The B/Ca and Cd/Ca of a subsurface-dwelling foraminifera Pulleniatina obliquiloculata in the tropical Indo-Pacific Ocean
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The B/Ca and Cd/Ca of a subsurface-dwelling foraminifera Pulleniatina obliquiloculata in the tropical Indo-Pacific Ocean: implications for the subsurface carbonate chemistry estimation Haowen Dang1, 2*, Tingting Wang1, Peijun Qiao1, Franck Bassinot2, Zhimin Jian1 1 State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China 2 Laboratoire des Sciences du Climat et de l’Environnement/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-
Yvette 91190, France Received 3 November 2017; accepted 7 December 2017 © Chinese Society for Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2019
Abstract
Pulleniatina obliquiloculata shells from 16 core-top samples from the tropical Indo-Pacific Oceans are analyzed for the ratios of boron and cadmium to calcium (B/Ca and Cd/Ca). The B/Ca ratios show a very weak positive £ ¤ and the dissolved carbonate species at the apparent calcification depth of P. relationship with B (OH)¡ 4 obliquiloculata. The boron partition coefficients (K D ) between P. obliquiloculata B/Ca and seawater £ ¤ £ ¤ ¡ distribute around 1.1×10 -3 –1.3×10 -3 with a mean value of (1.19±0.12)×10 -3 , and are B (OH)¡ 4 = HCO 3 significantly related to the nutrient concentration, especially phosphate. The lack of any clear correlation between the P. obliquiloculata B/Ca and seawater carbonate chemical parameters suggests that the physiochemical controls on boron incorporation are masked by the complexity of natural seawater condition. But the significant dependence of KD on nutrient may likely be explained by a nutrient related growth-rate effect. Cd/Ca of P. obliquiloculata shows significant correlation with seawater phosphate concentration, and its partition coefficients (DCd) are significantly related to temperature. A first-principle methodology of P. obliquiloculata B/Ca is applied, with the aid of Cd/Ca as a phosphate proxy and a constraint on KD, to estimating sea water carbonate chemistry (e.g., pH). The results are fairly promising and allow us to propose the possibility to apply the combination of B/Ca and Cd/Ca proxies (and also Mg/Ca and δ18O for estimating temperature and salinity) for the paleoreconstruction of seawater carbonate chemistry. Key words: Pulleniatina obliquiloculata, B/Ca, Cd/Ca, paleo-pH estimation Citation: Dang Haowen, Wang Tingting, Qiao Peijun, Bassinot Franck, Jian Zhimin. 2019. The B/Ca and Cd/Ca of a subsurface-dwelling foraminifera Pulleniatina obliquiloculata in the tropical Indo-Pacific Ocean: implications for the subsurface carbonate chemistry estimation. Acta Oceanologica Sinica, 38(3): 138–150, doi: 10.1007/s13131-019-1406-6
1 Introduction The ratio of boron to calcium contents (B/Ca) in foraminiferal shells may be used to reconstruct seawater carbon chemistry because of the theoretical pH-dependency of boron content in carbonate. When dissolved in sea water, boron exists as tetrahedrally-coordinating B (OH)¡ 4 and trigonally-coordinating B(OH)3, and their relative abundances are pH-dependent due to the reac¡ ¡ + tion: B(OH) 3 +
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