Abundance of planktonic foraminifera from IODP Hole 339-U1386C

The base of the Faro contouritic drift (IODP Site U1386) on the northern margin of the Gulf of Cádiz (Algarve margin) is characterized by two sequences of frequent gravity deposits with different ages and compositions. Among these gravity deposits, several relatively thick debrites (up to 12 m) have been observed and studied in detail. Sedimentological analyses have been performed and because of non-turbulent behavior of debris flows, detailed micropaleontological studies could be realized. Planktonic foraminifera thus allowed establishing a detailed biostratigraphy of these deposits. Benthic foraminifer and ostracod assemblageswere used to evaluate the origin of the sediment composing these debris flows and estimate their run-out distance. These debrites are dated fromEarly Pliocene and early Pleistocene, andwere deposited in amesobathyal environment. They comprise silty mud clasts and matrixes with sand content up to 34%. The Early Pliocene debrites are bioclast-rich whereas the Early Pleistocene debrite is enriched in terrigenous particles. The data indicates that these debrites were triggered on the continental shelf and traveled less than 100 km, eroding the seafloor all along their path for the Early Pliocene debrites and only the first part of their path for the early Pleistocene debrite.Matrixes originate fromfailure areaswhereas eroded sediments along the flow pathway are incorporated into the flow as clasts. High abundance of shelf fauna during the Early Pliocene and great supply of terrigenous particles from rivers during the early Pleistocene in the south-western Iberian margin have favored gravity flows fromthe continental shelf to the slope. The contouritic paleo-moat of the Faro drift has been a determining channeling feature for gravity flows along the Algarve margin during the early Pleistocene, testifying of the strong interaction between MOW circulation and down-slope processes. Tectonic and diapiric activities were significant during Early Pliocene and early Pleistocene on the Algarve margin and could have been triggering parameters of failures related to these debris flows.

位于加的斯湾北缘（阿尔加维陆缘）的法罗等深流漂积体（Faro contouritic drift）的国际大洋发现计划（International Ocean Discovery Program, IODP）U1386站位基底，发育两套时代与组分各异的高频重力流沉积序列。在该类重力流沉积中，已被发现并开展详尽研究的数套最大厚度可达12米的厚层碎屑流沉积（debrite）。研究人员已完成沉积学分析，且由于碎屑流呈非湍流运动特性，本次研究得以开展细致的微古生物学研究。其中，浮游有孔虫（planktonic foraminifera）为这套沉积建立了高精度生物地层学（biostratigraphy）格架。研究人员利用底栖有孔虫（benthic foraminifer）与介形类（ostracod）组合，分析了构成碎屑流沉积物的物源，并估算了其运移距离（run-out distance）。这批碎屑流沉积的年代为上新世早期与更新世早期，形成于中深海底环境（mesobathyal environment）。其组分包含粉砂质泥砾以及砂含量最高可达34%的基质。上新世早期的碎屑流沉积富生物碎屑，而更新世早期的碎屑流沉积则以陆源颗粒（terrigenous particles）富集为特征。数据表明，这批碎屑流沉积均由大陆架（continental shelf）区域的失稳事件触发，运移距离不足100千米；其中上新世早期的碎屑流沉积在整条运移路径上均对海底进行了侵蚀，而更新世早期的碎屑流沉积仅在运移路径的初始段发生海底侵蚀。碎屑流基质来源于沉积物失稳区域，而运移路径上被侵蚀的沉积物则以砾石的形式被裹挟进入碎屑流中。伊比利亚西南缘在上新世早期陆架生物种群繁盛，更新世早期则有充足的河流陆源颗粒补给，二者均促进了大陆架向陆坡（slope）的重力流活动。法罗等深流漂积体发育的古等深流凹槽，在更新世早期成为阿尔加维陆缘重力流运移的关键导向通道，证实了地中海流出水（Mediterranean Outflow Water, MOW）环流与陆坡下坡过程之间存在强烈的相互作用。阿尔加维陆缘在上新世早期与更新世早期发生了显著的构造活动与底辟活动，这些活动可能是触发本次研究涉及的碎屑流沉积失稳事件的诱因。