Strontium in coral aragonite: 2. Sr coordination and the long-term stability of coral environmental records

We have used X-ray Diffraction (XRD) and Sr K-edge Extended X-ray Absorption Fine Structure (EXAFS) to determine the structural state of Sr in a suite of coral aragonite samples. Our samples encompassed a selection of coral species (Porites lobata, Porites lutea, Pocillopora eydouxi, Montastrea annularis, Pavona gigantea and Pavona clavus) including some commonly used for palaeoenvironmental reconstruction. Aragonite was the only carbonate observed by XRD. We refined the isolated EXAFS against structural models for Sr in aragonite and two-phase strontianite/aragonite mixes. Our data are indistinguishable from Sr ideally substituted in aragonite and strontianite was present below detection levels (estimated at <5% of Sr present). Comparisons of recent and ancient coral aragonite show no sign of exsolution, either by spinodal decomposition or by the direct nucleation of strontianite domains. Published diffusion rates of Sr in ionic solids support the view that exsolution would occur prohibitively slowly. Coral aragonites are metastable materials with slow diffusion kinetics that have the potential to encode environments over timescales of millions of years.     

Coral skeleton P/Ca proxy for seawater phosphate: Multi-colony calibration with a contemporaneous seawater phosphate record

A geochemical proxy for surface ocean nutrient concentrations recorded in coral skeleton could provide new insight into the connections between sub-seasonal to centennial scale nutrient dynamics, ocean physics, and primary production in the past. Previous work showed that coralline P/Ca, a novel seawater phosphate proxy, varies synchronously with annual upwelling-driven cycles in surface water phosphate concentration. However, paired contemporaneous seawater phosphate time-series data, needed for rigorous calibration of the new proxy, were lacking. Here we present further development of the P/Ca proxy in Porites lutea and Montastrea sp. corals, showing that skeletal P/Ca in colonies from geographically distinct oceanic nutrient regimes is a linear function of seawater phosphate (PO_4 SW) concentration. Further, high-resolution P/Ca records in multiple colonies of Pavona gigantea and Porites lobata corals grown at the same upwelling location in the Gulf of Panamá were strongly correlated to a contemporaneous time-series record of surface water PO_4 SW at this site (r² = 0.7-0.9). This study supports application of the following multi-colony calibration equations to down-core records from comparable upwelling sites, resulting in ±0.2 and ±0.1 μmol/kg uncertainties in PO_4 SW reconstructions from P. lobata and P. gigantea, respectively.

     

Strontium in coral aragonite: 3. Sr coordination and geochemistry in relation to skeletal architecture

Use of the coral Sr palaeothermometer assumes that the Sr in coral skeletons is substituted randomly for Ca in the aragonite structure. The presence of Sr in additional phases e.g., strontianite, or the non random distribution of Sr across metal sites in aragonite, would complicate the Sr/Ca-sea surface temperature relationship. We have used Sr K-edge microEXAFS (extended X-ray absorption fine structure) to determine the structural state of Sr across selected microvolumes of four coral skeletons (Porites lobata, Acropora palmata, Pavona clavus, and Montastrea annularis). We used a 5 × 3 μm beam to analyse specific areas of the coral skeletal architecture, i.e., centres of calcification, fasciculi, and dissepiments. All EXAFS analyses refine, within error, to an ideally substituted Sr in aragonite, and we found no evidence of strontianite or partly ordered structural states. Anisotropy in the first shell responses results from the fact that the analysed microvolumes are not necessarily averaged for the responses of all crystal orientations in the aragonite. Although secondary ion mass spectrometry confirmed that Sr/Ca composition can vary substantially between skeletal components, we find no evidence for any contrast in Sr structural state. Sr heterogeneity may result from kinetic effects, reflecting complex disequilibrium processes during crystal precipitation, or biological effects, resulting from variations in the composition of the calcifying fluid which are biologically mediated.     

Calcium isotope fractionation in modern scleractinian corals

The ⁴⁴Ca/⁴⁰Ca ratios of cultured (Acropora sp.) and open ocean (Pavona clavus, Porites sp.) tropical reef corals are positively correlated with growth temperature. The slope of the temperature-fractionation relation is similar to inorganic aragonite precipitates. However, δ^44/40Ca of the coral aragonite is offset from inorganic and sclerosponge aragonite by about +0.5‰. This offset can neither be explained by the very fast, biologically controlled calcification of scleractinian corals, nor as a consequence of calcification from a partly closed volume of fluid. As corals actively transport calcium through several cell layers to the site of calcification, the most likely explanation for the offset is a biologically induced fractionation. Our results indicate a limited use of Ca isotopes in scleractinian corals as temperature proxy.     

Strontium heterogeneity and speciation in coral aragonite: implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO₃ (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 μm scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment.     

Compositional variations at ultra-structure length scales in coral skeleton

Distributions of Mg and Sr in the skeletons of a deep-sea coral (Caryophyllia ambrosia) and a shallow-water, reef-building coral (Pavona clavus) have been obtained with a spatial resolution of 150 nm, using the NanoSIMS ion microprobe at the Muséum National d’Histoire Naturelle in Paris. These trace element analyses focus on the two primary ultra-structural components in the skeleton: centers of calcification (COC) and fibrous aragonite. In fibrous aragonite, the trace element variations are typically on the order of 10% or more, on length scales on the order of 1-10 μm. Sr/Ca and Mg/Ca variations are not correlated. However, Mg/Ca variations in Pavona are strongly correlated with the layered organization of the skeleton.These data allow for a direct comparison of trace element variations in zooxanthellate and non-zooxanthellate corals. In both corals, all trace elements show variations far beyond what can be attributed to variations in the marine environment. Furthermore, the observed trace element variations in the fibrous (bulk) part of the skeletons are not related to the activity of zooxanthellae, but result from other biological activity in the coral organism. To a large degree, this biological forcing is independent of the ambient marine environment, which is essentially constant on the growth timescales considered here.Finally, we discuss the possible detection of a new high-Mg calcium carbonate phase, which appears to be present in both deep-sea and reef-building corals and is neither aragonite nor calcite.     

Deep-sea coral geochemistry: Implication for the vital effect

Deep-sea corals hold a great potential as a key to important aspects of paleoceanography for at least two reasons, 1) they offer temporal high resolution records of deep-sea environment, because they have growth banding structures, 2) and they are well suited for studying vital effects, because the deep-sea environment does not change over short time scales. However, the relationship between the chemical composition of deep-sea coral skeletons and environmental factors is not well understood. In this study, the chemical composition of deep-sea corals was measured in bulk individuals and along skeletal micro-structures. Among the bulk individuals, δ¹⁸O value and Sr / Ca ratio show a negative but weak correlation with ambient temperature. On the other hand, the Mg / Ca ratio has a positive, weak correlation with the temperature. Large variations were found among samples collected from similar temperature. The variation is up to 3.8‰ for δ¹⁸O, 0.9 mmol/mol for Sr / Ca ratios, and 0.78 mmol/mol for Mg / Ca ratios among samples with ambient average temperature within 1 °C. This variation may be due to a large vital effect. The centers of calcification (COCs), which were formed at high calcification rate, have lower Sr / Ca, U / Ca and higher Mg / Ca ratios than surrounding fasciculi. This chemical distribution supports the model that elemental incorporation depends on calcification rate. This suggests that calcification rate is a very important factor for the chemical composition in deep-sea corals and is one of the most significant mechanisms of the vital effect. Because of this large vital effect, further investigations are essential to use the deep-sea coral as a temperature proxy.     

δB, Sr, Mg and B in a modern Porites coral: the relationship between calcification site pH and skeletal chemistry

Sr/Ca, B/Ca, Mg/Ca and δ¹¹B were determined at high spatial resolution across ∼1 year of a modern Hawaiian Porites lobata coral by secondary ion mass spectrometry (SIMS). We observe significant variations in B/Ca, Mg/Ca, Sr/Ca and δ¹¹B over short skeletal distances (nominally equivalent to periods of <20 days). This heterogeneity probably reflects variations in the composition of the extracellular calcifying fluid (ECF) from which the skeleton precipitates. Calcification site pH (total scale) was estimated from skeletal δ¹¹B and ranged from 8.3 to 8.8 (± ∼0.1) with a mean of ∼8.6. Sr/Ca and B/Ca heterogeneity is not simply correlated with calcification site pH, as might be expected if Ca-ATPase activity increases the pH and decreases the Sr/Ca and B(OH)₄⁻/CO₃²⁻ ratios of the ECF. We produced a simple model of the ECF composition and the skeleton deposited from it, over a range of calcium transport and carbonate scenarios, which can account for these observed geochemical variations. The relationship between the pH and Sr/Ca of the ECF is dependent on the concentration of DIC at the calcification site. At higher DIC concentrations the ECF has a high capacity to buffer the [H⁺] changes induced by Ca-ATPase pumping. Conversely, at low DIC concentrations, this buffering capacity is reduced and ECF pH changes more rapidly in response to Ca-ATPase pumping. The absence of a simple correlation between ECF pH and skeletal Sr/Ca implies that calcification occurred under a range of DIC concentrations, reflecting variations in the respiration and photosynthesis of the coral and symbiotic zooxanthellate in the overlying coral tissues. Our observations have important implications for the use of coral skeletons as indicators of palaeo-ocean pH.     

Controls on Sr/Ca and Mg/Ca in scleractinian corals: The effects of Ca-ATPase and transcellular Ca channels on skeletal chemistry

The Sr/Ca of aragonitic coral skeletons is a commonly used palaeothermometer. However skeletal Sr/Ca is typically dominated by weekly-monthly oscillations which do not reflect temperature or seawater composition and the origins of which are currently unknown. To test the impact of transcellular Ca²⁺ transport processes on skeletal Sr/Ca, colonies of the branching coral, Pocillopora damicornis, were cultured in the presence of inhibitors of Ca-ATPase (ruthenium red) and Ca channels (verapamil hydrochloride). The photosynthesis, respiration and calcification rates of the colonies were monitored throughout the experiment. The skeleton deposited in the presence of the inhibitors was identified (by ⁴²Ca spike) and analysed for Sr/Ca and Mg/Ca by secondary ion mass spectrometry. The Sr/Ca of the aragonite deposited in the presence of either of the inhibitors was not significantly different from that of the solvent (dimethyl sulfoxide) control, although the coral calcification rate was reduced by up to 66% and 73% in the ruthenium red and verapamil treatments, respectively. The typical precision (95% confidence limits) of mean Sr/Ca determinations within any treatment was <±1% and differences in skeletal Sr/Ca between treatments were correspondingly small. Either Ca-ATPase and Ca channels transport Sr²⁺ and Ca²⁺ in virtually the same ratio in which they are present in seawater or transcellular processes contribute little Ca²⁺ to the skeleton and most Ca is derived from seawater transported directly to the calcification site. Variations in the activities of Ca-ATPase and Ca-channels are not responsible for the weekly-monthly Sr/Ca oscillations observed in skeletal chronologies, assuming that the specificities of Ca transcellular transport processes are similar between coral genera.     

Stable carbon isotope ratios in Astrangia danae: evidence for algal modification of carbon pools used in calcification

Stable carbon isotope ratios have been measured in skeletons of the temperature shallow water scleractinian coral, Astrangia danae. δ¹³C values ranging from ?5.42 to ?7.30%. revealed the expected depletion of ¹³C in skeletal carbonate relative to sea water bicarbonate. Differences among the ratios could not be attributed to collection site and were not correlated to skeletal morphology. Values of δ¹³C were directly related to zooxanthellae density for all colonies, so that as zooxanthellae concentration increased, δ¹³C valued increased. Colonies maintained under high temperature conditions were offset from the normal, exhibiting ratios less enriched in ¹³C than similar colonies from natural conditions. These trends supported the models of Weber and Goreau in which the carbon pools used in calcification are modified by algal photosynthesis. Direct evidence of physiological differences between symbiotic and asymbiotic colonies of A. danae has also been provided.     

Metal contents of Porites corals from Khang Khao Island,Gulf of Thailand: Anthropogenic input of river runoff into a coral reef from urbanized areas,Bangkok

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been used to assess the impact of metal pollution on Porites skeletons taken from the Gulf of Thailand since the 1980s. The collection period coincided with a series of laws enacted by the Thai government to curb environmental pollution. The extent of metal pollution by riverine input, including aerosol deposits, was assessed by comparing the metal/Ca (Me/Ca) ratios in the Thai corals to the ratios of another colony of corals sampled from Rukan-sho, a relatively unpolluted coral reef located in Okinawa, Japan. In this comparison, high riverine inputs of Ba, V, Cd and Pb were observed in the Thai coral samples. Higher V/Ca ratios found in the Thai corals compared to the Rukan-sho coral suggest anthropogenic V inputs due to fuel oil pollution in the Gulf since the late 1990s. The levels of Cd in the coral suggest a gradual decrease in the Gulf in the late 1990s, with a drastic drop in concentration from the 1980s. The historical variation in Pb/Ca ratios recorded in the coral skeletons suggests that exposure to anthropogenic Pb was a result of discharge from urbanized areas from 1984 to 1998, which has been gradually reduced since Thailand prohibited the use of leaded gasoline in the late 1990s.     

Mg/Ca, Sr/Ca, and stable isotopes in modern and Holocene Protothaca staminea shells from a northern California coastal upwelling region

This study explores the potential of intertidal Protothaca staminea shells as high-resolution geochemical archives of environmental change in a coastal upwelling region. Mg/Ca and Sr/Ca ratios were analyzed by excimer laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) at sub-weekly temporal resolution in shells growing ∼1 mm per month. Growth patterns of a modern P. staminea shell from Humboldt Bay, California, collected in December 1999 made it possible to infer a lifespan from 1993 to 1998. Growth hiatuses in the shell may have excluded records of extreme events. Mg/Ca ratios appeared to be partly controlled by water temperature; the correlation coefficient between temperature and Mg/Ca was r = 0.71 in one of four growth increments. Significant year-to-year differences in the sensitivity of Mg/Ca to temperature in P. staminea could not be explained, however. Sr/Ca ratios appeared to be more closely related to shell growth rate. Oxygen isotopes, measured at 2-week temporal resolution in the same shell, did not show a clear relation to local temperature in summer, possibly because temperatures were higher and less variable at the King Salmon mudflat, where the shell was collected, than in the main channel of Humboldt Bay, where water properties were monitored. Negative shell δ¹³C values (<−0.5‰) marked spring and summer coastal upwelling events.The Mg contents of P. staminea midden shells dated to ∼3 ka and ∼9 ka were significantly lower than in the modern shell. This may have resulted from degradation of a Mg-rich shell organic matrix and precluded quantitative interpretation of the older high-resolution records. Elevated δ¹³C values in the ∼3 ka shell suggested that the individual grew in highly productive or stratified environment, such as a shallow coastal embayment or lagoon.     

Vertical versus horizontal growth strategies of coral frameworks (Tulamben, Bali, Indonesia)

Two different coral framework structures located in a shallow subtidal area on the east coast of Bali are described in this study. One structure is a typical coral carpet with a distinct internal succession of coral taxa and growth forms. It starts with a variety of coral species exhibiting massive, tabular, branching, and platy growth forms settling on volcanic boulders and cobbles. The main body of the coral carpet is composed almost monospecifically of Acropora cf. vaughani, which has filled all accommodating spaces up to the low-water sea level. Mostof this carpet died during the bleaching event of 1998 and the resultant dead Acropora framework is now capped by a platy Montipora assemblage. Some of the Acropora branches within the dead carpet, however, are still alive and display active growth. The Montipora cover protects the dead Acropora framework against mechanical and biological destruction. The few still growing Acropora branches may also contribute to the strength of the framework. The second coral framework is made up almost monogenerically of Montipora. One species of Montipora is of a laminar growth form and produces whorl-like colonies. Within this framework, only part of the Montipora colonies are dead; however, these are intensively fragmented. The fragments have been rapidly settled by a platy Montipora species, which has stabilized the fragments. In this case, the fragment shedding of the Montipora offers the opportunity for progradation of the framework on these fragments. Concerning the Acropora carpet, similar examples from the fossil record of the Miocene era of Spain and Austria have been reported.     

Facies, geometry and geological significance of Late Ordovician (early Caradocian) coral bioherms: Lourdes Formation, western Newfoundland

Late Ordovician coral bioherms in the Lourdes Formation of western Newfoundland exhibit a complex mixing of architectural elements, including framework, boundstone and suspension deposits. The bioherms occur within a narrow (16 m) stratigraphic interval, and a prominent unconformity truncates the interval of bioherm growth and tops of many of the bioherms. The buildups developed along a carbonate ramp. They occur isolated and in groups, individuals in groups are aligned in parallel orientation. The sizes of the bioherms range from small (50–100 cm) coral piles to columnar and dome‐shaped masses (1–15 m); however, topographic relief was never more than ≈1 m. Bioherm construction reflects: (i) stacking of the tabulate coral Labyrinthites chidlensis, and less common stromatoporoids; (ii) accumulation of microbial‐stromatoporoid boundstone and suspension deposits within shelter cavities between corals; and (iii) detrital bioherm‐flank skeletal grainstone beds. Trypanites borings are common in the tops of coral heads. The bioherms exhibit three growth‐development stages: (i) seafloor stabilization, wherein rare, abraded coral colonies lie scattered within pelmatozoan/skeletal grainstone lenses; (ii) colonization, wherein corals (L. chidlensis), rare stromatoporoids (Labechia sp.), and other biota (bryozoans) produced a bioherm overlying the basal sediment base; and (iii) diversification, which is marked by a more diverse range of fauna and flora as well as occurrence of shelter‐cavity deposits. The diversification stage usually makes up more than 70% of a bioherm structure, and, in some defines multiple periods of start‐up and shut‐down of bioherm growth. The latter is defined by bored omission surfaces and/or deposition of inter‐bioherm sediment. The Lourdes bioherms have a similar ecological structure, biotic diversity and depositional environment to patch reefs in the equivalent Carters Limestone in Tennessee. The mixture of coral stacking and boundstone as architectural elements identify an Early Palaeozoic transition of reef‐design development along shallow‐water platforms that began to displace the muddy (boundstone, bafflestone) carbonate buildups more typical of the Early and Middle Ordovician time.     

Effects of diagenesis on paleoclimate reconstructions from modern and young fossil corals

The integrity of coral-based reconstructions of past climate variability depends on a comprehensive knowledge of the effects of post-depositional alteration on coral skeletal geochemistry. Here we combine millimeter-scale and micro-scale coral Sr/Ca data, scanning electron microscopy (SEM) images, and X-ray diffraction with previously published δ¹⁸O records to investigate the effects of submarine and subaerial diagenesis on paleoclimate reconstructions in modern and young sub-fossil corals from the central tropical Pacific. In a 40-year-old modern coral, we find secondary aragonite is associated with relatively high coral δ¹⁸O and Sr/Ca, equivalent to sea-surface temperature (SST) artifacts as large as −3 and −5 °C, respectively. Secondary aragonite observed in a 350-year-old fossil coral is associated with relatively high δ¹⁸O and Sr/Ca, resulting in apparent paleo-SST offsets of up to −2 and −4 °C, respectively. Secondary Ion Mass Spectrometry (SIMS) analyses of secondary aragonite yield Sr/Ca ratios ranging from 10.78 to 12.39 mmol/mol, significantly higher compared to 9.15 ± 0.37 mmol/mol measured in more pristine sections of the same fossil coral. Widespread dissolution and secondary calcite observed in a 750-year-old fossil coral is associated with relatively low δ¹⁸O and Sr/Ca. SIMS Sr/Ca measurements of the secondary calcite (1.96-9.74 mmol/mol) are significantly lower and more variable than Sr/Ca values from more pristine portions of the same fossil coral (8.22 ± 0.13 mmol/mol). Our results indicate that while diagenesis has a much larger impact on Sr/Ca-based paleoclimate reconstructions than δ¹⁸O-based reconstructions at our site, SIMS analyses of relatively pristine skeletal elements in an altered coral may provide robust estimates of Sr/Ca which can be used to derive paleo-SSTs.     

Calcite-filled borings in the most recently deposited skeleton in live-collected Porites (Scleractinia): Implications for trace element archives

Skeletons of the scleractinian coral Porites are widely utilized as archives of geochemical proxies for, among other things, sea surface temperature in paleoclimate studies. Here, we document live-collected Porites lobata specimens wherein as much as 60% of the most recently deposited skeletal aragonite, i.e., the part of the skeleton that projects into the layer of living polyps and thus is still in direct contact with living coral tissue, has been bored and replaced by calcite cement. Calcite and aragonite were identified in situ using Raman microspectroscopy. The boring-filling calcite cement has significantly different trace element ratios (Sr/Ca_(mmol/mol) = 6.3 ± 1.4; Mg/Ca_(mmol/mol) = 12.0 ± 5.1) than the host coral skeletal aragonite (Sr/Ca_(mmol/mol) = 9.9 ± 1.3; Mg/Ca_(mmol/mol) = 4.5 ± 2.3). The borings appear to have been excavated by a coccoid cyanobacterium that dissolved aragonite at one end and induced calcite precipitation at the other end as it migrated through the coral skeleton. Boring activity and cement precipitation occurred concomitantly with coral skeleton growth, thus replacing skeletal aragonite that was only days to weeks old in some cases. Although the cement-filled borings were observed in only ∼20% of sampled corals, their occurrence in some of the most recently produced coral skeleton suggests that any corallum could contain such cements, irrespective of the coral’s subsequent diagenetic history. In other words, pristine skeletal aragonite was not preserved in parts of some corals for even a few weeks. Although not well documented in coral skeletons, microbes that concomitantly excavate carbonate while inducing cement precipitation in their borings may be common in the ubiquitous communities that carry out micritization of carbonate grains in shallow carbonate settings. Thus, such phenomena may be widespread, and failure to recognize even very small quantities of early cement-filled borings in corals used for paleoclimate studies could compromise high resolution paleotemperature reconstructions. The inability to predict the occurrence of cement-filled borings in coralla combined with the difficulty in recognizing them on polished blocks highlights the great care that must be taken in vetting samples both for bulk and microanalysis of geochemistry.     

Seawater temperature proxies based on DSr, DMg, and DU from culture experiments using the branching coral Porites cylindrica

In order to investigate the incorporation of Sr, Mg, and U into coral skeletons and its temperature dependency, we performed a culture experiment in which specimens of the branching coral (Porites cylindrica) were grown for 1 month at three seawater temperatures (22, 26, and 30 °C). The results of this study showed that the linear extension rate of P. cylindrica has little effect on the skeletal Sr/Ca, Mg/Ca, and U/Ca ratios. The following temperature equations were derived: Sr/Ca (mmol/mol) = 10.214(±0.229) − 0.0642(±0.00897) × T (°C) (r² = 0.59, p < 0.05); Mg/Ca (mmol/mol) = 1.973(±0.302) + 0.1002(±0.0118) × T (°C) (r² = 0.67, p < 0.05); and U/Ca (μmol/mol) = 1.488(±0.0484) − 0.0212(±0.00189) × T (°C) (r² = 0.78, p < 0.05). We calculated the distribution coefficient (D) of Sr, Mg, and U relative to seawater temperature and compared the results with previous data from massive Porites corals. The seawater temperature proxies based on D calibrations of P. cylindrica established in this study are generally similar to those for massive Porites corals, despite a difference in the slope of D_U calibration. The calibration sensitivity of D_Sr, D_Mg, and D_U to seawater temperature change during the experiment was 0.64%/°C, 1.93%/°C, and 1.97%/°C, respectively. These results suggest that the skeletal Sr/Ca ratio (and possibly the Mg/Ca and/or U/Ca ratio) of the branching coral P. cylindrica can be used as a potential paleothermometer.     

Laser ablation ICP-MS screening of corals for diagenetically affected areas applied to Tahiti corals from the last deglaciation

Fossil corals are unique archives of past seasonal climate variability, providing vital information about seasonal climate phenomena such as ENSO and monsoons. However, submarine diagenetic processes can potentially obscure the original climate signals and lead to false interpretations. Here we demonstrate the potential of laser ablation ICP-MS to rapidly detect secondary aragonite precipitates in fossil Porites colonies recovered by Integrated Ocean Drilling Program (IODP) Expedition 310 from submerged deglacial reefs off Tahiti. High resolution (100 μm) measurements of coralline B/Ca, Mg/Ca, S/Ca, and U/Ca ratios are used to distinguish areas of pristine skeleton from those afflicted with secondary aragonite. Measurements of coralline Sr/Ca, U/Ca and oxygen isotope ratios, from areas identified as pristine, reveal that the seasonal range of sea surface temperature in the tropical south Pacific during the last deglaciation (14.7 and 11 ka) was similar to that of today.     

Diagenesis and geochemistry of porites corals from Papua New Guinea: Implications for paleoclimate reconstruction

Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post-depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, δ¹⁸O, and δ¹³C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and δ¹⁸O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and −5.2 to −8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in δ¹⁸O of calcite relative to coral aragonite is a function of the δ¹⁸O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from −2.5 to −10.4%. The variability of δ¹³C in secondary calcite reflects the amount of soil CO₂ contributing ¹³C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on δ¹⁸O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral δ¹⁸O-SST is relatively small (−0.2 to 0.2°C per percent calcite). We show that large shifts in δ¹⁸O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions.     

Rare earth elements in corals from the Isla de Sacrificios Reef,Veracruz, Mexico

Rare earth elements (REE) analysis was carried out in two coral species Diploria strigosa and Copophyllia natans from Isla de Sacrificios Reef (ISR) (19° 10′ 51.6″N; 96° 5′ 45.6″W) Veracruz, Mexico. Both corals were cut at the top, middle and bottom parts to detect possible differences in REE concentrations related to water masses and sediment inputs. An enrichment in heavy rare elements (HREE) compared to light rare elements (LREE) at the top of Diploria strigosa and Copophyllia natans, evidenced by (La/Lu)_SN <0.5, (La/Yb)_SN <0.5 and (Pr/Yb)_SN <0.5 is observed. This HREE enrichment in both corals is probably due to the high pH and CO₃^2? content in the seawater. A negative Ce anomaly is observed throughout Diploria strigosa and Copophyllia natans, probably linked with well oxygenated, highly oxidative modern shallow waters, and high nutrients related to suspended matter. Positive Eu anomalies in both corals are due to development of the ISR in shallow waters. Ce/Ce* vs. (Pr/Yb)_SN diagram suggests the input of terrigenous material, as all samples have Ce/Ce* and Pr/Yb values outside the seawater range signature. However, the Nd/Yb and (Nd/Yb)_SN suggest that the top of Diploria strigosa and Copophyllia natans are associated with coastal waters at about 50 m depth.