Smear slide analysis of sediment core GeoB5836-2

Laminated sediments spanning the last 20,000 years (though not continuously) in the Shaban Deep, a brine-filled basin in the northern Red Sea, were analyzed microscopically and with backscattered electron imagery in order to determine laminae composition with emphasis on the diatomaceous component. Based on this detailed study, we present schematic models to propose paleoflux scenarios for laminae formation at different time-slices. The investigated core (GeoB 5836-2; 26°12.61'N, 35°21.56'E; water depth 1475 m) shows light and dark alternating laminae that are easily distinguishable in the mid-Holocene and at the end of the deglaciation (13-15 ka) period. Light layers are mainly composed of coccoliths, terrigenous material and diatom fragments, while dark layers consist almost exclusively of diatom frustules (monospecific or mixed assemblages). The regularity in the occurrence of coccolith/diatom couplets points to an annual deposition cycle where contrasting seasons and associated plankton blooms are represented (diatoms-fall/winter deposition, coccoliths-summer signal). We propose that, for the past ~15,000 years, the laminations represent two-season annual varves. Strong dissolution of carbonate, with the concomitant loss of the coccolith-rich layer in sediments older than 15 ka, prevents us from presenting a schematic model of annual deposition. However, the diatomaceous component reveals a marked switch in species composition between Last Glacial Maximum (LGM) sediments (dominated by Chaetoceros resting spores) and sediments somewhat younger (18-19 ka; dominated by Rhizosolenia). We propose that different diatom assemblages reflect changing conditions in stratification in the northern Red Sea: Strong stratification conditions, such as during two meltwater pulses at 14.5 and 11.4 ka, are reflected in the sediment by Rhizosolenia layers, while Chaetoceros-dominated assemblages represent deep convection conditions.针对红海北部卤水充填盆地沙班海槽（Shaban Deep）内覆盖近2万年的沉积序列（虽非连续沉积），我们通过显微分析结合背散射电子成像（backscattered electron imagery）技术，对其中的纹层沉积物（Laminated sediments）展开研究，重点解析纹层组成中的硅藻组分。基于此项精细化工作，我们构建了模式示意图，提出不同时间切片下纹层形成的古通量情景（paleoflux scenarios）。本次研究的岩心（GeoB 5836-2；北纬26°12.61′，东经35°21.56′；水深1475 m）显示，全新世中期与冰消期末期（13~15 ka）的沉积物中，清晰可见易于分辨的明暗交替纹层。浅色纹层主要由颗石藻、陆源物质及硅藻碎片构成，而深色纹层则几乎完全由硅藻壳体（单种或混合组合）组成。颗石藻-硅藻纹层对的出现规律，指示了一个年沉积周期：对应季节迥异的浮游植物水华事件，其中硅藻对应秋季/冬季沉积，颗石藻代表夏季信号。我们提出，在过去约1.5万年中，该纹层沉积代表双季成年纹层（varves）。对于15 ka以上的沉积物，碳酸盐发生强烈溶蚀，伴随富颗石藻纹层的缺失，使得我们无法构建年沉积周期的示意图模型。不过，硅藻组分清晰揭示出，末次冰盛期（Last Glacial Maximum，LGM）沉积物（以角毛藻休眠孢子（Chaetoceros resting spores）为主）与稍年轻沉积物（18~19 ka，以根管藻属（Rhizosolenia）为主）之间存在显著的物种组成转变。我们认为，不同的硅藻组合反映了红海水体层化（stratification）条件的变化：如14.5 ka与11.4 ka两次融水事件（meltwater pulses）期间的强层化条件，在沉积物中以根管藻纹层体现；而以角毛藻为主的组合则代表深水对流环境。