(Table 1) Stable isotope record of Middle Cretaceous benthic foraminifera@en

Preservation of planktic foraminiferal calcite has received widespread attention in recent years, but the taphonomy of benthic foraminiferal calcite and its influence on the deep-sea palaeotemperature record have gone comparatively unreported. Numerical modeling indicates that the carbonate recrystallization histories of deep-sea sections are dominated by events in their early burial history, meaning that the degree of exchange between sediments and pore fluids during the early postburial phase holds the key to determining the palaeotemperature significance of diagenetic alteration of benthic foraminifera. Postburial sedimentation rate and lithology are likely to be important determinants of the paleoceanographic significance of this sediment–pore fluid interaction. Here we report an investigation of the impact of extreme change in sedimentation rate (a prolonged and widespread Upper Cretaceous hiatus in the North Atlantic Ocean) on the preservation and d18O of benthic foraminifera of Middle Cretaceous age (nannofossil zone NC10, uppermost Albian/lowermost Cenomanian, ~99 Ma ago) from multiple drill sites. At sites where this hiatus immediately overlies NC10, benthic foraminifera appear to display at least moderate preservation of the whole test. However, on closer inspection, these tests are shown to be extremely poorly preserved internally and yield d18O values substantially higher than those from contemporaneous better preserved benthic foraminifera at sites without an immediately overlying hiatus. These high d18O values are interpreted to indicate alteration close to the seafloor in cooler waters during the Late Cretaceous hiatus. Intersite differences in lithology modulate the diagenetic impact of this extreme change in sedimentation rate. Our results highlight the importance of thorough examination of benthic foraminiferal wall structures and lend support to the view that sedimentation rate and lithology are key factors controlling the paleoceanographic significance of diagenetic alteration of biogenic carbonates.近年来，浮游有孔虫方解石（planktic foraminiferal calcite）的保存状态已获得广泛关注，但底栖有孔虫方解石（benthic foraminiferal calcite）的埋藏学（taphonomy）特征及其对深海古温度记录的影响，相对而言鲜有报道。数值模拟结果表明，深海剖面的碳酸盐重结晶（carbonate recrystallization）历史主要受控于其早期埋藏史中的事件，这意味着沉积物与孔隙流体（pore fluids）在埋藏后早期阶段的交换程度，是决定底栖有孔虫成岩蚀变（diagenetic alteration）的古温度指示意义的关键因素。埋藏后沉积速率（postburial sedimentation rate）与岩性（lithology），或是调控该沉积物-孔隙流体相互作用的古海洋学指示意义的重要决定因素。本文报道了一项针对沉积速率极端变化——北大西洋地区长期广泛分布的白垩纪晚期沉积间断（Upper Cretaceous hiatus）——对白垩纪中期底栖有孔虫保存状况及δ¹⁸O值影响的研究，研究样品取自多个钻探站位，其时代对应超微化石带NC10（nannofossil zone NC10），即阿尔布阶顶部/塞诺曼阶底部（uppermost Albian/lowermost Cenomanian），距今约9900万年（~99 Ma ago）。在该沉积间断直接覆于NC10地层之上的站位，底栖有孔虫完整壳体（whole test）整体呈现出至少中等程度的保存状态，但经进一步细致观察可见，此类壳体的内部保存状况极差，其测得的δ¹⁸O值显著高于同期无直接覆于其上的沉积间断、且保存状况更佳的底栖有孔虫样品。这些偏高的δ¹⁸O值被解释为，在白垩纪晚期沉积间断期间，壳体在近海底冷水环境中发生了蚀变。不同站位间的岩性差异，会调控该沉积速率极端变化所带来的成岩影响。本研究结果凸显了全面检视底栖有孔虫壳体壁结构的重要性，并支持了如下观点：沉积速率与岩性是控制生物成因碳酸盐成岩蚀变的古海洋学指示意义的核心因素。