Data from: Evidence for stratigraphy in molluscan death assemblages preserved in seagrass beds: St. Croix, U.S. Virgin Islands

Death assemblages that occupy the upper tens of centimeters of sediment in shallow-marine settings are often subject to extensive mixing, thereby limiting their usefulness in assessing environmentally mediated compositional changes through time in the local biota. Here, we provide evidence that dense, Thalassia-rich seagrass beds preserve a stratigraphic record of biotic variation because their dense root–rhizome mats inhibit mixing. We sampled benthic mollusk assemblages at seven localities in Thalassia-rich beds around St. Croix, USVI, collecting three separate sediment intervals of ~13 cm each to a total depth of ~40 cm below the sediment–water interface, and found evidence that sedimentary intervals preserved compositional stratigraphy. Further, some localities displayed systematic, directional changes down-core. An examination of interval-to-interval changes in composition revealed that compositional variation was unique from locality to locality rather than reflecting coordinated, island-wide transitions. In general, however, relative abundances of epifaunal gastropods and small lucinid bivalves tended to decrease with depth below the sediment–water interface. Quantitative comparisons of life-to-death assemblages from each successive sedimentary interval demonstrated that the shallowest death assemblages were typically more similar to the life assemblages than were deeper assemblages, suggesting that deeper intervals provide records of earlier community states.

栖息于浅海沉积环境沉积物表层数十厘米范围内的死亡组合（death assemblage）常遭受强烈混合作用，这限制了其用于评估本地生物群随时间推移所发生的、由环境介导的组成变化的有效性。本研究证实，致密的富泰来草（Thalassia）海草床能够保存生物变异的地层记录，因其致密的根状茎垫层会抑制混合作用。我们于美属维尔京群岛圣克罗伊岛周边的富泰来草海草床的7个采样点开展底栖软体动物组合采样，采集了沉积物-水界面以下总深度约40 cm的3个独立沉积层段，每个层段厚度约13 cm，结果证实沉积层段保留了组成地层结构。此外，部分采样点的组成随岩芯深度增加呈现出系统性的定向变化。对层段间组成变化的分析显示，不同采样点的组成变异各具特征，而非反映全岛范围的协同过渡过程。但总体而言，表栖腹足类与小型满月蛤科双壳类的相对丰度往往随沉积物-水界面以下深度的增加而降低。对每个连续沉积层段的现生-死亡组合进行定量对比后发现，最浅层的死亡组合通常比深层组合更接近现生生物组合，这表明深层沉积层段记录了更早的群落状态。