Non-allophanic Andosols of Trindade Island, south Atlantic: a new soil order in Brazil

ABSTRACT The pedological studies carried out so far in Trindade Island (TI) have obtained patchy evidences of allophane, without detailed mineralogical and micromorphological studies to confirm the occurrence of Andosols in TI. Therefore, in this study, the mineralogical, micromorphological, physical and chemical characterization of four soil profiles from Vulcão do Paredão (P1) and Morro Vermelho formations (P2, P3, and P4) were carried on the latest volcanic events in Brazil from Trindade Island (TI) with the aim of to evaluate the presence of Andosols in this oceanic island. Profiles P1 and P2 are developed on pyroclastic bombs, and show, respectively, A-Bi- C and decapitated A-C horizons, whereas P3 and P4 are developed on lapillitic and bomb pyroclasts, show A-C horizons. The soil profiles have a reddish and brownish clayey matrix, are highly friable and show a plastic consistency. Their microstructures are granular, single grain and intergrain microaggregate, in which aggregates display an undifferentiated b-fabric. The mineralogical constituents of the bulk fraction are biotite, hematite, magnetite, ilmenite, pyroxene, olivine, halloysite, goethite, anatase, and rutile. The clay fraction is marked by the presence of halloysite, ferrihydrite, and little amounts of allophane. All soils presented andic properties and can be classified as non-allophanic Andosols. In addition, micromorphological features closely resemble those reported in Andosols from other volcanic islands from elsewhere. The predominance of halloysite in the clay fraction formed by alteration of sideromelane, suggests that allophane would be an intermediate phase of this rapid transformation favored by the wet climate conditions of the highest parts of TI.

摘要
迄今为止针对特林达迪岛（Trindade Island, TI）开展的土壤学研究仅获取了水铝英石（allophane）的零散观测证据，尚未开展详细的矿物学与微形态学研究以证实该岛存在火山灰土（Andosols）。为此，本研究针对采自巴西特林达迪岛帕雷当火山（Vulcão do Paredão，P1）与红峰地层（Morro Vermelho，P2、P3、P4）的4个土壤剖面，开展矿物学、微形态学、物理与化学特性表征，这些剖面均形成于该岛的最新火山活动事件，旨在探究该大洋岛屿中火山灰土的赋存情况。
剖面P1与P2发育于火山碎屑弹之上，分别呈现A-Bi-C与削顶A-C土层序列；而P3与P4发育于火山砾质与火山碎屑基质之上，仅具有A-C土层序列。供试土壤剖面均具红棕色黏粒基质，质地疏松易碎且呈塑性稠度。其微结构以粒状、单颗粒及粒间微团聚体为主，团聚体呈现未分化基质组构（undifferentiated b-fabric）。
全粒级组分的矿物组成包括黑云母、赤铁矿、磁铁矿、钛铁矿、辉石、橄榄石、埃洛石（halloysite）、针铁矿、锐钛矿与金红石。黏粒组分则以埃洛石、水铁矿及少量水铝英石为特征。所有供试土壤均表现出火山灰特性（andic properties），可归类为非水铝英石型火山灰土（non-allophanic Andosols）。此外，其微形态特征与全球其他火山岛发育的火山灰土的已有报道特征高度相似。
黏粒组分中以铁镁质火山玻璃（sideromelane）蚀变形成的埃洛石占主导，这表明水铝英石是该快速蚀变过程的中间相，而特林达迪岛最高区域的湿润气候条件促进了这一转化过程。