Strontium isotope fractionation of planktic foraminifera and inorganic calcite
Strontium isotope fractionation of planktic foraminifera and inorganic calcite
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DOI：
10.1016/j.gca.2012.04.038
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发表时间：
2010-12
影响因子：
5
通讯作者：
F. Böhm;A. Eisenhauer;Jianwu Tang;M. Dietzel;A. Krabbenhöft;B. Kisakurek;C. Horn
中科院分区：
文献类型：
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作者：
F. Böhm;A. Eisenhauer;Jianwu Tang;M. Dietzel;A. Krabbenhöft;B. Kisakurek;C. Horn
本文研究了无机方解石和水相Sr2+离子在25℃恒温、沉淀速率(R) 102.3 ~ 104.2μmol/m2/h下的锶同位素分馏（Δ88/86Srcarb-aq）。采用87sr - 84sr双尖峰技术测定了锶同位素比值。研究发现，这些方解石中的锶同位素分馏与降水速率密切相关：测量的δ88/86Sr值与先前测量的相同方解石样品的δ44/40Ca和Sr/Ca值显著相关：88sr /86Sr与us44ca /40Ca分馏斜率为0.18±0.04，与锶和钙离子脱水过程中的动力学分馏相适应，但与扩散边界层中的同位素分馏不相容。利用已公布的平衡值Δ44/40Cacarb-aqand KdSrvalues，我们估计锶的平衡同位素分馏非常接近于零（Δ88/86Sreq（碳水化合物- aq）= - 0.01±0.06‰）。海洋地壳玄武岩中缓慢沉淀的天然无机方解石的锶同位素值证实了这一估计。无机方解石的结果被用来解释浮游有孔虫的锶同位素分馏。从加勒比海沉积物岩心全新世剖面中采集了两个暖水物种（gloigerinoides rubber和gloigerinoides sacullifer）的标本。我们发现两种物种的δ88/86Sr没有显著差异。另外，同一岩心海相同位素2期G. ruber样品的δ88/86Sr值与全新世样品相同。与已发表的其他主要海洋钙化生物的数据相比，这两种有孔虫的锶同位素有很强的分异（Δ88/86Srcarb-aq =−0.248±0.005‰）。应用无机沉淀实验的结果，我们发现强有孔虫锶同位素分馏可以用高沉淀率下的很大程度上开放系统的钙化来解释，其量级与从硬核珊瑚中已知的速率相当。这一解释与kisak<e:1> rek等人（2011）基于钙同位素和元素Sr/Ca比值的浮游有孔虫动力学钙化模型非常吻合。
We have investigated the strontium isotope fractionation (Δ88/86Srcarb–aq) between inorganic calcite and aqueous Sr2+ions by precipitation experiments at a constant temperature of 25°C and precipitation rates (R) ranging from 102.3to 104.2μmol/m2/h. Strontium isotope ratios were measured using the87Sr–84Sr double spike technique. It was found that strontium isotope fractionation in these calcites is strongly dependent on the precipitation rate:The measured δ88/86Sr values are significantly correlated with previously measured δ44/40Ca and Sr/Ca values of the same calcite samples: The slope of88Sr/86Sr versus44Ca/40Ca fractionation is 0.18±0.04 and compatible with a kinetic fractionation during dehydration of the strontium and calcium ions, but not with isotope fractionation in a diffusive boundary layer. Using published equilibrium Δ44/40Cacarb–aqand KdSrvalues we estimate the equilibrium isotope fractionation of strontium to be very close to zero (Δ88/86Sreq(carb–aq)=−0.01±0.06‰). This estimate is confirmed by strontium isotope values of natural inorganic calcites that precipitated very slowly in basalts of the ocean crust. The results from the inorganic calcites are used to explain strontium isotope fractionation of planktic foraminifera. Specimens of two warm water species (Globigerinoides ruber and Globigerinoides sacculifer) were picked from the Holocene section of a Caribbean sediment core. We found no significant difference in δ88/86Sr between the two species. In addition, G. ruber specimens from Marine Isotope Stage 2 in the same core show δ88/86Sr values identical to the Holocene specimens. The strontium isotopes of both foraminifera species are strongly fractionated (Δ88/86Srcarb–aq=−0.248±0.005‰) when compared to published data of other major marine calcifiers. Applying the results from the inorganic precipitation experiments we find that the strong foraminiferal strontium isotope fractionation can be explained by calcification in a largely open system at high precipitation rates, comparable in magnitude to rates known from scleractinian reef corals. This interpretation is in good agreement with the kinetic calcification model for planktic foraminifera by Kisakürek et al. (2011), which was based on calcium isotopes and elemental Sr/Ca ratios.