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Environmental constraints on Holocene cold‐water coral reef growth off Norway: Insights from a multiproxy approach

2016; American Geophysical Union; Volume: 31; Issue: 10 Linguagem: Inglês

10.1002/2016pa002974

1944-9186

Jacek Raddatz, Volker Liebetrau, Julie Trotter, Andres Rüggeberg, Sascha Flögel, Wolf‐Christian Dullo, Anton Eisenhauer, Silke Voigt, Malcolm T. McCulloch,

Geology and Paleoclimatology Research

PaleoceanographyVolume 31, Issue 10 p. 1350-1367 Research ArticleFree Access Environmental constraints on Holocene cold-water coral reef growth off Norway: Insights from a multiproxy approach Jacek Raddatz, Corresponding Author Jacek Raddatz raddatz@em.uni-frankfurt.de Goethe University Frankfurt, Institute of Geosciences, Frankfurt am Main, Germany GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany Correspondence to: J. Raddatz, raddatz@em.uni-frankfurt.deSearch for more papers by this authorVolker Liebetrau, Volker Liebetrau GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorJulie Trotter, Julie Trotter The UWA Oceans Institute and School of Earth and Environment, University of Western Australia, Crawley, Western Australia, AustraliaSearch for more papers by this authorAndres Rüggeberg, Andres Rüggeberg Renard Centre of Marine Geology, Department of Geology and Soil Sciences, Ghent University, Gent, Belgium Department of Geosciences, University of Fribourg, Fribourg, SwitzerlandSearch for more papers by this authorSascha Flögel, Sascha Flögel GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorWolf-Christian Dullo, Wolf-Christian Dullo GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorAnton Eisenhauer, Anton Eisenhauer GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorSilke Voigt, Silke Voigt Goethe University Frankfurt, Institute of Geosciences, Frankfurt am Main, GermanySearch for more papers by this authorMalcolm McCulloch, Malcolm McCulloch The UWA Oceans Institute and School of Earth and Environment, University of Western Australia, Crawley, Western Australia, Australia ARC Centre of Excellence in Coral Reef Studies, University of Western Australia, Crawley, Western Australia, AustraliaSearch for more papers by this author Jacek Raddatz, Corresponding Author Jacek Raddatz raddatz@em.uni-frankfurt.de Goethe University Frankfurt, Institute of Geosciences, Frankfurt am Main, Germany GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany Correspondence to: J. Raddatz, raddatz@em.uni-frankfurt.deSearch for more papers by this authorVolker Liebetrau, Volker Liebetrau GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorJulie Trotter, Julie Trotter The UWA Oceans Institute and School of Earth and Environment, University of Western Australia, Crawley, Western Australia, AustraliaSearch for more papers by this authorAndres Rüggeberg, Andres Rüggeberg Renard Centre of Marine Geology, Department of Geology and Soil Sciences, Ghent University, Gent, Belgium Department of Geosciences, University of Fribourg, Fribourg, SwitzerlandSearch for more papers by this authorSascha Flögel, Sascha Flögel GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorWolf-Christian Dullo, Wolf-Christian Dullo GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorAnton Eisenhauer, Anton Eisenhauer GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanySearch for more papers by this authorSilke Voigt, Silke Voigt Goethe University Frankfurt, Institute of Geosciences, Frankfurt am Main, GermanySearch for more papers by this authorMalcolm McCulloch, Malcolm McCulloch The UWA Oceans Institute and School of Earth and Environment, University of Western Australia, Crawley, Western Australia, Australia ARC Centre of Excellence in Coral Reef Studies, University of Western Australia, Crawley, Western Australia, AustraliaSearch for more papers by this author First published: 10 September 2016 https://doi.org/10.1002/2016PA002974Citations: 22AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract High-latitude cold-water coral (CWC) reefs are particularly susceptible due to enhanced CO2 uptake in these regions. Using precisely dated (U/Th) CWCs (Lophelia pertusa) retrieved during research cruise POS 391 (Lopphavet 70.6°N, Oslofjord 59°N) we applied boron isotopes (δ11B), Ba/Ca, Li/Mg, and U/Ca ratios to reconstruct the environmental boundary conditions of CWC reef growth. The sedimentary record from these CWC reefs reveals a lack of corals between ~6.4 and 4.8 ka. The question remains if this phenomenon is related to changes in the carbonate system or other causes. The initial postglacial setting had elevated Ba/Ca ratios, indicative of meltwater fluxes showing a decreasing trend toward cessation at 6.4 ka with an oscillation pattern similar to continental glacier fluctuations. Downcore U/Ca ratios reveal an increasing trend, which is outside the range of modern U/Ca variability in L. pertusa, suggesting changes of seawater pH near 6.4 ka. The reconstructed bottom water temperature at Lopphavet reveals a striking similarity to Barent sea surface and subsea surface temperature records. We infer that meltwater pulses weakened the North Atlantic Current system, resulting in southward advances of cold and CO2-rich Arctic waters. A corresponding shift in the δ11B record from ~25.0‰ to ~27.0‰ probably implies enhanced pH up-regulation of the CWCs due to the higher pCO2 concentrations of ambient seawater, which hastened mid-Holocene CWC reef decline on the Norwegian margin. Key Points Multiproxy approach in Lophelia pertusa reveals changes in seawater properties throughout the Holocene Coupled boron isotopes and U/Ca approach CWC reef decline due to advances of Arctic waters 1 Introduction Cold-water coral (CWC) reefs on the European continental margin are hot spots of marine biodiversity, but due to ongoing climate change, they are under serious threat [Guinotte et al., 2006; Roberts and Cairns, 2014]. In particular, the current rise in atmospheric carbon dioxide concentrations (CO2) dissolves into the oceans and therefore leads to a decrease in seawater pH, known as "ocean acidification" [e.g., Doney et al., 2009]. This phenomenon probably limits the growth and survival of cold-water corals due to shoaling of the aragonite saturation horizon (ASH, aragonite saturation state = Ωarag = [Ca2+][CO32−]/K*arag, where K*arag is the stoichiometric solubility product of aragonite). Given their proximity to the carbonate saturation horizon, CWCs are particularly vulnerable to the impact of climate change and the consequent effects on seawater chemistry [Guinotte et al., 2006]. The main framework builder of CWC reefs in the NE Atlantic is Lophelia pertusa [Linnaeus, 1758 (Desmophyullum pertusum [Addamo et al., 2016; Freiwald et al., 2004, Figure 1]). Single L. pertusa polyps can be found within a seawater temperature range of 4–14°C [Freiwald et al., 2004, 2009; Roberts et al., 2006] with only a few exceptions in the western Atlantic [Mienis et al., 2014], whereas flourishing reefs thrive between >6°C on the Norwegian margin and 98% aragonite) indicate that all samples have retained their primary aragonite skeletons. Before subsampling for dating and elemental ratios, the fossil samples were chemically cleaning followed the protocol of Cheng et al. [2000a]; this protocol was not applied to samples prepared for boron isotope analysis. Furthermore, we did not employ alkaline diethylenetriamine-pentaacetic acid for barite removal, similar to other studies investigating Ba/Ca ratios in marine carbonates [e.g., Bahr et al., 2013; Hönisch et al., 2011, and references therein]. 2.1 U/Th Age Determinations The uranium/thorium coral ages were performed on a VG Axiom multicollector inductively coupled plasma–mass spectrometer (MC-ICP-MS) at GEOMAR Helmholtz Centre for Ocean Research Kiel (Germany) following the protocols of Fietzke et al. [2005] and a quadrupole ICP-MS Agilent Series 7200cs (GEOMAR) according to the method of Douville et al. [2010]. Calculations ware based on decay constants of Cheng et al. [2000b]. The measurements were carried out with a combined 233/236U/229Th spike. The stock solutions were calibrated against National Institute of Standards and Technology Standard Reference Material (NIST-SRM) 3164 (U) and NIST-SRM 3159 (Th), as a combined spike calibrated against CRM-145 uranium standard solution for U-isotope compositions, and against a secular equilibrium standard (HU-1, uranium ore solution) for determination of 230Th/234U activity ratios. Full procedural blanks were around 60 pg for U, 6 to 9 pg for 232Th, and 0.5 to 5 fg for 230Th. A correction for detrital Th has been carried out, applying mean crustal composition values for U and Th and a 230Th/232Th activity ratio of 0.75 ± 0.2, according to Wedepohl [1995]. The latter correction appears to be negligible due to sufficiently high 230Th/232Th activity ratios and relatively low Th concentrations. Modern scleractinian cold-water corals reveal δ234U(0) values of 145.5 ± 2.3‰ [Cheng et al., 2000b] and 146.3 ± 3.9‰ [Liebetrau et al., 2010]. We refer to the approach of Wienberg et al. [2010, and references therein] to consider samples of a δ234U(T) value of 146‰ ± 10‰ (modern seawater [Henderson and Anderson, 2003]) as reliable, which is apparently only for sample 822-13, not the case (δ234U(t) = 246.52 ± 14.19) (Table 1). Table 1. U/Th Geochronology of Lophelia pertusa Samples Retrieved During Research Cruise POS 391a Lab Code Station Number/Gear /Water Depth (m) Core Depth (cm) Description Sampling Region Location U-Th Prepared Sample Aliqout (mg) MC = M 238U Concentration (µg/g) 232Th Concentration (ng/g) (230Th/232Th) (230Th/234U) ∂234U (0) ∂234U (t) Age (ka) ICP-MS Type Latitude (°N) Longitude (°E) Quadr. = Q M = MC. Q = Quadr ICP-MS 429-11/M 550-1/JAGO/220 reef recent red Lopphavet 70.61 21.22 49 M 2.43 ± 0.01 b. d. l. ± - b. d. l. ± - 0.00015 ± 0.00004 143 ± 4 143 ± 4 0.02 ± 0.01 808-13/Q 551-2/GC/244 6 Lopphavet 70.61 21.22 158 Q 4.39 ± 0.03 2.99 ± 0.24 344.4 ± 28.5 344.4 ± 28.5 144 ± 10 147 ± 10 7.5 ± 0.2 439-11/M 551-3/GC/? 200 Lopphavet 70.61 21.22 50 M 2.89 ± 0.01 0.55 ± 0.02 1626.5 ± 0.001 0.0883 ± 0.0010 142 ± 4 146 ± 4 10.1 ± 0.1 809-13/Q 557-3/GC/268 1 Lopphavet 70.68 21.23 176 Q 3.10 ± 0.02 1.38 ± 0.19 712.9 ± 101.1 0.0895 ± 0.0014 151 ± 7 155 ± 7 10.2 ± 0.2 809-13 wpr/Q 557-3/GC/268 1 Lopphavet 70.68 21.23 182 Q 3.10 ± 0.03 1.34 ± 0.08 709.8 ± 45.4 0.0877 ± 0.0021 142 ± 13 146 ± 14 10.0 ± 0.3 497-12/M 559-2/GC/225 17 Lopphavet 70.69 21.23 39 M 6.09 ± 0.03 47.41 ± 0.28 25.4 ± 0.2 0.0570 ± 0.0007 137 ± 5 139 ± 5 6.4 ± 0.1 497-12/Q 559-2/GC/225 17 Lopphavet 70.69 21.23 197 Q 6.06 ± 0.14 44.81 ± 0.93 27.6 ± 3.2 0.0571 ± 0.0025 127 ± 33 129 ± 33 6.4 ± 0.3 810-13/M 559-2/GC/225 27 Lopphavet 70.69 21.23 54 M 4.84 ± 0.03 1.94 ± 0.25 582.9 ± 77.3 0.0662 ± 0.0021 147 ± 9 150 ± 9 7.5 ± 0.2 811-13/M 559-2/GC/225 38 Lopphavet 70.69 21.23 25 M 5.40 ± 0.06 12.41 ± 0.81 96.9 ± 7.0 0.0630 ± 0.0023 141 ± 17 144 ± 17 7.1 ± 0.3 812-13/Q 559-2/GC/225 56 Lopphavet 70.69 21.23 194 Q 4.52 ± 0.06 3.92 ± 0.31 277.4 ± 23.1 0.0682 ± 0.0022 146 ± 18 150 ± 18 7.7 ± 0.3 813-13/Q 559-2/GC/225 86 Lopphavet 70.69 21.23 165 Q 4.11 ± 0.03 1.85 ± 0.10 545.1 ± 29.0 0.0699 ± 0.0010 140 ± 11 143 ± 11 7.9 ± 0.1 814-13/Q 559-2/GC/225 103 Lopphavet 70.69 21.23 177 Q 3.96 ± 0.03 4.28 ± 0.50 244.1 ± 28.9 0.0751 ± 0.0020 141 ± 10 144 ± 10 8.5 ± 0.2 816-13/Q 559-2/GC/225 184 Lopphavet 70.69 21.23 150 Q 4.33 ± 0.02 4.67 ± 0.10 260.7 ± 8.6 0.0796 ± 0.0020 148 ± 4 152 ± 5 9.0 ± 0.2 816-13 wpr/Q 559-2/GC/225 184 Lopphavet 70.69 21.23 147 Q 4.70 ± 0.03 5.86 ± 0.13 217.8 ± 5.3 0.0777 ± 0.0010 135 ± 8 139 ± 8 8.8 ± 0.1 817-13/Q 559-2/GC/225 221 Lopphavet 70.69 21.23 163 Q 3.33 ± 0.02 10.48 ± 0.34 85.1 ± 3.5 0.0764 ± 0.0022 133 ± 11 136 ± 11 8.7 ± 0.3 818-13/Q 559-2/GC/225 235 Lopphavet 70.69 21.23 160 Q 3.50 ± 0.02 2.16 ± 0.12 515.2 ± 29.7 0.0903 ± 0.0011 147 ± 10 152 ± 10 10.3 ± 0.1 819-13/Q 559-2/GC/225 1 Lopphavet 70.69 21.23 160 Q 8.28 ± 0.04 5.39 ± 0.19 235.2 ± 8.8 0.0433 ± 0.0005 148 ± 4 150 ± 4 4.8 ± 0.1 820-13/Q 559-3/GC/237 8 Lopphavet 70.69 21.23 186 Q 4.56 ± 0.03 1.66 ± 0.02 625.8 ± 7.9 0.0656 ± 0.0008 131 ± 8 134 ± 8 7.4 ± 0.1 433-11/M 559-3/GC/237 sed. surf./rec. Sula 64.33 8.12 49 M 2.83 ± 0.01 b. d. l. ± - b. d. l. ± - 0.0001 ± 0.0001 146 ± 5 146 ± 5 0.02 ± 0.01 431-11/M 563-1/v.V. Gr./285 sed. surf./rec. Oslo Fjord 59.1 10.8 49 M 3.29 ± 0.01 b. d. l. ± - b. d. l. ± - 0.0002 ± 0.0001 145 ± 4 145 ± 4 0.02 ± 0.01 821-13/Q 571-1/JAGO/100 1 Oslo Fjord 59.1 10.8 225 M 3.52 ± 0.02 1.27 ± 0.05 402.2 ± 15.9 0.0416 ± 0.0005 139 ± 5 140 ± 5 4.6 ± 0.1 821-13 wpr/Q 575-2/GC/99 1 Oslo Fjord 59.1 10.8 173 M 3.54 ± 0.02 10.54 ± 0.19 50.0 ± 2.0 0.0418 ± 0.0015 141 ± 7 143 ± 7 4.7 ± 0.2 488-12/M 575-2/GC/99 12 Oslo Fjord 59.1 10.8 39 M 4.35 ± 0.03 2.16 ± 0.01 101.0 ± 1.4 0.0143 ± 0.0002 142 ± 7 143 ± 7 1.58 ± 0.02 488-12/Q 576-1/GC/101 12 Oslo Fjord 59.1 10.8 155 Q 4.24 ± 0.04 2.30 ± 0.04 98.6 ± 2.5 0.0148 ± 0.0004 168 ± 13 168 ± 13 1.63 ± 0.04 498/M 576-1/GC/101 134 Oslo Fjord 59.1 10.8 31 M 4.26 ± 0.02 44.76 ± 0.31 9.1 ± 0.1 0.0252 ± 0.0007 140 ± 3 141 ± 3 2.78 ± 0.08 494-12/M 576-1/GC/101 212 Oslo Fjord 59.1 10.8 42 M 4.33 ± 0.03 7.27 ± 0.05 64.7 ± 0.8 0.0309 ± 0.0004 145 ± 6 146 ± 6 3.4 ± 0.1 494-12/Q 576-1/GC/101 212 Oslo Fjord 59.1 10.8 209 Q 4.22 ± 0.05 7.49 ± 0.24 65.0 ± 5.8 0.0317 ± 0.0017 170 ± 18 172 ± 18 3.51 ± 0.2 a The bdl denotes below detection limit, and wpr denotes whole procedure blank, heterogeneity, and 232Th correction test; all uncertainties are presented at two SEM levels. The δ234U(0) value represents the originally today measured (234U/238U) activity ratio, given in delta notation (δ234U(0) = ((234Uact/238Uact) − 1) × 1000). Displayed δ234U(T) values reflect the age-corrected (234U/238U) activity ratios by recalculating the decay of 234U for the time interval T (δ234U(T) = δ234U(0) × exp (λ234 · T)), determined from 230Th/234U age of each individual sample. All errors are deduced on 2σ level. 2.2 Elemental Ratios (Ba/Ca, U/Ca, Li/Ca, and Mg/Ca) Elemental ratios were measured on a quadrupole ICP-MS Agilent Series 7200cs (GEOMAR) and Thermo X-series II quadrupole ICP-MS at University of Western Australia (UWA). Fossil samples were analyzed at GEOMAR, and the living modern samples were analyzed at UWA. Solutions were analyzed for elemental ratios according to the method of Rosenthal et al. [1999]. In particular, elemental ratios were calibrated against multielement standards made from single-element solutions. For comparison, the Porites coral powder reference material JCp-1 was analyzed at both labs and yielded the expected absolute ratios and uncertainties presented by Hathorne et al. [2013], reflecting both interlaboratory consistency and the robustness of this coral standard. Typical reproducibility (2 standard deviations) is 1.2% for Mg/Ca, 5% Li/Ca, 10% Ba/Ca, and 1.2% U/Ca. 2.2.1 Ba/Ca Ratios Open ocean dissolved barium has a similar distribution as nutrient and silicate concentrations [Chan et al., 1977] and short residence time of 9 ka [Broecker and Peng, 1982]. The use of Ba as a paleo-proxy has been discussed controversial [McManus et al., 1998]. However, in marginal seas and close to rivers, Ba/Ca has been successfully used as a tracer for fluctuating terrigenous input [e.g., Bahr et al., 2013; Hoffmann et al., 2014; Weldeab et al., 2007] and for tracing meltwater fluxes [Hall and Chan, 2004]. Ba/Ca can also be applied to scleractinian warm-water corals [e.g., Lavigne et al., 2016; Sinclair and Mcculloch, 2004] and cold-water corals [Anagnostou et al., 2011; Montagna et al., 2005; Raddatz et al., 2014a]. The study by Anagnostou et al. [2011] is to our knowledge the only existing CWC (scleractinian) calibration for estimating Ba/Ca partition coefficients between seawater and coral aragonite, but was measured in the solitary slow growing Desmophyllum dianthus, hence may not be directly applicable to the fast-growing reef-forming CWC, L. pertusa. Here we use the Ba/Ca record to infer relative trends of terrigenous input into the coral reef accompanied by meltwater fluxes at sea surface. 2.2.2 Li/Mg Ratios The Li/Mg ratios were calculated from the Mg/Ca and Li/Ca ratios. The Li/Mg ratio has previously been shown to serve as a robust temperature proxy in scleractinian cold-water corals [Case et al., 2010; Raddatz et al., 2013; Montagna et al., 2014]. Here we use the multispecies calibration of Montagna et al. [2014] to calculate bottom water temperatures (BWTs) with the following equation: Li/Mgcoral = 5.41 exp (−0.049 × BWT), with a resulting uncertainty of ±0.8°C. Lithium and magnesium are conservative in the ocean and have a residence time of about 1 and 10 Myr, respectively [HuH et al., 1998; Berner and Berner, 1996]. It has been shown that Li/Ca and Mg/Ca twofold increase over the last 2–3 Myr [Hathorne and James, 2006; Fantle and DePaolo, 2006], largely reflecting the shorter residence time of Ca compared to Mg (and Li). However, considering the relatively long residence time of Mg and hence Li in the oceans, changes in the Li/Mg ratio of seawater are likely to be minimal even on the time scale of 107 years. Accordingly, given that we have reconstructed environmental changes only over the last 11 kyr, we consider seawater changes in Li/Mg as not being relevant. 2.2.3 U/Ca Ratios Recent studies have shown that U/Ca ratios in scleractinian corals can be used to reconstruct changes in the carbonate system of the ocean [Anagnostou et al., 2011; Inoue et al., 2011; Raddatz et al., 2014b]. There is an ongoing discussion whether U/Ca ratios of aragonite precipitates reflect seawater pH and/or the carbonate ion concentration (CO32−) [DeCarlo et al., 2015]. In the seawater carbonate system, carbonate ion concentration generally decreases with decreasing seawater pH. As all prior U/Ca studies reveal that U/Ca ratios increase with either decreasing seawater pH or carbonate ion concentration, this parameter appears to be largely controlled by seawater carbonate chemistry. Here we use the U/Ca-seawater pH relationship of Raddatz et al. [2014b] with the following equation: pHseawater = (U/CaLophelia − 16.18)/−1.82, with a relatively large uncertainty of ±0.15 for the reconstructed seawater pH. However, we caution that U/Ca-based seawater pH reconstructions might be biased by potentially large differences in growth or precipitation rates [Raddatz et al., 2014b]. Given these uncertainties, we followed a conservative and more robust approach by mainly focusing on trends rather than absolute seawater pH values. The residence time of U is calculated to be 200–400 kyr and is therefore considered to be of minor importance controlling U/Ca variations in our record [e.g., Ku et al., 1977; Henderson, 2002; Chabaux et al., 2003; Dunk et al., 2002]. For comparison, modern in situ seawater pH and BWT values of the Oslo fjord and Sula reef were taken from Flögel et al. [2014]. For the Lopphavet CWC reef BWT were taken from Raddatz et al. [2013], but measured seawater pH values are unavailable and were calculated from the CARINA data set by using CO2SYS [Lewis and Wallace, 1998] and are therefore probably not as accurate as the in situ measured values (Table 3). 2.2.4 Boron Isotope Measurements Samples were processed for boron isotope analysis and measured on an NU Plasma II MC-ICP-MS at the University of Western Australia following the protocols of McCulloch et al. [2014]. Briefly, for this study, about 5 mg of sample material was used for each determination. All samples were measured in duplicate, except for sample 809-13. Generally, 11B was measured with a signal intensity of approximately 1.5 V. The boron method is based on a gravimetrically prepared laboratory standard, UWA24.7, calibrated against the international reference standard NIST-SRM 951 (δ11B = 0‰). UWA24.7 standard solutions between 50 to 500 ppb, equivalent to coral sites samples between 2 and 10 mg, exhibit consistent results for δ11B (24.7 ± 0.3‰ 2σ). The reproducibility of this method ranges from ±0.44 to 0.08‰, for solutions between 50 to 300 ppb, respectively, equivalent to 2–10 mg size coral samples. The external reproducibility of this method is demonstrated by the repeated measurements of the international carbonate standard JCp-1 δ11B = 24.3 ± 0.34‰ (2 standard deviations), which is consistent with the reported value of 24.2 ± 0.35‰ by the boron isotope interlaboratory comparison project [Gutjahr et al., 2014]. Boron isotopes are most commonly used for reconstructing ambient seawater pH from foraminifera [e.g., Hönisch et al., 2009; Rae et al., 2011; Foster et al., 2012]. Boron isotope compositions of scleractinian cold-water corals, however, appear to record the internal pH of the calcifying fluid of the coral (pHcf), which is up-regulated by physiological processes and strongly correlated to changes in ambient seawater saturation [McCulloch et al., 2012]. In particular, pH up-regulation is enhanced with decreasing seawater saturation state/seawater pH. Here we exploit this to calculate the internal pHcf of the coral by using the equation of Zeebe and Wolf-Gladow [2001] based on the measured δ11Bcarb in the coral: where δ11Bcarb represents the measured δ11B value, α is the isotopic fractionation factor between the boric and borate species, and δ11Bsw is the boron isotopic composition of seawater (=39.61 [Foster et al., 2010]). In seawater, boron exists as (1) the tetrahedrally coordinated borate [B(OH)−4] ion and (2) as the trigonal boric acid [B(OH)3] with a fractionation factor of 27.2‰ (=α(B3-B4) = 1.0272 [Klochko et al., 2006]). Marine calcifiers appear to exclusively incorporate the borate ion into their aragonite skeleton during calcification. A pKB value of 8.597 is the stoichiometric dissociation constant for boric acid at 25°C and salinity of 35. In principle, pKB is temperature, salinity, and pressure dependent, but the effect of these parameters is considerably small [Foster et al., 2012]. Furthermore, we calculate the difference between the δ11B-derived pHcf values and the reconstructed seawater pH, where ΔpH = pHcf − pHsw, to quantify coral pH up-regulation as well as the physiological stress on the coral/reef. The ΔpH was determined using a combination of U/Ca (=seawater pH) and δ11B (=calcifying fluid pH) analyses. 3 Results 3.1 Stratigraphic Constraints The U/Th age determinations reveal that our coral samples are Holocene in age, ranging from 10.3 ka to 4.6 ka for the Lopphavet cores and ~4.6–1.6 ka for the Oslo fjord (Figure 2 and Table 1). The Lopphavet reef core 559-2 shows an almost continuous record from 10.3 ka to 6.4 ka and hence a sedimentation rate of 55 cm/kyr, apparently slower than >100 cm/kyr reported for other CWC reefs on the Norwegian shelf [López Correa et al., 2012; Titschack et al., 2015]. Furthermore, the nearby core 559-3 (a few hundred meters away) indicates a gap of ~2.6 kyr, from 7.4 to 4.8 ka between 1 and 8 cm core depth. Combining the results of core 559-2 and 559-3, the gap can be reduced to ~1.6 kyr, from 6.4 to 4.8 ka, for this site (Figure 2). Figure 2Open in figure viewerPowerPoint U/Th ages determined in L. pertusa samples from various sediment cores from Lopphavet and Oslofjord reef. Note that sediment cores 559-2 and 559-3 are neighboring cores, but only core 559-3 reveals a possible hiatus between 1 and 8 cm. Considering all investigated samples no coral ages were found between 6.4 and 4.8 ka. The Oslofjord core 576-1 also shows a relatively continuous record from 3.5 ka to 1.6 ka, and an accumulation rate of 95 cm/kyr, significantly faster than