However, the composition of an intraskeletal calcite would have to be quite different from that observed in the interskeletal pore spaces (Table 5) in order to affect both the Mg and Ba of the primary coral. For example a volume of 0.6% of the interskeletal calcite cement would increase the Mg of the primary coral by the amount observed in Fig. 4 but would not significantly increase the Ba (or Sr, B or U) of the primary coral. Given the high compositional variability of inorganic calcites and cements (Mucci, 1987; Table 5) it is reasonable to assume that variations in the precipitating microenvironment and the fabric of the cement produced could significantly affect the cement geochemistry.
2004). Age rejuvenation effects are also affected by the U concentration of the cement and our observation of high U concentrations in a calcite cement indicates that these may be particularly significant in affecting coral age estimates. 3.3. Effects of cementation on primary aragonite composition The selective analysis of pristine areas of altered fossil skeletons by SIMS offers a route to the reconstruction of accurate SSTs from coral skeletons (Cohen and Hart, 2004; Allison et al., 2005). To investigate if cementation affects the geochemistry of the primary coral aragonite, we compared the trace element composition of coral fasciculi (the bundles of acicular needle crystals) in adjacent cemented and uncemented areas of 3 coral skeletons. In two specimens pristine coral and secondary aragonite cemented areas occurred on opposite sides of a dissepiment (as in Fig. 1) and the primary coral aragonite in each area would have been deposited at subtly different times, probably approximately one month apart. This may subtly affect the geochemistry of the two areas. In the third sample (T313-R23) pristine and Mg calcite cemented areas occurred within the same pair of dissepiments. We differentiated between cemented areas containing minor (<10% by volume, as in Fig. 1c) and major amounts of cement (>10% by volume, e.g. Fig. 1d). We observed no significant differences in the Sr/Ca, B/ Ca and U/Ca composition of the primary coral aragonite in cemented and pristine areas of all the corals (Fig. 5). The Sr/Ca error bars are ± 1% and reflect the natural heterogeneity of the primary aragonite at this scale (e.g. Allison and Finch, 2004). We observed no significant differences in the Mg/Ca and Ba/Ca composition of primary coral in areas infilled with aragonite cement compared to pristine areas. However, the Mg/Ca and Ba/Ca concentrations of the coral aragonite in sections of the coral infilled with calcite cements (T313-R23) were significantly higher than that of coral aragonite in pristine areas. Analyses in cemented and uncemented areas were made between the same pair of dissepiments in this coral and we have assumed that the primary coral in each area was deposited at approximately the same time and should have the same
-1
>10% calcite
No cement <10% aragonite
No cement <10% aragonite
3.6
4.2
9.0
9.2
9.4
-1
U/Ca μmol mol
B/Ca mmol mol (x10-2)
Sr/Ca mmol mol 4.8 8.8
We conclude that diagenesis and the precipitation of both aragonite and calcite cements can significantly affect coral trace element geochemistry. Contamination of a pristine skeleton by 1% calcite (the approximate detection limit of XRD) could increase Mg and Sr derived SST estimates by 1 C or more and decrease U derived estimates by 3 C. Aragonite cements cannot be detected by XRD screening. The inclusion of 5% aragonite cement will affect Sr, Mg and U derived SST estimates by 1 C and B derived SST estimates by +1 C. Of particular concern is the observation that inclusion of aragonite cements affects SST estimates by a similar magnitude in different geochemical proxies, i.e. Mg, Sr and U, leading to consistent but erroneous SST estimates from all proxies. We conclude that XRD screening may not detect geochemically significant amounts of cement in fossil corals. Thorough petrographic examination of fossil corals is necessary to confirm that they are pristine before drilled samples are analysed. We also note that cement may be difficult to observe in thin section. The main units of Porites coral skeletons, the
5.0 9.6
5.4
5.8
6.2
1.0 6.6
1.2
1.4
1.6
Ba/Ca μmol mol 1.8 1
-1
10
5
T313-R23 T301-R43 T304-R3
No cement <10% calcite
3.0
4. CONCLUSIONS
-1
-1
Mg/Ca mmol mol 2.4
4701
Fig. 5. Trace element concentrations of the primary coral aragonite in cemented and uncemented areas of each skeleton. The abundances and types of cement in the interskeletal pore spaces were estimated by petrographic analysis. Values are means and 95% confidence limits of 19–21 analyses, with the exception of U/Ca where means are of 7–12 analyses.
4702
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
cylindrical vertically growing trabeculae, are typically 100– 150 lm in diameter with a cross sectional area of 3– 7 · 104 lm2. The inclusion of 1% cement (by volume) would increase the diameter of the trabeculae by an average of only 0.5–0.75 lm. The distribution of cements is usually heterogeneous (Figs. 1 and 2) resulting in the precipitation of irregularly sized rims around skeletal pores, however careful petrographic examination may be required to identify these. The Sr/Ca, B/Ca or U/Ca of primary coral aragonite is not affected by the precipitation of cements in adjacent interskeletal pore spaces. The Mg/Ca and Ba/Ca of primary coral structures may be affected by calcite precipitation in pore spaces. This is an area for further investigation. ACKNOWLEDGMENTS This work was supported by the UK Natural Environment Research Council (award NER/A/S/2003/00473 to A.A.F. and N.A.). J.M.W. and D.A.C. acknowledge the support of the David and Lucile Packard Foundation through a grant to MBARI. Access to the ion probe was provided by NERC Scientific Services and we are indebted to Simone Kasemann for her assistance with the analyses. We thank Angus Calder and Andrew Rothwell for completing the XRD analyses. We thank Anne Cohen (AE) and three anonymous reviewers for comments which improved this manuscript.
REFERENCES Al-Horani F. A., Al-Moghrabi S. M. and de Beer D. (2003) The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis. Mar. Biol. 142, 419–426. Allison N. (1996a) Geochemical anomalies in coral skeletons and their possible implications for palaeoenvironmental analysis. Mar. Chem. 55, 367–379. Allison N. (1996b) Quantitative determinations of trace and minor elements in coral aragonite by ion microprobe analysis, with preliminary results from Phuket, South Thailand. Geochim. Cosmochim. Acta 60, 3457–3470. Allison N. and Finch A. A. (2004) High resolution Sr/Ca records in modern Porites lobata corals: effects of skeletal extension rate and architecture. Geochem. Geophys. Geosyst. 5. doi:10.1029/ 2004GC000696. Allison N. and Finch A. A. (2007) High temporal resolution Mg/ Ca and Ba/Ca records in modern Porites lobata corals, Geochem. Geophys. Geosyst. 8, Q05001, doi:10.1029/ 2006GC001477. Allison, N., Finch, A. A., Tudhope, A. W., Newville, M., Sutton, S. R., Ellam, R. (2005) Reconstruction of deglacial sea surface temperatures in the tropical Pacific from selective analysis of a fossil coral. Geophys. Res. Lett. 32, L17609, doi: 10.1029/ 2005GL023183. Bar-Matthews M., Wasserburg G. J. and Chen J. H. (1993) Diagenesis of fossil coral skeletons—correlation between traceelements, textures, and U-234/U-238. Geochim. Cosmochim. Acta 57, 257–276. Beck J. W., Edwards R. L. and Ito E., et al. (1992) Sea-surface temperature from coral skeletal strontium calcium ratios. Science 257, 644–647. Buster N. A. and Holmes C. W. (2006) Magnesium content within the skeletal architecture of the coral Montastraea faveolata: locations of brucite precipitation and implications to fine-scale data fluctuations. Coral Reefs 25, 243–253.
Cohen A. L., Lane G. D., Hart S. R. and Lobel P. S. (2001) Kinetic control of skeletal Sr/Ca in a symbiotic coral: implications for the palaeotemperature proxy. Paleoceanography 16, 20–26. Cohen A. L. and Hart S. R. (2004) Deglacial sea surface temperatures of the western tropical Pacific: a new look at old coral. Paeoceanography 19, PA4031. de Villiers S., Shen G. T. and Nelson B. K. (1994) The Sr/Catemperature relationship in coralline aragonite: influence of variability in (Sr/Ca)seawater and skeletal growth parameters. Geochim. Cosmochim. Acta 58, 197–208. Enmar R., Stein M., Bar-Matthews M., Sass E., Katz A. and Lazar B. (2000) Diagenesis in live corals from the Gulf of Aqaba. I. The effect on paleo-oceanography tracers. Geochim. Cosmochim. Acta 64, 3123–3132. Fallon S. J., McCulloch M. T., van Woesik R. and Sinclair D. J. (1999) Corals at their latitudinal limits: laser ablation trace element systematics in Porites from Shirigai Bay, Japan. Earth Planet. Sci. Lett. 172, 221–238. Hemming N. G. and Hanson G. N. (1992) Boron isotopic composition and concentration in modern marine carbonates. Geochim. Cosmochim. Acta 56, 537–543. Henderson G. M., Cohen A. S. and Onions R. K. (1993) U-234/U238 ratios and TH-230 ages for Hateruma Atoll corals— implications for coral diagenesis and seawater U-234/U-238 ratios. Earth Planet. Sci. Lett. 115, 65–73. Hobbs M. Y. and Reardon E. J. (1999) Effect of pH on boron coprecipitation by calcite: further evidence for nonequilibrium partitioning of trace elements. Geochim. Cosmochim. Acta 63, 1013–1021. Honisch B., Hemming N. G. and Grottoli A. G., et al. (2004) Assessing scleractinian corals as recorders for paleo-pH: empirical calibration and vital effects. Geochim. Cosmochim. Acta 68, 3675–3685. Lazar B., Enmar R., Schossberger M., Bar-Matthews M., Halicz L. and Stein M. (2004) Diagenetic effects on the distribution of uranium in live and Holocene corals from the Gulf of Aqaba. Geochim. Cosmochim. Acta 68, 4583–4593. Ludwig K. R., Szabo B. J., Moore J. G. and Simmons K. R. (1991) Crustal subsidence rate off Hawaii determined from 234U/238U ages of drowned coral reefs. Geology 19, 171–174. MacIntyre I. G. (1977) Distribution of submarine cements in a modern Caribbean fringing reef, Galeta point, Panama. J. Sed. Pet. 47, 503–516. MacIntyre I. G. and Towe K. M. (1976) Skeletal calcite in living scleractinian corals: microboring fillings, not primary skeletal deposits. Science 193, 701–702. McGregor H. V. and Gagan M. K. (2003) Diagenesis and geochemistry of Porites corals from Papua New Guinea: implications for paleoclimate reconstruction. Geochim. Cosmochim. Acta 67, 2147–2156. Meece D. E. and Benninger L. K. (1993) The coprecipitation of Pu and other radionuclides with CaCO3. Geochim. Cosmochim. Acta 57, 1447–1458. Meibom, A., Cuif, J. P., Hillion, F. O., et al. 2004. Distribution of magnesium in coral skeleton, Geophys. Res. Lett. 31(23), Art. No. L23306. Min G. R., Edwards R. L., Taylor F. W., Recy J., Gallup C. D. and Beck J. W. (1995) Annual cycles of U/Ca in coral skeletons and U/Ca thermometry. Geochim. Cosmochim. Acta 59, 2025–2042. Mitsuguchi T., Matsumoto E., Abe O., Uchida T. and Isdale P. J. (1996) Mg/Ca thermometry in coral-skeletons. Science 274, 961–963. Moore J. G., Clague D. A., Ludwig K. R. and Mark R. (1990a) Subsidence and volcanism of the Haleakala Ridge, Hawaii. J. Volcanol. Geoth. Res. 42, 273–284.
Effects of submarine coral diagenesis on climate estimates Moore J. G. and Fornari D. J. (1984) Drowned reefs as indicators of the rate of subsidence of the island of Hawaii. J. Geol. 92, 752–759. Mucci A. (1987) Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater. Geochim. Cosmochim. Acta 51, 1977–1984. Muller A., Gagan M. K. and McCulloch M. T. (2001) Early marine diagenesis in corals and geochemical consequences for paleoceanographic reconstructions. Geophys. Res. Lett. 28, 4471–4474. Pagani M., Lemarchand D. and Spivack A., et al. (2005) A critical evaluation of the boron isotope-pH proxy: the accuracy of ancient ocean pH estimates. Geochim. Cosmochim. Acta 69, 953–961. Perrin C. (2003) Compositional heterogeneity and microstructural diversity of coral skeletons: implications for taxonomy and control on early diagenesis. Coral Reefs 22, 109–120. Quinn, T. M., Sampson, D. E. (2002) A multiproxy approach to reconstructing sea surface conditions using coral skeleton geochemistry. Paleoceanography 17, Art. No. 1062. Quinn T. M. and Taylor F. W. (2006) SST artifacts in coral proxy records produced by early marine diagenesis in a modern coral from Rabaul, Papua New Guinea. Geophys. Res. Lett. 33, L04601, doi:10.1029/2005GL024972. Reeder R. J., Nugent M., Lamble G. M., Tait C. D. and Morris D. E. (2000) Uranyl incorporation into calcite and aragonite: XAFS and luminescence studies. Environ. Sci. Technol. 34, 638–644. Riker-Coleman, K., Gallup, C., Clague, D., Webster, J. M., Edwards, L., Cheng, H. (2005) New 230Th ages from the 400 m reef of northwestern Hawaii (2005). AGU Fall meeting: PP21C-1574. Sanyal A., Nugent M., Reeder R. J. and Buma J. (2000) Seawater pH control on the boron isotopic composition of calcite: evidence from inorganic calcite precipitation experiments. Geochim. Cosmochim. Acta 64, 1551–1555. Schrag D. P. (1999) Rapid analysis of high-precision Sr/Ca ratios in corals and other marine carbonates. Paleoceanography 14, 97–102.
4703
Shen G. T., Cole J. E., Lea D. W., Linn L. J., McConnaughy T. A. and Fairbanks R. G. (1992) Surface ocean variability at Galapagos from 1936–1982: calibration of geochemical tracers in corals. Paleoceanography 7, 563–588. Shen G. T. and Sanford C. L. (1990) Trace element indicators of climate change in annually banded corals. In Global Ecological Consequences of the 1982–83 El Nino (ed. P. W. Glynn). Elsevier, New York, pp. 255–283. Sherman C. E., Fletcher C. H. and Rubin K. H. (1999) Marine and meteoric diagenesis of Pleistocene carbonates from a nearshore submarine terrace, Oahu, Hawaii. J. Sed. Res. 69, 1083–1097. Sinclair D. J., Kinsley L. P. J. and McCulloch M. T. (1998) High resolution analysis of trace elements in corals by laser ablation ICP-MS. Geochim. Cosmochim. Acta 62, 1889–1901. Sturchio N. C., Antonio M. R., Saderholm L., Sutton S. R. and Brannon J. C. (1998) Tetravalent uranium in calcite. Science 281, 971–973. Tribble G. W., Sansone F. J. and Smith S. V. (1990) Stoichiometric modeling of carbon diagenesis within a coral-reef framework. Geochim. Cosmochim. Acta 54, 2439–2449. Webster J. M., Clague D. A., Riker-Coleman K., Gallup C., Braga J. C., Potts D., Moore J. G., Winterer E. L. and Paull C. K. (2004) Drowning of the 150 m reef off Hawaii: a casualty of global meltwater pulse 1A? Geology 32, 249–252. Wei G. J., Sun M., Li X. H. and Nei B. F. (2000) Mg/Ca, Sr/Ca and U/Ca ratios of a porites coral from Sanya Bay, Hainan Island, South China Sea and their relationships to sea surface temperature. Palaeogeogr. Palaeoclimatol. Palaeoecol. 162, 59– 74. Yokoyama Y. et al. (2001) Last glacial sea-level change deduced from uplifted coral terraces of Huon Peninsula, Papua New Guinea. Q. Int. 83(5), 275–283. Associate editor: Anne Cohen
Geochimica et Cosmochimica Acta 71 (2007) 4693–4703 www.elsevier.com/locate/gca
Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates Nicola Allison a
a,*
, Adrian A. Finch a, Jody M. Webster
b,c
, David A. Clague
c
School of Geography & Geosciences, University of St. Andrews, St. Andrews, Fife KY16 9AL, UK b School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia c Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA
Received 10 September 2006; accepted in revised form 8 July 2007; available online 7 September 2007
Abstract The geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry. We used secondary ion mass spectrometry (SIMS) to measure Sr, Mg, B, U and Ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil Porites corals from submerged reefs around the Hawaiian Islands. Cement and primary coral geochemistry were significantly different in all corals. We estimate the effects of cement inclusion on climate estimates from drilled coral samples, which combine cements and primary coral aragonite. Secondary 1% calcite or 2% aragonite cement contamination significantly affects Sr/Ca SST estimates by +1 C and 0.4 to 0.9 C, respectively. Cement inclusion also significantly affects Mg/Ca, B/Ca and U/Ca SST estimates in some corals. X-ray diffraction (XRD) will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection (1%) of the technique. Thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed. To confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton. Sr/Ca, B/Ca and U/Ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces i.e. the coral structures maintain their original composition and selective SIMS analysis of these structures offers a route to the reconstruction of accurate SSTs from altered coral skeletons. However, Mg/Ca and Ba/Ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high Mg calcite cement. We hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure. 2007 Elsevier Ltd. All rights reserved.
1. INTRODUCTION The geochemistry of coral skeletons is affected by the environmental factors prevailing at the time of skeletal deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However, fossil corals can undergo post-depositional diagenesis during which parts of the original coral skeleton (the primary aragonite) may dissolve and secondary minerals (e.g. aragonite, low- and
*
Corresponding author. E-mail address: [email protected] (N. Allison).
0016-7037/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.07.026
high-Mg calcite, depending on the diagenetic environment) may be deposited. For the purposes of this work, we collectively term all secondary minerals as cements although total infilling of skeletal porosity is rarely observed in the corals examined here. For palaeoenvironmental reconstruction corals are typically sampled by microdrill and several mg of carbonate are drilled from successive positions in the skeleton to create a time series. Cements may have significantly different geochemistry compared to the primary coral aragonite (e.g. Muller et al., 2001) and their inclusion in drilled coral samples may lead to erroneous estimates of past climate. Few studies have quantified the effects of diagenesis on palaeoenvironmental estimates (Enmar et al.,
4694
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
2000; Muller et al., 2001; McGregor and Gagan, 2003; Lazar et al., 2004; Quinn and Taylor, 2006). Cement compositions have been determined by electron microprobe (e.g. MacIntyre, 1977; Bar-Matthews et al., 1993; Sherman et al., 1999; Enmar et al., 2000) but these studies are restricted to elements above the limit of detection of the technique (notably Sr and Mg are of palaeoenvironmental significance) and are hampered by poor analytical precision (e.g. typically 5% for Sr). Other authors used bulk methods to analyse drilled samples (combining primary coral and cements) and have estimated the relative composition of the cement based on estimates of cement abundance made by XRD (McGregor and Gagan, 2003) or by petrography (Enmar et al., 2000). In this paper, we assess the significance of cements on climate reconstructions using a suite of fossil Porites spp. corals collected from submerged reefs around the Hawaiian Islands. Many drowned Hawaiian reefs are well preserved offering excellent opportunities for palaeoenvironmental reconstruction in the sub-tropical Northern Pacific Ocean. We have used SIMS to determine the concentrations of a range of potential palaeoenvironment proxies (Sr, Mg, B, Ba and U) in primary coral aragonite and secondary cements. The high spatial resolution of SIMS allows the independent analysis of small areas of secondary minerals or primary skeleton. We also compare the geochemistry of primary coral aragonite in cemented and uncemented areas of the skeleton to investigate if the trace element concentrations of the primary aragonite are affected by local cement precipitation. We estimate the likely effects of cements on climate estimates from drilled coral samples. Coralline Sr/ Ca is commonly used to indicate SST (e.g. Beck et al., 1992) while Ba is a combined indicator of SST and of seawater Ba/Ca composition which reflects upwelling and freshwater runoff (Shen and Sanford, 1990). Mg, B and U have been related to SST in some corals (e.g. Min et al., 1995; Mitsuguchi et al., 1996; Sinclair et al., 1998, respectively) although some correlations are poor (e.g. Quinn and Sampson, 2002) and the robustness of these proxies has yet to be proved. U series dating in corals is an important tool in both dating palaeoenvironmental specimens and in developing models of sea levels changes from raised coral terraces (e.g. Yokoyama et al., 2001) while B isotopic composition is a potential indicator of local seawater pH (Honisch et al., 2004). A comparison of the boron and uranium isotopic compositions of the primary skeleton and cements is beyond the scope of this study, however our data on the concentrations of these elements provides some insight into the likely impact of diagenesis on isotope-based proxies. 2. MATERIALS AND METHODS 2.1. Corals Fossil Porites samples were collected using a submersible (Pisces) and a ROV (Tiburon) from a series of drowned reefs around the Big Island and Oahu, Hawaii. Porites is an annually banded reef-building coral and is the genus most commonly used for palaeoenvironmental reconstruction. We examined 20 coral specimens by scanning electron
microscopy and selected diagenetically altered sections of seven specimens for detailed analysis. Sample sites, mineralogy and specimen ages are summarised in Table 1. 14C and U/Th ages were made on apparently pristine sections of two specimens and the other corals were dated stratigraphically (Table 1). 2.2. Sample characterisation A 10 mm thick slice was cut through the centre of each sample and two thin strips were cut along the axis of maximum skeletal growth from each slice. One strip was finely ground for XRD analysis with a Philips PW1050 diffractometer using SIROQUANT software (Sietronics Pty. Ltd.) to provide quantitative estimates of modal concentrations of all phases. Analysis of standard mixtures indicated that the limit of detection of calcite in aragonite was <1%. The second strip was divided into 15 mm lengths to produce thin sections suitable for SIMS. Samples were mounted onto 2.5 cm diameter glass slides with epoxy resin (Buehler Epothin) and ground to a thickness of 50 lm. For scanning electron microscopy (SEM) additional pieces of the skeletons were fractured to expose the internal structure, cleaned in an ultrasonic bath, dried and gold coated. Samples were photographed in a JEOL JSM 5600 microscope in the School of Chemistry at the University of St Andrews, UK. 2.3. SIMS Primary coral aragonite and cements were identified in the thin sections and analysed by SIMS. To investigate if the precipitation of cements in skeletal pore spaces affects the geochemistry of the primary coral aragonite we compared the trace element composition of coral fasciculi (the bundles of acicular needle crystals) in adjacent areas of the skeleton which did and did not contain cements. Trace element/Ca ratios were determined using a Cameca ims-4f ion microprobe in the School of GeoSciences at the University of Edinburgh, UK. Thin sections were gold coated and analysed with a 16O ion beam, accelerated at 14.5 kV. Instrument conditions were primary beam current = 20 nA, energy offset = 75 eV, imaged field = 25 lm, field aperture 1 (1800 lm) and contrast aperture 3 (150 lm). The analytical diameter was 25 lm. Secondary ion counts were measured using an electron multiplier by sequential stepping of the magnetic field through a cycle of relative atomic masses. Each analysis is the sum of ten cycles. Count times and statistics are summarised in Table 2. Background counts were measured at mass 4.7 and were typically 0.02 cps. The total time per analysis (including other isotopes not reported here) was 15 min. We used a pre-analysis burn-in time of 3 min to remove surface contamination. Trace element/Ca ratios on the sample did not change significantly over the 10 cycle period. Internal reproducibility (the precision at a single point) was calculated from the standard deviation (s) of the trace element/ p Ca ratio in the ten cycles in each analysis as (t*(s/ 10)), (t = 2.26). Mass interferences for Sr (Allison and Finch, 2004) and Mg and Ba (Allison, 1996b) are insignificant.
Effects of submarine coral diagenesis on climate estimates
4695
Table 1 Details of fossil corals studied. Bold dates are 14C or U/Th age estimates from actual specimens and range dates indicate predicted age ranges based on the sample’s stratigraphic relationship to dated samples (Ludwig et al., 1991; Moore et al., 1990a; Riker-Coleman et al., 2005; Webster et al., 2004) Sample
Age (ky) a
P5-67-3 P5-78-5 T304-R3 T277-R8 T286-R8 T301-R43 T313-R23
13.48 +0.35/0.43 13.9 ± 0.3b 14–125 130–140 225–276 370–400 1000–1500
Depth of fossil reef (m)
Mineralogy
175 207 250 412 593 967 499
98% 94% 99% 77% 98% 99% 92%
aragonite, aragonite, aragonite aragonite aragonite, aragonite, aragonite,
2% Mg calcite 4% Mg calcite
2% Mg calcite 1% calcite 8% calcite
Mineralogy of specimens was determined by X-ray diffraction. All specimens were collected from reefs around the Big Island with the exception of T313-R23 which was sampled off Oahu. a 14 C age, Moore et al., 1990a, calibrated in Webster et al., 2004. b U/Th age, Webster et al., 2004.
Table 2 SIMS count times, count rates and analysis statistics on coral Isotope
Count time per cycle (s)
Total counts per analysis
Internal precision, M/Ca (%)
Total counts per analysis in epoxy resin
11
5 3 2 2 15 20
25000 80000 6.9 · 106 2.4 · 106 4000 450
4.5 3.5 — 0.5 7 10
680 2000 216 40 6 0
B Mg 44 Ca 88 Sr 138 Ba 238 U 26
Internal precision was calculated from the trace element(M)/Ca ratio, see text.
We observe no counts at mass 238 when analysing on areas of the section filled with epoxy resin, indicating that the 197 Au40Ca1H trimer does not contribute significantly to counts attributed to 238U. We anticipate no significant interference for 11B. Data were collected over a total of four analytical sessions (each of one week duration). Trace element count rates were normalised to 44Ca and compared to standards (Table 3). The relative ion yields (RIYs) of Mg, Sr and Ba to Ca were estimated daily after multiple analyses (n = 3–10) on the carbonate standard OKA carbonatite (Ca concentration = 40.04%). The standard was usually analysed at the start of each day. Occasional analyses on the standard at the end of the day confirmed that RIYs did not vary significantly within a day and we observed
Table 3 Details of standards used for SIMS analysis Element
Standard
Std concentration, ± 1s (mmol mol1)
External precision, M/Ca (%)
RIY
B Mg Sr
Apatite OKA OKA
3.3 0.7 0.5
— 0.2895 1.0497
Ba U
OKA Apatite
Unknown 2.75 ± 0.13 (n = 3) 13.66 ± 1.65 (n = 3) 0.95 ± 0.22 (n = 4) 4.84 · 103
0.9 2.1
1.0104 0.3575
no significant variations in RIYs within an analytical session. Sr and Ba to Ca RIYs did not vary significantly between analytical sessions while the Mg to Ca RIY varied by up to 4%. B and U counts in this standard are low and we analysed an apatite standard to check instrument stability for these elements. The B concentration of the standard is unknown but 11B counts are relatively high (300 cps). Analyses on the apatite were made on 1 or 2 days in each session and we have assumed no significant changes in RIY over the remainder of the session. The p external reproducibility of analysis (t*(s/ 10)) was calculated from the standard deviation (s) of 10 analyses on each standard (Table 3).We estimate a B/Ca RIY of 0.095 by making multiple (n = 58) SIMS analyses across a 1 year transect of a modern Porites lobata coral collected from Lanikai, Oahu and assuming a B/Ca concentration of 0.391 mmol mol1 (reported for a shallow water Porites compressa from a similar locality (Honisch et al., 2004)). The accuracy of our SIMS estimates is affected by uncertainty in the composition of the standards (e.g. reflecting interlaboratory differences and/or variations in the different OKA batches analysed by each laboratory, Table 3) and by potential matrix effects resulting from chemical and physical differences between the standards and sample, i.e. aragonite, calcite and apatite. We have compared estimates of trace element concentration along adjacent skeletal transects by SIMS and by bulk methods of analysis. We analysed a 10 mm transect (equivalent to 1 years growth) of a fossil Porites from Papua New Guinea (our reference H96-18), previously drilled at 1 mm resolution and precleaned and analysed for Mg/Ca, Sr/Ca and U/Ca by ICP-MS in David Lea’s laboratory, University California, Santa Barbara, USA. We also analysed a 17 mm transect (2 years) of a Pavona clavus from the Galapagos previously analysed for Ba/Ca (Shen et al., 1992). The Ba composition of this specimen (Table 4) is similar to that of modern (Allison and Finch, 2007) and fossil (Table 5) Porites. We standardised all our SIMS data to these bulk values (Table 4). We were unable to compare SIMS and bulk B/Ca estimates for the same coral and accept that data for this element may be inaccurate. However our % estimates of analytical precision of SIMS are good (Tables 2 and 3).
4696
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
Table 4 Mean trace element concentrations and 95% confidence limits determined by SIMS and bulk analyses of parallel coral transects SIMS
Bulk analysis
Standardisation factor
Porites fossil Mg/Ca mmol mol1 Sr/Ca mmol mol1 U/Ca lmol mol1
4.88 ± 0.28 (n = 34) 8.630 ± 0.053 (n = 34) 0.344 ± 0.014 (n = 29)
3.919 ± 0.090 (n = 11) 8.994 ± 0.056 (n = 11) 1.166 ± 0.018 (n = 11)
0.80 1.04 3.39
Pavona clavus Ba/Ca lmol mol1
3.97 ± 0.13 (n = 95)
4.32 ± 0.14 (n = 8)
1.09
We have considered the likely impact of contamination of some of our analyses by small volumes of epoxy resin, e.g. where the diameter of the cement may be <25 lm or where spaces between acicular secondary aragonites have filled with epoxy resin. We observed no significant differences in total Ca counts in analyses on primary coral and secondary minerals indicating that contribution of epoxy resin to the analysis was low (<10%). We made several SIMS analyses on the epoxy resin under the same sputtering conditions as used on the carbonate samples. Count rates in the epoxy resin are low for all elements (Table 2) and we estimate that introducing a 10% volume of epoxy into a coral analysis would affect the original trace element/Ca ratio by <0.3% for B/Ca and Mg/Ca and by <0.02% for Sr/Ca, Ba/Ca and U/Ca. Count rates for B, Mg and Sr may be lower on some cements if they are present in lower concentrations compared to the coral (see Section 3). We estimate that a 10% volume of epoxy into cements of low B, Mg and Sr concentration would affect the original trace element/Ca ratio by <2% for B/Ca and Mg/Ca and by <0.02% for Sr/Ca. 3. RESULTS AND DISCUSSION 3.1. Distribution of cements All of the corals selected contain cements (Figs. 1 and 2). Secondary aragonite occurs as acicular cements syntaxial to the primary coral aragonite (Figs. 1a–b and 2a and c), i.e. the secondary aragonites precipitate onto and have the same lattice orientation as the original coral aragonite. The interface between the primary and secondary aragonite is best observed in thin section in transmitted light (e.g. Fig. 1a). Primary aragonite crystals of Porites corals are densely packed with little space between adjacent crystals (Fig. 1b and 2b). Secondary aragonite crystals become interspaced with voids as the crystals grow and extend away from the primary aragonite. This is shown in Fig. 2c where individual secondary aragonite crystals at the outermost edge of the circular skeletal unit may be separated by spaces of 5 lm or more. Similar spaces can be observed in Fig. 2f. When viewed in thin section by transmitted light these voids scatter light resulting in dark patches or lines between crystals. The exact interface between the primary and secondary aragonite may not be apparent by SEM (Fig. 2c) but is usually clear in transmitted light (Fig. 1a). We note that in some cases the interface may be difficult to discern, e.g. the interface to the bottom of Fig. 1b is obvious but is less clear at the top of the image. For the purposes of this
work we have restricted SIMS analyses to crystal types which can be identified clearly. Secondary calcite occurs in a range of textures: as micrite and as steep-sided rhombs, continued growth of which produced dog tooth crystals (Fig. 1c), suitable for SIMS. Calcite completely infills some interskeletal pore spaces and appears dark (Fig. 1d). The distribution of cements was very heterogeneous and we observe large variations in the abundance and types of cement within even single coral thin sections (15 · 15 mm, e.g. Fig. 1). Zones of different cement texture and/or abundance are frequently separated by dissepiments (Fig. 1a and 2a and d) which are thin layers of primary skeleton accreted approximately 2– 3 mm from the coral surface and parallel to it. They are produced approximately monthly and seal off the bulk of the skeleton from the uppermost skeletal layer, which is still occupied by the coral tissue. We did not observe neomorphism, i.e. the replacement of the primary coral aragonite by secondary calcite (Sherman et al., 1999) and found no evidence of barite or of brucite which has been observed in modern coral (Buster and Holmes, 2006). SIMS (see next section) indicates that all calcite cements analysed are high Mg calcite. High Mg calcite and secondary aragonite are typical shallow marine reef cements (MacIntyre, 1977). Aragonite cements may be precipitated within decades of accretion of the original skeleton (Enmar et al., 2000; Muller et al., 2001). Similarly secondary calcite has been observed along microborings of living corals (MacIntyre and Towe, 1976). We observe no consistent pattern between abundance or type of cement and age/depth of coral. Cementation is most pronounced in exposed reefs (e.g. MacIntyre, 1977) and the local positions of the coral specimens in each reef will have affected their exposure to wave activity. We observe dissolution of some of the centres of calcification (COCs, primary coral aragonite structures) in the fossil corals (Fig. 2b–d) leaving voids of 20 lm diameter. COCs in unaltered modern corals are composed of granular submicron crystals (Cohen et al., 2001) forming a structure of diameter 5–10 lm (Fig. 3a and b). We observed no alteration of the primary coral fasciculi which are formed from bundles of acicular crystals (e.g. Fig. 2b and h). Dissolution and cementation may occur simultaneously in different parts of submerged modern reefs reflecting variations in the composition of interstitial waters (Tribble et al., 1990). Dissolution below the aragonite compensation depth (1000 m at present) is unlikely to have been a significant factor at the depths of most of the corals studied here (Table 1) but may also occur in the vadose environment. The
1.62 ± 0.20 1.03 ± 0.80 1.42 ± 0.12 (n = 12) 15.33 ± 1.89
3.2. Composition of primary coral aragonite and cements and effects of cement inclusion on palaeoenvironmental estimates
T313-R23
Mean concentrations and 95% confidence limits are shown. Nd means not determined.
2.74 ± 0.19 144.2 ± 9.0 2.56 ± 0.19 59.9 ± 7.7 0.55 ± 0.05 0.25 ± 0.06 (n = 3) 0.56 ± 0.06 (n = 6) 0.13 ± 0.05 7 7 13 9 Primary coral Cement Primary coral Cement
4697
reefs sampled here were probably accreted during ice sheet growth when sea level fall and island subsidence occurred at similar rates, and stable shorelines conditions prevailed (Moore and Fornari, 1984). During deglaciations, sea level rose rapidly, drowning the reefs. It is possible that eustatic sea level changes exposed the reefs to the vadose environment before subsequent drowning, e.g. Sherman et al. (1999) infer subsequent periods of marine, meteoric and post-meteoric marine diagenesis in a nearshore submarine reef terrace in Oahu. However, we observe no low Mg calcite cements in these specimens which argues against subaerial exposure and meteoric diagenesis.
9.23 ± 0.22 1.51 ± 0.50 9.42 ± 0.14 0.99 ± 0.24
2.11 ± 0.37 2.73 ± 0.94 1.54 ± 0.30 1.64 ± 0.32
T313-R23
T286-R8
T304-R3
P5-78-5
Calcite cements T277-R8
coral
coral
coral
coral
11 12 10 6 8 8 9 8 13 8 Primary Cement Primary Cement Primary Cement Primary Cement Primary Cement
coral P5-67-3
nd nd 0.60 ± 0.04 0.15 ± 0.11 0.53 ± 0.08 0.12 ± 0.04 0.56 ± 0.15 0.20 ± 0.07 (n = 3) 0.56 ± 0.06 (n = 6) 0.15 ± 0.02 (n = 6)
2.93 ± 0.25 0.71 ± 0.26 3.16 ± 0.20 0.74 ± 0.41 3.41 ± 0.29 0.78 ± 0.27 3.04 ± 0.15 0.49 ± 0.15 2.56 ± 0.19 0.54 ± 0.53
9.47 ± 0.21 11.37 ± 0.49 8.95 ± 0.12 12.96 ± 1.66 9.26 ± 0.13 11.46 ± 0.37 9.40 ± 0.17 10.90 ± 0.33 9.42 ± 0.14 11.52 ± 0.38
(n = 6)
(n = 12) (n = 7)
1.42 ± 0.12 1.67 ± 0.17 1.13 ± 0.28 1.48 ± 1.03 1.27 ± 0.26 1.96 ± 0.40 1.46 ± 0.12 2.52 ± 0.46 1.42 ± 0.12 2.18 ± 0.44 2.57 ± 0.50 2.94 ± 0.37 2.24 ± 0.42 6.28 ± 2.31 5.26 ± 1.62 (n = 6) 4.16 ± 0.59 2.12 ± 0.31 3.27 ± 0.90 1.54 ± 0.30 3.35 ± 1.46
(n = 10)
U/Ca (lmol mol1) Mg/Ca (mmol mol1) B/Ca (mmol mol1) n Coral aragonite cements
Table 5 Composition of primary coral aragonites and adjacent cements, deposited in skeletal pores
Sr/Ca (mmol mol1)
Ba/Ca (lmol mol1)
Effects of submarine coral diagenesis on climate estimates
We performed SIMS analyses on Mg calcite and aragonite cements and on adjacent coral fasciculi. We examined thin sections in transmitted light before SIMS to select suitable areas for analyses. We avoided COCs and microborings, both of which are often associated with significantly different geochemistry compared to the surrounding skeleton (Allison, 1996a; Cohen et al., 2001; Allison and Finch, 2004; Meibom et al., 2004). Primary and secondary aragonites were readily distinguished in transmitted light (Fig. 1a and b) and we examined cemented areas carefully to avoid secondary calcite crystals interspersed between secondary aragonite (as in Fig. 2f). Most cement analyses were performed on substantially infilled pore spaces (e.g. Fig. 1a and d) to reduce any effects of contamination by epoxy resin. However some analyses were also performed on small areas or thin rims (e.g. Fig. 1c) of cement to check their composition. We observe no significant differences in the composition of these small areas compared to the more infilled areas and we have grouped together all analyses of each cement type in each coral for brevity. B was introduced into the analytical programme relatively late so fewer points have been analysed. We observe significant differences between the composition of both types of cements and the primary coral aragonite (Table 5 and Fig. 4). We also observe considerable variability in the composition of the cements. The coral and cement compositions can be interpreted in the light of likely controls on their chemistry, namely the composition of the precipitating solution and the trace element/ Ca partition coefficient. The cements precipitate from reef pore waters which reflect the chemistry of local seawater potentially modified by the precipitation and/or dissolution of other minerals in the reef pores. The coral skeleton precipitates from a calcifying fluid the composition of which is determined by the relative rates of active and passive ion transport to the calcification site. Carbonate composition may also be affected by pH and/or redox changes in the precipitating fluid, which can alter trace element speciation and affect the relative abundance of species which participate in carbonate precipitation. The pH of coral porewaters is considerably lower (7.5–7.7 Tribble et al., 1990; Enmar et al., 2000) than the pH of the coral calcifying fluid (ranging from 8 in dark conditions to >9 in the light, Al-Horani et al., 2003). Finally trace element/Ca partition coefficients
4698
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
Fig. 1. Transmitted light micrographs of coral T313-R23. The types and abundance of cements was heterogeneous and primary coral aragonite (PA), secondary aragonite (SA) and secondary calcite (SC) were observed within one thin section. Dissepiments (D) are marked. The interface between the primary and secondary aragonites is marked in (b).
Fig. 2. Scanning electron micrographs of corals T286-R8 (a–c), T313-R23 (d–f) and T277-R8 (g–i). Notation as for Fig. 1. The distribution of cements is heterogeneous. (a) No cement is visible to the left of the dissepiment (shown at higher magnification in (b) while aragonite cement is abundant on the right (shown at higher magnification in (c)). (d) Mg calcite crystals are distributed across the skeletal wall to the left of the dissepiment (shown at higher magnification in (e), while to the right of the dissepiment, aragonite cements are dominant with some Mg calcite crystals deposited between the aragonite crystals (shown at higher magnification in (f)). (g and h) Coral T277-R8 is almost completely infilled with micritic Mg calcite cement (shown at higher magnification in i). Centres of calcification in b, c and d are marked COC. Scale bars are 100 lm (a, d and g), 20 lm (b, c and h) and 10 lm (e, f and i).
may be affected by temperature, the carbonate phase, crystal shape and other kinetic factors, e.g. precipitation rate. For example, the trace element geochemistry of naturally precipitated non-biogenic shallow marine carbonates can
be highly variable, e.g. see Mucci (1987) for a review of Mg concentrations in calcites. We conclude that the variable composition of the cements reflects a wide range of pore water conditions.
Effects of submarine coral diagenesis on climate estimates
4699
Fig. 3. Scanning electron micrographs of a modern coral indicating the normal appearance and dimensions of a centre of calcification (COC). Scale bars are 20 lm (a) and 10 lm (b). Microboring holes (MB) are visible in this coral.
3.2.1. Sr/Ca All the aragonite cements contain significantly more Sr than the primary coral aragonite while all calcite cements contain significantly less Sr. Our estimates of Sr/Ca in aragonite cements are similar to one report (10.8 mmol mol1, MacIntyre, 1977) but higher than 2 other reports (9.0 and 10.1 mmol mol1, Sherman et al., 1999; Enmar et al., 2000, respectively). This may reflect real variations in coral composition or a difference in the calibration of the techniques used (e.g. previous estimates were made by electron probe microanalysis). The magnitude of Sr enrichment in the aragonite cements compared to the primary coral skeleton, is similar (typically 20%) in this and other studies (MacIntyre, 1977; Enmar et al., 2000) indicating that aragonite cements increase the Sr content of the whole skeleton (primary skeleton plus cements) by a similar amount in all cases. Using the mean compositions of the primary coral aragonite and cements in Table 5 we have estimated the effects of inclusion of 1% cement (by mass) on SST estimates from drilled coral samples (Table 6). Contamination of a pristine skeleton by 1% calcite cement causes an increase in SST estimates of 1 C while 1% aragonite cement decreases SST estimates by 0.2–0.3 C (although a larger decrease was observed in one coral). Sr/Ca determinations of drilled coral samples are usually made by thermal ionisation mass spectrometry (TIMS) or inductively coupled plasma atomic
Primary skeleton
0
B/Ca mmol mol-1 0.2 0.4 0.6
emission spectrophotometry (ICP-AES) which have typical analytical precision (2s) of <0.03% and <0.4%, respectively (Beck et al., 1992; Schrag, 1999). The inclusion of <1% or 2% of aragonite cement in drilled samples will significantly affect Sr/Ca estimates by each of these techniques respectively while <1% calcite cement samples will significantly affect Sr/Ca estimates by either of these techniques. 3.2.2. Mg/Ca The aragonite cements contain significantly less Mg than the primary coral aragonite while the calcite cements contain significantly more Mg. Estimates of the temperature dependence of Mg/Ca in Porites corals range from 0.088 to 0.164 mmol mol1 C1 (Fallon et al., 1999; Wei et al., 2000). Using an estimate in the middle of this range (Mitsuguchi et al., 1996), we calculate that the contamination of a pristine skeleton by 1% aragonite cement reduces SST estimates by 0.2 C while 1% calcite cement increases SST estimates by 4 and 11 C (Table 6). The typical precision (2s) of Mg/Ca analysis of drilled samples by ICPAES is <0.07 mmol mol1 (Mitsuguchi et al., 1996) and the inclusion of <1% calcite and 3% aragonite cement in drilled samples will significantly affect Mg/Ca estimates. 3.2.3. Ba/Ca We observe significantly higher Ba concentrations in the aragonite cements of 2 corals, compared to the primary
Cement
0
Mg/Ca mmol mol-1 2 4
Sr/Ca mmol mol-1 4 8 12
0
Ba/Ca μmol mol-1 2 4 6 8
0
U/Ca μmol mol-1 1 2 3 Aragonite cements
P5-67-3
0
P5-78-5 T304-R3 T286-R8 T313-R23
15.3
144
T313-R23
60
Calcite cements
T277-R8
Fig. 4. Composition of the primary coral aragonite and calcite cements of each skeleton. Values are means and 95% confidence limits.
4700
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
Table 6 Estimated effect of 1% cement contamination on SST estimates from the Mg/Ca, Sr/Ca, U/Ca and B/Ca composition of each coral Coral sample
Mg/Ca (C)
Sr/Ca (C)
U/Ca (C)
B/Ca (C)
Aragonite cements P5-67-3 P5-78-5 T304-R3 T286-R8 T313-R23
0.17 0.19 0.20 0.20 0.16
0.24 0.47 0.27 0.19 0.26
ns ns 0.15 0.23 0.16
nd +0.22 +0.20 +0.17 +0.21
Mg calcite cement T277-R8 T313-R23
+11.0 +4.44
+0.96 +1.05
ns 2.96
+0.14 +0.20
The temperature dependences used for the calculations for Sr/Ca (in Hawaiian Porites) and Mg/Ca, U/Ca and B/Ca in coral aragonite are 0.080 mmol mol1 C1, 0.13 mmol mol1 C1, 0.047 lmol mol1 C and 0.021 mmol mol1 C1 (de Villiers et al., 1994; Mitsuguchi et al., 1996; Min et al., 1995; Sinclair et al., 1998, respectively). ns indicates that the composition of the cements and coral were not significantly different (at p = 0.05). Nd, not determined.
aragonite (P5-78-5 and T313-R23) but no significant differences in the Ba compositions of the coral aragonite and cement of the remaining corals. In one case (coral p5-785) the Ba concentrations of aragonite cements were nearly 3x the concentrations observed on the primary coral aragonite. The inclusion of 5% cement in this coral sample can increase total Ba by up to 10%. The magnitude of the annual Ba signal in pristine coral is 20% in a Pavona clavus specimen from the Galapagos (Shen et al., 1992) indicating that cement inclusion can significantly affect total Ba and seasonal Ba trends. 3.2.4. B/Ca All the cements (aragonite and Mg calcite) contain significantly less B/Ca than the primary coral. The B concentration of inorganically precipitated calcite decreases as seawater pH decreases (Sanyal et al., 2000). The pH of coral porewaters is considerably lower (7.5–7.7) than that of the coral calcifying fluid (8 to >9 in the light) and the large decrease in the B/Ca concentration of the cements is therefore consistent with a pH effect. The pH dependence of B carbonate concentration may reflect the increase in the relative abundance of borate at higher pH if borate is the species predominantly incorporated in carbonates (see Pagani et al., 2005 for a review) or may reflect other factors e.g. crystal growth rate (Hobbs and Reardon, 1999). Variations in the B/Ca composition of the aragonite and Mg calcite cements are small and inclusion of 1% of either of these cements increases SST estimates by 0.2 C (Table 6). We estimate that d11B of coral aragonite is relatively robust to the effects of cement precipitation. d11B of abiotic shallow marine aragonites (22‰, Hemming and Hanson, 1992) and inorganically precipitated calcite (19‰ at pH 7.9, Sanyal et al., 2000) are less than that of coral aragonite (24‰, Hemming and Hanson, 1992). The reproducibility of d11B analysis is typically 0.5‰ and we estimate that 25% calcite cement by mass would be required to significantly reduce coral d11B using these estimates of carbonate d11B and the B concentrations observed in Table 5. The inclusion of 25% aragonite cement by mass reduces d11B by <0.2‰.
3.2.5. U/Ca U/Ca concentrations in the aragonite cements are higher than in the adjacent primary coral (although the difference is significant in only 3 of the 5 specimens) and are similar to a previous estimate (Lazar et al., 2004) in diagenetically altered living corals. The concentrations of U/Ca in the calcite cements were considerably higher than initially expected. The partition coefficient of U in inorganically precipitated calcite (0.3) is at least an order of magnitude less than that in aragonite (2–10, Meece and Benninger, 1993) and U(VI) in calcite is incorporated in a disordered and apparently less stable co-ordination environment than in aragonite (Reeder et al., 2000). We had therefore anticipated that U/Ca concentrations would be significantly less in the calcite cements. However in one specimen the calcite cements had similar concentrations to the primary aragonite and in the other coral (T323-R23) concentrations were significantly higher (15.3 lmol mol1) than in the primary aragonite or in the secondary aragonite cements analysed in other corals. High U/Ca partition coefficients (60– 430) have been estimated in spar calcite in zinc ores and the authors concluded that U (IV) substitutes for divalent Ca to form a stable complex under reducing conditions (Sturchio et al., 1998). Reducing environments can occur in the interior of living reefs (Tribble et al., 1990) and we hypothesise that the relatively high U concentrations observed in the Mg calcites reflect precipitation under these conditions. Inclusion of 1% aragonite cement decreased U/Ca based SST estimates by 0.2 C while inclusion of 1% of the Mg calcite cement, which had a significantly different U/Ca compared to the coral, decreased the SST estimate by 3 C. The typical precision (2s) of U/Ca analysis of drilled samples by TIMS is 2‰ (Min et al., 1995) and contamination of a pristine skeleton by <1% calcite or aragonite cement in drilled samples will significantly affect U/Ca estimates. Both high Mg calcite and aragonite cements can significantly enrich coral d234U (Henderson et al., 1993) so that true coral ages are underestimated. The magnitude of this effect is dependent on the timing and duration of diagenesis and will be insignificant if diagenesis occurs during or shortly after the life of the coral (Lazar et al.,
Effects of submarine coral diagenesis on climate estimates
geochemistry. We hypothesise that some alteration of the primary aragonite has occurred in this coral. Coral skeletons contain micropores at a range of scales from the micron sized borings produced by micro-organisms, e.g. algae and fungi (Fig. 3) to the sub micron spaces which can occur between skeletal fibres (Perrin, 2003). Infilling of these intraskeletal spaces by calcite could significantly affect the geochemistry of the primary skeleton. However, the composition of an intraskeletal calcite would have to be quite different from that observed in the interskeletal pore spaces (Table 5) in order to affect both the Mg and Ba of the primary coral. For example a volume of 0.6% of the interskeletal calcite cement would increase the Mg of the primary coral by the amount observed in Fig. 4 but would not significantly increase the Ba (or Sr, B or U) of the primary coral. Given the high compositional variability of inorganic calcites and cements (Mucci, 1987; Table 5) it is reasonable to assume that variations in the precipitating microenvironment and the fabric of the cement produced could significantly affect the cement geochemistry.
2004). Age rejuvenation effects are also affected by the U concentration of the cement and our observation of high U concentrations in a calcite cement indicates that these may be particularly significant in affecting coral age estimates. 3.3. Effects of cementation on primary aragonite composition The selective analysis of pristine areas of altered fossil skeletons by SIMS offers a route to the reconstruction of accurate SSTs from coral skeletons (Cohen and Hart, 2004; Allison et al., 2005). To investigate if cementation affects the geochemistry of the primary coral aragonite, we compared the trace element composition of coral fasciculi (the bundles of acicular needle crystals) in adjacent cemented and uncemented areas of 3 coral skeletons. In two specimens pristine coral and secondary aragonite cemented areas occurred on opposite sides of a dissepiment (as in Fig. 1) and the primary coral aragonite in each area would have been deposited at subtly different times, probably approximately one month apart. This may subtly affect the geochemistry of the two areas. In the third sample (T313-R23) pristine and Mg calcite cemented areas occurred within the same pair of dissepiments. We differentiated between cemented areas containing minor (<10% by volume, as in Fig. 1c) and major amounts of cement (>10% by volume, e.g. Fig. 1d). We observed no significant differences in the Sr/Ca, B/ Ca and U/Ca composition of the primary coral aragonite in cemented and pristine areas of all the corals (Fig. 5). The Sr/Ca error bars are ± 1% and reflect the natural heterogeneity of the primary aragonite at this scale (e.g. Allison and Finch, 2004). We observed no significant differences in the Mg/Ca and Ba/Ca composition of primary coral in areas infilled with aragonite cement compared to pristine areas. However, the Mg/Ca and Ba/Ca concentrations of the coral aragonite in sections of the coral infilled with calcite cements (T313-R23) were significantly higher than that of coral aragonite in pristine areas. Analyses in cemented and uncemented areas were made between the same pair of dissepiments in this coral and we have assumed that the primary coral in each area was deposited at approximately the same time and should have the same
-1
>10% calcite
No cement <10% aragonite
No cement <10% aragonite
3.6
4.2
9.0
9.2
9.4
-1
U/Ca μmol mol
B/Ca mmol mol (x10-2)
Sr/Ca mmol mol 4.8 8.8
We conclude that diagenesis and the precipitation of both aragonite and calcite cements can significantly affect coral trace element geochemistry. Contamination of a pristine skeleton by 1% calcite (the approximate detection limit of XRD) could increase Mg and Sr derived SST estimates by 1 C or more and decrease U derived estimates by 3 C. Aragonite cements cannot be detected by XRD screening. The inclusion of 5% aragonite cement will affect Sr, Mg and U derived SST estimates by 1 C and B derived SST estimates by +1 C. Of particular concern is the observation that inclusion of aragonite cements affects SST estimates by a similar magnitude in different geochemical proxies, i.e. Mg, Sr and U, leading to consistent but erroneous SST estimates from all proxies. We conclude that XRD screening may not detect geochemically significant amounts of cement in fossil corals. Thorough petrographic examination of fossil corals is necessary to confirm that they are pristine before drilled samples are analysed. We also note that cement may be difficult to observe in thin section. The main units of Porites coral skeletons, the
5.0 9.6
5.4
5.8
6.2
1.0 6.6
1.2
1.4
1.6
Ba/Ca μmol mol 1.8 1
-1
10
5
T313-R23 T301-R43 T304-R3
No cement <10% calcite
3.0
4. CONCLUSIONS
-1
-1
Mg/Ca mmol mol 2.4
4701
Fig. 5. Trace element concentrations of the primary coral aragonite in cemented and uncemented areas of each skeleton. The abundances and types of cement in the interskeletal pore spaces were estimated by petrographic analysis. Values are means and 95% confidence limits of 19–21 analyses, with the exception of U/Ca where means are of 7–12 analyses.
4702
N. Allison et al. / Geochimica et Cosmochimica Acta 71 (2007) 4693–4703
cylindrical vertically growing trabeculae, are typically 100– 150 lm in diameter with a cross sectional area of 3– 7 · 104 lm2. The inclusion of 1% cement (by volume) would increase the diameter of the trabeculae by an average of only 0.5–0.75 lm. The distribution of cements is usually heterogeneous (Figs. 1 and 2) resulting in the precipitation of irregularly sized rims around skeletal pores, however careful petrographic examination may be required to identify these. The Sr/Ca, B/Ca or U/Ca of primary coral aragonite is not affected by the precipitation of cements in adjacent interskeletal pore spaces. The Mg/Ca and Ba/Ca of primary coral structures may be affected by calcite precipitation in pore spaces. This is an area for further investigation. ACKNOWLEDGMENTS This work was supported by the UK Natural Environment Research Council (award NER/A/S/2003/00473 to A.A.F. and N.A.). J.M.W. and D.A.C. acknowledge the support of the David and Lucile Packard Foundation through a grant to MBARI. Access to the ion probe was provided by NERC Scientific Services and we are indebted to Simone Kasemann for her assistance with the analyses. We thank Angus Calder and Andrew Rothwell for completing the XRD analyses. We thank Anne Cohen (AE) and three anonymous reviewers for comments which improved this manuscript.
REFERENCES Al-Horani F. A., Al-Moghrabi S. M. and de Beer D. (2003) The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis. Mar. Biol. 142, 419–426. Allison N. (1996a) Geochemical anomalies in coral skeletons and their possible implications for palaeoenvironmental analysis. Mar. Chem. 55, 367–379. Allison N. (1996b) Quantitative determinations of trace and minor elements in coral aragonite by ion microprobe analysis, with preliminary results from Phuket, South Thailand. Geochim. Cosmochim. Acta 60, 3457–3470. Allison N. and Finch A. A. (2004) High resolution Sr/Ca records in modern Porites lobata corals: effects of skeletal extension rate and architecture. Geochem. Geophys. Geosyst. 5. doi:10.1029/ 2004GC000696. Allison N. and Finch A. A. (2007) High temporal resolution Mg/ Ca and Ba/Ca records in modern Porites lobata corals, Geochem. Geophys. Geosyst. 8, Q05001, doi:10.1029/ 2006GC001477. Allison, N., Finch, A. A., Tudhope, A. W., Newville, M., Sutton, S. R., Ellam, R. (2005) Reconstruction of deglacial sea surface temperatures in the tropical Pacific from selective analysis of a fossil coral. Geophys. Res. Lett. 32, L17609, doi: 10.1029/ 2005GL023183. Bar-Matthews M., Wasserburg G. J. and Chen J. H. (1993) Diagenesis of fossil coral skeletons—correlation between traceelements, textures, and U-234/U-238. Geochim. Cosmochim. Acta 57, 257–276. Beck J. W., Edwards R. L. and Ito E., et al. (1992) Sea-surface temperature from coral skeletal strontium calcium ratios. Science 257, 644–647. Buster N. A. and Holmes C. W. (2006) Magnesium content within the skeletal architecture of the coral Montastraea faveolata: locations of brucite precipitation and implications to fine-scale data fluctuations. Coral Reefs 25, 243–253.
Cohen A. L., Lane G. D., Hart S. R. and Lobel P. S. (2001) Kinetic control of skeletal Sr/Ca in a symbiotic coral: implications for the palaeotemperature proxy. Paleoceanography 16, 20–26. Cohen A. L. and Hart S. R. (2004) Deglacial sea surface temperatures of the western tropical Pacific: a new look at old coral. Paeoceanography 19, PA4031. de Villiers S., Shen G. T. and Nelson B. K. (1994) The Sr/Catemperature relationship in coralline aragonite: influence of variability in (Sr/Ca)seawater and skeletal growth parameters. Geochim. Cosmochim. Acta 58, 197–208. Enmar R., Stein M., Bar-Matthews M., Sass E., Katz A. and Lazar B. (2000) Diagenesis in live corals from the Gulf of Aqaba. I. The effect on paleo-oceanography tracers. Geochim. Cosmochim. Acta 64, 3123–3132. Fallon S. J., McCulloch M. T., van Woesik R. and Sinclair D. J. (1999) Corals at their latitudinal limits: laser ablation trace element systematics in Porites from Shirigai Bay, Japan. Earth Planet. Sci. Lett. 172, 221–238. Hemming N. G. and Hanson G. N. (1992) Boron isotopic composition and concentration in modern marine carbonates. Geochim. Cosmochim. Acta 56, 537–543. Henderson G. M., Cohen A. S. and Onions R. K. (1993) U-234/U238 ratios and TH-230 ages for Hateruma Atoll corals— implications for coral diagenesis and seawater U-234/U-238 ratios. Earth Planet. Sci. Lett. 115, 65–73. Hobbs M. Y. and Reardon E. J. (1999) Effect of pH on boron coprecipitation by calcite: further evidence for nonequilibrium partitioning of trace elements. Geochim. Cosmochim. Acta 63, 1013–1021. Honisch B., Hemming N. G. and Grottoli A. G., et al. (2004) Assessing scleractinian corals as recorders for paleo-pH: empirical calibration and vital effects. Geochim. Cosmochim. Acta 68, 3675–3685. Lazar B., Enmar R., Schossberger M., Bar-Matthews M., Halicz L. and Stein M. (2004) Diagenetic effects on the distribution of uranium in live and Holocene corals from the Gulf of Aqaba. Geochim. Cosmochim. Acta 68, 4583–4593. Ludwig K. R., Szabo B. J., Moore J. G. and Simmons K. R. (1991) Crustal subsidence rate off Hawaii determined from 234U/238U ages of drowned coral reefs. Geology 19, 171–174. MacIntyre I. G. (1977) Distribution of submarine cements in a modern Caribbean fringing reef, Galeta point, Panama. J. Sed. Pet. 47, 503–516. MacIntyre I. G. and Towe K. M. (1976) Skeletal calcite in living scleractinian corals: microboring fillings, not primary skeletal deposits. Science 193, 701–702. McGregor H. V. and Gagan M. K. (2003) Diagenesis and geochemistry of Porites corals from Papua New Guinea: implications for paleoclimate reconstruction. Geochim. Cosmochim. Acta 67, 2147–2156. Meece D. E. and Benninger L. K. (1993) The coprecipitation of Pu and other radionuclides with CaCO3. Geochim. Cosmochim. Acta 57, 1447–1458. Meibom, A., Cuif, J. P., Hillion, F. O., et al. 2004. Distribution of magnesium in coral skeleton, Geophys. Res. Lett. 31(23), Art. No. L23306. Min G. R., Edwards R. L., Taylor F. W., Recy J., Gallup C. D. and Beck J. W. (1995) Annual cycles of U/Ca in coral skeletons and U/Ca thermometry. Geochim. Cosmochim. Acta 59, 2025–2042. Mitsuguchi T., Matsumoto E., Abe O., Uchida T. and Isdale P. J. (1996) Mg/Ca thermometry in coral-skeletons. Science 274, 961–963. Moore J. G., Clague D. A., Ludwig K. R. and Mark R. (1990a) Subsidence and volcanism of the Haleakala Ridge, Hawaii. J. Volcanol. Geoth. Res. 42, 273–284.
Effects of submarine coral diagenesis on climate estimates Moore J. G. and Fornari D. J. (1984) Drowned reefs as indicators of the rate of subsidence of the island of Hawaii. J. Geol. 92, 752–759. Mucci A. (1987) Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater. Geochim. Cosmochim. Acta 51, 1977–1984. Muller A., Gagan M. K. and McCulloch M. T. (2001) Early marine diagenesis in corals and geochemical consequences for paleoceanographic reconstructions. Geophys. Res. Lett. 28, 4471–4474. Pagani M., Lemarchand D. and Spivack A., et al. (2005) A critical evaluation of the boron isotope-pH proxy: the accuracy of ancient ocean pH estimates. Geochim. Cosmochim. Acta 69, 953–961. Perrin C. (2003) Compositional heterogeneity and microstructural diversity of coral skeletons: implications for taxonomy and control on early diagenesis. Coral Reefs 22, 109–120. Quinn, T. M., Sampson, D. E. (2002) A multiproxy approach to reconstructing sea surface conditions using coral skeleton geochemistry. Paleoceanography 17, Art. No. 1062. Quinn T. M. and Taylor F. W. (2006) SST artifacts in coral proxy records produced by early marine diagenesis in a modern coral from Rabaul, Papua New Guinea. Geophys. Res. Lett. 33, L04601, doi:10.1029/2005GL024972. Reeder R. J., Nugent M., Lamble G. M., Tait C. D. and Morris D. E. (2000) Uranyl incorporation into calcite and aragonite: XAFS and luminescence studies. Environ. Sci. Technol. 34, 638–644. Riker-Coleman, K., Gallup, C., Clague, D., Webster, J. M., Edwards, L., Cheng, H. (2005) New 230Th ages from the 400 m reef of northwestern Hawaii (2005). AGU Fall meeting: PP21C-1574. Sanyal A., Nugent M., Reeder R. J. and Buma J. (2000) Seawater pH control on the boron isotopic composition of calcite: evidence from inorganic calcite precipitation experiments. Geochim. Cosmochim. Acta 64, 1551–1555. Schrag D. P. (1999) Rapid analysis of high-precision Sr/Ca ratios in corals and other marine carbonates. Paleoceanography 14, 97–102.
4703
Shen G. T., Cole J. E., Lea D. W., Linn L. J., McConnaughy T. A. and Fairbanks R. G. (1992) Surface ocean variability at Galapagos from 1936–1982: calibration of geochemical tracers in corals. Paleoceanography 7, 563–588. Shen G. T. and Sanford C. L. (1990) Trace element indicators of climate change in annually banded corals. In Global Ecological Consequences of the 1982–83 El Nino (ed. P. W. Glynn). Elsevier, New York, pp. 255–283. Sherman C. E., Fletcher C. H. and Rubin K. H. (1999) Marine and meteoric diagenesis of Pleistocene carbonates from a nearshore submarine terrace, Oahu, Hawaii. J. Sed. Res. 69, 1083–1097. Sinclair D. J., Kinsley L. P. J. and McCulloch M. T. (1998) High resolution analysis of trace elements in corals by laser ablation ICP-MS. Geochim. Cosmochim. Acta 62, 1889–1901. Sturchio N. C., Antonio M. R., Saderholm L., Sutton S. R. and Brannon J. C. (1998) Tetravalent uranium in calcite. Science 281, 971–973. Tribble G. W., Sansone F. J. and Smith S. V. (1990) Stoichiometric modeling of carbon diagenesis within a coral-reef framework. Geochim. Cosmochim. Acta 54, 2439–2449. Webster J. M., Clague D. A., Riker-Coleman K., Gallup C., Braga J. C., Potts D., Moore J. G., Winterer E. L. and Paull C. K. (2004) Drowning of the 150 m reef off Hawaii: a casualty of global meltwater pulse 1A? Geology 32, 249–252. Wei G. J., Sun M., Li X. H. and Nei B. F. (2000) Mg/Ca, Sr/Ca and U/Ca ratios of a porites coral from Sanya Bay, Hainan Island, South China Sea and their relationships to sea surface temperature. Palaeogeogr. Palaeoclimatol. Palaeoecol. 162, 59– 74. Yokoyama Y. et al. (2001) Last glacial sea-level change deduced from uplifted coral terraces of Huon Peninsula, Papua New Guinea. Q. Int. 83(5), 275–283. Associate editor: Anne Cohen