Methods for reconstruction of paleo-seawater pH based on boron isotopes in evaporative depositional sequences: case study using the Cambrian–Lower Ordovician evaporite sequence in the Tarim Block, NW China

Evaluation of paleo-seawater pH is an important aspect to study the paleo-ocean environment. The Paleozoic strata lack foraminiferal shells; therefore, reconstruction of paleo-seawater pH is difficult. In this paper, the Cambrian–Lower Ordovician evaporate sequence present in the Tarim Basin is used as an example to work out methods for reconstruction of paleo-seawater pH for evaporate sequences through study of boron isotopes. Analysis of diagenetically unaltered samples yields δ11B values for normal seawater in an open environment ranging from 7.5 to 12.6‰ (average = 9.4‰) and those of evaporative salt-lake facies ranging from − 4.7 to − 1.8‰ (average = − 3.3‰). With increase in the rate of evaporation of seawater, its pH decreases gradually and the δ11B values of tricoordinated and tetracoordinated compounds decrease synchronously. Using a salt-lake brine with pH 7, δ11B value in sedimentary rock of − 3.3‰ and unfractionated δ11B in tetracoordinated B(OH)4 in solution, the average δ11B of the Middle Cambrian–Early Ordovician paleo-seawater came to be 16.2‰. It establishes a workable relationship between δ11B in sedimentary rock and seawater pH. The Middle Cambrian–Early Ordovician normal paleo-seawater pH estimated using the δ11B value of sedimentary rock representing the weakest evaporation intensity is ~ 9.1 that is 0.9 higher than pH of modern seawater. In these calculations, it is assumed that the total dissolved inorganic carbon is unchanged between the Cambrian–Early Ordovician paleo-seawater and modern seawater and the [CO32−] content in the paleo-seawater is greater by a factor of 2–3 than that in modern seawater. This increase in [CO32−] content is inferred to be one of the main factors for the widespread development of dolomite.

Yang J., Zhang Y., Chen Y., Zhang L., Zhu C. & Wang Z., in press. Methods for reconstruction of paleo-seawater pH based on boron isotopes in evaporative depositional sequences: case study using the Cambrian–Lower Ordovician evaporite sequence in the Tarim Block, NW China. Carbonates and Evaporites. Article (subscription required).

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