Stable oxygen isotope ratios of marine barite from Cenozoic sediments

We report new data on oxygen isotopes in marine sulfate (delta18O[SO4]), measured in marine barite (BaSO4), over the Cenozoic. The delta18O[SO4] varies by 6x over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The delta18O[SO4] does not co-vary with the delta18O[SO4], emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the delta18O[SO4] over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The delta18O[SO4] also does not co-vary with the d18O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the delta18O[SO4] to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the delta18O[SO4]. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.本研究报道了新生代（Cenozoic）时期通过海相重晶石（marine barite, BaSO₄）测得的海洋硫酸盐氧同位素（δ¹⁸O[SO₄]）新数据集。该δ¹⁸O[SO₄]值在新生代期间的变化幅度达6倍，主要峰值出现在3 Ma、15 Ma、30 Ma及55 Ma时期。δ¹⁸O[SO₄]与硫酸盐硫同位素（δ³⁴S[SO₄]）无协变关系，这表明控制硫酸盐氧、硫同位素组成的地球化学过程存在差异。这意味着新生代时期δ¹⁸O[SO₄]的时间演化，必然反映了与硫化物再氧化途径相关的同位素分馏变化。该结果提示，不同类型富有机质沉积物的空间分布差异，可能对生物地球化学硫循环产生显著影响；同时也表明，硫循环对净有机碳埋藏的敏感性，低于其对有机碳埋藏环境变化的敏感性。δ¹⁸O[SO₄]同样与底栖有孔虫（benthic foraminifera）测得的δ¹⁸O值无协变关系，这表明在海洋硫酸盐的存续周期内，海水与硫酸盐中的氧同位素始终处于非平衡状态。本研究采用简单箱式模型，探究了新生代时期海洋硫循环在氧、硫同位素维度的动力学特征。我们将δ¹⁸O[SO₄]的变化解释为富有机质沉积物内部环境的空间分布演变：从以局域化富有机质沉积物为主的阶段，过渡到有机碳埋藏更为弥散的时期。尽管这些变化可能不会改变净有机碳埋藏总量，但会极大改变富有机质沉积物内的硫转化过程，进而影响硫化物再氧化途径，最终调控δ¹⁸O[SO₄]值。我们对该记录的定性分析表明，新生代早期的海洋硫酸盐浓度可能更低。