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Abstract. Mineral specific surface area (SSA) increases as primaryminerals weather and restructure into secondary phyllosilicate, oxide, andoxyhydroxide minerals. SSA is a measurable property that captures cumulativeeffects of many physical and chemical weathering processes in a singlemeasurement and has meaningful implications for many soil processes,including water-holding capacity and nutrient availability. Here we reportour measurements of SSA and mineralogy of two 21 m deep SSA profiles attwo landscape positions, in which the emergence of a very small mass percent(<0.1 %) of secondary oxide generated 36 %–81 % of the total SSAin both drill cores. The SSA transition occurred near 3 m at bothlocations and did not coincide with the boundary of soil to weathered rock. The3 m boundary in each weathering profile coincides with the depth extentof secondary iron oxide minerals and secondary phyllosilicates. Althoughelemental depletions in both profiles extend to 7 and 10 m depth, themineralogical changes did not result in SSA increase until 3 m depth. Theemergence of secondary oxide minerals at 3 m suggests that this boundary may bethe depth extent of oxidation weathering reactions. Our results suggest thatoxidation weathering reactions may be the primary limitation in thecoevolution of both secondary silicate and secondary oxide minerals. Wevalue element depletion profiles to understand weathering, but our findingof nested weathering fronts driven by different chemical processes (e.g.,oxidation to 3 m and acid dissolution to 10 m) warrants the recognition thatelement depletion profiles are not able to identify the full set ofprocesses that occur in weathering profiles. 

ABSTRACT Stone and earthen architecture is nearly ubiquitous in the archaeological record of Pacific islands. The construction of this architecture is tied to a range of socio-political processes, and the temporal patterning of these features is useful for understanding the rate at which populations grew, innovation occurred, and social inequality emerged. Unfortunately, this temporal patterning is poorly understood for many areas of the region, including the Sāmoan archipelago. Here, we describe a project directed toward establishing a robust chronology for the construction of these earthen and stone terraces and linear mounds on Ta‘ū Island. Using recent methodological improvements, we highlight the tempo at which different architectural types were constructed on the island and the implications for understanding demographic expansion and changing land tenure practices in the last 1500 years. This research suggests the construction of architecture was largely confined to the 2nd millennium AD with a small number of terraces plausibly built in the 1st millennium AD. This temporal patterning suggests that a reconfiguration of settlement patterns occurred within West Polynesia as people there moved into other regions of Oceania. 

Phosphorus (P) is an essential nutrient for life. Deficits in soil P reduce primary production and alter biodiversity. A soil P paradigm based on studies of soils that form on flat topography, where erosion rates are minimal, indicates P is supplied to soil mainly as apatite from the underlying parent material and over time is lost via weathering or transformed into labile and less-bioavailable secondary forms. However, little is systematically known about P transformation and bioavailability on eroding hillslopes, which make up the majority of Earth's surface. By linking soil residence time to P fractions in soils and parent material, we show that the traditional concept of P transformation as a function of time has limited applicability to hillslope soils of the western Southern Alps (New Zealand) and Northern Sierra Nevada (USA). Instead, the P inventory of eroding soils at these sites is dominated by secondary P forms across a range of soil residence times, an observation consistent with previously published soil P data. The findings for hillslope soils contrast with those from minimally eroding soils used in chronosequence studies, where the soil P paradigm originated, because chronosequences are often located on landforms where parent materials are less chemically altered and therefore richer in apatite P compared to soils on hillslopes, which are generally underlain by pre-weathered parent material (e.g., saprolite). The geomorphic history of the soil parent material is the likely cause of soil P inventory differences for eroding hillslope soils versus geomorphically stable chronosequence soils. Additionally, plants and dust seem to play an important role in vertically redistributing P in hillslope soils. Given the dominance of secondary soil P in hillslope soils, limits to ecosystem development caused by an undersupply of bio-available P may be more relevant to hillslopes than previously thought.