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.基于全新世（Holocene）有孔虫（foraminifera）的定量海平面重建，通常依托同一站位现代盐沼有孔虫的潮位相关分带所推导得到的高程转移函数（transfer function）估算值开展。新西兰境内多数盐沼因近期海平面上升超出沉积物供给量而处于活跃侵蚀状态，这使得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），本研究证实侵蚀岸坡产出的重塑盐沼有孔虫，已与原位（in situ）生物群发生显著混合：(1) 分布于盐沼向海侧的沙滩中；(2) 赋存于80厘米高岸坡顶部的大潮（spring tide）盐沼内（与岸坡下方沙体中的中潮（mid tide）带有孔虫混合）；(3) 被横向搬运至附近一处低海拔且未受侵蚀的灯芯草盐沼中。岸坡富含纤维状有机质的盐沼泥质沉积物已被侵蚀掏空，大块岸坡坍塌至沙滩更低高程处，原地残余有孔虫生物群正与新定植的低海拔盐沼有孔虫发生混合。本研究结果表明，受侵蚀的盐沼可导致大量有孔虫介壳被重塑并混入现代生物群，进而损害其在海平面重建研究中的应用价值。本次研究促使我们重新审视附近卡特林斯湖（Catlins Lake）的部分岩芯序列（cored sequence），并将此前推断的距今5~4.5千年的地震位移，重新解释为盐沼岸坡侵蚀坍塌的结果。