Geochemistry and age determination of sediments from teh Rainbow hydrothermal plume

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of "hydrothermal" elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr-1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores' surface mixed layers are presented which, typically, are an order of magnitude greater than "background" authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time - perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.

本研究针对采自彩虹热液田（Rainbow hydrothermal field）2至25km范围内、直接位于热液羽流（hydrothermal plume）下方的4套沉积物岩芯（sediment cores）开展了地球化学调查。这些沉积物除含有占比超80%的碳酸钙（CaCO3）生物组分外，还记录了热液羽流沉降带来的铁（Fe）、铜（Cu）、锰（Mn）、钒（V）、磷（P）及砷（As）元素的显著富集，但仅含少量碎屑背景物质。随热液喷口（vent site）距离增加，‘热液源’元素的丰度呈现系统性变化，这与扩散羽流的化学演化过程相符。此外，在距喷口最近的2个岩芯的特定层位中，镍（Ni）与铬（Cr）出现显著富集，表明这两处近场区域曾存在超基性岩物质（ultrabasic material）的间歇性额外输入事件。放射性碳定年法（radiocarbon dating）结果显示，4套岩芯的全新世（Holocene）平均沉积速率为2.7至3.7厘米·千年⁻¹，表层混合层（surface mixed layers）厚度为7至10+厘米，据此可反推彩虹热液羽流的沉积历史。热液羽流沉降可为下伏沉积物提供两类元素：一类直接来自热液活动（如Fe、Mn、Cu），另一类则通过热液羽流从海水中吸附获得（如V、P、As）。本研究还给出了岩芯表层混合层的全新世元素通量，其数值通常较北大西洋开阔海域的‘背景’自生通量（authigenic fluxes）高出一个数量级。其中1套采自距喷口最近的岩芯显示，在距今8至12个¹⁴C千年的时段内，热液源物质的浓度与通量均出现显著提升；主流解释认为，这一变化反映了同期彩虹热液田热液活动的启动或增强——这或许与该受断层控制的热液环境（fault-controlled hydrothermal setting）中发生的特定构造事件（tectonic event）存在关联。