Impact of reworked foraminifera from an eroding salt marsh on sea-level studies, New Zealand

Quantitative sea-level reconstructions using Holocene foraminifera are usually based on transfer function estimates of elevation derived from the tidal-elevation-related zonation of modern salt marsh foraminifera at the same site. Many New Zealand salt marshes are actively eroding as a result of recent sea-level rise exceeding sediment supply, making it difficult to find transects for modern analogue studies that are unaffected by erosion at their seaward ends. At Waikawa Harbour, South Island, New Zealand we show that reworked salt marsh foraminifera from the eroding bank have been mixed in significant quantities with in situ faunas: (1) in the sand flat seaward of the marsh; (2) in the spring tidal marsh on top of the 80-cm-high bank (together with mid tide foraminifera from the sand below the bank); and (3) transported laterally into a nearby non-eroding rush marsh at lower elevation. The fibrous organic-rich salt marsh mud of the bank has been undermined by erosion and large blocks have collapsed to a lower level on the sand flat where the in situ relict faunas are being mixed with newly colonised lower elevation salt marsh foraminifera. These studies show that eroding salt marshes may result in substantial reworking and mixing of foraminiferal tests into the modern faunas thereby compromising their use in sea-level reconstructions. This study prompted us to revisit part of a cored sequence at nearby Catlins Lake and reinterpret a previously inferred earthquake displacement (5–4.5 ka) to be the result of an eroding and collapsing salt marsh bank instead.

采用全新世（Holocene）有孔虫（foraminifera）开展的定量海平面重建研究，通常基于同一站位现代盐沼有孔虫的潮位相关分带特征推导得到的高程转移函数（transfer function）估计结果。新西兰诸多盐沼因近期海平面上升速率超过沉积物供给速率而处于活跃侵蚀状态，导致难以找到向海端未受侵蚀影响的现代类比研究样带。在新西兰南岛怀卡沃港（Waikawa Harbour），我们的研究发现：来自侵蚀岸滩的再沉积盐沼有孔虫，与原位群落发生了显著混合，具体表现为：（1）在盐沼向海侧的砂质潮滩中；（2）在80厘米高岸滩顶部的大潮盐沼环境中（与岸滩下方砂层中的中潮带有孔虫混合）；（3）被横向搬运至附近一处海拔更低、未受侵蚀的灯芯草盐沼内。岸滩处的纤维状富有机质盐沼泥质沉积物被侵蚀掏空，大块岸体崩塌至下方的砂质潮滩，原位残存的有孔虫群落与新近定植于该低海拔区域的盐沼有孔虫发生混合。上述研究表明，侵蚀中的盐沼会导致大量有孔虫壳体（foraminiferal tests）被再沉积并混入现代有孔虫群落，进而损害其在海平面重建研究中的应用价值。本研究促使我们重新审视附近卡蒂恩斯湖（Catlins Lake）的部分岩心序列，并将此前推断的5.0~4.5 ka地震地表位移，重新解释为盐沼岸滩侵蚀崩塌所导致的结果。