Data for: Oxfordian carbon-isotope stratigraphy and pelagic biofacies of the Middle–Upper Jurassic transition in the Tethys–Central Atlantic connection

A bulk carbon-isotope stratigraphy, based on high-resolution sampling of five stratigraphic ammonite-dated sections from pelagic swells of the Subbetic basin (External Zones of the Betic Cordillera, southern Spain) is presented. The studied sections are characterized by Callovian‒Oxfordian stratigraphic successions located in the South Iberian palaeomargin, a key area connecting the Central Atlantic to the Tethys oceans. The rocks are mainly nodular limestones with common extreme condensation (rosso ammonitico facies). Discontinuities with hiatuses of variable duration, submarine “hardgrounds”, Fe-Mn ooids, limonite crusts, and neptunian dykes are observed around the Callovian/Oxfordian boundary. The stratigraphic record, although very time-averaged, allows for a consistent and accurate ammonite chronostratigraphy. The carbon isotopes of marine carbonates show a marked trend towards high δ13C values from Lower Oxfordian to the Middle Oxfordian (~4.3 ‰, near the Plicatilis/Transversarium boundary). The isotopic values stand between 2.7–3.0 ‰ at the Upper Oxfordian (Bifurcatus and Hypselum Zones) and lowermost Kimmeridgian (Bimammatum Zone); afterward δ13C values decrease and reach a relative minimum (~2.3–2.5 ‰) in the Lower Kimmeridgian (Bimammatum/Planula boundary). Finally, isotopic values increase again (~2.6–2.7 ‰) in the lower Kimmeridgian (mid part of the Planula Zone). Comparison of carbon-isotope stratigraphy between Subbetic and other Tethyan areas shows similar trends. The replacement of thin-shelled bivalves by planktonic foraminifers and radiolarians that took place in the Callovian/Oxfordian boundary can be associated to the widening of the trans- Pangaean seaway . This, in turn, triggered the global warming which increased nutrients concentration in upper water column due to intensified nutrient input by river plumes. These phenomena are concomitant with the major Oxfordian δ13C positive excursion and with a transgressive cycle.

本研究呈现了一套基于高分辨率采样的块状碳同位素地层学（bulk carbon-isotope stratigraphy），采样对象为西班牙南部贝蒂科山脉（Betic Cordillera）外带的苏贝蒂克盆地（Subbetic basin）远洋隆起带中5个以菊石定年的地层剖面。所研究的剖面均位于南伊比利亚古陆缘（South Iberian palaeomargin）——连接中大西洋与特提斯洋的关键区域，其地层序列均属卡洛维阶-牛津阶。研究区岩石以结核状灰岩（nodular limestones）为主，普遍发育极强的沉积凝缩作用，对应罗斯索菊石相（rosso ammonitico facies）。在卡洛维阶/牛津阶界线附近，可见不同持续时长的沉积间断与间断面、海底硬底（submarine hardgrounds）、铁锰鲕粒（Fe-Mn ooids）、针铁矿壳（limonite crusts）以及 Neptunian 岩脉（neptunian dykes）。尽管该地层记录存在显著的时间平均效应，但仍可建立可靠且精确的菊石生物年代地层格架。海相碳酸盐的碳同位素值从中牛津阶底部到中牛津阶呈现显著升高的趋势，峰值约4.3‰，位于普列卡蒂利斯带/特兰斯韦拉里乌姆带（Plicatilis/Transversarium boundary）附近。上牛津阶（比弗卡图斯带与希普瑟勒姆带）及最底部基默里奇阶（比马马图姆带）的同位素值介于2.7‰~3.0‰之间；随后δ13C值逐渐降低，在下基默里奇阶（比马马图姆带/普兰努拉带界线处）达到相对低值（约2.3‰~2.5‰）。最终在基默里奇阶下部（普兰努拉带中部），同位素值再次回升至约2.6‰~2.7‰。将苏贝蒂克盆地的碳同位素地层学与其他特提斯区域进行对比后发现，二者呈现相似的演化趋势。在卡洛维阶/牛津阶界线处发生的薄壳双壳类被浮游有孔虫与放射虫替代的事件，可与横跨泛大陆的海道拓宽相关联。这一过程继而引发全球变暖，河流羽状流带来的营养物质输入增强，导致上层水体的营养盐浓度升高。上述现象与牛津阶δ13C正偏移以及海侵旋回同步发生。