Abstract Fossil shells of benthos and nektobenthos have been shown to be faithful recorders of seawater carbon- and oxygen-isotope geochemistry, and thus also useful to track the relationship between carbon cycle and palaeotemperature. In this study we present an extensive dataset from Lower Jurassic (Hettangian and lower Sinemurian) mollusc and brachiopod hard parts collected from biostratigraphically well-calibrated UK coastal outcrops (Bristol Channel and Hebrides basins). These basins lay palaeogeographically in the southern part of the Laurasian Seaway that connected the Tethys and Boreal oceans. All samples have been subject to screening for diagenesis on the basis of elemental composition, light microscopy, and SEM observations. In the case of some localities within the Hebrides Basin, alteration by hydrothermal systems around Paleogene intrusions has led to re-setting of carbonate oxygen isotopes, but the original carbon isotope values from the shells are largely preserved. Above the prominent and apparently short-lived, ~3 per mil δ13Ccarb amplitude positive carbon-isotope excursion (CIE) that occurs immediately above the Triassic-Jurasic (T-J) boundary (in the tilmanni ammonite biozone), a pronounced negative CIE (the so-called Main Negative CIE) spans the entire Hettangian Stage. At the Hettangian-Sinemurian boundary, and through the lower Sinemurian, the carbon-isotope values of the skeletal carbonate again trend towards progressively more positive values, but representing a time of several million years. The heaviest δ13Ccarb values of about +4.3 per mil are evident towards the top of the lower Sinemurian, and are comparable with values observed from the tilmanni Zone, and from the lower Toarcian, higher in the Jurassic. This long-term positive hump, which confirms trends derived from bulk organic matter carbon-isotope records, is supporting evidence of prolonged enhanced organic carbon burial that is inferred to have occurred in the extensive system of lacustrine and marine rifts that traversed a fragmenting Pangaea after emplacement of the Central Atlantic Magmatic Province. In parallel, oxygen-isotope values of the skeletal carbonate show a continuous downward trend from the lower part of the Hettangian (~ −1 per mil δ18Ocarb in the planorbis Zone) to the top of the lower Sinemurian (~ −2.5 per mil δ18Ocarb in the higher turneri Zone). Oxygen-isotope values may be interpreted as due to gradually increasing palaeotemperature, and/or addition of a meteoric or cryospheric water component; in the case of the Laurasian Seaway, palaeoceanographic and palaeoecological considerations point towards a dominant palaeotemperature signal. Consequently, any atmospheric carbon-dioxide drawdown effect on global palaeotemperatures, as suggested by progressively increasing δ13Ccarb values, and assuming a constant silicate weathering sink, was more than counterbalanced in the seaway by regional processes that led to significantly warmer bottom water temperatures.

中文翻译：

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三叠纪-侏罗纪边界后欧亚海道南部的碳和氧同位素记录：平行长期增强的碳埋藏和海水变暖

摘要 底栖动物和底栖动物的化石壳已被证明是海水碳和氧同位素地球化学的忠实记录者，因此也有助于追踪碳循环与古温度之间的关系。在这项研究中，我们提供了一个广泛的数据集，这些数据集来自下侏罗纪（赫唐阶和下 Sinemurian）软体动物和腕足动物的坚硬部分，这些部分是从生物地层学校准良好的英国沿海露头（布里斯托尔海峡和赫布里底斯盆地）收集的。这些盆地在古地理上位于连接特提斯洋和北方海洋的劳拉西亚海道的南部。所有样品都根据元素组成、光学显微镜和 SEM 观察进行了成岩作用的筛选。就赫布里底群岛内的一些地方而言，古近纪侵入体周围热液系统的改变导致碳酸盐氧同位素的重新设置，但壳中的原始碳同位素值在很大程度上得以保留。在显着且明显短暂的、~3 per mil δ13Ccarb 振幅正碳同位素偏移 (CIE) 之上，发生在三叠纪-侏罗纪 (TJ) 边界上方（在 tilmanni 菊石生物区），一个明显的负 CIE（所以-称为主要负 CIE）跨越整个 Hettangian 阶段。在 Hettangian-Sinemurian 边界处，通过下 Sinemurian，骨骼碳酸盐的碳同位素值再次趋向于逐渐增加的正值，但代表了几百万年的时间。最重的 δ13Ccarb 值约为每 mil +4.3，在 Sinemurian 下部的顶部很明显，并且与从蒂尔曼尼带观察到的值以​​及从较低的托阿尔阶观察到的值相当，在侏罗纪较高。这个长期的正驼峰证实了来自大量有机物质碳同位素记录的趋势，支持了长期增强的有机碳埋藏的证据，推测发生在广泛的湖泊和海洋裂谷系统中，该系统在穿越破碎的盘古大陆之后中大西洋岩浆省的进驻。同时，骨骼碳酸盐的氧同位素值显示出从 Hettangian 下部（平面带中~-1 per mil δ18Ocarb）到 Sinemurian 下部（~ -2.5 per mil δ18Ocarb）的持续下降趋势。较高的特纳里区）。氧同位素值可以解释为由于逐渐增加的古温度，和/或添加大气或冰冻圈水成分；在劳拉西亚海道的情况下，古海洋学和古生态学考虑指向主要的古温度信号。因此，任何大气二氧化碳下降对全球古温度的影响，正如逐渐增加的 δ13Ccarb 值所表明的，并假设一个恒定的硅酸盐风化汇，在海道中被区域过程所抵消，导致底部水温显着升高。