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Spatial and temporal variability of seabed disturbance in the York River subestuary

机译：约克河河口海床扰动的时空变化

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摘要

Seabed and water column observations from the microtidal York River subestuary were used to examine temporal changes in this fine-grained environment and compare the impact of infrequent disturbance events (annual to interannual time scales) with more frequent events (fortnightly to seasonal time scales). An experimental across-channel transect is described by time-series observations of seabed radiochemical profiles, X-radiographs, sidescan sonar, and current meter data for 1999. Four morphological subenvironments were distinguished based on the average maximum ~(137)Cs penetration depth and the character of excess ~(210)Pb profiles: southern shoal, flank, secondary channel, and main channel. Various combinations of long- and short-term events (e.g. spring-neap tidal cycle, spring freshet, storms) influence subeuvironment sediment structure and physical mixing depths. Shoal sediments were eroded/deposited on the spring neap tidal cycle during the early spring months, whereas the other three subenvironments were not noticeably influenced by tides on a fortnightly basis. Shortly after the peak currents of the annual spring freshet, sediments were temporarily deposited on the shoal and flank. Major events, such as Hurricane Floyd, did not affect this section of the river except as a result of subsequent longitudinal sediment transport along the estuary main channel that resulted in rapid sediment erosion and deposition of ～ 100 cm in ～1 month. The formation and infilling of longitudinal furrows have previously been implicated as contributing to shallow (25 cm) seabed mixing in the secondary channel. However, interannual variations in river discharge and estuarine circulation likely control sediment supply to this section of the river and may augment the location and frequency of furrow formation within the secondary channel and the main channel. Overall, the data indicate that the seabed at the upper York transect is frequently physically disturbed such that bioturbation structures are not preserved in surface sediments (0-30cm). However, the presence of bioturbation structures below 30 cm in some of these subenvironments suggests either that this section of the river has experienced long-term changes in the dominant mixing mechanism or that the sedimentary stratigraphy is event-driven.
机译：来自潮汐约克河河口的海床和水柱观测数据被用来检查这种细粒度环境中的时间变化，并比较不频繁的干扰事件（每年至每年的时间尺度）与更频繁的事件（每两周至季节性的时间尺度）的影响。通过对海床放射化学剖面的时间序列观测，X射线照相，侧扫声纳和1999年的电流表数据来描述实验性的跨通道断面。根据平均最大〜（137）Cs穿透深度和〜（210）Pb过量剖面的特征：南部浅滩，侧面，次要通道和主要通道。长期和短期事件的各种组合（例如，春季-潮汐周期，春季新生，风暴）会影响次环境沉积物的结构和物理混合深度。在春季的早春月份，浅滩沉积物在春季的潮汐周期中被侵蚀/沉积，而其他三个亚环境在两周的时间内没有受到潮汐的明显影响。在一年一度的春季新生高峰期过后不久，沉积物暂时沉积在浅滩和侧翼。诸如弗洛伊德飓风之类的主要事件并未影响这部分河段，除非随后沿河口主河道进行了纵向泥沙输送，从而导致泥沙迅速侵蚀，并在约1个月内沉积了约100 cm。纵向沟的形成和填充以前被认为与辅助通道中浅层（25厘米）海床混合有关。但是，河道流量和河口环流的年际变化可能会控制该河段的沉积物供应，并可能增加次要通道和主要通道内形成沟的位置和频率。总体而言，数据表明，约克横断面上游的海床经常受到物理干扰，以致在表层沉积物（0-30厘米）中未保留生物扰动结构。但是，在其中一些子环境中，低于30厘米的生物扰动结构的存在表明这条河的主要混合机制经历了长期变化，或者沉积地层是事件驱动的。

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