<p>Massive tropical corals represent one of the most important natural archives of modern climate change. Coral based reconstructions give us the possibility to extend the instrumental oceanographic records and observe hydrographic variability on seasonal to interdecadal scales in tropical oceans. South Pacific convergence zone (SPCZ) variability, Interdecadal Pacific Oscillation (IPO) and El Ni&#241;o-Southern Oscillation (ENSO) events are major drivers of global climate and may exert control on regional CO<sub>2</sub> absorption, outgassing and pH variability.</p><p><em>Porites</em> sp. corals from Tonga and Rotuma (Fijian dependency) are being analyzed for multi-proxy (e.g. Sr/Ca, &#948;<sup>18</sup>O, &#948;<sup>13</sup>C, &#948;<sup>11</sup>B, B/Ca) reconstructions of sea surface temperature and salinity (SST, SSS) and carbonate chemistry, on a monthly to annual resolution. Preliminary data of the Rotuma <em>Porites</em> sp. coral shows &#948;<sup>18</sup>O has been decreasing by 0.004 &#8240; per year at the end of the 20th century, suggesting freshening and/or warming of the surface water. In the same period, we observe a &#948;<sup>13</sup>C decrease of 0.017 &#8240; per year in-line with the anthropogenic CO<sub>2</sub> driven Suess effect. Initial results of the &#948;<sup>11</sup>B Tonga <em>Porites</em> sp. show high interannual variability, and a strong trend of decrease of -0.0626 &#8240; per year in the last five decades of the record (1949-2004) suggesting acidification. The results are in agreement with published coral-based reconstructions from the region.</p><p>When completed, the new records will facilitate exploring the effects of modern anthropogenic influence on ocean carbonate system and pH variation, and the relationship between them and interannual and decadal-interdecadal climatic fluctuations.</p>
<p>Whether or not Arctic regions remain(ed) a carbon sink or source to the atmosphere during rapidly warming climates (in the past) is a fundamental question with regards to future global warming and ocean acidification. The boron isotopic composition of planktonic foraminiferal shell calcite (&#948;<sup>11</sup>B<sub>Cc</sub>) can potentially provide valuable information of past seawater pH if information on a second carbonate system parameter, temperature, and salinity is available. However, most applications of palaeoceanographic proxies to the cold polar oceans are limited due to a paucity of calibration data, limited information on the calcification habitat, and secondary effects of the carbonate system on the temperature recorded by Mg/Ca values measured in the dominant Arctic species Neogloboquadrina pachyderma sinistral (NPS). Here we present a new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) &#948;<sup>11</sup>B dataset measured on live NPS collected via plankton tows from the Labrador Sea and Baffin Bay. We compare our results with &#948;<sup>11</sup>B<sub>borate</sub> derived from pH measurements, &#948;<sup>13</sup>C DIC seawater values, temperature and salinity collected at the time and depth the foraminifera calcified. To quantify the control of low carbonate ion concentration on Mg/Ca derived temperatures we measured B/Ca alongside Mg/Ca in the calibration dataset. We are thus able to present a new geochemical correction scheme that can isolate non-thermal controls on the Mg/Ca-temperature relationship for NPS, allowing us for the first time the reconstruction of carbonate system parameters in the Arctic Ocean.</p>
Abstract. In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity, are required. The boron isotopic composition (δ11B) of planktonic foraminifera shells is a powerful tool for reconstructing changes in past surface ocean pH. As B(OH)4− is substituted into the biogenic calcite lattice in place of CO32−, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B ∕ Ca ratios in biogenic calcite may serve as a proxy for [CO32−]. Although several recent studies have shown that a direct connection of B ∕ Ca to carbonate system parameters may be masked by other environmental factors in the field, there is ample evidence for a mechanistic relationship between B ∕ Ca and carbonate system parameters. Here, we focus on investigating the primary relationship to develop a mechanistic understanding of boron uptake. Differentiating between the effects of pH and [CO32−] is problematic, as they co-vary closely in natural systems, so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO32−] and to investigate their impact on the B ∕ Ca ratio and δ11B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO32−] (238, 286 and 534 µmol kg−1 CO32−) and at constant [CO32−] (276 ± 19.5 µmol kg−1) and varying pH (7.7, 7.9 and 8.05). Measurements of the isotopic composition of boron and the B ∕ Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to a MC-ICP-MS (multiple-collector inductively coupled plasma mass spectrometer). Our results show that, as expected, δ11B is controlled by pH but it is also modulated by [CO32−]. On the other hand, the B ∕ Ca ratio is driven by [HCO3−], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete paleo-carbonate system reconstructions. This is discussed in light of recent literature demonstrating that the primary relationship between B ∕ Ca and [HCO3−] can be obscured by other environmental parameters.
In field studies of active hydrocarbon seeps the carbon isotopic composition of Rose Bengal stained benthic foraminiferal tests (δ13Ctest) and bottom water DIC (δ13CDIC) deviates from their normal marine ratios. This circumstance led to ongoing discussions on whether aerobic foraminifers like Cibicides wuellerstorfi are capable of living at seepage sites and, more importantly, if their tests reflect the low δ13C values of emanating methane. To evaluate the discrepancy between δ13CDIC and δ13Ctest, we conducted methane seepage-emulating culture experiments on undepressurized sediments from the Håkon Mosby Mud Volcano, a modern methane seepage structure that hosts living C. wuellerstorfi with distinct negative δ13C values. The collected sediments were cultured at a site-alike pressure and mean bottom water methane concentration using newly developed high-pressure aquaria. Over an experimental period of 5 months our novel technology enabled a successful reproduction of all calcareous deep-sea benthic foraminiferal species living at that site, notably the first C. wuellerstorfi cultured in the laboratory. To show the influence of methane on δ13Ctest, we ran parallel experiments with > 99% 12C- and 99% 13C-methane in the experimental "bottom water". During the experimental running time methanotrophs in the water column obviously converted the experimentally added methane source to δ13C-enriched and -depleted DIC, respectively. Since whole sediment cores were cultured, it was impossible to keep δ13CDIC constant over the 5-month duration, which is reflected in a variability of δ13Ctest in foraminiferal shells. Irrespective of that, the methane source is reflected in δ13Ctest of foraminiferal shells, and for the natural seep-conditions simulating 12C-experiment the mean δ13CDIC and δ13Ctest in C. wuellerstorfi were equal. Although for future culturing experiments improvements of the experimental conditions are advisable, our first results are evidence that persistent methane emanation impacts the carbon isotopic composition of deep-sea benthic foraminifera.
A new matrix‐matched reference material has been developed – NFHS‐2‐NP (NIOZ Foraminifera House Standard‐2‐Nano‐Pellet) – with element mass fractions, and isotope ratios resembling that of natural foraminiferal calcium carbonate. A 180–355 µm size fraction of planktic foraminifera was milled to nano‐particles and pressed to pellets. We report reference and information values for mass fractions of forty‐six elements measured by six laboratories as well as for 87 Sr/ 86 Sr (three laboratories), δ 13 C, δ 18 O (five laboratories) and 206,207,208 Pb/ 204 Pb isotope ratios (one laboratory) determined by ICP‐MS, ICP‐OES, MC‐ICP‐MS, isotope ratio mass spectrometry, WD‐XRF and TIMS. Inter‐ and intra‐pellet elemental homogeneity was tested using multiple LA‐ICP‐MS analyses in two laboratories applying spot sizes of 60 and 70 µm. The LA‐ICP‐MS results for most of the elements relevant as proxies for palaeoclimate research show RSD values < 3%, demonstrating a satisfactory homogeneous composition. Homogeneity of 87 Sr/ 86 Sr ratios of the pellet was verified by repeated LA‐MC‐ICP‐MS by two laboratories. Information values are reported for Pb isotope ratios and δ 13 C, δ 18 O values. The homogeneity for these isotope systems remains to be tested by LA‐MC‐ICP‐MS and secondary‐ion mass spectrometry. Overall, our results confirm the suitability of NFHS‐2‐NP for calibration or monitoring the quality of in situ geochemical techniques.
