ALBERT

Description: The boron isotopic composition (δ11B) of marine carbonates (e.g. corals) has been established as a reliable proxy for paleo-pH, with the strong correlation between δ11B of marine calcifiers and seawater pH being now well documented. However, further investigations are needed in order to better quantify other environmental parameters potentially impacting boron isotopic composition and boron concentration into coral aragonite. To achieve this goal the tropical scleractinian coral Acropora sp. was cultured under 3 different temperature (22, 25 and 28 °C) and two light conditions (200 and 400 μmol photon m−2 s−1). The δ11B indicates an internal increase in pH from ambient seawater under both light conditions. Changes in light intensities from 200 to 400 μmol photon m−2 s−1 could bias pH reconstructions by about 0.05 units. For both light conditions, a significant impact of temperature on δ11B can be observed between 22 and 25 °C corresponding to enhancements of about 0.02 pH-units, while no further δ11B increase can be observed between 25 and 28 °C. This non-linear temperature effect complicates the determination of a correcting factor. B/Ca ratios decrease with increasing light, confirming the decrease in pH at the site of calcification under enhanced light intensities. When all the other parameters are maintained constant, boron concentrations in Acropora sp. increase with increasing temperature and increasing carbonate ions concentrations. These observations contradict previous studies where B/Ca in corals was found to vary inversely with temperature suggesting that the controlling factors driving boron concentrations have not yet been adequately identified and might be influenced by other seawater variables and species specific responses.

Description: The "δ11B-pH" technique was applied to modern and ancient Porites from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20 720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector-Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25‰, allowing a precision of ±0.025 pH-units. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient KD for different aragonite species. Modern coral δ11B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the Sea Water Scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.20 and 8.26 for the Holocene and reached 8.31 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50±0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the CO2SYS program, we recalculated the aqueous pCO2 to be 400±24 ppmV at around 11.5 kyr BP for corals at Marquesas and ~500 ppmV near Tahiti where it was assumed that pCO2 in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in pCO2 between the ocean and the atmosphere at Marquesas (ΔpCO2) indicates that the surface waters behave as a moderate CO2 sink (−67 to −11 ppmV) during El Niño-like conditions. In contrast, during the last glacial/interglacial transition, this area was a moderate source of CO2 (−9 to 56 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO2 released with ΔpCO2 of +140 ppmV. This last finding provides further evidence of the marked changes to the water mass pH and temperature properties in the equatorial Pacific at the Younger Dryas- Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO2 content.

Description: The "δ11B-pH" technique was applied to modern and ancient corals Porites from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector – Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25‰, allowing a precision of about ±0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient KD for different aragonite species. Modern coral δ11B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50±0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous pCO2 to be 440±25 ppmV at around 11.5 kyr BP for corals at Marquesas and ~500 ppmV near Tahiti where it was assumed that pCO2 in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in pCO2 between the ocean and the atmosphere at Marquesas (ΔpCO2) indicates that the surface waters behave as a moderate CO2 sink or source (−53 to 20 ppmV) during El Niño-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO2 (21 to 92 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO2 released with ΔpCO2 of +185±25 ppmV. This last finding provides further evidence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO2 content.

Description: Skeletal isotopic and metabolic measurements of the branching coral Acropora cultured in constant conditions and subjected to two light intensities were revisited. We individually compared the data recorded at low light (LL) and high light (HL) for 24 colonies, all derived from the same parent colony. Metabolic and isotopic responses to the different light levels were highly variable. High light led to productivity enhancement, reduction of surface extension, doubling of aragonite deposited weight and increased δ18O levels in all nubbins; responses in respiration and δ13C were not clear. The partitioning of the colonies into two groups, one showing a δ13C increase and the other a δ13C decrease with increased light, revealed common behaviors. Samples showing an increase in δ13C were associated with the co-variation of low surface extension and high productivity while samples showing a decrease in δ13C were associated with the co-variation of higher surface extension and limited productivity. This experiment, which allowed for the separation of temperature and light effects on the coral, highlighted the significant light influences on both skeletal δ18O and δ13C. The high scattering of inter-colony δ18O observed at one site could be due to the differing photosynthetic responses of symbiotic algal assemblages. The δ13C responses could also be related to differing algal distributions in different skeletal portions. Our results were compared to observations by Gladfelter on Acropora cervicornis (1982). Both set of results highlight the relationships between coral-growth rates, micro-structures and photosynthetic activity. It appears that extension growth and accretion are two separate growth modes, and accretion is light-enhanced while extension is light-repressed. There are multiple consequences of these findings for paleoclimatic reconstructions involving corals.

Description: The boron isotopic composition (δ11B) of marine carbonates (e.g. corals) is increasingly utilised as a proxy for paleo-pH, with the strong correlation between δ11B of marine calcifiers and seawater pH now well documented. However, the potential roles of other environmental parameters that may also influence both the boron isotopic composition and boron concentration into coral aragonite are poorly known. To overcome this, the tropical scleractinian coral Acropora sp. was cultured under 3 different temperatures (22, 25 and 28 °C) and two light conditions (200 and 400 μmol photon m−2 s−1). The δ11B indicates an increase in internal pH that is dependent on the light conditions. Changes in light intensities from 200 to 400 μmol photon m−2 s−1 seem to indicate an apparent decrease in pH at the site of calcification, contrary to what is expected in most models of light-enhanced calcification. Thus, variations in light conditions chosen to mimic average annual variations of the natural environments where Acropora sp. colonies can be found could bias pH reconstructions by about 0.05 units. For both light conditions, a significant impact of temperature on δ11B can be observed between 22 and 25 °C, corresponding to an increase of about 0.02 pH-units, while no further δ11B increase can be observed from 25 to 28 °C. This non-linear temperature effect complicates the determination of a correction factor. B / Ca ratios decrease with increasing light, consistent with the decrease in pH at the site of calcification under enhanced light intensities. When all the other parameters are constant, boron concentrations in Acropora sp. increase with increasing temperatures and increasing carbonate ion concentrations. These observations contradict previous studies where B / Ca in corals was found to vary inversely with temperature, suggesting that the controlling factors driving boron concentrations have not yet been adequately identified and might be influenced by other environmental variables and/or species-specific responses.

Description: Light, an environmental parameter playing a crucial role in coral aragonite growth and δ18O formulation, is always neglected in the geochemical literature. However, by revisiting already published studies, we demonstrated that light might be considered as a vital effect affecting coral aragonite oxygen isotopic ratios. Re-examining data series included in a publication by Weber and Woodhead (1972), we stressed that annual δ18O–annual temperature calibrations of all considered coral genera may be compared because their assessment assumes homogenous light levels. Temperature prevails on δ18O because it influences δ18O in two ways: firstly it acts as is thermodynamically predicted implying a δ18O decrease; and secondly it induces an enhancement of photosynthesis causing δ18O increase. When the highest annual temperature occurs simultaneously with the highest annual irradiation, the annual δ18O amplitude is shortened. The annual δ18O–annual temperature calibration is also explained by the relative distribution of microstructures, centres of calcification or COC and fibers, according to morphology, and in turn taxonomy. We also investigated monthly δ18O–monthly temperature calibrations of Porites grown at the same sites as by Stephans and Quinn (2002), Linsley et al. (1999, 2000) and Maier et al. (2004). Multiple evidence showed that temperature is the prevailing environment forcing on δ18O and that the mixture of temperature and light also determines the relative distribution of microstructures, explaining the relationships between Porites calibration constants. By examining monthly and annual δ18O–monthly and annual temperature calibrations, we revealed that monthly calibration results from the superimposition of seasonal and annual variability over time. Seasonal δ18O strongly impacted by seasonal light fluctuations, may be obtained by removing interannual δ18O only weakly affected by light. Such features necessitate the reconstitution of tools frequently utilised, such as the coupled δ18O–Sr/Ca or pseudo-coral concepts.

Description: Skeletal isotopic and metabolic measurements of the branching coral Acropora cultured in constant conditions and subjected to two light intensities were revisited. We individually compared the data recorded at low light (LL) and high light (HL) for 24 colonies, all derived from the same parent colony. Metabolic and isotopic responses to the different light levels were highly variable. High light led to productivity enhancement, reduction of surface extension, doubling of aragonite deposited weight and increased δ18O levels in all nubbins; responses in respiration and δ13C were not clear. The partitioning of the colonies cultured at HL into two groups, one showing a δ13C enrichment and the other a δ13C decrease revealed common behaviors. Samples showing an increase in δ13C were associated with the co-variation of low surface extension and high productivity while samples showing a decrease in δ13C were associated with the co-variation of higher surface extension and limited productivity. This experiment, which allowed for the separation of temperature and light effects on the coral, highlighted the significant light influences on both skeletal δ18O and δ13C. The high scattering of inter-colony δ18O observed at one site could be due to the differing photosynthetic responses of symbiotic algal assemblages. We compared our results with observations by Gladfelter on Acropora cervicornis (1982). Both set of results highlight the relationships between coral-growth rates, micro-structures and photosynthetic activity. It appears that extension growth and skeleton thickening are two separate growth modes, and thickening is light-enhanced while extension is light-suppressed. There are multiple consequences of these findings for paleoclimatic reconstructions involving corals.