Coastal lagoons as archives of event deposits, Belize, Caribbean Sea

在该项目的第一阶段，在伯利兹中部沿海的五个地点收集了26个可控震源，以回收浅泻湖(包括红树林沼泽、沼泽、洪泛平原、沙坑)及其周围的第四纪沉积物。从沉积学、古生态学和年代学的角度对这些岩心进行了研究，以了解海岸的发展，并整理出晚更新世至全新世期间主要气旋的全新世记录。沿海泻湖以细粒背景沉积为特征，因此粗粒事件沉积，如风暴床，有可能突出识别。过去几十年来，大西洋飓风强度增加的原因尚不确定。因此，在更长的时间序列中研究这一现象的历史是至关重要的，因为它超出了人类记录的时间。由于现有的全新世气旋档案远远不完整，仅覆盖当地地区，而且也存在争议，因此探索更多的记录是重要的。伯利兹对于这样的研究特别有趣，因为可以评估事件沉积对公元800-1000年左右崩溃的经典玛雅文明可能产生的影响。到目前为止，已对伯利兹4个沿海地区的22个岩心进行了分析。通常在岩心中形成底部的更新世相是黄色到微红色的硬壤土。上覆全新世相包括富含有机质的泥灰泥和粉砂、石英砂和泥炭/泥炭沉积。Colson Point和Manatee泻湖泥质单元内的壳层主要由耐盐双壳类楔形无名心虫主导。这种观察和大小-频率图表明贝壳是原地/泻湖沉积的。软体动物沿岩心的分布表明，在全新世海平面上升期间可能发生了相的退变。商业海湾泻湖的近岸泻湖岩心含有丰富的石英砂层，与更细颗粒的沉积物和波利茨珊瑚等海洋生物相间。虽然薄石英砂层的起源尚不完全清楚，但珊瑚沉积和1米厚的沙层被解释为在高能事件期间过冲过沙质屏障的结果。泥炭、软体动物贝壳和珊瑚骨骼的放射性碳测年允许进行相对比，并指示最大值。全新世沉积物的年龄分别为~5.5kyr(Commerce Bight)、~6kyrBP(Colson Point)、~6.5kyrBP(Sapodilla，Manatee)。珊瑚事件层的年龄分别为~1.5和4.95 kyr BP。相比之下，先前的研究发现伯利兹海岸风暴活动增加，约为1KYR BP和4.5-2.5KYR BP。对于第三年，我们计划从五个地点之一分析剩余的7个核心，对结果进行总体解释，在两次会议上提出结果，并准备两份手稿以供出版。

During the first phase of this project, 26 vibracores at five locations were collected along the coast of central Belize recovering Quaternary deposits from and around shallow lagoons (including mangrove swamps, marshes, floodplains, sand spits). The cores are being studied sedimentologically, paleoecologically, and chronologically in order to understand the coastal development and to put together a Holocene record of major cyclones during the late Pleistocene to Holocene. Coastal lagoons are characterized by fine-grained background sedimentation so that coarse-grained event deposits, such as storm beds, potentially stand out for identification. The reasons for the increase in hurricane intensity in the Atlantic over the past few decades are uncertain. Therefore, it is crucial to study the history of the phenomenon in longer time series that go beyond the time of human recording. Because existing Holocene cyclone archives are far from complete, cover only local areas, and are also controversial it is important that additional records be explored. Belize is especially interesting for such a study because the possible influence of event sedimentation on the classic Maya civilization, which collapsed around AD 800-1000, can be evaluated. So far, 22 cores from 4 coastal locations in Belize have been analyzed. Pleistocene facies, that usually form the base in cores, are stiff loams with yellow to reddish stain. Overlying Holocene facies include organic-rich marly muds and silts, quartz sands and peat/peaty sediments. Shell layers within the muddy units of Colson Point and Manatee Lagoons are mostly dominated by the salinity-tolerant bivalve Anomalocardia cuneimeris. This observation and size-frequency diagrams suggest autochthonous/ lagoonal deposition of the shells. Mollusk distribution along the cores indicates retrogradation of facies that may have occurred during Holocene sea-level rise. Nearshore lagoonal cores from Commerce Bight Lagoon contain abundant quartz sand layers alternating with finer grained sediments and marine organisms like Porites corals. Whereas the origin of thin quartz sand layers is not entirely clear as yet, coral deposits and a 1-m-thick sand layer are interpreted as the result of overwash across sandy barriers during high-energy events. Radiocarbon dating of peat, mollusc shells and coral skeletons allows facies correlation and indicates a max. age of Holocene sediments of ~5.5 kyrs (Commerce Bight), ~6 kyrs BP (Colson Point), ~6.5 kyrs BP (Sapodilla, Manatee). Coral event layers were dated as ~1.5 and 4.95 kyrs BP. For comparison, previous studies found increased storm activity along the Belize coast around 1 kyr BP and from 4.5-2.5 kyrs BP. For the third year, we plan to analyze the remaining 7 cores from one of the five location, to make an overall interpretation of the results, to present the results on two conferences, and to prepare two manuscripts for publication.