Approximately 13 km south of Gulf Shores, Alabama (United States), divers found in situ baldcypress (Taxodium distichum) stumps 18 m below the ocean surface. These trees could have only lived when sea level fell during the Pleistocene subaerially exposing the tectonically stable continental shelf. Here we investigate the geophysical properties along with microfossil and stratigraphical analyses of sediment cores to understand the factors that lead to this wood’s preservation. The stumps are exposed in an elongated depression (~100 m long, ~1 m deep) nested in a trough of the northwest–southeast trending Holocene sand ridges and troughs with 2–5 m vertical relief and ~0.5 km wavelength. Radiocarbon ages of the wood were infinite thus optically stimulated luminescence (OSL) dating was used to constrain the site’s age. Below the Holocene sands (~0.1–4 m thick), separated by a regional erosional unconformity, are Late Pleistocene mud-peat (72±8 ka OSL), mud-sand (63±5, 73±6 ka OSL), and palaeosol (56±5 ka OSL) facies that grade laterally from west to east, respectively. Foraminiferal analysis reveals the location of the terrestrial-marine transitional layer above the Pleistocene facies in an interbedded sand and mud facies (3940±30 (1σ) ¹⁴C a BP), which is part of a lower shoreface or marine-dominated estuarine environment. The occurrence of palaeosol and swamp facies of broadly similar ages and elevation suggests the glacial landscape possessed topographic relief that allowed wood, mud and peats to be preserved for ~50 ka of subaerial exposure before transitioning to the modern marine environment. We hypothesize that rapid sea-level rise occurring ~60 or ~40 ka ago provided opportunities for local flood-plain aggradation to bury the swamp thus preserving the stumps and that other sites may exist in the northern Gulf of Mexico shelf.

中文翻译：

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墨西哥湾北部大陆架上的晚更新世 baldcypress (Taxodium distichum) 森林沉积物

在美国阿拉巴马州格尔夫海岸以南约 13 公里处，潜水员在原地发现了白柏（Taxodium distichum) 树桩位于海面以下 18 m。这些树木只有在更新世地下暴露出构造稳定的大陆架期间海平面下降时才能生存。在这里，我们调查了地球物理特性以及沉积物核心的微化石和地层分析，以了解导致这种木材得以保存的因素。树桩暴露在细长的凹陷中（~100 m 长，~1 m 深），嵌套在西北-东南走向的全新世沙脊和槽中，垂直起伏为 2-5 m，波长约为 0.5 km。木材的放射性碳年龄是无限的，因此使用光受激发光 (OSL) 测年来限制该地点的年龄。在全新世砂层（~0.1-4 m 厚）之下，被区域性侵蚀不整合面隔开，是晚更新世泥炭（72±8 ka OSL）、泥砂（63±5, 73±6 ka OSL) 和从西向东横向分级的古土壤 (56±5 ka OSL) 相。有孔虫分析揭示了更新世相上方的陆海过渡层位于砂泥互层(3940±30 (1σ)¹⁴ C a BP)，它是低岸面或以海洋为主的河口环境的一部分。年龄和海拔高度相似的古土壤和沼泽相的出现表明冰川景观具有地形起伏，允许木材、泥浆和泥炭在过渡到现代海洋环境之前保存约 50 ka 的地下暴露。我们假设大约 60 或大约 40 ka 前发生的快速海平面上升为当地洪泛区加积提供了掩埋沼泽从而保护树桩的机会，并且墨西哥湾北部陆架可能存在其他地点。