A rock-glacier – pond system (NW Italian Alps): Soil and sediment properties, geochemistry, and trace-metal bioavailability

Highlights

• Different rock-glacier geochemistry and ecology with respect to surrounding areas.

• Rock-glacier debris transfer and meltwater inflow affected pond geochemistry.

• Enhanced bioavailability of trace metals for aquatic organisms was found.

• Rock-glacier advance created a dam and modified the level of impounded water.

• The dam constituted a sediment trap with geochemical and ecological implications.

Abstract

Rock-glacier sediment transfer and ice melting can impact surface waters located downstream. However, there is a lack of knowledge on the influence of rock-glacier dynamics on the geochemical, hydrochemical, and ecological characteristics of adjacent impounded surface waters. In the Col d'Olen area (Long-Term Ecological Research site, NW Italian Alps), an intact rock glacier terminates into a pond and solute-enriched waters originating from the rock glacier flow into the pond through a subsurface hydrological window. In this study, we performed geophysical and ground surface temperature measurements. Moreover, we sampled soils and sediments in different compartments of the investigated rock-glacier - pond system and we further sampled benthic invertebrates in the pond. Cold ground thermal regime, ground-ice presence, and coarse debris cover on the rock glacier together with its lithology (serpentinites) influence the rock-glacier geochemistry and ecology with respect to surrounding areas. Pond geochemistry is affected by transfer of trace-metal-enriched fine-grained debris and meltwaters from the rock glacier. Enhanced bioavailability of serpentinite-associated trace metals was proved, with concentrations of Ni and Cr in benthic invertebrates up to 384 and 110 mg kg⁻¹ d.w., respectively, potentially exerting toxic effects on pond biota. The advancing movement of the rock glacier not only has delivered sediments to the pond, but it has progressively filled the valley depression where the pond is located, creating a dam that could have modified the level of impounded water. This process likely constituted a sediment trap in which serpentinitic rock-glacier sediments could be deposited at the pond bottom, with related geochemical and ecological implications. This study illustrates the importance of rock glaciers in influencing the characteristics of downstream freshwater bodies and highlights the need to improve our knowledge about climate-change-related impacts of rock-glacier dynamics on alpine headwaters.

Introduction

Rock glaciers are slowly flowing mixtures of rock debris and ice, and constitute prominent sedimentary linkages within the alpine environment (Giardino et al., 1987). Rock glaciers act as efficient sediment conveyors in mountainous environments (Delaloye et al., 2010, Gärtner-Roer, 2012), transferring large quantities of debris from their rooting zone (upslope area) to their fronts (Kummert et al., 2018), and potentially impacting surface waters located downstream (Kummert and Delaloye, 2018a). Lakes and ponds can also form as a consequence of stream and river channel interruption due to rock-glacier advancing movement (Blöthe et al., 2019).

Melting ice in rock glaciers can impact the chemical characteristics of surface freshwater (Colombo et al., 2018a). Indeed, several studies report higher concentrations of trace metals and nutrients in their meltwaters in comparison to surface waters not affected by rock-glacier presence (Williams et al., 2006, Williams et al., 2007, Thies et al., 2013, Ilyashuk et al., 2014, Ilyashuk et al., 2018, Fegel et al., 2016, Colombo et al., 2018b, Colombo et al., 2019). Metals in rock-glacier-affected surface waters have been also shown to bioaccumulate, potentially impacting aquatic communities (Thies et al., 2013, Ilyashuk et al., 2014, Ilyashuk et al., 2018).

High-elevation impounded surface waters are key freshwater reference sites for global scale processes, due to minimal direct human influence and because of their rapid response to climate-related changes (Adrian et al., 2009, Salerno et al., 2014). Recent studies have demonstrated that lakes and ponds (defined as a water bodies < 2 × 10⁴ m², Hamerlík et al., 2014) affected by rock-glacier ice melting have suffered changes in their chemical and biological status (Ilyashuk et al., 2014, Ilyashuk et al., 2018), especially in the context of climate change during the last decades (Thies et al., 2007). Recent climatic changes have caused strong modifications in glacial and permafrost environments at global scale (Harris et al., 2009, Zemp et al., 2015, Biskaborn et al., 2019) and, specifically, in the NW Italian Alps (Giaccone et al., 2015, Colombo et al., 2016a, Colombo et al., 2016b).

Although a growing literature has documented the role of rock-glacier ice melting in impacting the alpine headwater hydrochemistry, there is a paucity of studies that investigated the influence of rock glaciers on sedimentological, hydrochemical, and ecological dynamics of surface waters located downstream. In this context, the Col d’Olen area (NW Italian Alps) represents a suitable model system. In fact, in this area, an intact rock glacier terminates into a pond and solute-enriched waters originating from the rock glacier flow into the pond through a subsurface hydrological window (Colombo et al., 2018b, Colombo et al., 2018c).

Our hypothesis is that the rock-glacier dynamics may influence the pond in terms of lacustrine sediment geochemistry and bioavailability of trace metals for aquatic organisms, potentially exerting adverse effects. In order to test this hypothesis, we sampled both soils and sediments in different compartments of the rock-glacier - pond system and we further sampled benthic invertebrates in the pond to: (i) assess the potential influence of the rock glacier and the surrounding soils on the lacustrine sediment geochemistry, (ii) investigate trace-metal bioaccumulation in aquatic organisms, and (iii) attempt to assess the effect of rock-glacier dynamics on potential pond systemic modifications.