The oxygen-18 isotopic composition (δ¹⁸O) of silica preserved in oceanic sediments is an important archive of Earth’s temperature and/or seawater δ¹⁸O from the Archean to present. Recent advances in high-precision measurements of both δ¹⁸O and δ¹⁷O values have been used to provide additional constraints on what the oxygen isotopic composition of chert reflects about past conditions. Here, we examine the effects on the triple oxygen isotopic composition of chert that occurs during transformation and recrystallization of biogenic opal-A to opal-CT to microquartz in deep sea sediments. We studied late Miocene to present samples from the Sea of Japan at ODP Site 795 and measured biogenic diatom opal-A, opal-CT, microquartz chert, and ‘altered’ opal-A samples—previously measured for δ¹⁸O values only—for both δ¹⁸O and δ¹⁷O values. We find that δ¹⁸O decreases and Δ’¹⁷O increases (where Δ’¹⁷O = δ¹⁷O – 0.528×δ¹⁸O) with depth, coincident with the conversions from diatom opal-A to opal-CT to microquartz. Silica samples deviate from the trend expected for triple oxygen isotopic equilibrium with modern seawater. To explain these data, we developed a model that shows that local temperature gradients and pore fluid δ¹⁸O profiles in combination lower the measured opal-CT and microquartz δ¹⁸O values and raise the Δ’¹⁷O values relative to the initial opal-A, but deviate from triple oxygen isotopic equilibrium with seawater. We find that a steeper local temperature gradient and a larger influence of hydrothermal alteration of basalt at the base of the sediment column (which lowers pore fluid δ¹⁸O values) in the past explain both the measured δ¹⁸O and Δ’¹⁷O values of the opal-CT and microquartz.

These data and our modeling show that the transformation of opal-A to microquartz in marine sediments at elevated temperatures and in the presence of lowered pore water δ¹⁸O values leads to an array that falls below the theoretical triple oxygen isotope line of equilibrium for SiO₂ with modern seawater. Further, our model indicates that opal-CT and microquartz do form in triple oxygen isotopic equilibrium with pore fluids that are offset in their triple oxygen isotopic composition compared to seawater due to fluid-rock alteration of igneous rocks at the base of the sediment column. The diagenetic processes taking place in the Japan Sea do not explain a large portion of the existing Archean to present triple oxygen isotope chert data, which likely require either changes in the oxygen isotopic composition of the source water (i.e., ocean water) and/or alteration by meteoric fluids. Further, our data demonstrate that the triple oxygen isotopic composition of preserved chert need not represent surface conditions, but instead may reflect processes that occur in subsurface sediments at elevated temperatures and with modified pore fluid oxygen isotopic compositions.

保存在海洋沉积物中的二氧化硅的氧 18 同位素组成 (δ ¹⁸ O) 是地球温度和/或从太古代至今的海水 δ ¹⁸ O的重要档案。δ ¹⁸ O 和 δ ¹⁷高精度测量的最新进展O 值已用于对燧石的氧同位素组成反映过去条件的内容提供额外的限制。在这里，我们研究了在深海沉积物中生物蛋白石 A 到蛋白石 CT 到微石英的转化和重结晶过程中发生的燧石三氧同位素组成的影响。我们研究了晚中新世以在 ODP 站点 795 展示来自日本海的样本，并测量了生物硅藻蛋白石-A、蛋白石-CT、微石英燧石和“改变的”蛋白石-A 样本（之前仅测量 δ ¹⁸ O 值） δ ¹⁸ O 和δ ¹⁷ O 值。我们发现 δ ¹⁸ O 减少而 Δ' ¹⁷ O 增加（其中 Δ' ¹⁷ O = δ ¹⁷O – 0.528×δ ¹⁸ O) 与深度，与从硅藻蛋白石-A 到蛋白石-CT 到微石英的转换一致。二氧化硅样品偏离了现代海水三氧同位素平衡的预期趋势。为了解释这些数据，我们开发了一个模型，该模型显示局部温度梯度和孔隙流体 δ ¹⁸ O 剖面组合降低了测得的蛋白石 CT 和微石英 δ ¹⁸ O 值，并提高了相对于初始蛋白石的 Δ' ¹⁷ O 值- A，但偏离海水的三氧同位素平衡。我们发现沉积柱底部更陡峭的局部温度梯度和更大的玄武岩热液蚀变影响（这降低了孔隙流体 δ ¹⁸O 值）在过去解释了蛋白石 CT 和微石英的测量 δ ¹⁸ O 和 Δ' ¹⁷ O 值。

这些数据和我们的模型表明，在升高的温度和孔隙水 δ ¹⁸ O 值降低的情况下，海洋沉积物中蛋白石-A 向微石英的转变导致阵列低于 SiO 的理论三氧同位素平衡线₂用现代海水。此外，我们的模型表明，由于沉积柱底部火成岩的流体岩蚀变，蛋白石 CT 和微石英确实在三氧同位素平衡中形成，孔隙流体的三氧同位素组成与海水相比有所抵消。日本海发生的成岩过程并不能解释现有太古代的大部分呈现三重氧同位素燧石数据，这可能需要改变源水（即海水）的氧同位素组成和/或由大气流体改变。此外，我们的数据表明，保存的燧石的三氧同位素组成不需要代表表面条件，