Geochemistry of a Late Quaternary loess-paleosol sequence in central Argentina: Implications for weathering, sedimentary recycling and provenance

Abstract

A loess-paleosol sequence in the Pampean Plain, central Argentina was analyzed geochemically, in order to evaluate the weathering signature and sedimentary recycling as well as to analyze the origin of these loessic sediments. Corralito I sequence is composed of three paleosols and a buried soil separated by loess layers. The loess-paleosol sequence shows a similar geochemical composition throughout, suggesting that pedogenesis has not been intense enough to mask the chemical signature of the parent material. Weathering indices (CIA, CIW, PIA and LWI), elemental ratios (Rb/Sr, CaO/TiO 2 and Na 2 O/TiO 2), and the A-CN-K ternary diagram indicate incipient chemical alteration for this sequence, compatible with a weathering-limited regime. As expected, paleosols (CIAs~60) exhibit a slightly higher chemical alteration than loess layers (CIAs~58). Mass balance calculations reveal the losses of some major oxides (i.e., CaO, Na 2 O) and Sr in the buried soil and paleosols compared to its loess mantles, reflecting the weathering of plagioclase. Besides, the losses of K 2 O and Ba in the buried soil and paleosols I and II, when compared to its loess layers, suggest the chemical alteration of Kfeldspar. The chemical alteration of volcanic glass is evidenced by the relative losses of As in almost all levels. Conversely, the relative gains of Fe 2 O 3 and MgO reveal that pyroxenes and amphiboles or Fe oxy-hydroxides are concentrated in paleosols and the buried soil compared to its loess mantles. The geochemical approach used to constrain the origin of these Pampean Plain loessic sediments indicates they derive mainly from the Andean region. Conversely, the Pampean ranges, the Paraná River basin and Uruguayan Precambrian outcrops, which were pointed out as minor local sources, do not contribute significantly to the loess deposits. This work also shows that Corralito I loess-paleosol sequence and other loess samples from Argentina do not evidence sedimentary recycling, suggesting that these Pampean Plain loess deposits are mainly composed by young materials derived from undifferentiated volcanic rocks. The chemical composition of loessic sediments reveals the composition of the dust sources and reflects the modifications induced during transport and post-depositional weathering (e.g., Ahmad and Chandra, 2013). Different geochemical approaches have been used to analyze chemical weathering and provenance in loess-paleosol sequences in various regions of the world (e.g.,

Disentangling Sedimentary Pathways for the Pleniglacial Lower Danube Loess Based on Geochemical Signatures

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Last glacial loess dynamics in the Southern Caucasus (NE-Armenia) and the phenomenon of missing loess deposition during MIS-2

“…5 ) 50 . Likewise the potential source areas of the Armenian loess exhibit high χ lf values suggesting that the parent material of the Armenian loess is not comparable to that of loess common in SE Europe or China, but is strongly affected by weathered volcanogenic minerals as it is known from the vast loess areas of Argentina and southern Brazil 51 , 52 . The high magnetic susceptibility (MS) in LPS of NE-Armenia may relate to high proportions of metallic ores within volcanic glasses that attain background levels of up to 20% within unaltered loess 53 .…”

Sand and mud generation from continental flood basalts in contrasting landscapes and climatic conditions (Paraná–Etendeka conjugate igneous provinces, Uruguay and Namibia)

“…Ratios of non‐mobile elements in the studied sediments and comparison with regional low‐Ti basalts (based on Bellieni et al ., 1984, Peate, 1997, Kirstein et al ., 2000, Marsh et al ., 2001, Iacumin et al ., 2003, Ewart et al ., 2004, and Hartmann et al ., 2013) and loess sequences at similar latitude (based on Pasquini et al ., 2017 and Campodonico et al ., 2019). …”