Sulfurization, acid-sulfate soils and active layer monitoring at the semiarid Seymour Island, Antarctica

Abstract

Soils and permafrost occurrence and behavior at the Weddell Sea zone, located in the key zone of climatic transition between Maritime Antarctica and Continental Antarctica, remain little studied. We studied the main chemical, physical and mineralogical properties of representative soil profiles of the northern part of Seymour Island, discussing soil formation processes and the active-layer thermal and water regimes of a representative site. Soil sampling and installation of the monitoring site was carried out in the northern part of Seymour Island. Permafrost was generally detected within the first 100 cm of depth, except at coastal areas, although typical periglacial features (cryoturbation), vegetation growth and organic matter accumulation were negligible or absent. Well-drained soils showed low Electric Conductivity (EC), indicating leaching of salts, whereas near-permafrost layers showed higher values of EC. Salt content and EC increased at lower altitudes. Two major groups of soils were identified: (i) Acid Sulfate Soils (ASS); and (ii) Weakly Developed Alkaline Soils (WDAS). Acid Sulfate Soils develop under the influence of sulfide-containing material under oxidizing conditions, and are the most developed and weathered in the area, presenting significant morphological, mineralogical and chemical traits, such as low pH values, sulfuric B horizons, and the presence of secondary minerals indicating sulfurization process, such as jarosite and low crystallinity iron oxide. Weakly Developed Alkaline Soils develop under low chemical and mineralogical transformation, where physical weathering is a key process. These soils present high pH and high content of primary minerals in the clay fraction, mainly K-feldspars and plagioclase. The active-layer thermal regime is typical of periglacial environments, with seasonal temperature variations of soil around 0 °C, with prevailing negative temperatures. Regional climate was comparable with Continental Antarctica, whereas the pedoclimate (active layer thickness, soil water content) showed closer affinity with Maritime Antarctica. Some soil forming processes are driven by pedoclimate, and favored by the unfrozen and free-drainage conditions during the summer season; however, the main factor influencing soil development is parent material. The main soil formation processes are sulfurization, cryoclasty and halomorphism, since the semiarid dry climate does not favor cryoturbation and organic matter accumulation.