Arsenic — an indicator for post-depositional water exchange in sediments at neutral to alkaline conditions

Sediment archives are invaluable for reconstructing past environmental conditions. Clastic sediments are formed through physical processes, such as weathering of rocks, during which minerals are released from the parent material, transported, and eventually deposited. However, both transport and depositional processes—such as abrasion, mixing, or chemical dissolution—can alter the original signals preserved in the sediment. Water plays a central role in these transformations by promoting processes like dissolution and recrystallization of minerals. This is particularly relevant for sediments deposited in stagnant waters such as lakes or marine basins. Similarly, infiltration of surface water flows into sediments, enhances chemical reactions and facilitates the transformation of minerals. Fine-grained sediments, such as sands or clays, are especially reactive due to their large specific surface area, which increases their interaction with fluids.

To determine the original composition, isotopic signatures, or depositional and source age of the sediment and the contained minerals, it is crucial to know whether, and to what extent, chemical alterations have occurred during and after the deposition. A critical factor in the analysis is whether the sediment layer represents a closed or open system. In closed systems, the original chemical signatures are preserved, as no significant element exchange occurs with the surrounding environment. In contrast, in open systems, interactions with water can lead to the loss or accumulation of elements, which can alter the sediment's original composition at the time of deposition, making it difficult to interpret its geochemical and mineralogical history.

In a comprehensive mineralogical and geochemical study combined with field observations, we analyzed solids and eluates of 250 sediment samples from the Miocene the Northern Alpine Foreland basin. Additionally, we separated and analyzed the mineral chemistry of over 30 samples (also clay fraction) using SEM. We examined the mineral textures with high-resolution microscopy. Furthermore, we performed extensive leaching experiments to study the mobilization behavior of the contained elements under “open system conditions”.

The results of this study revealed the following: 1) A significant proportion of the minerals in the sediments are newly formed. 2) Experiments showed that processes like dissolution and precipitation can proceed very quickly (within hours to days), depending on factors such as pH, Eh and concentration of dissolved elements. However, these processes can be halted when the sediment runs dry or when chemical equilibrium in the system is achieved. 3) In open-system sediment profiles, water infiltration causes the leaching of elements that are transported to deeper stratigraphic layers, where they precipitate and form new mineral phases.

A key observation is the elution behavior of arsenic: In sediments known to represent open systems, arsenate is not mobilized in the elution tests, because it is fixed in the mineral phases or is already desorbed during the water exchange. In closed systems, elevated arsenic concentrations in the eluate indicate that these sediments have not undergone significant post-depositional water exchanges. The analysis demonstrated that arsenic is predominantly adsorbed on phyllosilicates. Accordingly, an elution test is a reliable indicator of water exchange in sediments after their deposit.